bridge and bridge authority proposal
arma at mit.edu
Sun Nov 11 20:54:52 UTC 2007
On Sun, May 06, 2007 at 11:42:25PM -0400, Roger Dingledine wrote:
> Here are some details on my plans for bridges and bridge authorities.
> They're still fluid because I haven't actually built it, so it's hard
> to know if they will turn out to be the right plans when it comes down
> to coding, but it's at least a start.
Here is a much more fleshed out version, describing the current design
as well as the current state of implementation.
Behavior for bridge users, bridge relays, and bridge authorities
This document describes the design decisions around support for bridge
users, bridge relays, and bridge authorities. It acts as an overview
of the bridge design and deployment for developers, and it also tries
to point out limitations in the current design and implementation.
For more details on what all of these mean, look at blocking.tex in
1. Bridge relays
Bridge relays are just like normal Tor relays except they don't publish
their server descriptors to the main directory authorities.
To configure your relay to be a bridge relay, just add
to your torrc. This will cause your relay to publish its descriptor
to the bridge authorities rather than to the default authorities.
Alternatively, you can say
which will cause your relay to not publish anywhere. This could be
useful for private bridges.
1.2. Defining DirPort
Bridges need to answer BEGIN_DIR requests, both so they can answer
"/server/authority" questions ("what's your descriptor?") and so they
can supply their bridge users with cached copies of all the various
Tor network information.
Right now (0.2.0.11-alpha) we require that bridges turn their DirPort on
-- which means both that we answer BEGIN_DIR requests and that we fetch
and cache directory information in an aggressive way like other servers.
a) we don't enforce that DirPort is on, since it's not clear how to
detect if the user meant to be a bridge. So it's easy to set up a bridge
relay that silently refuses BEGIN_DIR requests and is thus useless.
b) We don't actually care if they have an open or reachable DirPort. So
at some point we should separate having an open DirPort from answering
directory questions. Which leads to:
c) We need to investigate if there are any anonymity worries with
answering BEGIN_DIR requests when our DirPort is off. If there aren't,
we should drop the DirPort requirement.
I claim that we don't open any new attacks by answering BEGIN_DIR
questions when DirPort is off: it's still a fine question to ask what
partitioning attacks there are when you can query a Tor client about
its current directory opinions, but these attacks already exist when
DirPort is on.
We need to answer this issue in 0.2.0.x.
1.3. Exit policy
Bridge relays should use an exit policy of "reject *:*". This is
because they only need to relay traffic between the bridge users
and the rest of the Tor network, so there's no need to let people
exit directly from them.
1.4. RelayBandwidthRate / RelayBandwidthBurst
We invented the RelayBandwidth* options for this situation: Tor clients
who want to allow relaying too. See proposal 111 for details. Relay
operators should feel free to rate-limit their relayed traffic.
1.5. Helping the user with port forwarding, NAT, etc.
Just as for operating normal relays, our documentation and hints for
how to make your ORPort reachable are inadequate for normal users.
We need to work harder on this step, perhaps in 0.2.1.x.
1.6. Vidalia integration
Vidalia 0.0.15 has turned its "Relay" settings page into a tri-state
"Don't relay" / "Relay for the Tor network" / "Help censored users".
If you click the third choice, it forces your exit policy to reject *:*,
and it forces your DirPort to 9030 (but see Sec 1.2 above about DirPort).
If all the bridges end up on port 9001, that's not so good. On the
other hand, putting the bridges on a low-numbered port in the Unix
world requires jumping through extra hoops. The current compromise is
that Vidalia makes the ORPort default to 443 on Windows, and 9001 on
At the bottom of the relay config settings window, Vidalia displays
the bridge identifier to the operator (see Section 3.1) so he can pass
it on to bridge users.
1.7. What if the default ORPort is already used?
If the user already has a webserver or some other application
bound to port 443, then Tor will fail to bind it and complain to the
user, probably in a cryptic way. Rather than just working on a better
error message (though we should do this), we should consider an
"ORPort auto" option that tells Tor to try to find something that's
bindable and reachable. This would also help us tolerate ISPs that
filter incoming connections on port 80 and port 443. But this should
be a different proposal, and can wait until 0.2.1.x.
