[client] Deduplicate SSH PTY session setup and host key verification

Extract the identical PTY session setup shared by the wasm and Android
terminal clients into ssh.StartPTYSession, and move the stored-key host
verification onto the engine so the embed client delegates and the
Android client passes the engine directly as HostKeyVerifier.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

client/android/ssh_client.go

@@ -59,19 +59,6 @@ func (e *errHostKeyUnknown) Error() string {
 	return HostKeyUnknownMarker + ":" + e.fingerprint
 }
 
-// engineHostKeyVerifier adapts *internal.Engine to nbssh.HostKeyVerifier.
-type engineHostKeyVerifier struct {
-	engine *internal.Engine
-}
-
-func (v *engineHostKeyVerifier) VerifySSHHostKey(peerAddress string, presented []byte) error {
-	storedKey, found := v.engine.GetPeerSSHKey(peerAddress)
-	if !found {
-		return nbssh.ErrPeerNotFound
-	}
-	return nbssh.VerifyHostKey(storedKey, presented, peerAddress)
-}
-
 // SSHTerminalListener receives SSH session events. It is implemented in Java.
 //
 // All callbacks are invoked from goroutines and may run concurrently with each
@@ -329,58 +316,24 @@ func (s *SSHClient) startSession(cols, rows int) error {
 		return errors.New("ssh client not connected")
 	}
 
-	session, err := sshClient.NewSession()
+	pty, err := nbssh.StartPTYSession(sshClient, cols, rows)
 	if err != nil {
-		return fmt.Errorf("new session: %w", err)
-	}
-
-	modes := gossh.TerminalModes{
-		gossh.ECHO:          1,
-		gossh.TTY_OP_ISPEED: 14400,
-		gossh.TTY_OP_OSPEED: 14400,
-		gossh.VINTR:         3,
-		gossh.VQUIT:         28,
-		gossh.VERASE:        127,
-	}
-	if err := session.RequestPty("xterm-256color", rows, cols, modes); err != nil {
-		closeQuiet(session, "session after pty error")
-		return fmt.Errorf("request pty: %w", err)
-	}
-
-	stdin, err := session.StdinPipe()
-	if err != nil {
-		closeQuiet(session, "session after stdin error")
-		return fmt.Errorf("stdin pipe: %w", err)
-	}
-	stdout, err := session.StdoutPipe()
-	if err != nil {
-		closeQuiet(session, "session after stdout error")
-		return fmt.Errorf("stdout pipe: %w", err)
-	}
-	stderr, err := session.StderrPipe()
-	if err != nil {
-		closeQuiet(session, "session after stderr error")
-		return fmt.Errorf("stderr pipe: %w", err)
-	}
-
-	if err := session.Shell(); err != nil {
-		closeQuiet(session, "session after shell error")
-		return fmt.Errorf("start shell: %w", err)
+		return err
 	}
 
 	s.mu.Lock()
 	if gen != s.gen {
 		s.mu.Unlock()
-		closeQuiet(session, "stale session")
+		closeQuiet(pty.Session, "stale session")
 		return errClientClosed
 	}
-	s.session = session
-	s.stdin = stdin
+	s.session = pty.Session
+	s.stdin = pty.Stdin
 	s.mu.Unlock()
 
 	readerDone := make(chan string, 2)
-	go func() { readerDone <- s.readLoop(stdout, "stdout") }()
-	go func() { readerDone <- s.readLoop(stderr, "stderr") }()
+	go func() { readerDone <- s.readLoop(pty.Stdout, "stdout") }()
+	go func() { readerDone <- s.readLoop(pty.Stderr, "stderr") }()
 	go func() {
 		reason := <-readerDone
 		if second := <-readerDone; reason == "" {
@@ -402,7 +355,7 @@ func (s *SSHClient) buildAuth(cfg *profilemanager.Config, engine *internal.Engin
 			return nil, nil, fmt.Errorf("jwt: %w", err)
 		}
 		auths := []gossh.AuthMethod{gossh.Password(token)}
-		return auths, nbssh.CreateHostKeyCallback(&engineHostKeyVerifier{engine: engine}), nil
+		return auths, nbssh.CreateHostKeyCallback(engine), nil
 
