[網路]DNS解析

DNS用戶端的設定檔DNS用戶端的設定檔主要有： 1) /etc/hosts 2) /etc/resolv.conf 3) /etc/nsswitch.conf
/etc/hosts檔案記錄了IP和主機名稱的對應關係，/etc/resolv.conf用於配置DNS伺服器，如下所示：

$ cat /etc/resolv.conf nameserver 8.8.4.4nameserver 8.8.8.8

第一個是主DNS伺服器，第二個是備DNS伺服器。一般情況下，是先解析/etc/hosts，沒找到再到DNS伺服器裡找，這是因為/etc/nsswitch.conf的預設配置是：

#hosts:     db files nisplus nis dnshosts:      files dns

所以建議把內網的IP和主機名稱的對應關係寫到/etc/hosts檔案裡。DNS相關命令 1. host host命令後面加上網域名稱就可以查到該網域名稱的IP：

# host baidu.combaidu.com has address 123.125.114.144baidu.com has address 220.181.111.85baidu.com has address 220.181.111.86baidu.com mail is handled by 20 mx1.baidu.com.baidu.com mail is handled by 20 jpmx.baidu.com.baidu.com mail is handled by 20 mx50.baidu.com.baidu.com mail is handled by 10 mx.mailcdn.baidu.com.

2. nslookup nslookup和host類似，只是它還列出使用的DNS伺服器資訊（Server部分）

# nslookup baidu.comServer:         8.8.4.4Address:        8.8.4.4#53Non-authoritative answer:Name:   baidu.comAddress: 123.125.114.144Name:   baidu.comAddress: 220.181.111.85Name:   baidu.comAddress: 220.181.111.86

3. dig dig命令會詳細列出DNS查詢的這個過程：

# dig +trace baidu.com; <<>> DiG 9.3.6-P1-RedHat-9.3.6-20.P1.el5 <<>> +trace baidu.com;; global options:  printcmd.                       19079   IN      NS      m.root-servers.net..                       19079   IN      NS      i.root-servers.net..                       19079   IN      NS      g.root-servers.net..                       19079   IN      NS      a.root-servers.net..                       19079   IN      NS      f.root-servers.net..                       19079   IN      NS      e.root-servers.net..                       19079   IN      NS      l.root-servers.net..                       19079   IN      NS      j.root-servers.net..                       19079   IN      NS      d.root-servers.net..                       19079   IN      NS      b.root-servers.net..                       19079   IN      NS      h.root-servers.net..                       19079   IN      NS      c.root-servers.net..                       19079   IN      NS      k.root-servers.net.;; Received 228 bytes from 8.8.4.4#53(8.8.4.4) in 162 mscom.                    172800  IN      NS      h.gtld-servers.net.com.                    172800  IN      NS      j.gtld-servers.net.com.                    172800  IN      NS      a.gtld-servers.net.com.                    172800  IN      NS      m.gtld-servers.net.com.                    172800  IN      NS      e.gtld-servers.net.com.                    172800  IN      NS      k.gtld-servers.net.com.                    172800  IN      NS      l.gtld-servers.net.com.                    172800  IN      NS      d.gtld-servers.net.com.                    172800  IN      NS      c.gtld-servers.net.com.                    172800  IN      NS      i.gtld-servers.net.com.                    172800  IN      NS      f.gtld-servers.net.com.                    172800  IN      NS      g.gtld-servers.net.com.                    172800  IN      NS      b.gtld-servers.net.;; Received 487 bytes from 202.12.27.33#53(m.root-servers.net) in 174 msbaidu.com.              172800  IN      NS      dns.baidu.com.baidu.com.              172800  IN      NS      ns2.baidu.com.baidu.com.              172800  IN      NS      ns3.baidu.com.baidu.com.              172800  IN      NS      ns4.baidu.com.baidu.com.              172800  IN      NS      ns7.baidu.com.;; Received 197 bytes from 192.54.112.30#53(h.gtld-servers.net) in 483 msbaidu.com.              600     IN      A       220.181.111.86baidu.com.              600     IN      A       123.125.114.144baidu.com.              600     IN      A       220.181.111.85baidu.com.              86400   IN      NS      ns3.baidu.com.baidu.com.              86400   IN      NS      ns2.baidu.com.baidu.com.              86400   IN      NS      ns4.baidu.com.baidu.com.              86400   IN      NS      dns.baidu.com.baidu.com.              86400   IN      NS      ns7.baidu.com.;; Received 245 bytes from 202.108.22.220#53(dns.baidu.com) in 129 ms

從上面的輸出可以看出，DNS是一個分布式的、階層式的架構，先訪問根伺服器(.)，根據根伺服器返回的結果，再訪問下一層DNS伺服器，以此類推，直至找到目標主機的IP地址為止。