2. Bridge authorities.
Bridge authorities are like normal directory authorities, except they
don't create their own network-status documents or votes. So if you
ask an authority for a network-status document or consensus, they
behave like a directory mirror: they give you one from one of the main
authorities. But if you ask the bridge authority for the descriptor
corresponding to a particular identity fingerprint, it will happily
give you the latest descriptor for that fingerprint.
To become a bridge authority, add these lines to your torrc:
Right now there's one bridge authority, running on the Tonga relay.
2.1. Exporting bridge-purpose descriptors
We've added a new purpose for server descriptors: the "bridge"
purpose. With the new router-descriptors file format that includes
annotations, it's easy to look through it and find the bridge-purpose
We should work with Tonga to export its router-descriptors file to
some place where we can distribute the bridge addresses according to
the policies in blocking.pdf. It might even be easier to have it write
out a separate file, just for export, that includes only the bridge
descriptors; or maybe a six-liner perl postprocessing script is the
better plan there to create a file for export.
2.2. Reachability/uptime testing
Right now the bridge authorities just passively collect bridge
descriptors, and give them out on request. At some point we are going
to want to recommend new bridges to users, and we'll want to have
some way of deciding which ones are up right now, which ones have
been around for a while, etc. We should have the bridge authorities
do active measurements of bridges just as the normal authorities do
active measurements of normal relays. Then we can export the results
just like in Section 2.1. above.
In the design document, we suggested that bridges should publish
anonymously (i.e. via Tor) to the bridge authority, so somebody watching
the bridge authority can't just enumerate all the bridges. But if we're
doing active measurement, the game is up. Perhaps we should back off on
this goal, or perhaps we should do our active measurement anonymously?
Answering this issue is scheduled for 0.2.1.x.
2.3. Migrating to multiple bridge authorities
Having only one bridge authority is both a trust bottleneck (if you
break into one place you learn about every single bridge we've got)
and a robustness bottleneck (when it's down, bridge users become sad).
Right now if we put up a second bridge authority, all the bridges would
publish to it, and (assuming the code works) bridge users would query
a random bridge authority. This resolves the robustness bottleneck,
but makes the trust bottleneck even worse.
In 0.2.1.x and later we should think about better ways to have multiple
3. Bridge users.
Bridge users are like ordinary Tor users except they use encrypted
directory connections by default, and they use bridge relays as both
entry guards (their first hop) and directory guards (the source of
all their directory information).
To become a bridge user, add the following two lines to your torrc:
and then add at least one "Bridge" line to your torrc based on the
3.1. Format of the bridge identifier.
The canonical format for a bridge identifier contains an IP address,
an ORPort, and an identity fingerprint:
bridge 18.104.22.168:9009 4C17 FB53 2E20 B2A8 AC19 9441 ECD2 B017 7B39 E4B1
However, the identity fingerprint can be left out, in which case the
bridge user will connect to that relay and use it as a bridge regardless
of what identity key it presents:
This might be useful for cases where only short bridge identifiers
can be communicated to bridge users.
In a future version we may also support bridge identifiers that are
only a key fingerprint:
bridge 4C17 FB53 2E20 B2A8 AC19 9441 ECD2 B017 7B39 E4B1
and the bridge user can fetch the latest descriptor from the bridge
authority (see Section 3.4).
3.2. Bridges as entry guards
For now, bridge users add their bridge relays to their list of "entry
guards" (see path-spec.txt for background on entry guards). They are
managed by the entry guard algorithms exactly as if they were a normal
entry guard -- their keys and timing get cached in the "state" file,
etc. This means that when the Tor user starts up with "UseBridges"
disabled, he will skip past the bridge entries since they won't be
listed as up and usable in his networkstatus consensus. But to be clear,
the "entry_guards" list doesn't currently distinguish guards by purpose.
