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250 lines
11 KiB
Plaintext
250 lines
11 KiB
Plaintext
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Tor bridges specification
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0. Preface
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This document describes the design decisions around support for bridge
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users, bridge relays, and bridge authorities. It acts as an overview
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of the bridge design and deployment for developers, and it also tries
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to point out limitations in the current design and implementation.
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For more details on what all of these mean, look at blocking.tex in
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/doc/design-paper/
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1. Bridge relays
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Bridge relays are just like normal Tor relays except they don't publish
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their server descriptors to the main directory authorities.
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1.1. PublishServerDescriptor
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To configure your relay to be a bridge relay, just add
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BridgeRelay 1
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PublishServerDescriptor bridge
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to your torrc. This will cause your relay to publish its descriptor
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to the bridge authorities rather than to the default authorities.
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Alternatively, you can say
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BridgeRelay 1
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PublishServerDescriptor 0
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which will cause your relay to not publish anywhere. This could be
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useful for private bridges.
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1.2. Recommendations.
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Bridge relays should use an exit policy of "reject *:*". This is
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because they only need to relay traffic between the bridge users
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and the rest of the Tor network, so there's no need to let people
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exit directly from them.
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We invented the RelayBandwidth* options for this situation: Tor clients
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who want to allow relaying too. See proposal 111 for details. Relay
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operators should feel free to rate-limit their relayed traffic.
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1.3. Implementation note.
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Vidalia 0.0.15 has turned its "Relay" settings page into a tri-state
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"Don't relay" / "Relay for the Tor network" / "Help censored users".
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If you click the third choice, it forces your exit policy to reject *:*.
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If all the bridges end up on port 9001, that's not so good. On the
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other hand, putting the bridges on a low-numbered port in the Unix
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world requires jumping through extra hoops. The current compromise is
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that Vidalia makes the ORPort default to 443 on Windows, and 9001 on
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other platforms.
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At the bottom of the relay config settings window, Vidalia displays
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the bridge identifier to the operator (see Section 3.1) so he can pass
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it on to bridge users.
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2. Bridge authorities.
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Bridge authorities are like normal v3 directory authorities, except
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they don't create their own network-status documents or votes. So if
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you ask a bridge authority for a network-status document or consensus,
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they behave like a directory mirror: they give you one from one of
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the main authorities. But if you ask the bridge authority for the
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descriptor corresponding to a particular identity fingerprint, it will
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happily give you the latest descriptor for that fingerprint.
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To become a bridge authority, add these lines to your torrc:
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AuthoritativeDirectory 1
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BridgeAuthoritativeDir 1
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Right now there's one bridge authority, running on the Tonga relay.
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2.1. Exporting bridge-purpose descriptors
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We've added a new purpose for server descriptors: the "bridge"
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purpose. With the new router-descriptors file format that includes
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annotations, it's easy to look through it and find the bridge-purpose
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descriptors.
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Currently we export the bridge descriptors from Tonga to the
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BridgeDB server, so it can give them out according to the policies
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in blocking.pdf.
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2.2. Reachability/uptime testing
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Right now the bridge authorities do active reachability testing of
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bridges, so we know which ones to recommend for users.
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But in the design document, we suggested that bridges should publish
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anonymously (i.e. via Tor) to the bridge authority, so somebody watching
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the bridge authority can't just enumerate all the bridges. But if we're
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doing active measurement, the game is up. Perhaps we should back off on
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this goal, or perhaps we should do our active measurement anonymously?
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Answering this issue is scheduled for 0.2.1.x.
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2.3. Future work: migrating to multiple bridge authorities
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Having only one bridge authority is both a trust bottleneck (if you
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break into one place you learn about every single bridge we've got)
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and a robustness bottleneck (when it's down, bridge users become sad).
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Right now if we put up a second bridge authority, all the bridges would
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publish to it, and (assuming the code works) bridge users would query
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a random bridge authority. This resolves the robustness bottleneck,
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but makes the trust bottleneck even worse.
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In 0.2.2.x and later we should think about better ways to have multiple
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bridge authorities.
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3. Bridge users.
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Bridge users are like ordinary Tor users except they use encrypted
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directory connections by default, and they use bridge relays as both
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entry guards (their first hop) and directory guards (the source of
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all their directory information).
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To become a bridge user, add the following line to your torrc:
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UseBridges 1
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and then add at least one "Bridge" line to your torrc based on the
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format below.
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3.1. Format of the bridge identifier.
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The canonical format for a bridge identifier contains an IP address,
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an ORPort, and an identity fingerprint:
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bridge 128.31.0.34:9009 4C17 FB53 2E20 B2A8 AC19 9441 ECD2 B017 7B39 E4B1
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However, the identity fingerprint can be left out, in which case the
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bridge user will connect to that relay and use it as a bridge regardless
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of what identity key it presents:
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bridge 128.31.0.34:9009
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This might be useful for cases where only short bridge identifiers
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can be communicated to bridge users.
