Hello World for an existing DHT

Question

I am familiar with the theory of how a Distributed Hash Table (DHT) works. Is it possible to write a program that stores data to an existing DHT (such as Kademlia or Mainline DHT) ? Is there a simple 'Hello World' type of program that would show the simplest possible way to do this?

Answer 1

The best hello world for DHT would be to send a 'ping' on Bittorrent's DHT to a bootstrap node. The steps are:

1. Bencode a KRPC PING message.
2. Send it over UDP to a bootstrap node.
3. Wait for a reply.

These are the steps I just took before I began working on my own DHT implementation.

Answer 2

The question is might be outdated but anyway.

As was mentioned, the simplest way to say "Hello" to an existing DHT is to send a ping message to one of DHT nodes. Let's consider Kademlia-based Mainline DHT (MDHT).

There is a bootstrap server at address router.bittorrent.com on port 6881. You can think about this server as a general DHT node which is permanently online. Also, you can use another node such as locally run torrent client, which uses DHT.

I've written a small example in Python:

import bencode
import random
import socket


# Generate a 160-bit (20-byte) random node ID.
my_id = ''.join([chr(random.randint(0, 255)) for _ in range(20)])

# Create ping query and bencode it.
# "'y': 'q'" is for "query".
# "'t': '0f'" is a transaction ID which will be echoed in the response.
# "'q': 'ping'" is a query type.
# "'a': {'id': my_id}" is arguments. In this case there is only one argument -
# our node ID.
ping_query = {'y': 'q',
              't': '0f',
              'q': 'ping',
              'a': {'id': my_id}}
ping_query_bencoded = bencode.bencode(ping_query)

# Send a datagram to a server and recieve a response.
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.sendto(ping_query_bencoded,
         (socket.gethostbyname('router.bittorrent.com'), 6881))
r = s.recvfrom(1024)

ping_response = bencode.bdecode(r[0])

print(ping_response)

I've used bencode module to bencode and bdecode messages.

More information on Mainline DHT protocol can be in this document. (Note that the protocol is slightly different from original Kademlia protocol.)