A new client must have knowledge of some "node" bittorrent lingo for a peer of the DHT to bootstrap its information of the DHT. Here the arguments given by allquixotic apply. As the MDHT currently consists of over 7 million peers a sustained denial of service attack seems unlikely. It can then query the DHT with respect to an infohash and doesn't have to use a tracker or know a peer that is part of the swarm before. If one of the peers he contacts supports sharing metadata it only needs the infohash can retrieve the.
Most p2p network clients are bootstrapped from a list of seed peers they initially connect to It connects to the bootstrap peer and downloads its DHT list of peers and then goes to each of those and does the same, etc. This is similar to how a tracker works, except pretty much anyone can be a bootstrap node so its nearly impossible to shut them down. Bitcoin core wallet works the same way.
It lets you change the bootstrap seed peers if for some reason the default ones get shutdown. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
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Improve this question. Community Bot 1. Steve V. Add a comment. Active Oldest Votes. Summary 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. 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.
When your torrent client joins the swarm to share and gather file pieces, how exactly does it know where all its peers are? Read on as we poke around inside the mechanisms that undergird the BitTorrent protocol.
SuperUser reader Steve V. 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?
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? But how did they bootstrap themselves? Bittorrent DHT is implemented via a protocol known as Kademlia , which is a special case of theoretical concept of a Distributed hash table.
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. I think your actual question in bold — that of how to join a Kademlia DHT network without knowing any other members — is based on a false assumption.
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. 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. Wham ; every single bootstrapping DHT node is down all in one fell swoop. Now what? So, as each interior node disconnects from the DHT over time, either due to people shutting down their computers, rebooting for updates, etc. 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.
Not only did we learn the answer to the original question but we also learned quite a bit about the nature of the BitTorrent system and its vulnerabilities. Start new topic. Recommended Posts. Ghoulio Posted March 8, Posted March 8, Link to comment Share on other sites More sharing options Harold Feit Posted March 9, Posted March 9, This also allows peers who do not upload and only download to download the file, even if they refuse to cooperate.
Albeit, they will download at a much slower speed. What happens if all peers uploading to another peer decide to choke it? We then have to find new peers, but the optimistic unchoking mechanism only checks one unused connection every 30 seconds. To help the download rate recover more, BitTorrent has snubbing. Following the mentality of tit-for-tat, we retaliate and refuse to upload to that peer except if they become an optimistic unchoke. We see that using the choking algorithm implemented in BitTorrent we favour peers who are kind to us.
If I can download fast from them, we allow them to upload fast from me. What about no downloads? When a download is completed, we use a new choking algorithm. This new choking algorithm unchokes peers with the highest upload rate. This ensures that pieces get uploaded faster, and they get replicated faster. Since the creation of the distributed hash table method for trackerless torrents, BitTorrent trackers are largely redundant. The Pirate Bay operated one of the most popular public trackers until disabling it in , opting only for magnet links discussed soon.
Private trackers are private. They restrict use by requiring users to register with the site. The method for controlling registration is often an invitation system. To use this tracker we need an invitation. Multi-tracker torrents contain multiple trackers in a single torrent file. This provides redundancy if one tracker fails, the other trackers can continue to maintain the swarm for the torrent. With this configuration, it is possible to have multiple unconnected swarms for a single torrent - which is bad.
Some users can connect to one specific tracker while being unable to connect to another. This can create a disjoint set which can impede the efficiency of a torrent to transfer the files it describes. Earlier, I talked about how the Pirate Bay got rid of trackers and started using trackerless torrents. When we download a torrent, we get a hash of that torrent. To download the torrent without a tracker, we need to find other peers also downloading the torrent.
To do this, we need to use a distributed hash table. Distributed Hash Tables DHT give us a dictionary-like interface, but the nodes are distributed across a network.
The trick with DHTs is that the node that gets to store a particular key is found by hashing that key. We choose node IDs at random from the same bit space as BitTorrent infohashes.
Infohashes are a SHA-1 hash of:. Nodes know about each other in the DHT. They know many nodes with IDs that are close to their own but few with far-away IDs. When a node wants to find peers for a torrent, they use the distance metric to compare the infohash of the torrent with the IDs of the nodes in its routing table or the ID of one node with the ID of another node. Then they contact the nodes in the routing table closet to the infohash and asks them for contact information of peers downloading the torrent.
If a contacted node knows about peers for the torrent, they return the peer contact information with the response. Otherwise, the contacted node must respond with the contact information of the nodes in its routing table closet to the infohash of the torrent.
The original node queries nodes that are closer to the target infohash until it cannot find any closer nodes. After the node exhausts the search, the client then inserts the peer contact information for itself onto the responding nodes with IDs closest to the infohash of the torrent. In the future, other nodes can easily find us.
This is to prevent malicious hosts from signing up other hosts for torrents. The querying node returns the token to the same node that they receive the token from. We must accept tokens for a reasonable amount of time after they have been distributed. The BitTorrent implementation uses the SHA-1 hash of the IP address concatenated onto a secret that changes every five minutes and tokens up to ten minutes old are accepted. Every node maintains a routing table of known good nodes.
We use the routing table starting points for queries in the DHT. We return nodes from the routing table in response to queries from other nodes. Not all nodes we learn about are equal. Many nodes using the DHT can send queries and receive responses, but cannot respond to queries from other nodes. A good node is a node has responded to one of our queries within the last 15 minutes.
A node is also good if it has ever responded to our queries and has sent us a query within the last 15 minutes. After 15 minutes of inactivity, a node becomes questionable. Nodes become bad when they fail to respond to multiple queries in a row.
Nodes that we see are good are given priority over nodes with an unknown status. The routing table covers the entire node ID space from 0 to An empty table has only one bucket so any node must fit within it.
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