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       dhcrelay - Dynamic Host Configuration Protocol Relay Agent


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


       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


       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.


       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 possible, 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 provide backup service for each other's networks.

       If  dhcrelay  should  listen  and  transmit  on  a  port other 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.


       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 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.

       When forwarding packets, dhcrelay discards packets which have reached a
       hop count of 10.  If a  lower  or  higher  threshold  (up  to  255)  is
       desired,  depending  on  your  environment, you can specify the max hop
       count threshold as a number following the -c option.

       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  Systems  Consortium  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
       specifying 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 between the
       server and the client is less than 1500, as long as the hosts on  which
       the  server  and  client are running 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   Systems
       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  suboption
       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.


       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.


       dhclient(8),   dhcpd(8),   RFC2132,   RFC2131,    draft-ietf-dhc-agent-


       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 network - if they do, the server
       will receive duplicate packets.   In order to fix  this,  however,  the
       relay agent needs to be able to learn about the network topology, which
       requires that it have a configuration file.


       dhcrelay(8) has been written for Internet  Systems  Consortium  by  Ted
       Lemon  in  cooperation  with  Vixie  Enterprises.   To learn more about
       Internet Systems Consortium,  see   To  learn
       more about Vixie Enterprises, see