Mailing List Archive: Qmail: users

Odd tcpserver issue

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tim at icukhosting

Jul 1, 2009, 7:39 AM

Post #1 of 3 (691 views)
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Odd tcpserver issue

Hi,

Does anyone know why tcpserver would, after running with the same
configuration for 2 years, suddenly hang on every connection when
listening on a private IP address, unless the -l option was specified?

We have tcpserver running on a 10 node cluster of qmail machines, with
the below:

tcpserver -R -H -v -p -x /var/qmail/tcp.smtp.cdb -c "$MAXSMTPD" \
-u "$QMAILDUID" -g "$NOFILESGID" 192.168.2.11 smtp qmail-smtpd \
checkpassword true 2>&1 5>&1 4>&1

The system has been running almost flawlessly for close to 2 years, and
no changes have been made to the configuration, the local network, the
local nameservers, or anything in the data centre.

Adding -l 0 to the command instantly resolved the problem, but what we
can't understand, is why tcpserver would stop accepting the NXDOMAIN
responses when trying to query the private IP and hang, after being
seemingly happy with them up until now!

Any insight into what might have caused this would be greatly appreciated.

Thanks,
Tim

sgifford at suspectclass

Jul 1, 2009, 7:50 AM

Post #2 of 3 (626 views)
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Re: Odd tcpserver issue [In reply to]

Tim O'Donovan <tim[at]icukhosting.co.uk> writes:

[...]

> Adding -l 0 to the command instantly resolved the problem, but what we
> can't understand, is why tcpserver would stop accepting the NXDOMAIN
> responses when trying to query the private IP and hang, after being
> seemingly happy with them up until now!

As you suggest, this is really a DNS issue, so any part of the DNS
that's used could affect this. Perhaps instead of receiving NXDOMAIN
it is receiving no response and so waiting for a timeout. Perhaps the
local name resolution library, local recursive resolver, an upstream
recursive resolver, the end name server for these IP addresses, or any
of the parent name servers has changed its behavior.

tcpserver should be doing basically the same thing as the djbdns tool
"dnsname" or the BIND tool "dig", so you should be able to
troubleshoot that way.

But really, hardcoding a name with -l is more efficient anyways, so
you could just leave it like that instead.

-----Scott.

kyle-qmail at memoryhole

Jul 1, 2009, 8:36 AM

Post #3 of 3 (631 views)
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Re: Odd tcpserver issue [In reply to]

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Hash: SHA256

On Wednesday, July 1 at 03:39 PM, quoth Tim O'Donovan:
> Does anyone know why tcpserver would, after running with the same
> configuration for 2 years, suddenly hang on every connection when
> listening on a private IP address, unless the -l option was
> specified?

Sounds like something DNS-related changed, such that it could no
longer quickly resolve its own IP address.

> The system has been running almost flawlessly for close to 2 years,
> and no changes have been made to the configuration, the local
> network, the local nameservers, or anything in the data centre.

Were any changes of any kind made? As you know, the -l option merely
tells tcpserver not to look up its own name in DNS. So, if your server
cannot look up its own name in DNS, that will cause the problem you
describe. Perhaps the machine uses DHCP and got a new IP address?
Perhaps the machine uses some sort of distributed /etc/hosts mechanism
that was wrong? I don't know... but it'll be something like that.

> Adding -l 0 to the command instantly resolved the problem, but what
> we can't understand, is why tcpserver would stop accepting the
> NXDOMAIN responses when trying to query the private IP and hang,
> after being seemingly happy with them up until now!

It sounds like it wasn't actually *getting* the NXDOMAIN responses;
because the hang is the result of waiting for the DNS server to
respond.

~Kyle
- --
You must be willing to accept the fact that pain is a part of the
process of revelation.
-- Stanley Crouch
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