A Retracker (also sometimes referred to as a pseudotracker) is a technology for optimizing BitTorrent traffic exchange within local area networks (LANs). [1] [2] A retracker enables direct BitTorrent protocol connections between subscribers of the same Internet Service Provider (ISP) (or multiple ISPs with local traffic exchange agreements), where each subscriber uses an internal (local) IP address. Technically, a retracker is a very simple BitTorrent tracker, typically anonymous, operating within the address space of an ISP's local network. Using a retracker allows peers to exchange traffic at higher speeds using the ISP's internal local network addressing, as access to the local network for many Russian ISPs is permitted at a higher speed than internet access. Furthermore, using a retracker reduces the load on backbone links because subscribers do not need to download the same data separately.
For a BitTorrent client to take advantage of a retracker, the retracker's announce URL must be added to the torrent's list of announce URLs. Some trackers do this automatically, adding the addresses of authorized retrackers based on an analysis of the user's IP address. In other cases, users have to add the retracker's announce URL to .torrent files themselves or by using special programs called torrent patchers. [3] Some ISPs (see below) attempt to install transparent proxies that intercept HTTP requests to popular trackers. Such a proxy modifies .torrent files during download and adds a local retracker to them. Additionally, some trackers add the address "retracker.local" [4] to all .torrent files, which is then resolved to the IP of an active retracker by the ISP's DNS servers. Other solutions to the problem of local BitTorrent traffic exchange also exist.
It is important not to confuse retrackers with regular local trackers for LANs. A retracker does not aim to facilitate the exchange of .torrent files, organize them, enable searching, etc. A retracker serves exclusively as a means to increase data exchange speed between peers.
Retracker support is only possible in multitracker-capable BitTorrent clients, such as μTorrent. Torrent clients that do not support multitracker functionality, like Vuze, will only contact the first tracker that responds; thus, the retracker will either not be used or only it will be used.
On August 24, 2007, the idea of a retracker was described and implemented by a subscriber under the pseudonym unxed on the Corbina Telecom network (Saint Petersburg). [5] On September 21, 2007, the administration of the tracker rutracker.org came forward with a similar initiative. [6] [7] However, the rutracker.org administration was highly critical [8] of attempts by local network users to organize their own retrackers and proposed its own centralized installation of retrackers on ISP technical platforms. The first retracker from rutracker.org began operating in the peering space of the TiERA home network in Saint Petersburg on February 5, 2008. [9]
The rutracker.org approach did not satisfy users whose ISPs were unwilling to officially install retrackers. Therefore, as an alternative to "official provider" retrackers, a system of local retrackers (Re-Tracker.ru, created by an initiative group known by the nicks FreeM@N (Dmitry) and Shift (Stanislav)) was launched in September 2008, again on the Corbina Telecom network in Saint Petersburg by users of that network. From October 2008, a similar system began operating in the Moscow network of Corbina Telecom, covering almost the entire city's local network.
Subsequently, the administration of the rutracker.org tracker abandoned the practice of adding individual retracker addresses based on the client's IP range. Instead, the address retracker.local is added to all torrents (thus, willing ISPs can redirect this address to their retracker via their DNS servers).
The retracker.local address is not supported in all local networks and, furthermore, does not work if the user uses DNS servers other than their ISP's. Therefore, the problem of creating user-run retrackers in local networks remains relevant.
The system of local retrackers works as follows: a .torrent file downloaded by a user is intercepted by the Torrent Patcher program, after which the modified file is passed to a torrent client, such as μTorrent. During patching, lists of retrackers are added. The program also supports a "safe" method of adding retrackers via a Magnet link (works only on μTorrent version 1.8.1 and higher). After this, the torrent client makes requests to the trackers and to the local retrackers. After a request to a local retracker, it receives a list of local peers, enabling data transfer via the BitTorrent protocol at the speed of the ISP's local network. The local retrackers themselves are usually set up by enthusiast users.
If the network where a local retracker is installed uses dual addressing (i.e., a subscriber receives two IP addresses—a public one and an internal network one), the retracker must operate on the network interface with the internal network address. Otherwise, each retracker user would need to configure a static route so that traffic to the retracker goes through the local interface—otherwise, the retracker's advantages would be nullified.
Advantages of creating retrackers on subscriber computers (as opposed to retrackers installed on the ISP's platform):
Disadvantages of this approach:
The original implementation of the retracker idea was published on March 10, 2009, on the nag.ru website. [10] The essence is to automatically add an address like http://retracker.local/ to all torrents, either on the tracker side or on the BitTorrent client side. Thus, each ISP can independently specify the address of its retracker server through its DNS server settings (and if the ISP does not have a retracker server, user-run retrackers can be used by editing the hosts file). Since September 18, 2009, the popular tracker rutracker.org has been adding http://retracker.local to torrents for Russian IP addresses. [11]
A proposal for a BitTorrent protocol extension for automatic discovery of local trackers was published on bittorrent.org. [14] The popular BitTorrent client μTorrent supports this protocol extension by enabling the isp.bep22 option in the "Advanced" section of the settings window. In the latest version as of 2011-10-14 (version 3.0.25756), this option is enabled by default. To check if this technology is supported in your network:
If you receive an error message, the technology is not supported.
The Saint Petersburg ISP Ozerki.net (later acquired by the operator Summa Telecom as part of the operator Severen-Home, which operated under the brand Tsifra) used its own technology for adding a retracker address to .torrent files. According to a network administrator's message, traffic from torrent trackers was redirected using the firewall iptables to a proxy server based on Squid, which in turn processed all .torrent files downloaded by users with a patcher program. [15]
On November 13, 2009, the Saint Petersburg ISP SmartHome announced the launch of its implementation for intercepting and adding a retracker address, publishing a full description of the technology on the nag.ru forum, [16] and also announced support for the isp.bep22 extension in its local network. [17]
The "Local Peer Discovery" feature, present in the BitTorrent client μTorrent since version 1.7 build 1065 (released April 5, 2007), [18] provides capabilities similar to using retrackers. Since this function uses multicast packets, which are blocked by most ISPs to protect against virus epidemics, its application is limited to small local networks consisting of a single segment.
The transition to IP version 6 addresses the problem of local address connectivity, as the number of possible IPv6 addresses is so vast that it, in principle, eliminates the need for local IP addresses. However, not all software (both client and server-side) currently supports IPv6. As a temporary solution, tunneling protocols such as Teredo can be used. Starting with Windows Vista, Teredo support is enabled in the OS by default. For earlier versions, the μTorrent client for Windows OS offers the option to install Teredo automatically via the settings menu.
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