dir-spec.txt 79 KB

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  1. $Id$
  2. Tor directory protocol, version 3
  3. 0. Scope and preliminaries
  4. This directory protocol is used by Tor version 0.2.0.x-alpha and later.
  5. See dir-spec-v1.txt for information on the protocol used up to the
  6. 0.1.0.x series, and dir-spec-v2.txt for information on the protocol
  7. used by the 0.1.1.x and 0.1.2.x series.
  8. Caches and authorities must still support older versions of the
  9. directory protocols, until the versions of Tor that require them are
  10. finally out of commission. See Section XXXX on backward compatibility.
  11. This document merges and supersedes the following proposals:
  12. 101 Voting on the Tor Directory System
  13. 103 Splitting identity key from regularly used signing key
  14. 104 Long and Short Router Descriptors
  15. AS OF 14 JUNE 2007, THIS SPECIFICATION HAS NOT YET BEEN COMPLETELY
  16. IMPLEMENTED, OR COMPLETELY COMPLETED.
  17. XXX when to download certificates.
  18. XXX timeline
  19. XXX fill in XXXXs
  20. 0.1. History
  21. The earliest versions of Onion Routing shipped with a list of known
  22. routers and their keys. When the set of routers changed, users needed to
  23. fetch a new list.
  24. The Version 1 Directory protocol
  25. --------------------------------
  26. Early versions of Tor (0.0.2) introduced "Directory authorities": servers
  27. that served signed "directory" documents containing a list of signed
  28. "router descriptors", along with short summary of the status of each
  29. router. Thus, clients could get up-to-date information on the state of
  30. the network automatically, and be certain that the list they were getting
  31. was attested by a trusted directory authority.
  32. Later versions (0.0.8) added directory caches, which download
  33. directories from the authorities and serve them to clients. Non-caches
  34. fetch from the caches in preference to fetching from the authorities, thus
  35. distributing bandwidth requirements.
  36. Also added during the version 1 directory protocol were "router status"
  37. documents: short documents that listed only the up/down status of the
  38. routers on the network, rather than a complete list of all the
  39. descriptors. Clients and caches would fetch these documents far more
  40. frequently than they would fetch full directories.
  41. The Version 2 Directory Protocol
  42. --------------------------------
  43. During the Tor 0.1.1.x series, Tor revised its handling of directory
  44. documents in order to address two major problems:
  45. * Directories had grown quite large (over 1MB), and most directory
  46. downloads consisted mainly of router descriptors that clients
  47. already had.
  48. * Every directory authority was a trust bottleneck: if a single
  49. directory authority lied, it could make clients believe for a time
  50. an arbitrarily distorted view of the Tor network. (Clients
  51. trusted the most recent signed document they downloaded.) Thus,
  52. adding more authorities would make the system less secure, not
  53. more.
  54. To address these, we extended the directory protocol so that
  55. authorities now published signed "network status" documents. Each
  56. network status listed, for every router in the network: a hash of its
  57. identity key, a hash of its most recent descriptor, and a summary of
  58. what the authority believed about its status. Clients would download
  59. the authorities' network status documents in turn, and believe
  60. statements about routers iff they were attested to by more than half of
  61. the authorities.
  62. Instead of downloading all router descriptors at once, clients
  63. downloaded only the descriptors that they did not have. Descriptors
  64. were indexed by their digests, in order to prevent malicious caches
  65. from giving different versions of a router descriptor to different
  66. clients.
  67. Routers began working harder to upload new descriptors only when their
  68. contents were substantially changed.
  69. 0.2. Goals of the version 3 protocol
  70. Version 3 of the Tor directory protocol tries to solve the following
  71. issues:
  72. * A great deal of bandwidth used to transmit router descriptors was
  73. used by two fields that are not actually used by Tor routers
  74. (namely read-history and write-history). We save about 60% by
  75. moving them into a separate document that most clients do not
  76. fetch or use.
  77. * It was possible under certain perverse circumstances for clients
  78. to download an unusual set of network status documents, thus
  79. partitioning themselves from clients who have a more recent and/or
  80. typical set of documents. Even under the best of circumstances,
  81. clients were sensitive to the ages of the network status documents
  82. they downloaded. Therefore, instead of having the clients
  83. correlate multiple network status documents, we have the
  84. authorities collectively vote on a single consensus network status
  85. document.
  86. * The most sensitive data in the entire network (the identity keys
  87. of the directory authorities) needed to be stored unencrypted so
  88. that the authorities can sign network-status documents on the fly.
  89. Now, the authorities' identity keys are stored offline, and used
  90. to certify medium-term signing keys that can be rotated.
  91. 0.3. Some Remaining questions
  92. Things we could solve on a v3 timeframe:
  93. The SHA-1 hash is showing its age. We should do something about our
  94. dependency on it. We could probably future-proof ourselves here in
  95. this revision, at least so far as documents from the authorities are
  96. concerned.
  97. Too many things about the authorities are hardcoded by IP.
  98. Perhaps we should start accepting longer identity keys for routers
  99. too.
  100. Things to solve eventually:
  101. Requiring every client to know about every router won't scale forever.
  102. Requiring every directory cache to know every router won't scale
  103. forever.
  104. 1. Outline
  105. There is a small set (say, around 5-10) of semi-trusted directory
  106. authorities. A default list of authorities is shipped with the Tor
  107. software. Users can change this list, but are encouraged not to do so,
  108. in order to avoid partitioning attacks.
  109. Every authority has a very-secret, long-term "Authority Identity Key".
  110. This is stored encrypted and/or offline, and is used to sign "key
  111. certificate" documents. Every key certificate contains a medium-term
  112. (3-12 months) "authority signing key", that is used by the authority to
  113. sign other directory information. (Note that the authority identity
  114. key is distinct from the router identity key that the authority uses
  115. in its role as an ordinary router.)
  116. Routers periodically upload signed "routers descriptors" to the
  117. directory authorities describing their keys, capabilities, and other
  118. information. Routers may also upload signed "extra info documents"
  119. containing information that is not required for the Tor protocol.
  120. Directory authorities serve router descriptors indexed by router
  121. identity, or by hash of the descriptor.
  122. Routers may act as directory caches to reduce load on the directory
  123. authorities. They announce this in their descriptors.
  124. Periodically, each directory authority generates a view of
  125. the current descriptors and status for known routers. They send a
  126. signed summary of this view (a "status vote") to the other
  127. authorities. The authorities compute the result of this vote, and sign
  128. a "consensus status" document containing the result of the vote.
  129. Directory caches download, cache, and re-serve consensus documents.
  130. Clients, directory caches, and directory authorities all use consensus
  131. documents to find out when their list of routers is out-of-date.
  132. (Directory authorities also use vote statuses.) If it is, they download
  133. any missing router descriptors. Clients download missing descriptors
  134. from caches; caches and authorities download from authorities.
  135. Descriptors are downloaded by the hash of the descriptor, not by the
  136. server's identity key: this prevents servers from attacking clients by
  137. giving them descriptors nobody else uses.
  138. All directory information is uploaded and downloaded with HTTP.
  139. [Authorities also generate and caches also cache documents produced and
  140. used by earlier versions of this protocol; see section XXX for notes.]
  141. 1.1. What's different from version 2?
  142. Clients used to download multiple network status documents,
  143. corresponding roughly to "status votes" above. They would compute the
  144. result of the vote on the client side.
  145. Authorities used to sign documents using the same private keys they used
  146. for their roles as routers. This forced them to keep these extremely
  147. sensitive keys in memory unencrypted.
  148. All of the information in extra-info documents used to be kept in the
  149. main descriptors.
  150. 1.2. Document meta-format
  151. Router descriptors, directories, and running-routers documents all obey the
  152. following lightweight extensible information format.
  153. The highest level object is a Document, which consists of one or more
  154. Items. Every Item begins with a KeywordLine, followed by zero or more
  155. Objects. A KeywordLine begins with a Keyword, optionally followed by
  156. whitespace and more non-newline characters, and ends with a newline. A
  157. Keyword is a sequence of one or more characters in the set [A-Za-z0-9-].
  158. An Object is a block of encoded data in pseudo-Open-PGP-style
  159. armor. (cf. RFC 2440)
  160. More formally:
  161. NL = The ascii LF character (hex value 0x0a).
  162. Document ::= (Item | NL)+
  163. Item ::= KeywordLine Object*
  164. KeywordLine ::= Keyword NL | Keyword WS ArgumentChar+ NL
  165. Keyword = KeywordChar+
  166. KeywordChar ::= 'A' ... 'Z' | 'a' ... 'z' | '0' ... '9' | '-'
  167. ArgumentChar ::= any printing ASCII character except NL.
  168. WS = (SP | TAB)+
  169. Object ::= BeginLine Base-64-encoded-data EndLine
  170. BeginLine ::= "-----BEGIN " Keyword "-----" NL
  171. EndLine ::= "-----END " Keyword "-----" NL
  172. The BeginLine and EndLine of an Object must use the same keyword.
  173. When interpreting a Document, software MUST ignore any KeywordLine that
  174. starts with a keyword it doesn't recognize; future implementations MUST NOT
  175. require current clients to understand any KeywordLine not currently
  176. described.
  177. The "opt" keyword was used until Tor 0.1.2.5-alpha for non-critical future
  178. extensions. All implementations MUST ignore any item of the form "opt
  179. keyword ....." when they would not recognize "keyword ....."; and MUST
  180. treat "opt keyword ....." as synonymous with "keyword ......" when keyword
  181. is recognized.
