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SSH Proxy Command -- connect.c

connect.c is the simple relaying command to make network connection via SOCKS and https proxy. It is mainly intended to be used as proxy command of OpenSSH. You can make SSH session beyond the firewall with this command,

Features of connect.c are:

  • Supports SOCKS (version 4/4a/5) and https CONNECT method.
  • Supports NO-AUTH and USERPASS authentication of SOCKS
  • You can input password from tty, ssh-askpass or environment variable.
  • Run on UNIX or Windows platform.
  • You can compile with various C compiler (cc, gcc, Visual C, Borland C. etc.)
  • Simple and general program independent from OpenSSH.
  • You can also relay local socket stream instead of standard I/O.

What is 'proxy command'

OpenSSH development team decides to stop supporting SOCKS and any other tunneling mechanism. It was aimed to separate complexity to support various mechanism of proxying from core code. And they recommends more flexible mechanism: ProxyCommand option instead.

Proxy command mechanism is delegation of network stream communication. If ProxyCommand options is specified, SSH invoke specified external command and talk with standard I/O of thid command. Invoked command undertakes network communication with relaying to/from standard input/output including iniitial communication or negotiation for proxying. Thus, ssh can split out proxying code into external command.

The connect.c program was made for this purpose.

How to Use

Get Source

Download source code from here. If you are MS Windows user, you can get pre-compiled binary from here.

Compile and Install

In most environment, you can compile connect.c simply. On UNIX environment, you can use cc or gcc. On Windows environment, you can use Microsoft Visual C, Borland C or Cygwin gcc.

Compilercommand line to compile
UNIX cccc connect.c -o connect
UNIX gccgcc connect.c -o connect
Solarisgcc connect.c -o connect -lnsl -lsocket -lresolv
Microsoft Visual C/C++cl connect.c wsock32.lib advapi32.lib
Borland Cbcc32 connect.c wsock32.lib advapi32.lib
Cygwin gccgcc connect.c -o connect
Mac OS/Darwingcc connect.c -o connect -lresolv

To install connect command, simply copy compiled binary to directory in your PATH (ex. /usr/local/bin). Like this:

  $ cp connect /usr/local/bin

Modify your /.ssh/config

Modify your ~/.ssh/config file to use connect command as proxy command. For the case of SOCKS server is running on firewall host with port 1080, you can add ProxyCommand option in ~/.ssh/config, like this:

    ProxyCommand connect -S %h %p

%h and %p will be replaced on invoking proxy command with target hostname and port specified to SSH command.

If you hate writing many entries of remote hosts, following example may help you.

  ## Outside of the firewall, use connect command with SOCKS conenction.
  Host *
    ProxyCommand connect -S %h %p

  ## Inside of the firewall, use connect command with direct connection.
  Host *
    ProxyCommand connect %h %p

If you want to use http proxy, use -H option instead of -S option in examle above, like this:

  ## Outside of the firewall, with HTTP proxy
  Host *
    ProxyCommand connect -H %h %p

  ## Inside of the firewall, direct
  Host *
    ProxyCommand connect %h %p


After editing your /.ssh/config file, you are ready to use ssh. You can execute ssh without any special options as if remote host is IP reachable host. Following is an example to execute hostname command on host

  $ ssh hostname

Have trouble?

If you have trouble, execute connect command from command line with -d option to see what is happened. Some debug message may appear and reports progress. This information may tell you what is wrong. In this example, error has occurred on authentication stage of SOCKS5 protocol.

  $ connect -d -S 110
  DEBUG: relay_method = SOCKS (2)
  DEBUG: relay_port=1080
  DEBUG: relay_user=gotoh
  DEBUG: socks_version=5
  DEBUG: socks_resolve=REMOTE (2)
  DEBUG: local_type=stdio
  DEBUG: dest_port=110
  DEBUG: Program is $Revision: 1.20 $
  DEBUG: connecting to
  DEBUG: begin_socks_relay()
  DEBUG: atomic_out()  [4 bytes]
  DEBUG: >>> 05 02 00 02
  DEBUG: atomic_in() [2 bytes]
  DEBUG: <<< 05 02
  DEBUG: auth method: USERPASS
  DEBUG: atomic_out()  [some bytes]
  DEBUG: >>> xx xx xx xx ...
  DEBUG: atomic_in() [2 bytes]
  DEBUG: <<< 01 01
  ERROR: Authentication faield.
  FATAL: failed to begin relaying via SOCKS.

More Detail

Command line usage is here:

  usage:  connect [-dnhs45] [-R resolve] [-p local-port] [-w sec]
		  [-H [user@]proxy-server[:port]]
		  [-S [user@]socks-server[:port]]
		  host port

host and port is target hostname and port-number to connect.

-H option specify hostname and port number of http proxy server to relay. If port is omitted, 80 is used. You can specify this value by environment variable HTTP_PROXY and give -h option to use it.

-S option specify hostname and port number of SOCKS server to relay. Like -H option, port number can be omit and default is 1080. You can also specify this value pair by environment variable SOCKS5_SERVER and give -s option to use it.

