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395 lines
19 KiB
Plaintext
395 lines
19 KiB
Plaintext
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BridgeDB specification
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Karsten Loesing
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Nick Mathewson
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0. Preliminaries
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This document specifies how BridgeDB processes bridge descriptor files
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to learn about new bridges, maintains persistent assignments of bridges
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to distributors, and decides which bridges to give out upon user
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requests.
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Some of the decisions here may be suboptimal: this document is meant to
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specify current behavior as of August 2013, not to specify ideal
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behavior.
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1. Importing bridge network statuses and bridge descriptors
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BridgeDB learns about bridges by parsing bridge network statuses,
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bridge descriptors, and extra info documents as specified in Tor's
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directory protocol. BridgeDB parses one bridge network status file
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first and at least one bridge descriptor file and potentially one extra
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info file afterwards.
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BridgeDB scans its files on sighup.
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BridgeDB does not validate signatures on descriptors or networkstatus
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files: the operator needs to make sure that these documents have come
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from a Tor instance that did the validation for us.
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1.1. Parsing bridge network statuses
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Bridge network status documents contain the information of which bridges
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are known to the bridge authority and which flags the bridge authority
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assigns to them.
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We expect bridge network statuses to contain at least the following two
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lines for every bridge in the given order (format fully specified in Tor's
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directory protocol):
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"r" SP nickname SP identity SP digest SP publication SP IP SP ORPort
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SP DirPort NL
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"a" SP address ":" port NL (no more than 8 instances)
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"s" SP Flags NL
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BridgeDB parses the identity and the publication timestamp from the "r"
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line, the OR address(es) and ORPort(s) from the "a" line(s), and the
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assigned flags from the "s" line, specifically checking the assignment
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of the "Running" and "Stable" flags.
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BridgeDB memorizes all bridges that have the Running flag as the set of
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running bridges that can be given out to bridge users.
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BridgeDB memorizes assigned flags if it wants to ensure that sets of
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bridges given out should contain at least a given number of bridges
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with these flags.
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1.2. Parsing bridge descriptors
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BridgeDB learns about a bridge's most recent IP address and OR port
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from parsing bridge descriptors.
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In theory, both IP address and OR port of a bridge are also contained
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in the "r" line of the bridge network status, so there is no mandatory
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reason for parsing bridge descriptors. But the functionality described
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in this section is still implemented in case we need data from the
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bridge descriptor in the future.
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Bridge descriptor files may contain one or more bridge descriptors.
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We expect a bridge descriptor to contain at least the following lines in
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the stated order:
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"@purpose" SP purpose NL
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"router" SP nickname SP IP SP ORPort SP SOCKSPort SP DirPort NL
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"published" SP timestamp
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["opt" SP] "fingerprint" SP fingerprint NL
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"router-signature" NL Signature NL
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BridgeDB parses the purpose, IP, ORPort, nickname, and fingerprint
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from these lines.
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BridgeDB skips bridge descriptors if the fingerprint is not contained
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in the bridge network status parsed earlier or if the bridge does not
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have the Running flag.
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BridgeDB discards bridge descriptors which have a different purpose
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than "bridge". BridgeDB can be configured to only accept descriptors
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with another purpose or not discard descriptors based on purpose at
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all.
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BridgeDB memorizes the IP addresses and OR ports of the remaining
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bridges.
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If there is more than one bridge descriptor with the same fingerprint,
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BridgeDB memorizes the IP address and OR port of the most recently
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parsed bridge descriptor.
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If BridgeDB does not find a bridge descriptor for a bridge contained in
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the bridge network status parsed before, it does not add that bridge
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to the set of bridges to be given out to bridge users.
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1.3. Parsing extra-info documents
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BridgeDB learns if a bridge supports a pluggable transport by parsing
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extra-info documents.
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Extra-info documents contain the name of the bridge (but only if it is
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named), the bridge's fingerprint, the type of pluggable transport(s) it
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supports, and the IP address and port number on which each transport
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listens, respectively.
