Securely Communicating with Remote Servers via PHP

Securely Communicating with Remote Servers via PHP

  • 2016-10-04
  • 803

PHP has an SSH2 library which provides access to resources (shell, remote exec, tunneling, file transfer) on a remote machine using a secure cryptographic transport. Objectively, it is a tedious and highly frustrating task for a developer to implement it due to its overwhelming configuration options and complex API with little documentation.

Connection between client and server

The phpseclib (PHP Secure Communications Library) package has a developer friendly API. It uses some optional PHP extensions if they’re available and falls back on an internal PHP implementation otherwise. To use this package, you don’t need any non-default PHP extensions installed.

Installation

composer require phpseclib/phpseclib

This will install the most recent stable version of the library via Composer.

Use-cases

Before diving in blindly, I’d like to list some use-cases appropriate for using this library:

  1. Executing deployment scripts on a remote server
  2. Downloading and uploading files via SFTP
  3. Generating SSH keys dynamically in an application
  4. Displaying live output for remote commands executed on a server
  5. Testing an SSH or SFTP connection

Connecting to the Remote Server

Using phpseclib, you can connect to your remote server with any of the following authentication methods:

  1. RSA key
  2. Password Protected RSA key
  3. Username and Password (Not recommended)

RSA Key

We will assume that you have a secure RSA key already generated. If you are not familiar with generating a secure RSA key pair, you can go through this article. For a video explanation, you can refer to Creating and Using SSH Keys from Servers For Hackers.

To log in to a remote server using RSA key authentication:

namespace App;

    use phpseclib\Crypt\RSA;
    use phpseclib\Net\SSH2;

    $key = new RSA();
    $key->loadKey(file_get_contents('privatekey'));

    //Remote server's ip address or hostname
    $ssh = new SSH2('192.168.0.1');

    if (!$ssh->login('username', $key)) {
        exit('Login Failed');
    }

Password Protected RSA Key

If your RSA keys are password protected, do not worry. Phpseclib takes care of this particular use case:

namespace App;

    use phpseclib\Crypt\RSA;
    use phpseclib\Net\SSH2;

    $key = new RSA();
    $key->setPassword('your-secure-password');
    $key->loadKey(file_get_contents('privatekey'));

    //Remote server's ip address or hostname
    $ssh = new SSH2('192.168.0.1');

    if (!$ssh->login('username', $key)) {
        exit('Login Failed');
    }

Username and Password

Alternatively, to log in to your remote server using a username and password (we don’t recommend this practice):

namespace App;

    use phpseclib\Net\SSH2;

    //Remote server's ip address or hostname
    $ssh = new SSH2('192.168.0.1');

    if (!$ssh->login('username', 'password')) {
        exit('Login Failed');
    }

For other options such as No Authentication or Multi-Factor authentication please refer to the documentation

Executing Commands on the Remote Server

The code to execute commands on a remote server is pretty simple. You call the $ssh->exec($cmd) method with the command to execute as the parameter.

namespace App;

    use phpseclib\Crypt\RSA;
    use phpseclib\Net\SSH2;

    $key = new RSA();
    $key->loadKey(file_get_contents('privatekey'));

    //Remote server's ip address or hostname
    $ssh = new SSH2('192.168.0.1');

    if (!$ssh->login('username', $key)) {
        exit('Login Failed');
    }

    $ssh->exec('ls -la');

Executing Multiple Commands on Remote Server

In real life applications, we rarely execute a single command. We often need to traverse around the server using cd and execute many other commands. If you try to execute multiple commands on the remote server as below, it won’t give you the desired output:

$ssh->exec('pwd'); //outputs /home/username

    $ssh->exec('cd mysite.com');

    $ssh->exec('pwd'); //outputs /home/username

The reason for above is that a call to the exec method does not carry state forward to the next exec call. To execute multiple commands without losing state:

$ssh->exec('cd /home/username; ls -la'); //Lists all files at /home/username

You can append as many commands as you wish with a semicolon or new line character PHP_EOL.

For example, if you want to run a full deployment script for Laravel:

$ssh->exec(
          "git pull origin master" . PHP_EOL
            . "composer install --no-interaction --no-dev --prefer-dist" . PHP_EOL
            . "composer dump-autoload -o" . PHP_EOL
            . "php artisan optimize" . PHP_EOL
            . "php artisan migrate --force"
    );

Exiting on First Error

In the above script, the whole set of commands is executed as a single shell script. Every command will be executed, even if some of them fail, just like in a regular shell script. As a default, this is fine, but if we need to exit on the first error, we have to alter our bash script. This is not something specific to phpseclib, it is related to bash scripting.

