[tbb-dev] Port regularity

David Fifield david at bamsoftware.com
Tue Oct 4 01:36:35 UTC 2016

On Mon, Oct 03, 2016 at 12:45:04PM -0600, Ben Mixon-Baca wrote:
> I am trying to determine how meaningful port numbers are for Tor. It
> seems like using fteproxy makes use of port 8080 regularly, while
> obfsproxy4 doesn't appear to use any regular port number? Is this an
> artifact or does obfs4 use a random port and fte use port 8080 by default?

obfs4proxy by default uses a random port number. (Actually it just binds
to port 0 the first time it runs, and takes whatever ephemeral port the
OS gives it. After that, it remembers and keeps using the same port.)
Details: https://trac.torproject.org/projects/tor/wiki/doc/PluggableTransports/obfs4proxy

Some bridge operators configure their obfs4proxy to listen on a
well-known port like 443. There's a torrc option to control this:
ServerTransportListenAddr. It's also common to use an iptables rule to
redirect a well-known port to the random obfs4 port.

You can get an idea by looking at the port numbers of the current
default bridges:
	1 × 10527
	1 × 13764
	1 × 1894
	1 × 2413
	1 × 2934
	1 × 4319
	1 × 50000
	1 × 5881
	4 × 80
	7 × 443
In general, I'd say it's common for obfs4 to run either on a random port
or on 80 or 443.

There are fewer fte bridges than obfs4 bridges, so it's harder to draw
general conclusions. All the default bridges are on 8080, it's true.
There are two server setup guides:
	says to choose your own port with ServerTransportListenAddr: https://fteproxy.org/help-server-with-tor
	uses a random ephemeral port: https://trac.torproject.org/projects/tor/wiki/doc/fte/setup
So it depends on how the operator sets it up. I don't know what's more

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