 	case detection.ServerTypeNetBirdNoJWT:
 		if cfg.SSHKey == "" {
@@ -413,7 +366,7 @@ func (s *SSHClient) buildAuth(cfg *profilemanager.Config, engine *internal.Engin
 			return nil, nil, fmt.Errorf("parse netbird ssh key: %w", err)
 		}
 		auths := []gossh.AuthMethod{gossh.PublicKeys(signer)}
-		return auths, nbssh.CreateHostKeyCallback(&engineHostKeyVerifier{engine: engine}), nil
+		return auths, nbssh.CreateHostKeyCallback(engine), nil
 
 	case detection.ServerTypeRegular:
 		var auths []gossh.AuthMethod

client/embed/embed.go

@@ -21,7 +21,6 @@ import (
 	"github.com/netbirdio/netbird/client/internal/auth"
 	"github.com/netbirdio/netbird/client/internal/peer"
 	"github.com/netbirdio/netbird/client/internal/profilemanager"
-	sshcommon "github.com/netbirdio/netbird/client/ssh"
 	"github.com/netbirdio/netbird/client/system"
 	"github.com/netbirdio/netbird/shared/management/domain"
 	mgmProto "github.com/netbirdio/netbird/shared/management/proto"
@@ -521,12 +520,7 @@ func (c *Client) VerifySSHHostKey(peerAddress string, key []byte) error {
 		return err
 	}
 
-	storedKey, found := engine.GetPeerSSHKey(peerAddress)
-	if !found {
-		return sshcommon.ErrPeerNotFound
-	}
-
-	return sshcommon.VerifyHostKey(storedKey, key, peerAddress)
+	return engine.VerifySSHHostKey(peerAddress, key)
 }
 
 // SetPerformance retunes a running Client. Only PreallocatedBuffersPerPool

client/internal/engine_ssh.go

@@ -12,6 +12,7 @@ import (
 	firewallManager "github.com/netbirdio/netbird/client/firewall/manager"
 	"github.com/netbirdio/netbird/client/iface/netstack"
 	nftypes "github.com/netbirdio/netbird/client/internal/netflow/types"
+	nbssh "github.com/netbirdio/netbird/client/ssh"
 	sshauth "github.com/netbirdio/netbird/client/ssh/auth"
 	sshconfig "github.com/netbirdio/netbird/client/ssh/config"
 	sshserver "github.com/netbirdio/netbird/client/ssh/server"
@@ -216,6 +217,16 @@ func (e *Engine) GetPeerSSHKey(peerAddress string) ([]byte, bool) {
 	return nil, false
 }
 
+// VerifySSHHostKey verifies a presented SSH host key against the stored key of
+// the peer at peerAddress. It implements ssh.HostKeyVerifier.
+func (e *Engine) VerifySSHHostKey(peerAddress string, presentedKey []byte) error {
+	storedKey, found := e.GetPeerSSHKey(peerAddress)
+	if !found {
+		return nbssh.ErrPeerNotFound
+	}
+	return nbssh.VerifyHostKey(storedKey, presentedKey, peerAddress)
+}
+
 // cleanupSSHConfig removes NetBird SSH client configuration on shutdown
 func (e *Engine) cleanupSSHConfig() {
 	if netstack.IsEnabled() {

