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BitTorrent kills 837

A couple of our lads installed uTorrent on their PCs, and very quickly we started having problems with DNS lookups failing. Eventually I traced it to a NAT problem on our 837 router. The size of the NAT table (from show ip nat statistics) would suddenly jump to about 1500 entries, all UDP, and at that point DNS lookups to external servers would fail and wouldn't work again until the router was reloaded.

Is this sort of thing normal? I use uTorrent at home behind a 837 and I've never encountered a problem. However at work it was quite reproducible. Using a packet sniffer I could actually watch the big burst of UDP packets from the PCs running uTorrent and I could see the NAT table filling up at the same time, and DNS then stop working. I didn't check whether it was just DNS that was affected or whether other UDP traffic was killed too.

I listed the router config. The guilty PCs weren't any of the statically NATed addresses, so they would have been dynamically NATed to the router's address.

JR

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Reply to
John Rennie
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I probably can't be of too much help as I'm not that experienced though I run an 837 here, and BitTorrent (uTorrent client) on occasion, and have found it can cause massive NAT tables if left unchecked (which I've seen consume all available DRAM). I use the command 'ip nat translation max-entries 2048' to place a limit here and see no problems occur when running with this many translations in use. It seems low to me the 1500 number you mention as I don't have any problems with this many (possibly a lack of available DRAM on your side?).

Oddly enough I've not had any problems with the 837 and the BitTorrent protocol... other than YESTERDAY... where for some reason the router kept 'dying', requiring a reload to fix, in that after a period of time I was losing net access completely other than with already established connections. It wasn't a DNS issue this though I don't believe as I couldn't even traceroute, or access a website by IP address, from inside the LAN. The router, however, had no problem tracing a remote site from console (IOS 'trace' command). It wasn't a memory issue either.

It's a mystery to me what happened exactly yesterday as this hasn't happened before. I did recently remove a line; 'ip nat translation tcp-timeout 900' from my configuration though, so maybe this had something to do with it (?).

Reply to
user

Followup: I just ran into this NAT problem again I think. Net connection stopped working correctly although I didn't have BitTorrent running at the time, though executing the command 'show ip nat statistics' in the router showed all 2048 NAT translations in use, probably due to other services I have running here. To clear this fault, rather than perform a reload, I just cleared out the translations tables using 'clear ip nat translation *' which brought back the connection immediately.

I don't know if that's similar to the problem you're seeing there though you might want to try clearing those NAT entries out to see if it cures the loss of connectivity. I'm going to re-add the 'tcp-timeout' command I removed recently (ip nat translation tcp-timeout 900) as I guess this was helping keep this problem from ocurring. You might want to look into the various timeouts and have a play with them unless somebody else knows a better way of preventing this (assuming it's causing the problem of course).

The Default NAT entry timeout values are;

timeout: 86,400 seconds (24 hours) udp-timeout: 300 seconds (5 minutes) dns-timeout: 60 seconds (1 minute) tcp-timeout: 86,400 seconds (24 hours) finrst-timeout: 60 seconds (1 minute) icmp-timeout: 60 seconds (1 minute) pptp-timeout: 86,400 seconds (24 hours) syn-timeout: 60 seconds (1 minute) seconds: 0 (never)

hth.

Reply to
user

Thanks, it does sound as if your experience is similar. One contributing factor may be that at home I statically NAT (strictly speaking PAT I suppose) the BitTorrent port to allow incoming connections, while at the office the port isn't statically NATed, and the lads may not have set up uTorrent correctly for this scenario.

I've fixed the problem by banning uTorrent! However I'll have a play with the settings you suggest and see what effect is has.

JR

Reply to
John Rennie

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