[tor-bugs] #32314 [Core Tor/Tor]: Can't connect to literal IPv6 address containing double colon

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#32314: Can't connect to literal IPv6 address containing double colon
--------------------------+------------------------------
 Reporter:  liberat       |          Owner:  (none)
     Type:  defect        |         Status:  new
 Priority:  Medium        |      Component:  Core Tor/Tor
  Version:  Tor: 0.4.1.6  |       Severity:  Normal
 Keywords:                |  Actual Points:
Parent ID:                |         Points:
 Reviewer:                |        Sponsor:
--------------------------+------------------------------
 When an application wants to use Tor's SOCKS port to connect to a known
 IPv6 address, it has a couple of options:

 - It can specify a 16-byte binary address using address type 4.

 - It can specify the address as an ASCII string using address type 3.

 If the address is specified as a string, Tor accepts IPv6 addresses either
 with or without brackets.  For example, Tor will accept either
 "2a01:4f8:fff0:4f:266:37ff:fe2c:5d19" or
 "[2a01:4f8:fff0:4f:266:37ff:fe2c:5d19]".

 However, if the address is abbreviated using double-colon notation, it
 only works if enclosed in brackets: "[2a00:1450:4001:800::200e]" works,
 but "2a00:1450:4001:800::200e" does not.  On the other hand, the
 unabbreviated form "2a00:1450:4001:800:0:0:0:200e" does work.

 The problem appears to be:

 - The destination is transmitted to the exit relay as a string of the form
 "<hostname>:<port>".

 - The exit relay tries to parse this string by calling the function
 tor_addr_port_split.

 - The string "2a00:1450:4001:800::200e:80" is a valid IPv6 literal, so
 tor_addr_port_split interprets it as an address with no port number.

 - The relay refuses to connect to an address with no port number.

 Note that if the application uses the binary form (address type 4), this
 is internally converted into a string enclosed in brackets.  However, it
 seems to be more common for applications to use the ASCII form, without
 brackets.  For example, if you try to visit
 http://[2a00:1450:4001:800::200e]/ in Tor Browser, it will fail, whereas
 http://[2a01:4f8:fff0:4f:266:37ff:fe2c:5d19]/ succeeds.

 So there are a few ways this could be fixed:

 (a) applications could be changed to use either the binary form or wrap
 the address in brackets;

 (b) the Tor proxy could automatically add brackets around IPv6 addresses;

 (c) the exit relay could be smarter about parsing IPv6 addresses.

 It seems to me that (b) would be the most sensible option, but it might be
 reasonable to do (c) as well.

 In the long term, I think it'd be wise to deprecate the use of IPv6
 addresses without brackets in RELAY_BEGIN, as well as any other places
 where tor_addr_port_split is used, because it's just confusing.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/32314>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online

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