Provided by: isc-dhcp-server_4.4.1-2.1ubuntu5.20.04.5_amd64 bug


       dhcpd.conf - dhcpd configuration file


       The  dhcpd.conf  file  contains  configuration information for dhcpd, the Internet Systems
       Consortium DHCP Server.

       The dhcpd.conf file is a free-form ASCII text file.  It is parsed by the recursive-descent
       parser  built  into  dhcpd.   The  file may contain extra tabs and newlines for formatting
       purposes.  Keywords in the file are case-insensitive.  Comments  may  be  placed  anywhere
       within  the  file  (except within quotes).  Comments begin with the # character and end at
       the end of the line.

       The file essentially consists of a list of statements.  Statements  fall  into  two  broad
       categories - parameters and declarations.

       Parameter  statements  either  say  how to do something (e.g., how long a lease to offer),
       whether to do something (e.g., should dhcpd provide addresses to unknown clients), or what
       parameters to provide to the client (e.g., use gateway

       Declarations  are used to describe the topology of the network, to describe clients on the
       network, to provide addresses that can be assigned to clients, or  to  apply  a  group  of
       parameters  to  a group of declarations.  In any group of parameters and declarations, all
       parameters must be specified before any declarations which depend on those parameters  may
       be specified.

       Declarations   about   network   topology   include  the  shared-network  and  the  subnet
       declarations.  If clients on a subnet are to be assigned addresses  dynamically,  a  range
       declaration  must  appear  within  the  subnet  declaration.   For clients with statically
       assigned addresses, or for installations where only known clients  will  be  served,  each
       such  client  must have a host declaration.  If parameters are to be applied to a group of
       declarations which are not related strictly on a per-subnet basis, the  group  declaration
       can be used.

       For  every  subnet  which will be served, and for every subnet to which the dhcp server is
       connected, there must be one subnet declaration, which tells dhcpd how to  recognize  that
       an address is on that subnet.  A subnet declaration is required for each subnet even if no
       addresses will be dynamically allocated on that subnet.

       Some installations have physical networks on which more than one IP subnet operates.   For
       example,  if  there  is  a  site-wide  requirement  that 8-bit subnet masks be used, but a
       department with a single physical ethernet network expands to the point where it has  more
       than  254  nodes,  it may be necessary to run two 8-bit subnets on the same ethernet until
       such time as a new physical network can be added.  In this case, the  subnet  declarations
       for these two networks must be enclosed in a shared-network declaration.

       Note  that  even when the shared-network declaration is absent, an empty one is created by
       the server to contain the subnet (and any scoped parameters included in the subnet).   For
       practical  purposes,  this  means  that  "stateless"  DHCP  clients, which are not tied to
       addresses (and therefore subnets) will receive the same configuration as stateful ones.

       Some sites may have departments which have clients on more than one subnet, but it may  be
       desirable to offer those clients a uniform set of parameters which are different than what
       would be offered to clients from other departments on the same subnet.  For clients  which
       will  be declared explicitly with host declarations, these declarations can be enclosed in
       a group declaration along with the parameters which are common to  that  department.   For
       clients  whose  addresses will be dynamically assigned, class declarations and conditional
       declarations may be used to group parameter assignments based on  information  the  client

       When  a  client  is  to  be  booted, its boot parameters are determined by consulting that
       client's host declaration (if any), and then consulting any  class  declarations  matching
       the  client,  followed  by  the  pool,  subnet  and shared-network declarations for the IP
       address assigned to the client.  Each  of  these  declarations  itself  appears  within  a
       lexical scope, and all declarations at less specific lexical scopes are also consulted for
       client option declarations.  Scopes are never considered  twice,  and  if  parameters  are
       declared  in more than one scope, the parameter declared in the most specific scope is the
       one that is used.

       When dhcpd tries to find a host declaration for a  client,  it  first  looks  for  a  host
       declaration  which  has a fixed-address declaration that lists an IP address that is valid
       for the subnet or shared network on which the client is booting.  If it doesn't  find  any
       such entry, it tries to find an entry which has no fixed-address declaration.


       A typical dhcpd.conf file will look something like this:

       global parameters...

       subnet netmask {
         subnet-specific parameters...

       subnet netmask {
         subnet-specific parameters...

       subnet netmask {
         subnet-specific parameters...

       group {
         group-specific parameters...
         host {
           host-specific parameters...
         host {
           host-specific parameters...
         host {
           host-specific parameters...

                                                Figure 1

       Notice  that  at  the beginning of the file, there's a place for global parameters.  These
       might be things like the organization's domain name, the addresses of the name servers (if
       they are common to the entire organization), and so on.  So, for example:

            option domain-name "";
            option domain-name-servers,;

                                                Figure 2

       As  you  can  see  in  Figure  2, you can specify host addresses in parameters using their
       domain names rather than their numeric IP addresses.  If a given hostname resolves to more
       than  one  IP  address (for example, if that host has two ethernet interfaces), then where
       possible, both addresses are supplied to the client.

       The most obvious reason for having subnet-specific parameters as shown in Figure 1 is that
       each  subnet,  of  necessity,  has  its own router.  So for the first subnet, for example,
       there should be something like:

            option routers;

       Note that the address here is specified numerically.  This is not required - if you have a
       different  domain name for each interface on your router, it's perfectly legitimate to use
       the domain name for that interface instead of the numeric address.  However, in many cases
       there  may be only one domain name for all of a router's IP addresses, and it would not be
       appropriate to use that name here.

       In Figure 1 there is also a group statement, which provides common parameters for a set of
       three  hosts  -  zappo,  beppo  and  harpo.   As  you  can see, these hosts are all in the domain, so it might make sense for a group-specific parameter to override the
       domain name supplied to these hosts:

            option domain-name "";

       Also, given the domain they're in, these are probably test machines.  If we wanted to test
       the DHCP leasing mechanism, we might set the  lease  timeout  somewhat  shorter  than  the

            max-lease-time 120;
            default-lease-time 120;

       You  may  have  noticed  that while some parameters start with the option keyword, some do
       not.  Parameters starting with the option keyword correspond to actual DHCP options, while
       parameters  that  do  not start with the option keyword either control the behavior of the
       DHCP server (e.g., how long a lease dhcpd will give out),  or  specify  client  parameters
       that are not optional in the DHCP protocol (for example, server-name and filename).

       In  Figure  1, each host had host-specific parameters.  These could include such things as
       the hostname option, the name of a file to upload (the filename parameter) and the address
       of  the server from which to upload the file (the next-server parameter).  In general, any
       parameter can appear anywhere that parameters are allowed, and will be  applied  according
       to the scope in which the parameter appears.

       Imagine  that  you  have  a site with a lot of NCD X-Terminals.  These terminals come in a
       variety of models, and you want to specify the boot files for each model.  One way  to  do
       this would be to have host declarations for each server and group them by model:

       group {
         filename "Xncd19r";
         next-server ncd-booter;

         host ncd1 { hardware ethernet 0:c0:c3:49:2b:57; }
         host ncd4 { hardware ethernet 0:c0:c3:80:fc:32; }
         host ncd8 { hardware ethernet 0:c0:c3:22:46:81; }

       group {
         filename "Xncd19c";
         next-server ncd-booter;

         host ncd2 { hardware ethernet 0:c0:c3:88:2d:81; }
         host ncd3 { hardware ethernet 0:c0:c3:00:14:11; }

       group {
         filename "XncdHMX";
         next-server ncd-booter;

         host ncd1 { hardware ethernet 0:c0:c3:11:90:23; }
         host ncd4 { hardware ethernet 0:c0:c3:91:a7:8; }
         host ncd8 { hardware ethernet 0:c0:c3:cc:a:8f; }


       The  pool  and  pool6 declarations can be used to specify a pool of addresses that will be
       treated differently than another pool of addresses, even on the same  network  segment  or
       subnet.   For  example,  you  may  want  to  provide  a large set of addresses that can be
       assigned to DHCP clients that are registered  to  your  DHCP  server,  while  providing  a
       smaller  set of addresses, possibly with short lease times, that are available for unknown
       clients.  If you have a firewall, you may be able to arrange for addresses from  one  pool
       to  be  allowed  access  to  the  Internet,  while addresses in another pool are not, thus
       encouraging users to register their DHCP clients.  To do this, you would set up a pair  of
       pool declarations:

       subnet netmask {
         option routers;

         # Unknown clients get this pool.
         pool {
           option domain-name-servers;
           max-lease-time 300;
           allow unknown-clients;

         # Known clients get this pool.
         pool {
           option domain-name-servers,;
           max-lease-time 28800;
           deny unknown-clients;

       It  is  also possible to set up entirely different subnets for known and unknown clients -
       address pools exist at the level  of  shared  networks,  so  address  ranges  within  pool
       declarations can be on different subnets.

       As  you  can  see in the preceding example, pools can have permit lists that control which
       clients are allowed access to the pool and which aren't.  Each entry in  a  pool's  permit
       list is introduced with the allow or deny keyword.  If a pool has a permit list, then only
       those clients that match specific entries on the  permit  list  will  be  eligible  to  be
       assigned addresses from the pool.  If a pool has a deny list, then only those clients that
       do not match any entries on the deny list will be eligible.    If  both  permit  and  deny
       lists  exist for a pool, then only clients that match the permit list and do not match the
       deny list will be allowed access.

       The pool6 declaration is similar to the pool declaration.  Currently it  is  only  allowed
       within  a  subnet6  declaration,  and  may  not  be  included directly in a shared network
       declaration.  In addition to the range6 statement it allows the prefix6  statement  to  be
       included.  You may include range6 statements for both NA and TA and prefixy6 statements in
       a single pool6 statement.


       Address allocation is actually only done when a client is in the INIT state and has sent a
       DHCPDISCOVER  message.   If the client thinks it has a valid lease and sends a DHCPREQUEST
       to initiate or renew that lease, the server has only three choices -  it  can  ignore  the
       DHCPREQUEST, send a DHCPNAK to tell the client it should stop using the address, or send a
       DHCPACK, telling the client to go ahead and use the address for a while.

       If the server finds the address the client is requesting, and that address is available to
       the client, the server will send a DHCPACK.  If the address is no longer available, or the
       client isn't permitted to have it, the server will send a DHCPNAK.  If  the  server  knows
       nothing  about the address, it will remain silent, unless the address is incorrect for the
       network segment to which the client has been attached and the server is authoritative  for
       that  network segment, in which case the server will send a DHCPNAK even though it doesn't
       know about the address.

       There may be a host declaration  matching  the  client's  identification.   If  that  host
       declaration  contains  a  fixed-address declaration that lists an IP address that is valid
       for the network segment to which the client is connected, the DHCP server  will  never  do
       dynamic  address  allocation.   In  this  case, the client is required to take the address
       specified in the host declaration.  If the client  sends  a  DHCPREQUEST  for  some  other
       address, the server will respond with a DHCPNAK.

       When  the DHCP server allocates a new address for a client (remember, this only happens if
       the client has sent a DHCPDISCOVER), it first looks to see if the  client  already  has  a
       valid  lease on an IP address, or if there is an old IP address the client had before that
       hasn't yet been reassigned.  In that case, the server will take that address and check  it
       to  see  if the client is still permitted to use it.  If the client is no longer permitted
       to use it, the lease is freed if the server thought it was still in use -  the  fact  that
       the client has sent a DHCPDISCOVER proves to the server that the client is no longer using
       the lease.

       If no existing lease is found, or if the client  is  forbidden  to  receive  the  existing
       lease,  then  the server will look in the list of address pools for the network segment to
       which the client is attached for a lease that is  not  in  use  and  that  the  client  is
       permitted  to  have.   It  looks  through  each  pool  declaration  in sequence (all range
       declarations that appear outside of pool declarations are grouped into a single pool  with
       no  permit  list).   If  the permit list for the pool allows the client to be allocated an
       address from that pool, the pool is examined to see if there is an address available.   If
       so,  then  the  client  is tentatively assigned that address.  Otherwise, the next pool is
       tested.  If no addresses are found that can be assigned to the client, no response is sent
       to the client.

       If  an  address  is  found  that  the client is permitted to have, and that has never been
       assigned to any client before, the address is immediately allocated to the client.  If the
       address  is  available  for  allocation  but  has  been previously assigned to a different
       client, the server will keep looking in hopes of finding an address that has never  before
       been assigned to a client.

       The  DHCP  server  generates  the  list of available IP addresses from a hash table.  This
       means that the addresses are not sorted in any particular order, and so it is not possible
       to  predict  the  order  in  which  the  DHCP server will allocate IP addresses.  Users of
       previous versions of the ISC DHCP server may have become accustomed  to  the  DHCP  server
       allocating  IP  addresses in ascending order, but this is no longer possible, and there is
       no way to configure this behavior with version 3 of the ISC DHCP server.


       The DHCP server checks IP addresses to see if they are in use before  allocating  them  to
       clients.   It  does  this  by sending an ICMP Echo request message to the IP address being
       allocated.  If no ICMP Echo reply is received within a second, the address is  assumed  to
       be  free.   This is only done for leases that have been specified in range statements, and
       only when the lease is thought by the DHCP server to be free - i.e., the  DHCP  server  or
       its failover peer has not listed the lease as in use.

       If a response is received to an ICMP Echo request, the DHCP server assumes that there is a
       configuration error - the IP address is in use by some host on the network that is  not  a
       DHCP  client.   It  marks the address as abandoned, and will not assign it to clients. The
       lease will remain abandoned for a minimum of abandon-lease-time seconds.

       If a DHCP client tries to get an IP  address,  but  none  are  available,  but  there  are
       abandoned  IP  addresses,  then  the  DHCP  server will attempt to reclaim an abandoned IP
       address.  It marks one IP address as free, and then does the same ICMP Echo request  check
       described  previously.   If  there  is  no answer to the ICMP Echo request, the address is
       assigned to the client.

       The DHCP server does not cycle through abandoned IP addresses if the first IP  address  it
       tries to reclaim is free.  Rather, when the next DHCPDISCOVER comes in from the client, it
       will attempt a new allocation using the same method described here, and will typically try
       a new IP address.


       This  version  of the ISC DHCP server supports the DHCP failover protocol as documented in
       draft-ietf-dhc-failover-12.txt.  This is not a final protocol document, and  we  have  not
       done interoperability testing with other vendors' implementations of this protocol, so you
       must not assume that this implementation conforms to the standard.  If you wish to use the
       failover  protocol, make sure that both failover peers are running the same version of the
       ISC DHCP server.