  182. Implementations before 0.1.2.5-alpha rejected any document with a
  183. KeywordLine that started with a keyword that they didn't recognize.
  184. When generating documents that need to be read by older versions of Tor,
  185. implementations MUST prefix items not recognized by older versions of
  186. Tor with an "opt" until those versions of Tor are obsolete. [Note that
  187. key certificates, status vote documents, extra info documents, and
  188. status consensus documents will never be read by older versions of Tor.]
  189. Other implementations that want to extend Tor's directory format MAY
  190. introduce their own items. The keywords for extension items SHOULD start
  191. with the characters "x-" or "X-", to guarantee that they will not conflict
  192. with keywords used by future versions of Tor.
  193. In our document descriptions below, we tag Items with a multiplicity in
  194. brackets. Possible tags are:
  195. "At start, exactly once": These items MUST occur in every instance of
  196. the document type, and MUST appear exactly once, and MUST be the
  197. first item in their documents.
  198. "Exactly once": These items MUST occur exactly one time in every
  199. instance of the document type.
  200. "At end, exactly once": These items MUST occur in every instance of
  201. the document type, and MUST appear exactly once, and MUST be the
  202. last item in their documents.
  203. "At most once": These items MAY occur zero or one times in any
  204. instance of the document type, but MUST NOT occur more than once.
  205. "Any number": These items MAY occur zero, one, or more times in any
  206. instance of the document type.
  207. "Once or more": These items MUST occur at least once in any instance
  208. of the document type, and MAY occur more.
  209. 1.3. Signing documents
  210. Every signable document below is signed in a similar manner, using a
  211. given "Initial Item", a final "Signature Item", a digest algorithm, and
  212. a signing key.
  213. The Initial Item must be the first item in the document.
  214. The Signature Item has the following format:
  215. <signature item keyword> [arguments] NL SIGNATURE NL
  216. The "SIGNATURE" Object contains a signature (using the signing key) of
  217. the PKCS1-padded digest of the entire document, taken from the
  218. beginning of the Initial item, through the newline after the Signature
  219. Item's keyword and its arguments.
  220. Unless otherwise, the digest algorithm is SHA-1.
  221. All documents are invalid unless signed with the correct signing key.
  222. The "Digest" of a document, unless stated otherwise, is its digest *as
  223. signed by this signature scheme*.
  224. 1.4. Voting timeline
  225. Every consensus document has a "valid-after" (VA) time, a "fresh-until"
  226. (FU) time and a "valid-until" (VU) time. VA MUST precede FU, which MUST
  227. in turn precede VU. Times are chosen so that every consensus will be
  228. "fresh" until the next consensus becomes valid, and "valid" for a while
  229. after. At least 3 consensuses should be valid at any given time.
  230. The timeline for a given consensus is as follows:
  231. VA-DistSeconds-VoteSeconds: The authorities exchange votes.
  232. VA-DistSeconds-VoteSeconds/2: The authorities try to download any
  233. votes they don't have.
  234. VA-DistSeconds: The authorities calculate the consensus and exchange
  235. signatures.
  236. VA-DistSeconds/2: The authorities try to download any signatures
  237. they don't have.
  238. VA: All authorities have a multiply signed consensus.
  239. VA ... FU: Caches download the consensus. (Note that since caches have
  240. no way of telling what VA and FU are until they have downloaded
  241. the consensus, they assume that the present consensus's VA is
  242. equal to the previous one's FU, and that its FU is one interval after
  243. that.)
  244. FU: The consensus is no longer the freshest consensus.
  245. FU ... (the current consensus's VU): Clients download the consensus.
  246. (See note above: clients guess that the next consensus's FU will be
  247. two intervals after the current VA.)
  248. VU: The consensus is no longer valid.
  249. VoteSeconds and DistSeconds MUST each be at least 20 seconds; FU-VA and
  250. VU-FU MUST each be at least 5 minutes.
  251. 2. Router operation and formats
  252. ORs SHOULD generate a new router descriptor and a new extra-info
  253. document whenever any of the following events have occurred:
  254. - A period of time (18 hrs by default) has passed since the last
  255. time a descriptor was generated.
  256. - A descriptor field other than bandwidth or uptime has changed.
  257. - Bandwidth has changed by a factor of 2 from the last time a
  258. descriptor was generated, and at least a given interval of time
  259. (20 mins by default) has passed since then.
  260. - Its uptime has been reset (by restarting).
  261. [XXX this list is incomplete; see router_differences_are_cosmetic()
  262. in routerlist.c for others]
  263. ORs SHOULD NOT publish a new router descriptor or extra-info document
  264. if none of the above events have occurred and not much time has passed
  265. (12 hours by default).
  266. After generating a descriptor, ORs upload them to every directory
  267. authority they know, by posting them (in order) to the URL
  268. http://<hostname:port>/tor/
  269. 2.1. Router descriptor format
  270. Router descriptors consist of the following items. For backward
  271. compatibility, there should be an extra NL at the end of each router
  272. descriptor.
  273. In lines that take multiple arguments, extra arguments SHOULD be
  274. accepted and ignored. Many of the nonterminals below are defined in
  275. section 2.3.
  276. "router" nickname address ORPort SOCKSPort DirPort NL
  277. [At start, exactly once.]
  278. Indicates the beginning of a router descriptor. "nickname" must be a
  279. valid router nickname as specified in 2.3. "address" must be an IPv4
  280. address in dotted-quad format. The last three numbers indicate the
  281. TCP ports at which this OR exposes functionality. ORPort is a port at
  282. which this OR accepts TLS connections for the main OR protocol;
  283. SOCKSPort is deprecated and should always be 0; and DirPort is the
  284. port at which this OR accepts directory-related HTTP connections. If
  285. any port is not supported, the value 0 is given instead of a port
  286. number. (At least one of DirPort and ORPort SHOULD be set;
  287. authorities MAY reject any descriptor with both DirPort and ORPort of
  288. 0.)
  289. "bandwidth" bandwidth-avg bandwidth-burst bandwidth-observed NL
  290. [Exactly once]
  291. Estimated bandwidth for this router, in bytes per second. The
  292. "average" bandwidth is the volume per second that the OR is willing to
  293. sustain over long periods; the "burst" bandwidth is the volume that
  294. the OR is willing to sustain in very short intervals. The "observed"
  295. value is an estimate of the capacity this server can handle. The
  296. server remembers the max bandwidth sustained output over any ten
  297. second period in the past day, and another sustained input. The
  298. "observed" value is the lesser of these two numbers.
  299. "platform" string NL
  300. [At most once]
  301. A human-readable string describing the system on which this OR is
  302. running. This MAY include the operating system, and SHOULD include
  303. the name and version of the software implementing the Tor protocol.
  304. "published" YYYY-MM-DD HH:MM:SS NL
  305. [Exactly once]
  306. The time, in GMT, when this descriptor (and its corresponding
  307. extra-info document if any) was generated.
  308. "fingerprint" fingerprint NL
  309. [At most once]
  310. A fingerprint (a HASH_LEN-byte of asn1 encoded public key, encoded in
  311. hex, with a single space after every 4 characters) for this router's
  312. identity key. A descriptor is considered invalid (and MUST be
  313. rejected) if the fingerprint line does not match the public key.
  314. [We didn't start parsing this line until Tor 0.1.0.6-rc; it should
  315. be marked with "opt" until earlier versions of Tor are obsolete.]
  316. "hibernating" bool NL
  317. [At most once]
  318. If the value is 1, then the Tor server was hibernating when the
  319. descriptor was published, and shouldn't be used to build circuits.
  320. [We didn't start parsing this line until Tor 0.1.0.6-rc; it should be
  321. marked with "opt" until earlier versions of Tor are obsolete.]
  322. "uptime" number NL
  323. [At most once]
  324. The number of seconds that this OR process has been running.
  325. "onion-key" NL a public key in PEM format
  326. [Exactly once]
  327. This key is used to encrypt EXTEND cells for this OR. The key MUST be
  328. accepted for at least 1 week after any new key is published in a
  329. subsequent descriptor. It MUST be 1024 bits.
  330. "signing-key" NL a public key in PEM format
  331. [Exactly once]
  332. The OR's long-term identity key. It MUST be 1024 bits.
  333. "accept" exitpattern NL
  334. "reject" exitpattern NL
  335. [Any number]
  336. These lines describe an "exit policy": the rules that an OR follows
  337. when deciding whether to allow a new stream to a given address. The
  338. 'exitpattern' syntax is described below. There MUST be at least one
  339. such entry. The rules are considered in order; if no rule matches,
  340. the address will be accepted. For clarity, the last such entry SHOULD
  341. be accept *:* or reject *:*.
  342. "router-signature" NL Signature NL
  343. [At end, exactly once]
  344. The "SIGNATURE" object contains a signature of the PKCS1-padded
  345. hash of the entire router descriptor, taken from the beginning of the
  346. "router" line, through the newline after the "router-signature" line.
  347. The router descriptor is invalid unless the signature is performed
  348. with the router's identity key.
  349. "contact" info NL
  350. [At most once]
  351. Describes a way to contact the server's administrator, preferably
  352. including an email address and a PGP key fingerprint.
