2004-06-17 20:11:31 +02:00
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$Id$
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Tor's extensions to the SOCKS protocol
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1. Overview
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The SOCKS protocol provides a generic interface for TCP proxies. Client
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software connects to a SOCKS server via TCP, and requests a TCP connection
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to another address and port. The SOCKS server establishes the connection,
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and reports success or failure to the client. After the connection has
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been established, the client application uses the TCP stream as usual.
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Tor supports SOCKS4 as defined in [1], SOCKS4A as defined in [2], and
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SOCKS5 as defined in [3].
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The stickiest issue for Tor in supporting clients, in practice, is forcing
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DNS lookups to occur at the OR side: if clients do their own DNS lookup,
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the DNS server can learn which addresses the client wants to reach.
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SOCKS4 supports addressing by IPv4 address; SOCKS4A is a kludge on top of
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SOCKS4 to allow addressing by hostname; SOCKS5 supports IPv4, IPv6, and
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hostnames.
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1.1. Extent of support
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Tor supports the SOCKS4, SOCKS4A, and SOCKS5 standards, except as follows:
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BOTH:
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- The BIND command is not supported.
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SOCKS4,4A:
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- SOCKS4 usernames are ignored.
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SOCKS5:
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- The (SOCKS5) "UDP ASSOCIATE" command is not supported.
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- IPv6 is not supported in CONNECT commands.
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- Only the "NO AUTHENTICATION" (SOCKS5) authentication method [00] is
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supported.
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2. Name lookup
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As an extension to SOCKS4A and SOCKS5, Tor implements a new command value,
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"RESOLVE" [F0]. When Tor receives a "RESOLVE" SOCKS command, it initiates
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a remote lookup of the hostname provided as the target address in the SOCKS
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request. The reply is either an error (if the address couldn't be
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resolved) or a success response. In the case of success, the address is
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stored in the portion of the SOCKS response reserved for remote IP address.
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2004-06-17 23:05:46 +02:00
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(We support RESOLVE in SOCKS4 too, even though it is unnecessary.)
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2004-06-17 20:11:31 +02:00
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3. HTTP-resistance
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Tor checks the first byte of each socks request to see whether it looks
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more like an HTTP request (that is, it starts with a "G", "H", or "P"). If
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so, Tor returns a small webpage, telling the user that his/her browser is
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misconfigured. This is helpful for the many users who mistakenly try to
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use Tor as an HTTP proxy instead of a SOCKS proxy.
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References:
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[1] http://archive.socks.permeo.com/protocol/socks4.protocol
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[2] http://archive.socks.permeo.com/protocol/socks4a.protocol
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[3] SOCKS5: RFC1928
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