       The failover protocol allows two DHCP servers (and no more than two)  to  share  a  common
       address  pool.  Each server will have about half of the available IP addresses in the pool
       at any given time for allocation.  If one server fails, the other server will continue  to
       renew  leases  out of the pool, and will allocate new addresses out of the roughly half of
       available addresses that it had when communications with the other server were lost.

       It is possible during a prolonged failure to tell the  remaining  server  that  the  other
       server  is  down,  in  which  case  the  remaining server will (over time) reclaim all the
       addresses the other server had available for allocation, and begin to reuse them.  This is
       called putting the server into the PARTNER-DOWN state.

       You can put the server into the PARTNER-DOWN state either by using the omshell (1) command
       or by stopping the server, editing the last failover state declaration in the lease  file,
       and restarting the server.  If you use this last method, change the "my state" line to:

       failover peer name state {
       my state partner-down;.
       peer state state at date;

       It is only required to change "my state" as shown above.

       When  the  other server comes back online, it should automatically detect that it has been
       offline and request a complete update from the server that was running in the PARTNER-DOWN
       state, and then both servers will resume processing together.

       It  is possible to get into a dangerous situation: if you put one server into the PARTNER-
       DOWN state, and then *that* server goes down, and the other  server  comes  back  up,  the
       other  server  will  not know that the first server was in the PARTNER-DOWN state, and may
       issue addresses previously issued by the other server to different clients,  resulting  in
       IP  address  conflicts.   Before putting a server into PARTNER-DOWN state, therefore, make
       sure that the other server will not restart automatically.

       The failover protocol defines a primary server role and a secondary  server  role.   There
       are  some  differences  in  how primaries and secondaries act, but most of the differences
       simply have to do with providing a way for each peer to behave in the  opposite  way  from
       the  other.  So one server must be configured as primary, and the other must be configured
       as secondary, and it doesn't matter too much which one is which.


       When a server starts that has not previously communicated with its failover peer, it  must
       establish  communications  with  its  failover  peer and synchronize with it before it can
       serve clients.  This can happen either because you have just configured your DHCP  servers
       to perform failover for the first time, or because one of your failover servers has failed
       catastrophically and lost its database.

       The initial recovery process is designed to ensure that when one failover peer  loses  its
       database  and  then  resynchronizes,  any leases that the failed server gave out before it
       failed will be honored.  When the failed server starts up, it notices that it has no saved
       failover state, and attempts to contact its peer.

       When  it  has  established  contact, it asks the peer for a complete copy its peer's lease
       database.  The peer then sends its complete database, and sends a message indicating  that
       it  is done.  The failed server then waits until MCLT has passed, and once MCLT has passed
       both servers make the transition back into normal operation.  This waiting period  ensures
       that any leases the failed server may have given out while out of contact with its partner
       will have expired.

       While the failed server is recovering, its partner  remains  in  the  partner-down  state,
       which  means that it is serving all clients.  The failed server provides no service at all
       to DHCP clients until it has made the transition into normal operation.

       In the case where both servers detect that they have never before communicated with  their
       partner,  they  both  come up in this recovery state and follow the procedure we have just
       described.  In this case, no service will be provided  to  DHCP  clients  until  MCLT  has


       In  order  to configure failover, you need to write a peer declaration that configures the
       failover protocol, and you need to write peer references  in  each  pool  declaration  for
       which  you  want  to do failover.  You do not have to do failover for all pools on a given
       network segment.   You must not tell one  server  it's  doing  failover  on  a  particular
       address  pool and tell the other it is not.  You must not have any common address pools on
       which you are not doing failover.  A pool declaration that utilizes  failover  would  look
       like this:

       pool {
            failover peer "foo";
            pool specific parameters

       The  server currently  does very  little  sanity checking,  so if  you configure it wrong,
       it will just  fail in odd ways.  I would recommend therefore that you either do   failover
       or  don't  do  failover,  but  don't  do  any  mixed  pools.   Also,   use the same master
       configuration file for both  servers,  and  have  a  separate file   that   contains   the
       peer  declaration and includes the master file.  This will help you to avoid configuration
       mismatches.  As our  implementation evolves,  this will become  less of   a   problem.   A
       basic  sample dhcpd.conf  file for  a primary server might look like this:

       failover peer "foo" {
         port 519;
         peer address;
         peer port 520;
         max-response-delay 60;
         max-unacked-updates 10;
         mclt 3600;
         split 128;
         load balance max seconds 3;

       include "/etc/dhcpd.master";

       The statements in the peer declaration are as follows:

       The primary and secondary statements

         [ primary | secondary ];

         This  determines  whether the server is primary or secondary, as described earlier under

       The address statement

         address address;

         The address statement declares the IP address or DNS name on  which  the  server  should
         listen  for  connections  from its failover peer, and also the value to use for the DHCP
         Failover Protocol server identifier.  Because this value is used as  an  identifier,  it
         may not be omitted.

       The peer address statement

         peer address address;

         The  peer  address  statement  declares  the  IP address or DNS name to which the server
         should connect to reach its failover peer for failover messages.

       The port statement

         port port-number;

         The port statement declares  the  TCP  port  on  which  the  server  should  listen  for
         connections  from  its  failover peer.  This statement may be omitted, in which case the
         IANA assigned port number 647 will be used by default.

       The peer port statement

         peer port port-number;

         The peer port statement declares the TCP port to which  the  server  should  connect  to
         reach  its failover peer for failover messages.  This statement may be omitted, in which
         case the IANA assigned port number 647 will be used by default.

       The max-response-delay statement

         max-response-delay seconds;

         The max-response-delay statement tells the DHCP server how many seconds may pass without
         receiving a message from its failover peer before it assumes that connection has failed.
         This number should be small enough that a transient  network  failure  that  breaks  the
         connection  will  not  result in the servers being out of communication for a long time,
         but large enough that the server isn't constantly making and breaking connections.  This
         parameter must be specified.

       The max-unacked-updates statement

         max-unacked-updates count;

         The  max-unacked-updates statement tells the remote DHCP server how many BNDUPD messages
         it can send before it receives a BNDACK from the local system.   We  don't  have  enough
         operational experience to say what a good value for this is, but 10 seems to work.  This
         parameter must be specified.

       The mclt statement

         mclt seconds;

         The mclt statement defines the Maximum Client Lead Time.  It must be  specified  on  the
         primary,  and  may  not  be  specified on the secondary.  This is the length of time for
         which a lease may be renewed by either failover peer without contacting the other.   The
         longer  you  set  this,  the  longer  it  will take for the running server to recover IP
         addresses after moving into PARTNER-DOWN state.  The shorter you set it, the  more  load
         your servers will experience when they are not communicating.  A value of something like
         3600 is probably reasonable, but again bear in mind that we  have  no  real  operational
         experience with this.

       The split statement

         split bits;

         The  split  statement  specifies  the  split  between  the primary and secondary for the
         purposes of load balancing.  Whenever a client makes a DHCP  request,  the  DHCP  server
         runs  a  hash  on  the client identification, resulting in value from 0 to 255.  This is
         used as an index into a 256 bit field.  If the bit at that index is set, the primary  is
         responsible.   If  the  bit at that index is not set, the secondary is responsible.  The
         split value determines how many of the leading bits are set to one.   So,  in  practice,
         higher  split  values  will  cause the primary to serve more clients than the secondary.
         Lower split values, the converse.  Legal values are between  0  and  256  inclusive,  of
         which  the  most  reasonable  is  128.   Note  that  a  value  of  0 makes the secondary
         responsible for all clients and a value of 256 makes the  primary  responsible  for  all

       The hba statement

         hba colon-separated-hex-list;

         The  hba  statement  specifies  the  split between the primary and secondary as a bitmap
         rather than  a  cutoff,  which  theoretically  allows  for  finer-grained  control.   In
         practice,  there is probably no need for such fine-grained control, however.  An example
         hba statement:

           hba ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:

         This is equivalent to a split 128; statement, and identical.  The following two examples
         are also equivalent to a split of 128, but are not identical:

           hba aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:

           hba 55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:

         They  are equivalent, because half the bits are set to 0, half are set to 1 (0xa and 0x5
         are 1010 and 0101 binary respectively) and consequently this would  roughly  divide  the
         clients  equally  between the servers.  They are not identical, because the actual peers
         this would load balance to each server are different for each example.

         You must only have split or hba defined, never both.  For most cases,  the  fine-grained
         control that hba offers isn't necessary, and split should be used.

       The load balance max seconds statement

         load balance max seconds seconds;

         This  statement allows you to configure a cutoff after which load balancing is disabled.
         The cutoff is  based  on  the  number  of  seconds  since  the  client  sent  its  first
         DHCPDISCOVER  or  DHCPREQUEST  message,  and  only  works  with  clients  that correctly
         implement the secs field - fortunately most clients do.  We recommend  setting  this  to
         something  like  3  or  5.  The effect of this is that if one of the failover peers gets
         into a state where it is responding to failover messages  but  not  responding  to  some
         client requests, the other failover peer will take over its client load automatically as
         the clients retry.

         It is possible to disable load balancing between peers by setting this  value  to  0  on
         both  peers.   Bear in mind that this means both peers will respond to all DHCPDISCOVERs
         or DHCPREQUESTs.

       The auto-partner-down statement

         auto-partner-down seconds;

         This statement instructs the  server  to  initiate  a  timed  delay  upon  entering  the
         communications-interrupted  state (any situation of being out-of-contact with the remote
         failover peer).  At the conclusion of the timer, the server will automatically enter the
         partner-down  state.  This permits the server to allocate leases from the partner's free
         lease pool after an STOS+MCLT timer expires, which can be dangerous if the partner is in
         fact operating at the time (the two servers will give conflicting bindings).

         Think very carefully before enabling this feature.  The partner-down and communications-
         interrupted states are intentionally segregated because there do exist situations  where
         a  failover  server  can fail to communicate with its peer, but still has the ability to
         receive and reply to requests from DHCP clients.  In general, this feature  should  only
         be  used  in  those deployments where the failover servers are directly connected to one
         another, such as by a dedicated hardwired link ("a heartbeat cable").

         A zero value disables the auto-partner-down feature (also the default), and any positive
         value indicates the time in seconds to wait before automatically entering partner-down.

       The Failover pool balance statements.

          max-lease-misbalance percentage;
          max-lease-ownership percentage;
          min-balance seconds;
          max-balance seconds;

         This  version  of  the  DHCP Server evaluates pool balance on a schedule, rather than on
         demand as leases are allocated.  The latter approach proved to be slightly  klunky  when
         pool  misbalanced  reach total saturation — when any server ran out of leases to assign,
         it also lost its ability to notice it had run dry.

         In order to understand pool balance, some elements of its operation  first  need  to  be
         defined.  First, there are ´free´ and ´backup´ leases.  Both of these are referred to as
         ´free state leases´.  ´free´ and ´backup´ are ´the free states´ for the purpose of  this
         document.   The  difference  is  that  only  the primary may allocate from ´free´ leases
         unless under special circumstances, and only the secondary may allocate ´backup´ leases.

         When pool balance is performed, the only plausible expectation is  to  provide  a  50/50
         split  of  the  free  state  leases between the two servers.  This is because no one can
         predict which server will fail, regardless of the relative  load  placed  upon  the  two
         servers,  so  giving  each  server half the leases gives both servers the same amount of
         ´failure endurance´.  Therefore, there is no way to configure any  different  behaviour,
         outside of some very small windows we will describe shortly.

         The  first  thing calculated on any pool balance run is a value referred to as ´lts´, or
         "Leases To Send".  This, simply, is the difference in  the  count  of  free  and  backup
         leases,  divided by two.  For the secondary, it is the difference in the backup and free
         leases, divided by two.  The resulting value is signed: if it  is  positive,  the  local
         server is expected to hand out leases to retain a 50/50 balance.  If it is negative, the
         remote server would need to send leases to balance the pool.  Once the lts value reaches
         zero,  the  pool  is  perfectly  balanced  (give or take one lease in the case of an odd
         number of total free state leases).

         The current approach is still something of a hybrid of the old approach, marked  by  the
         presence  of the max-lease-misbalance statement.  This parameter configures what used to
         be a 10% fixed value in previous versions: if lts is less than free+backup *  max-lease-
         misbalance  percent,  then  the server will skip balancing a given pool (it won't bother
         moving any leases, even if some leases "should" be moved).  The meaning of this value is
         also  somewhat  overloaded,  however,  in that it also governs the estimation of when to
         attempt to balance the pool (which may then also be skipped over).  The oldest leases in
         the free and backup states are examined.  The time they have resided in their respective
         queues is used as an estimate to indicate how much time it is  probable  it  would  take
         before  the leases at the top of the list would be consumed (and thus, how long it would
         take to use all leases in that state).  This percentage is directly multiplied  by  this
         time,  and  fit  into  the  schedule  if it falls within the min-balance and max-balance
         configured values.  The  scheduled  pool  check  time  is  only  moved  in  a  downwards
         direction, it is never increased.  Lastly, if the lts is more than double this number in
         the negative direction, the local server will ´panic´ and transmit a  Failover  protocol
         POOLREQ message, in the hopes that the remote system will be woken up into action.

         Once  the  lts  value  exceeds  the  max-lease-misbalance percentage of total free state
         leases as described above, leases are moved to the remote server.  This is done  in  two

         In  the first pass, only leases whose most recent bound client would have been served by
         the remote server - according to the Load Balance Algorithm (see  above  split  and  hba
         configuration  statements)  -  are given away to the peer.  This first pass will happily
         continue to give away leases, decrementing the lts value by one for each, until the  lts
         value  has  reached  the  negative  of the total number of leases multiplied by the max-
         lease-ownership percentage.  So it is through this value that you  can  permit  a  small
         misbalance  of  the  lease  pools - for the purpose of giving the peer more than a 50/50
         share of leases in the hopes that their clients might some day return and  be  allocated
         by  the  peer  (operating  normally).   This  process  is  referred  to  as ´MAC Address
         Affinity´, but this is somewhat misnamed: it applies equally to DHCP  Client  Identifier
         options.   Note also that affinity is applied to leases when they enter the state ´free´
         from ´expired´ or ´released´.  In this case also, leases will not be moved from free  to
         backup if the secondary already has more than its share.

