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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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