Provided by: isc-dhcp-server_4.2.4-7ubuntu12_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  declaration  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.


       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.

       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 index;

         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 255, of which the most
         reasonable is 128.

       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.

       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.

       Two DNS update schemes are currently implemented, and another is planned.   The  two  that
       are  currently  implemented  are  the  ad-hoc  DNS  update  mode  and the interim DHCP-DNS
       interaction draft update mode.  In the future we plan to add a third mode  which  will  be
       the  standard  DNS  update method based on the RFCS for DHCP-DNS interaction and DHCID The
       DHCP server must be configured to use one of the two currently-supported methods,  or  not
       to do dns updates.  This can be done with the ddns-update-style configuration parameter.


       The  ad-hoc  Dynamic  DNS  update  scheme  is now deprecated and does not work.  In future
       releases of the ISC DHCP server, this scheme will not likely be  available.   The  interim
       scheme  works,  allows for failover, and should now be used.  The following description is
       left here for informational purposes only.

       The ad-hoc Dynamic DNS update scheme implemented in this version of the ISC DHCP server is
       a prototype design, which does not have much to do with the standard update method that is
       being standardized in the IETF DHC working group, but rather implements some  very  basic,
       yet  useful,  update  capabilities.    This  mode does not work with the failover protocol
       because it does not account for the possibility of two different DHCP servers updating the
       same set of DNS records.

       For  the ad-hoc DNS update method, the client's FQDN is derived in two parts.   First, the
       hostname is determined.   Then, the  domain  name  is  determined,  and  appended  to  the

       The  DHCP  server  determines  the  client's hostname by first looking for a ddns-hostname
       configuration option, and using that if it is present.  If no such option is present,  the
       server looks for a valid hostname in the FQDN option sent by the client.  If one is found,
       it is used; otherwise, if the client sent a host-name option, that is used.  Otherwise, if
       there  is  a  host  declaration that applies to the client, the name from that declaration
       will be used.  If none of these applies, the server will  not  have  a  hostname  for  the
       client, and will not be able to do a DNS update.

       The  domain name is determined from the ddns-domainname configuration option.  The default
       configuration for this option is:

         option server.ddns-domainname = config-option domain-name;

       So if this configuration option is not configured to a different  value  (over-riding  the
       above default), or if a domain-name option has not been configured for the client's scope,
       then the server will not attempt to perform a DNS update.

       The client's fully-qualified domain name, derived as we have described,  is  used  as  the
       name on which an "A" record will be stored.  The A record will contain the IP address that
       the client was assigned in its lease.   If there is already an A record with the same name
       in  the  DNS server, no update of either the A or PTR records will occur - this prevents a
       client from claiming that its hostname is the name of some network server.   For  example,
       if you have a fileserver called "", and the client claims its hostname is
       "fs", no DNS update will be done for that client, and an error message will be logged.

       If the A record update succeeds, a PTR record update for the assigned IP address  will  be
       done,  pointing  to the A record.   This update is unconditional - it will be done even if
       another PTR record of the same name exists.   Since the IP address has  been  assigned  to
       the DHCP server, this should be safe.

       Please  note  that  the  current implementation assumes clients only have a single network
       interface.   A client with two network interfaces will see unpredictable behavior.    This
       is  considered  a bug, and will be fixed in a later release.   It may be helpful to enable
       the one-lease-per-client parameter so that  roaming  clients  do  not  trigger  this  same

       The  DHCP  protocol  normally  involves  a four-packet exchange - first the client sends a
       DHCPDISCOVER message, then  the  server  sends  a  DHCPOFFER,  then  the  client  sends  a
       DHCPREQUEST,  then the server sends a DHCPACK.   In the current version of the server, the
       server will do a DNS update after it has received the DHCPREQUEST, and before it has  sent
       the  DHCPACK.    It  only  sends  the  DNS  update if it has not sent one for the client's
       address before, in order to minimize the impact on the DHCP server.

       When the client's lease expires, the DHCP server (if it is operating at the time, or  when
       next  it  operates) will remove the client's A and PTR records from the DNS database.   If
       the client releases its lease by sending a DHCPRELEASE message, the server  will  likewise
       remove the A and PTR records.


       The  interim  DNS  update scheme operates mostly according to several drafts considered by
       the IETF.  While the drafts have since become RFCs the code was written before  they  were
       finalized  and there are some differences between our code and the final RFCs.  We plan to
       update our code, probably adding a standard DNS update option, at some  time.   The  basic
       framework  is similar with the main material difference being that a DHCID RR was assigned
       in the RFCs whereas our code continues to use an experimental TXT record.  The  format  of
       the  TXT record bears a resemblance to the DHCID RR but it is not equivalent (MD5 vs SHA1,
       field length differences etc).  The standard RFCs are:

                                      RFC 4701 (updated by RF5494)
                                                RFC 4702
                                                RFC 4703

       And the corresponding drafts were:


       Because our implementation is slightly  different  than  the  standard,  we  will  briefly
       document the operation of this update style here.

