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For some reason, all DNS queries on one of my Ubuntu 9.04 boxes get routed to localhost, which results in no domain names being resolved. When trying for instance dig example.com, it hangs for ~20 seconds, before quitting with the message ";; connection timed out; no servers could be reached". However, dig example.com @193.75.75.75 returns the correct result immediately. This problem applies to all DNS queries, not only from dig.

$ cat /etc/resolv.conf
193.75.75.75
193.75.75.193

I do not have a .digrc, NetworkManager has been uninstalled, and the network is otherwise working (e.g. I can use it normally for hosts in /etc/resolv.conf). I do not have a local DNS server installed, and nmap confirms that nothing is listening on port 53. I have several other boxen running Ubuntu 9.04 on the same network, DNS is working fine on all of them.

I have used strace to confirm that dig actually reads resolv.conf:

open("/etc/resolv.conf", O_RDONLY|O_LARGEFILE) = 6
(...)
read(6, "193.75.75.75\n193.75.75.193\n"..., 4096) = 27

I have also used tcpdump to confirm that the problem is indeed DNS queries getting routed to localhost:

$ sudo tcpdump -i lo
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on lo, link-type EN10MB (Ethernet), capture size 96 bytes
14:11:54.175580 IP localhost.45910 > localhost.domain: 38233+ A? example.com. (29)
14:11:54.175633 IP localhost > localhost: ICMP localhost udp port domain unreachable, length 65
14:11:55.175542 IP6 ip6-localhost.54963 > ip6-localhost.domain: 38233+[|domain]
14:11:55.175575 IP6 ip6-localhost > ip6-localhost: ICMP6, destination unreachable[|icmp6]

I am out of ideas for resolving this issue, so any input and suggestions are most welcome.

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3 Answers 3

up vote 4 down vote accepted

A record in /etc/resolv.conf should look like

nameserver 193.75.75.75
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I cannot belive i didn't see that, and went off on a wild hunt with strace, nc, tcpdump, and friends... Thanks anyway! –  knatten Sep 12 '09 at 13:53
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The format of /etc/resolv.conf is incorrect, it should read:

search domain.com
nameserver 193.75.75.75
nameserver 193.75.75.193

The search line is of course optional. If it exists it will be used to qualify hostnames, eg. looking up the address for router will return the address for router.domain.com

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If you've changed this recently, have you tried "service network restart" or restarting all of the server?

Sounds pretty newb, but that's what it takes when resolv.conf is modified.

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You don't have to restart any networking after modifying /etc/resolv.conf –  Anonymous Sep 12 '09 at 13:03
    
Ubuntu doesn't even have a service command. If anything restarting networking will most likely overwrite resolv.conf –  theotherreceive Sep 12 '09 at 13:19
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