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dhcpd(8) dhcpd(8)


[править] NAME

      dhcpd - Dynamic Host Configuration Protocol Server

[править] SYNOPSIS

      dhcpd  [  -p  port  ] [ -f ] [ -d ] [ -q ] [ -t | -T ] [ -4 | -6 ] [ -s
      server ] [ -cf config-file ] [ -lf lease-file ]  [  -pf  pid-file  ]  [
      --no-pid  ]  [  -tf trace-output-file ] [ -play trace-playback-file ] [
      if0 [ ...ifN ] ]
      dhcpd --version


      The Internet Systems Consortium  DHCP  Server,  dhcpd,  implements  the
      Dynamic  Host  Configuration Protocol (DHCP) and the Internet Bootstrap
      Protocol (BOOTP).  DHCP allows hosts on a TCP/IP network to request and
      be  assigned  IP  addresses, and also to discover information about the
      network to which they are attached.  BOOTP provides similar functional-
      ity, with certain restrictions.

[править] OPERATION

      The  DHCP protocol allows a host which is unknown to the network admin-
      istrator to be automatically assigned a new IP address out of a pool of
      IP  addresses for its network.   In order for this to work, the network
      administrator allocates address pools in each subnet  and  enters  them
      into the dhcpd.conf(5) file.
      There  are  two  versions  of  the DHCP protocol DHCPv4 and DHCPv6.  At
      startup the server  may be started for one or the other via the  -4  or
      -6 arguments.
      On startup, dhcpd reads the dhcpd.conf file and stores a list of avail-
      able addresses on each subnet in memory.  When  a  client  requests  an
      address  using  the  DHCP  protocol, dhcpd allocates an address for it.
      Each client is assigned a lease, which expires after an amount of  time
      chosen  by  the  administrator  (by  default,  one day).  Before leases
      expire, the clients to which leases are assigned are expected to  renew
      them  in  order  to  continue  to  use the addresses.  Once a lease has
      expired, the client to which that lease was assigned is no longer  per-
      mitted to use the leased IP address.
      In  order  to  keep  track  of  leases across system reboots and server
      restarts, dhcpd  keeps  a  list  of  leases  it  has  assigned  in  the
      dhcpd.leases(5)  file.    Before  dhcpd  grants  a  lease to a host, it
      records the lease in this file and makes sure that the contents of  the
      file  are  flushed  to disk.   This ensures that even in the event of a
      system crash, dhcpd will not forget about a lease that it has assigned.
      On  startup,  after  reading  the  dhcpd.conf  file,  dhcpd  reads  the
      dhcpd.leases file to refresh its memory about  what  leases  have  been
      New leases are appended to the end of the dhcpd.leases file.   In order
      to prevent the file from becoming arbitrarily large, from time to  time
      dhcpd  creates a new dhcpd.leases file from its in-core lease database.
      Once this file has been written  to  disk,  the  old  file  is  renamed
      dhcpd.leases~,  and the new file is renamed dhcpd.leases.   If the sys-
      tem crashes in the middle of this process, whichever dhcpd.leases  file
      remains will contain all the lease information, so there is no need for
      a special crash recovery process.
      BOOTP support is also provided by this server.  Unlike DHCP, the  BOOTP
      protocol  does  not  provide  a  protocol  for  recovering dynamically-
      assigned addresses once they are no longer needed.   It is still possi-
      ble to dynamically assign addresses to BOOTP clients, but some adminis-
      trative process for reclaiming addresses  is  required.    By  default,
      leases are granted to BOOTP clients in perpetuity, although the network
      administrator may set an earlier cutoff date or a shorter lease  length
      for BOOTP leases if that makes sense.
      BOOTP  clients  may also be served in the old standard way, which is to
      simply provide a declaration in the  dhcpd.conf  file  for  each  BOOTP
      client, permanently assigning an address to each client.
      Whenever  changes  are  made  to  the  dhcpd.conf  file,  dhcpd must be
      restarted.   To restart dhcpd,  send  a  SIGTERM  (signal  15)  to  the
      process  ID  contained  in  RUNDIR/, and then re-invoke dhcpd.
      Because the DHCP server database is not as lightweight as a BOOTP data-
      base, dhcpd does not automatically restart itself when it sees a change
      to the dhcpd.conf file.
      Note: We get a lot of complaints about this.   We realize that it would
      be nice if one could send a SIGHUP to the server and have it reload the
      database.   This is not technically impossible, but it would require  a
      great  deal  of work, our resources are extremely limited, and they can
      be better spent elsewhere.   So please don't complain about this on the
      mailing list unless you're prepared to fund a project to implement this
      feature, or prepared to do it yourself.

