dhclient and /etc/resolv.conf being overwritten.

In ubuntu 12.12 (the one I'm currently using), dhclient is used to get a dhcp lease.  For most people this works just dandy.  However if you use it for routing, and/or running your own dns server inside your network, or just want to have some dns servers used for whatever reason that the dhcp server doesn't issue you can do the following.  

If you go into the /etc/dhcp directory you will see a file called dhclient.conf

somewhere (anywhere really), in the file put the following line:

prepend domain-name-servers 8.8.8.8;

if you want just your custom dns server:
supersede domain-name-servers 8.8.8.8;

This will put the 8.8.8.8 dns server at the top of the list of dns servers in the /etc/resolv.conf file, in addition the to usual ones that the dhcp server gave to your computer. 

Of course for your network you will probably replace 8.8.8.8 with the ip address of another dns server (unless you really want to use one of google's dns servers).  

In fedora you have to go to /etc/sysconfig/network-scripts/, and find the file for your network device (something like ifcfg-ethX, where X is a number usually 0).

change the line PEERDNS=yes to =no.