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Re: [tor-bugs] #24367 [Core Tor/Tor]: Changing pluggable transports (during start-up) in Tor Browser is broken
#24367: Changing pluggable transports (during start-up) in Tor Browser is broken
-------------------------------------------------+-------------------------
Reporter: gk | Owner: (none)
Type: defect | Status:
| needs_information
Priority: Medium | Milestone: Tor:
| 0.3.2.x-final
Component: Core Tor/Tor | Version: Tor:
| 0.3.0.1-alpha
Severity: Normal | Resolution:
Keywords: 030-backport, 031-backport, | Actual Points:
regression, tor-bridge-client, s8-errors, |
tbb-wants |
Parent ID: | Points: 0.5
Reviewer: | Sponsor:
| Sponsor8-can
-------------------------------------------------+-------------------------
Comment (by gk):
We debugged this a bit on IRC today and it turned out that a crucial part
in this bug seems to be that one of the `obfs3` and one of the `obfs4`
bridges share the same ID. So, if one chooses an `obfs3` bridge Tor
Launcher issues a `SETCONF` command like
{{{
[12-12 16:21:14] TorLauncher DBUG: Sending Tor command: SETCONF
UseBridges=1 Bridge="obfs3 109.105.109.163:38980
1E05F577A0EC0213F971D81BF4D86A9E4E8229ED" Bridge="obfs3
109.105.109.163:47779 4C331FA9B3D1D6D8FB0D8FBBF0C259C360D97E6A"
Bridge="obfs3 83.212.101.3:80 A09D536DD1752D542E1FBB3C9CE4449D51298239"
Bridge="obfs3 169.229.59.75:46328
AF9F66B7B04F8FF6F32D455F05135250A16543C9"
Bridge="obfs3 169.229.59.74:31493
AF9F66B7B04F8FF6F32D455F05135250A16543C9"
}}}
. When pressing `Cancel` during the bootstrap process and selecting the
`obfs4` option instead, Tor Launcher is sending a similar `SETCONF`
command. However, later on we easily get a bootstrap error like
{{{
Dec 12 17:21:35.000 [warn] Problem bootstrapping. Stuck at 85%:
Finishing handshake with first hop. (DONE; DONE; count 1;
recommendation warn; host A09D536DD1752D542E1FBB3C9CE4449D51298239 at
83.212.101.3:80)
}}}
The ID A09D536DD1752D542E1FBB3C9CE4449D51298239 is used by one of the
`obfs4` bridges as well (the IP is the same but the port is different
(50002 in this case)). It seems things are working much better if I remove
the `obfs3` bridge with A09D536DD1752D542E1FBB3C9CE4449D51298239 first
before starting. I did some tests and I never ran into the bootstrap
error. FWIW it is visible as well if one starts with `obfs4` first and
switches to `obfs3`, although it seems the error does not pop up outright
(as in the `obfs3` -> `obfs4` switch) but gets visible once I press
`Cancel` in Tor Launcher after the bootstrap process is stuck.
According to Nick the issue seems to be that Tor Browser will have a guard
entry for the bridge it was using before but it seems the lookup which is
done by fingerprint _and_ address is not working as expected.
So, that's the error case in 4) in my comment:description. I am not sure
about the stalling part yet. My current plan is to open a new ticket once
all open issues related to that one are fixed/merged and I can still come
up with a scenario to reproduce a stalled bootstrap process.
--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/24367#comment:26>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online
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