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dhcrelay(8)                 System Manager's Manual                dhcrelay(8)

       dhcrelay - Dynamic Host Configuration Protocol Relay Agent

       dhcrelay  [ -p port ] [ -d ] [ -q ] [ -i if0 [ ...  -i ifN ] ] [ -a ] [
       -A length ] [ -D ] [ -m append | replace | forward | discard ]  server0
       [ ...serverN ]

       The Internet Software Consortium DHCP Relay Agent, dhcrelay, provides a
       means for relaying DHCP and BOOTP requests from a subnet  to  which  no
       DHCP  server is directly connected to one or more DHCP servers on other

       You must have the Berkeley Packet Filter (bpf) configured in your  Net-
       BSD kernel.   You must have at least one /dev/bpf* file for each broad-
       cast network interface that is attached to your system.

       The DHCP Relay Agent listens for DHCP and BOOTP queries and  responses.
       When  a  query  is  received from a client, dhcrelay forwards it to the
       list of DHCP servers specified on the command line.  When  a  reply  is
       received  from  a  server, it is broadcast or unicast (according to the
       relay agent's ability or the client's  request)  on  the  network  from
       which the original request came.

       The  names  of  the  network interfaces that dhcrelay should attempt to
       configure may be specified on the command line using the -i option.  If
       no  interface  names  are  specified  on the command line dhcrelay will
       identify all network interfaces, elimininating non-broadcast interfaces
       if possible, and attempt to configure each interface.

       The  -i flag can be used to specify the network interfaces on which the
       relay agent should listen.   In general, it must  listen  not  only  on
       those  network  interfaces  to  which clients are attached, but also on
       those network interfaces to  which  the  server  (or  the  router  that
       reaches  the  server)  is  attached.   However, in some cases it may be
       necessary to exclude some networks; in this case,  you  must  list  all
       those network interfaces that should not be excluded using the -i flag.

       In  some  cases  it  is helpful for the relay agent to forward requests
       from networks on which a DHCP server is running to other DHCP  servers.
       This  would  be the case if two DHCP servers on different networks were
       being used to provide backup service for each other's networks.

       If dhcrelay should listen and transmit on a port other than  the  stan-
       dard (port 67), the -p flag may used.  It should be followed by the udp
       port number that dhcrelay should use.  This is mostly useful for debug-
       ging purposes.

       Dhcrelay will normally run in the foreground until it has configured an
       interface, and then will revert to running in the background.  To force
       dhcrelay  to  always run as a foreground process, the -d flag should be
       specified.  This is useful when running dhcrelay under a  debugger,  or
       when running it out of inittab on System V systems.

       Dhcrelay  will  normally  print  its  network configuration on startup.
       This can be unhelpful in a system startup script - to disable this  be-
       haviour, specify the -q flag.

       If the -a flag is set the relay agent will append an agent option field
       to each request before forwarding it  to  the  server.    Agent  option
       fields  in  responses  sent  from  servers  to clients will be stripped
       before forwarding such responses back to the client.

       The agent option field will contain two agent options: the  Circuit  ID
       suboption  and  the Agent ID suboption.  Currently, the Circuit ID will
       be the printable name of the interface on which the client request  was
       received.    The Agent ID will be the value that the relay agent stores
       in the DHCP packet's giaddr field.   The client supports inclusion of a
       Remote ID suboption as well, but this is not used by default.

       Note:  The Agent ID suboption is not defined in the current Relay Agent
       Information Option draft (draft-ietf-dhc-agent-options-03.txt), but has
       been proposed for inclusion in the next draft.

       Relay Agent options are added to a DHCP packet without the knowledge of
       the DHCP client.   The client may have filled the  DHCP  packet  option
       buffer completely, in which case there theoretically isn't any space to
       add Agent options.   However, the DHCP server may be able to  handle  a
       much  larger  packet  than  most DHCP clients would send.   The current
       Agent Options draft requires that the relay agent use a maximum  packet
       size of 576 bytes.

       It  is  recommended  that  with  the  Internet Software Consortium DHCP
       server, the maximum packet size be set to about 1400,  allowing  plenty
       of extra space in which the relay agent can put the agent option field,
       while still fitting into the Ethernet MTU size.  This can  be  done  by
       specifying  the  -A  flag,  followed by the desired maximum packet size
       (e.g., 1400).

       Note that this is reasonably safe to do even if  the  MTU  between  the
       server  and the client is less than 1500, as long as the hosts on which
       the server and client are running support IP  fragmentation  (and  they
       should).   With  some knowledge as to how large the agent options might
       get in a particular configuration,  this  parameter  can  be  tuned  as
       finely as necessary.

       It is possible for a relay agent to receive a packet which already con-
       tains an agent option field.  If this packet does  not  have  a  giaddr
       set, the standard requires that the packet be discarded.

       If  giaddr  is  set, the server may handle the situation in one of four
       ways: it may append its own set of relay options to the packet, leaving
       the  supplied  option field intact.   It may replace the existing agent
       option field.  It may forward the packet unchanged.   Or, it  may  dis-
       card it.

       Which  of these behaviours is followed by the Internet Software Consor-
       tium DHCP Relay Agent may be configured with the -m flag,  followed  by
       one of the four keywords specified in italics above.

       When  the relay agent receives a reply from a server that it's supposed
       to forward to a client, and Relay Agent Information  option  processing
       is  enabled,  the relay agent scans the packet for Relay Agent Informa-
       tion options and removes them.   As it's scanning, if it finds a  Relay
       Agent  Information  option  field containing an Agent ID suboption that
       matches one of its IP addresses, that option is recognized as its  own.
       If no such option is found, the relay agent can either drop the packet,
       or relay it anyway.   If the -D option is specified, all  packets  that
       don't contain a match will be dropped.

       The  name  or  IP address of at least one DHCP server to which DHCP and
       BOOTP requests should be relayed must be specified on the command line.

       dhclient(8),   dhcpd(8),   RFC2132,   RFC2131,    draft-ietf-dhc-agent-

       It  should be possible for the user to define the Circuit ID and Remote
       ID values on a per-interface basis.

       The relay agent should not relay packets received on a physical network
       to  DHCP  servers on the same physical network - if they do, the server
       will receive duplicate packets.   In order to fix  this,  however,  the
       relay agent needs to be able to learn about the network topology, which
       requires that it have a configuration file.

       dhcrelay(8) has been written for the Internet  Software  Consortium  by
       Ted  Lemon  in cooperation with Vixie Enterprises.  To learn more about
       the Internet Software Consortium, see http://www.isc.org/isc.  To learn
       more about Vixie Enterprises, see http://www.vix.com.