-4 and -5 is for specifying SOCKS protocol version. It is valid only using with -s or -S. Default is -5 (protocol version 5)

-R is for specifying method to resolve hostname. 3 keywords (local, remote, both) or dot-notation IP address is allowed. Keyword both means; "Try local first, then remote". If dot-notation IP address is specified, use this host as nameserver (UNIX only). Default is remote for SOCKS5 or local for others. On SOCKS4 protocol, remote resolving method (remote and both) use protocol version 4a.

The -p option will forward a local TCP port instead of using the standard input and output.

The -w option specifys timeout seconds for making connection with TARGET host.

The -a option specifiys user intended authentication methods separated by comma. Currently userpass and none are supported. Default is userpass. You can also specifying this parameter by the environment variable SOCKS5_AUTH.

The -d option is used for debug. If you fail to connect, use this and check request to and response from server.

You can omit port argument when program name is special format containing port number itself. For example:

  $ ln -s connect connect-25
  $ ./connect-25
  220 ESMTP Sendmail
  221 2.0.0 closing connection

This example means that the command name "connect-25" contains port number 25 so you can omit 2nd argument (and used if specified explicitly).

Specifying user name via environment variables

There are 5 environemnt variables to specify user name without command line option. This mechanism is usefull for the user who using another user name different from system account.

SOCKS5_USERUsed for SOCKS v5 access.
SOCKS4_USERUsed for SOCKS v4 access.
SOCKS_USERUsed for SOCKS v5 or v4 access and varaibles above are not defined.
HTTP_PROXY_USERUsed for HTTP proxy access.
CONNECT_USERUsed for all type of access if all above are not defined.

Following table describes how user name is determined. Left most number is order to check. If variable is not defined, check next variable, and so on.


Specifying password via environment variables

There are 5 environemnt variables to specify password. If you use this feature, please note that it is not secure way.

SOCKS5_PASSWDUsed for SOCKS v5 access. This variables is compatible with NEC SOCKS implementation.
SOCKS5_PASSWORDUsed for SOCKS v5 access if SOCKS5_PASSWD is not defined.
SOCKS_PASSWORDUsed for SOCKS v5 (or v4) access all above is not defined.
HTTP_PROXY_PASSWORDUsed for HTTP proxy access.
CONNECT_PASSWORDUsed for all type of access if all above are not defined.

Following table describes how password is determined. Left most number is order to check. If variable is not defined, check next variable, and so on. Finally ask to user interactively using external program or tty input.

SOCKS v5HTTP proxy
4(ask to user)(ask to user)


SOCKS5 authentication

Only NO-AUTH and USER/PASSWORD authentications are supported. GSSAPI authentication (RFC 1961) and other draft authentications (CHAP, EAP, MAF, etc.) is not supported.

HTTP authentication

BASIC authentication is supported but DIGEST authentication is not.

Switching proxy server

There is no mechanism to switch proxy server regarding to PC environment. This limitation might be bad news for mobile user. Since I do not want to make this program complex, I do not want to support although this feature is already requested. Please advice me if there is good idea of detecting environment to swich and simple way to specify conditioned directive of servers.

One tricky workaround exists. It is replacing ~/.ssh/config file by script on ppp up/down.

There's another example of wrapper script (contributed by Darren Tucker). This script costs executing ifconfig and grep to detect current environment, but it works. (NOTE: you should modify addresses if you use it.)

  ## ~/bin/myconnect --- Proxy server switching wrapper

  if ifconfig eth0 |grep "inet addr:192\.168\.1" >/dev/null; then
  elif ifconfig eth0 |grep "inet addr:10\." >/dev/null; then
  exec /usr/local/bin/connect $opts $@


Proxying socket connection

In usual, connect.c relays network connection to/from standard input/output. By specifying -p option, however, connect.c relays local network stream instead of standard input/output. With this option, connect command waits connection from other program, then start relaying between both network stream.

This feature may be useful for the program which is hard to SOCKSify.

Use with ssh-askpass command

connect.c ask you password when authentication is required. If you are using on tty/pty terminal, connect can input from terminal with prompt. But you can also use ssh-askpass program to input password. If you are graphical environment like X Window or MS Windows, and program does not have tty/pty, and environment variable SSH_ASKPASS is specified, then connect.c invoke command specified by environment variable SSH_ASKPASS to input password. ssh-askpass program might be installed if you are using OpenSSH on UNIX environment. On Windows environment, pre-compiled binary is available from here.

This feature is limited on window system environment.

And also useful on Emacs on MS Windows (NT Emacs or Meadow). It is hard to send passphrase to connect command (and also ssh) because external command is invoked on hidden terminal and do I/O with this terminal. Using ssh-askpass avoids this problem.

Use for Network Stream of Emacs

Although connect.c is made for OpenSSH, it is generic and independent from OpenSSH. So we can use this for other purpose. For example, you can use this command in Emacs to open network connection with remote host over the firewall via SOCKS or HTTP proxy without SOCKSifying Emacs itself.

There is sample code:

With this code, you can use relay-open-network-stream function instead of open-network-stream to make network connection. See top comments of source for more detail.

