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2b2ad532ec
Incorporates changes based on atagar's review on #24742. Documents the contents of the manually modified initial fallback version 2.0.0 list, and future generated lists. Documents the format changes in the children of #22271. Closes #24742.
452 lines
16 KiB
Plaintext
452 lines
16 KiB
Plaintext
Tor Directory List Format
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Tim Wilson-Brown (teor)
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1. Scope and Preliminaries
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This document describes the format of Tor's directory lists, which are
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compiled and hard-coded into the tor binary. There is currently one
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list: the fallback directory mirrors. This list is also parsed by other
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libraries, like stem and metrics-lib. Alternate Tor implementations can
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use this list to bootstrap from the latest public Tor directory
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information.
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The FallbackDir feature was introduced by proposal 210, and was first
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supported by Tor in Tor version 0.2.4.7-alpha. The first hard-coded
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list was shipped in 0.2.8.1-alpha.
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The hard-coded fallback directory list is located in the tor source
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repository at:
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src/or/fallback_dirs.inc
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This document describes version 2.0.0 and later of the directory list
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format.
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Legacy, semi-structured versions of the fallback list were released with
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Tor 0.2.8.1-alpha through Tor 0.3.1.9. We call this format version 1.
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Stem and Relay Search have parsers for this legacy format.
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1.1. Format Overview
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A directory list is a C code fragment containing an array of C string
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constants. Each double-quoted C string constant is a valid torrc
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FallbackDir entry. Each entry contains various data fields.
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Directory lists do not include the C array's declaration, or the array's
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terminating NULL. Entries in directory lists do not include the
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FallbackDir torrc option. These are handled by the including C code.
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Directlry lists also include C-style comments and whitespace. The
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presence of whitespace may be significant, but the amount of whitespace
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is never significant. The type of whitespace is not significant to the
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C compiler or Tor C string parser. However, other parsers MAY rely on
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the distinction between newlines and spaces. (And that the only
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whitespace characters in the list are newlines and spaces.)
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The directory entry C string constants are split over multiple lines for
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readability. Structured C-style comments are used to provide additional
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data fields. This information is not used by Tor, but may be of interest
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to other libraries.
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The order of directory entries and data fields is not significant,
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except where noted below.
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1.2. Acknowledgements
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The original fallback directory script and format was created by
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weasel. The current script uses code written by gsathya & karsten.
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This specification was revised after feedback from:
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Damian Johnson ("atagar")
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Iain R. Learmonth ("irl")
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1.3. Format Versions
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1.0.0 - The legacy fallback directory list format
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2.0.0 - Adds name and extrainfo structured comments, and section separator
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comments to make the list easier to parse.
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2. Format Details
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Directory lists contain the following sections:
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- List Header (exactly once)
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- List Generation (exactly once, may be empty)
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- Directory Entry (zero or more times)
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Each section (or entry) ends with a separator.
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2.1. Nonterminals
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The following nonterminals are defined in the Onionoo details document
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specification:
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dir_address
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fingerprint
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nickname
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See https://metrics.torproject.org/onionoo.html#details
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The following nonterminals are defined in the "Tor directory protocol"
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specification in dir-spec.txt:
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Keyword
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ArgumentChar
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NL (newline)
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SP (space)
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bool (must not be confused with Onionoo's JSON "boolean")
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We derive the following nonterminals from Onionoo and dir-spec.txt:
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ipv4_or_port ::= port from an IPv4 or_addresses item
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The ipv4_or_port is the port part of an IPv4 address from the
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Onionoo or_addresses list.
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ipv6_or_address ::= an IPv6 or_addresses item
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The ipv6_or_address is an IPv6 address and port from the Onionoo
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or_addresses list. The address MAY be in the canonical RFC 5952
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IPv6 address format.
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A key-value pair:
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value ::= Zero or more ArgumentChar, excluding the following strings:
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* a double quotation mark (DQUOTE), and
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* the C comment terminators ("/*" and "*/").
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Note that the C++ comment ("//") and equals sign ("=") are
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not excluded, because they are reserved for future use in
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base64 values.
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key_value ::= Keyword "=" value
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We also define these additional nonterminals:
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number ::= An optional negative sign ("-"), followed by one or more
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numeric characters ([0-9]), with an optional decimal part
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(".", followed by one or more numeric characters).
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separator ::= "/*" SP+ "=====" SP+ "*/"
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2.2. List Header
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The list header consists of a number of key-value pairs, embedded in
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C-style comments.
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2.2.1 List Header Format
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"/*" SP+ "type=" Keyword SP+ "*/" SP* NL
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[At start, exactly once.]
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The type of directory entries in the list. Parsers SHOULD exit with
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an error if this is not the first line of the list, or if the value
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is anything other than "fallback".
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"/*" SP+ "version=" version_number SP+ "*/" SP* NL
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[In second position, exactly once.]
