On centos, everytime I reboot /etc/resolv.conf resets itself. Turns out it is the PEERDNS var inside the network config.

[root@cmlin02 ~]# more /usr/share/doc/initscripts-9.03.27/sysconfig.txt
PEERDNS=yes|no

modify /etc/resolv.conf if peer uses msdns extension (PPP only) or
DNS{1,2} are set, or if using dhclient. default to “yes”.

[root@cmlin02 ~]# more /etc/sysconfig/network-scripts/ifcfg-eth0 | grep PEERDNS
[root@cmlin02 ~]# vi /etc/sysconfig/network-scripts/ifcfg-eth0
...
PEERDNS=no
...

then reboot