  353. "family" names NL
  354. [At most once]
  355. 'Names' is a space-separated list of server nicknames or
  356. hexdigests. If two ORs list one another in their "family" entries,
  357. then OPs should treat them as a single OR for the purpose of path
  358. selection.
  359. For example, if node A's descriptor contains "family B", and node B's
  360. descriptor contains "family A", then node A and node B should never
  361. be used on the same circuit.
  362. "read-history" YYYY-MM-DD HH:MM:SS (NSEC s) NUM,NUM,NUM,NUM,NUM... NL
  363. [At most once]
  364. "write-history" YYYY-MM-DD HH:MM:SS (NSEC s) NUM,NUM,NUM,NUM,NUM... NL
  365. [At most once]
  366. Declare how much bandwidth the OR has used recently. Usage is divided
  367. into intervals of NSEC seconds. The YYYY-MM-DD HH:MM:SS field
  368. defines the end of the most recent interval. The numbers are the
  369. number of bytes used in the most recent intervals, ordered from
  370. oldest to newest.
  371. [We didn't start parsing these lines until Tor 0.1.0.6-rc; they should
  372. be marked with "opt" until earlier versions of Tor are obsolete.]
  373. [See also migration notes in section 2.2.1.]
  374. "eventdns" bool NL
  375. [At most once]
  376. Declare whether this version of Tor is using the newer enhanced
  377. dns logic. Versions of Tor with this field set to false SHOULD NOT
  378. be used for reverse hostname lookups.
  379. [All versions of Tor before 0.1.2.2-alpha should be assumed to have
  380. this option set to 0 if it is not present. All Tor versions at
  381. 0.1.2.2-alpha or later should be assumed to have this option set to
  382. 1 if it is not present. Until 0.1.2.1-alpha-dev, this option was
  383. not generated, even when the new DNS code was in use. Versions of Tor
  384. before 0.1.2.1-alpha-dev did not parse this option, so it should be
  385. marked "opt". The dnsworker logic has been removed, so this option
  386. should not be used by new server code. However, it can still be
  387. used, and should still be recognized by new code until Tor 0.1.2.x
  388. is obsolete.]
  389. "caches-extra-info" NL
  390. [At most once.]
  391. Present only if this router is a directory cache that provides
  392. extra-info documents.
  393. [Versions before 0.2.0.1-alpha don't recognize this, and versions
  394. before 0.1.2.5-alpha will reject descriptors containing it unless
  395. it is prefixed with "opt"; it should be so prefixed until these
  396. versions are obsolete.]
  397. "extra-info-digest" digest NL
  398. [At most once]
  399. "Digest" is a hex-encoded digest (using upper-case characters) of the
  400. router's extra-info document, as signed in the router's extra-info
  401. (that is, not including the signature). (If this field is absent, the
  402. router is not uploading a corresponding extra-info document.)
  403. [Versions before 0.2.0.1-alpha don't recognize this, and versions
  404. before 0.1.2.5-alpha will reject descriptors containing it unless
  405. it is prefixed with "opt"; it should be so prefixed until these
  406. versions are obsolete.]
  407. "hidden-service-dir" *(SP VersionNum) NL
  408. [At most once.]
  409. Present only if this router stores and serves hidden service
  410. descriptors. If any VersionNum(s) are specified, this router
  411. supports those descriptor versions. If none are specified, it
  412. defaults to version 2 descriptors.
  413. [Versions of Tor before 0.1.2.5-alpha rejected router descriptors
  414. with unrecognized items; the protocols line should be preceded with
  415. an "opt" until these Tors are obsolete.]
  416. "protocols" SP "Link" SP LINK-VERSION-LIST SP "Circuit" SP
  417. CIRCUIT-VERSION-LIST NL
  418. [At most once.]
  419. Both lists are space-separated sequences of numbers, to indicate which
  420. protocols the server supports. As of 30 Mar 2008, specified
  421. protocols are "Link 1 2 Circuit 1". See section 4.1 of tor-spec.txt
  422. for more information about link protocol versions.
  423. [Versions of Tor before 0.1.2.5-alpha rejected router descriptors
  424. with unrecognized items; the protocols line should be preceded with
  425. an "opt" until these Tors are obsolete.]
  426. "allow-single-hop-exits"
  427. [At most one.]
  428. Present only if the router allows single-hop circuits to make exit
  429. connections. Most Tor servers do not support this: this is
  430. included for specialized controllers designed to support perspective
  431. access and such.
  432. 2.2. Extra-info documents
  433. Extra-info documents consist of the following items:
  434. "extra-info" Nickname Fingerprint NL
  435. [At start, exactly once.]
  436. Identifies what router this is an extra info descriptor for.
  437. Fingerprint is encoded in hex (using upper-case letters), with
  438. no spaces.
  439. "published"
  440. [Exactly once.]
  441. The time, in GMT, when this document (and its corresponding router
  442. descriptor if any) was generated. It MUST match the published time
  443. in the corresponding router descriptor.
  444. "read-history" YYYY-MM-DD HH:MM:SS (NSEC s) NUM,NUM,NUM,NUM,NUM... NL
  445. [At most once.]
  446. "write-history" YYYY-MM-DD HH:MM:SS (NSEC s) NUM,NUM,NUM,NUM,NUM... NL
  447. [At most once.]
  448. As documented in 2.1 above. See migration notes in section 2.2.1.
  449. "geoip-start" YYYY-MM-DD HH:MM:SS NL
  450. "geoip-client-origins" CC=N,CC=N,... NL
  451. Only generated by bridge routers (see blocking.pdf), and only
  452. when they have been configured with a geoip database.
  453. Non-bridges SHOULD NOT generate these fields. Contains a list
  454. of mappings from two-letter country codes (CC) to the number
  455. of clients that have connected to that bridge from that
  456. country (approximate, and rounded up to the nearest multiple of 8
  457. in order to hamper traffic analysis). A country is included
  458. only if it has at least one address. The time in
  459. "geoip-start" is the time at which we began collecting geoip
  460. statistics.
  461. "router-signature" NL Signature NL
  462. [At end, exactly once.]
  463. A document signature as documented in section 1.3, using the
  464. initial item "extra-info" and the final item "router-signature",
  465. signed with the router's identity key.
  466. 2.2.1. Moving history fields to extra-info documents.
  467. Tools that want to use the read-history and write-history values SHOULD
  468. download extra-info documents as well as router descriptors. Such
  469. tools SHOULD accept history values from both sources; if they appear in
  470. both documents, the values in the extra-info documents are authoritative.
  471. New versions of Tor no longer generate router descriptors
  472. containing read-history or write-history. Tools should continue to
  473. accept read-history and write-history values in router descriptors
  474. produced by older versions of Tor until all Tor versions earlier
  475. than 0.2.0.x are obsolete.
  476. 2.3. Nonterminals in router descriptors
  477. nickname ::= between 1 and 19 alphanumeric characters ([A-Za-z0-9]),
  478. case-insensitive.
  479. hexdigest ::= a '$', followed by 40 hexadecimal characters
  480. ([A-Fa-f0-9]). [Represents a server by the digest of its identity
  481. key.]
  482. exitpattern ::= addrspec ":" portspec
  483. portspec ::= "*" | port | port "-" port
  484. port ::= an integer between 1 and 65535, inclusive.
  485. [Some implementations incorrectly generate ports with value 0.
  486. Implementations SHOULD accept this, and SHOULD NOT generate it.
  487. Connections to port 0 are never permitted.]
  488. addrspec ::= "*" | ip4spec | ip6spec
  489. ipv4spec ::= ip4 | ip4 "/" num_ip4_bits | ip4 "/" ip4mask
  490. ip4 ::= an IPv4 address in dotted-quad format
  491. ip4mask ::= an IPv4 mask in dotted-quad format
  492. num_ip4_bits ::= an integer between 0 and 32
  493. ip6spec ::= ip6 | ip6 "/" num_ip6_bits
  494. ip6 ::= an IPv6 address, surrounded by square brackets.
  495. num_ip6_bits ::= an integer between 0 and 128
  496. bool ::= "0" | "1"
  497. 3. Formats produced by directory authorities.
  498. Every authority has two keys used in this protocol: a signing key, and
  499. an authority identity key. (Authorities also have a router identity
  500. key used in their role as a router and by earlier versions of the
  501. directory protocol.) The identity key is used from time to time to
  502. sign new key certificates using new signing keys; it is very sensitive.
  503. The signing key is used to sign key certificates and status documents.
  504. There are three kinds of documents generated by directory authorities:
  505. Key certificates
  506. Status votes
  507. Status consensuses
  508. Each is discussed below.
  509. 3.1. Key certificates
  510. Key certificates consist of the following items:
  511. "dir-key-certificate-version" version NL
  512. [At start, exactly once.]
  513. Determines the version of the key certificate. MUST be "3" for
  514. the protocol described in this document. Implementations MUST
  515. reject formats they don't understand.
  516. "dir-address" IPPort NL
  517. [At most once]
  518. An IP:Port for this authority's directory port.
  519. "fingerprint" fingerprint NL
  520. [Exactly once.]
  521. Hexadecimal encoding without spaces based on the authority's
  522. identity key.
  523. "dir-identity-key" NL a public key in PEM format
  524. [Exactly once.]
  525. The long-term authority identity key for this authority. This key
  526. SHOULD be at least 2048 bits long; it MUST NOT be shorter than
  527. 1024 bits.
