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

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 ] -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  uni-
       cast 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.

       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.

       -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)