         The  second  pass  is  only  entered  into  if  the  first  pass fails to reduce the lts
         underneath the total number of free state leases multiplied by  the  max-lease-ownership
         percentage.   In  this pass, the oldest leases are given over to the peer without second
         thought about the Load Balance Algorithm, and this continues until the lts  falls  under
         this  value.  In this way, the local server will also happily keep a small percentage of
         the leases that would normally load balance to itself.

         So, the max-lease-misbalance value acts as a  behavioural  gate.   Smaller  values  will
         cause  more  leases  to  transition states to balance the pools over time, higher values
         will decrease the amount of change (but may lead to pool starvation if there's a run  on

         The  max-lease-ownership value permits a small (percentage) skew in the lease balance of
         a percentage of the total number of free state leases.

         Finally, the min-balance and max-balance make certain that a scheduled  rebalance  event
         happens  within  a  reasonable timeframe (not to be thrown off by, for example, a 7 year
         old free lease).

         Plausible values for the percentages lie between 0 and 100, inclusive, but  values  over
         50  are  indistinguishable  from  one  another  (once  lts exceeds 50% of the free state
         leases, one server must therefore have 100% of the leases in its respective free state).
         It  is  recommended  to  select a max-lease-ownership value that is lower than the value
         selected for the max-lease-misbalance value.  max-lease-ownership defaults  to  10,  and
         max-lease-misbalance defaults to 15.

         Plausible values for the min-balance and max-balance times also range from 0 to (2^32)-1
         (or the limit  of  your  local  time_t  value),  but  default  to  values  60  and  3600
         respectively (to place balance events between 1 minute and 1 hour).


       Clients  can  be  separated  into classes, and treated differently depending on what class
       they are in.  This separation can be done either with a conditional statement, or  with  a
       match  statement  within  the class declaration.  It is possible to specify a limit on the
       total number of clients within a particular class or subclass that may hold leases at  one
       time,  and  it  is  possible to specify automatic subclassing based on the contents of the
       client packet.

       Classing support for DHCPv6 clients was added in 4.3.0.  It follows the same rules as  for
       DHCPv4 except that support for billing classes has not been added yet.

       To  add  clients  to  classes  based on conditional evaluation, you can specify a matching
       expression in the class statement:

       class "ras-clients" {
         match if substring (option dhcp-client-identifier, 1, 3) = "RAS";

       Please note that the values used in match expressions may only come from data  or  options
       that  are  part of the client packet. It is not possible to use values constructed through
       one or more executable statements.  This stems from the fact  that  client  classification
       occurs  before  any  statements are executed. Attempting to do so will yield indeterminate

       Note that whether you use matching expressions or add statements  (or  both)  to  classify
       clients,  you  must always write a class declaration for any class that you use.  If there
       will be no match statement and no in-scope statements for a class, the declaration  should
       look like this:

       class "ras-clients" {


       In  addition to classes, it is possible to declare subclasses.  A subclass is a class with
       the same name as a regular class, but with a specific submatch expression which is  hashed
       for  quick  matching.  This is essentially a speed hack - the main difference between five
       classes with match expressions and one class with five  subclasses  is  that  it  will  be
       quicker to find the subclasses.  Subclasses work as follows:

       class "allocation-class-1" {
         match pick-first-value (option dhcp-client-identifier, hardware);

       class "allocation-class-2" {
         match pick-first-value (option dhcp-client-identifier, hardware);

       subclass "allocation-class-1" 1:8:0:2b:4c:39:ad;
       subclass "allocation-class-2" 1:8:0:2b:a9:cc:e3;
       subclass "allocation-class-1" 1:0:0:c4:aa:29:44;

       subnet netmask {
         pool {
           allow members of "allocation-class-1";
         pool {
           allow members of "allocation-class-2";

       The  data  following the class name in the subclass declaration is a constant value to use
       in matching the match expression for the class.  When class matching is done,  the  server
       will  evaluate  the match expression and then look the result up in the hash table.  If it
       finds a match, the client is considered a member of both the class and the subclass.

       Subclasses can be declared with or without scope.  In the above example, the sole  purpose
       of  the  subclass is to allow some clients access to one address pool, while other clients
       are given access to the other pool, so these subclasses are declared without  scopes.   If
       part  of  the  purpose  of the subclass were to define different parameter values for some
       clients, you might want to declare some subclasses with scopes.

       In the above  example,  if  you  had  a  single  client  that  needed  some  configuration
       parameters, while most didn't, you might write the following subclass declaration for that

       subclass "allocation-class-2" 1:08:00:2b:a1:11:31 {
         option root-path "samsara:/var/diskless/alphapc";
         filename "/tftpboot/netbsd.alphapc-diskless";

       In this example, we've used subclassing as a way to control address allocation on  a  per-
       client  basis.   However,  it's  also  possible  to  use  subclassing in ways that are not
       specific to clients - for example, to use the value of the vendor-class-identifier  option
       to determine what values to send in the vendor-encapsulated-options option.  An example of
       this is shown under the VENDOR ENCAPSULATED OPTIONS head  in  the  dhcp-options(5)  manual


       You  may  specify a limit to the number of clients in a class that can be assigned leases.
       The effect of this will be to make it difficult for a new client in  a  class  to  get  an
       address.   Once a class with such a limit has reached its limit, the only way a new client
       in that class can get a lease is for an existing client to relinquish its lease, either by
       letting  it  expire,  or  by  sending a DHCPRELEASE packet.  Classes with lease limits are
       specified as follows:

       class "limited-1" {
         lease limit 4;

       This will produce a class in which a maximum of four members may hold a lease at one time.


       It is  possible  to  declare  a  spawning  class.   A  spawning  class  is  a  class  that
       automatically  produces  subclasses  based  on  what  the  client  sends.  The reason that
       spawning classes were created was to make it possible to create lease-limited  classes  on
       the  fly.  The envisioned application is a cable-modem environment where the ISP wishes to
       provide clients at a particular site with more than one IP address, but does not  wish  to
       provide  such  clients  with  their  own  subnet,  nor give them an unlimited number of IP
       addresses from the network segment to which they are connected.

       Many cable modem head-end systems can be configured  to  add  a  Relay  Agent  Information
       option to DHCP packets when relaying them to the DHCP server.  These systems typically add
       a circuit ID or remote ID option that uniquely identifies  the  customer  site.   To  take
       advantage of this, you can write a class declaration as follows:

       class "customer" {
         spawn with option agent.circuit-id;
         lease limit 4;

       Now  whenever  a  request  comes  in  from  a customer site, the circuit ID option will be
       checked against the class´s hash table.  If a subclass is found that matches  the  circuit
       ID,  the  client  will  be  classified  in  that  subclass and treated accordingly.  If no
       subclass is found matching the circuit ID, a new one will be created  and  logged  in  the
       dhcpd.leases  file,  and the client will be classified in this new class.  Once the client
       has been classified, it will be treated according to the rules of the class, including, in
       this case, being subject to the per-site limit of four leases.

       The use of the subclass spawning mechanism is not restricted to relay agent options - this
       particular example is given only because it is a fairly straightforward one.


       In some cases, it may be useful to use one expression to assign a client to  a  particular
       class,  and a second expression to put it into a subclass of that class.  This can be done
       by combining the match if and spawn with statements, or the match if and match statements.
       For example:

       class "jr-cable-modems" {
         match if option dhcp-vendor-identifier = "jrcm";
         spawn with option agent.circuit-id;
         lease limit 4;

       class "dv-dsl-modems" {
         match if option dhcp-vendor-identifier = "dvdsl";
         spawn with option agent.circuit-id;
         lease limit 16;

       This  allows you to have two classes that both have the same spawn with expression without
       getting the clients in the two classes confused with each other.


       The DHCP server has the ability to dynamically update the Domain Name System.  Within  the
       configuration  files,  you  can  define how you want the Domain Name System to be updated.
       These updates are RFC 2136 compliant so any DNS server supporting RFC 2136 should be  able
       to accept updates from the DHCP server.

       There  are two DNS schemes implemented.  The interim option is based on draft revisions of
       the DDNS documents while the standard option is based on the RFCs for DHCP-DNS interaction
       and DHCIDs.  A third option, ad-hoc, was deprecated and has now been removed from the code
       base.  The DHCP server must be configured  to  use  one  of  the  two  currently-supported
       methods, or not to do DNS updates.

       New installations should use the standard option. Older installations may want to continue
       using the interim option for backwards compatibility  with  the  DNS  database  until  the
       database  can  be  updated.   This  can  be  done with the ddns-update-style configuration


       the interim and standard DNS update schemes operate mostly  according  to  work  from  the
       IETF.   The  interim  version  was  based  on the drafts in progress at the time while the
       standard is based on the completed RFCs.  The standard RFCs are:

                                      RFC 4701 (updated by RF5494)
                                                RFC 4702
                                                RFC 4703

       And the corresponding drafts were:


       The basic framework for the two schemes is similar with the main material difference being
       that  a DHCID RR is used in the standard version while the interim versions uses a TXT RR.
       The format of the TXT record bears a resemblance to the DHCID RR but it is not  equivalent
       (MD5 vs SHA2, field length differences etc).

       In these two schemes the DHCP server does not necessarily always update both the A and the
       PTR records.  The FQDN option includes a flag which, when sent by  the  client,  indicates
       that  the  client  wishes  to  update  its  own A record.  In that case, the server can be
       configured either to honor the client´s intentions or ignore them.  This is done with  the
       statement  allow  client-updates;  or  the  statement ignore client-updates;.  By default,
       client updates are allowed.

       If the server is configured to allow client updates, then if the  client  sends  a  fully-
       qualified domain name in the FQDN option, the server will use that name the client sent in
       the FQDN option to update the PTR record.  For example, let us say that the  client  is  a
       visitor  from the "" domain, whose hostname is "jschmoe".  The server is for the
       "" domain.  The DHCP client indicates in the  FQDN  option  that  its  FQDN  is
       "".   It  also indicates that it wants to update its own A record.  The
       DHCP server therefore does not attempt to set up an A record for the client, but does  set
       up   a   PTR  record  for  the  IP  address  that  it  assigns  the  client,  pointing  at  Once the DHCP client has an IP address,  it  can  update  its  own  A
       record, assuming that the "" DNS server will allow it to do so.

       If  the server is configured not to allow client updates, or if the client doesn´t want to
       do its own update, the server will simply choose a name for the client.  By  default,  the
       server will choose from the following three values:

            1. fqdn option (if present)
            2. hostname option (if present)
            3. Configured hostname option (if defined).

       If  these  defaults  for choosing the host name are not appropriate you can write your own
       statement to set the ddns-hostname variable as you wish.  If none of the above  are  found
       the server will use the host declaration name (if one) and use-host-decl-names is on.

       It  will  use  its own domain name for the client.  It will then update both the A and PTR
       record, using the name that it chose for  the  client.   If  the  client  sends  a  fully-
       qualified  domain  name  in the fqdn option, the server uses only the leftmost part of the
       domain name - in the example above, "jschmoe" instead of "".

       Further, if the ignore client-updates; directive is used, then the server will in addition
       send a response in the DHCP packet, using the FQDN Option, that implies to the client that
       it should perform its own updates if it chooses to do so.  With  deny  client-updates;,  a
       response is sent which indicates the client may not perform updates.

       Both  the  standard  and interim options also include a method to allow more than one DHCP
       server to update the DNS database without accidentally deleting A records  that  shouldn´t
       be deleted nor failing to add A records that should be added.  For the standard option the
       method works as follows:

       When the DHCP server issues a client a new lease, it creates a text string that is an  SHA
       hash over the DHCP client´s identification (see RFCs 4701 & 4702 for details).  The update
       attempts to add an A record with the name the server chose and a DHCID  record  containing
       the hashed identifier string (hashid).  If this update succeeds, the server is done.

       If  the update fails because the A record already exists, then the DHCP server attempts to
       add the A record with the prerequisite that there must be a DHCID record in the same  name
       as  the  new  A record, and that DHCID record´s contents must be equal to hashid.  If this
       update succeeds, then the client has its A record and PTR record.  If it fails,  then  the
       name  the  client  has  been  assigned  (or requested) is in use, and can´t be used by the
       client.  At this point the DHCP server gives up trying to do a DNS update for  the  client
       until the client chooses a new name.

       The  server  also  does  not update very aggressively.  Because each DNS update involves a
       round trip to the DNS server, there is a cost associated with doing updates even  if  they
       do  not actually modify the DNS database.  So the DHCP server tracks whether or not it has
       updated the record in the past (this information is stored on  the  lease)  and  does  not
       attempt to update records that it thinks it has already updated.

       This can lead to cases where the DHCP server adds a record, and then the record is deleted
       through some other mechanism, but the server never again updates the DNS because it thinks
       the  data  is  already there.  In this case the data can be removed from the lease through
       operator intervention, and once this has been done, the DNS will be updated the next  time
       the client renews.

       The  interim  DNS  update  scheme  was written before the RFCs were finalized and does not
       quite follow them.  The RFCs call for a new DHCID RRtype  while  the  interim  DNS  update
       scheme  uses  a  TXT  record.   In  addition the ddns-resolution draft called for the DHCP
       server to put a DHCID RR on the PTR record, but the interim  update  method  does  not  do
       this.  In the final RFC this requirement was relaxed such that a server may add a DHCID RR
       to the PTR record.


       As described  in  RFC  4703,  section  5.2,  in  order  to  perform  DDNS  in  dual  stack
       environments,  both  IPv4 and IPv6 servers would need to be configured to use the standard
       update style and participating IPv4 clients MUST convey DUIDs as described  in  RFC  4361,
       section 6.1., in their dhcp-client-identifiers.

       In  a  nutshell,  this mechanism is intended to use globally unique DUIDs to idenfity both
       IPv4 and IPv6 clients, and where a device has both IPv4 and IPv6 leases it  is  identified
       by the same DUID.  This allows a dual stack client to use the same FQDN for both mappings,
       while being protected from updates for other clients by the rules of conflict detection.

       However, not all IPv4 clients implement this behavior which  makes  supporting  them  dual
       stack  environments  problematic.   In  order to address this issue ISC DHCP (as of 4.4.0)
       supports a new mode of DDNS conflict resolution referred  to  as  Dual  Stack  Mixed  Mode

       The  concept  behind DSMM is relatively simple.  All dhcp servers of one protocol (IPv4 or
       v6) use one ddns-update-style (interim or standard)  while  all  servers  of  the  "other"
       protocol  will  use  the  "other"  ddns-udpate-style.  In this way, all servers of a given
       protocol are using the same record type (TXT or DHCID) for their DHCID RR  entries.   This
       allows  conflict  detection  to be enforced within each protocol without interferring with
       the other's entries.

