Community ID spec appears to rely on underlying application implementation for src/dst direction determination

[conorpwr]

Hey,

While doing some research on various data we have, we've seen across two separate applications which saw the same flow, they each had their own interpretation of src/dst ordering.

You'll have to excuse the formatting but in application 1 (the data is fake but the example has happened), the flow was seen as:

src,srcp,dst,dstp
aaa,65000,bbb,443

While in application 2 it was reversed:

src,srcp,dst,dstp
bbb,443,aaa,65000

Looking at the spec itself, we appear to leave the ordering of the src/dst combination up to the individual application and if we had implemented community ID in both applications, we would have ended up with a different community ID for the same flow.

I would suggest that the spec is updated to specify how the src/dst combination should be calculated/sorted in order to ensure a consistent community ID is generated for the same flow (regardless of what the underlying application thinks the direction is).

Thanks,
Conor

[ckreibich]

Thanks for flagging, Conor. I looked over the spec as well as the three implementations (reference, Suricata, and Zeek). The spec indeed does not mention address/port ordering — a clear omission that we'll need to fix.

However, all three implementations actually do apply an address/port ordering function. I modeled the reference implementation's on Zeek's addr_port_canon_lt(), and @victorjulien mirrored the reference implementation in Suricata. So whatever's going on here is a bit more subtle — a corner case, byte ordering, some kind of mid-flow problem, etc. Are you able to share more details about the offending flow?

[conorpwr]

I did see some references in the scripts but my day to day knowledge of Bro & Suricata scripting left my unsure if this was happening or not. I can do some further debugging on the flows but that will happen outside this issue.

Thanks!
Conor

[ckreibich]

Sure thing, thanks Conor. I'll leave this ticket open for the time being ... I'm sure as we get more experience with producing the ID, we'll get more such feedback.