Provided by: isc-dhcp-server_4.3.5-3ubuntu7_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.  In this case, 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.

       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";

       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.


       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

         adandon-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 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 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-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 28.  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 not compiled in by default, but must be  enabled  at  compile
         time   with   ´./configure  --enable-delayed-ack´.   While  we  no  longer  consider  it
         experimental and we don't know of any issues with it, in order to minimize problems with
         existing configuration files we have left it disabled by default.

       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.

         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 assigns the same lease (i.e. reuses it)
         with  no  modifications  except  for CLTT (Client Last Transmission Time) which does not
         require disk operations. This feature applies to IPv4 only.

         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
                c. The host declaration associated with the lease
                d. The client id - this may happen if a client boots without
                a client id and then starts using one in subsequent requests.

         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

       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

       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

       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  should  be  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.
         Furthermore, it has no effect if  overridden  by  the  -lf  flag  or  the  PATH_DHCPD_DB
         environment variable.

       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

       The dhcpv6-lease-file-name statement

         dhcpv6-lease-file-name name;

         Name is the name of the lease file to use if and only if the server is running in DHCPv6
         mode.  By default, this is /var/lib/dhcp/dhcpd6.leases.   This  statement,  like  lease-
         file-name,  must  appear in the outer scope of the configuration file.  It has no effect
         if  overridden  by  the  -lf  flag  or  the  PATH_DHCPD6_DB  environment  variable.   If
         dhcpv6-lease-file-name  is  not specified, but lease-file-name is, the latter value will
         be used.

       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 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 startup:

          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 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 uncontrollably.

         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

       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

         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 addresses.

       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

       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.  It has no effect if overridden by the -pf
         flag or the 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.  It  has  no
            effect if overridden by the -pf flag or the PATH_DHCPD6_PID environment variable.  If
            dhcpv6-pid-file-name is not specified, but pid-file-name is, the latter value will be

         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

            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.

            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.  This is the default behavior.

            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 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 arrived.

            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

         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

         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.

         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  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.

         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