client/ssh/session.go

@@ -0,0 +1,73 @@
+package ssh
+
+import (
+	"fmt"
+	"io"
+
+	log "github.com/sirupsen/logrus"
+	"golang.org/x/crypto/ssh"
+)
+
+// defaultTerminalModes are the PTY modes used by the interactive terminal clients.
+var defaultTerminalModes = ssh.TerminalModes{
+	ssh.ECHO:          1,
+	ssh.TTY_OP_ISPEED: 14400,
+	ssh.TTY_OP_OSPEED: 14400,
+	ssh.VINTR:         3,
+	ssh.VQUIT:         28,
+	ssh.VERASE:        127,
+}
+
+// PTYSession is an interactive shell session with a PTY and its I/O pipes.
+type PTYSession struct {
+	Session *ssh.Session
+	Stdin   io.WriteCloser
+	Stdout  io.Reader
+	Stderr  io.Reader
+}
+
+// StartPTYSession opens a session on the client, requests an xterm-256color PTY
+// with the default terminal modes, wires up the I/O pipes and starts a shell.
+// The session is closed on any error.
+func StartPTYSession(client *ssh.Client, cols, rows int) (*PTYSession, error) {
+	session, err := client.NewSession()
+	if err != nil {
+		return nil, fmt.Errorf("new session: %w", err)
+	}
+
+	pty, err := setupPTYSession(session, cols, rows)
+	if err != nil {
+		if closeErr := session.Close(); closeErr != nil {
+			log.Debugf("ssh: session close after setup error: %v", closeErr)
+		}
+		return nil, err
+	}
+	return pty, nil
+}
+
+// setupPTYSession requests the PTY, opens the pipes and starts the shell on an
+// already created session.
+func setupPTYSession(session *ssh.Session, cols, rows int) (*PTYSession, error) {
+	if err := session.RequestPty("xterm-256color", rows, cols, defaultTerminalModes); err != nil {
+		return nil, fmt.Errorf("request pty: %w", err)
+	}
+
+	stdin, err := session.StdinPipe()
+	if err != nil {
+		return nil, fmt.Errorf("stdin pipe: %w", err)
+	}
+	stdout, err := session.StdoutPipe()
+	if err != nil {
+		return nil, fmt.Errorf("stdout pipe: %w", err)
+	}
+	stderr, err := session.StderrPipe()
+	if err != nil {
+		return nil, fmt.Errorf("stderr pipe: %w", err)
+	}
+
+	if err := session.Shell(); err != nil {
+		return nil, fmt.Errorf("start shell: %w", err)
+	}
+
+	return &PTYSession{Session: session, Stdin: stdin, Stdout: stdout, Stderr: stderr}, nil
+}

client/wasm/internal/ssh/client.go

@@ -125,51 +125,17 @@ func (c *Client) StartSession(cols, rows int) error {
 		return fmt.Errorf("SSH client not connected")
 	}
 
-	session, err := c.sshClient.NewSession()
+	pty, err := nbssh.StartPTYSession(c.sshClient, cols, rows)
 	if err != nil {
-		return fmt.Errorf("create session: %w", err)
+		return err
 	}
 
 	c.mu.Lock()
 	defer c.mu.Unlock()
-	c.session = session
-
-	modes := ssh.TerminalModes{
-		ssh.ECHO:          1,
-		ssh.TTY_OP_ISPEED: 14400,
-		ssh.TTY_OP_OSPEED: 14400,
-		ssh.VINTR:         3,
-		ssh.VQUIT:         28,
-		ssh.VERASE:        127,
-	}
-
-	if err := session.RequestPty("xterm-256color", rows, cols, modes); err != nil {
-		closeWithLog(session, "session after PTY error")
-		return fmt.Errorf("PTY request: %w", err)
-	}
-
-	c.stdin, err = session.StdinPipe()
-	if err != nil {
-		closeWithLog(session, "session after stdin error")
-		return fmt.Errorf("get stdin: %w", err)
-	}
-
-	c.stdout, err = session.StdoutPipe()
-	if err != nil {
-		closeWithLog(session, "session after stdout error")
-		return fmt.Errorf("get stdout: %w", err)
-	}
-
-	c.stderr, err = session.StderrPipe()
-	if err != nil {
-		closeWithLog(session, "session after stderr error")
-		return fmt.Errorf("get stderr: %w", err)
-	}
-
-	if err := session.Shell(); err != nil {
-		closeWithLog(session, "session after shell error")
-		return fmt.Errorf("start shell: %w", err)
-	}
+	c.session = pty.Session
+	c.stdin = pty.Stdin
+	c.stdout = pty.Stdout
+	c.stderr = pty.Stderr
 
 	logrus.Info("SSH: Session started with PTY")
 	return nil