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Extra-info documents may contain zero or more entries per bridge. We expect
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an extra-info entry to contain the following lines in the stated order:
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"extra-info" SP name SP fingerprint NL
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"transport" SP transport SP IP ":" PORT ARGS NL
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BridgeDB parses the fingerprint, transport type, IP address, port and any
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arguments that are specified on these lines. BridgeDB skips the name. If
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the fingerprint is invalid, BridgeDB skips the entry. BridgeDB memorizes
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the transport type, IP address, port number, and any arguments that are be
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provided and then it assigns them to the corresponding bridge based on the
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fingerprint. Arguments are comma-separated and are of the form k=v,k=v.
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Bridges that do not have an associated extra-info entry are not invalid.
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2. Assigning bridges to distributors
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A "distributor" is a mechanism by which bridges are given (or not
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given) to clients. The current distributors are "email", "https",
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and "unallocated".
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BridgeDB assigns bridges to distributors based on an HMAC hash of the
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bridge's ID and a secret and makes these assignments persistent.
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Persistence is achieved by using a database to map node ID to
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distributor.
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Each bridge is assigned to exactly one distributor (including
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the "unallocated" distributor).
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BridgeDB may be configured to support only a non-empty subset of the
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distributors specified in this document.
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BridgeDB may be configured to use different probabilities for assigning
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new bridges to distributors.
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BridgeDB does not change existing assignments of bridges to
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distributors, even if probabilities for assigning bridges to
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distributors change or distributors are disabled entirely.
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3. Giving out bridges upon requests
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Upon receiving a client request, a BridgeDB distributor provides a
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subset of the bridges assigned to it.
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BridgeDB only gives out bridges that are contained in the most recently
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parsed bridge network status and that have the Running flag set (see
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Section 1).
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BridgeDB may be configured to give out a different number of bridges
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(typically 4) depending on the distributor.
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BridgeDB may define an arbitrary number of rules. These rules may
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specify the criteria by which a bridge is selected. Specifically,
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the available rules restrict the IP address version, OR port number,
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transport type, bridge relay flag, or country in which the bridge
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should not be blocked.
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4. Selecting bridges to be given out based on IP addresses
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BridgeDB may be configured to support one or more distributors which
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gives out bridges based on the requestor's IP address. Currently, this
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is how the HTTPS distributor works.
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The goal is to avoid handing out all the bridges to users in a similar
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IP space and time.
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# Someone else should look at proposals/ideas/old/xxx-bridge-disbursement
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# to see if this section is missing relevant pieces from it. -KL
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BridgeDB fixes the set of bridges to be returned for a defined time
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period.
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BridgeDB considers all IP addresses coming from the same /24 network
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as the same IP address and returns the same set of bridges. From here on,
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this non-unique address will be referred to as the IP address's 'area'.
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BridgeDB divides the IP address space equally into a small number of
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# Note, changed term from "areas" to "disjoint clusters" -MF
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disjoint clusters (typically 4) and returns different results for requests
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coming from addresses that are placed into different clusters.
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# I found that BridgeDB is not strict in returning only bridges for a
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# given area. If a ring is empty, it considers the next one. Is this
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# expected behavior? -KL
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#
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# This does not appear to be the case, anymore. If a ring is empty, then
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# BridgeDB simply returns an empty set of bridges. -MF
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#
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# I also found that BridgeDB does not make the assignment to areas
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# persistent in the database. So, if we change the number of rings, it
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# will assign bridges to other rings. I assume this is okay? -KL
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BridgeDB maintains a list of proxy IP addresses and returns the same
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set of bridges to requests coming from these IP addresses.
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The bridges returned to proxy IP addresses do not come from the same
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set as those for the general IP address space.
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BridgeDB can be configured to include bridge fingerprints in replies
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along with bridge IP addresses and OR ports.
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BridgeDB can be configured to display a CAPTCHA which the user must solve
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prior to returning the requested bridges.
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The current algorithm is as follows. An IP-based distributor splits
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the bridges uniformly into a set of "rings" based on an HMAC of their
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ID. Some of these rings are "area" rings for parts of IP space; some
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are "category" rings for categories of IPs (like proxies). When a
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client makes a request from an IP, the distributor first sees whether
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the IP is in one of the categories it knows. If so, the distributor
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returns an IP from the category rings. If not, the distributor
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maps the IP into an "area" (that is, a /24), and then uses an HMAC to
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map the area to one of the area rings.
