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dhcrelay(8)							   dhcrelay(8)

       dhcrelay	- Dynamic Host Configuration Protocol Relay Agent

       dhcrelay	 [  -4 ] [ -dqaD ] [ -p	port ] [ -c count ] [ -A length	] [ -m
       append |	replace	| forward | discard ] [	-i interface0 [	...  -i	inter-
       faceN ] ] server0 [ ...serverN ]

       dhcrelay	 -6 [ -dqI ] [ -p port ] [ -c count ] -l lower0	[ ...  -l low-
       erN ] -u	upper0 [ ...  -u upperN	]

       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.	 It supports both DHCPv4/BOOTP and DHCPv6 protocols.

       The  DHCP Relay Agent listens for DHCPv4	or DHCPv6 queries from clients
       or other	relay agents on	one or more interfaces,	passing	them along  to
       ``upstream''  servers or	relay agents as	specified on the command line.
       When a reply is received	from upstream, it is multicast or unicast back
       downstream to the source	of the original	request.

       Protocol	selection options:

       -6     Run  dhcrelay as a DHCPv6	relay agent.  Incompatible with	the -4

       -4     Run dhcrelay as a	DHCPv4/BOOTP relay agent.  This	is the default
	      mode  of operation, so the argument is not necessary, but	may be
	      specified	for clarity.  Incompatible with	-6.

       Specifying DHCPv4/BOOTP servers

       In DHCPv4 mode, a list of one or	more server addresses must  be	speci-
       fied  on	 the  command  line,  to  which	 DHCP/BOOTP  queries should be

       Options available for both DHCPv4 and DHCPv6:

       -c COUNT
	      Maximum hop count.  When forwarding packets,  dhcrelay  discards
	      packets which have reached a hop count of	COUNT.	Default	is 10.
	      Maximum is 255.

       -d     Force dhcrelay to	run as a foreground process.  Useful when run-
	      ning  dhcrelay  under  a	debugger, or running out of inittab on
	      System V systems.

       -p PORT
	      Listen and transmit on port PORT.	 This  is  mostly  useful  for
	      debugging	 purposes.   Default  is  port 67 for DHCPv4/BOOTP, or
	      port 547 for DHCPv6.

       -q     Quiet mode.  Prevents dhcrelay6 from printing its	 network  con-
	      figuration on startup.

       Options available in DHCPv4 mode	only:

       -a     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  con-
	      tain  two	agent options: the Circuit ID suboption	and the	Remote
	      ID suboption.  Currently,	the Circuit ID will be	the  printable
	      name  of the interface 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.

       -A LENGTH
	      Specify  the  maximum  packet  size  to  send  to	a DHCPv4/BOOTP
	      server.  This might be done to allow sufficient space for	 addi-
	      tion  of relay agent options while still fitting into the	Ether-
	      net MTU size.

       -D     Drop packets from	upstream servers if they contain  Relay	 Agent
	      Information   options  that  indicate  they  were	 generated  in
	      response to a query that came via	a different relay  agent.   If
	      this  option is not specified, such packets will be relayed any-

       -i ifname
	      Listen for DHCPv4/BOOTP queries on interface  ifname.   Multiple
	      interfaces  may  be  specified by	using more than	one -i option.
	      If no interfaces are specified on	 the  command  line,  dhcrelay
	      will  identify all network interfaces, eliminating non-broadcast
	      interfaces if possible, and attempt to listen on all of them.

       -m append|replace|forward|discard
	      Control the handling of incoming DHCPv4  packets	which  already
	      contain  relay  agent  options.	If such	a packet does not have
	      giaddr set in its	header,	the DHCP standard  requires  that  the
	      packet be	discarded.  However, if	giaddr is set, the relay agent
	      may handle the situation in 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.

       Options available in DHCPv6 mode	only:

       -I     Force  use  of  the  DHCPv6 Interface-ID option.	This option is
	      automatically sent when there are	two or more downstream	inter-
	      faces  in	 use,  to  disambiguate	 between  them.	 The -I	option
	      causes dhcrelay to send the option even if  there	 is  only  one
	      downstream interface.

       -l [address%]ifname[#index]
	      Specifies	the ``lower'' network interface	for DHCPv6 relay mode:
	      the interface on which queries will be received from clients  or
	      from  other  relay  agents.   At	least  one  -l	option must be
	      included in the command line when	running	in DHCPv6  mode.   The
	      interface	 name  ifname  is  a  mandatory	 parameter.   The link
	      address can be specified by address%; if it isn't, dhcrelay will
	      use  the	first  non-link-local address configured on the	inter-
	      face.  The optional #index  parameter  specifies	the  interface

       -u [address%]ifname
	      Specifies	the ``upper'' network interface	for DHCPv6 relay mode:
	      the interface to which queries  from  clients  and  other	 relay
	      agents  should  be  forwarded.   At  least one -u	option must be
	      included in the command line when	running	in DHCPv6  mode.   The
	      interface	 name ifname is	a mandatory parameter. The destination
	      unicast or multicast address can be specified  by	 address%;  if
	      not  specified,  the  relay  agent  will	forward	 to the	DHCPv6
	      All_DHCP_Relay_Agents_and_Servers	multicast address.

       It is possible to specify the same interface with  different  addresses
       more  than once,	and even, when the system supports it, to use the same
       interface as both upper and lower interfaces.

       dhclient(8), dhcpd(8), RFC3315, RFC2132,	RFC2131.

       Using the same interface	on both	upper and lower	sides may cause	loops,
       so  when	running	this way, the maximum hop count	should be set to a low

       The loopback interface is not (yet) recognized as a valid interface.

       dhcrelay(8) To  learn  more  about  Internet  Systems  Consortium,  see



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