bootstrap-dht

The DHT bootstrap server can be used as an introducer to the bittorrent DHT network. Like the ones running at router.utorrent.com and router.bittorrent.com.

BitTorrent clients can use this server to join the DHT, assuming some number of clients are agreeing on using the same server.

The command line options are:

usage: dht-bootstrap <external-IP> [options]

OPTIONS:
--help                prints this message.
--threads <n>         spawns <n> threads (defaults to the
                      number of hardware cores).
--nodes <n>           sets the number of nodes to keep in
                      the node buffer. Once full, the oldest
                      nodes are replaced as new nodes come in.
--ping-queue <n>      sets the max number of nodes to keep in
                      the ping queue. Nodes are held in the queue
                      for 15 minutes.
--no-verify-id        disable filtering nodes based on their node ID
                      and external IP (allow any node in on the
                      node list to hand out).
--ipv6 <ip>           listen for IPv6 packets on the given address
                      can be specified more than once
--version <version>   The client version to insert into all outgoing
                      packets. The version format must be 2 characters
                      followed by a dash and an integer.
--dir <path>          specify the directory where the node buckets are
                      stored on disk. Defaults to ".".
--port <listen-port>  Sets the port to listen on (for all interfaces)
                      defaults to 6881
--x-pollinate <ip> <port>
                      if the ping queue becomes too small, request more
                      nodes from this DHT node.

The first argument when launching the server is its own IP. This is not only relevant for binding the socket to this interface but is also used when generating its own DHT node ID.

The number of threads defaults to the number of CPU cores but can be overridden by the --threads argument.

The number of DHT nodes to keep in the cache of nodes to hand out defaults to 10000000 and can be overridden by --nodes. It may not be lower than 1000.

The ping queue is the max number of nodes allowed in the queue waiting to be pinged. Nodes are (ideally) pinged 15 minutes after seen. If too many nodes are pinging the dht-bootstrapper to hold them all for 15 minutes in the ping buffer, the rate of pinging is throttled to match.

For example, the default of 5000000 nodes in the ping buffer restricts the ping rate to (5000000 / 900 = ) 5555 pings per second. Experience from router.utorrent.com suggests that about 45% of pings succeeds and end up being added to the node buffer. At the default rate, that means the entire node buffer is replaced every 66 minutes. i.e. once you're in the node buffer, you'll be handed out to nodes for abut 66 minutes before you are replaced by someone else.

The current rate at router.utorrent.com is about 20000 requests per second. Every request returns 16 nodes. That means every node in the node buffer is handed out once every (10000000 / 16 / 20000 ~=) 31 seconds.

Only nodes whose node ID match their external IP address (according to this) are pinged.

dht-bootstrap listens on port 6881 by default. You may specify a different port using the --port command line option.

A more detailed description of how it's implemented can be found in the header of main.cpp.

The source for the bootstrap server is released under the MIT license. Please contribute back fixes and improvements!

building

To build, you need boost and boost-build installed. Build by running: b2.

If you prefer to build manually, just compile main.cpp, bdecode.cpp and link against the boost.system library as well as adding boost to your include path.

To build you need a compiler with C++11 support.

acknowledgments

Thanks to following people who have contributed code or discovered bugs in bootstrap-dht:

Team_LPJ@BoB working with Zero Day Initiative (ZDI)