SSH Proxy Command — connect.c

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.

You can get source code from project download page. Pre-compiled MS Windows binary is also available there.

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.

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

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

%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.

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

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 remote.outside.net.

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.

Command line usage is here:

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

-d: Run with debug message output. If you fail to connect, use this option to see what is done.

As additional feature, you can omit port argument when program name is special format containing port number itself like "connect-25". For example:

This example means that the command name "connect-25" indicates port number 25 so you can omit 2nd argument (and used if specified explicitly). This is usefull for the application which invokes only with hostname argument.

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.

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.

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

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.

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

BASIC authentication is supported but DIGEST authentication is not.

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 that you should modify addresses if you use it.

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.

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.

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: http://bitbucket.org/gotoh/connect/src/tip/relay.el

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

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.

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

This method is useful for the situations like:

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.

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

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.

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:

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.

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.

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:

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