Remote resolver

If you are SOCKS4 user on UNIX environment, you might want specify nameserver to resolve remote hostname. You can do it specifying -R option followed by IP address of resolver.

Hopping Connection via SSH

Conbination of ssh and connect command have more interesting usage. Following command makes indirect connection to host2:port from your current host via host1.

  ssh host1 connect host2 port

This method is useful for the situations like:

  • You are outside of organizasion now, but you want to access an internal host barriered by firewall.
  • You want to use some service which is allowed only from some limited hosts.

For example, I want to use local NetNews service in my office from home. I cannot make NNTP session directly because NNTP host is barriered by firewall. Fortunately, I have ssh account on internal host and allowed using SOCKS5 on firewall from outside. So I use following command to connect to NNTP service.

  $ ssh host1 connect news 119
  200 InterNetNews NNRP server INN 2.3.2 ready (posting ok).
  205 .

By combinating hopping connection and relay.el, I can read NetNews using Wanderlust on Emacs at home.

      External (internet) | Internal (office)
  +------+           +----------+          +-------+           +-----------+
  | HOME |           | firewall |          | host1 |           | NNTP host |
  +------+           +----------+          +-------+           +-----------+
   emacs <-------------- ssh ---------------> sshd <-- connect --> nntpd
	 <-- connect --> socksd <-- SOCKS -->

As an advanced example, you can use SSH hopping as fetchmail's plug-in program to access via secure tunnel. This method requires that connect program is insatalled on remote host. There's example of .fetchmailrc bellow. When fetchmail access to mail-server, you will login to remote host using SSH then execute connect program on remote host to relay conversation with pop server. Thus fetchmail can retrieve mails in secure.

  poll mail-server
  protocol pop3
  plugin "ssh %h connect localhost %p"
  username "username"
  password "password"

Break The More Restricted Wall

If firewall does not provide SOCKS nor HTTPS other than port 443, you cannot break the wall in usual way. But if you have you own host which is accessible from internet, you can make ssh connection to your own host by configuring sshd as waiting at port 443 instead of standard 22. By this, you can login to your own host via port 443. Once you have logged-in to extenal home machine, you can execute connect as second hop to make connection from your own host to final target host, like this:

  $ cat ~/.ssh/config
  Host home
    ProxyCommand connect -H firewall:8080 %h 443
  Host server
    ProxyCommand ssh home connect %h %p 
  $ ssh home
  You are logged in to home!
  home# exit
  internal$ ssh server
  You are logged in to server!
  server# exit

This way is similar to "Hopping connection via SSH" except configuring outer sshd as waiting at port 443 (https). This means that you have a capability to break the strongly restricted wall if you have own host out side of the wall.

	Internal (office) | External (internet)
  +--------+         +----------+                 +------+          +--------+
  | office |         | firewall |                 | home |          | server |
  +--------+         +----------+                 +------+          +--------+
     <------------------ ssh --------------------->sshd:443
      <-- connect --> http-proxy <-- https:443 -->                      any
						   connect <-- tcp -->  port

NOTE: If you wanna use this, you should give up hosting https service at port 443 on you external host 'home'.


Difference between SOCKS versions.

SOCKS version 4 is first popular implementation which is documented here. Since this protocol provide IP address based requesting, client program should resolve name of outer host by itself. Version 4a (documented here) is enhanced to allow request by hostname instead of IP address.

SOCKS version 5 is re-designed protocol stands on experience of version 4 and 4a. There is no compativility with previous versions. Instead, there's some improvement: IPv6 support, request by hostname, UDP proxying, etc.

Configuration to use HTTPS

Many http proxy servers implementation supports https CONNECT method (SLL). You might add configuration to allow using https. For the example of DeleGate (DeleGate is a multi-purpose application level gateway, or a proxy server) , you should add https to REMITTABLE parameter to allow HTTP-Proxy like this:

  delegated -Pxxxx ...... REMITTABLE='+,https' ...

For the case of Squid, you should allow target ports via https by ACL, and so on.

SOCKS5 Servers

NEC SOCKS Reference Implementation ... Reference implementation of SOKCS server and library.
Dante ... Dante is free implementation of SOKCS server and library. Many enhancements and modulalized.
DeleGate ... DeleGate is multi function proxy service provider. DeleGate 5.x.x or earlier can be SOCKS4 server, and 6.x.x can be SOCKS5 and SOCKS4 server. and 7.7.0 or later can be SOCKS5 and SOCKS4a server.


socks4.protocol.txt ... SOCKS: A protocol for TCP proxy across firewalls
socks4a.protocol.txt ... SOCKS 4A: A Simple Extension to SOCKS 4 Protocol
RFC 1928 ... SOCKS Protocol Version 5
RFC 1929 ... Username/Password Authentication for SOCKS V5
RFC 2616 ... Hypertext Transfer Protocol -- HTTP/1.1
RFC 2617 ... HTTP Authentication: Basic and Digest Access Authentication


  • Proxy Tunnel -- Proxying command using https CONNECT.
  • stunnel -- Proxy through an https tunnel (Perl script)