Author: | Arvid Norberg, arvid@rasterbar.com

# Mainline DHT extensions

libtorrent implements a few extensions to the Mainline DHT protocol.

## client identification

In each DHT packet, an extra key is inserted named "v". This is a string describing the client and version used. This can help alot when debugging and finding errors in client implementations. The string is encoded as four characters, two characters describing the client and two characters interpreted as a binary number describing the client version.

Currently known clients:

uTorrent | `UT`
libtorrent | `LT`
MooPolice | `MP`
GetRight | `GR`

## IPv6 support

The DHT messages that don't support IPv6 are the `nodes` replies. They encode all the contacts as 6 bytes packed together in sequence in a string. The problem is that IPv6 endpoints cannot be encoded as 6 bytes, but needs 18 bytes. The extension libtorrent applies is to add another key, called `nodes2`.

`nodes2` may be present in replies that contains a `nodes` key. It is encoded as a list of strings. Each string represents one contact and is encoded as 20 bytes node-id and then a variable length encoded IP address (6 bytes in IPv4 case and 18 bytes in IPv6 case).

As an optimization, libtorrent does not include the extra key in case there are only IPv4 nodes present.