magnet is a URI scheme that 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. 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 IETFURI standards as closely as possible.
Design
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", which is generally a URN formed from the content hash of a particular file, e.g.: magnet:?xt=urn:btih:c12fe1c06bba254a9dc9f519b335aa7c1367a88a 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. 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 Magnet URIs can contain multiple parameters, in any order, separated from each other by &. The format for a tiger tree hashed file therefore looks like: The following parameters are supported:
dn : a filename to display to the user, for convenience
The standard also allows for application-specific experimental parameters, which must begin with "x."
URN, containing hash (xt)
"xt" : the most important part of a Magnet link and is used to find and verify the specified files. ; Tiger Tree Hash : These hashes are used on Direct Connect and G2, among others. ; Secure Hash Algorithm 1 : These hash sums are used on gnutella and G2. ; 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. ; ED2K hash: These hash sums are used on eDonkey2000. ; Advanced Intelligent Corruption Handler : 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. ; Kazaa hash: Used on FastTrack, these hash sums are vulnerable to hash collision attacks. ; BitTorrent info hash : 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. Some clients require Base32 of info_hash. ; Message Digest 5 : Supported by G2, such hashes are vulnerable to hash collision attacks.
Web links to the file
There are two types of download links that a Magnet link can include as a direct or backup source. ; "as" : 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, 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. ; "xs" : either an HTTP 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, 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 -based link is used by gnutella as well as G2 applications. ;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. ;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. ;Reference to an eD2k source
This field specifies a string of search keywords to search for in P2P networks.
Address tracker (tr)
Tracker URL; used to obtain resources for BitTorrent downloads without a need for DHT support. The value must be URL encoded.
Supplement format (x.)
For experimental and self-complementing informal options, the prefix followed by a chosen suffix letter can be used. These names are guaranteed to never be standardized.
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.
