16.4 BOOTP Server Design

The BOOTP client is normally provided in read-only memory on the diskless system. It is interesting to see how the server is normally implemented.

First, the server reads UDP datagrams from its well-known port (67). Nothing special is required. This differs from an RARP server (Section 5.4), which we said had to read Ethernet frames with a type field of "RARP request." The BOOTP protocol also made it easy for the server to obtain the client's hardware address, by placing it into the BOOTP packet (Figure 16.2).

An interesting problem arises: how can the server send a response directly back to the client? The response is a UDP datagram, and the server knows the client's IP address (probably read from a configuration file on the server). But if the BOOTP server sends a UDP datagram to that IP address (the normal way UDP output is handled), the server's host will probably issue an ARP request for that IP address. But the client can't respond to the ARP request since it doesn't know its IP address yet! (This is called the "chicken and egg" issue in RFC 951.)

There are two solutions. The first, commonly used by Unix servers, is for the server to issue an ioctl (2) request to the kernel, to place an entry into the ARP cache for this client. (This is what the arp -s command does, Section 4.8.) The server can do this since it knows the client's hardware address and IP address. This means that when the server sends the UDP datagram (the BOOTP reply), the server's ARP module will find the client's IP address in the ARP cache.

An alternative solution is for the server to broadcast the BOOTP reply, instead of sending it directly to the client. Since reducing the number of broadcasts on a network is always desirable, this solution should be used only if the server cannot make an entry into its ARP cache. Normally it requires superuser permission to make an entry into the ARP cache, requiring a broadcast reply if the server is nonprivileged.