Zig

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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const std = @import("std");
const chilkat = @import("chilkat");

pub fn main(init: std.process.Init) !void {
    const alloc = init.arena.allocator();

    // 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.

    const tunnel = try chilkat.SshTunnel.init();
    defer tunnel.deinit();

    // List the allowed algorithms for each category, in order of preference.
    const json = try chilkat.JsonObject.init();
    defer json.deinit();
    json.updateString("kex", "curve25519-sha256@libssh.org,ecdh-sha2-nistp256") catch {};
    json.updateString("hostKey", "ssh-ed25519,ecdsa-sha2-nistp256") catch {};
    json.updateString("cipher", "chacha20-poly1305@openssh.com,aes256-ctr") catch {};
    json.updateString("mac", "hmac-sha2-256,hmac-sha2-512") catch {};

    // Apply the allow-list before connecting.
    tunnel.setAllowedAlgorithms(json) catch {
        std.debug.print("{s}\n", .{try tunnel.getLastErrorText(alloc)});
        return;
    };

    tunnel.setDestHostname("db.internal.example.com");
    tunnel.setDestPort(5432);

    const ssh_port = 22;
    tunnel.connect("ssh.example.com", ssh_port) catch {
        std.debug.print("{s}\n", .{try tunnel.getLastErrorText(alloc)});
        return;
    };

    std.debug.print("Connected.\n", .{});

    const wait_for_thread_exit = true;
    tunnel.closeTunnel(wait_for_thread_exit) catch {
        std.debug.print("{s}\n", .{try tunnel.getLastErrorText(alloc)});
        return;
    };
}