From: Brian Candler <b.candler@...>
Date: 2010-12-17T05:58:47+09:00
Subject: Re: How to do UDP calls

Luis G. wrote in post #968897:
> connect input
> 64-bit integer  connection_id  0x41727101980
> 32-bit integer  action          0
> 32-bit integer  transaction_id  16
>
> You told me that Array#pack will help me. This means that I need to put
> the the content of the connect input in an array and pack it so I can
> send it, right?
> Actually I thought we need to have something like:
> connection_id =>0x41727101980
> action => 0
> ...
>
> Anyway, see this example:
>
> data = Array.new
> data << 0x41727101980 #connection_id
> data << 0 #action
> data << 16 #transaction_id
>
> host = 'tracker.openbittorrent.com'
> port = 80
>
> s = data.pack('q') #q?!

There are three elements in the array 'data', so:
s = data.pack('qNN')

However 'q' may be wrong here, if you're on a little-endian machine. 
Unfortunately, Array#pack doesn't have a 64-bit network-order flag.

I think you should use something like this:

conn_id = 0x41727101780
s = [conn_id >> 32, conn_id & 0xffffffff, 0, 16].pack('NNNN')

Then for debugging, to check you're sending what you think:

STDERR.puts s.unpack("H*").first

> BTW, I tried
>
> echo "GET /announce" | nc -u tracker.openbittorrent.com 80
> sudo tcpdump -i eth0 -nn -s0 udp port 80
>
> and I got results:
> 18:09:14.719638 IP 192.168.182.123.57232 > 95.215.62.26.80: UDP, length
> 14

That's the outbound packet (from 192.168.182.123 local port 57232, to 
95.215.62.26 port 80), and you got no response.

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