[tor-bugs] #20943 [Obfuscation/Obfsproxy]: Clarify documentation for obfs4 setup

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#20943: Clarify documentation for obfs4 setup
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 Reporter:  kaie                   |          Owner:  asn
     Type:  task                   |         Status:  new
 Priority:  Medium                 |      Milestone:
Component:  Obfuscation/Obfsproxy  |        Version:
 Severity:  Normal                 |     Resolution:
 Keywords:                         |  Actual Points:
Parent ID:                         |         Points:
 Reviewer:                         |        Sponsor:
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Comment (by dcf):

 Replying to [comment:5 kaie]:
 > ... but  ExtORPort auto  will listen on all network interfaces!

 That's not right. `ExtORPort` and `ServerTransportListenAddr` are
 completely separate things. You are correct, that
 `ServerTransportListenAddr` controls the external listening address and
 port for obfs4. `ExtORPort` is something else: it's the localhost port to
 which obfs4 forwards client traffic received from the outside. `ExtORPort`
 has nothing to do with the external listening address. `ExtORPort` can
 even change every time you run tor; it doesn't matter because the outside
 world never sees it. The point of `ExtORPort auto` is not to generate a
 random port number for censorship resistance; obfs4 already does that if
 you don't set `ServerTransportListenAddr` (and caches the port number so
 it will be stable).

 `ExtORPort` is not an external listening port. ("Ext" stands for
 "extended", not "external".) You should always set `ExtORPort auto` when
 running a bridge.

 It sounds like you might be having a problem with `OutboundBindAddress`
 instead. It is indeed a bug that pluggable transport plugins do not know
 about `OutboundBindAddress`: #5304.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/20943#comment:6>
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