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In a future version we may also support bridge identifiers that are
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only a key fingerprint:
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bridge 4C17 FB53 2E20 B2A8 AC19 9441 ECD2 B017 7B39 E4B1
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and the bridge user can fetch the latest descriptor from the bridge
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authority (see Section 3.4).
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3.2. Bridges as entry guards
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For now, bridge users add their bridge relays to their list of "entry
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guards" (see path-spec.txt for background on entry guards). They are
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managed by the entry guard algorithms exactly as if they were a normal
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entry guard -- their keys and timing get cached in the "state" file,
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etc. This means that when the Tor user starts up with "UseBridges"
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disabled, he will skip past the bridge entries since they won't be
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listed as up and usable in his networkstatus consensus. But to be clear,
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the "entry_guards" list doesn't currently distinguish guards by purpose.
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Internally, each bridge user keeps a smartlist of "bridge_info_t"
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that reflects the "bridge" lines from his torrc along with a download
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schedule (see Section 3.5 below). When he starts Tor, he attempts
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to fetch a descriptor for each configured bridge (see Section 3.4
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below). When he succeeds at getting a descriptor for one of the bridges
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in his list, he adds it directly to the entry guard list using the
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normal add_an_entry_guard() interface. Once a bridge descriptor has
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been added, should_delay_dir_fetches() will stop delaying further
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directory fetches, and the user begins to bootstrap his directory
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information from that bridge (see Section 3.3).
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Currently bridge users cache their bridge descriptors to the
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"cached-descriptors" file (annotated with purpose "bridge"), but
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they don't make any attempt to reuse descriptors they find in this
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file. The theory is that either the bridge is available now, in which
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case you can get a fresh descriptor, or it's not, in which case an
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old descriptor won't do you much good.
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We could disable writing out the bridge lines to the state file, if
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we think this is a problem.
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As an exception, if we get an application request when we have one
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or more bridge descriptors but we believe none of them are running,
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we mark them all as running again. This is similar to the exception
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already in place to help long-idle Tor clients realize they should
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fetch fresh directory information rather than just refuse requests.
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3.3. Bridges as directory guards
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In addition to using bridges as the first hop in their circuits, bridge
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users also use them to fetch directory updates. Other than initial
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bootstrapping to find a working bridge descriptor (see Section 3.4
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below), all further non-anonymized directory fetches will be redirected
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to the bridge.
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This means that bridge relays need to have cached answers for all
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questions the bridge user might ask. This makes the upgrade path
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tricky --- for example, if we migrate to a v4 directory design, the
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bridge user would need to keep using v3 so long as his bridge relays
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only knew how to answer v3 queries.
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In a future design, for cases where the user has enough information
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to build circuits yet the chosen bridge doesn't know how to answer a
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given query, we might teach bridge users to make an anonymized request
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to a more suitable directory server.
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3.4. How bridge users get their bridge descriptor
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Bridge users can fetch bridge descriptors in two ways: by going directly
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to the bridge and asking for "/tor/server/authority", or by going to
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the bridge authority and asking for "/tor/server/fp/ID". By default,
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they will only try the direct queries. If the user sets
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UpdateBridgesFromAuthority 1
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in his config file, then he will try querying the bridge authority
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first for bridges where he knows a digest (if he only knows an IP
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address and ORPort, then his only option is a direct query).
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If the user has at least one working bridge, then he will do further
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queries to the bridge authority through a full three-hop Tor circuit.
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But when bootstrapping, he will make a direct begin_dir-style connection
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to the bridge authority.
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As of Tor 0.2.0.10-alpha, if the user attempts to fetch a descriptor
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from the bridge authority and it returns a 404 not found, the user
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will automatically fall back to trying a direct query. Therefore it is
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recommended that bridge users always set UpdateBridgesFromAuthority,
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since at worst it will delay their fetches a little bit and notify
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the bridge authority of the identity fingerprint (but not location)
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of their intended bridges.
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3.5. Bridge descriptor retry schedule
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Bridge users try to fetch a descriptor for each bridge (using the
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steps in Section 3.4 above) on startup. Whenever they receive a
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bridge descriptor, they reschedule a new descriptor download for 1
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hour from then.
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If on the other hand it fails, they try again after 15 minutes for the
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first attempt, after 15 minutes for the second attempt, and after 60
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minutes for subsequent attempts.
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In 0.2.2.x we should come up with some smarter retry schedules.
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3.6. Implementation note.
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Vidalia 0.1.0 has a new checkbox in its Network config window called
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"My ISP blocks connections to the Tor network." Users who click that
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box change their configuration to:
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UseBridges 1
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UpdateBridgesFromAuthority 1
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and should add at least one bridge identifier.
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