PHP

SSH Tunnel Set Allowed SSH Algorithms

Demonstrates the Chilkat SshTunnel.SetAllowedAlgorithms method, which restricts the SSH connection to a specific set of algorithms. A JsonObject supplies comma-separated allow-lists for the kex, hostKey, cipher, and mac categories. It must be called before Connect.

Important: You typically should not set allowed algorithms explicitly. By default Chilkat orders algorithms according to best practices and accounts for known vulnerabilities.

Background: SSH negotiates a mutually supported algorithm from each category at connection time, and pinning that choice makes an application brittle: if the server later drops a weak algorithm, the two sides may fail to agree and the tunnel stops working. The legitimate reasons to override are a security policy mandating specific algorithms or a compatibility problem with an old server; otherwise let the library's defaults evolve with current guidance.

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Chilkat2 PHP

<?php

use Chilkat2\JsonObject;
use Chilkat2\SshTunnel;

// Demonstrates the SshTunnel.SetAllowedAlgorithms method, which restricts the SSH connection to a
// specific set of algorithms.  The only argument is a JsonObject.  It must be called before
// Connect.
//
// Note: You typically should NOT set allowed algorithms explicitly.  By default Chilkat orders
// algorithms according to best practices and accounts for known vulnerabilities.  Hard-coding
// them can make an application brittle if a server later changes its allowed algorithms.

$tunnel = new SshTunnel();

// List the allowed algorithms for each category, in order of preference.
$json = new JsonObject();
$json->UpdateString('kex', 'curve25519-sha256@libssh.org,ecdh-sha2-nistp256');
$json->UpdateString('hostKey', 'ssh-ed25519,ecdsa-sha2-nistp256');
$json->UpdateString('cipher', 'chacha20-poly1305@openssh.com,aes256-ctr');
$json->UpdateString('mac', 'hmac-sha2-256,hmac-sha2-512');

// Apply the allow-list before connecting.
try {
    $tunnel->SetAllowedAlgorithms($json);

    $tunnel->DestHostname = 'db.internal.example.com';
    $tunnel->DestPort = 5432;

    $sshPort = 22;
    $tunnel->Connect('ssh.example.com', $sshPort);
} catch (\Chilkat2\Exception $e) {
    echo $e->getMessage(), "\n";
    exit;
}

echo "Connected.\n";

$waitForThreadExit = true;
try {
    $tunnel->CloseTunnel($waitForThreadExit);
} catch (\Chilkat2\Exception $e) {
    echo $e->getMessage(), "\n";
    exit;
}