[править] COMMAND LINE

      The names of the network interfaces on which dhcpd  should  listen  for
      broadcasts  may  be specified on the command line.  This should be done
      on systems where dhcpd is unable to identify non-broadcast  interfaces,
      but should not be required on other systems.  If no interface names are
      specified on the command line dhcpd will identify  all  network  inter-
      faces  which  are up, eliminating non-broadcast interfaces if possible,
      and listen for DHCP broadcasts on each interface.


      -4     Run as a DHCP server. This is the default and cannot be combined
             with -6.
      -6     Run as a DHCPv6 server. This cannot be combined with -4.
      -p port
             The  udp  port number on which dhcpd should listen.  If unspeci-
             fied dhcpd uses the default port of 67.  This is  mostly  useful
             for debugging purposes.
      -s address
             Specify  an  address  or  host  name  to which dhcpd should send
             replies rather than  the  broadcast  address  (
             This option is only supported in IPv4.
      -f     Force  dhcpd to run as a foreground process instead of as a dae-
             mon in the background.  This is useful when running dhcpd  under
             a  debugger,  or when running it out of inittab on System V sys-
      -d     Send log messages to the standard error descriptor.  This can be
             useful  for debugging, and also at sites where a complete log of
             all dhcp activity must be kept but syslogd is  not  reliable  or
             otherwise  cannot be used.   Normally, dhcpd will log all output
             using the syslog(3)  function  with  the  log  facility  set  to
             LOG_DAEMON.   Note  that -d implies -f (the daemon will not fork
             itself into the background).
      -q     Be quiet at startup.  This suppresses the printing of the entire
             copyright  message during startup.  This might be desirable when
             starting dhcpd from a system startup script (e.g., /etc/rc).
      -t     Test the configuration file.  The server tests the configuration
             file  for  correct  syntax,  but will not attempt to perform any
             network operations.   This can be used to test a new  configura-
             tion file automatically before installing it.
      -T     Test  the  lease file.  The server tests the lease file for cor-
             rect syntax, but will not attempt to perform any network  opera-
             tions.   This can be used to test a new leaes file automatically
             before installing it.
      -tf tracefile
             Specify a file into which the entire startup state of the server
             and  all  the transactions it processes are logged.  This can be
             useful in submitting bug reports - if you  are  getting  a  core
             dump  every  so  often,  you  can  start the server with the -tf
             option and then, when the server dumps core, the trace file will
             contain  all the transactions that led up to it dumping core, so
             that the problem can be easily debugged with -play.
      -play playfile
             Specify a file from which the entire startup state of the server
             and  all  the  transactions  it  processed  are read.  The -play
             option must be specified with an alternate lease file, using the
             -lf switch, so that the DHCP server doesn't wipe out your exist-
             ing lease file with its test data.  The DHCP server will  refuse
             to  operate  in  playback  mode  unless you specify an alternate
             lease file.
             Print version number and exit.
      Modifying default file locations: The following options can be used  to
      modify  the locations dhcpd uses for it's files.  Because of the impor-
      tance of using the same lease database at all times when running  dhcpd
      in  production,  these  options  should  be used only for testing lease
      files or database files in a non-production environment.
      -cf config-file
             Path to alternate configuration file.
      -lf lease-file
             Path to alternate lease file.
      -pf pid-file
             Path to alternate pid file.
             Option to disable writing pid files.   By  default  the  program
             will  write  a  pid  file.   If the program is invoked with this
             option it will not check for an existing server process.


      The syntax of the dhcpd.conf(5) file is  discussed  separately.    This
      section should be used as an overview of the configuration process, and
      the dhcpd.conf(5) documentation should be consulted for detailed refer-
      ence information.

