[tor-dev] "Seeing through Network-Protocol Obfuscation"

Yawning Angel yawning at schwanenlied.me
Sat Aug 22 23:34:53 UTC 2015

On Sat, 22 Aug 2015 14:40:08 -0700
Kevin P Dyer <kpdyer at gmail.com> wrote:

> Ah, gotcha. It's not RFC compliant. RFC2616 was created in 1999 and
> there are tons of HTTP-like implementations since then that,
> ostensibly, don't need to follow it. (e.g., an HTTP-like
> client/server that only talk to each other.) A network monitor must
> deal with these cases too, and they'll broadcast HTTP/1.1 in their
> headers.
> This [1] paper is a bit dated (2007) but my intuition is that
> real-world implementations have drifted even further from the RFC
> over the last 8 years. I swear there's a more recent paper on this
> topic, but I couldn't find it...

I'd be surprised if there were lots of clients that advertise HTTP/1.1
that don't include a Host header, since clients that are broken in that
manner will not be able to talk to apache/ngnix/tomcat/etc[0].

Then again, fteproxy is an example of such a thing, so I may be rather
sad at the results of an actual survey.

> > Since requests of that sort should invoke the error path on RFC
> > compliant servers it's a really good distinguisher since legitimate
> > clients will not do such a thing.  Existing realistic adversaries
> > already have "identify 'suspicious behavior', call back to confirm"
> > style filtering in production, so false positive rate can be reduce
> > to 0 if needed.
> >
> Based on our exploration of data, we found there's a wide range of
> implementations and most of which have non-RFC-compliant behaviors.
> See Section 4 of our paper for more details. For that reason I'd be
> very surprised if a host-header-check could result in a 0 FP rate.

The point isn't to use non-compliance as the sole discriminator (since
people do write broken code), but to cut down the candidate IP/Port
list down to something that's reasonable for whatever active probing
infrastructure that exists to manage.

From there, delta-T later with separate infrastructure attempt a full
FTE + Tor handshake, and blacklist/RST inject/etc target candidates that

The second step gets to 0 FP, and precisely this sort of thing is how
China currently handles obfs3. The delay (anecdotal) is about 10 mins.

Intuitively I think that "missing Host header" will be extremely rare
but I don't have a way to get traces to prove/disprove it.

> With that being said, I'll add the host-header-check to the list of
> experiments that we want to do for the full version of our paper.
> Would be interesting to learn what the data tells us.

I would be interested in seeing the results.

Yawning Angel

[0]: Ngnix supports hooking the error handler rather easily, apache
less so.
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