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Magnet URI scheme

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Magnet URI scheme

The Magnet URI scheme, defines the format of magnet links, a de facto standard for identifying files by their content, via cryptographic hash value rather than by their location.

Contents

Although magnet links can be used in a number of contexts, they are particularly useful in peer-to-peer file sharing networks because they allow resources to be referred to without the need for a continuously available host, and can be generated by anyone who already has the file, without the need for a central authority to issue them. This makes them popular for use as "guaranteed" search terms within the file sharing community where anyone can distribute a magnet link to ensure that the resource retrieved by that link is the one intended, regardless of how it is retrieved.

History

The standard for Magnet URIs was developed by Bitzi in 2002, partly as a "vendor- and project-neutral generalization" of the ed2k: and freenet: URI schemes used by eDonkey2000 and Freenet, respectively, and attempts to follow official IETF URI standards as closely as possible.

Technical description

Magnet URIs consist of a series of one or more parameters, the order of which is not significant, formatted in the same way as query strings that ordinarily terminate HTTP URLs. The most common parameter is "xt" ("exact topic"), which is generally a URN formed from the content hash of a particular file, e.g.:

magnet:?xt=urn:btih:c12fe1c06bba254a9dc9f519b335aa7c1367a88a&dn

This refers to the hex-encoded SHA-1 hash of the torrent file info section in question. Note that, although a particular file is indicated, an availability search for it must still be carried out by the client application.

Other parameters defined by the draft standard are:

  • "dn" ("display name"): a filename to display to the user, for convenience
  • "kt" ("keyword topic"): a more general search, specifying search terms, rather than a particular file
  • "mt" ("manifest topic"): a URI pointing to a "manifest", e.g. a list of further items
  • application-specific experimental parameters, which must begin "x."

    • The standard also allows for multiple parameters of the same type to be used by appending ".1", ".2", etc. to the parameter name, e.g.: magnet:?xt.1=urn:sha1:YNCKHTQCWBTRNJIV4WNAE52SJUQCZO5C&xt.2=urn:sha1:TXGCZQTH26NL6OUQAJJPFALHG2LTGBC7

    Design

    Magnet URIs can contain multiple parameters, in any order, separated from each other by '&'.

    magnet:? xl = [Size in Bytes] & dn = [file name (URL encoded)] & xt = urn: tree: tiger: [ TTH hash (Base32) ]

    Parameters

  • dn (Display Name) – Filename
  • xl (eXact Length) – Size in bytes
  • xt (eXact Topic) – URN containing file hash
  • as (Acceptable Source) – Web link to the file online
  • xs (eXact Source) – P2P link.
  • kt (Keyword Topic) – Key words for search
  • mt (Manifest Topic) – link to the metafile that contains a list of magneto (MAGMA – MAGnet MAnifest)
  • tr (address TRacker) – Tracker URL for BitTorrent downloads

    • URN, containing hash (xt)

    "xt" ("exact topic"): the most important part of a Magnet link, this parameter is used to find and verify the specified files.

    Tiger Tree Hash (TTH)

    These hashes are used on Direct Connect and G2 (Gnutella2), among others.

    xt=urn:tree:tiger:[ TTH Hash (Base32) ]

    Secure Hash Algorithm 1 (SHA-1)

    These hash sums are used on gnutella and G2 (Gnutella2).

    xt=urn:sha1:[ SHA-1 Hash (Base32) ]

    BitPrint

    Such hash sums consist of an SHA-1 Hash, followed by a TTH Hash, delimited by a point; they are used on gnutella and G2 (Gnutella2).

    xt=urn:bitprint:[ SHA-1 Hash (Base32) ].[ TTH Hash (Base32) ]

    ED2K (eDonkey2000) hash

    These hash sums are used on eDonkey2000.

    xt=urn:ed2k:[ ED2K Hash (Hex) ]

    Advanced Intelligent Corruption Handler (AICH)

    Not formal URNs for Magnet links, such hash sums are used by eDonkey2000 to restore and control the integrity of downloading and already downloaded files.

    xt=urn:aich:[ aich Hash (Base32) ]

    Kazaa hash

    Used on FastTrack, these hash sums are vulnerable to hash collision attacks.

    xt=urn:kzhash:[ Kazaa Hash (Hex) ]

    BitTorrent info hash (BTIH)

    These are hex-encoded SHA-1 hash sums of the "info" sections of BitTorrent metafiles as used by BitTorrent to identify downloadable files or sets of files. For backwards compatibility with existing links, clients should also support the Base32 encoded version of the hash.

    xt=urn:btih:[ BitTorrent Info Hash (Hex) ]

    Some clients require Base32 of info_hash (e.g., Vuze).

    Message Digest 5 (MD5)

    Supported by G2 (Gnutella Peanut Butter), such hashes are vulnerable to hash collision attacks.

    xt=urn:md5:[ MD5 Hash (Hex) ]

    There are two types of download links that a Magnet link can include as a direct or backup source.