[править] Subnets

      dhcpd  needs to know the subnet numbers and netmasks of all subnets for
      which it will be providing service.   In addition, in order to  dynami-
      cally  allocate  addresses,  it  must be assigned one or more ranges of
      addresses on each subnet which it can in turn assign to client hosts as
      they  boot.    Thus, a very simple configuration providing DHCP support
      might look like this:
           subnet netmask {
      Multiple address ranges may be specified like this:
           subnet netmask {
      If a subnet will only be provided with BOOTP  service  and  no  dynamic
      address  assignment, the range clause can be left out entirely, but the
      subnet statement must appear.

[править] Lease Lengths

      DHCP leases can be assigned almost any  length  from  zero  seconds  to
      infinity.    What lease length makes sense for any given subnet, or for
      any given installation, will vary depending on the kinds of hosts being
      For example, in an office environment where systems are added from time
      to time and removed from time  to  time,  but  move  relatively  infre-
      quently,  it  might make sense to allow lease times of a month or more.
      In a final test environment on a manufacturing floor, it may make  more
      sense  to  assign a maximum lease length of 30 minutes - enough time to
      go through a simple test procedure on a network appliance before  pack-
      aging it up for delivery.
      It  is  possible  to specify two lease lengths: the default length that
      will be assigned if a client  doesn't  ask  for  any  particular  lease
      length, and a maximum lease length.   These are specified as clauses to
      the subnet command:
           subnet netmask {
             default-lease-time 600;
             max-lease-time 7200;
      This particular subnet declaration specifies a default  lease  time  of
      600  seconds  (ten  minutes),  and a maximum lease time of 7200 seconds
      (two hours).   Other common values would be  86400  (one  day),  604800
      (one week) and 2592000 (30 days).
      Each  subnet  need  not  have  the same lease--in the case of an office
      environment and a manufacturing environment served  by  the  same  DHCP
      server, it might make sense to have widely disparate values for default
      and maximum lease times on each subnet.

[править] BOOTP Support

      Each BOOTP client must be explicitly declared in the  dhcpd.conf  file.
      A  very basic client declaration will specify the client network inter-
      face's hardware address and the IP address to assign  to  that  client.
      If  the  client  needs  to be able to load a boot file from the server,
      that file's name must be specified.   A simple bootp client declaration
      might look like this:
           host haagen {
             hardware ethernet 08:00:2b:4c:59:23;
             filename "/tftpboot/haagen.boot";

[править] Options

      DHCP  (and  also  BOOTP  with  Vendor  Extensions)  provide a mechanism
      whereby the server can provide the client with information about how to
      configure  its  network interface (e.g., subnet mask), and also how the
      client can access various network services (e.g., DNS, IP routers,  and
      so on).
      These  options  can  be specified on a per-subnet basis, and, for BOOTP
      clients, also on a per-client basis.   In the event that a BOOTP client
      declaration  specifies  options  that  are also specified in its subnet
      declaration, the options  specified  in  the  client  declaration  take
      precedence.   A reasonably complete DHCP configuration might look some-
      thing like this:
           subnet netmask {
             default-lease-time 600 max-lease-time 7200;
             option subnet-mask;
             option broadcast-address;
             option routers;
             option domain-name-servers,;
             option domain-name "";
      A bootp host on that subnet that needs to be in a different domain  and
      use a different name server might be declared as follows:
           host haagen {
             hardware ethernet 08:00:2b:4c:59:23;
             filename "/tftpboot/haagen.boot";
             option domain-name-servers;
             option domain-name "";
      A  more  complete description of the dhcpd.conf file syntax is provided
      in dhcpd.conf(5).

[править] OMAPI

      The DHCP server provides the capability to modify some of its  configu-
      ration while it is running, without stopping it, modifying its database
      files, and restarting it.  This capability is currently provided  using
      OMAPI  - an API for manipulating remote objects.  OMAPI clients connect
      to the server using TCP/IP, authenticate,  and  can  then  examine  the
      server's current status and make changes to it.
      Rather  than  implementing the underlying OMAPI protocol directly, user
      programs should use the dhcpctl API or OMAPI  itself.    Dhcpctl  is  a
      wrapper  that  handles  some of the housekeeping chores that OMAPI does
      not do automatically.   Dhcpctl and OMAPI are documented in  dhcpctl(3)
      and omapi(3).
      OMAPI  exports  objects, which can then be examined and modified.   The
      DHCP server exports the following objects: lease, host,  failover-state
      and  group.    Each  object  has a number of methods that are provided:
      lookup, create, and destroy.   In addition, it is possible to  look  at
      attributes  that  are  stored  on  objects, and in some cases to modify
      those attributes.


