Go App Served via SSH

[Bob Henkel]

I'm starting to experiment where the edges of an unikernel environment are. I didn't expect this to work and it doesn't. I wrote a simple ssh server in go that in theory serves out a simple app, it doesn't allow the user to execute any commands, only to hit the ssh server and be served content much like a http hello world REST API. The app listens for ssh connections on port 23234. When it's running you can connect to it with ssh localhost -p23234 and it will create a PTY and print some text back.

I'm curious if it's possible with a nanos instance? If not great no big deal, I'll move onto using nanos in a more typical manner. If it can work what's needed? Thanks!

When NOT ran in a nanos unikernel this is the output when connecting to the the ssh server:

Hello, world!
Term: xterm-256color
PTY: /dev/pts/5
Connection to localhost closed.

When ran in a nanos unikernel I get this error when I deploy it to a google cloud instance:

PTY allocation request failed on channel 0
Requires an active PTY
Connection to 104.999.999.63 closed.

Here's the code:

package main

import (
        "context"
        "errors"
        "net"
        "os"
        "os/signal"
        "syscall"
        "time"

        "github.com/charmbracelet/log"
        "github.com/charmbracelet/ssh"
        "github.com/charmbracelet/wish"
        "github.com/charmbracelet/wish/activeterm"
        "github.com/charmbracelet/wish/logging"
)

const (
        host = ""
        port = "23234"
)

func main() {
        srv, err := wish.NewServer(
                wish.WithAddress(net.JoinHostPort(host, port)),
        //      wish.WithHostKeyPath(".ssh/id_ed25519"),

                // Wish can allocate a PTY per user session.
                ssh.AllocatePty(),

                wish.WithMiddleware(
                        func(next ssh.Handler) ssh.Handler {
                                return func(sess ssh.Session) {
                                        pty, _, _ := sess.Pty()
                                        wish.Printf(sess, "Hello, world!\r\n")
                                        wish.Printf(sess, "Term: "+pty.Term+"\r\n")
                                        wish.Printf(sess, "PTY: "+pty.Slave.Name()+"\r\n")
                                        next(sess)
                                }
                        },

                        activeterm.Middleware(),
                        logging.Middleware(),
                ),
        )
        if err != nil {
                log.Error("Could not start server", "error", err)
        }

        done := make(chan os.Signal, 1)
        signal.Notify(done, os.Interrupt, syscall.SIGINT, syscall.SIGTERM)

        go func() {
                log.Info("Starting SSH server", "host", host, "port", port)
                if err = srv.ListenAndServe(); err != nil && !errors.Is(err, ssh.ErrServerClosed) {
                        log.Error("Could not start server", "error", err)
                        done <- nil
                }
        }()

        <-done

        ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
        defer func() { cancel() }()
        log.Info("Stopping SSH server")
        if err := srv.Shutdown(ctx); err != nil && !errors.Is(err, ssh.ErrServerClosed) {
                log.Error("Could not stop server", "error", err)
        }
}

[Rinor Hoxha]

it doesn't allow the user to execute any commands, only to hit the ssh server and be served content much like a http hello world REST API

you don't need a "real" PTY then.

Can you try and check if the following changes work for you?

1. comment/delete ssh.AllocatePty(), or change it to ssh.EmulatePty(),
2. remove wish.Printf(sess, "PTY: "+pty.Slave.Name()+"\r\n") since there are no fd allocated or change it to smth like if !pty.IsZero() { wish.Printf(sess, "PTY: "+pty.Slave.Name()+"\r\n") }

[Bob Henkel]

Thanks for your time, it's appreciated! Seems so obvious now that someone showed me the solution 😄. For anyone else that is trying to use the Charm modules to make ssh apps like the Wish module here's a more interesting app from the Wish module examples that just works out of the box. I see myself leaning into Nano unikernels more for some of my services and apps.

package main

// An example Bubble Tea server. This will put an ssh session into alt screen
// and continually print up to date terminal information.

