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379 lines
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379 lines
16 KiB
Plaintext
Filename: 154-automatic-updates.txt
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Title: Automatic Software Update Protocol
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Version: $Revision$
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Last-Modified: $Date$
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Author: Matt Edman
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Created: 30-July-2008
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Status: Open
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Target: 0.2.1.x
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Scope
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This proposal specifies the method by which an automatic update client can
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determine the most recent recommended Tor installation package for the
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user's platform, download the package, and then verify that the package was
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downloaded successfully. While this proposal focuses on only the Tor
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software, the protocol defined is sufficiently extensible such that other
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components of the Tor bundles, like Vidalia, Polipo, and Torbutton, can be
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managed and updated by the automatic update client as well.
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The initial target platform for the automatic update framework is Windows,
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given that's the platform used by a majority of our users and that it lacks
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a sane package management system that many Linux distributions already have.
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Our second target platform will be Mac OS X, and so the protocol will be
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designed with this near-future direction in mind.
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Other client-side aspects of the automatic update process, such as user
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interaction, the interface presented, and actual package installation
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procedure, are outside the scope of this proposal.
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Motivation
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Tor releases new versions frequently, often with important security,
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anonymity, and stability fixes. Thus, it is important for users to be able
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to promptly recognize when new versions are available and to easily
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download, authenticate, and install updated Tor and Tor-related software
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packages.
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Tor's control protocol [2] provides a method by which controllers can
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identify when the user's Tor software is obsolete or otherwise no longer
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recommended. Currently, however, no mechanism exists for clients to
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automatically download and install updated Tor and Tor-related software for
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the user.
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Design Overview
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The core of the automatic update framework is a well-defined file called a
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"recommended-packages" file. The recommended-packages file is accessible via
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HTTP[S] at one or more well-defined URLs. An example recommended-packages
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URL may be:
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https://updates.torproject.org/recommended-packages
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The recommended-packages document is formatted according to Section 1.2
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below and specifies the most recent recommended installation package
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versions for Tor or Tor-related software, as well as URLs at which the
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packages and their signatures can be downloaded.
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An automatic update client process runs on the Tor user's computer and
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periodically retrieves the recommended-packages file according to the method
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described in Section 2.0. As described further in Section 1.2, the
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recommended-packages file is signed and can be verified by the automatic
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update client with one or more public keys included in the client software.
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Since it is signed, the recommended-packages file can be mirrored by
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multiple hosts (e.g., Tor directory authorities), whose URLs are included in
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the automatic update client's configuration.
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After retrieving and verifying the recommended-packages file, the automatic
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update client compares the versions of the recommended software packages
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listed in the file with those currently installed on the end-user's
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computer. If one or more of the installed packages is determined to be out
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of date, an updated package and its signature will be downloaded from one of
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the package URLs listed in the recommended-packages file as described in
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Section 2.2.
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The automatic update system uses a multilevel signing key scheme for package
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signatures. There are a small number of entities we call "packaging
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authorities" that each have their own signing key. A packaging authority is
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responsible for signing and publishing the recommended-packages file.
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Additionally, each individual packager responsible for producing an
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installation package for one or more platforms has their own signing key.
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Every packager's signing key must be signed by at least one of the packaging
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authority keys.
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Specification
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1. recommended-packages Specification
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In this section we formally specify the format of the published
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recommended-packages file.
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1.1. Document Meta-format
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The recommended-packages document follows the lightweight extensible
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information format defined in Tor's directory protocol specification [1]. In
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the interest of self-containment, we have reproduced the relevant portions
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of that format's specification in this Section. (Credits to Nick Mathewson
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for much of the original format definition language.)
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The highest level object is a Document, which consists of one or more
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Items. Every Item begins with a KeywordLine, followed by zero or more
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Objects. A KeywordLine begins with a Keyword, optionally followed by
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whitespace and more non-newline characters, and ends with a newline. A
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Keyword is a sequence of one or more characters in the set [A-Za-z0-9-].
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An Object is a block of encoded data in pseudo-Open-PGP-style
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armor. (cf. RFC 2440)
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More formally:
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Document ::= (Item | NL)+
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Item ::= KeywordLine Object*
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KeywordLine ::= Keyword NL | Keyword WS ArgumentChar+ NL
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Keyword ::= KeywordChar+
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KeywordChar ::= 'A' ... 'Z' | 'a' ... 'z' | '0' ... '9' | '-'
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ArgumentChar ::= any printing ASCII character except NL.
