precog / fs2-ssh   1.1.0


A wrapper around Apache SSHD targeting cats-effect and fs2

Scala versions: 2.13 2.12

fs2-ssh Discord

A lightweight wrapper around Apache SSHD. The primary purpose of this library is to provide a resource-safe, functional, thread-pool-safe API for SSH within the Cats Effect ecosystem.

At present, only limited client functionality is offered, but future functionality is likely to include further client functionality (such as shell access, tunneling, and signals) as well as a server API.


libraryDependencies += "com.precog" %% "fs2-ssh" % <version>

fs2-ssh is currently published for Scala 2.12. It depends on Cats Effect 1.4.0, Cats MTL 0.5.0, fs2 1.0.5, SSHD 2.3.0, and Netty 4.1.39.

What follows is a simple example of connecting to a remote server, running a command, printing the result and the status code, and then cleaning everything up:

import cats.effect._
import cats.mtl.instances.all._
import fs2.Sink
import{Auth, Client, ConnectionConfig}

object Example extends IOApp {
  def run(args: List[String]) = {
    val r = for {
      blocker <- Blocker[IO]
      client <- Client[IO]

      host <- Resource.liftF(Client.resolve[IO]("", 22, blocker))

      // you're free to use something other than EitherT
      // so long as it forms a FunctorRaise with Client.Error
      et = client.exec[EitherT[IO, Client.Error, ?]](
        "whoami",   // or really any other command

      code <- et.value map {
        case Left(Client.Error.Authentication) => 
          Resource.liftF(IO(println("authentication error!"))).as(ExitCode(-1))

        case Right(p) => 
          for {
            // also available: p.stderr and p.stdin
            _ <- p.stdout.through(Sink.showLinesStdOut).compile.resource
            statusCode <- Resource.liftF(p.join)
            _ <- Resource.liftF(IO(println(s"remote exited with status $statusCode")))
          } yield ExitCode(0)
    } yield code


Replace the obvious host, user, and password stubs with something real and this will work. Public/private key authentication (including support for password-protected keys) is also supported with the Auth.KeyFile case.

Perhaps more realistically, here's an Ansible-style use-case where we fire-and-forget the command to a zillion servers in parallel:

import cats.implicits._
import java.nio.file.Paths

val servers = List(/* a zillion hostnames here */)

for {
  client <- Client[IO]
  blocker <- Blocker[IO]

  _ <- Resource liftF {
    servers parTraverse_ { host =>
      val ret = for {
        // the boilerplate here is... regretable
        isa <- Resource.liftF(
            Client.resolve[IO](host, 22, blocker)))
        _ <- client.exec(
            Auth.KeyFile(Paths.get("id_rsa"), None)),
          "nohup ./",
      } yield ()

      // discard errors
      ret.use(_ => unit).value.void
} yield ()

(there is an analogous Auth.KeyBytes case if you happen to have the private key already in memory)

The only limitation on the above is really memory. Due to the fact that the client is entirely asynchronous, no threads will be retained to manage active connections, and so it's really not that absurd to open millions of these things. Note that if you would like to do this in a memory-incremental fashion, you probably want to use the Stream.resource constructor and parJoinUnbounded in fs2, rather than going through cats.Parallel (as in the above), but this is just a simple example.


Everything is encapsulated within the object. Currently, the only available functionality (ignoring the resolve helper function) is exec, which executes a command on the remote server and exits. Password and public/private key authentication are both supported, as are password-protected private keys, via the Auth ADT. The command is given as a String, and in my experience most remote servers seem to parse it with shell-style functionality (e.g. wildcards and piping seem to be supported most of the time).

exec returns a Process, which provides access to stdin/stderr/stdout, represented as Streams, and a single effect, join, which blocks until the command completes and produces the exit status code. It's theoretically possible to add more functionality here, such as signal handling, but that hasn't yet been required.

It's worth noting that there is (currently) no way to send EOF to the destination. This is technically possible internally (for example, perhaps when the stream being piped to stdin completes), but that would create a situation where resource safety is compromised. Instead, it's better to rely on terminating the resource which manages the Process. This corresponds to sending Ctrl-C to the destination.

The known_hosts file is respected, but not written. It's worth noting that SSHD does some weird juju with known_hosts and appears to read it in unmanaged threads, which can't be helped without completely ignoring the file.

It's also worth noting that SSHD's Netty usage seems to be somewhat naive, and it does a lot of work on the event dispatcher pool (most notably, decrypting and parsing the SSH protocol itself). This is deeply unfortunate, but there's not much we can do about it other than switching to MINA for as an upstream NIO framework, and even then I suspect that they still wouldn't be context shifting correctly. As a result, throughput is somewhat lower under load then you might expect, given the other internals.


Apache License v2. Don't format code bizarrely. Don't force push to PRs. You know the drill. The weird and unfortunate thing is that external contributors will not be able to run the "unit" tests (which are really integration tests) without private credentials. The reason for this is the tests actually shell into a server hosted in EC2 (generously sponsored by Precog) in order to test that the client functionality is compatible with OpenSSH. This is all well and good, but the credentials cannot be made public for reasons that likely involve bitcoin mining and botnets.

Travis does run the integration tests, but only on branches which are pushed to the upstream fork. This means that third-party PRs will never build successfully in CI. Sorry. 😔


Some future expected functionality:

  • Shell access
  • Tunneling
  • Jump hosting
  • Server things