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       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.

       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 sends.

       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 default:

            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

       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

       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

       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


       This version of the ISC DHCP server supports the DHCP failover protocol
       as  documented in draft-ietf-dhc-failover-07.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 peer
       state declaration in the lease file, and restarting  the  server.    If
       you  use  this  last  method, be sure to leave the date and time of the
       start of the state blank:

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

       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

       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 expired.


       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 DHCP FAILOVER.

       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
          not currently be omitted, because the failover protocol does not yet
          have a reserved TCP port number.

       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 not be omitted because the failover protocol does
          not  yet have a reserved TCP port number.   The port number declared
          in the peer port statement may  be  the  same  as  the  port  number
          declared in the port statement.

       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

            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 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 passes.

          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  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 (percenatge) skew in
          the lease balance of a percentage of the total number of free  state

          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

          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

       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

       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

       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 client:

       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 page.


       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

       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 opton 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  available  are  the  ad-hoc  DNS
       update mode and the interim DHCP-DNS interaction draft update mode.  If
       and when the DHCP-DNS interaction draft and the  DHCID  draft  make  it
       through  the  IETF standards process, there will be a third mode, which
       will be the standard DNS update  method.    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 hostname.

       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

       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

       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 that are being considered by the IETF and are expected to become
       standards, but are not yet  standards,  and  may  not  be  standardized
       exactly as currently proposed.   These are:


       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 allowed.

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

       If  the  server  is  configured  not to allow client updates, or if the
       client doesn't want to do its own update, the server will simply choose
       a  name  for the client 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 done.

       If the update fails because the A record already exists, then the  DHCP
       server  attempts  to  add the A record with the prerequisite that there
       must be a 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

       The interim DNS update  scheme  is  called  interim  for  two  reasons.
       First,  it  does not quite follow the drafts.   The current versions of
       the drafts call for a new DHCID RRtype, but this is not yet  available.
       The  interim  DNS  update scheme uses a TXT record instead.   Also, the
       existing ddns-resolution draft calls for the DHCP server to put a DHCID
       RR  on  the PTR record, but the interim update method does not do this.
       It is our position that this is not useful, and we are working with the
       author  in  hopes of removing it from the next version of the draft, or
       better understanding why it is considered useful.

       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

       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.

       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

       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 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

       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 protocol.

       Please  be  aware  that  only the dhcp-client-identifier option and the
       hardware address can  be  used  to  match  a  host  declaration.    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 groups.


       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 requests.


       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

       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.

       This option does not satisfy the  requirement  of  failover  peers  for
       denying  dynamic bootp clients.  The deny dynamic bootp clients; option
       should be used instead. See the ALLOW AND DENY WITHIN POOL DECLARATIONS
       section of this man page for more details.

       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

       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 details.

       The leasequery keyword

        allow leasequery;
        deny leasequery;

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


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

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

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

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


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


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

        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.


       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 declarations.

       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

       The ddns-rev-domainname statement

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

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

       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 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.

       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 get-lease-hostnames statement

          get-lease-hostnames flag;

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

       The hardware statement

          hardware hardware-type hardware-address;

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

       The infinite-is-reserved statement

          infinite-is-reserved flag;

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

          The default is off.

       The lease-file-name statement

          lease-file-name name;

          Name should be the name  of  the  DHCP  server's  lease  file.    By
          default,  this is /var/lib/dhcp3/dhcpd.leases.   This statement must
          appear in the outer scope of the configuration file - if it  appears
          in some other scope, it will have no effect.

       The 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

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

           Listening on Socket/eth0

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

       The 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

          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.   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 min-secs statement

          min-secs seconds;

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

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

       The next-server statement

          next-server server-name;

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

       The omapi-port statement

          omapi-port port;

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

       The one-lease-per-client statement

          one-lease-per-client flag;

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

       The pid-file-name statement

          pid-file-name name;

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

       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 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-name statement

          server-name name ;

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

       The 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

       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 expires.

       The update-static-leases statement

          update-static-leases flag;

          The update-static-leases flag, if enabled, causes the DHCP server to
          do  DNS updates for clients even if those clients are being assigned
          their IP address using a fixed-address  statement  -  that  is,  the
          client is being given a static assignment.   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


       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,


       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