[tor-bugs] #33817 [Core Tor/Tor]: Perform all IPv4 and IPv6 extend checks in one place

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#33817: Perform all IPv4 and IPv6 extend checks in one place
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     Reporter:  teor            |      Owner:  teor
         Type:  task            |     Status:  assigned
     Priority:  Medium          |  Milestone:  Tor: 0.4.4.x-final
    Component:  Core Tor/Tor    |    Version:
     Severity:  Normal          |   Keywords:  ipv6, prop311
Actual Points:                  |  Parent ID:  #33220
       Points:  1               |   Reviewer:
      Sponsor:  Sponsor55-must  |
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 Currently, tor checks that extend cells have IPv4 addresses in:
 * some functions in circuitbuild_relay.c (a new file introduced by #33633)
 * check_extend_cell() in onion.c
 * extend_cell_from_extend2_cell_body() in onion.c
   * (note that all relays that support IPv6 extends should be using
 extend2 cells, but we want to modify this code for consistency)
 * and possibly other functions.

 We want to perform all these checks in the same place, so we can modify
 tor's behaviour based on:
 * tor's configuration
   * including consensus parameters
 * the reachability of a relay's own IPv6 ORPort, and
 * any other relevant factors.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/33817>
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