topic Re: flush DNS cache? in Operating System - HP-UX

flush DNS cache?

Dean M. Durant — Thu, 19 Jan 2006 17:01:46 GMT

Hello, is there a way to flush the DNS cache in hp-ux 11.11? We get reverse lookups that fail when people are trying to connect using exceed from laptops whose IP addresses have changed. They can still connect using xstart, but the question of flushing the DNS cache still came up. Apparently there is a command in that other OS to ipconfig /flushdns. People are asking, why not in HPUX? Thanks, Dean

Re: flush DNS cache?

Jeff_Traigle — Thu, 19 Jan 2006 17:09:17 GMT

I could be mistaken (and someone will correct me if I'm wrong)...

I don't recall ever seeing a way to flush DNS cache in HP-UX. The reason? I don't think an HP-UX DNS client caches anything. The DNS model doesn't account for caching on the client in any explanation I've ever read. I think that's one of those Microsoft "we're going to do things our own way" things. :)

Re: flush DNS cache?

Dean M. Durant — Thu, 19 Jan 2006 17:13:07 GMT

I also heard something like this could be done in Solaris. Don't know if that is true or not.

Re: flush DNS cache?

Jeff_Traigle — Thu, 19 Jan 2006 17:16:45 GMT

Note, I was referring to an HP-UX client. Now there is such a thing as a cache only DNS server on HP-UX and you *can* flush that cache. See the following thread:

http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=65348

Re: flush DNS cache?

rick jones — Thu, 19 Jan 2006 20:38:21 GMT

Indeed, the HP-UX DNS resolver code does not include any caching, so there is nothing to flush. And indeed, one can simulate some of the behaviour of other platform's resolver caches by running a local caching-only name server.

Now, one _might_ consider /etc/hosts a form of cache, and "flushing" that would involve the use of an editor :) The extent to which that is involved depends on your /etc/nsswitch.conf file.

Re: flush DNS cache?

Geoff Wild — Thu, 19 Jan 2006 22:11:56 GMT

If you run a BIND 9.2.0 or newer name server, you can flush the cache with rndc flush. With older name servers, you need to kill the name server and restart it to flush the cache. You can do that in one fell swoop with rndc restart or rndc exec.

Clearing the cache is really a side effect of killing the name server, since BIND name servers only store cached data in memory. Since restarting the name server takes time, especially if the name server is authoritative for many zones, rndc flush is a better option.

If you run multiple views on your BIND 9.2.0 or newer name server, you can flush the cache in only one view using rndc flush viewname. For example:

# rndc flush internal
BIND 9.3.0 will support flushing all of the records attached to a particular domain name with rndc flushname. For example:

# rndc flushname cnn.com

Rgds...Geoff

Re: flush DNS cache?

Dean M. Durant — Fri, 20 Jan 2006 08:18:23 GMT

These are all good, useful answers. Thanks.