What is Fully Qualified Domain Name FQDN

When you try to access one website on the internet, you must enter one complete domain name into that site. And this domain name is a valid fully qualified domain name (FQDN).

The FQDN (Fully Qualified Domain Name) of the system is the name that the resolver (3) returns for the host name, such as, ursula.example.com.

It is usually the hostname followed by the DNS domain name (the part after the first dot). You can check the FQDN using hostname –fqdn or the domain name using dnsdomainname.

You cannot change the FQDN with hostname or dnsdomainname.

The recommended method of setting the FQDN is to make the hostname be an alias for the fully qualified name using /etc/hosts, DNS, or NIS. For example, if the hostname was “ursula”, one might have a line in /etc/hosts which reads 127.0.1.1 ursula.example.com ursula

Technically: The FQDN is the name getaddrinfo(3) returns for the host name returned by gethostname(2). The DNS domain name is the part after the first dot.

Therefore it depends on the configuration of the resolver (usually in /etc/host.conf) how you can change it. Usually the hosts file is parsed before DNS or NIS, so it is most common to change the FQDN in /etc/hosts.

If a machine has multiple network interfaces/addresses or is used in a mobile environment, then it may either have multiple FQDNs/domain names or none at all. Therefore avoid using hostname –fqdn, hostname –domain and dnsdomainname. hostname –ip-address is subject to the same limi‐tations so it should be avoided as well.