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- $Id$
- Tor's extensions to the SOCKS protocol
- 1. Overview
- The SOCKS protocol provides a generic interface for TCP proxies. Client
- software connects to a SOCKS server via TCP, and requests a TCP connection
- to another address and port. The SOCKS server establishes the connection,
- and reports success or failure to the client. After the connection has
- been established, the client application uses the TCP stream as usual.
- Tor supports SOCKS4 as defined in [1], SOCKS4A as defined in [2], and
- SOCKS5 as defined in [3].
- The stickiest issue for Tor in supporting clients, in practice, is forcing
- DNS lookups to occur at the OR side: if clients do their own DNS lookup,
- the DNS server can learn which addresses the client wants to reach.
- SOCKS4 supports addressing by IPv4 address; SOCKS4A is a kludge on top of
- SOCKS4 to allow addressing by hostname; SOCKS5 supports IPv4, IPv6, and
- hostnames.
- 1.1. Extent of support
- Tor supports the SOCKS4, SOCKS4A, and SOCKS5 standards, except as follows:
- BOTH:
- - The BIND command is not supported.
- SOCKS4,4A:
- - SOCKS4 usernames are ignored.
- SOCKS5:
- - The (SOCKS5) "UDP ASSOCIATE" command is not supported.
- - IPv6 is not supported in CONNECT commands.
- - Only the "NO AUTHENTICATION" (SOCKS5) authentication method [00] is
- supported.
- 2. Name lookup
- As an extension to SOCKS4A and SOCKS5, Tor implements a new command value,
- "RESOLVE" [F0]. When Tor receives a "RESOLVE" SOCKS command, it initiates
- a remote lookup of the hostname provided as the target address in the SOCKS
- request. The reply is either an error (if the address couldn't be
- resolved) or a success response. In the case of success, the address is
- stored in the portion of the SOCKS response reserved for remote IP address.
- (We support RESOLVE in SOCKS4 too, even though it is unnecessary.)
- 3. HTTP-resistance
- Tor checks the first byte of each SOCKS request to see whether it looks
- more like an HTTP request (that is, it starts with a "G", "H", or "P"). If
- so, Tor returns a small webpage, telling the user that his/her browser is
- misconfigured. This is helpful for the many users who mistakenly try to
- use Tor as an HTTP proxy instead of a SOCKS proxy.
- References:
- [1] http://archive.socks.permeo.com/protocol/socks4.protocol
- [2] http://archive.socks.permeo.com/protocol/socks4a.protocol
- [3] SOCKS5: RFC1928
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