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dhcrelay(8)             FreeBSD System Manager's Manual            dhcrelay(8)

       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 ] -l interface ] -U interface ]
       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.

       -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

       -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.

       -U ifname
              Enables the addition of a RFC 3527 compliant link selection
              suboption 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
              forwarded 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
              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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