cmd/tailscale: generate known_hosts file for 'tailscale ssh'

Updates #3802

Change-Id: I7a0052392f000ee44fc8e719f6666756aab91f3d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/ssh.go

@@ -5,6 +5,7 @@
 package cli
 
 import (
+	"bytes"
 	"context"
 	"errors"
 	"fmt"
@@ -12,13 +13,16 @@ import (
 	"os"
 	"os/exec"
 	"os/user"
+	"path/filepath"
 	"runtime"
 	"strings"
 	"syscall"
 
 	"github.com/alessio/shellescape"
 	"github.com/peterbourgon/ff/v3/ffcli"
+	"tailscale.com/client/tailscale"
 	"tailscale.com/envknob"
+	"tailscale.com/ipn/ipnstate"
 )
 
 var sshCmd = &ffcli.Command{
@@ -52,9 +56,21 @@ func runSSH(ctx context.Context, args []string) error {
 	if err != nil {
 		return err
 	}
+	st, err := tailscale.Status(ctx)
+	if err != nil {
+		return err
+	}
+	knownHostsFile, err := writeKnownHosts(st)
+	if err != nil {
+		return err
+	}
+
 	argv := append([]string{
 		ssh,
 
+		"-o", fmt.Sprintf("UserKnownHostsFile %s",
+			shellescape.Quote(knownHostsFile),
+		),
 		"-o", fmt.Sprintf("ProxyCommand %s --socket=%s nc %%h %%p",
 			shellescape.Quote(tailscaleBin),
 			shellescape.Quote(rootArgs.socket),
@@ -95,3 +111,52 @@ func runSSH(ctx context.Context, args []string) error {
 	}
 	return errors.New("unreachable")
 }
+
+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]
+		if len(ps.SSH_HostKeys) == 0 {
+			continue
+		}
+		// addEntries adds one line per each of p's host keys.
+		addEntries := func(host string) {
+			for _, hk := range ps.SSH_HostKeys {
+				hostKey := strings.TrimSpace(hk)
+				if strings.ContainsAny(hostKey, "\n\r") { // invalid
+					continue
+				}
+				fmt.Fprintf(&buf, "%s %s\n", host, hostKey)
+			}
+		}
+		if ps.DNSName != "" {
+			addEntries(ps.DNSName)
+		}
+		if base, _, ok := strings.Cut(ps.DNSName, "."); ok {
+			addEntries(base)
+		}
+		for _, ip := range st.TailscaleIPs {
+			addEntries(ip.String())
+		}
+	}
+	return buf.Bytes()
+}