MAN page from Mandrake 9.X dhcp-relay-3.0-1.rc12.2mdk.i586.rpm

dhcrelay

Section: Maintenance Commands (8)
Index

NAME

dhcrelay - Dynamic Host Configuration Protocol Relay Agent

SYNOPSIS

dhcrelay[-pport][-d][-q][-iif0[...-iifN]][-a][-Alength][-D][-mappend|replace|forward|discard]server0[...serverN]

DESCRIPTION

The Internet Software Consortium DHCP Relay Agent, dhcrelay, provides ameans for relaying DHCP and BOOTP requests from a subnet to whichno DHCP server is directly connected to one or more DHCP servers on othersubnets.

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 thelist of DHCP servers specified on the command line. When a reply isreceived from a server, it is broadcast or unicast (according to therelay agent's ability or the client's request) on the network fromwhich the original request came.

COMMAND LINE

The names of the network interfaces that dhcrelay should attempt toconfigure may be specified on the command line using the-ioption. If no interface namesare specified on the command line dhcrelay will identify all networkinterfaces, elimininating non-broadcast interfaces if possible, andattempt to configure each interface.

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

In some cases itishelpful for the relay agent to forward requests from networks on whicha DHCP server is running to other DHCP servers. This would be thecase if two DHCP servers on different networks were being used toprovide backup service for each other's networks.

If dhcrelay should listen and transmit on a port other than thestandard (port 67), the-pflag may used. It should be followed by the udp port number thatdhcrelay should use. This is mostly useful for debugging purposes.

Dhcrelay will normally run in the foreground until it has configuredan interface, and then will revert to running in the background.To force dhcrelay to always run as a foreground process, the-dflag should be specified. This is useful when running dhcrelay undera 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 thisbehaviour, specify the-qflag.

RELAY AGENT INFORMATION OPTIONS

If the-aflag is set the relay agent will append an agent option field to eachrequest before forwarding it to the server. Agent option fields inresponses sent from servers to clients will be stripped beforeforwarding such responses back to the client.

The agent option field will contain two agent options: the Circuit IDsuboption and the Agent ID suboption. Currently, the Circuit ID willbe the printable name of the interface on which the client request wasreceived. The Agent ID will be the value that the relay agent storesin the DHCP packet's giaddr field. The client supports inclusion ofa Remote ID suboption as well, but this is not used by default.

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

Relay Agent options are added to a DHCP packet without the knowledgeof the DHCP client. The client may have filled the DHCP packetoption buffer completely, in which case there theoretically isn't anyspace to add Agent options. However, the DHCP server may be able tohandle a much larger packet than most DHCP clients would send. Thecurrent Agent Options draft requires that the relay agent use amaximum packet size of 576 bytes.

It is recommended that with the Internet Software Consortium DHCPserver, the maximum packet size be set to about 1400, allowing plentyof extra space in which the relay agent can put the agent optionfield, while still fitting into the Ethernet MTU size. This can bedone by specifying the-Aflag, followed by the desired maximum packet size (e.g., 1400).

Note that this is reasonably safe to do even if the MTU between theserver and the client is less than 1500, as long as the hosts on whichthe server and client are running support IP fragmentation (and theyshould). With some knowledge as to how large the agent options mightget in a particular configuration, this parameter can be tuned asfinely as necessary.

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

If giaddr is set, the server may handle the situation in one of fourways: it mayappendits own set of relay options to the packet, leaving thesupplied option field intact. It mayreplacethe existing agent option field.It mayforwardthe packet unchanged. Or, it maydiscardit.

Which of these behaviours is followed by the Internet SoftwareConsortium DHCP Relay Agent may be configured with the-mflag, followed by one of the four keywords specified initalicsabove.

When the relay agent receives a reply from a server that it's supposedto forward to a client, and Relay Agent Information option processingis enabled, the relay agent scans the packet for Relay AgentInformation options and removes them. As it's scanning, if it findsa Relay Agent Information option field containing an Agent IDsuboption that matches one of its IP addresses, that option isrecognized as its own. If no such option is found, the relay agentcan either drop the packet, or relay it anyway. If the-Doption is specified, all packets that don't contain a match will bedropped.

SPECIFYING DHCP SERVERS

The name or IP address of at least one DHCP server to which DHCP andBOOTP requests should be relayed must be specified on the commandline.

BUGS

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

The relay agent should not relay packets received on a physicalnetwork to DHCP servers on the same physical network - if they do, theserver will receive duplicate packets. In order to fix this,however, the relay agent needs to be able to learn about the networktopology, which requires that it have a configuration file.

AUTHOR

dhcrelay(8)has been written for the Internet Software Consortiumby Ted Lemon in cooperation with VixieEnterprises. To learn more about the Internet Software Consortium,seehttp://www.isc.org/isc.To learn more about VixieEnterprises, seehttp://www.vix.com.

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