[tor-bugs] #21310 [Core Tor/Tor]: Exits should tell clients when they are connecting to an IPv6-only hostname

[Tor Bug Tracker & Wiki]

#21310: Exits should tell clients when they are connecting to an IPv6-only hostname
-------------------------------------------------+-------------------------
 Reporter:  teor                                 |          Owner:  teor
     Type:  defect                               |         Status:
                                                 |  needs_revision
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  unspecified
Component:  Core Tor/Tor                         |        Version:  Tor:
                                                 |  0.2.4.7-alpha
 Severity:  Normal                               |     Resolution:
 Keywords:  ipv6, 034-triage-20180328,           |  Actual Points:  0.1
  034-removed-20180328                           |
Parent ID:  #17811                               |         Points:  1
 Reviewer:                                       |        Sponsor:
-------------------------------------------------+-------------------------
Description changed by teor:

Old description:

> Edit: Turns out that these IPv6Exit option checks prevent clients ever
> seeing that they're trying to connect to an IPv6-only hostname
>
> 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
> )~~
> (Apparently this works anyway.)
>
> {{{
>   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;
>     }
>   }
> }}}

New description:

 When #21311 is finished, we need to make exits tell clients that the
 hostname they asked for is IPv6-only

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21310#comment:21>
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