[tor-bugs] #31103 [Core Tor/Tor]: Support ORPort picking a random port that persists across restarts

[Tor Bug Tracker & Wiki]

#31103: Support ORPort picking a random port that persists across restarts
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     Reporter:  phw           |      Owner:  (none)
         Type:  enhancement   |     Status:  new
     Priority:  Medium        |  Milestone:
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:
Actual Points:                |  Parent ID:
       Points:  0.5           |   Reviewer:
      Sponsor:                |
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 A bridge's transport port and OR port are semi-secret. We don't want a
 bridge to listen on port 9001 because it would facilitate Internet-wide
 scanning: a censor could scan the entire IPv4 address space for port 9001
 and block all bridges they discover this way. We therefore encourage
 operators to not set `ServerTransportListenAddr`, which makes Tor pick a
 random port and write it to its state file, so it persists across
 restarts. Bridge operators can then whitelist this port in their firewall
 configuration.

 Bridge operators may welcome a similar option for `ORPort`. However, when
 setting `ORPort` to auto, Tor attempts to find a new port each time it
 starts. This means that operators would have to re-configure their
 firewalls after each restart.

 In the short term, we should instruct operators to pick their own ports
 and explicitly set them for both `ORPort` and `ServerTransportListenAddr`
 but in the long term we may want `ORPort` to be able to pick a random port
 and save it to Tor's state file.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/31103>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
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