  528. "dir-key-published" YYYY-MM-DD HH:MM:SS NL
  529. [Exactly once.]
  530. The time (in GMT) when this document and corresponding key were
  531. last generated.
  532. "dir-key-expires" YYYY-MM-DD HH:MM:SS NL
  533. [Exactly once.]
  534. A time (in GMT) after which this key is no longer valid.
  535. "dir-signing-key" NL a key in PEM format
  536. [Exactly once.]
  537. The directory server's public signing key. This key MUST be at
  538. least 1024 bits, and MAY be longer.
  539. "dir-key-crosscert" NL CrossSignature NL
  540. [At most once.]
  541. NOTE: Authorities MUST include this field in all newly generated
  542. certificates. A future version of this specification will make
  543. the field required.
  544. CrossSignature is a signature, made using the certificate's signing
  545. key, of the digest of the PKCS1-padded hash of the certificate's
  546. identity key. For backward compatibility with broken versions of the
  547. parser, we wrap the base64-encoded signature in -----BEGIN ID
  548. SIGNATURE---- and -----END ID SIGNATURE----- tags. Implementations
  549. MUST allow the "ID " portion to be omitted, however.
  550. When encountering a certificate with a dir-key-crosscert entry,
  551. implementations MUST verify that the signature is a correct signature
  552. of the hash of the identity key using the signing key.
  553. "dir-key-certification" NL Signature NL
  554. [At end, exactly once.]
  555. A document signature as documented in section 1.3, using the
  556. initial item "dir-key-certificate-version" and the final item
  557. "dir-key-certification", signed with the authority identity key.
  558. Authorities MUST generate a new signing key and corresponding
  559. certificate before the key expires.
  560. 3.2. Vote and consensus status documents
  561. Votes and consensuses are more strictly formatted then other documents
  562. in this specification, since different authorities must be able to
  563. generate exactly the same consensus given the same set of votes.
  564. The procedure for deciding when to generate vote and consensus status
  565. documents are described in section XXX below.
  566. Status documents contain a preamble, an authority section, a list of
  567. router status entries, and one more footers signature, in that order.
  568. Unlike other formats described above, a SP in these documents must be a
  569. single space character (hex 20).
  570. Some items appear only in votes, and some items appear only in
  571. consensuses. Unless specified, items occur in both.
  572. The preamble contains the following items. They MUST occur in the
  573. order given here:
  574. "network-status-version" SP version NL.
  575. [At start, exactly once.]
  576. A document format version. For this specification, the version is
  577. "3".
  578. "vote-status" SP type NL
  579. [Exactly once.]
  580. The status MUST be "vote" or "consensus", depending on the type of
  581. the document.
  582. "consensus-methods" SP IntegerList NL
  583. [Exactly once for votes; does not occur in consensuses.]
  584. A space-separated list of supported methods for generating
  585. consensuses from votes. See section 3.4.1 for details. Method "1"
  586. MUST be included.
  587. "consensus-method" SP Integer NL
  588. [Exactly once for consensuses; does not occur in votes.]
  589. See section 3.4.1 for details.
  590. (Only included when the vote is generated with consensus-method 2 or
  591. later.)
  592. "published" SP YYYY-MM-DD SP HH:MM:SS NL
  593. [Exactly once for votes; does not occur in consensuses.]
  594. The publication time for this status document (if a vote).
  595. "valid-after" SP YYYY-MM-DD SP HH:MM:SS NL
  596. [Exactly once.]
  597. The start of the Interval for this vote. Before this time, the
  598. consensus document produced from this vote should not be used.
  599. See 1.4 for voting timeline information.
  600. "fresh-until" SP YYYY-MM-DD SP HH:MM:SS NL
  601. [Exactly once.]
  602. The time at which the next consensus should be produced; before this
  603. time, there is no point in downloading another consensus, since there
  604. won't be a new one. See 1.4 for voting timeline information.
  605. "valid-until" SP YYYY-MM-DD SP HH:MM:SS NL
  606. [Exactly once.]
  607. The end of the Interval for this vote. After this time, the
  608. consensus produced by this vote should not be used. See 1.4 for
  609. voting timeline information.
  610. "voting-delay" SP VoteSeconds SP DistSeconds NL
  611. [Exactly once.]
  612. VoteSeconds is the number of seconds that we will allow to collect
  613. votes from all authorities; DistSeconds is the number of seconds
  614. we'll allow to collect signatures from all authorities. See 1.4 for
  615. voting timeline information.
  616. "client-versions" SP VersionList NL
  617. [At most once.]
  618. A comma-separated list of recommended client versions, in
  619. ascending order. If absent, no opinion is held about client
  620. versions.
  621. "server-versions" SP VersionList NL
  622. [At most once.]
  623. A comma-separated list of recommended server versions, in
  624. ascending order. If absent, no opinion is held about server
  625. versions.
  626. "known-flags" SP FlagList NL
  627. [Exactly once.]
  628. A space-separated list of all of the flags that this document
  629. might contain. A flag is "known" either because the authority
  630. knows about them and might set them (if in a vote), or because
  631. enough votes were counted for the consensus for an authoritative
  632. opinion to have been formed about their status.
  633. The authority section of a vote contains the following items, followed
  634. in turn by the authority's current key certificate:
  635. "dir-source" SP nickname SP identity SP address SP IP SP dirport SP
  636. orport NL
  637. [Exactly once, at start]
  638. Describes this authority. The nickname is a convenient identifier
  639. for the authority. The identity is an uppercase hex fingerprint of
  640. the authority's current (v3 authority) identity key. The address is
  641. the server's hostname. The IP is the server's current IP address,
  642. and dirport is its current directory port. XXXXorport
  643. "contact" SP string NL
  644. [At most once.]
  645. An arbitrary string describing how to contact the directory
  646. server's administrator. Administrators should include at least an
  647. email address and a PGP fingerprint.
  648. "legacy-key" SP FINGERPRINT NL
  649. [At most once]
  650. Lists a fingerprint for an obsolete _identity_ key still used
  651. by this authority to keep older clients working. This option
  652. is used to keep key around for a little while in case the
  653. authorities need to migrate many identity keys at once.
  654. (Generally, this would only happen because of a security
  655. vulnerability that affected multiple authorities, like the
  656. Debian OpenSSL RNG bug of May 2008.)
  657. The authority section of a consensus contains groups the following items,
  658. in the order given, with one group for each authority that contributed to
  659. the consensus, with groups sorted by authority identity digest:
  660. "dir-source" SP nickname SP identity SP address SP IP SP dirport SP
  661. orport NL
  662. [Exactly once, at start]
  663. As in the authority section of a vote.
  664. "contact" SP string NL
  665. [At most once.]
  666. As in the authority section of a vote.
  667. "vote-digest" SP digest NL
  668. [Exactly once.]
  669. A digest of the vote from the authority that contributed to this
  670. consensus, as signed (that is, not including the signature).
  671. (Hex, upper-case.)
  672. Each router status entry contains the following items. Router status
  673. entries are sorted in ascending order by identity digest.
  674. "r" SP nickname SP identity SP digest SP publication SP IP SP ORPort
  675. SP DirPort NL
  676. [At start, exactly once.]
  677. "Nickname" is the OR's nickname. "Identity" is a hash of its
  678. identity key, encoded in base64, with trailing equals sign(s)
  679. removed. "Digest" is a hash of its most recent descriptor as
  680. signed (that is, not including the signature), encoded in base64.
  681. "Publication" is the
  682. publication time of its most recent descriptor, in the form
  683. YYYY-MM-DD HH:MM:SS, in GMT. "IP" is its current IP address;
  684. ORPort is its current OR port, "DirPort" is it's current directory
  685. port, or "0" for "none".
  686. "s" SP Flags NL
  687. [At most once.]
  688. A series of space-separated status flags, in alphabetical order.
  689. Currently documented flags are:
  690. "Authority" if the router is a directory authority.
  691. "BadExit" if the router is believed to be useless as an exit node
  692. (because its ISP censors it, because it is behind a restrictive
  693. proxy, or for some similar reason).
  694. "BadDirectory" if the router is believed to be useless as a
  695. directory cache (because its directory port isn't working,
  696. its bandwidth is always throttled, or for some similar
  697. reason).
  698. "Exit" if the router is more useful for building
  699. general-purpose exit circuits than for relay circuits. The
  700. path building algorithm uses this flag; see path-spec.txt.
  701. "Fast" if the router is suitable for high-bandwidth circuits.
  702. "Guard" if the router is suitable for use as an entry guard.
  703. "HSDir" if the router is considered a v2 hidden service directory.
  704. "Named" if the router's identity-nickname mapping is canonical,
  705. and this authority binds names.
  706. "Stable" if the router is suitable for long-lived circuits.
  707. "Running" if the router is currently usable.
  708. "Unnamed" if another router has bound the name used by this
  709. router, and this authority binds names.
  710. "Valid" if the router has been 'validated'.
  711. "V2Dir" if the router implements the v2 directory protocol.
  712. "V3Dir" if the router implements this protocol.
  713. "v" SP version NL
  714. [At most once.]
  715. The version of the Tor protocol that this server is running. If
  716. the value begins with "Tor" SP, the rest of the string is a Tor
  717. version number, and the protocol is "The Tor protocol as supported
  718. by the given version of Tor." Otherwise, if the value begins with
  719. some other string, Tor has upgraded to a more sophisticated
  720. protocol versioning system, and the protocol is "a version of the
  721. Tor protocol more recent than any we recognize."
  722. Directory authorities SHOULD omit version strings they receive from
  723. descriptors if they would cause "v" lines to be over 128 characters
  724. long.