Internally, each bridge user keeps a smartlist of "bridge_info_t"
that reflects the "bridge" lines from his torrc along with a download
schedule (see Section 3.5 below). When he starts Tor, he attempts
to fetch a descriptor for each configured bridge (see Section 3.4
below). When he succeeds at getting a descriptor for one of the bridges
in his list, he adds it directly to the entry guard list using the
normal add_an_entry_guard() interface. Once a bridge descriptor has
been added, should_delay_dir_fetches() will stop delaying further
directory fetches, and the user begins to bootstrap his directory
information from that bridge (see Section 3.3).
Currently bridge users cache their bridge descriptors to the
"cached-descriptors" file (annotated with purpose "bridge"), but
they don't make any attempt to reuse descriptors they find in this
file. The theory is that either the bridge is available now, in which
case you can get a fresh descriptor, or it's not, in which case an
old descriptor won't do you much good.
We could disable writing out the bridge lines to the state file, if
we think this is a problem.
As an exception, if we get an application request when we have one
or more bridge descriptors but we believe none of them are running,
we mark them all as running again. This is similar to the exception
already in place to help long-idle Tor clients realize they should
fetch fresh directory information rather than just refuse requests.
3.3. Bridges as directory guards
In addition to using bridges as the first hop in their circuits, bridge
users also use them to fetch directory updates. Other than initial
bootstrapping to find a working bridge descriptor (see Section 3.4
below), all further non-anonymized directory fetches will be redirected
to the bridge.
This means that bridge relays need to have cached answers for all
questions the bridge user might ask. This makes the upgrade path
tricky --- for example, if we migrate to a v4 directory design, the
bridge user would need to keep using v3 so long as his bridge relays
only knew how to answer v3 queries.
In a future design, for cases where the user has enough information
to build circuits yet the chosen bridge doesn't know how to answer a
given query, we might teach bridge users to make an anonymized request
to a more suitable directory server.
3.4. How bridge users get their bridge descriptor
Bridge users can fetch bridge descriptors in two ways: by going directly
to the bridge and asking for "/tor/server/authority", or by going to
the bridge authority and asking for "/tor/server/fp/ID". By default,
they will only try the direct queries. If the user sets
in his config file, then he will try querying the bridge authority
first for bridges where he knows a digest (if he only knows an IP
address and ORPort, then his only option is a direct query).
If the user has at least one working bridge, then he will do further
queries to the bridge authority through a full three-hop Tor circuit.
But when bootstrapping, he will make a direct begin_dir-style connection
to the bridge authority.
As of Tor 0.2.0.10-alpha, if the user attempts to fetch a descriptor
from the bridge authority and it returns a 404 not found, the user
will automatically fall back to trying a direct query. Therefore it is
recommended that bridge users always set UpdateBridgesFromAuthority,
since at worst it will delay their fetches a little bit and notify
the bridge authority of the identity fingerprint (but not location)
of their intended bridges.
3.5. Bridge descriptor retry schedule
Bridge users try to fetch a descriptor for each bridge (using the
steps in Section 3.4 above) on startup. Whenever they receive a
bridge descriptor, they reschedule a new descriptor download for 1
hour from then.
If on the other hand it fails, they try again after 15 minutes for the
first attempt, after 15 minutes for the second attempt, and after 60
minutes for subsequent attempts.
In 0.2.1.x we should come up with some smarter retry schedules.
3.6. Vidalia integration
Vidalia 0.0.15 has a new checkbox in its Network config window called
"My ISP blocks connections to the Tor network." Users who click that
box change their configuration to:
Once the box is checked, there is also a section for adding bridge
identifiers. When at least one bridge identifier is present, Vidalia
also changes their config to:
and updates their Bridge config option accordingly.
3.7. When should we make TunnelDirConns default?
Right now Tor's directory requests can be filtered on the network,
and some tools used by Middle Eastern governments even do this. A user
who wants to circumvent these filters should click the above box in
Vidalia 0.0.15. But at what point should we make tunneled directory
requests the default?
Once proposal 124 (modified TLS handshake) is in place, we should
consider doing the switch. This might even be in the 0.2.0.x timeframe.
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