    Normal (as)

    "as" ("acceptable source") refers to a direct download from a web server. Regarded as only a fall-back source in case a client is unable to locate and/or download the linked-to file in its supported P2P network(s), most clients treat it equal to the "xs" token when it comes to priority, and ignore the timeout before contacting "as" sources denoted by the specs.

    as=[ a web link to the file(URL encoded) ]

    P2P (xs)

    "xs" ("exact source") is either an HTTP (or HTTPS, FTP, FTPS, etc.) download source for the file pointed to by the Magnet link, the address of a P2P source for the file or the address of a hub (in the case of DC++), by which a client tries to connect directly, asking for the file and/or its sources. This field is commonly used by P2P clients to store the source, and may include the file hash.

    Content-Addressable Web URL

    This type of RFC 2168-based link is used by gnutella as well as G2 applications.

    xs=http://[Client Address]:[Port of client]/uri-res/N2R?[ URN containing a file hash ]

    Example:

    xs=http://192.0.2.27:6346/uri-res/N2R?urn:sha1:FINYVGHENTHSMNDSQQYDNLPONVBZTICF
    Link to a DirectConnect hub to find sources for a file

    This type of link connects a DirectConnect client immediately to the hub in question.

    xs=dchub://[hub address]:[hub port]
    Reference to a web-based source cache for a file on Gnutella2

    In this case, the included link points, not to a client IP or direct source, but to a source cache which stores the IPs of other clients contacting it to download the same file. Once a client connects to the cache, it is served IPs for alternate sources, while its own IP is stored within the cache and forwarded to the next one connecting to the cache. This system operates similar to a BitTorrent tracker.

    xs=http://cache.freebase.be/[ SHA-1 hash ]
    Reference to an eD2k source
    xs=ed2kftp://[client address]:[client port]/[ed2k hash]/[file size]/

    Manifest (mt)

    This is a link to a list of links (see list). Perhaps as a web link...

    mt=http://weblog.foo/all-my-favorites.rss

    ...or a URN

    mt=urn:sha1:3I42H3S6NNFQ2MSVX7XZKYAYSCX5QBYJ

    Keywords (kt)

    This field specifies a string of search keywords to search for in P2P networks.

    kt=joe+blow+mp3

    Address tracker (tr)

    Tracker URL; used to obtain resources for BitTorrent downloads without a need for DHT support.

    tr=http://example.com/announce

    Supplement format (x.)

    For experimental and self-complementing informal options, the prefix x followed by a chosen second letter can be used.

    x.[name of the new parameter]=[data of the new parameter (URL encoded)]

    Group settings

    Multiple files and their URNs, names and hashes in the Magnet link can be included by adding a count number preceded by a dot (".") to each link parameter.

    magnet:?xt.1=[ URN of the first file]&xt.2=[ URN of the second file]magnet:?xt=urn:ed2k:31D6CFE0D16AE931B73C59D7E0C089C0&xl=0&dn=zero_len.fil&xt=urn:bitprint:3I42H3S6NNFQ2MSVX7XZKYAYSCX5QBYJ.LWPNACQDBZRYXW3VHJVCJ64QBZNGHOHHHZWCLNQ&xt=urn:md5:D41D8CD98F00B204E9800998ECF8427E

    mediawiki-1.15.1.tar.gz

    magnet:?xt=urn:ed2k:354B15E68FB8F36D7CD88FF94116CDC1&xt=urn:tree:tiger:7N5OAMRNGMSSEUE3ORHOKWN4WWIQ5X4EBOOTLJY&xt=urn:btih:QHQXPYWMACKDWKP47RRVIV7VOURXFE5Q&xl=10826029&dn=mediawiki-1.15.1.tar.gz&tr=udp%3A%2F%2Ftracker.openbittorrent.com%3A80%2Fannounce&as=http%3A%2F%2Fdownload.wikimedia.org%2Fmediawiki%2F1.15%2Fmediawiki-1.15.1.tar.gz&xs=http%3A%2F%2Fcache.example.org%2FXRX2PEFXOOEJFRVUCX6HMZMKS5TWG4K5&xs=dchub://example.org

    Features and clients

    Parameters:

    dn (Display Name) – Filenamexl (Exact Length) – Size in bytesxt (Exact Topic) – URN containing file hashas (Acceptable Source) – Web link to the file onlinexs (Exact Source) – P2P link.kt (Keyword Topic) – Key words for searchmt (Manifest Topic) – link to the metafile that contains a list of magneto (MAGMA – MAGnet MAnifest)tr (Address Tracker) – Tracker URL for BitTorrent downloads

    Shareaza

    This client is able to intercept links from within the web browser and can also handle Magnet links pasted into the search area or the "Download File or Torrent" dialogue.

    aMule 2.2

    Unable to intercept Magnet links from within a web browser, this client supports Magnets by offering a text box at the bottom of the client into which they can be copied and pasted. The client is able only to recognize eD2k hash, file size and name in a specific order and therefore, many valid links may not be suitable for downloading.

    References

    Magnet URI scheme Wikipedia
    (Text) CC BY-SA


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