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Old 11-21-2009, 06:26 PM
Jamie Thompson
Default Network stalls

I'm experiencing a strange problem that I think my Debian server may be
responsible for, but I've no idea how to troubleshoot it!

For the last month or so, I've been experiencing stalls - all access to
the internet stops for a minute or two. It happens frequently...but
intermittently. I assumed it was due to my ISP being rubbish...but then
I noticed that I couldn't access my DSL router's admin during these
stalls either.

Well, I decided to solve the mystery once and for all today, and I'm
even more confused now!

My setup is the rather conventional:
LAN<=>Eth0 - Server - Eth1<=>Router<=>Internet

The server is designated as the DMZ host on the router, and the server
NATs everything going out.

I found that when connected to the wireless router via wifi, internet
access remained constant throughout the stalls (pinging a remote host).
Pinging the same host from my server during the stalls was also fine.
Pinging the server's eth1, the router, or the remote host from my
workstation during a stall fails, but works otherwise.

This leads me to suspect that something's amiss with the NAT process
somehow. FWIW, both network cards on the server are gigabit, though the
router only supports 100Mb. Driver-wise, eth0 is using e1000, and eth1
is using r8169. An alternative is that the problem lies with the routing
between eth0 and eth1.

On the off-chance, I've tried turning off auto-negotiation, I've tried
updating the firmware on the router, and I've tried replacing the
cables, all to no avail.

I don't even know where to begin looking for info on what the problem
is. Any ideas?

- Jamie

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Old 11-30-2009, 12:57 PM
Jamie Thompson
Default Network stalls

So, I've spent all weekend looking into this, and I'm still no closer to
solving it.

I've tried replacing the NICs, swapping the switches, removing the
switches, isolating the machines, replacing the wiring, and logging the
iptables traffic.

That last one was quite interesting actually. I added log rules for ICMP
traffic to the nat table's prerouting and postrouting chains, and the
filter table's input, forward, and output chains.

When an outage is occurring, pinging the internal NIC from my
workstation shows up packets. Pinging the external NIC from my
workstation doesn't show a thing. The packets don't even seem to be
reaching the prerouting chain. Once the outage finishes, they start
appearing as normal. The server is able to ping both it's interfaces at
all times, as you'd expect. A laptop on the external network is able to
ping the external NIC, (but obviously not the internal one).

I get no reported dropped packets anywhere. I did notice some
rx_crc_errors on the internal NIC (using ethtool), (which is why I tried
replacing the wiring), but these don't seem to go up when the problem
occurs (i.e. they didn't increment at all during outages) - I'm going to
hazard that they're another issue entirely. ~1000 errors out of
~100000000 good receives doesn't suggest anything major

NIC-wise, I tried swapping to both being r8169, and both being e1000.
Identical results regardless of hardware involved.

My next port of call I guess has to be trying older kernels and seeing
if I get the same symptoms.

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