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The version of the directory list format. version_number uses
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semantic versioning: https://semver.org
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In particular:
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* major versions are used for incompatible changes, like
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removing non-optional fields
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* minor versions are used for compatible changes, like adding
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fields
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* patch versions are for bug fixes, like fixing an
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incorrectly-formatted Summary item
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Version 1.0.0 represents the undocumented, legacy fallback list
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format(s). Version 2.0.0 and later are documented by this
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specification.
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"/*" SP+ "timestamp=" number SP+ "*/" SP* NL
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[Exactly once.]
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A positive integer that indicates when this directory list was
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generated. This timestamp is guaranteed to increase for every
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version 2.0.0 and later directory list.
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The current timestamp format is YYYYMMDDHHMMSS, as an integer.
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"/*" SP+ key_value SP+ "*/" SP* NL
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[Zero or more times.]
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Future releases may include additional header fields. Parsers MUST NOT
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rely on the order of these additional fields. Additional header fields
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will be accompanied by a minor version increment.
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separator SP* NL
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The list header ends with the section separator.
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2.3. List Generation
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The list generation information consists of human-readable prose
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describing the content and origin of this directory list. It is contained
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in zero or more C-style comments, and may contain multi-line comments and
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uncommented C code.
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In particular, this section may contain C-style comments that contain
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an equals ("=") character. It may also be entirely empty.
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Future releases may arbitrarily change the content of this section.
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Parsers MUST NOT rely on a version increment when the format changes.
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2.3.1 List Generation Format
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In general, parsers MUST NOT rely on the format of this section.
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Parsers MAY rely on the following details:
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The list generation section MUST NOT be a valid directory entry.
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The list generation summary MUST end with a section separator:
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separator SP* NL
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There MUST NOT be any section separators in the list generation
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section, other than the terminating section separator.
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2.4. Directory Entry
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A directory entry consists of a C string constant, and one or more
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C-style comments. The C string constant is a valid argument to the
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DirAuthority or FallbackDir torrc option. The section also contains
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additional key-value fields in C-style comments.
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The list of fallback entries does not include the directory
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authorities: they are in a separate list. (The Tor implementation combines
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these lists after parsing them, and applies the DirAuthorityFallbackRate
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to their weights.)
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2.4.1 Directory Entry Format
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If a directory entry does not conform to this format, the entry SHOULD
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be ignored by parsers.
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DQUOTE dir_address SP+ "orport=" ipv4_or_port SP+
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"id=" fingerprint DQUOTE SP* NL
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[At start, exactly once, on a single line.]
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This line consists of the following fields:
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dir_address
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An IPv4 address and DirPort for this directory, as defined by
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Onionoo. In this format version, all IPv4 addresses and DirPorts
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are guaranteed to be non-zero. (For IPv4 addresses, this means
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that they are not equal to "0.0.0.0".)
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ipv4_or_port
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An IPv4 ORPort for this directory, derived from Onionoo. In this
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format version, all IPv4 ORPorts are guaranteed to be non-zero.
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fingerprint
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The relay fingerprint of this directory, as defined by Onionoo.
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All relay fingerprints are guaranteed to have one or more non-zero
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digits.
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Note:
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Each double-quoted C string line that occurs after the first line,
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starts with space inside the quotes. This is a requirement of the
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Tor implementation.
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DQUOTE SP+ "ipv6=" ipv6_or_address DQUOTE SP* NL
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[Zero or one time.]
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The IPv6 address and ORPort for this directory, as defined by
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Onionoo. If present, IPv6 addresses and ORPorts are guaranteed to be
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non-zero. (For IPv6 addresses, this means that they are not equal to
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"[::]".)
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DQUOTE SP+ "weight=" number DQUOTE SP* NL
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[Zero or one time.]
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A non-negative, real-numbered weight for this directory.
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The default fallback weight is 1.0, and the default
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DirAuthorityFallbackRate is 1.0 in legacy Tor versions, and 0.1 in
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recent Tor versions.
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weight was removed in version 2.0.0, but is documented because it
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may be of interest to libraries implementing Tor's fallaback
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behaviour.
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DQUOTE SP+ key_value DQUOTE SP* NL
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[Zero or more times.]
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Future releases may include additional data fields in double-quoted
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C string constants. Parsers MUST NOT rely on the order of these
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additional fields. Additional data fields will be accompanied by a
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minor version increment.
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"/*" SP+ "nickname=" nickname* SP+ "*/" SP* NL
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[Exactly once.]
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The nickname for this directory, as defined by Onionoo. An
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empty nickname indicates that the nickname is unknown.
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The first fallback list in the 2.0.0 format had nickname lines, but
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they were all empty.
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"/*" SP+ "extrainfo=" bool SP+ "*/" SP* NL
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[Exactly once.]
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An integer flag that indicates whether this directory caches
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extra-info documents. Set to 1 if the directory claimed that it
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cached extra-info documents in its descriptor when the list was
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created. 0 indicates that it did not, or its descriptor was not
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available.
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The first fallback list in the 2.0.0 format had extrainfo lines, but
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they were all zero.
