TCP/IP and Data Communications Administration Guide

BOOTP Relay Agents

Multiple networks, and the use of netmasks to identify them, complicate the functioning of TCP/IP-based networks. For instance, broadcasting using IP cannot take place through the gateways that link networks. So clients on one network cannot broadcast DHCP or BOOTP requests to servers on other networks. A BOOTP relay agent must direct the initial requests through the gateway to the server, then return the replies from the server to the clients.

If your router doesn't have a built-in BOOTP relay agent, and you want clients in other networks to have the advantage of the services installed on the DHCP server, you can install BOOTP relay agents on those networks. This allows clients to access DHCP servers from a network that is not running a DHCP server.

The in.dhcpd daemon can be run as a BOOTP relay agent. If you specify BOOTP relay agent mode, the option argument specifies a comma-separated list of IP addresses or the hostnames of DHCP or BOOTP servers to which the relay agent must forward BOOTP requests. When the daemon is started in this mode, any DHCP databases are ignored, and the daemon acts as a BOOTP relay agent.

A BOOTP relay agent listens to UDP port 68, and forwards BOOTP request packets received on this port to the destinations specified on the command line. The relay agent can run on any machine that has knowledge of local routers, so it does not have to be an Internet gateway machine.

The -r IPaddr | hostname ... option enables the BOOTP relay agent. The proper entries need to be made to the netmasks database so that the DHCP server served by the BOOTP relay agents can identify the subnet mask of the foreign BOOTP/DHCP client's network.

After you install the BOOTP relay agent, entries must be added to the distributed DHCP databases so the DHCP servers can service clients sending requests through the BOOTP relay agent.

The macro option (-M) for PNTADM command is a mechanism that permits the network administrator to select configuration parameters to be returned to clients using a particular IP address. It can also be used to deliver a macro with server-specific information by including this macro definition in all dhcp_network database entries owned by a specific server.