dhcrelay(8)                                                        dhcrelay(8)



NAME
       dhcrelay - Dynamic Host Configuration Protocol Relay Agent

SYNOPSIS
       dhcrelay  [  -4 ] [ -dqaD ] [ -p port | -rp relay-port ] [ -c count ] [
       -A length ] [ -pf pid-file ] [ --no-pid ] [ -m append | replace |  for-
       ward  | discard ] [ -i interface0 [ ...  -i interfaceN ] ] [ -iu inter-
       face0 [ ...  -iu interfaceN ] ] [ -id interface0 [ ...  -id  interfaceN
       ] ] [ -U interface ] server0 [ ...serverN ]

       dhcrelay  -6  [  -dqI ] [ -p port | -rp relay-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 speci-
       fied 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 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.  Incompatible with -rp.

       -rp relay-port
              Alternative  source  port  for  upstream (i.e toward the server)
              messages with DHCPv4 RAI relay-port sub-option or DHCPv6  relay-
              source-port  option. Relay port support is only available if the
              code was compiled  with  (./configure  --enable-relay-port)  and
              requires LPF or BPF link layer access.

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

       -i ifname
              Listen for DHCPv4/BOOTP traffic 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.

       -iu ifname
              Specifies an upstream network interface: an interface from which
              replies  from  servers  and other relay agents will be accepted.
              Multiple interfaces may be specified by using more than one  -iu
              option.  This argument is
               intended  to  be used in conjunction with one or more -i or -id
              arguments.

       -id ifname
              Specifies a downstream  network  interface:  an  interface  from
              which  requests  from  clients  and  other  relay agents will be
              accepted.  Multiple interfaces may be specified  by  using  more
              than  one  -id  option.  This argument is intended to be used in
              conjunction with one or more -i or -iu arguments.

       -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.

       -U ifname
              Enables the addition of a RFC 3527 compliant link selection sub-
              option for clients directly connected to the  relay.   This  RFC
              allows  a  relay  to specify two different IP addresses: one for
              the server to use when communicating with the relay (giaddr) the
              other  for  choosing  the subnet for the client (the suboption).
              This can be useful if the server is unable to  send  packets  to
              the relay via the address used for the subnet.

              When  enabled,  dhcrelay  will  add  an  agent option (as per -a
              above) that includes the link selection suboption  to  the  for-
              warded  packet.  This will only be done to packets received from
              clients that are directly connected to the relay (i.e. giaddr is
              zero).   The  address  used in the suboption will be that of the
              link upon which the inbound packet  was  received  (which  would
              otherwise  be  used for giaddr). The value of giaddr will be set
              to that of interface ifname.

              Only one interface should be marked in this fashion.   Currently
              enabling this option on an interface causes the relay to process
              all DHCP traffic similar to the -i option, in the future we  may
              split the two more completely.

              This  option  is off by default.  Note that enabling this option
              automatically enables the -a option.

              Keep in mind that using options such as -m replace or -m discard
              on  relays  upstream  from  one using -U can pose problems.  The
              upstream relay will wipe out the initial agent option containing
              the link selection while leaving the re-purposed giaddr value in
              place, causing packets to go astray.


       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.

       -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  inter-
              face.   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)

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