FQDN：(Fully Qualified Domain Name)全限定域名：同时带有主机名和域名的名称
其实就是标注一个主机的完整域名。比如我的域名为 ifrom.top 那么它的邮件服务器的主机名为 mail, 所以它的FQDN 为： mail.ifrom.top
Linux 获取主机名 的命令为：
➜ ~ hostname -f ➜ ~ hostname --fqdn ➜ ~ hostname --long
三个都返回 fqdn 的值。
我们通过 man hostname 可以发现，其中有对fqdn 有更问详细的讲解：
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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