Reverse Address Resolution Protocol

Before leaving the topic of configuration protocols, we should quickly mention Reverse Address Resolution Protocol (RARP). As the name implies, it is the reverse of ARP. Instead of asking for an Ethernet address in response to an IP addresses, this protocol broadcasts an Ethernet address, and asks for an IP address in response.

A RARP server uses the /etc/ethers file to map Ethernet addresses to IP addresses. It then sends the IP address from the ethers file to the client system. A sample /etc/ethers file is shown here:

00:00:C0:4F:3E:DD bluejay 00:10:4B:87:D4:A8 duck 08:00:20:82:D5:1D raven 00:00:0C:43:8D:FB osprey

Each line in the file contains an Ethernet address, followed by a hostname or IP address. Hostnames are most commonly used, but they must be valid names that map to IP addresses.

We mention this protocol because the nsswitch.conf file covered in Chapter 4, "Linux Name Services," includes /etc/ethers as a part of the NIS service, which might make you curious about it. However, you should not use RARP. RARP only provides the client with an IP address. No other configuration information is provided. Much better configuration servers are available for Linux, including DHCP, which is the right configuration server for most networks.

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