  725. "w" SP "Bandwidth=" INT NL
  726. [At most once.]
  727. An estimate of the bandwidth of this server, in an arbitrary
  728. unit (currently kilobytes per second). Used to weight router
  729. selection. Other weighting keywords may be added later.
  730. Clients MUST ignore keywords they do not recognize.
  731. "p" SP ("accept" / "reject") SP PortList NL
  732. [At most once.]
  733. PortList = PortOrRange
  734. PortList = PortList "," PortOrRange
  735. PortOrRange = INT "-" INT / INT
  736. A list of those ports that this router supports (if 'accept')
  737. or does not support (if 'reject') for exit to "most
  738. addresses".
  739. The signature section contains the following item, which appears
  740. Exactly Once for a vote, and At Least Once for a consensus.
  741. "directory-signature" SP identity SP signing-key-digest NL Signature
  742. This is a signature of the status document, with the initial item
  743. "network-status-version", and the signature item
  744. "directory-signature", using the signing key. (In this case, we take
  745. the hash through the _space_ after directory-signature, not the
  746. newline: this ensures that all authorities sign the same thing.)
  747. "identity" is the hex-encoded digest of the authority identity key of
  748. the signing authority, and "signing-key-digest" is the hex-encoded
  749. digest of the current authority signing key of the signing authority.
  750. 3.3. Deciding how to vote.
  751. (This section describes how directory authorities choose which status
  752. flags to apply to routers, as of Tor 0.2.0.0-alpha-dev. Later directory
  753. authorities MAY do things differently, so long as clients keep working
  754. well. Clients MUST NOT depend on the exact behaviors in this section.)
  755. In the below definitions, a router is considered "active" if it is
  756. running, valid, and not hibernating.
  757. "Valid" -- a router is 'Valid' if it is running a version of Tor not
  758. known to be broken, and the directory authority has not blacklisted
  759. it as suspicious.
  760. "Named" -- Directory authority administrators may decide to support name
  761. binding. If they do, then they must maintain a file of
  762. nickname-to-identity-key mappings, and try to keep this file consistent
  763. with other directory authorities. If they don't, they act as clients, and
  764. report bindings made by other directory authorities (name X is bound to
  765. identity Y if at least one binding directory lists it, and no directory
  766. binds X to some other Y'.) A router is called 'Named' if the router
  767. believes the given name should be bound to the given key.
  768. Two strategies exist on the current network for deciding on
  769. values for the Named flag. In the original version, server
  770. operators were asked to send nickname-identity pairs to a
  771. mailing list of Naming directory authorities operators. The
  772. operators were then supposed to add the pairs to their
  773. mapping files; in practice, they didn't get to this often.
  774. Newer Naming authorities run a script that registers routers
  775. in their mapping files once the routers have been online at
  776. least two weeks, no other router has that nickname, and no
  777. other router has wanted the nickname for a month. If a router
  778. has not been online for six months, the router is removed.
  779. "Unnamed" -- Directory authorities that support naming should vote for a
  780. router to be 'Unnamed' if its given nickname is mapped to a different
  781. identity.
  782. "Running" -- A router is 'Running' if the authority managed to connect to
  783. it successfully within the last 30 minutes.
  784. "Stable" -- A router is 'Stable' if it is active, and either its Weighted
  785. MTBF is at least the median for known active routers or its Weighted MTBF
  786. corresponds to at least 7 days. Routers are never called Stable if they are
  787. running a version of Tor known to drop circuits stupidly. (0.1.1.10-alpha
  788. through 0.1.1.16-rc are stupid this way.)
  789. To calculate weighted MTBF, compute the weighted mean of the lengths
  790. of all intervals when the router was observed to be up, weighting
  791. intervals by $\alpha^n$, where $n$ is the amount of time that has
  792. passed since the interval ended, and $\alpha$ is chosen so that
  793. measurements over approximately one month old no longer influence the
  794. weighted MTBF much.
  795. [XXXX what happens when we have less than 4 days of MTBF info.]
  796. "Exit" -- A router is called an 'Exit' iff it allows exits to at
  797. least two of the ports 80, 443, and 6667 and allows exits to at
  798. least one /8 address space.
  799. "Fast" -- A router is 'Fast' if it is active, and its bandwidth is
  800. either in the top 7/8ths for known active routers or at least 100KB/s.
  801. "Guard" -- A router is a possible 'Guard' if its Weighted Fractional
  802. Uptime is at least the median for "familiar" active routers, and if
  803. its bandwidth is at least median or at least 250KB/s.
  804. If the total bandwidth of active non-BadExit Exit servers is less
  805. than one third of the total bandwidth of all active servers, no Exit is
  806. listed as a Guard.
  807. To calculate weighted fractional uptime, compute the fraction
  808. of time that the router is up in any given day, weighting so that
  809. downtime and uptime in the past counts less.
  810. A node is 'familiar' if 1/8 of all active nodes have appeared more
  811. recently than it, OR it has been around for a few weeks.
  812. "Authority" -- A router is called an 'Authority' if the authority
  813. generating the network-status document believes it is an authority.
  814. "V2Dir" -- A router supports the v2 directory protocol if it has an open
  815. directory port, and it is running a version of the directory protocol that
  816. supports the functionality clients need. (Currently, this is
  817. 0.1.1.9-alpha or later.)
  818. "V3Dir" -- A router supports the v3 directory protocol if it has an open
  819. directory port, and it is running a version of the directory protocol that
  820. supports the functionality clients need. (Currently, this is
  821. 0.2.0.?????-alpha or later.)
  822. "HSDir" -- A router is a v2 hidden service directory if it stores and
  823. serves v2 hidden service descriptors and the authority managed to connect
  824. to it successfully within the last 24 hours.
  825. Directory server administrators may label some servers or IPs as
  826. blacklisted, and elect not to include them in their network-status lists.
  827. Authorities SHOULD 'disable' any servers in excess of 3 on any single IP.
  828. When there are more than 3 to choose from, authorities should first prefer
  829. authorities to non-authorities, then prefer Running to non-Running, and
  830. then prefer high-bandwidth to low-bandwidth. To 'disable' a server, the
  831. authority *should* advertise it without the Running or Valid flag.
  832. Thus, the network-status vote includes all non-blacklisted,
  833. non-expired, non-superseded descriptors.
  834. The bandwidth in a "w" line should be taken as the best estimate
  835. of the router's actual capacity that the authority has. For now,
  836. this should be the lesser of the observed bandwidth and bandwidth
  837. rate limit from the router descriptor. It is given in kilobytes
  838. per second, and capped at some arbitrary value (currently 10 MB/s).
  839. The ports listed in a "p" line should be taken as those ports for
  840. which the router's exit policy permits 'most' addresses, ignoring any
  841. accept not for all addresses, ignoring all rejects for private
  842. netblocks. "Most" addresses are permitted if no more than 2^25
  843. IPv4 addresses (two /8 networks) were blocked. The list is encoded
  844. as described in 3.4.2.
  845. 3.4. Computing a consensus from a set of votes
  846. Given a set of votes, authorities compute the contents of the consensus
  847. document as follows:
  848. The "valid-after", "valid-until", and "fresh-until" times are taken as
  849. the median of the respective values from all the votes.
  850. The times in the "voting-delay" line are taken as the median of the
  851. VoteSeconds and DistSeconds times in the votes.
  852. Known-flags is the union of all flags known by any voter.
  853. "client-versions" and "server-versions" are sorted in ascending
  854. order; A version is recommended in the consensus if it is recommended
  855. by more than half of the voting authorities that included a
  856. client-versions or server-versions lines in their votes.
  857. The authority item groups (dir-source, contact, fingerprint,
  858. vote-digest) are taken from the votes of the voting
  859. authorities. These groups are sorted by the digests of the
  860. authorities identity keys, in ascending order. If the consensus
  861. method is 3 or later, a dir-source line must be included for
  862. every vote with legacy-key entry, using the legacy-key's
  863. fingerprint, the voter's ordinary nickname with the string
  864. "-legacy" appended, and all other fields as from the original
  865. vote's dir-source line.
  866. A router status entry:
  867. * is included in the result if some router status entry with the same
  868. identity is included by more than half of the authorities (total
  869. authorities, not just those whose votes we have).
  870. * For any given identity, we include at most one router status entry.
  871. * A router entry has a flag set if that is included by more than half
  872. of the authorities who care about that flag.
  873. * Two router entries are "the same" if they have the same
  874. <descriptor digest, published time, nickname, IP, ports> tuple.
  875. We choose the tuple for a given router as whichever tuple appears
  876. for that router in the most votes. We break ties first in favor of
  877. the more recently published, then in favor of smaller server
  878. descriptor digest.
  879. * The Named flag appears if it is included for this routerstatus by
  880. _any_ authority, and if all authorities that list it list the same
  881. nickname. However, if consensus-method 2 or later is in use, and
  882. any authority calls this identity/nickname pair Unnamed, then
  883. this routerstatus does not get the Named flag.
  884. * If consensus-method 2 or later is in use, the Unnamed flag is
  885. set for a routerstatus if any authorities have voted for a different
  886. identities to be Named with that nickname, or if any authority
  887. lists that nickname/ID pair as Unnamed.
