ssh: add integration tests against a reference implementation

As in crypto/tls, we record the connection against a reference
implementation, OpenSSH in our case, and run part of our integration
tests by replaying these recorded SSH connections.

Change-Id: If042b5f650b267bd3ede34a05ec3a6fa6d1a86b3
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/644436
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>

ssh/test/banner_test.go

@@ -1,31 +0,0 @@
-// Copyright 2014 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris
-
-package test
-
-import (
-	"testing"
-)
-
-func TestBannerCallbackAgainstOpenSSH(t *testing.T) {
-	server := newServer(t)
-
-	clientConf := clientConfig()
-
-	var receivedBanner string
-	clientConf.BannerCallback = func(message string) error {
-		receivedBanner = message
-		return nil
-	}
-
-	conn := server.Dial(clientConf)
-	defer conn.Close()
-
-	expected := "Server Banner"
-	if receivedBanner != expected {
-		t.Fatalf("got %v; want %v", receivedBanner, expected)
-	}
-}

ssh/test/forward_unix_test.go

@@ -94,7 +94,7 @@ func testPortForward(t *testing.T, n, listenAddr string) {
 	if len(sent) != len(read) {
 		t.Fatalf("got %d bytes, want %d", len(read), len(sent))
 	}
-	if bytes.Compare(sent, read) != 0 {
+	if !bytes.Equal(sent, read) {
 		t.Fatalf("read back data does not match")
 	}
 

