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What's Changed

New Contributors

Full Changelog: 0.6.0...0.7.0


Citadel is a high level API around NIOSSH. It aims to add what's out of scope for NIOSSH, lending code from my private tools.

Citadel is in active development by our team or Swift experts. Get in touch with our Discord Community.

Do you need professional support? We're available at hello@unbeatable.software

Client Usage

Citadel's SSHClient needs a connection to a SSH server first:

let client = try await SSHClient.connect(
    host: "example.com",
    authenticationMethod: .passwordBased(username: "joannis", password: "s3cr3t"),
    hostKeyValidator: .acceptAnything(), // Please use another validator if at all possible, it's insecure
    reconnect: .never

Using that client, we support a couple types of operations:

TCP-IP Forwarding (Proxying)

// The address that is presented as the locally exposed interface
// This is purely communicated to the SSH server
let address = try SocketAddress(ipAddress: "fe80::1", port: 27017)
let configuredProxyChannel = try await client.createDirectTCPIPChannel(
    using: SSHChannelType.DirectTCPIP(
        targetHost: "localhost", // MongoDB host 
        targetPort: 27017, // MongoDB port
        originatorAddress: address
) { proxyChannel in

This will create a channel that is connected to the SSH server, and then forwarded to the target host. This is useful for proxying TCP-IP connections, such as MongoDB, Redis, MySQL, etc.

Executing Commands

You can execute a command through SSH using the following code:

let stdout = try await client.executeCommand("ls -la ~")

Additionally, a maximum responsive response size can be set, and stderr can be merged with stdout so that the answer contains the content of both streams:

let stdoutAndStderr = try await client.executeCommand("ls -la ~", maxResponseSize: 42, mergeStreams: true)

The executeCommand function accumulated information into a contiguous ByteBuffer. This is useful for non-interactive commands such as cat and ls.

The executeCommandPair function or executeCommandStream function can be used to access stdout and stderr independently. Both functions also accumulate information into contiguous separate ByteBuffers.

An example of how executeCommandPair can be used:

let streams = try await client.executeCommandPair("cat /foo/bar.log")

for try await blob in answer.stdout {
    // do something with blob

for try await blob in answer.stderr {
    // do something with blob

An example of how executeCommandStream can be used:

let streams = try await client.executeCommandStream("cat /foo/bar.log")
var asyncStreams = streams.makeAsyncIterator()

while let blob = try await asyncStreams.next() {
    switch blob {
        case .stdout(let stdout):
            // do something with stdout
        case .stderr(let stderr):
            // do something with stderr

Citadel currently does not expose APIs for streaming into a process' stdin. If you want this, please create an issue.

SFTP Client

To begin with SFTP, you must instantiate an SFTPClient based on your SSHClient:

// Open an SFTP session on the SSH client
let sftp = try await client.openSFTP()

// List the contents of the /etc directory
let directoryContents = try await sftp.listDirectory(atPath: "/etc")

// Create a directory
try await sftp.createDirectory(atPath: "/etc/custom-folder")

// Open a file
let resolv = try await sftp.openFile(filePath: "/etc/resolv.conf", flags: .read)

// Read a file in bulk
let resolvContents: ByteBuffer = try await resolv.readAll()

// Read a file in chunks
let chunk: ByteBuffer = try await resolv.read(from: index, length: maximumByteCount)

// Close a file
try await resolv.close()

// Write to a file
let file = try await sftp.openFile(filePath: "/etc/resolv.conf", flags: [.read, .write, .forceCreate])
let fileWriterIndex = 0
try await file.write(ByteBuffer(string: "Hello, world", at: fileWriterIndex)
try await file.close()

// Read a file using a helper. This closes the file automatically
let data = try await sftp.withFile(
    filePath: "/etc/resolv.conf",
    flags: .read
) { file in
    try await file.readAll()

// Close the SFTP session
try await sftp.close()


To use Citadel, first you need to create & start an SSH server, using your own authentication delegate:

import NIOSSH
import Citadel

// Create a custom authentication delegate that uses MongoDB to authenticate users
// This is just an example, you can use any database you want
// You can use public key authentication, password authentication, or both.
struct MyCustomMongoDBAuthDelegate: NIOSSHServerUserAuthenticationDelegate {
    let db: MongoKitten.Database

    let supportedAuthenticationMethods: NIOSSHAvailableUserAuthenticationMethods = [.password, .publicKey]
    func requestReceived(request: NIOSSHUserAuthenticationRequest, responsePromise: EventLoopPromise<NIOSSHUserAuthenticationOutcome>) {
        responsePromise.completeWithTask {
            // Authenticate the user
            guard let user = try await db[User.self].findOne(matching: { user in
                user.$username == username
            }) else {
                // User does not exist
                return .failure

            switch request.request {
            case .hostBased. none:
                // Not supported
                return .failure
            case .publicKey(let publicKey):
                // Check if the public key is correct
                guard publicKey.publicKey == user.publicKey else {
                    return .failure

                return .success
            case .password(let request):
                // Uses Vapor's Bcrypt library to verify the password
                guard try Bcrypt.verify(request.password, created: user.password) else {
                    return .failure
                return .success