  888. (With consensus-method 1, Unnamed is set like any other flag.)
  889. * The version is given as whichever version is listed by the most
  890. voters, with ties decided in favor of more recent versions.
  891. * If consensus-method 4 or later is in use, then routers that
  892. do not have the Running flag are not listed at all.
  893. * If consensus-method 5 or later is in use, then the "w" line
  894. is generated using a low-median of the bandwidth values from
  895. the votes that included "w" lines for this router.
  896. * If consensus-method 5 or later is in use, then the "p" line
  897. is taken from the votes that have the same policy summary
  898. for the descriptor we are listing. (They should all be the
  899. same. If they are not, we pick the most commonly listed
  900. one, breaking ties in favor of the lexicographically larger
  901. vote.) The port list is encoded as specified in 3.4.2.
  902. The signatures at the end of a consensus document are sorted in
  903. ascending order by identity digest.
  904. All ties in computing medians are broken in favor of the smaller or
  905. earlier item.
  906. 3.4.1. Forward compatibility
  907. Future versions of Tor will need to include new information in the
  908. consensus documents, but it is important that all authorities (or at least
  909. half) generate and sign the same signed consensus.
  910. To achieve this, authorities list in their votes their supported methods
  911. for generating consensuses from votes. Later methods will be assigned
  912. higher numbers. Currently recognized methods:
  913. "1" -- The first implemented version.
  914. "2" -- Added support for the Unnamed flag.
  915. "3" -- Added legacy ID key support to aid in authority ID key rollovers
  916. "4" -- No longer list routers that are not running in the consensus
  917. "5" -- adds support for "w" and "p" lines.
  918. Before generating a consensus, an authority must decide which consensus
  919. method to use. To do this, it looks for the highest version number
  920. supported by more than 2/3 of the authorities voting. If it supports this
  921. method, then it uses it. Otherwise, it falls back to method 1.
  922. (The consensuses generated by new methods must be parsable by
  923. implementations that only understand the old methods, and must not cause
  924. those implementations to compromise their anonymity. This is a means for
  925. making changes in the contents of consensus; not for making
  926. backward-incompatible changes in their format.)
  927. 3.4.2. Encoding port lists
  928. Whether the summary shows the list of accepted ports or the list of
  929. rejected ports depends on which list is shorter (has a shorter string
  930. representation). In case of ties we choose the list of accepted
  931. ports. As an exception to this rule an allow-all policy is
  932. represented as "accept 1-65535" instead of "reject " and a reject-all
  933. policy is similarly given as "reject 1-65535".
  934. Summary items are compressed, that is instead of "80-88,89-100" there
  935. only is a single item of "80-100", similarly instead of "20,21" a
  936. summary will say "20-21".
  937. Port lists are sorted in ascending order.
  938. The maximum allowed length of a policy summary (including the "accept "
  939. or "reject ") is 1000 characters. If a summary exceeds that length we
  940. use an accept-style summary and list as much of the port list as is
  941. possible within these 1000 bytes. [XXXX be more specific.]
  942. 3.5. Detached signatures
  943. Assuming full connectivity, every authority should compute and sign the
  944. same consensus directory in each period. Therefore, it isn't necessary to
  945. download the consensus computed by each authority; instead, the
  946. authorities only push/fetch each others' signatures. A "detached
  947. signature" document contains items as follows:
  948. "consensus-digest" SP Digest NL
  949. [At start, at most once.]
  950. The digest of the consensus being signed.
  951. "valid-after" SP YYYY-MM-DD SP HH:MM:SS NL
  952. "fresh-until" SP YYYY-MM-DD SP HH:MM:SS NL
  953. "valid-until" SP YYYY-MM-DD SP HH:MM:SS NL
  954. [As in the consensus]
  955. "directory-signature"
  956. [As in the consensus; the signature object is the same as in the
  957. consensus document.]
  958. 4. Directory server operation
  959. All directory authorities and directory caches ("directory servers")
  960. implement this section, except as noted.
  961. 4.1. Accepting uploads (authorities only)
  962. When a router posts a signed descriptor to a directory authority, the
  963. authority first checks whether it is well-formed and correctly
  964. self-signed. If it is, the authority next verifies that the nickname
  965. in question is not already assigned to a router with a different
  966. public key.
  967. Finally, the authority MAY check that the router is not blacklisted
  968. because of its key, IP, or another reason.
  969. If the descriptor passes these tests, and the authority does not already
  970. have a descriptor for a router with this public key, it accepts the
  971. descriptor and remembers it.
  972. If the authority _does_ have a descriptor with the same public key, the
  973. newly uploaded descriptor is remembered if its publication time is more
  974. recent than the most recent old descriptor for that router, and either:
  975. - There are non-cosmetic differences between the old descriptor and the
  976. new one.
  977. - Enough time has passed between the descriptors' publication times.
  978. (Currently, 12 hours.)
  979. Differences between router descriptors are "non-cosmetic" if they would be
  980. sufficient to force an upload as described in section 2 above.
  981. Note that the "cosmetic difference" test only applies to uploaded
  982. descriptors, not to descriptors that the authority downloads from other
  983. authorities.
  984. When a router posts a signed extra-info document to a directory authority,
  985. the authority again checks it for well-formedness and correct signature,
  986. and checks that its matches the extra-info-digest in some router
  987. descriptor that it believes is currently useful. If so, it accepts it and
  988. stores it and serves it as requested. If not, it drops it.
  989. 4.2. Voting (authorities only)
  990. Authorities divide time into Intervals. Authority administrators SHOULD
  991. try to all pick the same interval length, and SHOULD pick intervals that
  992. are commonly used divisions of time (e.g., 5 minutes, 15 minutes, 30
  993. minutes, 60 minutes, 90 minutes). Voting intervals SHOULD be chosen to
  994. divide evenly into a 24-hour day.
  995. Authorities SHOULD act according to interval and delays in the
  996. latest consensus. Lacking a latest consensus, they SHOULD default to a
  997. 30-minute Interval, a 5 minute VotingDelay, and a 5 minute DistDelay.
  998. Authorities MUST take pains to ensure that their clocks remain accurate
  999. within a few seconds. (Running NTP is usually sufficient.)
  1000. The first voting period of each day begins at 00:00 (midnight) GMT. If
  1001. the last period of the day would be truncated by one-half or more, it is
  1002. merged with the second-to-last period.
  1003. An authority SHOULD publish its vote immediately at the start of each voting
  1004. period (minus VoteSeconds+DistSeconds). It does this by making it
  1005. available at
  1006. http://<hostname>/tor/status-vote/next/authority.z
  1007. and sending it in an HTTP POST request to each other authority at the URL
  1008. http://<hostname>/tor/post/vote
  1009. If, at the start of the voting period, minus DistSeconds, an authority
  1010. does not have a current statement from another authority, the first
  1011. authority downloads the other's statement.
  1012. Once an authority has a vote from another authority, it makes it available
  1013. at
  1014. http://<hostname>/tor/status-vote/next/<fp>.z
  1015. where <fp> is the fingerprint of the other authority's identity key.
  1016. And at
  1017. http://<hostname>/tor/status-vote/next/d/<d>.z
  1018. where <d> is the digest of the vote document.
  1019. The consensus status, along with as many signatures as the server
  1020. currently knows, should be available at
  1021. http://<hostname>/tor/status-vote/next/consensus.z
  1022. All of the detached signatures it knows for consensus status should be
  1023. available at:
  1024. http://<hostname>/tor/status-vote/next/consensus-signatures.z
  1025. Once there are enough signatures, or once the voting period starts,
  1026. these documents are available at
  1027. http://<hostname>/tor/status-vote/current/consensus.z
  1028. and
  1029. http://<hostname>/tor/status-vote/current/consensus-signatures.z
  1030. [XXX current/consensus-signatures is not currently implemented, as it
  1031. is not used in the voting protocol.]
  1032. The other vote documents are analogously made available under
  1033. http://<hostname>/tor/status-vote/current/authority.z
  1034. http://<hostname>/tor/status-vote/current/<fp>.z
  1035. http://<hostname>/tor/status-vote/current/d/<d>.z
  1036. once the consensus is complete.
  1037. Once an authority has computed and signed a consensus network status, it
  1038. should send its detached signature to each other authority in an HTTP POST
  1039. request to the URL:
  1040. http://<hostname>/tor/post/consensus-signature
  1041. [XXX Note why we support push-and-then-pull.]
  1042. [XXX possible future features include support for downloading old
  1043. consensuses.]
  1044. 4.3. Downloading consensus status documents (caches only)
  1045. All directory servers (authorities and caches) try to keep a recent
  1046. network-status consensus document to serve to clients. A cache ALWAYS
  1047. downloads a network-status consensus if any of the following are true:
  1048. - The cache has no consensus document.
  1049. - The cache's consensus document is no longer valid.
  1050. Otherwise, the cache downloads a new consensus document at a randomly
  1051. chosen time in the first half-interval after its current consensus
  1052. stops being fresh. (This time is chosen at random to avoid swarming
  1053. the authorities at the start of each period. The interval size is
  1054. inferred from the difference between the valid-after time and the
  1055. fresh-until time on the consensus.)
  1056. [For example, if a cache has a consensus that became valid at 1:00,
  1057. and is fresh until 2:00, that cache will fetch a new consensus at
  1058. a random time between 2:00 and 2:30.]
