磁力連線是如何接入DHT網路並下載檔案的

大囚長發表於2019-02-13

你的bt客戶端連線到的第一個ip（除DNS外）提供一個公網ip的列表，這些ip被用來當作DHT網路的入口。這個列表一般由製作bt客戶端的人來提供。
當你接入DHT網路之後，你會通過DHT查詢找到更多的ip，這些ip一般會快取再你的磁碟上以便於下次你不用再下載一次ip列表。
磁力連結包含了一個hash值，就是一個唯一的id，用來識別你要查詢的檔案，它不包含任何ip，也不需要這麼做。這個hash就是從你要獲取的那個種子的內容來生成的。
DHT網路上的其他使用者知道這個hash的話，你的客戶端就會向他們索取響應的種子檔案，並藉由種子檔案來下載你想要的內容了。
比如uTorrent的索引地址就是router.utorrent.com

The first IP you connect to (besides a DNS) is a server that provides a list of public IP’s. These IPs can be used as entry points into the DHT network. This list is usually provided by the same people who make the bitTorrent client, though of course technically you can find a participating IP by any method that works (like asking a friend).

After you enter the network, you’ll discover more IPs by using DHT lookups. It’s likely that these IPs will be stored locally on your hard drive so you can just connect directly next time without fetching the bootstrap list first.

The magnet link contains a hash – basically a unique ID – which identifies the file you’re looking for. It doesn’t contain any IPs, nor does it need to. A hash is mathematically calculated from the exact contents of the file. Due to the extreme calculations involved, a hash nearly never refers to any other file on the planet. It’s almost impossible to spoof.

Other users on the DHT network know about this hash -> contents relation, so when you say “someone, gimme the exact sequence of bytes that match this hash”, they do, and you have your file!

Hope that explains things.

A client can learn about other DHT-capable peers through it’s interactions with them. A peer’s support for DHT is advertised in it’s Handshake. Once a client discovers at least one good, well-connected DHT peer, it can navigate the DHT to find more and closer DHT peers. It will remember these peers, called nodes in DHT-speak, between restarts of the software and maintain/update the list continuously while it is running. In the worse case where a client knows of no good DHT-capable peers, it will require you to download a tracker-based torrent so it can hopefully contact a few good DHT-capable peers it learns about through the tracker.

Update: For it’s initial list of DHT peers, as @Seppo points out, a torrent client can use one or more hard-coded DNS names to find the addresses for well-known peers, and it may also include a hard-coded list of peers as a final fallback as well. One limitation of DNS, however, it no port information is provided so a default port of 6881 is generally assumed whereas other means support peers operating on different ports.

The initial bootstrap DHT peer on uTorrent is router.utorrent.com

Nobody answered the OPs question, I had the same question myself.

After the initial load, a cache of DHT peers is kept and used.

磁力連線是如何接入DHT網路並下載檔案的

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