       DSMM modifications now ensure that IPv4 DSMM servers only ever  modify  A  records,  their
       associated PTR records and DHCID records, while DSMM IPv6 severs only modify AAAA records,
       their associated PTR records, and DHCID records.

       Note that DSMM is not a perfect solution, it is a compromise that can work  well  provided
       all participating DNS updaters play by DSMM rules.  As with anything else in life, it only
       works as well as those who particpate behave.

       While conflict detection is enabled by default, DSMM is not.  To enable DSMM, both update-
       conflict-detection and ddns-dual-stack-mixed-mode must be true.


       Built  into  conflict  resolution  is  the  protection of manually made entries for static
       clients.  Per the rules of conflict resolution,  a DNS updater may not alter  forward  DNS
       entries  unless  there  is  a  DHCID  RR  which matches for whom the update is being made.
       Therefore, any forward DNS entries without a corresponding DHCID RR cannot be  altered  by
       such an updater.

       In  some  environments, it may be desirable to use only this aspect of conflict resolution
       and allow DNS updaters to overwrite entries for dynamic clients regardless of what  client
       owns  them.   In  other  words,  the  presence  or lack of a DHCID RR is used to determine
       whether entries may or may not be overwritten.  Whether or not the client matches the data
       value  of  the  DHCID RR is irrelevant.   This behavior, off by default, can be configured
       through the parameter, ddns-guard-id-must-match.  As with DSMM, this behavior is can  only
       be  enabled  if  conflict  resolution  is  enabled.    This  behavior should be considered
       carefully before electing to use it.

       There is an additional parameter that can be used with  DSMM  ddns-other-guard-is-dynamic.
       When enabled along with DSMM, a server will regard the presence of a DHCID RR of the other
       style type as indicating that the forward DNS entries for that FQDN should be dynamic  and
       may  be  overwritten.   For example, such a server using interim style could overwrite the
       DNS entries for an FQDN if there is only a DHDID type DHDID RR for the FQDN.  Essentially,
       if  there  are  dynamic  entries  for  one protocol, that is enough to overcome the static
       protection  of  entries  for  the  other  protocol.   This   behavior   warrants   careful
       consideration before electing to use it.


       When you set your DNS server up to allow updates from the DHCP server, you may be exposing
       it to unauthorized updates.  To avoid this, you should use TSIG signatures - a  method  of
       cryptographically  signing  updates using a shared secret key.  As long as you protect the
       secrecy of this key, your updates should also be secure.  Note,  however,  that  the  DHCP
       protocol  itself  provides no security, and that clients can therefore provide information
       to the DHCP server which  the  DHCP  server  will  then  use  in  its  updates,  with  the
       constraints described previously.

       The  DNS server must be configured to allow updates for any zone that the DHCP server will
       be updating.  For example, let us say that clients in the  domain  will  be
       assigned  addresses  on  the  subnet.   In  that  case, you will need a key
       declaration for the TSIG key you will be using, and also two zone declarations -  one  for
       the  zone  containing  A  records that will be updates and one for the zone containing PTR
       records - for ISC BIND, something like this:

       key DHCP_UPDATER {
         algorithm HMAC-MD5.SIG-ALG.REG.INT;
         secret pRP5FapFoJ95JEL06sv4PQ==;

       zone "" {
            type master;
            file "";
            allow-update { key DHCP_UPDATER; };

       zone "" {
            type master;
            file "10.10.17.db";
            allow-update { key DHCP_UPDATER; };

       You will also have to configure your DHCP server to do updates to these zones.  To do  so,
       you need to add something like this to your dhcpd.conf file:

       key DHCP_UPDATER {
         algorithm HMAC-MD5.SIG-ALG.REG.INT;
         secret pRP5FapFoJ95JEL06sv4PQ==;

       zone EXAMPLE.ORG. {
         key DHCP_UPDATER;

       zone {
         key DHCP_UPDATER;

       The  primary  statement specifies the IP address of the name server whose zone information
       is to be updated.  In addition to the primary statement there  are  also  the  primary6  ,
       secondary and secondary6 statements.  The primary6 statement specifies an IPv6 address for
       the name server.  The secondaries provide for additional addresses for name servers to  be
       used  if  the  primary  does  not  respond.  The number of name servers the DDNS code will
       attempt to use before giving up is limited and is currently set to three.

       Note that the zone declarations have to correspond  to  authority  records  in  your  name
       server  -  in  the  above  example, there must be an SOA record for "" and for
       "".  For example, if there were a subdomain  ""  with
       no  separate SOA, you could not write a zone declaration for ""  Also keep
       in mind that zone names in your DHCP configuration should  end  in  a  ".";  this  is  the
       preferred  syntax.  If you do not end your zone name in a ".", the DHCP server will figure
       it out.  Also note that in the DHCP configuration, zone  names  are  not  encapsulated  in
       quotes where there are in the DNS configuration.

       You  should choose your own secret key, of course.  The ISC BIND 9 distribution comes with
       a program for generating secret keys called dnssec-keygen.  If  you  are  using  BIND  9´s
       dnssec-keygen, the above key would be created as follows:

            dnssec-keygen -a HMAC-MD5 -b 128 -n USER DHCP_UPDATER

       The key name, algorithm, and secret must match that being used by the DNS server. The DHCP
       server currently supports the following algorithms:


       You may wish to enable logging of DNS updates on your DNS server.  To  do  so,  you  might
       write a logging statement like the following:

       logging {
            channel update_debug {
                 file "/var/log/update-debug.log";
                 severity  debug 3;
                 print-category yes;
                 print-severity yes;
                 print-time     yes;
            channel security_info    {
                 file "/var/log/";
                 severity  info;
                 print-category yes;
                 print-severity yes;
                 print-time     yes;

            category update { update_debug; };
            category security { security_info; };

       You  must  create  the /var/log/ and /var/log/update-debug.log files before
       starting the name server.  For more information  on  configuring  ISC  BIND,  consult  the
       documentation that accompanies it.


       There  are  three kinds of events that can happen regarding a lease, and it is possible to
       declare statements that occur when any of these  events  happen.   These  events  are  the
       commit  event,  when  the server has made a commitment of a certain lease to a client, the
       release event, when the client has released the server from its commitment, and the expiry
       event, when the commitment expires.

       To  declare  a  set  of  statements  to execute when an event happens, you must use the on
       statement, followed by the name of the event,  followed  by  a  series  of  statements  to
       execute when the event happens, enclosed in braces.


       The include statement

        include "filename";

       The  include  statement  is used to read in a named file, and process the contents of that
       file as though it were entered in place of the include statement.

       The shared-network statement

        shared-network name {
          [ parameters ]
          [ declarations ]

       The shared-network statement is used to inform  the  DHCP  server  that  some  IP  subnets
       actually  share  the  same  physical  network.   Any subnets in a shared network should be
       declared within a shared-network statement.  Parameters specified  in  the  shared-network
       statement will be used when booting clients on those subnets unless parameters provided at
       the subnet or host level override them.  If any subnet in a shared network  has  addresses
       available  for  dynamic  allocation,  those addresses are collected into a common pool for
       that shared network and assigned to clients as needed.  There is no way to distinguish  on
       which subnet of a shared network a client should boot.

       Name  should be the name of the shared network.  This name is used when printing debugging
       messages, so it should be descriptive for the shared  network.   The  name  may  have  the
       syntax  of  a valid domain name (although it will never be used as such), or it may be any
       arbitrary name, enclosed in quotes.

       The subnet statement

        subnet subnet-number netmask netmask {
          [ parameters ]
          [ declarations ]

       The subnet statement is used to provide dhcpd with enough information to tell  whether  or
       not  an  IP  address  is  on  that subnet.  It may also be used to provide subnet-specific
       parameters and to specify what addresses may be dynamically allocated to  clients  booting
       on that subnet.  Such addresses are specified using the range declaration.

       The  subnet-number  should  be  an  IP address or domain name which resolves to the subnet
       number of the subnet being described.  The netmask should be an IP address or domain  name
       which  resolves  to  the  subnet  mask  of the subnet being described.  The subnet number,
       together with the netmask, are sufficient to determine whether any given IP address is  on
       the specified subnet.

       Although  a netmask must be given with every subnet declaration, it is recommended that if
       there is any variance in subnet masks at a site, a subnet-mask option statement be used in
       each  subnet  declaration  to  set  the  desired subnet mask, since any subnet-mask option
       statement will override the subnet mask declared in the subnet statement.

       The subnet6 statement

        subnet6 subnet6-number {
          [ parameters ]
          [ declarations ]

       The subnet6 statement is used to provide dhcpd with enough information to tell whether  or
       not  an  IPv6  address is on that subnet6.  It may also be used to provide subnet-specific
       parameters and to specify what addresses may be dynamically allocated to  clients  booting
       on that subnet.

       The subnet6-number should be an IPv6 network identifier, specified as ip6-address/bits.

       The range statement

       range [ dynamic-bootp ] low-address [ high-address];

       For any subnet on which addresses will be assigned dynamically, there must be at least one
       range statement.  The range statement gives the lowest  and  highest  IP  addresses  in  a
       range.  All IP addresses in the range should be in the subnet in which the range statement
       is declared.  The dynamic-bootp flag may be specified if addresses in the specified  range
       may  be  dynamically assigned to BOOTP clients as well as DHCP clients.  When specifying a
       single address, high-address can be omitted.

       The range6 statement

       range6 low-address high-address;
       range6 subnet6-number;
       range6 subnet6-number temporary;
       range6 address temporary;

       For any IPv6 subnet6 on which addresses will be assigned dynamically,  there  must  be  at
       least one range6 statement. The range6 statement can either be the lowest and highest IPv6
       addresses in a range6, or  use  CIDR  notation,  specified  as  ip6-address/bits.  All  IP
       addresses  in  the  range6  should  be  in  the  subnet6  in which the range6 statement is

       The temporary variant makes the prefix (by default on 64  bits)  available  for  temporary
       (RFC  4941)  addresses. A new address per prefix in the shared network is computed at each
       request with an IA_TA option. Release and Confirm ignores temporary addresses.

       Any IPv6 addresses given to hosts with fixed-address6 are excluded from the range6, as are
       IPv6 addresses on the server itself.

       The prefix6 statement

       prefix6 low-address high-address / bits;

       The  prefix6  is  the range6 equivalent for Prefix Delegation (RFC 3633). Prefixes of bits
       length are assigned between low-address and high-address.

       Any IPv6 prefixes given to static entries (hosts) with fixed-prefix6 are excluded from the

       This statement is currently global but it should have a shared-network scope.

       The host statement

        host hostname {
          [ parameters ]
          [ declarations ]

       The  host  declaration  provides  a  way  for  the DHCP server to identify a DHCP or BOOTP
       client.  This allows the server  to  provide  configuration  information  including  fixed
       addresses or, in DHCPv6, fixed prefixes for a specific client.

       If  it is desirable to be able to boot a DHCP or BOOTP client on more than one subnet with
       fixed v4  addresses,  more  than  one  address  may  be  specified  in  the  fixed-address
       declaration, or more than one host statement may be specified matching the same client.

       The  fixed-address6 declaration is used for v6 addresses.  At this time it only works with
       a single address.  For multiple addresses specify multiple host statements.

       If client-specific boot parameters must change based on the network to which the client is
       attached, then multiple host declarations should be used.  The host declarations will only
       match a client if one of their fixed-address statements is viable on the subnet (or shared
       network)  where  the  client  is  attached.  Conversely, for a host declaration to match a
       client being allocated a dynamic address, it must not have any  fixed-address  statements.
       You  may  therefore need a mixture of host declarations for any given client...some having
       fixed-address statements, others without.

       hostname should be a name identifying the host.  If a hostname option is not specified for
       the host, hostname is used.

       Host declarations are matched to actual DHCP or BOOTP clients by matching the dhcp-client-
       identifier option specified in the host declaration to the one supplied by the client, or,
       if the host declaration or the client does not provide a dhcp-client-identifier option, by
       matching the hardware parameter in the host declaration to the  network  hardware  address
       supplied  by  the client.  BOOTP clients do not normally provide a dhcp-client-identifier,
       so the hardware address must be used for  all  clients  that  may  boot  using  the  BOOTP

       DHCPv6  servers  can use the host-identifier option parameter in the host declaration, and
       specify any option with a fixed value to identify hosts.

       Please be aware that only the dhcp-client-identifier option and the hardware  address  can
       be  used  to  match a host declaration, or the host-identifier option parameter for DHCPv6
       servers.  For example, it is not possible to match  a  host  declaration  to  a  host-name
       option.   This  is  because the host-name option cannot be guaranteed to be unique for any
       given client, whereas both the hardware address and dhcp-client-identifier option  are  at
       least theoretically guaranteed to be unique to a given client.

       The group statement

        group {
          [ parameters ]
          [ declarations ]

       The  group  statement  is  used  simply  to  apply  one  or  more parameters to a group of
       declarations.  It can be used to group hosts, shared  networks,  subnets,  or  even  other


       The  allow  and  deny statements can be used to control the response of the DHCP server to
       various sorts of requests.  The allow and deny keywords actually have  different  meanings
       depending  on the context.  In a pool context, these keywords can be used to set up access
       lists for address allocation pools.   In  other  contexts,  the  keywords  simply  control
       general  server  behavior  with respect to clients based on scope.  In a non-pool context,
       the ignore keyword can be used in place of the deny keyword to prevent logging  of  denied


       The  following  usages  of  allow  and  deny  will  work  in any scope, although it is not
       recommended that they be used in pool declarations.

       The unknown-clients keyword

        allow unknown-clients;
        deny unknown-clients;
        ignore unknown-clients;

       The unknown-clients flag is used to tell  dhcpd  whether  or  not  to  dynamically  assign
       addresses to unknown clients.  Dynamic address assignment to unknown clients is allowed by
       default.  An unknown client is simply a client that has no host declaration.

       The use of this option is now deprecated.  If you are trying to restrict  access  on  your
       network  to  known  clients,  you  should use deny unknown-clients; inside of your address
       pool, as described under the heading ALLOW AND DENY WITHIN POOL DECLARATIONS.

       The bootp keyword

        allow bootp;
        deny bootp;
        ignore bootp;

       The bootp flag is used to tell dhcpd whether or not to respond to  bootp  queries.   Bootp
       queries are allowed by default.