  1059. 4.4. Downloading and storing router descriptors (authorities and caches)
  1060. Periodically (currently, every 10 seconds), directory servers check
  1061. whether there are any specific descriptors that they do not have and that
  1062. they are not currently trying to download. Caches identify these
  1063. descriptors by hash in the recent network-status consensus documents;
  1064. authorities identify them by hash in vote (if publication date is more
  1065. recent than the descriptor we currently have).
  1066. [XXXX need a way to fetch descriptors ahead of the vote? v2 status docs can
  1067. do that for now.]
  1068. If so, the directory server launches requests to the authorities for these
  1069. descriptors, such that each authority is only asked for descriptors listed
  1070. in its most recent vote (if the requester is an authority) or in the
  1071. consensus (if the requester is a cache). If we're an authority, and more
  1072. than one authority lists the descriptor, we choose which to ask at random.
  1073. If one of these downloads fails, we do not try to download that descriptor
  1074. from the authority that failed to serve it again unless we receive a newer
  1075. network-status (consensus or vote) from that authority that lists the same
  1076. descriptor.
  1077. Directory servers must potentially cache multiple descriptors for each
  1078. router. Servers must not discard any descriptor listed by any recent
  1079. consensus. If there is enough space to store additional descriptors,
  1080. servers SHOULD try to hold those which clients are likely to download the
  1081. most. (Currently, this is judged based on the interval for which each
  1082. descriptor seemed newest.)
  1083. [XXXX define recent]
  1084. Authorities SHOULD NOT download descriptors for routers that they would
  1085. immediately reject for reasons listed in 3.1.
  1086. 4.5. Downloading and storing extra-info documents
  1087. All authorities, and any cache that chooses to cache extra-info documents,
  1088. and any client that uses extra-info documents, should implement this
  1089. section.
  1090. Note that generally, clients don't need extra-info documents.
  1091. Periodically, the Tor instance checks whether it is missing any extra-info
  1092. documents: in other words, if it has any router descriptors with an
  1093. extra-info-digest field that does not match any of the extra-info
  1094. documents currently held. If so, it downloads whatever extra-info
  1095. documents are missing. Caches download from authorities; non-caches try
  1096. to download from caches. We follow the same splitting and back-off rules
  1097. as in 4.4 (if a cache) or 5.3 (if a client).
  1098. 4.6. General-use HTTP URLs
  1099. "Fingerprints" in these URLs are base-16-encoded SHA1 hashes.
  1100. The most recent v3 consensus should be available at:
  1101. http://<hostname>/tor/status-vote/current/consensus.z
  1102. Starting with Tor version 0.2.1.1-alpha is also available at:
  1103. http://<hostname>/tor/status-vote/current/consensus/<F1>+<F2>+<F3>.z
  1104. Where F1, F2, etc. are authority identity fingerprints the client trusts.
  1105. Servers will only return a consensus if more than half of the requested
  1106. authorities have signed the document, otherwise a 404 error will be sent
  1107. back. The fingerprints can be shortened to a length of any multiple of
  1108. two, using only the leftmost part of the encoded fingerprint. Tor uses
  1109. 3 bytes (6 hex characters) of the fingerprint.
  1110. Clients SHOULD sort the fingerprints in ascending order. Server MUST
  1111. accept any order.
  1112. Clients SHOULD use this format when requesting consensus documents from
  1113. directory authority servers and from caches running a version of Tor
  1114. that is known to support this URL format.
  1115. A concatenated set of all the current key certificates should be available
  1116. at:
  1117. http://<hostname>/tor/keys/all.z
  1118. The key certificate for this server (if it is an authority) should be
  1119. available at:
  1120. http://<hostname>/tor/keys/authority.z
  1121. The key certificate for an authority whose authority identity fingerprint
  1122. is <F> should be available at:
  1123. http://<hostname>/tor/keys/fp/<F>.z
  1124. The key certificate whose signing key fingerprint is <F> should be
  1125. available at:
  1126. http://<hostname>/tor/keys/sk/<F>.z
  1127. The key certificate whose identity key fingerprint is <F> and whose signing
  1128. key fingerprint is <S> should be available at:
  1129. http://<hostname>/tor/keys/fp-sk/<F>-<S>.z
  1130. (As usual, clients may request multiple certificates using:
  1131. http://<hostname>/tor/keys/fp-sk/<F1>-<S1>+<F2>-<S2>.z )
  1132. [The above fp-sk format was not supported before Tor 0.2.1.9-alpha.]
  1133. The most recent descriptor for a server whose identity key has a
  1134. fingerprint of <F> should be available at:
  1135. http://<hostname>/tor/server/fp/<F>.z
  1136. The most recent descriptors for servers with identity fingerprints
  1137. <F1>,<F2>,<F3> should be available at:
  1138. http://<hostname>/tor/server/fp/<F1>+<F2>+<F3>.z
  1139. (NOTE: Implementations SHOULD NOT download descriptors by identity key
  1140. fingerprint. This allows a corrupted server (in collusion with a cache) to
  1141. provide a unique descriptor to a client, and thereby partition that client
  1142. from the rest of the network.)
  1143. The server descriptor with (descriptor) digest <D> (in hex) should be
  1144. available at:
  1145. http://<hostname>/tor/server/d/<D>.z
  1146. The most recent descriptors with digests <D1>,<D2>,<D3> should be
  1147. available at:
  1148. http://<hostname>/tor/server/d/<D1>+<D2>+<D3>.z
  1149. The most recent descriptor for this server should be at:
  1150. http://<hostname>/tor/server/authority.z
  1151. [Nothing in the Tor protocol uses this resource yet, but it is useful
  1152. for debugging purposes. Also, the official Tor implementations
  1153. (starting at 0.1.1.x) use this resource to test whether a server's
  1154. own DirPort is reachable.]
  1155. A concatenated set of the most recent descriptors for all known servers
  1156. should be available at:
  1157. http://<hostname>/tor/server/all.z
  1158. Extra-info documents are available at the URLS
  1159. http://<hostname>/tor/extra/d/...
  1160. http://<hostname>/tor/extra/fp/...
  1161. http://<hostname>/tor/extra/all[.z]
  1162. http://<hostname>/tor/extra/authority[.z]
  1163. (As for /tor/server/ URLs: supports fetching extra-info
  1164. documents by their digest, by the fingerprint of their servers,
  1165. or all at once. When serving by fingerprint, we serve the
  1166. extra-info that corresponds to the descriptor we would serve by
  1167. that fingerprint. Only directory authorities of version
  1168. 0.2.0.1-alpha or later are guaranteed to support the first
  1169. three classes of URLs. Caches may support them, and MUST
  1170. support them if they have advertised "caches-extra-info".)
  1171. For debugging, directories SHOULD expose non-compressed objects at URLs like
  1172. the above, but without the final ".z".
  1173. Clients MUST handle compressed concatenated information in two forms:
  1174. - A concatenated list of zlib-compressed objects.
  1175. - A zlib-compressed concatenated list of objects.
  1176. Directory servers MAY generate either format: the former requires less
  1177. CPU, but the latter requires less bandwidth.
  1178. Clients SHOULD use upper case letters (A-F) when base16-encoding
  1179. fingerprints. Servers MUST accept both upper and lower case fingerprints
  1180. in requests.
  1181. 5. Client operation: downloading information
  1182. Every Tor that is not a directory server (that is, those that do
  1183. not have a DirPort set) implements this section.
  1184. 5.1. Downloading network-status documents
  1185. Each client maintains a list of directory authorities. Insofar as
  1186. possible, clients SHOULD all use the same list.
  1187. Clients try to have a live consensus network-status document at all times.
  1188. A network-status document is "live" if the time in its valid-until field
  1189. has not passed.
  1190. If a client is missing a live network-status document, it tries to fetch
  1191. it from a directory cache (or from an authority if it knows no caches).
  1192. On failure, the client waits briefly, then tries that network-status
  1193. document again from another cache. The client does not build circuits
  1194. until it has a live network-status consensus document, and it has
  1195. descriptors for more than 1/4 of the routers that it believes are running.
  1196. (Note: clients can and should pick caches based on the network-status
  1197. information they have: once they have first fetched network-status info
  1198. from an authority, they should not need to go to the authority directly
  1199. again.)
  1200. To avoid swarming the caches whenever a consensus expires, the
  1201. clients download new consensuses at a randomly chosen time after the
  1202. caches are expected to have a fresh consensus, but before their
  1203. consensus will expire. (This time is chosen uniformly at random from
  1204. the interval between the time 3/4 into the first interval after the
  1205. consensus is no longer fresh, and 7/8 of the time remaining after
  1206. that before the consensus is invalid.)
  1207. [For example, if a cache has a consensus that became valid at 1:00,
  1208. and is fresh until 2:00, and expires at 4:00, that cache will fetch
  1209. a new consensus at a random time between 2:45 and 3:50, since 3/4
  1210. of the one-hour interval is 45 minutes, and 7/8 of the remaining 75
  1211. minutes is 65 minutes.]
  1212. 5.2. Downloading and storing router descriptors
  1213. Clients try to have the best descriptor for each router. A descriptor is
  1214. "best" if:
  1215. * It is listed in the consensus network-status document.
  1216. Periodically (currently every 10 seconds) clients check whether there are
  1217. any "downloadable" descriptors. A descriptor is downloadable if:
  1218. - It is the "best" descriptor for some router.
