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105 lines
4.5 KiB
Plaintext
105 lines
4.5 KiB
Plaintext
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] and [4].
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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 used to implement stream isolation.
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SOCKS5:
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- The (SOCKS5) "UDP ASSOCIATE" command is not supported.
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- SOCKS5 BIND command is not supported.
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- IPv6 is not supported in CONNECT commands.
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- SOCKS5 GSSAPI subnegotiation is not supported.
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- The "NO AUTHENTICATION REQUIRED" (SOCKS5) authentication method [00] is
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supported; and as of Tor 0.2.3.2-alpha, the "USERNAME/PASSWORD" (SOCKS5)
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authentication method [02] is supported too, and used as a method to
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implement stream isolation. As an extension to support some broken clients,
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we allow clients to pass "USERNAME/PASSWORD" authentication message to us
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even if no authentication was selected. Furthermore, we allow
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username/password fields of this message to be empty. This technically
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violates RFC1929 [4], but ensures interoperability with somewhat broken
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SOCKS5 client implementations.
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(For more information on stream isolation, see IsolateSOCKSAuth on the Tor
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manpage.)
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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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(We support RESOLVE in SOCKS4 too, even though it is unnecessary.)
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For SOCKS5 only, we support reverse resolution with a new command value,
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"RESOLVE_PTR" [F1]. In response to a "RESOLVE_PTR" SOCKS5 command with
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an IPv4 address as its target, Tor attempts to find the canonical
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hostname for that IPv4 record, and returns it in the "server bound
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address" portion of the reply.
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(This command was not supported before Tor 0.1.2.2-alpha.)
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3. Other command extensions.
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Tor 0.1.2.4-alpha added a new command value: "CONNECT_DIR" [F2].
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In this case, Tor will open an encrypted direct TCP connection to the
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directory port of the Tor server specified by address:port (the port
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specified should be the ORPort of the server). It uses a one-hop tunnel
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and a "BEGIN_DIR" relay cell to accomplish this secure connection.
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The F2 command value was removed in Tor 0.2.0.10-alpha in favor of a
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new use_begindir flag in edge_connection_t.
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4. 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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5. Optimistic data
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Tor allows SOCKS clients to send connection data before Tor has sent a
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SOCKS response. When using an exit node that supports "optimistic data",
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Tor will send such data to the server without waiting to see whether the
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connection attempt succeeds. This behavior can save a single round-trip
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time when starting connections with a protocol where the client speaks
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first (like HTTP). Clients that do this must be ready to hear that
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their connection has succeeded or failed _after_ they have sent the
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data.
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References:
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[1] http://en.wikipedia.org/wiki/SOCKS#SOCKS4
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[2] http://en.wikipedia.org/wiki/SOCKS#SOCKS4a
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[3] SOCKS5: RFC 1928 https://www.ietf.org/rfc/rfc1928.txt
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[4] RFC 1929: https://www.ietf.org/rfc/rfc1929.txt
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