It turns out that this is actually a tricky problem. It’s super easy to use the OpenSSH command line stuff via PHP when you have key based authentication set up, but it’s not at all easy to use when you want to go the user/pass route. This is for a couple of reasons:

First you cannot specify the password on the command line. Second you cannot use the php process controls directly to give the password (well this isn’t 100% true, if you want to recompile your PHP binary with pty support then you probably could bypass everything I’m about to say and just use proc_open straight). And there’s a third reason that I’ll get to in a bit.

OpenSSH supports getting a password from an executable program via the SSH_ASKPASS environment variable — with two notable gotchas. First this only works if you also specify a DISPLAY environment variable, and second it does NOT work if the controlling process has a tty or pty.

The code below works… but if you just paste it into a script file and run it directly with php ./myscript.php it fails. Why?

function ssh_user_pass_port_forward( $hostname, $username, $password, $localport, $remotehost, $remoteport ) {
	$descriptorspec = array(
	        0 => array("pipe", "r"),  // stdin is a pipe that the child will read from
	        1 => array("pipe", "w"),  // stdout is a pipe that the child will write to
	        2 => array("pipe", "w")   // stderr is a file to write to
	);
	$script = tempnam( '/tmp/', 'askpass-');
	file_put_contents( 
		$script, 
		"#!/bin/bash
echo -n ".escapeshellarg( $password ) 
	);
	chmod( $script, 0755 );
	$env = array( 
		'DISPLAY' => '0', 
		'SSH_ASKPASS' => $script 
	);
	$forward = sprintf(
		"%d:%s:%d",
		$localport,
		$remotehost,
		$remoteport
	);
	$command = sprintf(
		"/usr/bin/ssh -n -o StrictHostKeyChecking=no -l %s -L %s -N %s",
		escapeshellarg( $username ),
		escapeshellarg( $forward ),
		escapeshellarg( $hostname )
	);
	return proc_open( $command, $descriptorspec, $pipes, getcwd(), $env);
}

The answer is that you are running it, and you’re running it from a shell which has a tty or pty attached, and the script inherits those and OpenSSH sees this and then ignores the SSH_ASKPASS variable completely. The trick to testing/using this code is to execute it without running it from a terminal. If you execute the command below (roughly) it looks like it hangs. but if you open another terminal you should be able to use the port forward just created.

ssh myserver.com “php /path/to/myscript.php”

This is because since you’re sshing for the sole purpose of running that command and not using a shell the system doesn’t allocate you a pty like it would normally. I’m guessing that executing it via an http request would do something similar.

Since I previously posted some hard-won advice for working with ssh2_* in php I thought I would share this equally tricky bit that I’d also figured out in the process.

Remember that there is ALWAYS more than one way to skin a problem, and every problem can be skinned with enough effort.

Again, this is itch-scratch-ware, YMMV, this is meant as a starting point on a journey to a solution and not a drop-in-works-everywhere bit of code. It’s just the hard stuff. The useful stuff is still up to you 😉