import (
	"context"
	"errors"
	"fmt"
	"net"
	"os"
	"os/signal"
	"syscall"
	"time"

	tea "github.com/charmbracelet/bubbletea"
	"github.com/charmbracelet/lipgloss"
	"github.com/charmbracelet/log"
	"github.com/charmbracelet/ssh"
	"github.com/charmbracelet/wish"
	"github.com/charmbracelet/wish/activeterm"
	"github.com/charmbracelet/wish/bubbletea"
	"github.com/charmbracelet/wish/logging"
)

const (
	host = ""
	port = "23234"
)

func main() {
	s, err := wish.NewServer(
		wish.WithAddress(net.JoinHostPort(host, port)),
		wish.WithMiddleware(
			bubbletea.Middleware(teaHandler),
			activeterm.Middleware(), // Bubble Tea apps usually require a PTY.
			logging.Middleware(),
		),
	)
	if err != nil {
		log.Error("Could not start server", "error", err)
	}

	done := make(chan os.Signal, 1)
	signal.Notify(done, os.Interrupt, syscall.SIGINT, syscall.SIGTERM)
	log.Info("Starting SSH server", "host", host, "port", port)
	go func() {
		if err = s.ListenAndServe(); err != nil && !errors.Is(err, ssh.ErrServerClosed) {
			log.Error("Could not start server", "error", err)
			done <- nil
		}
	}()

	<-done
	log.Info("Stopping SSH server")
	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
	defer func() { cancel() }()
	if err := s.Shutdown(ctx); err != nil && !errors.Is(err, ssh.ErrServerClosed) {
		log.Error("Could not stop server", "error", err)
	}
}

// You can wire any Bubble Tea model up to the middleware with a function that
// handles the incoming ssh.Session. Here we just grab the terminal info and
// pass it to the new model. You can also return tea.ProgramOptions (such as
// tea.WithAltScreen) on a session by session basis.
func teaHandler(s ssh.Session) (tea.Model, []tea.ProgramOption) {
	// This should never fail, as we are using the activeterm middleware.
	pty, _, _ := s.Pty()

	// When running a Bubble Tea app over SSH, you shouldn't use the default
	// lipgloss.NewStyle function.
	// That function will use the color profile from the os.Stdin, which is the
	// server, not the client.
	// We provide a MakeRenderer function in the bubbletea middleware package,
	// so you can easily get the correct renderer for the current session, and
	// use it to create the styles.
	// The recommended way to use these styles is to then pass them down to
	// your Bubble Tea model.
	renderer := bubbletea.MakeRenderer(s)
	txtStyle := renderer.NewStyle().Foreground(lipgloss.Color("10"))
	quitStyle := renderer.NewStyle().Foreground(lipgloss.Color("8"))

	m := model{
		term:      pty.Term,
		width:     pty.Window.Width,
		height:    pty.Window.Height,
		txtStyle:  txtStyle,
		quitStyle: quitStyle,
	}
	return m, []tea.ProgramOption{tea.WithAltScreen()}
}

// Just a generic tea.Model to demo terminal information of ssh.
type model struct {
	term      string
	width     int
	height    int
	txtStyle  lipgloss.Style
	quitStyle lipgloss.Style
}

func (m model) Init() tea.Cmd {
	return nil
}

func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
	switch msg := msg.(type) {
	case tea.WindowSizeMsg:
		m.height = msg.Height
		m.width = msg.Width
	case tea.KeyMsg:
		switch msg.String() {
		case "q", "ctrl+c":
			return m, tea.Quit
		}
	}
	return m, nil
}

func (m model) View() string {
	s := fmt.Sprintf("Your term is %s\nYour window size is %dx%d", m.term, m.width, m.height)
	return m.txtStyle.Render(s) + "\n\n" + m.quitStyle.Render("Press 'q' to quit\n")
}

And when you hit that port with ssh on an instance with a Nano unikernel you get this and pressing q works!

Your term is xterm-256color
Your window size is 180x51

Press 'q' to quit

[Bob Henkel]

I'll close this as the reply from @rinor solved it.