Combining partial bittorrent downloads.

October 5, 2009 6:54 PM

Can I combine two partial bittorrent downloads? (OSX Leopard, Transmission)

I have two part-downloaded files, each obtained with Transmission, on different macs. I'd like to combine. Is there any way to do this that doesn't involve the data going out over my 'net connection to the swarm? It's the end of my billing month, and my data allowance is all gone.

posted by pompomtom to computers & internet (13 answers total)

All you should need to do is start the torrent clients up again on both machines, then turn your router off as soon as you see they've found each other and started exchanging pieces.
posted by flabdablet at 7:05 PM on October 5, 2009

If your router is also your LAN switch, just yank the ADSL plug instead of turning it off.
posted by flabdablet at 7:06 PM on October 5, 2009

If you have a crossover cable you can just add the 2nd computer as a peer on your client and exchange data.
posted by saxamo at 8:53 PM on October 5, 2009

Does Transmission have a way to do that explicitly? Because if it does, and you don't actually need to talk to the tracker, your existing router with the ADSL cable disconnected will work the same way a crossover cable would.
posted by flabdablet at 9:18 PM on October 5, 2009

Transmission has no explicit way to do this. I'm pretty sure Azureus used to have a 'prefer clients on your LAN' setting, which would do what I'm after. Will certainly give the brute "yank the ADSL line" approach a go when I get home.
posted by pompomtom at 9:44 PM on October 5, 2009

Home now, have tried disconnecting the net at the modem, (if it matters, the setup is modem->WAP/switch->clients), and each client stops transferring, while still claiming to see peers. Then the peer count drops, slowly, to zero.

Checking the peer display on each client, at no stage do I see a local IP, or my house's (static) IP, so I don't think they're ever transferring to each other.

Perhaps a bit more patience is in order...
posted by pompomtom at 2:01 AM on October 6, 2009

at no stage do I see a local IP, or my house's (static) IP, so I don't think they're ever transferring to each other

The cable yanking magic won't work until you do see your own static IP. Once that happens, you should be golden. Keep hitting the tracker up for more peers until you see yourself.

Alternatively, you could install Azureus/Vuze and set up a local tracker inside your LAN.
posted by flabdablet at 4:37 AM on October 6, 2009

Well, patience (and sitting in a pub with a good connection) won out. For future reference if anyone finds this thread: the problem seems to be that you can't get a connection between two machines behind the same NAT. Which is a bummer. Setting up a local tracker is probably the real answer here.
posted by pompomtom at 3:37 PM on October 7, 2009

you can't get a connection between two machines behind the same NAT

Seems to me that this should just happen automagically, provided they have different ports forwarded from the public side. What breaks it?
posted by flabdablet at 4:24 PM on October 7, 2009

flabdablet - the tracker tells each peer the *external* addresses of all the other peers... so say there's 2 hosts, 192.168.1.97 and 192.168.1.82, both natted behind 60.240.161.74... neither of the 192.168 addresses are stored by the tracker, just the 60.240 one... so both hosts will be trying to reach the other not directly on the LAN but via. the external interface of the router... and due to the way most consumer routers configure their interfaces, NAT translation and port forwarding rules aren't applied to packets unless they actually come in through the external interface (there will be "in" rules on the ppp0 interface to take care of this stuff, so if the packets don't come in over ppp0 then the rules aren't applied)...
posted by russm at 3:15 AM on October 8, 2009

Well, I guess I must have one of the rare and beautiful routers that does allow NAT to work properly even from inside the LAN. I have PortaPuTTY installed on a USB key, set up to ssh into one of my home computers via a forwarded port on the router's public IP address, and it works from inside the LAN as well as from outside.

My router is a Billion 7404VGP. What's yours, pompomtom?
posted by flabdablet at 5:22 AM on October 8, 2009

It's an iConnect 621 (modem/router).
posted by pompomtom at 4:19 PM on October 8, 2009

...and like russm says, if I accidentally use the external IP from inside my LAN (which happens more often than I should really admit), I get connected to the internal interface of the modem - which doesn't apply the port forwards or anything. In my case, that usually means getting the modem admin page, rather than the webserver which is on local 9091, forwarded from external port 80.
posted by pompomtom at 4:25 PM on October 8, 2009