cmd/ssh-auth-none-demo: add demo SSH server that acts like Tailscale SSH

For SSH client authors to fix their clients without setting up
Tailscale stuff.

Change-Id: I8c7049398512de6cb91c13716d4dcebed4d47b9c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/ssh-auth-none-demo/ssh-auth-none-demo.go

@@ -0,0 +1,171 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// ssh-auth-none-demo is a demo SSH server that's meant to run on the
+// public internet and highlight the unique parts of the Tailscale SSH
+// server so SSH client authors can hit it easily and fix their SSH
+// clients without needing to set up Tailscale and Tailscale SSH.
+package main
+
+import (
+	"crypto/ecdsa"
+	"crypto/ed25519"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+	"flag"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"log"
+	"os"
+	"path/filepath"
+	"time"
+
+	gossh "github.com/tailscale/golang-x-crypto/ssh"
+	"tailscale.com/tempfork/gliderlabs/ssh"
+)
+
+// keyTypes are the SSH key types that we either try to read from the
+// system's OpenSSH keys.
+var keyTypes = []string{"rsa", "ecdsa", "ed25519"}
+
+var (
+	addr = flag.String("addr", ":2222", "address to listen on")
+)
+
+func main() {
+	flag.Parse()
+
+	cacheDir, err := os.UserCacheDir()
+	if err != nil {
+		log.Fatal(err)
+	}
+	dir := filepath.Join(cacheDir, "ssh-auth-none-demo")
+	if err := os.MkdirAll(dir, 0700); err != nil {
+		log.Fatal(err)
+	}
+
+	keys, err := getHostKeys(dir)
+	if err != nil {
+		log.Fatal(err)
+	}
+	if len(keys) == 0 {
+		log.Fatal("no host keys")
+	}
+
+	srv := &ssh.Server{
+		Addr:    *addr,
+		Version: "Tailscale",
+		Handler: handleSessionPostSSHAuth,
+		ServerConfigCallback: func(ctx ssh.Context) *gossh.ServerConfig {
+			return &gossh.ServerConfig{
+				ImplicitAuthMethod: "tailscale",
+				NoClientAuth:       true, // required for the NoClientAuthCallback to run
+				NoClientAuthCallback: func(gossh.ConnMetadata) (*gossh.Permissions, error) {
+					return nil, nil
+				},
+				BannerCallback: func(cm gossh.ConnMetadata) string {
+					log.Printf("Got connection from user %q, %q from %v", cm.User(), cm.ClientVersion(), cm.RemoteAddr())
+					return fmt.Sprintf("# Banner for user %q, %q\n", cm.User(), cm.ClientVersion())
+				},
+			}
+		},
+	}
+
+	for _, signer := range keys {
+		srv.AddHostKey(signer)
+	}
+
+	log.Printf("Running on %s ...", srv.Addr)
+	if err := srv.ListenAndServe(); err != nil {
+		log.Fatal(err)
+	}
+	log.Printf("done")
+}
+
+func handleSessionPostSSHAuth(s ssh.Session) {
+	log.Printf("Started session from user %q", s.User())
+	fmt.Fprintf(s, "Hello user %q, it worked.\n", s.User())
+
+	// Abort the session on Control-C or Control-D.
+	go func() {
+		buf := make([]byte, 1024)
+		for {
+			n, err := s.Read(buf)
+			for _, b := range buf[:n] {
+				if b <= 4 { // abort on Control-C (3) or Control-D (4)
+					io.WriteString(s, "bye\n")
+					s.Exit(1)
+				}
+			}
+			if err != nil {
+				return
+			}
+		}
+	}()
+
+	for i := 10; i > 0; i-- {
+		fmt.Fprintf(s, "%v ...\n", i)
+		time.Sleep(time.Second)
+	}
+	s.Exit(0)
+}
+
+func getHostKeys(dir string) (ret []ssh.Signer, err error) {
+	for _, typ := range keyTypes {
+		hostKey, err := hostKeyFileOrCreate(dir, typ)
+		if err != nil {
+			return nil, err
+		}
+		signer, err := gossh.ParsePrivateKey(hostKey)
+		if err != nil {
+			return nil, err
+		}
+		ret = append(ret, signer)
+	}
+	return ret, nil
+}
+
+func hostKeyFileOrCreate(keyDir, typ string) ([]byte, error) {
+	path := filepath.Join(keyDir, "ssh_host_"+typ+"_key")
+	v, err := ioutil.ReadFile(path)
+	if err == nil {
+		return v, nil
+	}
+	if !os.IsNotExist(err) {
+		return nil, err
+	}
+	var priv any
+	switch typ {
+	default:
+		return nil, fmt.Errorf("unsupported key type %q", typ)
+	case "ed25519":
+		_, priv, err = ed25519.GenerateKey(rand.Reader)
+	case "ecdsa":
+		// curve is arbitrary. We pick whatever will at
+		// least pacify clients as the actual encryption
+		// doesn't matter: it's all over WireGuard anyway.
+		curve := elliptic.P256()
+		priv, err = ecdsa.GenerateKey(curve, rand.Reader)
+	case "rsa":
+		// keySize is arbitrary. We pick whatever will at
+		// least pacify clients as the actual encryption
+		// doesn't matter: it's all over WireGuard anyway.
+		const keySize = 2048
+		priv, err = rsa.GenerateKey(rand.Reader, keySize)
+	}
+	if err != nil {
+		return nil, err
+	}
+	mk, err := x509.MarshalPKCS8PrivateKey(priv)
+	if err != nil {
+		return nil, err
+	}
+	pemGen := pem.EncodeToMemory(&pem.Block{Type: "PRIVATE KEY", Bytes: mk})
+	err = os.WriteFile(path, pemGen, 0700)
+	return pemGen, err
+}