FQDN：(Fully Qualified Domain Name)全限定域名：同時帶有主機名和域名的名稱
其實就是標註一個主機的完整域名。比如我的域名為 ifrom.top 那麼它的郵件伺服器的主機名為 mail, 所以它的FQDN 為： mail.ifrom.top
Linux 獲取主機名 的命令為：
➜ ~ hostname -f ➜ ~ hostname --fqdn ➜ ~ hostname --long
三個都返回 fqdn 的值。
我們通過 man hostname 可以發現，其中有對fqdn 有更問詳細的講解：
THE 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 com‐ mon 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 limitations so it should be avoided as well.
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