dhcrelay(8)                                           dhcrelay(8)


NAME
       dhcrelay - Dynamic Host Configuration Protocol Relay Agent

SYNOPSIS
       dhcrelay [ -p port ] [ -d ] [ -q ] [ -i if0 [ ...  -i  ifN
       ]  ]  [  -a ] [ -A length ] [ -D ] [ -m append | replace |
       forward | discard ] server0 [ ...serverN ]

DESCRIPTION
       The Internet Software Consortium DHCP Relay Agent,  dhcre-
       lay, provides a means for relaying DHCP and BOOTP requests
       from a subnet to which no DHCP  server  is  directly  con-
       nected to one or more DHCP servers on other subnets.

SYSTEM REQUIREMENTS
       You  must have the Berkeley Packet Filter (bpf) configured
       in your NetBSD  kernel.    You  must  have  at  least  one
       /dev/bpf*  file  for each broadcast network interface that
       is attached to your system.

OPERATION
       The DHCP Relay Agent listens for DHCP  and  BOOTP  queries
       and  responses.   When  a query is received from a client,
       dhcrelay forwards it to the list of DHCP servers specified
       on  the  command  line.   When  a reply is received from a
       server, it is broadcast or unicast (according to the relay
       agent's  ability  or  the client's request) on the network
       from which the original request came.

COMMAND LINE
       The names of the network interfaces that  dhcrelay  should
       attempt  to configure may be specified on the command line
       using the -i option.  If no interface names are  specified
       on  the  command  line  dhcrelay will identify all network
       interfaces, elimininating non-broadcast interfaces if pos-
       sible, and attempt to configure each interface.

       The  -i flag can be used to specify the network interfaces
       on which the relay agent should listen.   In  general,  it
       must  listen not only on those network interfaces to which
       clients are attached, but also on those network interfaces
       to  which  the  server  (or  the  router  that reaches the
       server) is attached.   However, in some cases  it  may  be
       necessary to exclude some networks; in this case, you must
       list all those  network  interfaces  that  should  not  be
       excluded using the -i flag.

       In some cases it is helpful for the relay agent to forward
       requests from networks on which a DHCP server  is  running
       to  other  DHCP  servers.    This would be the case if two
       DHCP servers on different networks were being used to pro-
       vide backup service for each other's networks.

       If  dhcrelay  should  listen  and transmit on a port other



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dhcrelay(8)                                           dhcrelay(8)


       than the standard (port 67), the -p  flag  may  used.   It
       should  be  followed  by the udp port number that dhcrelay
       should use.  This is mostly useful for debugging purposes.

       Dhcrelay  will normally run in the foreground until it has
       configured an interface, and then will revert  to  running
       in  the  background.  To force dhcrelay to always run as a
       foreground process, the -d flag should be specified.  This
       is  useful when running dhcrelay under a debugger, or when
       running it out of inittab on System V systems.

       Dhcrelay will normally print its network configuration  on
       startup.  This can be unhelpful in a system startup script
       - to disable this behaviour, specify the -q flag.

RELAY AGENT INFORMATION OPTIONS
       If the -a flag is set the relay agent will 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 Agent ID suboption.  Cur-
       rently, the Circuit ID will be the printable name  of  the
       interface  on which the client request was received.   The
       Agent ID will be the value that the relay agent stores  in
       the  DHCP  packet's  giaddr  field.    The client supports
       inclusion of a Remote ID suboption as well,  but  this  is
       not used by default.

       Note: The Agent ID suboption is not defined in the current
       Relay  Agent  Information  Option  draft  (draft-ietf-dhc-
       agent-options-03.txt), but has been proposed for inclusion
       in the next draft.

       Relay Agent options are added to a DHCP packet without the
       knowledge of the DHCP client.   The client may have filled
       the DHCP packet option buffer completely,  in  which  case
       there  theoretically isn't any space to add Agent options.
       However, the DHCP server may be  able  to  handle  a  much
       larger  packet  than  most  DHCP clients would send.   The
       current Agent Options draft requires that the relay  agent
       use a maximum packet size of 576 bytes.

       It  is recommended that with the Internet Software Consor-
       tium DHCP server, the maximum packet size be set to  about
       1400,  allowing  plenty  of extra space in which the relay
       agent can put the agent option field, while still  fitting
       into  the Ethernet MTU size.  This can be done by specify-
       ing the -A flag, followed by the  desired  maximum  packet
       size (e.g., 1400).

       Note  that  this  is reasonably safe to do even if the MTU



                                                                2





dhcrelay(8)                                           dhcrelay(8)


       between the server and the client is less  than  1500,  as
       long  as the hosts on which the server and client are run-
       ning support IP fragmentation  (and  they  should).   With
       some knowledge as to how large the agent options might get
       in a particular configuration, this parameter can be tuned
       as finely as necessary.

       It is possible for a relay agent to receive a packet which
       already contains an agent option field.   If  this  packet
       does not have a giaddr set, the standard requires that the
       packet be discarded.

       If giaddr is set, the server may handle the  situation  in
       one  of  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.

       Which  of  these  behaviours  is  followed by the Internet
       Software Consortium DHCP Relay  Agent  may  be  configured
       with  the  -m  flag,  followed by one of the four keywords
       specified in italics above.

       When the relay agent receives a reply from a  server  that
       it's  supposed  to  forward  to  a client, and Relay Agent
       Information option processing is enabled, the relay  agent
       scans  the  packet for Relay Agent Information options and
       removes them.   As it's scanning,  if  it  finds  a  Relay
       Agent Information option field containing an Agent ID sub-
       option that matches one of its IP addresses,  that  option
       is  recognized  as  its own.   If no such option is found,
       the relay agent can either drop the packet,  or  relay  it
       anyway.    If the -D option is specified, all packets that
       don't contain a match will be dropped.

SPECIFYING DHCP SERVERS
       The name or IP address of at  least  one  DHCP  server  to
       which  DHCP  and  BOOTP requests should be relayed must be
       specified on the command line.

SEE ALSO
       dhclient(8), dhcpd(8), RFC2132,  RFC2131,  draft-ietf-dhc-
       agent-options-03.txt.

BUGS
       It  should  be possible for the user to define the Circuit
       ID and Remote ID values on a per-interface basis.

       The relay agent should not relay  packets  received  on  a
       physical network to DHCP servers on the same physical net-
       work - if they do, the server will receive duplicate pack-
       ets.    In  order  to  fix  this, however, the relay agent
       needs to be able to  learn  about  the  network  topology,



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dhcrelay(8)                                           dhcrelay(8)


       which requires that it have a configuration file.

AUTHOR
       dhcrelay(8)  has  been  written  for the Internet Software
       Consortium by Ted Lemon <mellon@fugue.com> in  cooperation
       with  Vixie Enterprises.  To learn more about the Internet
       Software Consortium, see http://www.vix.com/isc.  To learn
       more about Vixie Enterprises, see http://www.vix.com.

















































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