If you put a set -e option at the beginning of the script, the script will terminate as soon as any command in the chain returns a non-zero value.

For example, the modified version of the above deployment script would be

$ssh->exec(
        "set -e" . PHP_EOL
            . "git pull origin master" . PHP_EOL 
            . "composer install --no-interaction --no-dev --prefer-dist" . PHP_EOL
            . "composer dump-autoload -o" . PHP_EOL
            . "php artisan optimize" . PHP_EOL
            . "php artisan migrate --force"
    );

The above script will terminate if any of the commands results in an error.

Gathering Output

The exec method returns the output of your remote script:

    $output = $ssh->exec('ls -la');
    echo $output;

Sometimes, however, it does not return the whole output. You can overcome this by passing a closure as a second argument to the exec method to make sure that any uncaught output will also be returned.

$ssh->exec(
        $deployment_script, function ($str) {
            $output .= $str;
    });

    echo $output;

Note: Error output, if any, will also be returned by the exec method or the underlying closure.

Displaying Live Output

If you want to execute the script via console commands and display live output, you can achieve this by echoing the output in the underlying closure.

$ssh->exec($deployment_script, function ($str) {
        echo $str;
    });

If you want to display it in a web browser, you need to flush (send) the output buffer with ob_flush().

$ssh->exec(
        $deployment_script, function ($str) {
            echo $str;
            flush();
            ob_flush();
    });

Other Configuration Options

It’s also possible to set many other available configuration options. You can call them as $ssh->{option}.

For example: $ssh->setTimeout(100).

All the options we haven’t covered yet are in the table below:

Option Use case
`setTimeout($seconds)` `$ssh->exec('ping 127.0.0.1');` on a Linux host will never return and will run indefinitely. `setTimeout()` makes it so it’ll timeout. Setting `$timeout` to false or 0 will mean there is no timeout.
`write($cmd)` Inputs a command into an interactive shell.
`read()` Returns the output of an interactive shell
`isTimeout()` Return true if the result of the last `$ssh->read()` or `$ssh->exec()` was due to a timeout. Otherwise it will return false.
`isConnected()` Returns true if the connection is still active
`enableQuietMode()` Suppresses stderr so no errors are returned
`disableQuietMode()` Includes stderr in output
`isQuiteModeEnabled()` Returns true if quiet mode is enabled
`enablePTY()` Enable request-pty when using `exec()`
`disablePty()` Disable request-pty when using `exec()`
`isPTYEnabled()` Returns true if request-pty is enabled
`getLog()` Returns a log of the packets that have been sent and received.
`getServerPublicHostKey()` Returns the server public host key. Returns false if the server signature is not signed correctly with the public host key.
`getExitStatus()` Returns the exit status of an SSH command or false.
`getLanguagesClient2Server()` Return a list of the languages the server supports, when receiving stuff from the client.
`getLanguagesServer2Client()` Return a list of the languages the server supports, when sending stuff to the client.
`getCompressionAlgorithmsServer2Client()` Return a list of the compression algorithms the server supports, when sending stuff to the client.
`getCompressionAlgorithmsClient2Server()` Return a list of the compression algorithms the server supports, when receiving stuff from the client.
`getMACAlgorithmsServer2Client()` Return a list of the MAC algorithms the server supports, when sending stuff to the client.
`getMACAlgorithmsClient2Server()` Return a list of the MAC algorithms the server supports, when receiving stuff from the client.
`getEncryptionAlgorithmsServer2Client()` Return a list of the (symmetric key) encryption algorithms the server supports, when sending stuff to the client.
`getEncryptionAlgorithmsClient2Server()` Return a list of the (symmetric key) encryption algorithms the server supports, when receiving stuff from the client.
`getServerHostKeyAlgorithms()` Return a list of the host key (public key) algorithms the server supports.
`getKexAlgorithms()` Return a list of the key exchange algorithms the server supports.

Alternatives

  1. LIBSSH2 – The SSH library – The library provides similar functionality, but is a little less intuitive to use, and it requires you to have libssh2 installed on the server, which most shared hosts don’t have.
  2. Process component – Symfony’s component for writing your own script for connecting and executing commands – as you would do in a normal terminal. This limits us in the configuration options that we might need to set. Achieving the same functionality the above configuration methods provide us with with Process would require in-depth bash knowledge. If your use-case involves only running a local script, however, this might prove to be a useful component.

Summary

In this article, we introduced phpseclib, a package which provides an alternative for SSH2. We have covered the configuration options necessary to get started, and the table above should help you fill in the gaps and give an overview of other configuration options available to you.

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