dhcrelay.8

open source dhcp server client etc...

.\"	dhcrelay.8
.\"
.\" Copyright (c) 1997, 1998 The Internet Software Consortium.
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.\" CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
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.\" This software has been written for the Internet Software Consortium
.\" by Ted Lemon in cooperation with Vixie
.\" Enterprises.  To learn more about the Internet Software Consortium,
.\" see ``http://www.isc.org/isc''.  To learn more about Vixie
.\" Enterprises, see ``http://www.vix.com''.
.TH dhcrelay 8
.SH NAME
dhcrelay - Dynamic Host Configuration Protocol Relay Agent
.SH SYNOPSIS
.B dhcrelay
[
.B -p
.I port
]
[
.B -d
]
[
.B -q
]
[
.B -i
.I if0
[
.B ...
.B -i
.I ifN
]
]
[
.B -a
]
[
.B -A
.I length
]
[
.B -D
]
[
.B -m
.I append
|
.I replace
|
.I forward
|
.I discard
]
.I server0
[
.I ...serverN
]
.SH DESCRIPTION
The Internet Software 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.
.SH OPERATION
.PP
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.
.SH COMMAND LINE
.PP
The names of the network interfaces that dhcrelay should attempt to
configure may be specified on the command line using the
.B -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.
.PP
The
.B -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 \fInot\fR be excluded using the \fB-i\fR flag.
.PP
In some cases it
.I 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.
.PP
If dhcrelay should listen and transmit on a port other than the
standard (port 67), the
.B -p
flag may used.  It should be followed by the udp port number that
dhcrelay should use.  This is mostly useful for debugging purposes.
.PP
Dhcrelay will normally run in the foreground until it has configured
an interface, and then will revert to running in the background.
To run force dhcrelay to always run as a foreground process, the
.B -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.
.PP
Dhcrelay will normally print its network configuration on startup.
This can be unhelpful in a system startup script - to disable this
behaviour, specify the
.B -q
flag.
.SH RELAY AGENT INFORMATION OPTIONS
If the
.B -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.
.PP
The agent option field will contain two agent options: the Circuit ID
suboption and the Agent ID suboption.  Currently, 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.
.PP
.I 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.
.PP
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.   
.PP
It is recommended that with the Internet Software 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
.B -A
flag, followed by the desired maximum packet size (e.g., 1400).
.PP
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.
.PP
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.
.PP
If giaddr is set, the server may handle the situation in one of four
ways: it may
.I append
its own set of relay options to the packet, leaving the
supplied option field intact.   It may
.I replace
the existing agent option field.
It may
.I forward
the packet unchanged.   Or, it may
.I discard
it.
.PP
Which of these behaviours is followed by the Internet Software
Consortium DHCP Relay Agent may be configured with the
.B -m
flag, followed by one of the four keywords specified in
.I italics
above.
.PP
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
.B -D
option is specified, all packets that don't contain a match will be
dropped.
.SH 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.
.SH SEE ALSO
dhclient(8), dhcpd(8), RFC2132, RFC2131, draft-ietf-dhc-agent-options-03.txt.
.SH BUGS
It should be possible for the user to define the Circuit ID and Remote
ID values on a per-interface basis.
.PP
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.
.SH AUTHOR
.B dhcrelay(8)
has been written for the Internet Software Consortium
by Ted Lemon in cooperation with Vixie
Enterprises.  To learn more about the Internet Software Consortium,
see
.B http://www.vix.com.
To learn more about Vixie
Enterprises, see
.B http://www.vix.com.