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