SOCKS: A protocol for TCP proxy across firewalls[SOCKS4]

	SOCKS: A protocol for TCP proxy across firewalls



			Ying-Da Lee

		Principal Member Technical Staff

		  NEC Systems Laboratory, CSTC

			ylee@syl.dl.nec.com



SOCKS was originally developed by David Koblas and subsequently modified

and extended by me to its current running version -- version 4. It is a

protocol that relays TCP sessions at a firewall host to allow application

users transparent access across the firewall. Because the protocol is

independent of application protocols, it can be (and has been) used for

many different services, such as telnet, ftp, finger, whois, gopher, WWW,

etc. Access control can be applied at the beginning of each TCP session;

thereafter the server simply relays the data between the client and the

application server, incurring minimum processing overhead. Since SOCKS

never has to know anything about the application protocol, it should also

be easy for it to accommodate applications which use encryption to protect

their traffic from nosey snoopers.



Two operations are defined: CONNECT and BIND.



1) CONNECT



The client connects to the SOCKS server and sends a CONNECT request when

it wants to establish a connection to an application server. The client

includes in the request packet the IP address and the port number of the

destination host, and userid, in the following format.



		+----+----+----+----+----+----+----+----+----+----+....+----+

		| VN | CD | DSTPORT |      DSTIP        | USERID       |NULL|

		+----+----+----+----+----+----+----+----+----+----+....+----+

 # of bytes:	   1    1      2              4           variable       1



VN is the SOCKS protocol version number and should be 4. CD is the

SOCKS command code and should be 1 for CONNECT request. NULL is a byte

of all zero bits.



The SOCKS server checks to see whether such a request should be granted

based on any combination of source IP address, destination IP address,

destination port number, the userid, and information it may obtain by

consulting IDENT, cf. RFC 1413.  If the request is granted, the SOCKS

server makes a connection to the specified port of the destination host.

A reply packet is sent to the client when this connection is established,

or when the request is rejected or the operation fails. 



		+----+----+----+----+----+----+----+----+

		| VN | CD | DSTPORT |      DSTIP        |

		+----+----+----+----+----+----+----+----+

 # of bytes:	   1    1      2              4



VN is the version of the reply code and should be 0. CD is the result

code with one of the following values:



	90: request granted

	91: request rejected or failed

	92: request rejected becasue SOCKS server cannot connect to

	    identd on the client

	93: request rejected because the client program and identd

	    report different user-ids



The remaining fields are ignored.



The SOCKS server closes its connection immediately after notifying

the client of a failed or rejected request. For a successful request,

the SOCKS server gets ready to relay traffic on both directions. This

enables the client to do I/O on its connection as if it were directly

connected to the application server.





2) BIND



The client connects to the SOCKS server and sends a BIND request when

it wants to prepare for an inbound connection from an application server.

This should only happen after a primary connection to the application

server has been established with a CONNECT.  Typically, this is part of

the sequence of actions:



-bind(): obtain a socket

-getsockname(): get the IP address and port number of the socket

-listen(): ready to accept call from the application server

-use the primary connection to inform the application server of

 the IP address and the port number that it should connect to.

-accept(): accept a connection from the application server



The purpose of SOCKS BIND operation is to support such a sequence

but using a socket on the SOCKS server rather than on the client.



The client includes in the request packet the IP address of the

application server, the destination port used in the primary connection,

and the userid.



		+----+----+----+----+----+----+----+----+----+----+....+----+

		| VN | CD | DSTPORT |      DSTIP        | USERID       |NULL|

		+----+----+----+----+----+----+----+----+----+----+....+----+

 # of bytes:	   1    1      2              4           variable       1



VN is again 4 for the SOCKS protocol version number. CD must be 2 to

indicate BIND request.



The SOCKS server uses the client information to decide whether the

request is to be granted. The reply it sends back to the client has

the same format as the reply for CONNECT request, i.e.,



		+----+----+----+----+----+----+----+----+

		| VN | CD | DSTPORT |      DSTIP        |

		+----+----+----+----+----+----+----+----+

 # of bytes:	   1    1      2              4



VN is the version of the reply code and should be 0. CD is the result

code with one of the following values:



	90: request granted

	91: request rejected or failed

	92: request rejected becasue SOCKS server cannot connect to

	    identd on the client

	93: request rejected because the client program and identd

	    report different user-ids.



However, for a granted request (CD is 90), the DSTPORT and DSTIP fields

are meaningful.  In that case, the SOCKS server obtains a socket to wait

for an incoming connection and sends the port number and the IP address

of that socket to the client in DSTPORT and DSTIP, respectively. If the

DSTIP in the reply is 0 (the value of constant INADDR_ANY), then the

client should replace it by the IP address of the SOCKS server to which

the cleint is connected. (This happens if the SOCKS server is not a

multi-homed host.)  In the typical scenario, these two numbers are

made available to the application client prgram via the result of the

subsequent getsockname() call.  The application protocol must provide a

way for these two pieces of information to be sent from the client to

the application server so that it can initiate the connection, which

connects it to the SOCKS server rather than directly to the application

client as it normally would.



The SOCKS server sends a second reply packet to the client when the

anticipated connection from the application server is established.

The SOCKS server checks the IP address of the originating host against

the value of DSTIP specified in the client's BIND request.  If a mismatch

is found, the CD field in the second reply is set to 91 and the SOCKS

server closes both connections.  If the two match, CD in the second

reply is set to 90 and the SOCKS server gets ready to relay the traffic

on its two connections. From then on the client does I/O on its connection

to the SOCKS server as if it were directly connected to the application

server.







For both CONNECT and BIND operations, the server sets a time limit

(2 minutes in current CSTC implementation) for the establishment of its

connection with the application server. If the connection is still not

establiched when the time limit expires, the server closes its connection

to the client and gives up.