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WS ::= (SP | TAB)+
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Object ::= BeginLine Base-64-encoded-data EndLine
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BeginLine ::= "-----BEGIN " Keyword "-----" NL
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EndLine ::= "-----END " Keyword "-----" NL
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The BeginLine and EndLine of an Object must use the same keyword.
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In our Document description below, we also tag Items with a multiplicity in
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brackets. Possible tags are:
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"At start, exactly once": These items MUST occur in every instance of the
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document type, and MUST appear exactly once, and MUST be the first item in
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their documents.
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"Exactly once": These items MUST occur exactly one time in every
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instance of the document type.
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"Once or more": These items MUST occur at least once in any instance
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of the document type, and MAY occur more than once.
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"At end, exactly once": These items MUST occur in every instance of
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the document type, and MUST appear exactly once, and MUST be the
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last item in their documents.
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1.2. recommended-packages Document Format
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When interpreting a recommended-packages Document, software MUST ignore
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any KeywordLine that starts with a keyword it doesn't recognize; future
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implementations MUST NOT require current automatic update clients to
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understand any KeywordLine not currently described.
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In lines that take multiple arguments, extra arguments SHOULD be
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accepted and ignored.
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The currently defined Items contained in a recommended-packages document
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are:
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"recommended-packages-format" SP number NL
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[Exactly once]
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This Item specifies the version of the recommended-packages format that
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is contained in the subsequent document. The version defined in this
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proposal is version "1". Subsequent iterations of this protocol MUST
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increment this value if they introduce incompatible changes to the
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document format and MAY increment this value if they only introduce
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additional Keywords.
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"published" SP YYYY-MM-DD SP HH:MM:SS NL
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[Exactly once]
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The time, in GMT, when this recommended-packages document was generated.
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Automatic update clients SHOULD ignore Documents over 60 days old.
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"tor-stable-win32-version" SP TorVersion NL
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[Exactly once]
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This keyword specifies the latest recommended release of Tor's "stable"
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branch for the Windows platform that has an installation package
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available. Note that this version does not necessarily correspond to the
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most recently tagged stable Tor version, since that version may not yet
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have an installer package available, or may have known issues on
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Windows.
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The TorVersion field is formatted according to Section 2 of Tor's
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version specification [3].
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"tor-stable-win32-package" SP Url NL
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[Once or more]
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This Item specifies the location from which the most recent
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recommended Windows installation package for Tor's stable branch can be
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downloaded.
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When this Item appears multiple times within the Document, automatic
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update clients SHOULD select randomly from the available package
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mirrors.
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"tor-dev-win32-version" SP TorVersion NL
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[Exactly once]
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This Item specifies the latest recommended release of Tor's
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"development" branch for the Windows platform that has an installation
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package available. The same caveats from the description of
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"tor-stable-win32-version" also apply to this keyword.
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The TorVersion field is formatted according to Section 2 of Tor's
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version specification [3].
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"tor-dev-win32-package" SP Url NL
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[Once or more]
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This Item specifies the location from which the most recent recommended
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Windows installation package and its signature for Tor's development
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branch can be downloaded.
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When this Keyword appears multiple times within the Document, automatic
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update clients SHOULD select randomly from the available package
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mirrors.
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"signature" NL SIGNATURE NL
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[At end, exactly once]
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The "SIGNATURE" Object contains a PGP signature (using a packaging
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authority signing key) of the entire document, taken from the beginning
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of the "recommended-packages-format" keyword, through the newline after
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the "signature" Keyword.
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2. Automatic Update Client Behavior
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The client-side component of the automatic update framework is an
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application that runs on the end-user's machine. It is responsible for
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fetching and verifying a recommended-packages document, as well as
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downloading, verifying, and subsequently installing any necessary updated
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software packages.
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2.1. Download and verify a recommended-packages document
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The first step in the automatic update process is for the client to download
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a copy of the recommended-packages file. The automatic update client
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contains a (hardcoded and/or user-configurable) list of URLs from which it
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will attempt to retrieve a recommended-packages file.
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Connections to each of the recommended-packages URLs SHOULD be attempted in
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the following order:
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1) HTTPS over Tor
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2) HTTP over Tor
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3) Direct HTTPS
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4) Direct HTTP
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If the client fails to retrieve a recommended-packages document via any of
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the above connection methods from any of the configured URLs, the client
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SHOULD retry its download attempts following an exponential back-off
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algorithm. After the first failed attempt, the client SHOULD delay one hour
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before attempting again, up to a maximum of 24 hours delay between retry
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attempts.
