Bit Torrent - won't download on specific computer

Question

I'm suddenly having issues with bit torrents downloading on my desktop computer.  I have tried multiple programs (deluge, transmission, and my daily use qbittorrent).  I've also tried with a separate user account.  On the same network, my laptop will work just fine (both connected wirelessly), so think its something system-related.  I even tried setting a manual port and port-forwarding via the router.  No matter the tracker it won't connect and just stall out.  I've tried reinstalling (libktorrent libtorrent libtorrent-rasterbar transmission-qt qbittorrent), but nothing seems to affect it.  What am I missing here?

Answer 1

It's not going to be an issue with port forwarding.  Bittorrent will work even if no ports are forwarded, you'll just see less peers connected.
You say it can't even connect to the tracker?  That sounds like you have a deeper networking problem not specific to Bittorrent.  I would check your network configuration.  Check your DNS servers, IP address (Make sure no two computers on your network have the same IP address.  That creates a conflict and causes all kinds of nasty and sometimes unpredictable problems), default gateway, and any firewall/iptables configuration you may have.
Another possibility if other internet applications are working on that machine is that your problem only affects UDP (HTTP web traffic uses TCP.)  A lot of Bittorrent trackers are switching from TCP to UDP.  There's a quick way to test UDP connectivity.  First, install nmap (pacman -S nmap)  Then, as root, run
nmap -p 60,80 -sU tracker.openbittorrent.com
This is the expected result:
Starting Nmap 6.46 ( http://nmap.org ) at 2014-07-10 04:56 CDT
Nmap scan report for tracker.openbittorrent.com (31.172.63.253)
Host is up (0.21s latency).
Other addresses for tracker.openbittorrent.com (not scanned): 31.172.63.252
PORT STATE SERVICE
60/udp closed unknown
80/udp open|filtered http
Nmap done: 1 IP address (1 host up) scanned in 5.74 seconds
Note the state of the ports.  The tracker is running on port 80 but since open UDP ports don't send a response back, nmap can't be sure if the port is truly open or if it's being filtered by a firewall.  That's why I also scanned the dummy port 60 so nmap would receive a closed response.  If nmap can differentiate between open and closed ports as demonstrated here, it means you're successfully sending and receiving UDP packets.  If it can't, you've got a deeper networking issue you need to solve.
Also, if you wish to test a TCP tracker, simply change the -sU to a -sS (and obviously change the ports and hostname since openbittorrent doesn't run a TCP tracker.)