[tor-bugs] #32637 [Core Tor/Tor]: SocksPort IPv6 flags differs in default config and in Torlauncher prefs, and exits can distinguish them

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#32637: SocksPort IPv6 flags differs in default config and in Torlauncher prefs,
and exits can distinguish them
-------------------------+------------------------------
 Reporter:  cypherpunks  |          Owner:  (none)
     Type:  defect       |         Status:  new
 Priority:  Medium       |      Component:  Core Tor/Tor
  Version:               |       Severity:  Normal
 Keywords:               |  Actual Points:
Parent ID:               |         Points:
 Reviewer:               |        Sponsor:
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 By default tor daemon set only IPv6Traffic flag in client sockets, but not
 PreferIPv6
 https://gitweb.torproject.org/tor.git/tree/src/app/config/config.c#n6094

 But Torlauncher sets them both when launch tor daemon with Tor Browser
 https://gitweb.torproject.org/tor-
 launcher.git/tree/src/defaults/preferences/torlauncher-prefs.js#n41

 As i can see in spec, this flags sets bits in "FLAGS" value in RELAY_BEGIN
 cell, and exit relays can recognize which flags sets in client port
 settings. So, exits can distinguish circuits from Tor Browser Bundle
 clients and, as example, from daemons in linux distro repositories, with
 high level of probability.
 https://gitweb.torproject.org/torspec.git/tree/tor-spec.txt#n1595

 Is there any reasons why PreferIPv6 flag is not turned on by default? This
 issue raises up in past (#21269), but solution is not efficient - anyway
 flags sent to exit by default differs from vast majority of real usecases
 - surfing web with TBB.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/32637>
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