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After successfully downloading a recommended-packages file, the automatic
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update client will verify the signature using one of the public keys
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distributed with the client software. If more than one recommended-packages
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file is downloaded and verified, the file with the most recent "published"
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date that is verified will be retained and the rest discarded.
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2.2. Download and verify the updated packages
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The automatic update client next compares the latest recommended package
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version from the recommended-packages document with the currently installed
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Tor version. If the user currently has installed a Tor version from Tor's
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"development" branch, then the version specified in "tor-dev-*-version" Item
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is used for comparison. Similarly, if the user currently has installed a Tor
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version from Tor's "stable" branch, then the version specified in the
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"tor-stable-*version" Item is used for comparison. Version comparisons are
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done according to Tor's version specification [3].
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If the automatic update client determines an installation package newer than
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the user's currently installed version is available, it will attempt to
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download a package appropriate for the user's platform and Tor branch from a
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URL specified by a "tor-[branch]-[platform]-package" Item. If more than one
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mirror for the selected package is available, a mirror will be chosen at
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random from all those available.
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The automatic update client must also download a ".asc" signature file for
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the retrieved package. The URL for the package signature is the same as that
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for the package itself, except with the extension ".asc" appended to the
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package URL.
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Connections to download the updated package and its signature SHOULD be
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attempted in the same order described in Section 2.1.
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After completing the steps described in Sections 2.1 and 2.2, the automatic
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update client will have downloaded and verified a copy of the latest Tor
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installation package. It can then take whatever subsequent platform-specific
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steps are necessary to install the downloaded software updates.
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2.3. Periodic checking for updates
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The automatic update client SHOULD maintain a local state file in which it
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records (at a minimum) the timestamp at which it last retrieved a
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recommended-packages file and the timestamp at which the client last
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successfully downloaded and installed a software update.
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Automatic update clients SHOULD check for an updated recommended-packages
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document at most once per day but at least once every 30 days.
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3. Future Extensions
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There are several possible areas for future extensions of this framework.
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The extensions below are merely suggestions and should be the subject of
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their own proposal before being implemented.
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3.1. Additional Software Updates
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There are several software packages often included in Tor bundles besides
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Tor, such as Vidalia, Privoxy or Polipo, and Torbutton. The versions and
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download locations of updated installation packages for these bundle
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components can be easily added to the recommended-packages document
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specification above.
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3.2. Including ChangeLog Information
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It may be useful for automatic update clients to be able to display for
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users a summary of the changes made in the latest Tor or Tor-related
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software release, before the user chooses to install the update. In the
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future, we can add keywords to the specification in Section 1.2 that specify
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the location of a ChangeLog file for the latest recommended package
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versions. It may also be desirable to allow localized ChangeLog information,
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so that the automatic update client can fetch release notes in the
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end-user's preferred language.
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3.3. Weighted Package Mirror Selection
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We defined in Section 1.2 a method by which automatic update clients can
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select from multiple available package mirrors. We may want to add a Weight
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argument to the "*-package" Items that allows the recommended-packages file
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to suggest to clients the probability with which a package mirror should be
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chosen. This will allow clients to more appropriately distribute package
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downloads across available mirrors proportional to their approximate
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bandwidth.
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Implementation
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Implementation of this proposal will consist of two separate components.
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The first component is a small "au-publish" tool that takes as input a
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configuration file specifying the information described in Section 1.2 and a
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private key. The tool is run by a "packaging authority" (someone responsible
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for publishing updated installation packages), who will be prompted to enter
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the passphrase for the private key used to sign the recommended-packages
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document. The output of the tool is a document formatted according to
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Section 1.2, with a signature appended at the end. The resulting document
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can then be published to any of the update mirrors.
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The second component is an "au-client" tool that is run on the end-user's
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machine. It periodically checks for updated installation packages according
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to Section 2 and fetches the packages if necessary. The public keys used
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to sign the recommended-packages file and any of the published packages are
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included in the "au-client" tool.
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References
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[1] Tor directory protocol (version 3),
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https://tor-svn.freehaven.net/svn/tor/trunk/doc/spec/dir-spec.txt
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[2] Tor control protocol (version 2),
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https://tor-svn.freehaven.net/svn/tor/trunk/doc/spec/control-spec.txt
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[3] Tor version specification,
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https://tor-svn.freehaven.net/svn/tor/trunk/doc/spec/version-spec.txt
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