I just noticed that KTorrent listens on udp6 when IPv6 is enabled for DHT, but disabling ipv6 forces it to listen on udpv4...

netstat -aln further reveals that it listens on another arbitrary UDP port (32768 in my case). A packet sniffer confirms that other torrent clients are responding to KTorrent on UDP port 32768 instead of my specified port 16881.

Disabling ipv6 corrected this behavior...

This appears like a bug to me.

For DHT we use KDE socket code, which uses IPv6. For the actual bittorrent protocol we only use IPv4 (we use our own socket code).

I'm not really sure how the KDE code handles all this. I will have to look into this.

Any update on this? I turned ipv6 back on the other day, after forgetting about this, and surely enough I see udp bound on 16891 ipv6 (my set port) and also on 32768 ipv4 (something I never set)

I'm currently looking at it

Well, I have found a way to make the KDE socket code only use IPv6 sockets, but forcing it to use only IPv4 doesn't seem to be possible.

The question is, how does this impacts port forwarding ?

I have done some looking around on the internet and it seems that if you open a IPv6 socket on linux, it will handle IPv4 without any problems.

As for this 32768 port, I also see that on my computer. So I fired up ethereal and it seems that this port is used for DNS communication.

So there is nothing to worry about.

Ah, ok, thanks for letting me know. That makes me feel better now.