      Leases can't currently be created or destroyed, but they can be  looked
      up to examine and modify their state.
      Leases have the following attributes:
      state integer lookup, examine
           1 = free
           2 = active
           3 = expired
           4 = released
           5 = abandoned
           6 = reset
           7 = backup
           8 = reserved
           9 = bootp
      ip-address data lookup, examine
           The IP address of the lease.
      dhcp-client-identifier data lookup, examine, update
           The  client  identifier  that the client used when it acquired the
           lease.  Not all clients send client identifiers, so  this  may  be
      client-hostname data examine, update
           The value the client sent in the host-name option.
      host handle examine
           the host declaration associated with this lease, if any.
      subnet handle examine
           the subnet object associated with this lease (the subnet object is
           not currently supported).
      pool handle examine
           the pool object associated with this lease (the pool object is not
           currently supported).
      billing-class handle examine
           the  handle  to the class to which this lease is currently billed,
           if any (the class object is not currently supported).
      hardware-address data examine, update
           the hardware address (chaddr) field sent by  the  client  when  it
           acquired its lease.
      hardware-type integer examine, update
           the type of the network interface that the client reported when it
           acquired its lease.
      ends time examine
           the time when the lease's current state ends, as understood by the
      tstp time examine
           the time when the lease's current state ends, as understood by the
      tsfp time examine
           the adjusted time when the lease's current state ends,  as  under-
           stood  by  the  failover  peer (if there is no failover peer, this
           value is undefined).  Generally this value is  only  adjusted  for
           expired,  released,  or reset leases while the server is operating
           in partner-down state, and otherwise is simply the value  supplied
           by the peer.
      atsfp time examine
           the actual tsfp value sent from the peer.  This value is forgotten
           when a lease binding state change is made, to facilitate  retrans-
           mission logic.
      cltt time examine
           The time of the last transaction with the client on this lease.


      Hosts  can be created, destroyed, looked up, examined and modified.  If
      a host declaration is created or deleted using OMAPI, that  information
      will  be  recorded  in  the  dhcpd.leases  file.   It is permissible to
      delete host declarations that are declared in the dhcpd.conf file.
      Hosts have the following attributes:
      name data lookup, examine, modify
           the name of the host declaration.   This name must be unique among
           all host declarations.
      group handle examine, modify
           the  named group associated with the host declaration, if there is
      hardware-address data lookup, examine, modify
           the link-layer address that will be used to match the  client,  if
           any.  Only valid if hardware-type is also present.
      hardware-type integer lookup, examine, modify
           the  type  of the network interface that will be used to match the
           client, if any.   Only valid if hardware-address is also  present.
      dhcp-client-identifier data lookup, examine, modify
           the  dhcp-client-identifier  option that will be used to match the
           client, if any.
      ip-address data examine, modify
           a fixed IP address which  is  reserved  for  a  DHCP  client  that
           matches  this  host  declaration.    The  IP  address will only be
           assigned to the client if it is valid for the network  segment  to
           which the client is connected.
      statements data modify
           a  list  of  statements  in the format of the dhcpd.conf file that
           will be executed whenever a message from the client is being  pro-
      known integer examine, modify
           if nonzero, indicates that a client matching this host declaration
           will be treated as known in pool  permit  lists.    If  zero,  the
           client will not be treated as known.


      Named  groups  can be created, destroyed, looked up, examined and modi-
      fied.  If a group declaration is created or deleted using  OMAPI,  that
      information will be recorded in the dhcpd.leases file.  It is permissi-
      ble to delete group declarations that are declared  in  the  dhcpd.conf
      Named  groups currently can only be associated with hosts - this allows
      one set of statements to be efficiently attached to more than one  host
      Groups have the following attributes:
      name data
           the  name  of  the group.  All groups that are created using OMAPI
           must have names, and the names must be unique among all groups.
      statements data
           a list of statements in the format of  the  dhcpd.conf  file  that
           will  be executed whenever a message from a client whose host dec-
           laration references this group is processed.


