當你的torrent客戶端加入swarm來共享和收集檔案片段時，它如何確切地知道所有的對等點在哪裡？請繼續閱讀，我們將深入瞭解支撐BitTorrent協議的機制。

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，是一個由社群驅動的問答網站分組。

問題

超級使用者讀者Steve V.對BitTorrent協議中的分散式雜湊表（DHT）系統有一個非常具體的問題：

I’ve already read this SuperUser answer and this Wikipedia article but both are too technical for me to really wrap my head around.

I understand the idea of a tracker: clients connect to a central server which maintains a list of peers in a swarm.

I also understand the idea of peer exchange: clients already in a swarm send the complete list of their peers to each other. If new peers are discovered, they are added to the list.

My question is, how does DHT work? That is, how can a new client join a swarm without either a tracker or the knowledge of at least one member of the swarm to exchange peers with?

(Note: simple explanati*** are best.)

他的問題反過來又引發了一個關於BitTorrent系統不同功能的非常詳細的回答；現在讓我們來看看。

答案

超級使用者貢獻者Allquixotic提供了一個深入的解釋：

How can a new client join a swarm without either a tracker or the knowledge of at least one member of the swarm to exchange peers with?

You can’t. It is impossible.*

* (Unless a node on your local area network happens to already be a node in the DHT. In this case, you could use a broadcasting mechani**, such as Avahi, to “discover” this peer, and bootstrap from them. But how did they bootstrap themselves? Eventually, you’ll hit a situation where you need to connect to the public Internet. And the public Internet is unicast-only, not multicast, so you’re stuck with using pre-determined lists of peers.)

References

Bittorrent DHT is implemented via a protocol known as Kademlia, which is a special case of theoretical concept of a Distributed hash table.

Exposition

With the Kademlia protocol, when you join the network, you go through a bootstrapping procedure, which absolutely requires that you know, in advance, the IP address and port of at least one node already participating in the DHT network. The tracker that you connect to, for instance, may be itself a DHT node. Once you are connected to one DHT node, you then proceed to download information from the DHT, which provides you connectivity information for more nodes, and you then navigate that “graph” structure to obtain connecti*** to more and more nodes, who can provide both connectivity to other nodes, and payload data (chunks of the download).

I think your actual question in bold — that of how to join a Kademlia DHT network without knowing anyother members — is based on a false assumption.

The simple answer to your question in bold is, you don’t. If you do not know ANY information at all about even one host which might contain DHT metadata, you are stuck — you can’t even get started. I mean, sure, you could brute force attempt to discover an IP on the public internet with an open port that happens to broadcast DHT information. But more likely, your BT client is hard-coded to some specific static IP or DNS which resolves to a stable DHT node, which just provides the DHT metadata.

Basically, the DHT is only as decentralized as the joining mechani**, and because the joining mechani** is fairly brittle (there’s no way to “broadcast” over the entire Internet! so you have to unicastto an individual pre-assigned host to get the DHT data), Kademlia DHT isn’t really decentralized. Not in the strictest sense of the word.

Imagine this scenario: Someone who wants P2P to stop goes out and prepares an attack on all commonly used stable DHT nodes which are used for bootstrapping. Once they’ve staged their attack, they spring it on all nodes all at once. Wham; every single bootstrapping DHT node is down all in one fell swoop. Now what? You’re stuck with connecting to centralized trackers to download traditional lists of peers from those. Well, if they attack the trackers too, then you’re really, really up a creek. In other words, Kademlia and the entire BT network is c***trained by the limitati*** of the Internet itself, in that, there is a finite (and relatively **all) number of computers that you would have to successfully attack or take offline to prevent >90% of users from connecting to the network.

Once the “pseudo-centralized” bootstrapping nodes are all gone, the interior nodes of the DHT, which are not bootstrapping because nobody on the outside of the DHT knows about the interior nodes, are useless; they can’t bring new nodes into the DHT. So, as each interior node disconnects from the DHT over time, either due to people shutting down their computers, rebooting for updates, etc., the network would collapse.

Of course, to get around this, someone could deploy a patched BitTorrent client with a new list of pre-determined stable DHT nodes or DNS addresses, and loudly advertise to the P2P community to use this new list instead. But this would become a “whack-a-mole” situation where the aggressor (the node-eater) would progressively download these lists themselves, and target the brave new bootstrapping nodes, then take them offline, too.

我們不僅瞭解了原始問題的答案，還了解了BitTorrent系統的本質及其漏洞。

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