tailscale/cmd/tailscale/cli/ssh.go

// 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.

package cli

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"log"
	"net/netip"
	"os"
	"os/user"
	"path/filepath"
	"runtime"
	"strings"

	"github.com/peterbourgon/ff/v3/ffcli"
	"tailscale.com/envknob"
	"tailscale.com/ipn/ipnstate"
	"tailscale.com/net/tsaddr"
	"tailscale.com/paths"
	"tailscale.com/version"
)

var sshCmd = &ffcli.Command{
	Name:       "ssh",
	ShortUsage: "ssh [user@]<host> [args...]",
	ShortHelp:  "SSH to a Tailscale machine",
	LongHelp: strings.TrimSpace(`

The 'tailscale ssh' command is an optional wrapper around the system 'ssh'
command that's useful in some cases. Tailscale SSH does not require its use;
most users running the Tailscale SSH server will prefer to just use the normal
'ssh' command or their normal SSH client.

The 'tailscale ssh' wrapper adds a few things:

* It resolves the destination server name in its arugments using MagicDNS,
  even if --accept-dns=false.
* It works in userspace-networking mode, by supplying a ProxyCommand to the
  system 'ssh' command that connects via a pipe through tailscaled.
* It automatically checks the destination server's SSH host key against the
  node's SSH host key as advertised via the Tailscale coordination server.
`),
	Exec: runSSH,
}

func runSSH(ctx context.Context, args []string) error {
	if runtime.GOOS == "darwin" && version.IsSandboxedMacOS() && !envknob.UseWIPCode() {
		return errors.New("The 'tailscale ssh' subcommand is not available on sandboxed macOS builds.\nUse the regular 'ssh' client instead.")
	}
	if len(args) == 0 {
		return errors.New("usage: ssh [user@]<host>")
	}
	arg, argRest := args[0], args[1:]
	username, host, ok := strings.Cut(arg, "@")
	if !ok {
		host = arg
		lu, err := user.Current()
		if err != nil {
			return nil
		}
		username = lu.Username
	}

	st, err := localClient.Status(ctx)
	if err != nil {
		return err
	}

	// hostForSSH is the hostname we'll tell OpenSSH we're
	// connecting to, so we have to maintain fewer entries in the
	// known_hosts files.
	hostForSSH := host
	if v, ok := nodeDNSNameFromArg(st, host); ok {
		hostForSSH = v
	}

	ssh, err := findSSH()
	if err != nil {
		// TODO(bradfitz): use Go's crypto/ssh client instead
		// of failing. But for now:
		return fmt.Errorf("no system 'ssh' command found: %w", err)
	}
	tailscaleBin, err := os.Executable()
	if err != nil {
		return err
	}
	knownHostsFile, err := writeKnownHosts(st)
	if err != nil {
		return err
	}

	argv := []string{ssh}

	if envknob.Bool("TS_DEBUG_SSH_EXEC") {
		argv = append(argv, "-vvv")
	}
	argv = append(argv,
		// Only trust SSH hosts that we know about.
		"-o", fmt.Sprintf("UserKnownHostsFile %q", knownHostsFile),
		"-o", "UpdateHostKeys no",
		"-o", "StrictHostKeyChecking yes",
	)

	// TODO(bradfitz): nc is currently broken on macOS:
	// https://github.com/tailscale/tailscale/issues/4529
	// So don't use it for now. MagicDNS is usually working on macOS anyway
	// and they're not in userspace mode, so 'nc' isn't very useful.
	if runtime.GOOS != "darwin" {
		socketArg := ""
		if rootArgs.socket != "" && rootArgs.socket != paths.DefaultTailscaledSocket() {
			socketArg = fmt.Sprintf("--socket=%q", rootArgs.socket)
		}

		argv = append(argv,
			"-o", fmt.Sprintf("ProxyCommand %q %s nc %%h %%p",
				tailscaleBin,
				socketArg,
			))
	}

	// Explicitly rebuild the user@host argument rather than
	// passing it through.  In general, the use of OpenSSH's ssh
	// binary is a crutch for now.  We don't want to be
	// Hyrum-locked into passing through all OpenSSH flags to the
	// OpenSSH client forever. We try to make our flags and args
	// be compatible, but only a subset. The "tailscale ssh"
	// command should be a simple and portable one. If they want
	// to use a different one, we'll later be making stock ssh
	// work well by default too. (doing things like automatically
	// setting known_hosts, etc)
	argv = append(argv, username+"@"+hostForSSH)

	argv = append(argv, argRest...)

	if envknob.Bool("TS_DEBUG_SSH_EXEC") {
		log.Printf("Running: %q, %q ...", ssh, argv)
	}

	return execSSH(ssh, argv)
}

func writeKnownHosts(st *ipnstate.Status) (knownHostsFile string, err error) {
	confDir, err := os.UserConfigDir()
	if err != nil {
		return "", err
	}
	tsConfDir := filepath.Join(confDir, "tailscale")
	if err := os.MkdirAll(tsConfDir, 0700); err != nil {
		return "", err
	}
	knownHostsFile = filepath.Join(tsConfDir, "ssh_known_hosts")
	want := genKnownHosts(st)
	if cur, err := os.ReadFile(knownHostsFile); err != nil || !bytes.Equal(cur, want) {
		if err := os.WriteFile(knownHostsFile, want, 0644); err != nil {
			return "", err
		}
	}
	return knownHostsFile, nil
}

func genKnownHosts(st *ipnstate.Status) []byte {
	var buf bytes.Buffer
	for _, k := range st.Peers() {
		ps := st.Peer[k]
		for _, hk := range ps.SSH_HostKeys {
			hostKey := strings.TrimSpace(hk)
			if strings.ContainsAny(hostKey, "\n\r") { // invalid
				continue
			}
			fmt.Fprintf(&buf, "%s %s\n", ps.DNSName, hostKey)
		}
	}
	return buf.Bytes()
}

// nodeDNSNameFromArg returns the PeerStatus.DNSName value from a peer
// in st that matches the input arg which can be a base name, full
// DNS name, or an IP.
func nodeDNSNameFromArg(st *ipnstate.Status, arg string) (dnsName string, ok bool) {
	if arg == "" {
		return
	}
	argIP, _ := netip.ParseAddr(arg)
	for _, ps := range st.Peer {
		dnsName = ps.DNSName
		if argIP.IsValid() {
			for _, ip := range ps.TailscaleIPs {
				if ip == argIP {
					return dnsName, true
				}
			}
			continue
		}
		if strings.EqualFold(strings.TrimSuffix(arg, "."), strings.TrimSuffix(dnsName, ".")) {
			return dnsName, true
		}
		if base, _, ok := strings.Cut(ps.DNSName, "."); ok && strings.EqualFold(base, arg) {
			return dnsName, true
		}
	}
	return "", false
}

// getSSHClientEnvVar returns the "SSH_CLIENT" environment variable
// for the current process group, if any.
var getSSHClientEnvVar = func() string {
	return ""
}

// isSSHOverTailscale checks if the invocation is in a SSH session over Tailscale.
// It is used to detect if the user is about to take an action that might result in them
// disconnecting from the machine (e.g. disabling SSH)
func isSSHOverTailscale() bool {
	sshClient := getSSHClientEnvVar()
	if sshClient == "" {
		return false
	}
	ipStr, _, ok := strings.Cut(sshClient, " ")
	if !ok {
		return false
	}
	ip, err := netip.ParseAddr(ipStr)
	if err != nil {
		return false
	}
	return tsaddr.IsTailscaleIP(ip)
}