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NetBSD 6.1.5 - man page for dhcrelay (netbsd section 8)

dhcrelay(8)									      dhcrelay(8)

       dhcrelay - Dynamic Host Configuration Protocol Relay Agent

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

       The Internet Systems 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.

       You must have the Berkeley Packet Filter (bpf) configured in your NetBSD kernel.

       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 force dhcrelay to always run as a fore-
       ground  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
       Remote ID suboption.  Currently, the Circuit ID will be the printable name of  the  inter-
       face  on which the client request was received.	The client supports inclusion of a Remote
       ID suboption as well, but this is not used by default.

       When forwarding packets, dhcrelay discards packets which have reached a hop count  of  10.
       If  a lower or higher threshold (up to 255) is desired, depending on your environment, you
       can specify the max hop count threshold as a number following the -c option.

       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  Systems  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 Systems 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 Internet Systems Consortium by Ted Lemon  in  cooperation
       with   Vixie   Enterprises.    To  learn  more  about  Internet	Systems  Consortium,  see
       http://www.isc.org/isc.	To learn more about Vixie Enterprises, see http://www.vix.com.


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