BEP:24
Title:Tracker Returns External IP
Version:11085
Last-Modified:2008-05-14 14:54:49 -0700 (Wed, 14 May 2008)
Author:David Harrison <dave at bittorrent.com>
Status:Draft
Type:Standards track
Content-Type:text/x-rst
Created:12-May-2008
Post-History:

So that peers may easily determine their IP address. A BitTorrent tracker may return the requesting peer's public IP address.

Overview

A BitTorrent client can easily learn the IP address used when sending, but because of intervnening Network Address Translators (NATs) the IP address of the client's host seen inside the client's private network may differ from the IP address used to route the client's packets through the public Internet.

In BitTorrent as described in BEP 3 [1], peers wishing to transfer a file contact a central tracker. This tracker returns a list of peers that are currently transferring the file. For all peers that reach the tracker across the public Internet, the source address in each request seen by the tracker carries the publicly routable IP address of the requesting peer.

A tracker implementing this extension returns the key external ip with a bencoded string value containing the IPv4 address as a 32 bit binary or containing the IPv6 address as a 128 bit binary.

Using a 32 or 128 packed binary is idiosyncratic since the value for the ip key returned in a dict within the peers list is returned as a string using ipv4 dotted notation or a domain name. However the packed representation poses no semantic disadvantage and is negligibly smaller.

The IPv4 part of this extension has been supported by uTorrent since May of 2007.

The tracker is not unique in its ability to see a peer's public IP address. Any peer reached over the public Internet sees the public IP address of the peer on the far side of the connection. The Extension Protocol proposed in [3] provides a mechanism for a client to obtains its public IP from any of its peers. There is no conflict for a BitTorrent client to support both tracker and peer methods, and they have slightly different advantages. A peer can learn its external IP from a tracker even when there are no peers. For trackerless swarms, a peer can learn its IP from any peer whose reference was obtained from the DHT [2].

References

[1]BEP_0003. The BitTorrent Protocol Specification, Cohen. (http://www.bittorrent.org/beps/bep_0003.html)
[2]BEP_0005. DHT Protocol. Loewenstern. (http://www.bittorrent.org/beps/bep_0005.html)
[3]BEP_0010. Extension Protocol. Norberg, Strigeus, Hazel (http://www.bittorrent.org/beps/bep_0010.html)