For some users when I add new internal nameserver address in resolv.conf after some time it disappears. Any suggestions??

Edited out in favor of Firerat's soution, thanks Firerat for pointing this one out, I always use static IP adresses on my LAN's, so the phenomenon of a changing resolv.conf never happened to me...

I learned something here! Thanks dude

Thor

resolv.conf is usually replaced when the network 'comes up' often by those offered by the DHCP server

Solutions vary
Probably the simplest is to force a static DNS in your network manager's settings

you could also look in /etc/resolvconf/resolv.conf.d/ ( assuming it exists )

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'resolv.conf's should push and pop in an orderly fashion, so if you are saying that you make a change and then it permanently disappears, then something odd is happening. On the other hand, if, when some application such as NetworkManager or Wicd takes control of networking, you temporarily see something unconnected to what you originally set, then that is expected behaviour. But along with this expected behaviour, you should see some note along the lines of "### altered by network manager...do not edit" and probably a date/time that this last happened.

While NM/wicd are in control, any settings available from DHCP/mDNS might turn up in the file, although you can probably reconfigure them to only take selected parts of the information.

If, of course, you did edit the file while it had the "do not edit" comment present, you should expect things to go wrong, even if you don't know quite how soon this should happen.