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GSoC Week 10: Finding that damn little bug

Finally!! I found a bug which I was after for the last week. Now I finally got that little bas****.

The bug happened in my code for the Jingle SOCKS5 Bytestream Transport (XEP-0260). SOCKS5 proxies are used whenever the two endpoints can’t reach one another directly due to firewalls etc. In such a case, another entity (eg. the XMPP server) can jump in to act as a proxy between both endpoints. For that reason, the initiator (Alice) first collects available proxies, and sends them over to the responder (Bob). The responder does the same and sends its candidates back to the initiator. Both then try to connect to the candidates (in this case proxies) they got sent from their peer. In order for the proxy to know, who wants to talk to whom, both include a destination address, which is calculated as SHA1(sid, providerJid, targetJid), where the provider is the party which sent the candidates to the target.

The alert reader will know, that there are two different destination addresses in the game by now. The first one being SHA1(sid, Alice, Bob) and the second one SHA1(sid, Bob, Alice). The issue is that somewhere in the logs I ended up with 3 different destination addresses. How the hell did that happen. For the answer lets look at an example stanza:

<iq from=’romeo@montague.lit/orchard’
id=’xn28s7gk’
to=’juliet@capulet.lit/balcony’
type=’set’>
<jingle xmlns=’urn:xmpp:jingle:1′
action=’session-initiate’
initiator=’romeo@montague.lit/orchard’
sid=’a73sjjvkla37jfea’>
<content creator=’initiator’ name=’ex’>
<description xmlns=’urn:xmpp:example’/>
<transport xmlns=’urn:xmpp:jingle:transports:s5b:1′
dstaddr=’972b7bf47291ca609517f67f86b5081086052dad’
mode=’tcp’
sid=’vj3hs98y’>
<candidate cid=’hft54dqy’
host=’192.168.4.1′
jid=’romeo@montague.lit/orchard’
port=’5086′
priority=’8257636′
type=’direct’/>
</transport>
</content>
</jingle>
</iq>

Here we have a session initiation with a Jingle SOCKS5 Bytestream transport. The transport exists of one candidate. Now where was my error?

You might have noted, that there are two attributes with the name ‘sid’ in the stanza. The first one is the so called session id, the id of the session. This should not be of interest for our case. The second one however is the stream id. Thats the sid that gets crunched in the SHA1 algorithm to produce the destination address.

Well, yeah… In one tiny method used to update my transport with the candidates of the responder, I used session.getSid() instead of transport.getSid()… That was the bug, that cost me a week.

Now it wasn’t too bad. While I searched for the error, I read through the XEPs again and again, discovering some more issues which I discussed with other developers. Also I began testing my implementation against Gajim and I’m happy to tell you that the InBand Bytestream code is already working sort of. Sometimes a few bytes get lost, but we live in times of Big Data, so thats not too bad, am I right :P ?

In the last 3 weeks I plan to stabilize the API some more. Currently you can only receive data into files, but I plan to add another method which gives back a bytestream instead.

Also I need more tests. Things like that nasty sid bug can be prevented and found using junit tests, so I’ll definitely stock up on that front.

Thats all for now :) Happy Hacking!

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