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

NAME

       dhcrelay - Dynamic Host Configuration Protocol Relay Agent

SYNOPSIS

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

DESCRIPTION

       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.

OPERATION

       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.

COMMAND LINE

       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 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  be-
       haviour, specify the -q flag.

RELAY AGENT INFORMATION OPTIONS

       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  maxi-
       mum 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 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,  leav-
       ing  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 process-
       ing 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.

SPECIFYING DHCP SERVERS

       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.

SEE ALSO

       dhclient(8), dhcpd(8), RFC2132, RFC2131, draft-ietf-dhc-agent-options-03.txt.

BUGS

       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 topol-
       ogy, which requires that it have a configuration file.

AUTHOR

       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.vix.com.  To learn more about Vixie Enterprises, see http://www.vix.com.

																       dhcrelay(8)