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When the IP-based distributor determines from which area ring it is handing
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out bridges, it identifies which rules it will use to choose appropriate
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bridges. Using this information, it searches its cache of rings for one
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that already adheres to the criteria specified in this request. If one
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exists, then BridgeDB maps the current "epoch" (N-hour period) and the
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IP's area (/24) to a point on the ring based on HMAC, and hands out
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bridges at that point. If a ring does not already exist which satisfies this
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request, then a new ring is created and filled with bridges that fulfill
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the requirements. This ring is then used to select bridges as described.
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"Mapping X to Y based on an HMAC" above means one of the following:
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- We keep all of the elements of Y in some order, with a mapping
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from all 160-bit strings to positions in Y.
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- We take an HMAC of X using some fixed string as a key to get a
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160-bit value. We then map that value to the next position of Y.
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When giving out bridges based on a position in a ring, BridgeDB first
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looks at flag requirements and port requirements. For example,
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BridgeDB may be configured to "Give out at least L bridges with port
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443, and at least M bridges with Stable, and at most N bridges
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total." To do this, BridgeDB combines to the results:
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- The first L bridges in the ring after the position that have the
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port 443, and
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- The first M bridges in the ring after the position that have the
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flag stable and that it has not already decided to give out, and
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- The first N-L-M bridges in the ring after the position that it
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has not already decided to give out.
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After BridgeDB selects appropriate bridges to return to the requestor, it
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then prioritises the ordering of them in a list so that as many criteria
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are fulfilled as possible within the first few bridges. This list is then
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truncated to N bridges, if possible. N is currently defined as a
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piecewise function of the number of bridges in the ring such that:
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/
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| 1, if len(ring) < 20
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N = | 2, if 20 <= len(ring) <= 100
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| 3, if 100 <= len(ring)
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\
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The bridges in this sublist, containing no more than N bridges, are the
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bridges returned to the requestor.
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5. Selecting bridges to be given out based on email addresses
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BridgeDB can be configured to support one or more distributors that are
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giving out bridges based on the requestor's email address. Currently,
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this is how the email distributor works.
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The goal is to bootstrap based on one or more popular email service's
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sybil prevention algorithms.
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# Someone else should look at proposals/ideas/old/xxx-bridge-disbursement
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# to see if this section is missing relevant pieces from it. -KL
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BridgeDB rejects email addresses containing other characters than the
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ones that RFC2822 allows.
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BridgeDB may be configured to reject email addresses containing other
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characters it might not process correctly.
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# I don't think we do this, is it worthwhile? -MF
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BridgeDB rejects email addresses coming from other domains than a
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configured set of permitted domains.
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BridgeDB normalizes email addresses by removing "." characters and by
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removing parts after the first "+" character.
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BridgeDB can be configured to discard requests that do not have the
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value "pass" in their X-DKIM-Authentication-Result header or does not
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have this header. The X-DKIM-Authentication-Result header is set by
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the incoming mail stack that needs to check DKIM authentication.
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BridgeDB does not return a new set of bridges to the same email address
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until a given time period (typically a few hours) has passed.
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# Why don't we fix the bridges we give out for a global 3-hour time period
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# like we do for IP addresses? This way we could avoid storing email
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# addresses. -KL
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# The 3-hour value is probably much too short anyway. If we take longer
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# time values, then people get new bridges when bridges show up, as
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# opposed to then we decide to reset the bridges we give them. (Yes, this
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# problem exists for the IP distributor). -NM
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# I'm afraid I don't fully understand what you mean here. Can you
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# elaborate? -KL
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#
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# Assuming an average churn rate, if we use short time periods, then a
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# requestor will receive new bridges based on rate-limiting and will (likely)
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# eventually work their way around the ring; eventually exhausting all bridges
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# available to them from this distributor. If we use a longer time period,
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# then each time the period expires there will be more bridges in the ring
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# thus reducing the likelihood of all bridges being blocked and increasing
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# the time and effort required to enumerate all bridges. (This is my
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# understanding, not from Nick) -MF
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# Also, we presently need the cache to prevent replays and because if a user
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# sent multiple requests with different criteria in each then we would leak
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# additional bridges otherwise. -MF
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BridgeDB can be configured to include bridge fingerprints in replies
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along with bridge IP addresses and OR ports.
