NICs and Names

Host names

One of the great misunderstandings in networking is about the hostname of a machine. That’s because hostname can be defined in two distinct ways.

First, a hostname is the name an operating system uses to identify itself. This is what you see when you run the hostname command. The administrator of the machine can set that hostname and the default search domain. Together, the concatenation of the hostname and search domain is called the fully qualified domain name (FQDN).

The second definition of hostname pertains to the external name of the system. Other computers expect to connect to the target machine using that hostname. When a program tries to connect to a particular hostname, it resolves that name via DNS. DNS resolves the desired name, maybe through a recursive query up to higher authorities, and ultimately returns an IP address.

DNS and FQDN

Did you spot the discrepancy? There’s no guarantee that the machine’s own FQDN matches the FQDN that DNS has for its IP address. In other words, a machine may have its FQDN set to spock.example.com but have a DNS mapping as mail.example.com and www.example.com. The fundamental ...