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[liberationtech] Iran blocks MP3, MP4, AVI and SWF files

Fabio Pietrosanti (naif) lists at infosecurity.ch
Sat Oct 6 12:58:20 PDT 2012


On 10/6/12 9:41 PM, Collin Anderson wrote:
> What is particularly different in this case is that this filtering
> does not return a 403 Code or TCP RST -- when a GET request for a .mp3
> file is sent, the connection is simply blocked. The server can send
> whatever it likes, but the client will not see anymore traffic after
> that. This is abnormal, but it does not seem to be a DPI rule (zero'ed
> out .mp3: blocked, .mp3 renamed to .rawr: successful transmission). I
> pointed out a few days ago on Twitter that there will probably be new
> Internet interference in the wake of the rapid currency depreciation,
> this seems to align with that theory and the increased jamming of
> international broadcasting. It won't last but it is definitely a way
> to poke at the infrastructure.
>From technical point of view, what are they looking at?
File extension in URL requested, Content-Type or are they even finding
their own Content-Type?

Depending on how do they do that, it maybe easier to provide some
guidelines to workaround them.

And if they are not DPIing it with packet re-assembly, then we can use
the Philip Winter's techniques
(http://www.cs.kau.se/philwint/static/gfc/) of small-sized windows-size
on server-side to bypass their filter purely server-side
(https://github.com/NullHypothesis/brdgrd) that worked successfully in
stopping GFW Tor Bridge fingerprinting.

-naif



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