NAME

       dhcrelay - Dynamic Host Configuration Protocol Relay Agent

SYNOPSIS

       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 ]

DESCRIPTION

       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.

OPERATION

       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.

COMMAND LINE

       Protocol selection options:

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

       -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 relayed.

       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.

       --no-pid
              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.

       -A LENGTH
              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 anyway.

       -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.

SEE ALSO

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

BUGS

       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 value.

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

AUTHOR

       dhcrelay(8) To learn more about Internet Systems Consortium, see https://www.isc.org

                                                                                                     dhcrelay(8)