      The control object allows you to shut the server down.   If the  server
      is  doing  failover  with another peer, it will make a clean transition
      into the shutdown state and notify its peer, so that the  peer  can  go
      into  partner  down,  and  then record the "recover" state in the lease
      file so that when the server is restarted, it will automatically resyn-
      chronize with its peer.
      On shutdown the server will also attempt to cleanly shut down all OMAPI
      connections.  If these connections do not go down  cleanly  after  five
      seconds,  they  are  shut down preemptively.  It can take as much as 25
      seconds from the beginning of the shutdown process to the time that the
      server actually exits.
      To  shut  the  server  down,  open its control object and set the state
      attribute to 2.


      The failover-state object is the object that tracks the  state  of  the
      failover  protocol  as  it  is being managed for a given failover peer.
      The failover object has the following attributes (please see dhcpd.conf
      (5) for explanations about what these attributes mean):
      name data examine
           Indicates the name of the failover peer relationship, as described
           in the server's dhcpd.conf file.
      partner-address data examine
           Indicates the failover partner's IP address.
      local-address data examine
           Indicates the IP address that is being used by the DHCP server for
           this failover pair.
      partner-port data examine
           Indicates  the TCP port on which the failover partner is listening
           for failover protocol connections.
      local-port data examine
           Indicates the TCP port on which the DHCP server is  listening  for
           failover protocol connections for this failover pair.
      max-outstanding-updates integer examine
           Indicates  the number of updates that can be outstanding and unac-
           knowledged at any given time, in this failover relationship.
      mclt integer examine
           Indicates the maximum client lead time in this failover  relation-
      load-balance-max-secs integer examine
           Indicates the maximum value for the secs field in a client request
           before load balancing is bypassed.
      load-balance-hba data examine
           Indicates the load balancing hash bucket array for  this  failover
      local-state integer examine, modify
           Indicates  the  present  state of the DHCP server in this failover
           relationship.   Possible values for state are:
                1   - startup
                2   - normal
                3   - communications interrupted
                4   - partner down
                5   - potential conflict
                6   - recover
                7   - paused
                8   - shutdown
                9   - recover done
                10  - resolution interrupted
                11  - conflict done
                254 - recover wait
           (Note that some of  the  above  values  have  changed  since  DHCP
           In  general  it  is not a good idea to make changes to this state.
           However, in the case that the failover  partner  is  known  to  be
           down,  it can be useful to set the DHCP server's failover state to
           partner down.   At this point the DHCP server will take over  ser-
           vice  of  the  failover  partner's leases as soon as possible, and
           will give out normal leases, not leases  that  are  restricted  by
           MCLT.    If  you do put the DHCP server into the partner-down when
           the other DHCP server is not in the partner-down state, but is not
           reachable,  IP  address  assignment  conflicts  are possible, even
           likely.   Once a server has been put into partner-down  mode,  its
           failover  partner must not be brought back online until communica-
           tion is possible between the two servers.
      partner-state integer examine
           Indicates the present state of the failover partner.
      local-stos integer examine
           Indicates the time at which the DHCP server  entered  its  present
           state in this failover relationship.
      partner-stos integer examine
           Indicates  the  time  at  which  the  failover partner entered its
           present state.
      hierarchy integer examine
           Indicates whether the DHCP server is primary (0) or secondary  (1)
           in this failover relationship.
      last-packet-sent integer examine
           Indicates  the  time  at which the most recent failover packet was
           sent by this DHCP server to its failover partner.
      last-timestamp-received integer examine
           Indicates the timestamp that was  on  the  failover  message  most
           recently received from the failover partner.
      skew integer examine
           Indicates  the  skew between the failover partner's clock and this
           DHCP server's clock
      max-response-delay integer examine
           Indicates the time in  seconds  after  which,  if  no  message  is
           received  from  the failover partner, the partner is assumed to be
           out of communication.
      cur-unacked-updates integer examine
           Indicates the number of update messages that  have  been  received
           from the failover partner but not yet processed.

[править] FILES

      ETCDIR/dhcpd.conf,         DBDIR/dhcpd.leases,        RUNDIR/,

[править] SEE ALSO

dhclient(8), dhcrelay(8), dhcpd.conf(5), dhcpd.leases(5)

[править] AUTHOR

      dhcpd(8) was originally written by Ted  Lemon  under  a  contract  with
      Vixie  Labs.  Funding for this project was provided by Internet Systems
      Consortium.   Version 3 of the DHCP server was funded by Nominum,  Inc.
      Information   about   Internet   Systems  Consortium  is  available  at
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