ping域名报错unknown host，nslookup/systemd-resolve可以正常解析，ping公网地址通怎么解决？

异常现象

ping域名报错unknown host，nslookup/systemd-resolve可以正常解析，ping公网地址通。

[email protected]:/etc# ping baidu.com
ping: unknown host baidu.com

[email protected]:/etc# nslookup www.baidu.com
Server:         192.168.61.200
Address:        192.168.61.200#53

Non-authoritative answer:
www.baidu.com   canonical name = www.a.shifen.com.
Name:   www.a.shifen.com
Address: 183.232.231.174
Name:   www.a.shifen.com
Address: 183.232.231.172

[email protected]:/etc# systemd-resolve www.baidu.com
www.baidu.com: 183.232.231.172
               183.232.231.174
               (www.a.shifen.com)

-- Information acquired via protocol DNS in 74.1ms.
-- Data is authenticated: no

[email protected]:/etc# ping 183.232.231.174
PING 183.232.231.174 (183.232.231.174) 56(84) bytes of data.
64 bytes from 183.232.231.174: icmp_seq=1 ttl=54 time=33.4 ms
64 bytes from 183.232.231.174: icmp_seq=2 ttl=54 time=32.7 ms
64 bytes from 183.232.231.174: icmp_seq=3 ttl=54 time=33.9 ms
64 bytes from 183.232.231.174: icmp_seq=4 ttl=54 time=45.0 ms
^C
--- 183.232.231.174 ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3013ms
rtt min/avg/max/mdev = 32.723/36.281/45.043/5.077 ms

需要检查的异常点

1、检查/etc/nsswitch.conf

[email protected]:~# cat /etc/nsswitch.conf 
# /etc/nsswitch.conf
#
# Example configuration of GNU Name Service Switch functionality.
# If you have the `glibc-doc-reference' and `info' packages installed, try:
# `info libc "Name Service Switch"' for information about this file.

passwd:         compat
group:          compat
shadow:         compat
gshadow:        files

hosts:          files dns
networks:       files

protocols:      db files
services:       db files
ethers:         db files
rpc:            db files

netgroup:       nis

检查hosts行，是否为 files dns，files 表示使用 /etc/hosts 和 /etc/network 配置文件，dns 表示使用 /etc/resolv.conf文件中的DNS解析地址。

ping 程序解析域名依赖于C库中的函数 gethostbyname() 和 gethostbyaddr()，对应配置文件 /etc/nsswitch.conf 中的hosts 配置项。nslookup 是直接使用 /etc/resolv.conf 中的DNS解析域名，因此不受该文件影响。

2、检查/etc/resolv.conf文件权限

# Ubuntu系统
# Ubuntu系统中resolv.conf为一个软连接，如有问题，可以手动重建软连接ln -s /etc/resolvconf/run/resolv.conf /etc/resolv.conf
# 重建完成后注意检查/etc/resolvconf/run/resolv.conf配置是否正确
[email protected]:~# ll /etc/resolv.conf 
lrwxrwxrwx 1 root root 29 Apr 26 16:20 /etc/resolv.conf -> ../run/resolvconf/resolv.conf

# CentOS系统
[[email protected] ~]$ ll /etc/resolv.conf
-rw-r--r--. 1 root root 61 Nov  3  2021 /etc/resolv.conf

如果resolv.conf文件无读权限，也会造成ping uknown host的情况。