Tor bridge selection

Question

Say I wish to use a lot of bridges, and my torrc looks like this:

#...
# other settings 

UseBridges 1
#
#obfs4 bridges
Bridge obfs4 so-and-so1
Bridge obfs4 so-and-so2
# ... and so on
#
#obfs3 bridges
Bridge obfs3 so-and-so1
Bridge obfs3 so-and-so2
# ... and so on
#
# fte bridges
Bridge fte so-and-so1
Bridge fte so-and-so2
#
# and then there's a list of scramblesuit bridges, meek bridges and any other sort of bridges out there

When I actually run tor, how will it choose bridges it'd use? Would the choice be random? Would it prefer obfs4? Once the choice is made, would it "rotate" bridges?

Answer 1

It'll pick the first one listed (in the order they're defined in the config) that works, this was problematic for Tor Browser's default bridges list, because the first bridge(s) in the list would be overloaded and make bootstrapping slow(er). They had to implement functionality to shuffle the set of bridges to avoid this (#18113).

There was previously #19875 to address this in tor itself but it looks like this has been superseded by the work on prop-271.

Depending on the implementation status of prop-271 (which looks set for 0.3.x), this should change so as not to use them in the order entered but it will still adhere to trying to use one guard(/bridge) for longer, so it should also "rotate" through them but at the timescale of normal guard rotation (months).

Both under the current behaviour or under prop-271 the pluggable transport won't be a factor in the choice of bridge.