2.11 Adding a Multihomed Host

2.11.1 Problem

You want to add a multihomed host to DNS.

2.11.2 Solution

Add multiple A records to the host's domain name, one per IP address. For example, for a file server with two network interfaces, you might add these records:

fs.foo.example.    IN    A    10.0.0.9
fs.foo.example.    IN    A    192.168.0.9

To handle reverse mapping for the host, add one PTR record to each of the appropriate two reverse-mapping zones:

9.0.0.10.in-addr.arpa.    IN    PTR    fs.foo.example.

and

9.0.168.192.in-addr.arpa.    IN    PTR    fs.foo.example.

2.11.3 Discussion

Clients looking up the address of fs.foo.example will see both IP addresses, and can choose which one to use (though most clients will just use the first address returned). Remember that, by default, they'll be returned in round robin order.

For troubleshooting purposes, you may want to add two more A records, each of which maps to just one of your multihomed host's addresses. For example:

fs-eth0.foo.example.    IN    A    10.0.0.9
fs-eth1.foo.example.    IN    A    192.168.0.9

This lets you test whether a particular network interface on the file server is up, by pinging fs-eth0.foo.example, for instance. You probably shouldn't add PTR records mapping the addresses back to these interface-specific names, though: most software can't handle multiple reverse mappings for a single IP address.

2.11.4 See Also

Section 2.8 for the behavior of round robin, and Chapter 4 of DNS and BIND.