Phpseclib: Securely Communicating with Remote Servers via PHP

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Phpseclib: Securely Communicating with Remote Servers via PHP

Key Takeaways

  • Phpseclib, or PHP Secure Communications Library, offers a developer-friendly API for secure communication with remote servers, providing an alternative to SSH2. It can be installed via Composer and does not require any non-default PHP extensions.
  • With phpseclib, developers can execute deployment scripts on a remote server, download and upload files via SFTP, generate SSH keys dynamically, display live output for remote commands, and test SSH or SFTP connections. It supports RSA key, Password Protected RSA key, and Username and Password authentication methods.
  • Phpseclib allows developers to execute commands on a remote server, gather output, and display live output. It offers numerous configuration options including setting timeouts, enabling or disabling quiet mode, and enabling or disabling request-pty when using exec(). It also provides methods to retrieve information about the server and the connection.

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.

For an in-depth implementation of key-based communication, see our previous tutorial.

How do you execute remote commands? Can you think of any advanced use cases for this library? What are they? Let us know in the comments!

Frequently Asked Questions (FAQs) about PHPseclib

What is PHPseclib and why is it important for secure communication with remote servers?

PHPseclib is a library for PHP that facilitates secure communication with remote servers. It is important because it provides a set of tools for encrypting data, authenticating users, and ensuring the integrity of data during transmission. This is crucial in today’s digital world where data breaches and cyber-attacks are common. PHPseclib uses various cryptographic techniques to ensure data security, making it a reliable choice for developers.

How does PHPseclib ensure secure communication?

PHPseclib ensures secure communication by implementing various cryptographic protocols like SSH-2, SFTP, and X.509. These protocols encrypt the data before transmission, making it unreadable to anyone except the intended recipient. This way, even if the data is intercepted during transmission, it cannot be deciphered without the correct decryption key.

How can I install PHPseclib in my project?

PHPseclib can be easily installed in your project using Composer, a dependency management tool for PHP. You just need to run the command ‘composer require phpseclib/phpseclib’ in your project directory. This will download and install PHPseclib along with its dependencies.

How can I use PHPseclib to connect to a remote server?

To connect to a remote server using PHPseclib, you need to create an instance of the Net_SSH2 class and use the ‘login’ method with your username and password. If the login is successful, you can then use the ‘exec’ method to execute commands on the remote server.

Can I use PHPseclib for file transfer?

Yes, PHPseclib supports file transfer through the SFTP protocol. You can use the ‘put’ and ‘get’ methods of the Net_SFTP class to upload and download files respectively.

How can I handle errors in PHPseclib?

PHPseclib provides the ‘getErrors’ and ‘getLastError’ methods to retrieve the errors that occurred during the execution of commands. These methods return an array of error messages that can help you debug your code.

Is PHPseclib compatible with all versions of PHP?

PHPseclib is compatible with PHP 5.3.3 and above. However, it is recommended to use the latest version of PHP for better performance and security.

Can I use PHPseclib with frameworks like Laravel or Symfony?

Yes, PHPseclib can be used with any PHP framework that supports Composer. You just need to include the PHPseclib package in your project’s composer.json file.

How can I contribute to the PHPseclib project?

PHPseclib is an open-source project hosted on GitHub. You can contribute to the project by reporting bugs, suggesting new features, or submitting pull requests with code improvements.

Where can I find more information about PHPseclib?

You can find more information about PHPseclib on its official website and GitHub repository. The website provides detailed documentation on how to use the library, while the GitHub repository contains the source code and issues tracker.

Viraj KhatavkarViraj Khatavkar
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Viraj Khatavkar is a software developer, writer, speaker, and entrepreneur from Mumbai, India. He likes to challenge a complex set of problems with PHP. He’s fond of servers, swims and sometimes blogs

BrunoSdeploymentOOPHPPHPphpseclibremote serversecurityssh
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