DNS name resolution not working consistently with ssh

Question

Level 1

0 points

DNS name resolution not working consistently with ssh

I've been having an annoying problem on my OS X 10.6.2 box, where ssh name resolution does not work.

nslookup will always resolve correctly, but ping does not. Is it that ssh does not use DNS on OS X?

I can add a host (that is already configured in DNS) to my /etc/hosts file and ssh (or ping) will work, if I then comment that entry out, ssh and/or ping will stop working.

Here is my resolv.conf file.

#
search lab.steinhouse.com
nameserver 10.32.6.20
nameserver 10.32.6.21

Here is some output from nslookup and ping:

tsmacpro:etc toddstein$ nslookup labvma01
Server: 10.32.6.20
Address: 10.32.6.20#53

Name: labvma01.lab.steinhouse.com
Address: 10.32.6.29

tsmacpro:etc toddstein$ ping labvma01
ping: cannot resolve labvma01: Unknown host
tsmacpro:etc toddstein$

Mac Pro, Mac OS X (10.6.2)

Posted on Mar 10, 2010 9:20 AM

Reply

Answer 1

Top-ranking reply

cvmiller

Level 1

4 points

Apr 7, 2010 9:02 AM in response to Todd Stein

I have had a similar problem, where after several days of uptime, my local name resolution stops working. My primary and secondary name servers (running on Linux) are onsite. I have a tertiary DNS server offsite, which my MacBook Pro does resolve names (read: external names resolve, local names do not).

By using Wireshark, I was able to see that only the tertiary DNS server was being queried.

I believe there is a problem in 10.6.2 mDNSResponder.

I have found that if I restart mDNSResponder (sudo kill <pid of mDNSResponder>) my local name resolution is restored.

I hope this helps,

Craig...

Reply

Answer 2

Dogcow-Moof

Level 8

36,845 points

Mar 10, 2010 4:36 PM in response to Todd Stein

Mac OS X does not use DNS the same way a BIND-compiled utility like nslookup does, so the answers you receive from nslookup may be different from those the OS obtains.

There is also an issue where the Mac OS X resolver (mDNSResponder) may "switch" the order of hosts it uses for DNS service where utilities like nslookup and those compiled with BIND honor them.

So in your example:

nameserver 10.32.6.20
nameserver 10.32.6.21

I suspect that the DNS server at 10.32.6.20 can resolve your host but the one at 10.32.6.21, for whatever reason, cannot.

Reply

Answer 3

Dogcow-Moof

Level 8

36,845 points

Mar 10, 2010 7:31 PM in response to Todd Stein

Are you sure? Have you tried querying the other DNS server?

That's the most common reason for the failure.

Try querying 10.32.6.21 directly:

$ nslookup > server 10.32.6.21 > labvma01

and see if you get an answer.

Reply

Answer 4

Todd Stein Author

Level 1

0 points

Mar 11, 2010 5:12 AM in response to Dogcow-Moof

So the second DNS server (10.32.6.21) is a "secondary" server that is synced with the first. I did just recently clean-up any un-removed PTR table entires etc and knew that the particular nodes labvma01, labvma02, labvam03 were in both DNS servers... However, I decided to show you the nslookup output and sure enough the secondary was empty... It is a windows box, so I applied a little microsoft troubleshooting and gave it a good three finger salute. Now all the records are in sync again and the symptoms are gone...

I'm not certain that the problem is solved, because this has happened before. I'll begin to monitor my dns better and watch for this in the future...

thanks for the help

Reply

Answer 5

Dogcow-Moof

Level 8

36,845 points

Apr 7, 2010 11:31 AM in response to cvmiller

Why aren't you running 10.6.3?

Also, read the explanation of how mDNSResponder queries name servers here:

http://support.apple.com/kb/HT4030

Reply

Answer 6

Todd Stein Author

Level 1

0 points

Mar 10, 2010 6:31 PM in response to Dogcow-Moof

Thanks for the input William, but both DNS servers are identical. Is there a way to configure mDNSResponder with Snow Leopard?

Reply