[tor-bugs] #21310 [Core Tor/Tor]: Fix IPv6Exit support in connection_exit_begin_conn()

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#21310: Fix IPv6Exit support in connection_exit_begin_conn()
------------------------------+------------------------------
     Reporter:  teor          |      Owner:
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: unspecified
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  ipv6
Actual Points:                |  Parent ID:
       Points:  1             |   Reviewer:
      Sponsor:                |
------------------------------+------------------------------
 This code is wrong for at least two reasons:
 * it should also unset BEGIN_FLAG_IPV6_OK, and
 * it's way too early in the function: we might end up resolving an
 IPv6-only hostname, learn that it doesn't match our exit policy, and send
 the address back in the REASON_EXITPOLICY RELAY_END cell
 (See https://gitweb.torproject.org/torspec.git/tree/tor-spec.txt#n1436 )

 {{{
   if (! options->IPv6Exit) {
     /* I don't care if you prefer IPv6; I can't give you any. */
     bcell.flags &= ~BEGIN_FLAG_IPV6_PREFERRED;
     /* If you don't want IPv4, I can't help. */
     if (bcell.flags & BEGIN_FLAG_IPV4_NOT_OK) {
       tor_free(address);
       relay_send_end_cell_from_edge(rh.stream_id, circ,
                                     END_STREAM_REASON_EXITPOLICY, NULL);
       return 0;
     }
   }
 }}}

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21310>
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