[tor-bugs] #21346 [Core Tor/Tor]: Clients with NoIPv4Traffic should only choose IPv6-supporting Exits

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#21346: Clients with NoIPv4Traffic should only choose IPv6-supporting Exits
-------------------------------------------------+-------------------------
 Reporter:  teor                                 |          Owner:  neel
     Type:  defect                               |         Status:
                                                 |  needs_revision
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  0.3.4.x-final
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  ipv6, 031-deferred-20170425,         |  Actual Points:
  032-unreached                                  |
Parent ID:  #21311                               |         Points:  0.5
 Reviewer:  dgoulet                              |        Sponsor:
-------------------------------------------------+-------------------------
Changes (by dgoulet):

 * status:  needs_review => needs_revision


Comment:

 Hmmm... I think the approach here is not ideal. It seems that the patch is
 looking at the _choosen_ exit node if we can use it with IPv6 and then
 unmark it.

 Instead, shouldn't we *pick* the node in the first place with IPv6 exit
 policy and not fail later? The function `connection_ap_can_use_exit()`
 seems to already performing validation on the exit node if the we have no
 IPv4 but IPv6 enabled.

 So I think we should do this filtering around `circuit_get_best()`?

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21346#comment:17>
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