总结：linux 6的/etc/resolv.conf重启后修改的原因，是因为重启时如果NetworkManager服务自动启动则该服务会读取网卡信息中的NM_CONTROLLED和DNS配置，如果网卡中配置了NM_CONTROLLED=yes，如果网卡中配置了DNS则把网卡中的DNS写入到resolv.conf，如果网卡没有配置DNS则把空的DNS信息写入resolv.conf

如果不希望重启时/etc/resolv.conf被修改，则关闭NetworkManager服务service NetworkManager stop
chkconfig --level 0123456 NetworkManager off
并修改网卡NM_CONTROLLED=no

也就是说重启时，如果有Network Manager服务，且网卡配置了Network Manager服务，则该服务会根据网卡的DNS修改/etc/resolv.conf

而网卡中DNS配置，不需要每个有IP的网卡都配置，只要其中一个配置了DNS就可以，因为DNS是共享的，但是DNS必须配置在有IP的网卡上，如果有很多个网卡但是没有IP，就算这些网卡配置了DNS，则重启后/etc/resolv.conf也不会获取这个DNS

故障现象1，/etc/resolv.conf里面设置的nameserver信息丢失了
[root@bsdb ~]# cat /etc/resolv.conf
# Generated by NetworkManager
# No nameservers found; try putting DNS servers into your
# ifcfg files in /etc/sysconfig/network-scripts like so:
#
# DNS1=xxx.xxx.xxx.xxx
# DNS2=xxx.xxx.xxx.xxx
# DOMAIN=lab.foo.com bar.foo.com

查看文件属性，发现文件被修改了
[root@bsdb ~]# ll /etc/resolv.conf
-rw-r--r--. 1 root root 228 6月 19 11:06 /etc/resolv.conf

发现文件修改时间和重启时间匹配
[root@bsdb ~]# last |grep reboot
reboot system boot 2.6.32-431.el6.x Tue Jun 19 11:06 - 11:27 (2+00:20)

查看网卡的IP
[root@bsdb ~]# grep IPADDR /etc/sysconfig/network-scripts/ifcfg-eth*
/etc/sysconfig/network-scripts/ifcfg-eth0:IPADDR=192.168.21.16
/etc/sysconfig/network-scripts/ifcfg-eth3:IPADDR=192.167.120.11

查看网卡的DNS信息，没有配置DNS
[root@bsdb ~]# grep DNS /etc/sysconfig/network-scripts/ifcfg-eth*
/etc/sysconfig/network-scripts/ifcfg-eth1:PEERDNS=yes
/etc/sysconfig/network-scripts/ifcfg-eth2:PEERDNS=yes
/etc/sysconfig/network-scripts/ifcfg-eth4:PEERDNS=yes
/etc/sysconfig/network-scripts/ifcfg-eth5:PEERDNS=yes

查看网卡是否启用了NetworkManager管理，即NM_CONTROLLED=yes
[root@bsdb ~]# grep NM_CONTROLLED /etc/sysconfig/network-scripts/ifcfg-eth*
/etc/sysconfig/network-scripts/ifcfg-eth0:NM_CONTROLLED=yes
/etc/sysconfig/network-scripts/ifcfg-eth1:NM_CONTROLLED=yes
/etc/sysconfig/network-scripts/ifcfg-eth2:NM_CONTROLLED=yes
/etc/sysconfig/network-scripts/ifcfg-eth3:NM_CONTROLLED=yes
/etc/sysconfig/network-scripts/ifcfg-eth4:NM_CONTROLLED=yes
/etc/sysconfig/network-scripts/ifcfg-eth5:NM_CONTROLLED=yes

检查NetworkManager是否运行
[root@bsdb ~]# service NetworkManager status
NetworkManager (pid 6297) 正在运行...

OS版本
[oraprod@bsdb ~]$ cat /etc/issue
CentOS release 6.5 (Final)

故障现象2，/etc/resolv.conf里面设置的nameserver被修改了
[root@lmdb ~]# cat /etc/resolv.conf
# Generated by NetworkManager
nameserver 192.168.30.50
nameserver 192.168.130.50

查看文件属性，发现文件被修改了
[root@lmdb ~]# ll /etc/resolv.conf
-rw-r--r--. 1 root root 81 6月 19 11:04 /etc/resolv.conf

发现文件修改时间和重启时间匹配
[root@lmdb ~]# last |grep reboot
reboot system boot 2.6.32-642.el6.x Tue Jun 19 11:04 - 10:09 (1+23:04)

查看网卡的IP
[root@lmdb ~]# grep IPADDR /etc/sysconfig/network-scripts/ifcfg-eth*
/etc/sysconfig/network-scripts/ifcfg-eth0:IPADDR=192.168.148.200
/etc/sysconfig/network-scripts/ifcfg-eth1:IPADDR=192.168.130.200

查看网卡的DNS信息，配置了DNS，且配置DNS的是有IP的网卡之一
[root@lmdb ~]# grep DNS /etc/sysconfig/network-scripts/ifcfg-eth*
/etc/sysconfig/network-scripts/ifcfg-eth0:PEERDNS=yes
/etc/sysconfig/network-scripts/ifcfg-eth1:DNS1=192.168.30.50
/etc/sysconfig/network-scripts/ifcfg-eth1:DNS2=192.168.130.50
/etc/sysconfig/network-scripts/ifcfg-eth1:PEERDNS=yes
/etc/sysconfig/network-scripts/ifcfg-eth2:PEERDNS=yes
/etc/sysconfig/network-scripts/ifcfg-eth3:PEERDNS=yes
/etc/sysconfig/network-scripts/ifcfg-eth4:PEERDNS=yes
/etc/sysconfig/network-scripts/ifcfg-eth5:PEERDNS=yes

查看网卡是否启用了NetworkManager管理，即NM_CONTROLLED=yes
[root@lmdb ~]# grep NM_CONTROLLED /etc/sysconfig/network-scripts/ifcfg-eth*
/etc/sysconfig/network-scripts/ifcfg-eth0:NM_CONTROLLED=yes
/etc/sysconfig/network-scripts/ifcfg-eth1:NM_CONTROLLED=yes
/etc/sysconfig/network-scripts/ifcfg-eth2:NM_CONTROLLED=yes
/etc/sysconfig/network-scripts/ifcfg-eth3:NM_CONTROLLED=yes
/etc/sysconfig/network-scripts/ifcfg-eth4:NM_CONTROLLED=yes
/etc/sysconfig/network-scripts/ifcfg-eth5:NM_CONTROLLED=yes

检查NetworkManager是否运行
[root@lmdb ~]# service NetworkManager status
NetworkManager (pid 3173) 正在运行...

OS版本
[root@lmdb ~]# cat /etc/issue
CentOS release 6.8 (Final)
Kernel \r on an \m

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