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BridgeDB can be configured to sign all replies using a PGP signing key.
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BridgeDB periodically discards old email-address-to-bridge mappings.
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BridgeDB rejects too frequent email requests coming from the same
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normalized address.
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To map previously unseen email addresses to a set of bridges, BridgeDB
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proceeds as follows:
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- It normalizes the email address as above, by stripping out dots,
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removing all of the localpart after the +, and putting it all
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in lowercase. (Example: "John.Doe+bridges@example.COM" becomes
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"johndoe@example.com".)
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- It maps an HMAC of the normalized address to a position on its ring
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of bridges.
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- It hands out bridges starting at that position, based on the
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port/flag requirements, as specified at the end of section 4.
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See section 4 for the details of how bridges are selected from the ring
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and returned to the requestor.
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6. Selecting unallocated bridges to be stored in file buckets
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# Kaner should have a look at this section. -NM
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BridgeDB can be configured to reserve a subset of bridges and not give
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them out via one of the distributors.
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BridgeDB assigns reserved bridges to one or more file buckets of fixed
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sizes and write these file buckets to disk for manual distribution.
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BridgeDB ensures that a file bucket always contains the requested
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number of running bridges.
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If the requested number of bridges in a file bucket is reduced or the
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file bucket is not required anymore, the unassigned bridges are
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returned to the reserved set of bridges.
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If a bridge stops running, BridgeDB replaces it with another bridge
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from the reserved set of bridges.
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# I'm not sure if there's a design bug in file buckets. What happens if
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# we add a bridge X to file bucket A, and X goes offline? We would add
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# another bridge Y to file bucket A. OK, but what if A comes back? We
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# cannot put it back in file bucket A, because it's full. Are we going to
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# add it to a different file bucket? Doesn't that mean that most bridges
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# will be contained in most file buckets over time? -KL
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#
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# This should be handled the same as if the file bucket is reduced in size.
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# If X returns, then it should be added to the appropriate distributor. -MF
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7. Displaying Bridge Information
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After bridges are selected using one of the methods described in
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Sections 4 - 6, they are output in one of two formats. Bridges are
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formatted as:
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<address:port> NL
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Pluggable transports are formatted as:
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<transportname> SP <address:port> [SP arglist] NL
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where arglist is an optional space-separated list of key-value pairs in
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the form of k=v.
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Previously, each line was prepended with the "bridge" keyword, such as
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"bridge" SP <address:port> NL
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"bridge" SP <transportname> SP <address:port> [SP arglist] NL
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# We don't do this anymore because Vidalia and TorLauncher don't expect it.
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# See the commit message for b70347a9c5fd769c6d5d0c0eb5171ace2999a736.
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8. Writing bridge assignments for statistics
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BridgeDB can be configured to write bridge assignments to disk for
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statistical analysis.
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The start of a bridge assignment is marked by the following line:
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"bridge-pool-assignment" SP YYYY-MM-DD HH:MM:SS NL
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YYYY-MM-DD HH:MM:SS is the time, in UTC, when BridgeDB has completed
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loading new bridges and assigning them to distributors.
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For every running bridge there is a line with the following format:
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fingerprint SP distributor (SP key "=" value)* NL
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The distributor is one out of "email", "https", or "unallocated".
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Both "email" and "https" distributors support adding keys for "port",
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"flag" and "transport". Respectively, the port number, flag name, and
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transport types are the values. These are used to indicate that
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a bridge matches certain port, flag, transport criteria of requests.
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The "https" distributor also allows the key "ring" with a number as
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value to indicate to which IP address area the bridge is returned.
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The "unallocated" distributor allows the key "bucket" with the file
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bucket name as value to indicate which file bucket a bridge is assigned
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to.
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