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 , SOCKS4A as defined in , and SOCKS5 as defined in  and .
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.
Tor supports the SOCKS4, SOCKS4A, and SOCKS5 standards, except as follows:
- The BIND command is not supported.
- SOCKS4 usernames are used to implement stream isolation.
SOCKS5: - The (SOCKS5) "UDP ASSOCIATE" command is not supported. - SOCKS5 BIND command is not supported. - IPv6 is not supported in CONNECT commands. - SOCKS5 GSSAPI subnegotiation is not supported. - The "NO AUTHENTICATION REQUIRED" (SOCKS5) authentication method  is supported; and as of Tor 0.2.3.2-alpha, the "USERNAME/PASSWORD" (SOCKS5) authentication method  is supported too, and used as a method to implement stream isolation. As an extension to support some broken clients, we allow clients to pass "USERNAME/PASSWORD" authentication message to us even if no authentication was selected. Furthermore, we allow username/password fields of this message to be empty. This technically violates RFC1929 , but ensures interoperability with somewhat broken SOCKS5 client implementations. - Custom reply error code. The "REP" fields, as per the RFC, has unassigned values which are used to describe Tor internal errors. See ExtendedErrors in the tor.1 man page for more details. It is only sent back if this SocksPort flag is set.
(For more information on stream isolation, see IsolateSOCKSAuth on the Tor manpage.)
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.)
For SOCKS5 only, we support reverse resolution with a new command value, "RESOLVE_PTR" [F1]. In response to a "RESOLVE_PTR" SOCKS5 command with an IPv4 address as its target, Tor attempts to find the canonical hostname for that IPv4 record, and returns it in the "server bound address" portion of the reply. (This command was not supported before Tor 0.1.2.2-alpha.)
Tor 0.1.2.4-alpha added a new command value: "CONNECT_DIR" [F2]. In this case, Tor will open an encrypted direct TCP connection to the directory port of the Tor server specified by address:port (the port specified should be the ORPort of the server). It uses a one-hop tunnel and a "BEGIN_DIR" relay cell to accomplish this secure connection.
The F2 command value was removed in Tor 0.2.0.10-alpha in favor of a new use_begindir flag in edge_connection_t.
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.
Tor allows SOCKS clients to send connection data before Tor has sent a SOCKS response. When using an exit node that supports "optimistic data", Tor will send such data to the server without waiting to see whether the connection attempt succeeds. This behavior can save a single round-trip time when starting connections with a protocol where the client speaks first (like HTTP). Clients that do this must be ready to hear that their connection has succeeded or failed after they have sent the data.
We define a set of additional extension error codes that can be returned by our SOCKS implementation in response to failed onion service connections.
(In the C Tor implementation, these error codes can be disabled via the ExtendedErrors flag. In Arti, these error codes are enabled whenever onion services are.)
- X'F0' Onion Service Descriptor Can Not be Found
The requested onion service descriptor can't be found on the hashring and thus not reachable by the client. * X'F1' Onion Service Descriptor Is Invalid The requested onion service descriptor can't be parsed or signature validation failed. * X'F2' Onion Service Introduction Failed Client failed to introduce to the service meaning the descriptor was found but the service is not anymore at the introduction points. The service has likely changed its descriptor or is not running. * X'F3' Onion Service Rendezvous Failed Client failed to rendezvous with the service which means that the client is unable to finalize the connection. * X'F4' Onion Service Missing Client Authorization Tor was able to download the requested onion service descriptor but is unable to decrypt its content because it is missing client authorization information for it. * X'F5' Onion Service Wrong Client Authorization Tor was able to download the requested onion service descriptor but is unable to decrypt its content using the client authorization information it has. This means the client access were revoked. * X'F6' Onion Service Invalid Address The given .onion address is invalid. In one of these cases this error is returned: address checksum doesn't match, ed25519 public key is invalid or the encoding is invalid. * X'F7' Onion Service Introduction Timed Out Similar to X'F2' code but in this case, all introduction attempts have failed due to a time out.
(Note that not all of the above error codes are currently returned by Arti as of August 2023.)
References:  http://en.wikipedia.org/wiki/SOCKS#SOCKS4  http://en.wikipedia.org/wiki/SOCKS#SOCKS4a  SOCKS5: RFC 1928 https://www.ietf.org/rfc/rfc1928.txt  RFC 1929: https://www.ietf.org/rfc/rfc1929.txt