feat: SSH terminal - the agent connects by SSH to another host (password, key file or its own keys) and the shell opens in a tab with the connection's progress shown; host keys trusted on first use, a changed one refused

internal/agent/client.go

@@ -276,6 +276,10 @@ func (a *Agent) dispatch(ctx context.Context, msg proto.ControlMessage) {
 		go a.runForward(ctx, msg.SessionID, msg.Port)
 	case proto.CtrlTask:
 		go a.runTask(ctx, msg.TaskID)
+	case proto.CtrlOpenSSH:
+		if msg.SSH != nil {
+			go a.runSSH(ctx, msg.SessionID, *msg.SSH)
+		}
 	}
 }
 

internal/agent/ssh.go

@@ -0,0 +1,238 @@
+package agent
+
+import (
+	"bufio"
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"log"
+	"net"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+
+	"golang.org/x/crypto/ssh"
+
+	"github.com/x-grk/longpolling-remote-access/internal/proto"
+)
+
+// An SSH terminal: the agent connects to another host by SSH and bridges the
+// remote shell to the session, as it does a local one. The panel is told how
+// the connection goes (connecting, signing in, in) to show it.
+
+// sshKnownHosts holds the host keys seen, next to the agent's state: a host
+// is trusted the first time, and a changed key is refused.
+func (a *Agent) sshKnownHosts() string {
+	return filepath.Join(filepath.Dir(a.cfg.StatePath), "ssh_known_hosts")
+}
+
+var knownHostsMu sync.Mutex
+
+// hostKeyCheck trusts a new host and remembers its key; a host whose key
+// has changed since is refused, as ssh would.
+func (a *Agent) hostKeyCheck(seen *string) ssh.HostKeyCallback {
+	return func(hostname string, _ net.Addr, key ssh.PublicKey) error {
+		knownHostsMu.Lock()
+		defer knownHostsMu.Unlock()
+		fp := ssh.FingerprintSHA256(key)
+		*seen = fp
+		path := a.sshKnownHosts()
+		line := hostname + " " + key.Type() + " "
+		if f, err := os.Open(path); err == nil {
+			sc := bufio.NewScanner(f)
+			for sc.Scan() {
+				fields := strings.Fields(sc.Text())
+				if len(fields) != 3 || fields[0] != hostname {
+					continue
+				}
+				if fields[1] == key.Type() && fields[2] == fp {
+					f.Close()
+					return nil
+				}
+				if fields[1] == key.Type() {
+					f.Close()
+					return fmt.Errorf("ключ хоста %s изменился: был %s, теперь %s. Если так и должно быть, удалите строку с этим хостом из %s на агенте",
+						hostname, fields[2], fp, path)
+				}
+			}
+			f.Close()
+		}
+		_ = os.MkdirAll(filepath.Dir(path), 0o700)
+		f, err := os.OpenFile(path, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0o600)
+		if err == nil {
+			_, _ = f.WriteString(line + fp + "\n")
+			f.Close()
+		}
+		return nil
+	}
+}
+
+// sshAuth is how to sign in: the key given, the password given (also
+// answered to keyboard-interactive prompts), or the agent user's own keys.
+func sshAuth(t proto.SSHTarget) ([]ssh.AuthMethod, error) {
+	var methods []ssh.AuthMethod
+	if t.Key != "" {
+		var signer ssh.Signer
+		var err error
+		if t.Passphrase != "" {
+			signer, err = ssh.ParsePrivateKeyWithPassphrase([]byte(t.Key), []byte(t.Passphrase))
+		} else {
+			signer, err = ssh.ParsePrivateKey([]byte(t.Key))
+		}
+		var missing *ssh.PassphraseMissingError
+		if errors.As(err, &missing) {
+			return nil, errors.New("ключ защищён фразой - введите её")
+		}
+		if err != nil {
+			return nil, fmt.Errorf("ключ не читается: %v", err)
+		}
+		methods = append(methods, ssh.PublicKeys(signer))
+	}
+	if t.Password != "" {
+		pw := t.Password
+		methods = append(methods, ssh.Password(pw),
+			ssh.KeyboardInteractive(func(_, _ string, qs []string, _ []bool) ([]string, error) {
+				ans := make([]string, len(qs))
+				for i := range ans {
+					ans[i] = pw
+				}
+				return ans, nil
+			}))
+	}
+	if len(methods) == 0 {
+		var signers []ssh.Signer
+		for _, n := range []string{"id_ed25519", "id_ecdsa", "id_rsa"} {
+			b, err := os.ReadFile(filepath.Join(homeDir(""), ".ssh", n))
+			if err != nil {
+				continue
+			}
+			if s, err := ssh.ParsePrivateKey(b); err == nil {