       The booting keyword

        allow booting;
        deny booting;
        ignore booting;

       The  booting  flag  is  used  to  tell  dhcpd  whether or not to respond to queries from a
       particular client.  This keyword only has meaning when it appears in a  host  declaration.
       By  default,  booting is allowed, but if it is disabled for a particular client, then that
       client will not be able to get an address from the DHCP server.

       The duplicates keyword

        allow duplicates;
        deny duplicates;

       Host declarations can match client messages based on the DHCP Client Identifier option  or
       based  on the client's network hardware type and MAC address.  If the MAC address is used,
       the host declaration will match any client with that  MAC  address  -  even  clients  with
       different  client  identifiers.   This  doesn't  normally happen, but is possible when one
       computer has more than one operating system installed  on  it  -  for  example,  Microsoft
       Windows and NetBSD or Linux.

       The duplicates flag tells the DHCP server that if a request is received from a client that
       matches the MAC address of a host declaration, any other leases matching that MAC  address
       should  be  discarded by the server, even if the UID is not the same.  This is a violation
       of the DHCP protocol, but can prevent clients whose client  identifiers  change  regularly
       from holding many leases at the same time.  By default, duplicates are allowed.

       The declines keyword

        allow declines;
        deny declines;
        ignore declines;

       The  DHCPDECLINE message is used by DHCP clients to indicate that the lease the server has
       offered is not valid.  When the server receives a DHCPDECLINE for a particular address, it
       normally  abandons  that  address,  assuming  that  some  unauthorized system is using it.
       Unfortunately, a malicious or buggy client can,  using  DHCPDECLINE  messages,  completely
       exhaust  the  DHCP  server's  allocation  pool.   The server will eventually reclaim these
       leases, but not while the client is running through  the  pool.  This  may  cause  serious
       thrashing  in  the  DNS,  and it will also cause the DHCP server to forget old DHCP client
       address allocations.

       The declines flag tells the DHCP server whether or not to honor DHCPDECLINE messages.   If
       it  is  set  to  deny or ignore in a particular scope, the DHCP server will not respond to
       DHCPDECLINE messages.

       The declines flag is only supported by DHCPv4 servers.  Given the large IPv6 address space
       and  the  internal  limits  imposed  by the server's address generation mechanism we don't
       think it is necessary for DHCPv6 servers at this time.

       Currently, abandoned IPv6 addresses are reclaimed in one of two ways:
           a) Client renews a specific address:
           If a client using a given DUID submits a DHCP REQUEST containing
           the last address abandoned by that DUID, the address will be
           reassigned to that client.

           b) Upon the second restart following an address abandonment.  When
           an address is abandoned it is both recorded as such in the lease
           file and retained as abandoned in server memory until the server
           is restarted. Upon restart, the server will process the lease file
           and all addresses whose last known state is abandoned will be
           retained as such in memory but not rewritten to the lease file.
           This means that a subsequent restart of the server will not see the
           abandoned addresses in the lease file and therefore have no record
           of them as abandoned in memory and as such perceive them as free
           for assignment.

       The total number addresses in a pool, available for a  given  DUID  value,  is  internally
       limited  by the server's address generation mechanism.  If through mistaken configuration,
       multiple clients are using the same DUID  they  will  competing  for  the  same  addresses
       causing  the  server  to  reach  this  internal  limit rather quickly.  The internal limit
       isolates this type of activity such that address range is not  exhausted  for  other  DUID
       values.   The  appearance  of  the  following  error  log,  can  be  an indication of this

           "Best match for DUID <XX> is an abandoned address, This may be a
            result of multiple clients attempting to use this DUID"

           where <XX> is an actual DUID value depicted as colon separated
           string of bytes in hexadecimal values.

       The client-updates keyword

        allow client-updates;
        deny client-updates;

       The client-updates flag tells the DHCP  server  whether  or  not  to  honor  the  client's
       intention  to  do its own update of its A record.  See the documentation under the heading
       THE DNS UPDATE SCHEME for details.

       The leasequery keyword

        allow leasequery;
        deny leasequery;

       The leasequery flag tells the DHCP server whether or not to answer DHCPLEASEQUERY packets.
       The answer to a DHCPLEASEQUERY packet includes information about a specific lease, such as
       when it was issued and when it will expire. By default, the server  will  not  respond  to
       these packets.


       The uses of the allow and deny keywords shown in the previous section work pretty much the
       same way whether the client is sending a  DHCPDISCOVER  or  a  DHCPREQUEST  message  -  an
       address  will be allocated to the client (either the old address it's requesting, or a new
       address) and then that address will be tested to see if it's okay to let the  client  have
       it.   If  the  client  requested  it,  and  it's  not okay, the server will send a DHCPNAK
       message.  Otherwise, the server will simply not respond to the client.  If it is  okay  to
       give the address to the client, the server will send a DHCPACK message.

       The  primary motivation behind pool declarations is to have address allocation pools whose
       allocation policies are different.  A client may be denied access to one pool, but allowed
       access  to  another  pool  on the same network segment.  In order for this to work, access
       control has to be done during address allocation, not after address allocation is done.

       When a DHCPREQUEST message is processed, address allocation simply consists of looking  up
       the  address  the  client is requesting and seeing if it's still available for the client.
       If it is, then the DHCP server checks both the address pool permit lists and the  relevant
       in-scope  allow  and  deny statements to see if it's okay to give the lease to the client.
       In the case of a DHCPDISCOVER  message,  the  allocation  process  is  done  as  described
       previously in the ADDRESS ALLOCATION section.

       When  declaring  permit  lists  for  address  allocation pools, the following syntaxes are
       recognized following the allow or deny keywords:


       If specified, this statement either allows or prevents allocation from this  pool  to  any
       client  that  has a host declaration (i.e., is known).  A client is known if it has a host
       declaration in any scope, not just the current scope.


       If specified, this statement either allows or prevents allocation from this  pool  to  any
       client that has no host declaration (i.e., is not known).

        members of "class";

       If  specified,  this  statement either allows or prevents allocation from this pool to any
       client that is a member of the named class.

        dynamic bootp clients;

       If specified, this statement either allows or prevents allocation from this  pool  to  any
       bootp client.

        authenticated clients;

       If  specified,  this  statement either allows or prevents allocation from this pool to any
       client that has been authenticated using the DHCP authentication protocol.   This  is  not
       yet supported.

        unauthenticated clients;

       If  specified,  this  statement either allows or prevents allocation from this pool to any
       client that has not been authenticated using the DHCP authentication  protocol.   This  is
       not yet supported.

        all clients;

       If  specified,  this  statement either allows or prevents allocation from this pool to all
       clients.  This can be used when you want to write a pool declaration for some reason,  but
       hold  it  in reserve, or when you want to renumber your network quickly, and thus want the
       server to force all clients that have been allocated addresses from this  pool  to  obtain
       new addresses immediately when they next renew.

        after time;

       If  specified,  this statement either allows or prevents allocation from this pool after a
       given date. This can be used when you want to move clients from one pool to  another.  The
       server  adjusts  the  regular  lease  time  so that the latest expiry time is at the given
       time+min-lease-time.  A short min-lease-time enforces a step change, whereas a longer min-
       lease-time  allows for a gradual change.  time is either second since epoch, or a UTC time
       string e.g.  4 2007/08/24 09:14:32 or a string with time zone offset  in  seconds  e.g.  4
       2007/08/24 11:14:32 -7200


       The abandon-lease-time statement

         abandon-lease-time time;

         Time  should  be  the  maximum  amount of time (in seconds) that an abandoned IPv4 lease
         remains unavailable for assignment to a client.  Abandoned leases will only  be  offered
         to  clients if there are no free leases.  If not defined, the default abandon lease time
         is 86400 seconds (24 hours).  Note the  abandoned  lease  time  for  a  given  lease  is
         preserved across server restarts.  The parameter may only be set at the global scope and
         is evaluated only once during server startup.

         Values less than sixty seconds are not recommended as  this  is  below  the  ping  check
         threshold  and  can  cause leases once abandoned but since returned to the free state to
         not be pinged before being offered.  If the requested time is larger than 0x7FFFFFFF - 1
         or  the  sum of the current time plus the abandoned time isgreater than 0x7FFFFFFF it is
         treated as infinite.

       The adaptive-lease-time-threshold statement

         adaptive-lease-time-threshold percentage;

         When the number of allocated leases within a pool rises above the  percentage  given  in
         this  statement,  the DHCP server decreases the lease length for new clients within this
         pool to min-lease-time seconds. Clients renewing an already valid (long) leases  get  at
         least  the  remaining  time  from the current lease. Since the leases expire faster, the
         server may either recover more quickly or avoid  pool  exhaustion  entirely.   Once  the
         number  of  allocated leases drop below the threshold, the server reverts back to normal
         lease times.  Valid percentages are between 1 and 99.

       The always-broadcast statement

         always-broadcast flag;

         The DHCP and BOOTP protocols both require DHCP and BOOTP clients to  set  the  broadcast
         bit  in the flags field of the BOOTP message header.  Unfortunately, some DHCP and BOOTP
         clients do not do this, and therefore may not receive responses from  the  DHCP  server.
         The DHCP server can be made to always broadcast its responses to clients by setting this
         flag to ´on´ for the relevant scope; relevant  scopes  would  be  inside  a  conditional
         statement,  as  a  parameter  for a class, or as a parameter for a host declaration.  To
         avoid creating excess broadcast traffic on your network, we recommend that you  restrict
         the  use  of this option to as few clients as possible.  For example, the Microsoft DHCP
         client is known not to have this problem, as are the OpenTransport and ISC DHCP clients.

       The always-reply-rfc1048 statement

         always-reply-rfc1048 flag;

         Some BOOTP clients expect RFC1048-style  responses,  but  do  not  follow  RFC1048  when
         sending  their requests.  You can tell that a client is having this problem if it is not
         getting the options you have configured for it and if you see  in  the  server  log  the
         message "(non-rfc1048)" printed with each BOOTREQUEST that is logged.

         If  you  want  to  send  rfc1048 options to such a client, you can set the always-reply-
         rfc1048 option in that client's host declaration, and the DHCP server will respond  with
         an  RFC-1048-style  vendor  options  field.  This flag can be set in any scope, and will
         affect all clients covered by that scope.

       The authoritative statement


         not authoritative;

         The DHCP server will normally assume that the configuration information  about  a  given
         network segment is not known to be correct and is not authoritative.  This is so that if
         a naive user installs a DHCP server not fully understanding how to configure it, it does
         not  send  spurious  DHCPNAK  messages  to  clients  that have obtained addresses from a
         legitimate DHCP server on the network.

         Network administrators setting up authoritative DHCP servers for their  networks  should
         always  write authoritative; at the top of their configuration file to indicate that the
         DHCP server should send DHCPNAK messages to misconfigured clients.  If this is not done,
         clients  will  be  unable to get a correct IP address after changing subnets until their
         old lease has expired, which could take quite a long time.

         Usually, writing authoritative; at the top level  of  the  file  should  be  sufficient.
         However, if a DHCP server is to be set up so that it is aware of some networks for which
         it is authoritative and some networks for which it is not, it may be more appropriate to
         declare authority on a per-network-segment basis.

         Note  that the most specific scope for which the concept of authority makes any sense is
         the physical network segment - either a shared-network statement or a  subnet  statement
         that  is  not  contained  within  a  shared-network  statement.  It is not meaningful to
         specify that the server is authoritative for some subnets within a shared  network,  but
         not  authoritative  for  others,  nor  is  it  meaningful  to specify that the server is
         authoritative for some host declarations and not others.

       The boot-unknown-clients statement

         boot-unknown-clients flag;

         If the boot-unknown-clients statement is present and has a value of false or  off,  then
         clients  for  which  there  is  no  host  declaration  will  not be allowed to obtain IP
         addresses.  If this statement is not present or has a value of true or on, then  clients
         without  host  declarations  will  be  allowed  to obtain IP addresses, as long as those
         addresses  are  not  restricted  by  allow  and  deny  statements  within   their   pool

       The check-secs-byte-order statement

         check-secs-byte-order flag;

         When  check-secs-byte-order is enabled, the server will check for DHCPv4 clients that do
         the byte ordering on the secs field incorrectly. This field should be  in  network  byte
         order  but  some  clients  get  it wrong. When this parameter is enabled the server will
         examine the secs field and if it looks wrong (high byte non zero and low byte zero) swap
         the  bytes.   The  default  is  disabled.  This parameter is only useful when doing load
         balancing within failover. (Formerly, this behavior had to be enabled during compilation
         configuration via --enable-secs-byteorder).

         The db-time-format statement

            db-time-format [ default | local ] ;

            The DHCP server software outputs several timestamps when writing leases to persistent
            storage.  This configuration parameter  selects  one  of  two  output  formats.   The
            default  format  prints the day, date, and time in UTC, while the local format prints
            the system seconds-since-epoch, and helpfully provides the day and time in the system
            timezone   in   a  comment.   The  time  formats  are  described  in  detail  in  the
            dhcpd.leases(5) manpage.

         The ddns-hostname statement

            ddns-hostname name;

            The name parameter should be the hostname  that  will  be  used  in  setting  up  the
            client's  A  and  PTR  records.   If no ddns-hostname is specified in scope, then the
            server will derive the hostname automatically, using an  algorithm  that  varies  for
            each of the different update methods.

         The ddns-domainname statement

            ddns-domainname name;

            The  name  parameter  should be the domain name that will be appended to the client's
            hostname to form a fully-qualified domain-name (FQDN).

         The ddns-dual-stack-mixed-mode statement

            ddns-dual-stack-mixed-mode flag;

            The ddns-dual-stack-mixed-mode parameter controls whether or not the  server  applies
            Dual  Stack  Mixed Mode rules during DDNS conflict resolution.  This parameter is off
            by default, has no effect unless update-conflict-detection is enabled, and  may  only
            be specified at the global scope.

         The ddns-guard-id-must-match statement

            ddns-guard-id-must-match flag;

            The ddns-guard-id-must-match parameter controls whether or not a the client id within
            a DHCID RR must match  that  of  the  DNS  update's  client  to  permit  DNS  entries
            associated  with that DHCID RR to be ovewritten.  Proper conflict resolution requires
            ID matching and should only be disabled after careful consideration.  When  disabled,
            it is allows any DNS updater to replace DNS entries that have an associated DHCID RR,
            regardless of client identity. This parameter is on by default, has no effect  unless
            update-conflict-detection is enabled, and may only be specified at the global scope.

