dhcpd



dhcpd(8)                                                              dhcpd(8)




NAME

       dhcpd - Dynamic Host Configuration Protocol Server


SYNOPSIS

       dhcpd  [ -p port ] [ -f ] [ -d ] [ -q ] [ -t | -T ] [ -cf config-file ]
       [ -lf lease-file ] [ -tf trace-output-file ]  [  -play  trace-playback-
       file ] [ if0 [ ...ifN ] ]


DESCRIPTION

       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.


CONTRIBUTIONS

       This software is free software.  At various times its  development  has
       been underwritten by various organizations, including the ISC and Vixie
       Enterprises.  The development of 3.0 has been funded almost entirely by
       Nominum, Inc.

       At  this point development is being shepherded by Ted Lemon, and hosted
       by the ISC, but the future of this project depends on you.  If you have
       features you want, please consider implementing them.


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.

       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
       assigned.

       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  pro-
       cess  ID  contained  in  /var/run/dhcpd.pid,  and then re-invoke dhcpd.
       Because the DHCP server database is  not  as  lightweight  as  a  BOOTP
       database,  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.


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.

       If dhcpd should listen on a port other than the standard (port 67), the
       -p  flag  may  used.   It  should be followed by the udp port number on
       which dhcpd should listen.  This is mostly useful  for  debugging  pur-
       poses.

       To run dhcpd as a foreground process, rather than allowing it to run as
       a daemon in the background, the -f flag should be specified.   This  is
       useful  when  running dhcpd under a debugger, or when running it out of
       inittab on System V systems.

       To have dhcpd log to the standard  error  descriptor,  specify  the  -d
       flag.  This can be useful for debugging, and also at sites where a com-
       plete 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.

       Dhcpd can be made to use an alternate configuration file with  the  -cf
       flag,  or  an  alternate lease file with the -lf flag.   Because of the
       importance 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.

       When starting dhcpd up from a system startup script (e.g., /etc/rc), it
       may  not  be  desirable  to  print  out the entire copyright message on
       startup.   To avoid printing this message, the -q flag  may  be  speci-
       fied.

       The DHCP server reads two files on startup: a configuration file, and a
       lease database.   If the -t flag is specified, the server  will  simply
       test the configuration file for correct syntax, but will not attempt to
       perform any network operations.   This can be used to test  the  a  new
       configuration file automatically before installing it.

       The  -T  flag  can be used to test the lease database file in a similar
       way.

       The -tf and -play options allow you to specify a file  into  which  the
       entire  startup  state  of  the server and all the transactions it pro-
       cesses are either logged or played back from.  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.

       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  existing
       lease  file with its test data.  The DHCP server will refuse to operate
       in playback mode unless you specify an alternate lease file.


CONFIGURATION

       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.



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 239.252.197.0 netmask 255.255.255.0 {
              range 239.252.197.10 239.252.197.250;
            }

       Multiple address ranges may be specified like this:

            subnet 239.252.197.0 netmask 255.255.255.0 {
              range 239.252.197.10 239.252.197.107;
              range 239.252.197.113 239.252.197.250;
            }

       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.



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
       served.

       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 of 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 239.252.197.0 netmask 255.255.255.0 {
              range 239.252.197.10 239.252.197.107;
              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 envi-
       ronment 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.


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;
              fixed-address 239.252.197.9;
              filename "/tftpboot/haagen.boot";
            }


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 239.252.197.0 netmask 255.255.255.0 {
              range 239.252.197.10 239.252.197.250;
              default-lease-time 600 max-lease-time 7200;
              option subnet-mask 255.255.255.0;
              option broadcast-address 239.252.197.255;
              option routers 239.252.197.1;
              option domain-name-servers 239.252.197.2, 239.252.197.3;
              option domain-name "isc.org";
            }

       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;
              fixed-address 239.252.197.9;
              filename "/tftpboot/haagen.boot";
              option domain-name-servers 192.5.5.1;
              option domain-name "vix.com";
            }

       A  more  complete description of the dhcpd.conf file syntax is provided
       in dhcpd.conf(5).


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.


THE LEASE OBJECT

       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
            empty.

       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 associted 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
            client.

       tstp time examine
            the time when the lease’s current state ends, as understood by the
            server.
       tsfp time examine
            the time when the lease’s current state ends, as understood by the
            failover  peer  (if there is no failover peer, this value is unde-
            fined).

       cltt time examine
            The time of the last transaction with the client on this lease.


THE HOST OBJECT

       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
            one.

       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-
            cessed.

       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.


THE GROUP OBJECT

       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
       file.

       Named groups currently can only be associated with hosts - this  allows
       one  set of statements to be efficiently attached to more than one host
       declaration.

       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

       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 pre-emptively.  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

       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-
            ship.

       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
            relationship.

       local-state integer examine, modify
            Indicates the present state of the DHCP server  in  this  failover
            relationship.   Possible values for state are:

                 1  - partner down
                 2  - normal
                 3  - communications interrupted
                 4  - resolution interrupted
                 5  - potential conflict
                 6  - recover
                 7  - recover done
                 8  - shutdown
                 9  - paused
                 10 - startup
                 11 - recover wait

            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.


FILES

       /etc/dhcpd.conf,     /var/lib/dhcp/dhcpd.leases,    /var/run/dhcpd.pid,
       /var/lib/dhcp/dhcpd.leases~.


SEE ALSO

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


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
       http://www.isc.org/.   Information  about  Nominum  can  be  found   at
       http://www.nominum.com/.



                                                                      dhcpd(8)

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