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We have two internal DNS servers. One is located on a linux server box and the other is the router's DNS management. We set the linux box as primary DNS via DHCP and the router as secondary. We have a few Mac clients that are accessing our internal mail server (hostnamed "mail" internally).

When using IMAP or SMTP against the mail server internally, the mac boxes will sometimes fail to locate the server. If I use NSLOOKUP I can see that "mail" is pointed to the correct IP address and is being resolved via the correct DNS server, but if I ping "mail" it fails.

~ (bash)$ nslookup mail

Name:   mail.example.com

Note: I replaced our actual internal IP address with 254.254.254.*

If I wait a few minutes (3-5 minutes), somehow it resolves itself and sends successfully. This happens multiple times a day. The /etc/hosts file on the mac boxes is the default config.

# Host Database
# localhost is used to configure the loopback interface
# when the system is booting.  Do not change this entry.
##   localhost broadcasthost
::1             localhost 
fe80::1%lo0 localhost

Is there something about Mac clients I should know to prevent this failed DNS resolution?

Client boxes are: OSX 10.7.4, 8GB RAM, i5 MacBooks Server is: Ubuntu 12.04 Server

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What happens if you use nslookup, set server to the router IP address and then do the lookup? –  EightBitTony Jul 10 '12 at 18:57
It forwards to the 206 server. We have the router forwarding to the 206 server on failure to resolve and the server forwarding to google dns for resolution. –  Concordus Applications Jul 10 '12 at 19:35
I just ran wireshark and got this error Header checksum: 0x0000 [incorrect, should be 0xc13a (may be caused by "IP checksum offload"?)] –  Concordus Applications Jul 10 '12 at 19:37
My guess is that the Mac client is randomly picking between the DNS server and the router when using the resolver client library, and you're seeing broken behaviour from the router resolver. nslookup manually picks the first one, so it always looks fine. –  EightBitTony Jul 10 '12 at 21:43

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