         The dns-local-address4 and dns-local-address6 statements

            ddns-local-address4 address;

            ddns-local-address6 address;

            The  address parameter should be the local IPv4 or IPv6 address the server should use
            as the from address when sending DDNS update requests.

         The ddns-other-guard-is-dynamic statement

            ddns-other-guard-is-dynamic flag;

            The ddns-other-guard-is-dynamic parameter controls whether or not a a server  running
            DSMM  will  consider  the presence of the other update style DHCID RR as an indcation
            that a DNS entries may be overwritten. It should only be enabled after careful  study
            as  it  allows  DNS  entries  that  would  otherwise  be  protected  as static, to be
            overwritten in certain cases. This paramater is off by default, has no effect  unless
            ddns-dual-stack-mixed-mode is enabled, and may only be specified at the global scope.

         The ddns-rev-domainname statement

            ddns-rev-domainname name;

            The  name  parameter  should be the domain name that will be appended to the client's
            reversed IP address to produce a name  for  use  in  the  client's  PTR  record.   By
            default, this is "", but the default can be overridden here.

            The  reversed  IP  address  to  which  this  domain name is appended is always the IP
            address of the client, in dotted quad notation, reversed - for  example,  if  the  IP
            address  assigned  to  the  client  is,  then the reversed IP address is
    So a client with that IP address would, by  default,  be  given  a  PTR
            record of

         The ddns-update-style parameter

            ddns-update-style style;

            The  style parameter must be one of standard, interim or none.  The ddns-update-style
            statement is only meaningful in the outer scope - it is evaluated once after  reading
            the  dhcpd.conf  file,  rather  than each time a client is assigned an IP address, so
            there is no way to use different DNS update styles for different clients. The default
            is none.

         The ddns-updates statement

             ddns-updates flag;

            The  ddns-updates  parameter  controls whether or not the server will attempt to do a
            DNS update when a lease is confirmed.  Set this to  off  if  the  server  should  not
            attempt  to  do  updates within a certain scope.  The ddns-updates parameter is on by
            default.  To disable DNS updates in all scopes, it is preferable  to  use  the  ddns-
            update-style statement, setting the style to none.

         The default-lease-time statement

            default-lease-time time;

            Time  should  be the length in seconds that will be assigned to a lease if the client
            requesting the lease does not ask for a specific expiration time.  This is  used  for
            both  DHCPv4  and DHCPv6 leases (it is also known as the "valid lifetime" in DHCPv6).
            The default is 43200 seconds.

         The delayed-ack and max-ack-delay statements

            delayed-ack count;

            max-ack-delay microseconds;

            Count should be an integer value from zero to 2^16-1 and defaults to 0,  which  means
            that  the  feature  is  disabled.  Otherwise, 28 may be a sensible starting point for
            many configurations (SO_SNDBUF size / 576 bytes.)   The  count  represents  how  many
            DHCPv4  replies  maximum  will  be queued pending transmission until after a database
            commit event.  If this number is reached, a database commit event (commonly resulting
            in  fsync()  and  representing  a  performance  penalty)  will be made, and the reply
            packets will be transmitted in  a  batch  afterwards.   This  preserves  the  RFC2131
            direction  that "stable storage" be updated prior to replying to clients.  Should the
            DHCPv4 sockets "go dry" (select() returns immediately  with  no  read  sockets),  the
            commit is made and any queued packets are transmitted.

            Similarly,  microseconds  indicates  how  many  microseconds  are  permitted  to pass
            inbetween queuing a packet pending an fsync, and performing the fsync.  Valid  values
            range from 0 to 2^32-1, and defaults to 250,000 (1/4 of a second).

            The  delayed-ack  feature  is  compiled in by default, but can be disabled at compile
            time with ´./configure --disable-delayed-ack´.   Please  note  that  the  delayed-ack
            feature  is  not  currently  compatible with support for DHPCv4-over-DHCPv6 so when a
            4to6 port ommand line argument enables this in the server the  delayed-ack  value  is
            reset to 0.

         The dhcp-cache-threshold statement

            dhcp-cache-threshold percentage;

            The  dhcp-cache-threshold  statement  takes one integer parameter with allowed values
            between 0 and 100. The default value is 25 (25% of the lease  time).  This  parameter
            expresses the percentage of the total lease time, measured from the beginning, during
            which a client's attempt to renew its  lease  will  result  in  getting  the  already
            assigned  lease,  rather  than an extended lease.  This feature is supported for both
            IPv4 and IPv6 and down to the pool level and for IPv6 all three pool  types:  NA,  TA
            and PD.

            Clients  that attempt renewal frequently can cause the server to update and write the
            database frequently resulting in a performance impact on the server.  The dhcp-cache-
            threshold statement instructs the DHCP server to avoid updating leases too frequently
            thus avoiding this behavior.  Instead the server replies with the  same  lease  (i.e.
            reuses  it) with no modifications except for CLTT (Client Last Transmission Time) and
            for IPv4:

                the lease time sent to the client is shortened by the age of
                the lease

            while for IPv6:

                the preferred and valid lifetimes sent to the client are
                shortened by the age of the lease.

            None of these changes require writing the lease to disk.

            When an existing lease is matched to a renewing client, it will be reused if  all  of
            the following conditions are true:
                1. The dhcp-cache-threshold is larger than zero
                2. The current lease is active
                3. The percentage of the lease time that has elapsed is less than
                4. The client information provided in the renewal does not alter
                any of the following:
                   a. DNS information and DNS updates are enabled
                   b. Billing class to which the lease is associated (IPv4 only)
                   c. The host declaration associated with the lease (IPv4 only)
                   d. The client id - this may happen if a client boots without
                      a client id and then starts using one in subsequent
                      requests. (IPv4 only)

            While lease data is not written to disk when a lease is reused, the server will still
            execute any on-commit statements.

            Note that the lease can be reused if the options the client or relay agent sends  are
            changed.   These  changes  will not be recorded in the in-memory or on-disk databases
            until the client renews after the threshold time is reached.

         The do-forward-updates statement

            do-forward-updates flag;

            The do-forward-updates statement instructs the DHCP server as to  whether  it  should
            attempt  to  update  a  DHCP  client´s  A record when the client acquires or renews a
            lease.  This statement has no effect unless DNS updates are enabled.  Forward updates
            are  enabled  by  default.  If this statement is used to disable forward updates, the
            DHCP server will never attempt to update the client´s A record, and  will  only  ever
            attempt  to update the client´s PTR record if the client supplies an FQDN that should
            be placed in the PTR record using the fqdn option.  If forward updates  are  enabled,
            the DHCP server will still honor the setting of the client-updates flag.

         The dont-use-fsync statement

            dont-use-fsync flag;

            The dont-use-fsync statement instructs the DHCP server if it should call fsync() when
            writing leases to the lease file.  By default and if the flag is  set  to  false  the
            server  will  call  fsync().   Suppressing  the  call  to  fsync()  may  increase the
            performance of the server but it also adds a risk that a lease will not  be  properly
            written to the disk after it has been issued to a client and before the server stops.
            This can lead to duplicate leases being issued  to  different  clients.   Using  this
            option is not recommended.

         The dynamic-bootp-lease-cutoff statement

            dynamic-bootp-lease-cutoff date;

            The dynamic-bootp-lease-cutoff statement sets the ending time for all leases assigned
            dynamically to BOOTP clients.  Because BOOTP clients do not have any way of  renewing
            leases,  and  don't  know  that  their  leases could expire, by default dhcpd assigns
            infinite leases to all BOOTP clients.  However, it may make sense in some  situations
            to set a cutoff date for all BOOTP leases - for example, the end of a school term, or
            the time at night when a facility is closed and  all  machines  are  required  to  be
            powered off.

            Date  should  be  the  date on which all assigned BOOTP leases will end.  The date is
            specified in the form:

                                            W YYYY/MM/DD HH:MM:SS

            W is the day of the week expressed as a number from zero (Sunday) to six  (Saturday).
            YYYY  is the year, including the century.  MM is the month expressed as a number from
            1 to 12.  DD is the day of the month, counting from 1.  HH is the hour, from zero  to
            23.   MM  is  the  minute  and  SS  is the second.  The time is always in Coordinated
            Universal Time (UTC), not local time.

         The dynamic-bootp-lease-length statement

            dynamic-bootp-lease-length length;

            The dynamic-bootp-lease-length  statement  is  used  to  set  the  length  of  leases
            dynamically  assigned  to BOOTP clients.  At some sites, it may be possible to assume
            that a lease is no longer in use if its holder has not used BOOTP or DHCP to get  its
            address  within a certain time period.  The period is specified in length as a number
            of seconds.  If a client reboots using BOOTP during the  timeout  period,  the  lease
            duration  is  reset  to  length,  so a BOOTP client that boots frequently enough will
            never lose its lease.  Needless to  say,  this  parameter  should  be  adjusted  with
            extreme caution.

         The echo-client-id statement

            echo-client-id flag;

            The  echo-client-id  statement  is  used  to  enable  or  disable  RFC 6842 compliant
            behavior.  If the echo-client-id statement is present and has a value of true or  on,
            and  a  DHCP  DISCOVER  or  REQUEST  is received which contains the client identifier
            option (Option code 61), the server will copy the option into its response (DHCP  ACK
            or  NAK) per RFC 6842.  In other words if the client sends the option it will receive
            it back. By default, this flag is off and client identifiers will not echoed back  to
            the client.

         The filename statement

            filename "filename";

            The filename statement can be used to specify the name of the initial boot file which
            is to be loaded by a client.  The filename  should  be  a  filename  recognizable  to
            whatever file transfer protocol the client can be expected to use to load the file.

         The fixed-address declaration

            fixed-address address [, address ... ];

            The  fixed-address  declaration is used to assign one or more fixed IP addresses to a
            client.  It should only appear in a host declaration.  If more than  one  address  is
            supplied,  then  when  the  client  boots,  it  will  be  assigned  the  address that
            corresponds to the network on which it is booting.  If none of the addresses  in  the
            fixed-address  statement  are valid for the network to which the client is connected,
            that client will  not  match  the  host  declaration  containing  that  fixed-address
            declaration.   Each  address  in the fixed-address declaration should be either an IP
            address or a domain name that resolves to one or more IP addresses.

         The fixed-address6 declaration

            fixed-address6 ip6-address ;

            The fixed-address6 declaration is used to assign a fixed IPv6 addresses to a  client.
            It should only appear in a host declaration.

         The fixed-prefix6 declaration

            fixed-prefix6 low-address / bits;

            The  fixed-prefix6 declaration is used to assign a fixed IPv6 prefix to a client.  It
            should only appear in a host declaration, but multiple fixed-prefix6  statements  may
            appear in a single host declaration.

            The  low-address specifies the start of the prefix and the bits specifies the size of
            the prefix in bits.

            If there are multiple prefixes for a given host entry the server will choose one that
            matches the requested prefix size or, if none match, the first one.

            If  there  are multiple host declarations the server will try to choose a declaration
            where the fixed-address6 matches the client's subnet.  If none match it  will  choose
            one that doesn't have a fixed-address6 statement.

            Note  Well: Unlike the fixed address the fixed prefix does not need to match a subnet
            in order to be served.  This allows you to provide a  prefix  to  a  client  that  is
            outside of the subnet on which the client makes the request to the the server.

         The get-lease-hostnames statement

            get-lease-hostnames flag;

            The get-lease-hostnames statement is used to tell dhcpd whether or not to look up the
            domain name corresponding to the IP address of each address in the lease pool and use
            that address for the DHCP hostname option.  If flag is true, then this lookup is done
            for all addresses in the current scope.  By default, or if flag is false, no  lookups
            are done.

         The hardware statement

            hardware hardware-type hardware-address;

            In  order  for  a BOOTP client to be recognized, its network hardware address must be
            declared using a hardware clause in the host statement.  hardware-type  must  be  the
            name  of a physical hardware interface type.  Currently, only the ethernet and token-
            ring types are recognized, although support for a fddi  hardware  type  (and  others)
            would  also be desirable.  The hardware-address should be a set of hexadecimal octets
            (numbers from 0 through ff) separated by colons.  The hardware statement may also  be
            used for DHCP clients.

         The host-identifier option statement

            host-identifier option option-name option-data;


            host-identifier v6relopt number option-name option-data;

            This  identifies a DHCPv6 client in a host statement.  option-name is any option, and
            option-data is the value for the option that the client will  send.  The  option-data
            must  be  a constant value.  In the v6relopts case the additional number is the relay
            to examine for the specified option name and value.  The values are the same  as  for
            the  v6relay  option.  0 is a no-op, 1 is the relay closest to the client, 2 the next
            one in and so on.   Values  that  are  larger  than  the  maximum  number  of  relays
            (currently 32) indicate the relay closest to the server independent of number.

         The ignore-client-uids statement

            ignore-client-uids flag;

            If the ignore-client-uids statement is present and has a value of true or on, the UID
            for clients will not be recorded.  If this statement is not present or has a value of
            false or off, then client UIDs will be recorded.

         The infinite-is-reserved statement

            infinite-is-reserved flag;

            ISC  DHCP  now supports ´reserved´ leases.  See the section on RESERVED LEASES below.
            If this flag is on, the server will automatically reserve leases allocated to clients
            which requested an infinite (0xffffffff) lease-time.

            The default is off.

         The lease-file-name statement

            lease-file-name name;

            Name  Where  name  is  the  name of the DHCP server's lease file. By default, this is
            /var/lib/dhcp/dhcpd.leases.  This statement must appear in the  outer  scope  of  the
            configuration  file - if it appears in some other scope, it will have no effect.  The
            value must be the absolute path of the file to use.   The  order  of  precedence  the
            server uses for the lease file name is:

                1. lease-file-name configuration file statement.
                2. -lf command line flag.
                3. PATH_DHCPD_DB environment variable.

         The dhcpv6-lease-file-name statement

            dhcpv6-lease-file-name name;

            Where  name  is  the  name of the DHCP server's lease file when the server is running
            DHCPv6. By default, this is /var/lib/dhcp/dhcpd6.leases. This statement  must  appear
            in  the outer scope of the configuration file - if it appears in some other scope, it
            will have no effect.  The value must be the absolute path of the file  to  use.   The
            order of precedence the server uses for the lease file name is:

                1. dhcpv6-lease-file-name configuration file statement.
                2. -lf command line flag.
                3. PATH_DHCPD6_DB environment variable.

