I was talking on IRC with someone having strange symptoms using KTorrent on a DMZ'ed Ubuntu box. No, it's not the typical 99.9% stalling problem, but something more peculiar.

Basically, even with DHT off and sane limits on upload rate and max connections, KTorrent running on the machine will soon cause the machine to lose Internet connectivity. All other machines on his LAN are still able to access the net, and his DMZ machine is still able to make connections within the LAN, just not across the net. Stop KTorrent, and all returns to normal.

More interestingly, he reports all other torrent clients seem to work fine, without this symptom.

I can't think of anything that could cause this, except maybe KTorrent tries to be too aggressive in initiating new connections? Could he be hitting some half-open connection limit of his router's DMZ functionality, while other clients (obviously with Windows in mind) are minding the number of half-open connections they're keeping, before starting new ones?

I've asked for netstat output when he has a chance to reproduce this bug.

UPDATE:

Got the user to reproduce the problem for me, netstat shows 160 connections associated with KTorrent, 64 of which were in SYN_SENT state. I can see how so many half-open connections can potentially rain hell on routers. George/Ivan, perhaps we should have a config option for the max number of "in progress" outbound connections, and limit that to like 15 or so by default.

I think currently the number is 25 max simultaneous connection setups. So 64 shouldn't happen.

Was netstat taken on the DMZ box ? Or the router ?

netstat was taken on the DMZ box, and it clearly showed 60-70 connections in SYN_SENT state, all owned by KTorrent, even when max-per-torrent connections was limited down to 40. There seems to be something funny going on.

The limit is currently set to 20 per torrent, so with multiple torrents it is possible to get to that

Lets make this limit global, and the problem should go away.

OK, added a global outgoing connection setup limit, also made this configurable, so this problem should be fixed now.