dhcrelay man page on BSDOS

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dhcrelay(8)					      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,  dhcre-
       lay, provides a means for relaying DHCP and BOOTP requests
       from a subnet to which no DHCP  server  is  directly  con-
       nected 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 pos-
       sible, 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 pro-
       vide 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

								1

dhcrelay(8)					      dhcrelay(8)

       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.

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.   Cur-
       rently,	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  Consor-
       tium  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	 specify-
       ing  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  run-
       ning  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.

								2

dhcrelay(8)					      dhcrelay(8)

       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 sub-
       option  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 net-
       work - if they do, the server will receive duplicate pack-
       ets.   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.

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

								3

dhcrelay(8)					      dhcrelay(8)

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

								4

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