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"/*" SP+ key_value SP+ "*/" SP* NL
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[Zero or more times.]
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Future releases may include additional data fields in C-style
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comments. Parsers MUST NOT rely on the order of these additional
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fields. Additional data fields will be accompanied by a minor version
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increment.
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separator SP* NL
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[Exactly once.]
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Each directory entry ends with the section separator.
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"," SP* NL
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[Exactly once.]
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The comma terminates the C string constant. (Multiple C string
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constants separated by whitespace or comments are coalesced by
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the C compiler.)
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3. Usage Considerations
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This section contains recommended library behaviours. It does not affect
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the format of directory lists.
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3.1. Caching
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The fallback list typically changes once every 6-12 months. The data in
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the list represents the state of the fallback directory entries when the
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list was created. Fallbacks can and do change their details over time.
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Libraries SHOULD parse and cache the most recent version of these lists
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during their build or release processes. Libraries MUST NOT retrieve the
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lists by default every time they are deployed or executed.
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The latest fallback list can be retrieved from:
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https://gitweb.torproject.org/tor.git/plain/src/or/fallback_dirs.inc
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Libraries MUST NOT rely on the availability of the server that hosts
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these lists.
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The list can also be retrieved using:
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git clone https://git.torproject.org/tor.git
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If you just want the latest list, you may wish to perform a shallow
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clone.
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3.2. Retrieving Directory Information
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Some libraries retrieve directory documents directly from the Tor
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Directory Authorities. The directory authorities are designed to support
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Tor relay and client bootstrap, and MAY choose to rate-limit library
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access. Libraries MAY provide a user-agent in their requests, if they
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are not intended to support anonymous operation. (User agents are a
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fingerprinting vector.)
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Libraries SHOULD consider the potential load on the authorities, and
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whether other sources can meet their needs.
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Libraries that require high-uptime availablility of Tor directory
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information should investigate the following options:
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* OnionOO: https://metrics.torproject.org/onionoo.html
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* Third-party OnionOO mirrors are also available
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* CollecTor: https://collector.torproject.org/
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* Fallback Directory Mirrors
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Onionoo and CollecTor are typically updated every hour on a regular
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schedule. Fallbacks update their own directory information at random
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intervals, see dir-spec for details.
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3.3. Fallback Reliability
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The fallback list is typically regenerated when the fallback failure
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rate exceeds 25%. Libraries SHOULD NOT rely on any particular fallback
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being available, or some proportion of fallbacks being available.
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Libraries that use fallbacks MAY wish to query an authority after a
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few fallback queries fail. For example, Tor clients try 3-4 fallbacks
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before trying an authority.
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A.1. Sample Data
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A sample version 2.0.0 fallback list is available here:
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https://trac.torproject.org/projects/tor/raw-attachment/ticket/22759/fallback_dirs_new_format_version.4.inc
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A sample transitional version 2.0.0 fallback list is available here:
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https://raw.githubusercontent.com/teor2345/tor/fallback-format-2-v4/src/or/fallback_dirs.inc
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A.1.1. Sample Fallback List Header
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/* type=fallback */
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/* version=2.0.0 */
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/* ===== */
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A.1.2. Sample Fallback List Generation
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/* Whitelist & blacklist excluded 1326 of 1513 candidates. */
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/* Checked IPv4 DirPorts served a consensus within 15.0s. */
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/*
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Final Count: 151 (Eligible 187, Target 392 (1963 * 0.20), Max 200)
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Excluded: 36 (Same Operator 27, Failed/Skipped Download 9, Excess 0)
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Bandwidth Range: 1.3 - 40.0 MByte/s
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*/
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/*
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Onionoo Source: details Date: 2017-05-16 07:00:00 Version: 4.0
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URL: https:onionoo.torproject.orgdetails?fields=fingerprint%2Cnickname%2Ccontact%2Clast_changed_address_or_port%2Cconsensus_weight%2Cadvertised_bandwidth%2Cor_addresses%2Cdir_address%2Crecommended_version%2Cflags%2Ceffective_family%2Cplatform&flag=V2Dir&type=relay&last_seen_days=-0&first_seen_days=30-
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*/
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/*
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Onionoo Source: uptime Date: 2017-05-16 07:00:00 Version: 4.0
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URL: https:onionoo.torproject.orguptime?first_seen_days=30-&flag=V2Dir&type=relay&last_seen_days=-0
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*/
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/* ===== */
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A.1.3. Sample Fallback Entries
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"176.10.104.240:80 orport=443 id=0111BA9B604669E636FFD5B503F382A4B7AD6E80"
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/* nickname=foo */
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/* extrainfo=1 */
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/* ===== */
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,
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"5.9.110.236:9030 orport=9001 id=0756B7CD4DFC8182BE23143FAC0642F515182CEB"
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" ipv6=[2a01:4f8:162:51e2::2]:9001"
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/* nickname= */
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/* extrainfo=0 */
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/* ===== */
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,
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