parsing packets

[Thomas Rachel]

Am 10.07.2011 22:59 schrieb Littlefield, Tyler:
> Hello all:
> I'm working on a server that will need to parse packets sent from a
> client, and construct it's own packets.

Are these packets sent as separate UDP packets or embedded in a TCP 
stream? In the first case, you already have packets and only have to 
parse them. In a stream, you first have to split them up.

In the following, I will talk about UDP datagrams. For TCP, further work 
is needed.


> The setup is something like this: the first two bytes is the type of the
> packet.

Then you have

type = struct.unpack(">H", packet),
payload1 = packet[2:]

> So, lets say we have a packet set to connect. There are two types of
> connect packet: a auth packet and a connect packet.
> The connect packet then has two bytes with the type, another byte that
> notes that it is a connect packet, and 4 bytes that contains the version
> of the client.

if type == CONNECT:
     subtype = struct.unpack("B", payload1)
     payload2 = payload1[1:]
     if subtype == CONNECT:
         upx = payload2.split("\0")
         assert len(upx) == 3 and upx[-1] == ''
         username, password = upx[:2]
     else:
         assert len(payload2) == 4
         version = struct.unpack(">L", payload2)


> The auth packet has the two bytes that tells what packet it is, one byte
> denoting that it is an auth packet, then the username, a NULL character,
> and a password and a NULL character.


> With all of this said, I'm kind of curious how I would 1) parse out
> something like this (I am using twisted, so it'll just be passed to my
> Receive function),

I. e., you already have your packets distinct? That's fine.

 > and how I get the length of the packet with multiple NULL values.

With len(), how else?


 > I'm also looking to build a packet and send it back out, is
> there something that will allow me to designate two bytes, set
> individual bits, then put it altogether in a packet to be sent out?

The same: with struct.pack().


Thomas