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ping fails but nslookup works

By cpfeiffe ·
All,
Ping, traceroute, telnet, etc. fail from time to time (not always). They fail with "unknown host". But nslookup always works. If you try a ping after an nslookup on the same host it will always work. This is very strange.

I am running HP-UX 11i with a fairly empty hosts file (only the local server information) a properly configured resolv.conf and nsswitch.conf to go to files, then DNS. I have a lot of experience with DNS so I know nsswitch.conf and resolv.conf are right.

I noticed with tcpdump that on failures the behavior is that the host will open a port (random > 1024) to connect to port 53 on the DNS server. The DNS server responds to the randomly opened port and then the local server refuses the packet (says "udp port xxxxx unreachable for dns.server.com"). Its like the port was closed before the response got back, but the response was quick so it shouldn't be a timing issue.

Any thoughts?

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by sgt_shultz In reply to ping fails but nslookup w ...

i would if you could be describing fingering. i know nuth so forgive talking out loud...
don't we need to know more about how you used nslookup?

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by cpfeiffe In reply to

Poster rated this answer.

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by cpfeiffe In reply to ping fails but nslookup w ...

I think the key point here is what I saw in tcpdump. When ping works you can see the request going to the DNS server and the response coming back. When it doesn't work you see an extra packet from our server to the DNS server indicating that the port (the one we randomly open to talk to the DNS server) has been closed. Its like our server is closing the UDP port before it gets a response. It seems to do this for everything except nslookup/nsquery.

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by cpfeiffe In reply to ping fails but nslookup w ...

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