[tor-bugs] #25528 [Core Tor/Tor]: When ClientTransportPlugin is missing, tor connects directly to bridge addresses, even if they have a transport name

[Tor Bug Tracker & Wiki]

#25528: When ClientTransportPlugin is missing, tor connects directly to bridge
addresses, even if they have a transport name
-------------------------+-------------------------------------------------
     Reporter:  dcf      |      Owner:  (none)
         Type:  defect   |     Status:  new
     Priority:  Medium   |  Milestone:
    Component:  Core     |    Version:  Tor: unspecified
  Tor/Tor                |   Keywords:  pt bootstrap bridge-client bridge-
     Severity:  Normal   |  bypass
Actual Points:           |  Parent ID:
       Points:           |   Reviewer:
      Sponsor:           |
-------------------------+-------------------------------------------------
 Start `tcpdump -n host 83.212.101.3`

 Run tor with this torrc:
 {{{
 UseBridges 1
 Bridge obfs4 83.212.101.3:50002 A09D536DD1752D542E1FBB3C9CE4449D51298239
 cert=lPRQ/MXdD1t5SRZ9MquYQNT9m5DV757jtdXdlePmRCudUU9CFUOX1Tm7/meFSyPOsud7Cw
 iat-mode=0
 }}}

 See a connection to 83.212.101.3:50002, despite that, lacking a
 `ClientTransportPlugin` line, tor doesn't know how to connect to an
 "obfs4" bridge.

 Another way to see it is with this torrc, using a phony address like meek
 and snowflake do:
 {{{
 UseBridges 1
 Bridge dummy 0.0.3.0:1
 }}}
 tor actually tries to connect to the 0.0.3.0:1 address, and fails with an
 "Invalid argument" error:
 {{{
 [warn] Problem bootstrapping. Stuck at 5%: Connecting to directory server.
 (Invalid argument; RESOURCELIMIT; count 1; recommendation warn; host
 0000000000000000000000000000000000000000 at 0.0.3.0:1)
 }}}

 I expected instead that tor would not try to connect to the address, but
 rather would show
 [https://gitweb.torproject.org/tor.git/tree/src/or/connection_or.c?h=tor-0.3.2.10#n1231
 this error message]:
 > We were supposed to connect to bridge 'X' using pluggable transport 'Y',
 but we can't find a pluggable transport proxy supporting 'Y'. This can
 happen if you haven't provided a ClientTransportPlugin line, or if your
 pluggable transport proxy stopped running.

 The problem exists in both 0.2.9.14 and 0.3.4.0-alpha-dev, which are the
 two versions I tested.

 I found this problem through a user report at #25527. The user was trying
 to run the Tor Browser tor, but they were in the wrong directory, so they
 were only getting torrc and not torrc-defaults. torrc contains
 `UseBridges` and `Bridge`, but torrc-defaults contains
 `ClientTransportPlugin`.

 There was another ticket about tor occasionally connecting to PT bridges
 as if they were ordinary guards: #20532. It may be the same as this. At
 #25527 I speculated that the problem might have been caused by cached
 `Guard` entries, but that doesn't seem to be the case. All you have to do
 is omit the `ClientTransportPlugin` line.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25528>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online