+				signers = append(signers, s)
+			}
+		}
+		if len(signers) == 0 {
+			return nil, errors.New("не задан ни пароль, ни ключ, и в ~/.ssh агента нет ключей без фразы")
+		}
+		methods = append(methods, ssh.PublicKeys(signers...))
+	}
+	return methods, nil
+}
+
+// sshShell is the remote shell as a stream: its output (stdout and stderr,
+// one on a PTY) to read, keystrokes to write.
+type sshShell struct {
+	client *ssh.Client
+	sess   *ssh.Session
+	stdin  io.WriteCloser
+	out    *io.PipeReader
+	once   sync.Once
+}
+
+func (s *sshShell) Read(b []byte) (int, error)  { return s.out.Read(b) }
+func (s *sshShell) Write(b []byte) (int, error) { return s.stdin.Write(b) }
+func (s *sshShell) Resize(cols, rows uint16) {
+	if cols > 0 && rows > 0 {
+		_ = s.sess.WindowChange(int(rows), int(cols))
+	}
+}
+func (s *sshShell) Close() error {
+	s.once.Do(func() {
+		_ = s.sess.Close()
+		_ = s.client.Close()
+		_ = s.out.Close()
+	})
+	return nil
+}
+
+// runSSH connects to the target and bridges its shell to the session.
+func (a *Agent) runSSH(parent context.Context, sid string, t proto.SSHTarget) {
+	status := func(st, note string) {
+		a.postOut(context.Background(), sid, proto.OutMessage{Status: st, Note: note})
+	}
+	fail := func(err error) {
+		log.Printf("ssh %s: %v", sid, err)
+		status(proto.SSHErr, err.Error())
+		a.postOut(context.Background(), sid, proto.OutMessage{Data: b64.EncodeToString([]byte("\x1b[31m" + err.Error() + "\x1b[0m\r\n"))})
+		a.postOut(context.Background(), sid, proto.OutMessage{Exit: true})
+	}
+	if t.Port == 0 {
+		t.Port = 22
+	}
+	addr := net.JoinHostPort(t.Host, strconv.Itoa(t.Port))
+	auth, err := sshAuth(t)
+	if err != nil {
+		fail(err)
+		return
+	}
+	status(proto.SSHDial, "подключение к "+addr)
+	d := net.Dialer{Timeout: 15 * time.Second, KeepAlive: 30 * time.Second}
+	conn, err := d.DialContext(parent, "tcp", addr)
+	if err != nil {
+		fail(fmt.Errorf("не подключиться к %s: %v", addr, err))
+		return
+	}
+	status(proto.SSHAuth, "вход как "+t.User)
+	var fp string
+	_ = conn.SetDeadline(time.Now().Add(30 * time.Second))
+	c, chans, reqs, err := ssh.NewClientConn(conn, addr, &ssh.ClientConfig{
+		User:            t.User,
+		Auth:            auth,
+		HostKeyCallback: a.hostKeyCheck(&fp),
+		Timeout:         15 * time.Second,
+	})
+	if err != nil {
+		conn.Close()
+		msg := strings.TrimPrefix(err.Error(), "ssh: handshake failed: ")
+		if strings.Contains(msg, "unable to authenticate") {
+			msg = "сервер не принял вход как " + t.User + " - проверьте пароль или ключ"
+		}
+		fail(errors.New(msg))
+		return
+	}
+	_ = conn.SetDeadline(time.Time{})
+	client := ssh.NewClient(c, chans, reqs)
+	sess, err := client.NewSession()
+	if err != nil {
+		client.Close()
+		fail(fmt.Errorf("сессия не открылась: %v", err))
+		return
+	}
+	modes := ssh.TerminalModes{ssh.ECHO: 1, ssh.TTY_OP_ISPEED: 115200, ssh.TTY_OP_OSPEED: 115200}
+	if err := sess.RequestPty("xterm-256color", 24, 80, modes); err != nil {
+		sess.Close()
+		client.Close()
+		fail(fmt.Errorf("терминал не выделен: %v", err))
+		return
+	}
+	stdin, _ := sess.StdinPipe()
+	pr, pw := io.Pipe()
+	sess.Stdout = pw
+	sess.Stderr = pw
+	if err := sess.Shell(); err != nil {
+		sess.Close()
+		client.Close()
+		fail(fmt.Errorf("оболочка не запустилась: %v", err))
+		return
+	}
+	sh := &sshShell{client: client, sess: sess, stdin: stdin, out: pr}
+	go func() { _ = sess.Wait(); pw.Close() }()
+	// Keep-alives, so a dead host is noticed and the session ends.
+	go func() {
+		tk := time.NewTicker(30 * time.Second)
+		defer tk.Stop()
+		for range tk.C {
+			if _, _, err := client.SendRequest("keepalive@openssh.com", true, nil); err != nil {
+				sh.Close()
+				return
+			}
+		}
+	}()
+	log.Printf("ssh %s: %s@%s up (host key %s)", sid, t.User, addr, fp)
+	status(proto.SSHOK, t.User+"@"+addr+" · "+fp)
+	a.bridgeStream(parent, sid, sh, sh.Resize)
+	log.Printf("ssh %s: ended", sid)
+}