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dhcrelay(8)									      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 subnets.

       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 (accord-
       ing  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 spec-
       ified 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

       If dhcrelay should listen and transmit on a port other than the standard (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 debugging purposes.

       Dhcrelay  will  normally  run  in the foreground until it has configured an interface, and
       then will revert to running in the background.  To run 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 behaviour, 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.,

       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  neces-

       It  is  possible  for  a  relay	agent to receive a packet which already contains 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 discard it.

       Which of these behaviours is followed by the Internet Software Consortium 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 Information 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-options-03.txt.

       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 coopera-
       tion with Vixie Enterprises.  To learn more about the Internet  Software  Consortium,  see
       http://www.vix.com.  To learn more about Vixie Enterprises, see http://www.vix.com.

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