         The lease-id-format parameter

            lease-id-format format;

            The  format parameter must be either octal or hex.  This parameter governs the format
            used to write certain values to lease files. With the default format,  octal,  values
            are  written  as  quoted strings in which non-printable characters are represented as
            octal escapes - a backslash character followed by three octal digits.  When  the  hex
            format is specified, values are written as an unquoted series of pairs of hexadecimal
            digits, separated by colons.

            Currently, the values written out based on lease-id-format are the  server-duid,  the
            uid   (DHCPv4   leases),   and  the  IAID_DUID  (DHCPv6  leases).   Note  the  server
            automatically reads the values in either format.

         The limit-addrs-per-ia statement

            limit-addrs-per-ia number;

            By default, the DHCPv6 server will limit clients to one IAADDR per IA option, meaning
            one  address.   If  you  wish  to permit clients to hang onto multiple addresses at a
            time, configure a larger number here.

            Note that there is no present method to configure the server  to  forcibly  configure
            the  client with one IP address per each subnet on a shared network.  This is left to
            future work.

         The local-port statement

            local-port port;

            This statement causes the DHCP server to listen for DHCP requests  on  the  UDP  port
            specified in port, rather than on port 67.

         The local-address statement

            local-address address;

            This  statement  causes  the  DHCP  server  to  listen  for DHCP requests sent to the
            specified address, rather  than  requests  sent  to  all  addresses.   Since  serving
            directly  attached DHCP clients implies that the server must respond to requests sent
            to the all-ones IP address, this option cannot be used if  clients  are  on  directly
            attached  networks;  it  is only realistically useful for a server whose only clients
            are reached via unicasts, such as via DHCP relay agents.

            Note:  This statement is  only  effective  if  the  server  was  compiled  using  the
            USE_SOCKETS  #define  statement,  which  is  default  on  a small number of operating
            systems, and must be explicitly chosen at compile-time for all others.   You  can  be
            sure  if  your server is compiled with USE_SOCKETS if you see lines of this format at

             Listening on Socket/eth0

            Note also that since this bind()s all DHCP sockets to  the  specified  address,  that
            only one address may be supported in a daemon at a given time.

         The local-address6 and bind-local-address6 statements

            local-address6 address;

            bind-local-address6 flag;

            The  local-address6  statement  causes  the  DHCP  server  to  send  IPv6  packets as
            originating from the specified IPv6 address, rather than leaving the kernel  to  fill
            in the source address field.

            When  bind-local-address6  is  present and has a value of true or on, service sockets
            are bound to address too.

            By default address is the undefined address and the bind-local-address6 is  disabled,
            both may only be set at the global scope.

         The log-facility statement

            log-facility facility;

            This  statement  causes the DHCP server to do all of its logging on the specified log
            facility once the dhcpd.conf file has been read.  By default the DHCP server logs  to
            the  daemon  facility.  Possible log facilities include auth, authpriv, cron, daemon,
            ftp, kern, lpr, mail, mark, news, ntp,  security,  syslog,  user,  uucp,  and  local0
            through  local7.  Not all of these facilities are available on all systems, and there
            may be other facilities available on other systems.

            In addition to setting this value, you may need to modify your  syslog.conf  file  to
            configure logging of the DHCP server.  For example, you might add a line like this:

                 local7.debug /var/log/dhcpd.log

            The  syntax  of  the  syslog.conf  file  may be different on some operating systems -
            consult the syslog.conf manual page to be sure.  To get syslog to  start  logging  to
            the  new  file, you must first create the file with correct ownership and permissions
            (usually,  the  same   owner   and   permissions   of   your   /var/log/messages   or
            /usr/adm/messages  file  should  be fine) and send a SIGHUP to syslogd.  Some systems
            support log rollover using a shell script or program called newsyslog  or  logrotate,
            and  you  may  be  able  to configure this as well so that your log file doesn't grow

            Because the log-facility setting is controlled by the dhcpd.conf file,  log  messages
            printed  while  parsing  the  dhcpd.conf  file or before parsing it are logged to the
            default log facility.  To prevent this,  see  the  README  file  included  with  this
            distribution,  which describes BUG: where is that mentioned in README?  how to change
            the default log facility.  When this parameter is used, the DHCP  server  prints  its
            startup  message  a second time after parsing the configuration file, so that the log
            will be as complete as possible.

         The log-threshold-high and log-threshold-low statements

            log-threshold-high percentage;

            log-threshold-low percentage;

            The log-threshold-low and log-threshold-high statements are used to  control  when  a
            message  is output about pool usage.  The value for both of them is the percentage of
            the pool in use.  If the high threshold is 0 or has not been specified,  no  messages
            will  be  produced.   If a high threshold is given, a message is output once the pool
            usage passes that level.  After that, no more messages will be output until the  pool
            usage  falls  below the low threshold.  If the low threshold is not given, it default
            to a value of zero.

            A special case occurs when the low threshold is  set  to  be  higher  than  the  high
            threshold.   In  this  case,  a  message  will  be  generated  each  time  a lease is
            acknowledged when the pool usage is above the high threshold.

            Note that threshold logging will be automatically disabled for shared  subnets  whose
            total  number  of  addresses  is  larger  than  (2^64)-1.  The server will emit a log
            statement at startup when threshold logging is disabled as shown below:

                "Threshold logging disabled for shared subnet of ranges: <addresses>"

            This is likely to have no practical runtime effect as CPUs are unlikely to support  a
            server actually reaching such a large number of leases.

         The max-lease-time statement

            max-lease-time time;

            Time  should  be  the maximum length in seconds that will be assigned to a lease.  If
            not defined, the default maximum lease time is 86400.  The only exception to this  is
            that  Dynamic  BOOTP  lease  lengths,  which are not specified by the client, are not
            limited by this maximum.

         The min-lease-time statement

            min-lease-time time;

            Time should be the minimum length in seconds that will be assigned to a  lease.   The
            default is the minimum of 300 seconds or max-lease-time.

         The min-secs statement

            min-secs seconds;

            Seconds  should  be  the  minimum  number  of  seconds since a client began trying to
            acquire a new lease before the DHCP server will respond to its request.   The  number
            of seconds is based on what the client reports, and the maximum value that the client
            can report is 255 seconds.  Generally, setting this to one will result  in  the  DHCP
            server  not  responding  to  the client's first request, but always responding to its
            second request.

            This can be used to set up a secondary DHCP server which never offers an address to a
            client  until  the  primary  server has been given a chance to do so.  If the primary
            server is down, the client will bind to the secondary server, but  otherwise  clients
            should  always  bind  to  the  primary.  Note that this does not, by itself, permit a
            primary server and a secondary server to  share  a  pool  of  dynamically-allocatable

         The next-server statement

            next-server server-name;

            The  next-server  statement  is  used  to specify the host address of the server from
            which the initial boot file (specified in the filename statement) is  to  be  loaded.
            Server-name should be a numeric IP address or a domain name.

         The omapi-port statement

            omapi-port port;

            The  omapi-port  statement  causes the DHCP server to listen for OMAPI connections on
            the specified port.  This statement is required to enable the OMAPI  protocol,  which
            is used to examine and modify the state of the DHCP server as it is running.

         The one-lease-per-client statement

            one-lease-per-client flag;

            If  this  flag  is  enabled,  whenever  a client sends a DHCPREQUEST for a particular
            lease, the server will automatically free any other leases the  client  holds.   This
            presumes  that  when  the  client sends a DHCPREQUEST, it has forgotten any lease not
            mentioned in the DHCPREQUEST - i.e., the client has only a single  network  interface
            and it does not remember leases it's holding on networks to which it is not currently
            attached.  Neither of these assumptions  are  guaranteed  or  provable,  so  we  urge
            caution in the use of this statement.

         The persist-eui-64-leases statement

            persist-eui-64-leases flag;

            When  this  flag  is enabled, the server will write EUI-64 based leases to the leases
            file. Since such leases can only, ever be valid for a single DUID  value  it  can  be
            argued that writing them to the leases file isn't essential and not doing so may have
            perfomance advantages.  See use-eui-64 statement for more  details  on  EUI-64  based
            address allocation.  The flag is enabled by default and may only be set at the global

         The pid-file-name statement

            pid-file-name name;

            Name should be the name of the DHCP server's process ID file.  This is  the  file  in
            which  the  DHCP  server's  process ID is stored when the server starts.  By default,
            this is /var/run/  Like the lease-file-name statement, this statement  must
            appear  in the outer scope of the configuration file. The order of precedence used by
            the server is:

                1. pid-file-name configuration file statement.
                2. -lf command line flag.
                3. PATH_DHCPD_PID environment variable.

            The dhcpv6-pid-file-name statement

              dhcpv6-pid-file-name name;

              Name is the name of the pid file to use if and only if the  server  is  running  in
              DHCPv6  mode.   By default, this is /var/lib/dhcp/  This statement, like
              pid-file-name, must appear in the outer scope of the configuration file.  The order
              of precedence used by the server is:

                  1. dhcpv6-pid-file-name configuration file statement.
                  2. -lf command line flag.
                  3. PATH_DHCPD6_PID environment variable.

            The ping-check statement

              ping-check flag;

              When  the  DHCP  server  is  considering  dynamically allocating an IP address to a
              client, it first sends an ICMP Echo request (a ping) to the address being assigned.
              It  waits for a second, and if no ICMP Echo response has been heard, it assigns the
              address.  If a response is heard, the lease is abandoned, and the server  does  not
              respond  to  the client.  The lease will remain abandoned for a minimum of abandon-
              lease-time seconds.

              If a there are no free addresses but there are abandoned  IP  addresses,  the  DHCP
              server  will  attempt to reclaim an abandoned IP address regardless of the value of

              This ping check introduces a default one-second delay in responding to DHCPDISCOVER
              messages, which can be a problem for some clients.  The default delay of one second
              may be configured using the ping-timeout parameter.  The  ping-check  configuration
              parameter can be used to control checking - if its value is false, no ping check is

            The ping-timeout statement

              ping-timeout seconds;

              If the DHCP server determined it should send an ICMP echo request (a ping)  because
              the  ping-check  statement  is  true, ping-timeout allows you to configure how many
              seconds the DHCP server should wait for an ICMP Echo response to be  heard,  if  no
              ICMP  Echo  response  has  been received before the timeout expires, it assigns the
              address.  If a response is heard, the lease is abandoned, and the server  does  not
              respond to the client.  If no value is set, ping-timeout defaults to 1 second.

            The preferred-lifetime statement

              preferred-lifetime seconds;

              IPv6  addresses  have  ´valid´  and  ´preferred´  lifetimes.   The  valid  lifetime
              determines at what point at lease might be said to have expired, and is  no  longer
              useable.   A  preferred lifetime is an advisory condition to help applications move
              off of the address and onto currently valid addresses (should there  still  be  any
              open TCP sockets or similar).

              The preferred lifetime defaults to 5/8 the default lease time.

            The prefix-length-mode statement

              prefix-length-mode mode;

              According  to  RFC  3633,  DHCPv6  clients  may specify preferences when soliciting
              prefixes by including an IA_PD Prefix option within the  IA_PD  option.  Among  the
              preferences that may be conveyed is the "prefix-length". When non-zero it indicates
              a client's desired length for offered prefixes.  The RFC states that  servers  "MAY
              choose  to  use  the  select prefix(es)" but does not specify any
              particular rules for doing so. The prefix-length-mode statement can be used to  set
              the  prefix  selection  rules  employed by the server, when clients send a non-zero
              prefix-length value. The mode parameter must  be  one  of  ignore,  prefer,  exact,
              minimum, or maximum where:

              1.  ignore  -  The  requested  length  is  ignored. The server will offer the first
              available prefix.

              2. prefer - The server will offer the first available prefix with the  same  length
              as  the requested length.  If none are found then it will offer the first available
              prefix of any length.  This is the default behavior.

              3. exact - The server will offer the first available prefix with the same length as
              the  requested  length.   If  none are found, it will return a status indicating no
              prefixes available.

              4. minimum - The server will offer the first available prefix with the same  length
              as  the  requested  length.   If none are found, it will return the first available
              prefix whose length is greater than (e.g. longer than), the  requested  value.   If
              none  of those are found, it will return a status indicating no prefixes available.
              For example, if client requests a length of  /60,  and  the  server  has  available
              prefixes of lengths /56 and /64, it will offer prefix of length /64.

              5.  maximum - The server will offer the first available prefix with the same length
              as the requested length.  If none are found, it will  return  the  first  available
              prefix whose length is less than (e.g. shorter than), the requested value.  If none
              of those are found, it will return a status indicating no prefixes available.   For
              example,  if client requests a length of /60, and the server has available prefixes
              of lengths /56 and /64, it will offer a prefix of length /56.

              In general "first available" is determined by the order in which pools are  defined
              in  the  server's  configuration.   For  example, if a subnet is defined with three
              prefix pools A,B, and C:

              subnet 3000::/64 {
                   # pool A
                   pool6 {
                   # pool B
                   pool6 {
                   # pool C
                   pool6 {

              then the pools will be checked in the order A, B, C. For modes prefer, minimum, and
              maximum this may mean checking the pools in that order twice.  A first pass through
              is made looking for an available prefix of exactly the preferred length.   If  none
              are  found,  then  a  second  pass  is  performed  starting  with  pool  A but with
              appropriately adjusted length criteria.

            The release-on-roam statement

              release-on-roam flag;

              When enabled and the dhcpd server detects that  a  DHCPv6  client  (IAID+DUID)  has
              roamed to a new network, it will release the pre-existing leases on the old network
              and emit a log statement similiar to the following:

                    "Client: <id> roamed to new network, releasing lease: <address>"

              The server will carry out all of the same steps that would normally  occur  when  a
              client explicitly releases a lease.  When release-on-roam is disabled (the default)
              the server makes such leases  unavailable  until  they  expire  or  the  server  is
              restarted.  Clients that need leases in multiple networks must supply a unique IAID
              in each IA.  This parameter may only be specified at the global level.

            The remote-port statement

              remote-port port;

              This statement causes the DHCP server to transmit DHCP responses  to  DHCP  clients
              upon the UDP port specified in port, rather than on port 68.  In the event that the
              UDP response is transmitted to a DHCP Relay, the server generally uses  the  local-
              port  configuration  value.   Should  the  DHCP  Relay  happen  to  be addressed as
    , however, the DHCP Server  transmits  its  response  to  the  remote-port
              configuration  value.  This is generally only useful for testing purposes, and this
              configuration value should generally not be used.

