https://stackoverflow.com/questions/16333165
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Russian2019 Update: Yes, you now can! In 2017 a draft BEP was released that covers the question's behaviour for magnet URIs! This is great, as it creates a standard that keeps a consistent info_hash between a magnet URI pointing to the multi-file torrent, and a magnet URI pointing to a single file within that multi-file torrent. They will share a swarm, which means you can, as the question asks "[generate] an almost new torrent/magnet link to download only that single file but using the same source".
The draft BEP: http://www.bittorrent.org/beps/bep_0053.html BEP 53: "Magnet URI extension - Select specific file indices for download"
Example URI to request files 0, 2, 4 and the inclusive range 6 through to 8:
magnet:?xt=urn:btih:HASH&dn=NAME&tr=TRACKER&so=0,2,4,6-8
And the draft BEP is making it's way into bittorrent libraries:
- https://gitlab.com/proninyaroslav/libretorrent/tags/1.9 LibreTorrent 1.9 2018-NOV-26
- https://github.com/webtorrent/webtorrent/issues/1395 Webtorrent 0.100.0 2018-MAY-23
2013-MAY-03 Original Answer: Sometimes yes, but not often, and the resulting swarm has no peers.
Firstly, you need the original .torrent file, so if you only have a magnet URI you need to resolve that to a .torrent using DHT. Any bittorrent library that supports magnet URIs has the code for that task.
Once you have the .torrent, you then need to get the hashes relating to the file you're interested in. The .torrent file contains a very long string, each 20 bytes representing the hash of each piece in the torrent. Piece length is fixed for a torrent, typically between 256KB and 1MB. If the file starts at exactly a piece offset, and is sized equal to a multiple of the piece size or is the last file in the torrent then you can reuse these hashes. You can then create a new .torrent file with that information, and generate a new magnet URI from the torrent file, re-using the announce or using a new one.
Torrent info structure: https://wiki.theory.org/BitTorrentSpecification#Metainfo_File_Structure
Being lucky enough to get that offset is unlikely, with a piece length generally varying between 256KB & 1MB, you have a 1/262144 to 1/268435456 chance of getting that offset (given that a file could start anywhere in a piece), so the circumstance is unlikely. If you can't re-use the hashes, you need to generate new hashes which means you can't re-use the .torrent and would need to download the files to generate the new piece hashes.
The killer is that in the end, the torrent created has a different info_hash. The info_hash is the hash of the info describing the torrent, which was a description of many files and now in your new hash is the description of a single file, thus is a new torrent so there's no-one available to leech from. Peers collect into swarms based on the info_hash, and if you create a new torrent based on one file from a multifile torrent, the peers from the multifile torrent don't know about it and won't be available to leech from.
Even if you're lucky enough to get the right piece offsets, you create a torrent that doesn't have anyone sharing the file.
So, could you instead re-use the magnet URI and just specify a file name within the torrent? No, the BEP that describes how Bittorrent uses magnet URIs doesn't cover this behaviour. http://www.bittorrent.org/beps/bep_0009.html
