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Old 05-21-2008, 05:13 PM   #1
Registered: Nov 2003
Posts: 70

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Very Flaky, Weird, and Infuriating Network Anomalies on Ubuntu Box

Whoever fixes this gets a prize. :-)

I'm trying to (finally) replace my antique P3 home server with a newer box. So over the weekend I upgraded it from Ubuntu 7.04 to 7.10 (no problems, and I don't think this caused the problem ... see below), finished up my email server config (postfix with all the usual addons), all that happy stuff. Except I am seeing very strange problems:

1) When I try to send email from the box, postfix queues up the mail as a DNS failure. However, tcpdump shows no DNS request ever left the network interface. These outbound mail DNS lookups always fail. However, dig mx (or pick your domain name) works fine from bash.

2) When inbound mail connections come in, some work and some don't. Test messages from work. Messages from do not. Overall, about 50% of incoming mail from the world works, about 50% does not. It's all or nothing for each sending server; the ones that work always work, the ones that don't never do.

3) tcpdump shows the usual packet traces for the inbound mail that works. For the ones that fail, a TCP syn comes in from the source, and the box simply does not answer.

4) Before the Ubuntu version upgrade, I would sometimes run a torrent on this box (Linux mint .iso's). It would run for several hours, up to a day or so, and the NIC would simply stop working. I'm assuming that behavior and what I'm current wrestling with probably have the same cause.

Additional configuration info:

a) I have the MTU on eth0 set to 1420 bytes, to make sure my PPPOE DSL line doesn't fragment packets. The old server has the same MTU, and it works great.

b) File transfers, both on the local network and over the internet, work great. Perfect. No problems.

Here's the snippet from lspci identifying the NIC:

00:12.0 Ethernet controller: VIA Technologies, Inc. VT6102 [Rhine-II] (rev 78)

dmesg has nothing bad or unusual in it.

Kernel info:

[ 0.000000] Linux version 2.6.22-14-server (buildd@terranova) (gcc version 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2)) #1 SMP Tue Feb 12 08:27:05 UTC 2008 (Ubuntu 2.6.22-14.52-server)

Anyone have any idea at all?

Old 05-21-2008, 07:40 PM   #2
Registered: Nov 2006
Location: Seattle, WA
Distribution: Fedora
Posts: 407

Rep: Reputation: 35
Flaky, indeed. It sounds to me like a partition experiment is in order; you might have more than one problem. Have you tried replacing the offending Ethernet controller? Even if it's a chip on the mother board, a $15 PCI adapter might clear up some of the mist.

Then I would look pretty carefully at your outbound firewall rules to see if there is some rule blocking the Postfix MX record lookup. DNS is tricky, because it used to be that both source and destination ports were 53, but more recent BiNDs set destination port to 53 and source port to something above 1024. I'm not sure what dig does.
Old 05-21-2008, 08:34 PM   #3
Registered: Nov 2003
Posts: 70

Original Poster
Rep: Reputation: 17
Yep, a new NIC is the next troubleshooting step. The box isn't running a firewall, so that's not the issue. Not running BIND either; the lookups should simply go to the DNS server in resolv.conf.

Old 05-22-2008, 09:19 PM   #4
Registered: Nov 2003
Posts: 70

Original Poster
Rep: Reputation: 17
Well, just for drill, I backed up /etc/postfix and did apt-get --purge remove postfix. Reinstalled, restored the config files, and ... it's happy.

Go figure.



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