       The  first  point  to  understand about this style of DNS update is that unlike the ad-hoc
       style, 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

       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 from either the
       fqdn option (if present) or the hostname option (if present).  It will use its own  domain
       name  for the client, just as in the ad-hoc update scheme.  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.

       Also,   if  the  use-host-decl-names  configuration  option  is  enabled,  then  the  host
       declaration's hostname will be used in place of the hostname option, and  the  same  rules
       will apply as described above.

       The  other  difference  between  the ad-hoc scheme and the interim scheme is that with the
       interim scheme, a method is used that allows 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.   The scheme works as follows:

       When the DHCP server issues a client a new lease, it creates a text string that is an  MD5
       hash  over  the  DHCP  client's  identification (see draft-ietf-dnsext-dhcid-rr-??.txt for
       details).   The update adds an A record with the name the server chose and  a  TXT  record
       containing the hashed identifier string (hashid).   If this update succeeds, the server is

       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 TXT record in the same name as
       the new A record, and that TXT 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 interim DNS update scheme is called interim for two reasons.  First, it does not quite
       follow the RFCs.   The RFCs call for a new DHCID RRtype while he interim DNS update scheme
       uses a TXT record.  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.

       In addition to these differences, 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.


       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 8 and 9 distributions come
       with a program for generating secret keys called dnssec-keygen.  The  version  that  comes
       with  BIND 9 is likely to produce a substantially more random key, so we recommend you use
       that one even if you are not using BIND 9 as your DNS server.  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

       If  you  are using the BIND 8 dnskeygen program, the following command will generate a key
       as seen above:

            dnskeygen -H 128 -u -c -n DHCP_UPDATER

       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.   Events are used to implement DNS
       updates, so you should not define your own event handlers if you are  using  the  built-in
       DNS update mechanism.

       The  built-in  version  of the DNS update mechanism is in a text string towards the top of
       server/dhcpd.c.   If you want to use events for things other than  DNS  updates,  and  you
       also  want  DNS  updates,  you  will  have  to  start  out  by copying this code into your
       dhcpd.conf file and modifying it.


       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 scope in which to provide configuration information  about
       a  specific  client, and also provides a way to assign a client a fixed address.  The host
       declaration provides a way for the DHCP server to identify a DHCP  or  BOOTP  client,  and
       also a way to assign the client a static IP address.

       If  it is desirable to be able to boot a DHCP or BOOTP client on more than one subnet with
       fixed 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.

       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  reclaim  these  leases,  but
       while  the  client is running through the pool, it 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 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.  This is only relevant when doing interim
       DNS updates.   See the documentation under the heading THE INTERIM DNS UPDATE  SCHEME  for

       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

       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 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 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

         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 ad-hoc, 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).

         Please note that as delayed-ack is currently experimental, the  delayed-ack  feature  is
         not  compiled  in  by  default,  but  must  be enabled at compile time with ´./configure

       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 and ddns-update-style is set
         to  interim.    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 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 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 get-lease-hostnames statement

         get-lease-hostnames flag;

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

       The hardware statement

         hardware hardware-type hardware-address;

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

       The host-identifier option statement

         host-identifier option 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.

       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,  clients
         will  be  handled as though they provided no UID and the actual provided UID will not be
         recorded.  If this statement is not present or has a value of false or off, then  client
         UIDs will be parsed and used as normal.

       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
         DBDIR/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 DBDIR/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 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 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  how  to change the default log facility.  When this parameter is used,
         the DHCP server prints its startup message a second time after parsing the configuration
         file, so that the log will be as complete as possible.

       The max-lease-time statement

         max-lease-time time;

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

       The min-lease-time statement

         min-lease-time time;

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

       The min-secs statement

         min-secs seconds;

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

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

       The next-server statement

         next-server server-name;

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

       The omapi-port statement

         omapi-port port;

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

       The one-lease-per-client statement

         one-lease-per-client flag;

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

       The 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 RUNDIR/   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 DBDIR/  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.

            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 the renew+rebind timers, or 3/4 the default  lease
            time if none were specified.

         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-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-

            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 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.  This option only affects  the  behavior  of  the  interim  DNS
            update scheme, and has no effect on the ad-hoc DNS update scheme.   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

         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.    This
            can only work with the interim DNS update scheme.   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";

            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)  was  written  by  Ted Lemon under a contract with Vixie Labs.   Funding for
       this project was provided by Internet  Systems  Consortium.   Information  about  Internet
       Systems Consortium can be found at