0 前言

dns 配置文件 /etc/resolv.conf 中常看到有 search 设置，以前以为是根据 search 中的域去指定 nameserver，其实不是这样用的。它的一个用处是程序只需要知道主机名就可以解析到 ip，不必知道域名后缀 domain 是什么

FQDN (Fully Qualified Domain Name) 含义是完整的域名. 例如, 一台机器主机名 (hostname) 是 www, 域名后缀 (domain) 是 baidu.com, 那么该主机的 FQDN 应该是 www.baidu.com. 最后是以 . 来结尾的, 但是大部分的应用和服务器都允许忽略最后这个点 . 所有大家直接输入 www.baidu.com 也可以识别

https://www.man7.org/linux/man-pages/man5/resolv.conf.5.html

下面以几个示例演示一下 search 是如何工作的

/etc/resolv.conf 配置文件内容

nameserver 8.8.8.8
search foo.local bar.local

解析 test ，优先以 hostname 的形式拼接到 search 中配置的 domain 上进行查询，如果失败直接以 FQDN 的形式查询

[root@k8s-node1 ~]# host -a test
Trying "test.foo.local"
Trying "test.bar.local"
Trying "test"
Host test not found: 3(NXDOMAIN)
Received 97 bytes from 8.8.8.8#53 in 53 ms

解析 test.hello ，优先以 FQDN 的形式查询，如果失败则以 hostname 的形式拼接到 search 中配置的 domain 上进行查询

[root@k8s-node1 ~]# host -a test.hello
Trying "test.hello"
Received 103 bytes from 8.8.8.8#53 in 48 ms
Trying "test.hello.foo.local"
Trying "test.hello.bar.local"
Host test.hello not found: 3(NXDOMAIN)
Received 113 bytes from 8.8.8.8#53 in 49 ms

解析 test. ，直接认定为 FQDN ，以 FQDN 的形式查询，不会进行拼接查询

[root@k8s-node1 ~]# host -a test.
Trying "test"
Host test. not found: 3(NXDOMAIN)
Received 97 bytes from 8.8.8.8#53 in 54 ms

2 options ndots

可以发现，配置了 search 之后，除非以最后一种形式查询，总会将 hostname 和 search 进行拼接查询

其实它是由 options ndots:[number] 选项控制的：当查询的域名有 >= number 个 . 时，优先以 FQDN 的形式查询，如果失败再拼接查询

配置 /etc/resolv.conf

nameserver 8.8.8.8
search foo.local bar.local
options ndots:2

观察下述示例

• test

  [root@k8s-node1 ~]# host -a test
  Trying "test.foo.local"
  Trying "test.bar.local"
  Trying "test"
  Host test not found: 3(NXDOMAIN)
  Received 97 bytes from 8.8.8.8#53 in 45 ms
  

• test.hello

  [root@k8s-node1 ~]# host -a test.hello
  Trying "test.hello.foo.local"
  Trying "test.hello.bar.local"
  Trying "test.hello"
  Host test.hello not found: 3(NXDOMAIN)
  Received 103 bytes from 8.8.8.8#53 in 46 ms
  

• test.hello.world

  [root@k8s-node1 ~]# host -a test.hello.world
  Trying "test.hello.world"
  Received 119 bytes from 8.8.8.8#53 in 57 ms
  Trying "test.hello.world.foo.local"
  Trying "test.hello.world.bar.local"
  Host test.hello.world not found: 3(NXDOMAIN)
  Received 119 bytes from 8.8.8.8#53 in 45 ms
  

• test.

  [root@k8s-node1 ~]# host -a test.
  Trying "test"
  Host test. not found: 3(NXDOMAIN)
  Received 97 bytes from 8.8.8.8#53 in 45 ms
  

以上