Then, create the server:

let server = try await SSHServer.host(
    host: "",
    port: 22,
    hostKeys: [
        // This hostkey changes every app boot, it's more practical to use a pre-generated one
        NIOSSHPrivateKey(ed25519Key: .init())
    authenticationDelegate: MyCustomMongoDBAuthDelegate(db: mongokitten)

Then, enable the SFTP server or allow executing commands. Don't worry, these commands do not target the host system. You can implement filsystem and shell access yourself! So you get to dictate permissions, where it's actually stored, and do any shenanigans you need:

server.enableExec(withDelegate: MyExecDelegate())
server.enableSFTP(withDelegate: MySFTPDelegate())

If you're running the SSHServer from main.swift or an @main annotated type, make sure that Swift doesn't exit or deinit the server. A simple solution that is applicable most of the time is to use the server's closeFuture.

try await server.closeFuture.get()

Exec Server

When creating a command execution delegate, simply implement the ExecDelegate protocol and the following functions:

func setEnvironmentValue(_ value: String, forKey key: String) async throws
func start(command: String, outputHandler: ExecOutputHandler) async throws -> ExecCommandContext

The setEnvironmentValue function adds an environment variable, which you can pass onto child processes. The start command simply executed the command "in the shell". How and if you process that command is up to you. The executed command is inputted as the first argument, and the second argument (the ExecOutputHandler), contains the authenticated user, Pipes for stdin, stdout and stderr as well as some function calls for indicating a process has exited.

Whether you simulate a process, or hook up a real child-process, the requirements are the same. You must provide an exit code or throw an error out of the executing function. You can also fail on the outputHandler the process using an error. Finally, you'll have to return an ExecCommandContext that represents your process. This can receive remote terminate signals, or receive a notification that stdin was closed through inputClosed.

import Foundation

/// A context that represents a process that is being executed.
/// This can receive remote `terminate` signals, or receive a notification that `stdin` was closed through `inputClosed`.
struct ExecProcessContext: ExecCommandContext {
    let process: Process
    func terminate() async throws {
    func inputClosed() async throws {
        try process.stdin.close()

/// An example of a custom ExecDelegate that uses bash as the shell to execute commands
public final class MyExecDelegate: ExecDelegate {
    var environment: [String: String] = [:]

    public func setEnvironmentValue(_ value: String, forKey key: String) async throws {
        // Set the environment variable
        environment[key] = value

    public func start(command: String, outputHandler: ExecOutputHandler) async throws -> ExecCommandContext {
        // Start the command
        let process = Process()

        // This uses bash as the shell to execute the command
        // You can use any shell you want, or even a custom one
        // This is just an example, you can do whatever you want
        // as long as you provide an exit code
        process.executableURL = URL(fileURLWithPath: "/bin/bash")
        process.arguments = ["-c", command]
        process.environment = environment
        process.standardInput = outputHandler.stdin
        process.standardOutput = outputHandler.stdout
        process.standardError = outputHandler.stderr
        process.terminationHandler = { process in
            // Send the exit code
            outputHandler.exit(code: Int(process.terminationStatus))

        // Start the process
        try process.run()
        return ExecProcessContext(process: process)

SFTP Server

When you implement SFTP in Citadel, you're responsible for taking care of logistics. Be it through a backing MongoDB store, a real filesystem, or your S3 bucket.


The most important helper most people need is OpenSSH key parsing. We support extensions on PrivateKey types such as our own Insecure.RSA.PrivateKey, as well as existing SwiftCrypto types like Curve25519.Signing.PrivateKey:

// Parse an OpenSSH RSA private key. This is the same format as the one used by OpenSSH
let sshFile = try String(contentsOf: ..)
let privateKey = try Insecure.RSA.PrivateKey(sshRsa: sshFile)


If you can't connect to a server, it's likely that your server uses a deprecated set of algorithms that NIOSSH doesn't support. No worries though, as Citadel does implement these! Don't use these if you don't have to, as they're deprecated for good (security) reasons.

// Create a new set of algorithms
var algorithms = SSHAlgorithms()

algorithms.transportProtectionSchemes = .add([

algorithms.keyExchangeAlgorithms = .add([

You can then use these in an SSHClient, together with any other potential protocol configuration options:

// Connect to the server using the new algorithms and a password-based authentication method
let client = try await SSHClient.connect(
    host: "example.com",
    authenticationMethod: .passwordBased(username: "joannis", password: "s3cr3t"),
    hostKeyValidator: .acceptAnything(), // Please use another validator if at all possible, it's insecure
    reconnect: .never,
    algorithms: algorithms,
    protocolOptions: [
        .maximumPacketSize(1 << 20)


A couple of code is held back until further work in SwiftNIO SSH is completed. We're currently working with Apple to resolve these.

  • RSA Authentication (implemented & supported, but in a fork of NIOSSH)
  • Much more documentation & tutorials


I'm happy to accept ideas and PRs for new API's.


  • Swift Tools 5.5.0
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Last updated: Wed Apr 10 2024 09:09:14 GMT-0900 (Hawaii-Aleutian Daylight Time)