At some point, you may want to convert an IP address to a set of hostnames. This is known as a reverse lookup; it is implemented using PTR records in your DNS server. For example, say you have the IP address 17.149.160.49 and want to display its hostname(s) (apple.com). You have a few options:

• +[NSHost hostWithAddress:]
• gethostbyaddr(3) - A BSD function

Both of these functions accomplish the reverse lookup by having the resolver (lookupd on Tiger and earlier; DirectoryService on Leopard) hit up DNS for a PTR record matching the IP address. You can do the same from Terminal using dig -x 17.149.160.49. It’s a very long list.

So that works for registered DNS names. What about Bonjour (aka mDNS)? Well, Bonjour is just a distributed multicast DNS system. DirectoryService looks up entries in the local domain for you. This is why you don’t need to set up a DNS server but can still access all your machines with names like kitchen-imac.local. Bonjour also supports reverse lookups, but with one subtle caveat: your Mac wants to use IPv6, not IPv4. This means reverse lookups against IPv4 addresses in the local domain may fail if your network understands and prefers IPv6. This may be puzzling if you can verify that the IPv4 address you have matches a computer whose name you know.

Try it for yourself; mDNS(1) is a debugging tool that lets you see what’s being advertised on your network. mDNS -B _daap._tcp . will show you all the shared iTunes libraries on your network. Then use mDNS -L “Your Music Library Name” _daap._tcp . to see which IP addresses are advertised over Bonjour.