  1219. - The descriptor was published at least 10 minutes in the past.
  1220. (This prevents clients from trying to fetch descriptors that the
  1221. mirrors have probably not yet retrieved and cached.)
  1222. - The client does not currently have it.
  1223. - The client is not currently trying to download it.
  1224. - The client would not discard it immediately upon receiving it.
  1225. - The client thinks it is running and valid (see 6.1 below).
  1226. If at least 16 known routers have downloadable descriptors, or if
  1227. enough time (currently 10 minutes) has passed since the last time the
  1228. client tried to download descriptors, it launches requests for all
  1229. downloadable descriptors, as described in 5.3 below.
  1230. When a descriptor download fails, the client notes it, and does not
  1231. consider the descriptor downloadable again until a certain amount of time
  1232. has passed. (Currently 0 seconds for the first failure, 60 seconds for the
  1233. second, 5 minutes for the third, 10 minutes for the fourth, and 1 day
  1234. thereafter.) Periodically (currently once an hour) clients reset the
  1235. failure count.
  1236. Clients retain the most recent descriptor they have downloaded for each
  1237. router so long as it is not too old (currently, 48 hours), OR so long as
  1238. no better descriptor has been downloaded for the same router.
  1239. [Versions of Tor before 0.1.2.3-alpha would discard descriptors simply for
  1240. being published too far in the past.] [The code seems to discard
  1241. descriptors in all cases after they're 5 days old. True? -RD]
  1242. 5.3. Managing downloads
  1243. When a client has no consensus network-status document, it downloads it
  1244. from a randomly chosen authority. In all other cases, the client
  1245. downloads from caches randomly chosen from among those believed to be V2
  1246. directory servers. (This information comes from the network-status
  1247. documents; see 6 below.)
  1248. When downloading multiple router descriptors, the client chooses multiple
  1249. mirrors so that:
  1250. - At least 3 different mirrors are used, except when this would result
  1251. in more than one request for under 4 descriptors.
  1252. - No more than 128 descriptors are requested from a single mirror.
  1253. - Otherwise, as few mirrors as possible are used.
  1254. After choosing mirrors, the client divides the descriptors among them
  1255. randomly.
  1256. After receiving any response client MUST discard any network-status
  1257. documents and descriptors that it did not request.
  1258. 6. Using directory information
  1259. Everyone besides directory authorities uses the approaches in this section
  1260. to decide which servers to use and what their keys are likely to be.
  1261. (Directory authorities just believe their own opinions, as in 3.1 above.)
  1262. 6.1. Choosing routers for circuits.
  1263. Circuits SHOULD NOT be built until the client has enough directory
  1264. information: a live consensus network status [XXXX fallback?] and
  1265. descriptors for at least 1/4 of the servers believed to be running.
  1266. A server is "listed" if it is included by the consensus network-status
  1267. document. Clients SHOULD NOT use unlisted servers.
  1268. These flags are used as follows:
  1269. - Clients SHOULD NOT use non-'Valid' or non-'Running' routers unless
  1270. requested to do so.
  1271. - Clients SHOULD NOT use non-'Fast' routers for any purpose other than
  1272. very-low-bandwidth circuits (such as introduction circuits).
  1273. - Clients SHOULD NOT use non-'Stable' routers for circuits that are
  1274. likely to need to be open for a very long time (such as those used for
  1275. IRC or SSH connections).
  1276. - Clients SHOULD NOT choose non-'Guard' nodes when picking entry guard
  1277. nodes.
  1278. - Clients SHOULD NOT download directory information from non-'V2Dir'
  1279. caches.
  1280. See the "path-spec.txt" document for more details.
  1281. 6.2. Managing naming
  1282. In order to provide human-memorable names for individual server
  1283. identities, some directory servers bind names to IDs. Clients handle
  1284. names in two ways:
  1285. When a client encounters a name it has not mapped before:
  1286. If the consensus lists any router with that name as "Named", or if
  1287. consensus-method 2 or later is in use and the consensus lists any
  1288. router with that name as having the "Unnamed" flag, then the name is
  1289. bound. (It's bound to the ID listed in the entry with the Named,
  1290. or to an unknown ID if no name is found.)
  1291. When the user refers to a bound name, the implementation SHOULD provide
  1292. only the router with ID bound to that name, and no other router, even
  1293. if the router with the right ID can't be found.
  1294. When a user tries to refer to a non-bound name, the implementation SHOULD
  1295. warn the user. After warning the user, the implementation MAY use any
  1296. router that advertises the name.
  1297. Not every router needs a nickname. When a router doesn't configure a
  1298. nickname, it publishes with the default nickname "Unnamed". Authorities
  1299. SHOULD NOT ever mark a router with this nickname as Named; client software
  1300. SHOULD NOT ever use a router in response to a user request for a router
  1301. called "Unnamed".
  1302. 6.3. Software versions
  1303. An implementation of Tor SHOULD warn when it has fetched a consensus
  1304. network-status, and it is running a software version not listed.
  1305. 6.4. Warning about a router's status.
  1306. If a router tries to publish its descriptor to a Naming authority
  1307. that has its nickname mapped to another key, the router SHOULD
  1308. warn the operator that it is either using the wrong key or is using
  1309. an already claimed nickname.
  1310. If a router has fetched a consensus document,, and the
  1311. authorities do not publish a binding for the router's nickname, the
  1312. router MAY remind the operator that the chosen nickname is not
  1313. bound to this key at the authorities, and suggest contacting the
  1314. authority operators.
  1315. ...
  1316. 6.5. Router protocol versions
  1317. A client should believe that a router supports a given feature if that
  1318. feature is supported by the router or protocol versions in more than half
  1319. of the live networkstatuses' "v" entries for that router. In other words,
  1320. if the "v" entries for some router are:
  1321. v Tor 0.0.8pre1 (from authority 1)
  1322. v Tor 0.1.2.11 (from authority 2)
  1323. v FutureProtocolDescription 99 (from authority 3)
  1324. then the client should believe that the router supports any feature
  1325. supported by 0.1.2.11.
  1326. This is currently equivalent to believing the median declared version for
  1327. a router in all live networkstatuses.
  1328. 7. Standards compliance
  1329. All clients and servers MUST support HTTP 1.0. Clients and servers MAY
  1330. support later versions of HTTP as well.
  1331. 7.1. HTTP headers
  1332. Servers MAY set the Content-Length: header. Servers SHOULD set
  1333. Content-Encoding to "deflate" or "identity".
  1334. Servers MAY include an X-Your-Address-Is: header, whose value is the
  1335. apparent IP address of the client connecting to them (as a dotted quad).
  1336. For directory connections tunneled over a BEGIN_DIR stream, servers SHOULD
  1337. report the IP from which the circuit carrying the BEGIN_DIR stream reached
  1338. them. [Servers before version 0.1.2.5-alpha reported 127.0.0.1 for all
  1339. BEGIN_DIR-tunneled connections.]
  1340. Servers SHOULD disable caching of multiple network statuses or multiple
  1341. router descriptors. Servers MAY enable caching of single descriptors,
  1342. single network statuses, the list of all router descriptors, a v1
  1343. directory, or a v1 running routers document. XXX mention times.
  1344. 7.2. HTTP status codes
  1345. Tor delivers the following status codes. Some were chosen without much
  1346. thought; other code SHOULD NOT rely on specific status codes yet.
  1347. 200 -- the operation completed successfully
  1348. -- the user requested statuses or serverdescs, and none of the ones we
  1349. requested were found (0.2.0.4-alpha and earlier).
  1350. 304 -- the client specified an if-modified-since time, and none of the
  1351. requested resources have changed since that time.
  1352. 400 -- the request is malformed, or
  1353. -- the URL is for a malformed variation of one of the URLs we support,
  1354. or
  1355. -- the client tried to post to a non-authority, or
  1356. -- the authority rejected a malformed posted document, or
  1357. 404 -- the requested document was not found.
  1358. -- the user requested statuses or serverdescs, and none of the ones
  1359. requested were found (0.2.0.5-alpha and later).
  1360. 503 -- we are declining the request in order to save bandwidth
  1361. -- user requested some items that we ordinarily generate or store,
  1362. but we do not have any available.
  1363. 9. Backward compatibility and migration plans
  1364. Until Tor versions before 0.1.1.x are completely obsolete, directory
  1365. authorities should generate, and mirrors should download and cache, v1
  1366. directories and running-routers lists, and allow old clients to download
  1367. them. These documents and the rules for retrieving, serving, and caching
  1368. them are described in dir-spec-v1.txt.
  1369. Until Tor versions before 0.2.0.x are completely obsolete, directory
  1370. authorities should generate, mirrors should download and cache, v2
  1371. network-status documents, and allow old clients to download them.
  1372. Additionally, all directory servers and caches should download, store, and
  1373. serve any router descriptor that is required because of v2 network-status
  1374. documents. These documents and the rules for retrieving, serving, and
  1375. caching them are described in dir-spec-v1.txt.
  1376. A. Consensus-negotiation timeline.
  1377. Period begins: this is the Published time.
  1378. Everybody sends votes
  1379. Reconciliation: everybody tries to fetch missing votes.
  1380. consensus may exist at this point.
  1381. End of voting period:
  1382. everyone swaps signatures.
  1383. Now it's okay for caches to download
  1384. Now it's okay for clients to download.
  1385. Valid-after/valid-until switchover