            The server-identifier statement

              server-identifier hostname;

              The server-identifier statement can be used to define the value that is sent in the
              DHCP Server Identifier option for a given scope.  The value specified must be an IP
              address for the DHCP server, and must be reachable  by  all  clients  served  by  a
              particular scope.

              The  use of the server-identifier statement is not recommended - the only reason to
              use it is to force a value other than the default value to  be  sent  on  occasions
              where  the  default  value  would  be incorrect.  The default value is the first IP
              address associated with  the  physical  network  interface  on  which  the  request

              The  usual  case  where  the server-identifier statement needs to be sent is when a
              physical interface has more than one IP address, and the one being sent by  default
              isn't appropriate for some or all clients served by that interface.  Another common
              case is when an alias is defined for the purpose of having a consistent IP  address
              for  the  DHCP  server, and it is desired that the clients use this IP address when
              contacting the server.

              Supplying a value for the dhcp-server-identifier option is equivalent to using  the
              server-identifier statement.

            The server-id-check statement

              server-id-check flag;

              The  server-id-check  statement  is  used  to  control  whether  or  not  a server,
              participating in failover, verifies that the value  of  the  dhcp-server-identifier
              option  in  received  DHCP  REQUESTs  match  the  server's id before processing the
              request. Server id checking is disabled by default.  Setting this flag  enables  id
              checking and thereafter the server will only process requests that match.  Note the
              flag setting should be consistent between failover partners.

              Unless overridden by use of the server-identifier statement, the value  the  server
              uses  as  its  id will be the first IP address associated with the physical network
              interface on which the request arrived.

              In order to reduce runtime overhead the server only checks for a server  id  option
              in  the  global  and  subnet  scopes.   Complicated  configurations  may  result in
              different server ids for this check and when the server id for a  reply  packet  is
              determined, which would prohibit the server from responding.

              The  primary use for this option is when a client broadcasts a request but requires
              that the response come from a specific failover peer.  An example of this would  be
              when  a  client reboots while its lease is still active - in this case both servers
              will normally respond.  Most of the time the client won't check the server  id  and
              can use either of the responses.  However if the client does check the server id it
              may reject the response if it came from the wrong peer.  If the timing is such that
              the  "wrong" peer responds first most of the time the client may not get an address
              for some time.

              Care should be taken before enabling this option.

            The server-duid statement

              server-duid LLT [ hardware-type timestamp hardware-address ] ;

              server-duid EN enterprise-number enterprise-identifier ;

              server-duid LL [ hardware-type hardware-address ] ;

              The server-duid statement configures the server DUID. You may pick either LLT (link
              local address plus time), EN (enterprise), or LL (link local).

              If you choose LLT or LL, you may specify the exact contents of the DUID.  Otherwise
              the server will generate a DUID of the specified type.

              If you choose EN, you must  include  the  enterprise  number  and  the  enterprise-

              If  there is a server-duid statement in the lease file it will take precedence over
              the server-duid statement from the config file and a dhcp6.server-id option in  the
              config file will override both.

              The default server-duid type is LLT.

            The server-name statement

              server-name name ;

              The  server-name  statement  can  be  used  to inform the client of the name of the
              server from which it is booting.  Name should be the name that will be provided  to
              the client.

            The dhcpv6-set-tee-times statement

              dhcpv6-set-tee-times flag;

              The  dhcpv6-set-tee-times  statement  enables  setting  T1  and  T2  to  the values
              recommended in RFC 3315 (Section 22.4).  When setting T1 and T2,  the  server  will
              use dhcp-renewal-time and dhcp-rebinding-time, respectively.  A value of zero tells
              the client it may choose its own value.

              When those options are not defined then values will  be  set  to  zero  unless  the
              global  dhcpv6-set-tee-times is enabled.  When this option is enabled the times are
              calculated as recommended by RFC 3315, Section 22.4:

                    T1 will be set to 0.5 times the shortest preferred lifetime
                    in the reply.  If the "shortest" preferred lifetime is
                    0xFFFFFFFF,  T1 will set to 0xFFFFFFFF.

                    T2 will be set to 0.8 times the shortest preferred lifetime
                    in the reply.  If the "shortest" preferred lifetime is
                    0xFFFFFFFF,  T2 will set to 0xFFFFFFFF.

              Keep in mind that given sufficiently small lease lifetimes, the above  calculations
              will  result  in the two values being equal. For example, a 9 second lease lifetime
              would yield T1 = T2 = 4 seconds, which would cause clients to issue  rebinds  only.
              In such a case it would likely be better to explicitly define the values.

              Note  that  dhcpv6-set-tee-times  is intended to be transitional and will likely be
              removed in a future  release.  Once  removed  the  behavior  will  be  to  use  the
              configured  values  when  present or calculate them per the RFC. If you want zeros,
              define them as zeros.

            The site-option-space statement

              site-option-space name ;

              The site-option-space statement can be used to determine  from  what  option  space
              site-local  options  will  be  taken.  This can be used in much the same way as the
              vendor-option-space statement.  Site-local options in DHCP are those options  whose
              numeric  codes  are greater than 224.  These options are intended for site-specific
              uses, but are frequently used by vendors of embedded hardware  that  contains  DHCP
              clients.   Because  site-specific  options  are allocated on an ad hoc basis, it is
              quite possible that one vendor's DHCP client might use the same  option  code  that
              another vendor's client uses, for different purposes.  The site-option-space option
              can be used to assign a different  set  of  site-specific  options  for  each  such
              vendor, using conditional evaluation (see dhcp-eval (5) for details).

            The stash-agent-options statement

              stash-agent-options flag;

              If  the  stash-agent-options  parameter is true for a given client, the server will
              record the relay  agent  information  options  sent  during  the  client's  initial
              DHCPREQUEST  message  when  the  client was in the SELECTING state and behave as if
              those options are included in all  subsequent  DHCPREQUEST  messages  sent  in  the
              RENEWING  state.  This works around a problem with relay agent information options,
              which is that they usually not appear in DHCPREQUEST messages sent by the client in
              the  RENEWING  state,  because such messages are unicast directly to the server and
              not sent through a relay agent.

            The update-conflict-detection statement

              update-conflict-detection flag;

              If the  update-conflict-detection  parameter  is  true,  the  server  will  perform
              standard  DHCID multiple-client, one-name conflict detection.  If the parameter has
              been set false, the server will skip this check and instead simply  tear  down  any
              previous bindings to install the new binding without question.  The default is true
              and this parameter may only be specified at the global scope.

            The update-optimization statement

              update-optimization flag;

              If the update-optimization parameter is false for a given client, the  server  will
              attempt  a DNS update for that client each time the client renews its lease, rather
              than only attempting an update when it appears to be necessary.   This  will  allow
              the DNS to heal from database inconsistencies more easily, but the cost is that the
              DHCP server must do many more  DNS  updates.   We  recommend  leaving  this  option
              enabled,  which is the default. If this parameter is not specified, or is true, the
              DHCP server will only update when the client information changes, the client gets a
              different lease, or the client's lease expires.

            The update-static-leases statement

              update-static-leases flag;

              The update-static-leases flag, if enabled, causes the DHCP server to do DNS updates
              for clients even if those clients are being  assigned  their  IP  address  using  a
              fixed-address  or  fixed-address6  statement - that is, the client is being given a
              static assignment.  It is not recommended because the DHCP server  has  no  way  to
              tell  that  the update has been done, and therefore will not delete the record when
              it is not in use.  Also, the server must attempt the update each  time  the  client
              renews its lease, which could have a significant performance impact in environments
              that place heavy demands on the DHCP server.  This feature is  supported  for  both
              DHCPv4 and DHCPv6, and update modes standard or interim. It is disabled by default.

            The use-eui-64 statement

              use-eui-64 flag;

              (Support for this must be enabled at compile time, see EUI_64 in

              The use-eui-64 flag, if enabled, instructs the server to construct an address using
              the client's EUI-64 DUID (Type 3, HW Type EUI-64), rather than creating an  address
              using the dynamic algorithm.  This means that a given DUID will always generate the
              same address for a given pool and further that the  address  is  guaranteed  to  be
              unique  to  that  DUID.   The  IPv6 address will be calculated from the EUI-64 link
              layer address, conforming to RFC 2373, unless there is a host declaration  for  the

              The  range6  statement  for  EUI-64 must define full /64 bit ranges. Invalid ranges
              will be flagged during configuration parsing as errors.  See the following example:

                  subnet6 fc00:e4::/64 {
                      use-eui-64 true;
                      range6 fc00:e4::/64;

              The statement may be specified down to  the  pool  level,  allowing  a  mixture  of
              dynamic and EUI-64 based pools.

              During lease file parsing, any leases which map to an EUI-64 pool, that have a non-
              EUI-64 DUID or for which the lease address is not the EUI-64 address for that  DUID
              in that pool, will be discarded.

              If  a  host  declaration exists for the DUID, the server grants the address (fixed-
              prefix6, fixed-address6) according to the host declaration, regardless of the  DUID
              type of the client (even for EUI-64 DUIDs).

              If  a  client  request's  an  EUI-64  lease  for a given network, and the resultant
              address conflicts with a fixed address reservation, the server will send the client
              a "no addresses available" response.

              Any  client  with  a non-conforming DUID (not type 3 or not hw type EUI-64) that is
              not linked to a host declaration, which requests an address from an EUI-64  enabled
              pool will be ignored and the event will be logged.

              Pools  that  are  configured for EUI-64 will be skipped for dynamic allocation.  If
              there are no pools in the shared network from which to allocate,  the  client  will
              get back a no addresses available status.

              On  an  EUI-64 enabled pool, any client with a DUID 3, HW Type EUI-64, requesting a
              solicit/renew and including IA_NA that do not match the EUI-64 policy, they will be
              treated as though they are "outside" the subnet for a given client message:

                  Solicit - Server will advertise with EUI-64 ia suboption, but with rapid
                  commit off
                  Request - Server will send "an address not on link status", and no ia
                  suboption Renew/Rebind - Server will send the requested address ia
                  suboption with lifetimes of 0, plus an EUI-64 ia

              Whether  or  not   EUI-64 based leases are written out to the lease database may be
              controlled by persist-eui-64-leases statement.

            The use-host-decl-names statement

              use-host-decl-names flag;

              If the use-host-decl-names parameter is true in a given scope, then for every  host
              declaration  within  that scope, the name provided for the host declaration will be
              supplied to the client as its hostname.  So, for example,

                  group {
                    use-host-decl-names on;

                    host joe {
                      hardware ethernet 08:00:2b:4c:29:32;

              is equivalent to

                    host joe {
                      hardware ethernet 08:00:2b:4c:29:32;
                      option host-name "joe";

              Additionally, enabling use-host-decl-names instructs the server  to  use  the  host
              declaration  name  in  the  the forward DNS name, if no other values are available.
              This value selection process is discussed in more detail under DNS updates.

              An option host-name statement within a host declaration will override  the  use  of
              the name in the host declaration.

              It  should  be  noted  here  that most DHCP clients completely ignore the host-name
              option sent by the DHCP server, and there is no way to configure  them  not  to  do
              this.   So  you generally have a choice of either not having any hostname to client
              IP address mapping that the client will recognize, or doing  DNS  updates.   It  is
              beyond the scope of this document to describe how to make this determination.

            The use-lease-addr-for-default-route statement

              use-lease-addr-for-default-route flag;

              If  the  use-lease-addr-for-default-route  parameter is true in a given scope, then
              instead of sending the value specified in the routers option (or sending  no  value
              at  all),  the  IP address of the lease being assigned is sent to the client.  This
              supposedly causes Win95 machines to ARP for all IP addresses, which can be  helpful
              if  your  router  is  configured  for  proxy  ARP.   The use of this feature is not
              recommended, because it won't work for many DHCP clients.

            The vendor-option-space statement

              vendor-option-space string;

              The vendor-option-space parameter determines from what option space vendor  options
              are  taken.   The  use  of this configuration parameter is illustrated in the dhcp-
              options(5) manual page, in the VENDOR ENCAPSULATED OPTIONS section.


       Sometimes it's helpful to be able to set the value of a DHCP  server  parameter  based  on
       some  value that the client has sent.  To do this, you can use expression evaluation.  The
       dhcp-eval(5) manual page describes how to write expressions.  To assign the result  of  an
       evaluation to an option, define the option as follows:

         my-parameter = expression ;

       For example:

         ddns-hostname = binary-to-ascii (16, 8, "-",
                                          substring (hardware, 1, 6));


       It's  often  useful  to  allocate  a  single  address  to  a single client, in approximate
       perpetuity.  Host statements with fixed-address clauses exist to a certain extent to serve
       this   purpose,   but   because  host  statements  are  intended  to  approximate  ´static
       configuration´, they suffer from not  being  referenced  in  a  littany  of  other  Server
       Services, such as dynamic DNS, failover, ´on events´ and so forth.

       If  a  standard dynamic lease, as from any range statement, is marked ´reserved´, then the
       server will only allocate this lease to the client it is identified by (be that by  client
       identifier or hardware address).

       In  practice,  this  means  that  the lease follows the normal state engine, enters ACTIVE
       state when the client is bound to it, expires, or is released, and any events or  services
       that  would  normally  be supplied during these events are processed normally, as with any
       other dynamic lease.  The only difference is that failover servers treat  reserved  leases
       as  special when they enter the FREE or BACKUP states - each server applies the lease into
       the state it may allocate from - and the leases are not placed on the queue for allocation
       to  other  clients.   Instead  they may only be ´found´ by client identity.  The result is
       that the lease is only offered to the returning client.

       Care should probably be taken to ensure that the client only has one lease within a  given
       subnet that it is identified by.

       Leases  may  be set ´reserved´ either through OMAPI, or through the ´infinite-is-reserved´
       configuration option (if this is applicable to your environment and mixture of clients).

       It should also be noted that leases marked ´reserved´ are effectively treated the same  as
       leases marked ´bootp´.


       DHCP option statements are documented in the dhcp-options(5) manual page.


       Expressions  used  in  DHCP  option  statements  and elsewhere are documented in the dhcp-
       eval(5) manual page.


       dhcpd(8), dhcpd.leases(5), dhcp-options(5), dhcp-eval(5), RFC2132, RFC2131.


       dhcpd.conf(5) is maintained by ISC.  Information about Internet Systems Consortium can  be
       found at