From: Brian Candler <b.candler@...>
Date: 2010-12-17T00:13:28+09:00
Subject: Re: How to do UDP calls

Luis G. wrote in post #968857:
> As you might know, you can have this:
>
> http://9.rarbg.com:2710/announce
> udp://tracker.openbittorrent.com:80/announce

No, I haven't seen the udp:// URL format at all. Is there an RFC which 
describes it?

> For the first one (HTTP) I just use this:
...
> But for the second one (UDP) I can't use Mechanize, so I need to work
> with the standard library as you told before.

And not only that, you need to know what format data to put *inside* the 
UDP packet. That's what I was getting at. You need some documentation of 
the application-layer protocol, which sits inside the transport layer 
protocol.

I guess it *might* be HTTP, complete with a full set of HTTP headers. 
Worth a try, if you have a valid URL to point at.

You can use netcat (nc) to test:

    echo "GET /announce" | nc -u tracker.openbittorrent.com 80

and run

    sudo tcpdump -i eth0 -nn -s0 udp port 80

in another window to check for any replies. When I try that particular 
example, I get nothing back.

> 1- The main idea is to create a socket, connect it to the remote host,
> send the request and get the response, right?

I guess so...

> 2- do I need to include the 'udp://' in the host?

Nope. The socket functions take hostnames/IP addresses, not URLs. There 
is a 'URI' class you can use to parse it out of the URL though.

> 3- is ok to include the name of the field I want to send (?info_hash=)
> in the data variable?

That I have no idea, because I've not seen any documentation for this 
protocol.

> 4- how can I get the response from the server? something like:
> sock.recvfrom(1024)?

Yes, recvfrom returns a tuple of [data, address_info]. Or I believe you 
could 'connect' the socket first, send, recv.

> I tried several options and I had no success.
>
> Any clues?

Look at a C implementation of this protocol, and port it to ruby? 
tcpdump -X an existing working client, and see what it sends/receives? 
Ask on a bittorrent development mailing list?

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