/etc/resolv.conf文件中的search項做用

resolv.conf文件中有search項時，主機名解析規則順序： 
DNS配置文件以下：

# vi /etc/resolv.conf # Generated by NetworkManager search openstack.local dev.com example.local nameserver 192.168.122.21

例1：查詢主機名，由於主機名後面沒有點，就認爲是主機名，因此先添加search裏的每一項依次組成FQDN（徹底合格域名）來查詢，徹底合格域名查詢未找到，就再認爲主機名是徹底合格域名來查詢。

# host -a centos7-bind-1 Trying "centos7-bind-1.openstack.local" Trying "centos7-bind-1.dev.com" Trying "centos7-bind-1.example.local" Trying "centos7-bind-1" ;; connection timed out; no servers could be reached

 

例2：查詢主機名，由於主機名中有點（不是末尾有點），就認爲是徹底合格域名，先用它來查詢，查詢失敗就把它當成是主機名來進行，添加search裏的每一項組成FQDN（徹底合格域名）來查詢。

# host -a centos7-bind-1.com Trying "centos7-bind-1.com" Received 109 bytes from 192.168.122.21#53 in 177 ms Trying "centos7-bind-1.com.openstack.local" Trying "centos7-bind-1.com.dev.com" Trying "centos7-bind-1.com.example.local" Host centos7-bind-1.com not found: 3(NXDOMAIN) Received 125 bytes from 192.168.122.21#53 in 55 ms

 

例3：查詢主機名，由於主機名中末尾有點，則認爲是徹底合格域名，只用它來查詢（不會再添加search裏的每一項）。查詢次數會與search裏項域名個數有關。

# host -a centos7-bind-1. Trying "centos7-bind-1" ;; connection timed out; trying next origin Trying "centos7-bind-1" ;; connection timed out; trying next origin Trying "centos7-bind-1" ;; connection timed out; trying next origin Trying "centos7-bind-1" ;; connection timed out; no servers could be reached

 

DNS查詢工具：host命令介紹 
用法：host [-a] FQDN [server] 
host -l domain [server] 
-a參數：顯示查詢的全部詳細信息。 
-l參數：若是後面的domain容許allow-transfer時，則列出該domain所管理的全部主機數據。 
server參數：這個參數無關緊要，當想要利用非 /etc/resolv.conf 內的DNS主機，來查詢主機名與 IP 的對應時，就能夠利用這個參數了。

host命令使用例（列出該domain所管理的全部主機數據）：

# host -l example.local 192.168.122.84 Using domain server: Name: 192.168.122.84 Address: 192.168.122.84#53 Aliases: example.local name server ns.example.local. dns.example.local has address 192.168.123.10 ns.example.local has address 192.168.122.84 ns.example.local has IPv6 address ::1 sss.example.local has address 192.168.123.9 www.example.local has address 192.168.123.8