Tracker Returns External IP

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 [#BEP-3]_, 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 [#BEP-10]_ 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

[#BEP-5]_.

References

.. [#BEP-3] BEP_0003. The BitTorrent Protocol Specification, Cohen.

(http://www.bittorrent.org/beps/bep_0003.html)

.. [#BEP-5] BEP_0005. DHT Protocol. Loewenstern.

(http://www.bittorrent.org/beps/bep_0005.html)

.. [#BEP-10] BEP_0010. Extension Protocol. Norberg, Strigeus, Hazel

(http://www.bittorrent.org/beps/bep_0010.html)

Copyright

This document has been placed in the public domain.

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