dhcrelay - Dynamic Host Configuration Protocol Relay Agent


   dhcrelay  [ -4 ] [ -dqaD ] [ -p port ] [ -c count ] [ -A length ] [ -pf
   pid-file ] [ --no-pid ] [ -m append | replace | forward | discard  ]  [
   -i interface0 [ ...  -i interfaceN ] ] server0 [ ...serverN ]

   dhcrelay  -6 [ -dqI ] [ -p port ] [ -c count ] [ -pf pid-file ] [ --no-
   pid ] [ -s subscriber-id ] -l lower0 [ ...  -l lowerN ] -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
   specified 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
          running 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
          configuration on startup.

   -pf pid-file
          Path to alternate pid file.

          Option to disable writing pid files.   By  default  the  program
          will write a pid file.

   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
          contain  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.

          Specify  the  maximum  packet  size  to  send  to a DHCPv4/BOOTP
          server.  This might  be  done  to  allow  sufficient  space  for
          addition  of  relay  agent  options while still fitting into the
          Ethernet 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

   -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.

          To use this option you must also enable the -a option.

   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
          interfaces  in use, to disambiguate between them.  The -I option
          causes dhcrelay to send the option even if  there  is  only  one
          downstream interface.

   -s subscriber-id
          Add  an option with the specified subscriber-id into the packet.
          This feature is for testing rather than production  as  it  will
          put the same subscriber-id into the packet for all clients.

   -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
          interface.   The  optional  #index   parameter   specifies   the
          interface index.

   -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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