ssh/test/recording_client_test.go

@@ -0,0 +1,483 @@
+// Copyright 2025 The Go 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 test
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"golang.org/x/crypto/internal/testenv"
+	"golang.org/x/crypto/sha3"
+	"golang.org/x/crypto/ssh"
+	"golang.org/x/crypto/ssh/testdata"
+)
+
+// serverPort contains the port that OpenSSH will listen on. OpenSSH can't take
+// "0" as an argument here so we have to pick a number and hope that it's not in
+// use on the machine. Since this only occurs when -update is given and thus
+// when there's a human watching the test, this isn't too bad.
+const serverPort = 24222
+
+var (
+	storeUsernameOnce sync.Once
+)
+
+type clientTest struct {
+	// name is a freeform string identifying the test and the file in which
+	// the expected results will be stored.
+	name string
+	// config contains the client configuration to use for this test.
+	config *ssh.ClientConfig
+	// expectError defines the error string to check if the connection is
+	// expected to fail.
+	expectError string
+	// successCallback defines a callback to execute after the client connection
+	// is established.
+	successCallback func(t *testing.T, client *ssh.Client)
+}
+
+// connFromCommand starts the reference server process, connects to it and
+// returns a recordingConn for the connection. It must be closed before Waiting
+// for child.
+func (test *clientTest) connFromCommand(t *testing.T, config string) *recordingConn {
+	sshd, err := exec.LookPath("sshd")
+	if err != nil {
+		t.Skipf("sshd not found, skipping test: %v", err)
+	}
+	dir, err := os.MkdirTemp("", "sshtest")
+	if err != nil {
+		t.Fatal(err)
+	}
+	f, err := os.Create(filepath.Join(dir, "sshd_config"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := configTmpl[config]; ok == false {
+		t.Fatal(fmt.Errorf("Invalid server config '%s'", config))
+	}
+	configVars := map[string]string{
+		"Dir": dir,
+	}
+	err = configTmpl[config].Execute(f, configVars)
+	if err != nil {
+		t.Fatal(err)
+	}
+	f.Close()
+
+	writeFile(filepath.Join(dir, "banner"), []byte("Server Banner"))
+
+	for k, v := range testdata.PEMBytes {
+		filename := "id_" + k
+		writeFile(filepath.Join(dir, filename), v)
+		writeFile(filepath.Join(dir, filename+".pub"), ssh.MarshalAuthorizedKey(testPublicKeys[k]))
+	}
+
+	var authkeys bytes.Buffer
+	for k := range testdata.PEMBytes {
+		authkeys.Write(ssh.MarshalAuthorizedKey(testPublicKeys[k]))
+	}
+	writeFile(filepath.Join(dir, "authorized_keys"), authkeys.Bytes())
+	cmd := testenv.Command(t, sshd, "-D", "-e", "-f", f.Name(), "-p", strconv.Itoa(serverPort))
+	cmd.Stdin = nil
+	var output bytes.Buffer
+	cmd.Stdout = &output
+	cmd.Stderr = &output
+	if err := cmd.Start(); err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		if err := os.RemoveAll(dir); err != nil {
+			t.Error(err)
+		}
+		// Don't check for errors; if it fails it's most
+		// likely "os: process already finished", and we don't
+		// care about that. Use os.Interrupt, so child
+		// processes are killed too.
+		cmd.Process.Signal(os.Interrupt)
+		cmd.Wait()
+		if t.Failed() {
+			t.Logf("OpenSSH output:\n\n%s", cmd.Stdout)
+		}
+	})
+	var tcpConn net.Conn
+	for i := uint(0); i < 5; i++ {
+		tcpConn, err = net.DialTCP("tcp", nil, &net.TCPAddr{
+			IP:   net.IPv4(127, 0, 0, 1),
+			Port: serverPort,
+		})
+		if err == nil {
+			break
+		}
+		time.Sleep((1 << i) * 5 * time.Millisecond)
+	}
+	if err != nil {
+		t.Fatalf("error connecting to the OpenSSH server: %v (%v)\n\n%s", err, cmd.Wait(), output.Bytes())
+	}
+
+	record := &recordingConn{
+		Conn:           tcpConn,
+		clientToServer: true,
+	}
+
+	return record
+}
+
+func (test *clientTest) dataPath() string {
+	return filepath.Join("..", "testdata", "Client-"+test.name)
+}
+
+func (test *clientTest) usernameDataPath() string {
+	return filepath.Join("..", "testdata", "Client-username")
+}
+
+func (test *clientTest) loadData() (flows [][]byte, err error) {
+	in, err := os.Open(test.dataPath())
+	if err != nil {
+		return nil, err
+	}
+	defer in.Close()
+	return parseTestData(in)
+}
+
+func (test *clientTest) storeUsername() (err error) {
+	storeUsernameOnce.Do(func() {
+		err = os.WriteFile(test.usernameDataPath(), []byte(username()), 0666)
+	})
+	return err
+}
+
+func (test *clientTest) loadUsername() (string, error) {
+	data, err := os.ReadFile(test.usernameDataPath())
+	return string(data), err
+}
+
+func (test *clientTest) run(t *testing.T, write bool) {
+	var clientConn net.Conn
+	var recordingConn *recordingConn
+
+	if write {
+		// We store the username used when we record the connection so we can
+		// reuse the same username when running tests.
+		if err := test.storeUsername(); err != nil {
+			t.Fatalf("failed to store username to %q: %v", test.usernameDataPath(), err)
+		}
+		recordingConn = test.connFromCommand(t, "default")
+		clientConn = recordingConn
+	} else {
+		username, err := test.loadUsername()
+		if err != nil {
+			t.Fatalf("failed to load username from %q: %v", test.usernameDataPath(), err)
+		}
+		test.config.User = username
+		timer := time.AfterFunc(10*time.Second, func() {
+			fmt.Println("This test may be stuck, try running using -timeout 10s")
+		})
+		t.Cleanup(func() {
+			timer.Stop()
+		})
+		flows, err := test.loadData()
+		if err != nil {
+			t.Fatalf("failed to load data from %s: %v", test.dataPath(), err)
+		}
+		clientConn = newReplayingConn(t, flows)
+	}
+	c, chans, reqs, err := ssh.NewClientConn(clientConn, "", test.config)
+	if err != nil {
+		if test.expectError == "" {
+			t.Fatal(err)
+		} else {
+			if !strings.Contains(err.Error(), test.expectError) {
+				t.Fatalf("%q not found in %v", test.expectError, err)
+			}
+		}
+	} else {
+		if test.expectError != "" {
+			t.Error("dial should have failed.")
+		}
+		client := ssh.NewClient(c, chans, reqs)
+		if test.successCallback != nil {
+			test.successCallback(t, client)
+		}
+		if err := client.Close(); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	if write {
+		path := test.dataPath()
+		out, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0644)
+		if err != nil {
+			t.Fatalf("Failed to create output file: %v", err)
+		}
+		defer out.Close()
+		recordingConn.Close()
+
+		recordingConn.WriteTo(out)
+		t.Logf("Wrote %s\n", path)
+	}
+}
+
+func recordingsClientConfig() *ssh.ClientConfig {
+	config := clientConfig()
+	config.Rand = sha3.NewShake128()
+	config.Auth = []ssh.AuthMethod{
+		ssh.PublicKeys(testSigners["rsa"]),
+	}
+	return config
+}
+
+func TestClientKeyExchanges(t *testing.T) {
+	config := ssh.ClientConfig{}
+	config.SetDefaults()
+
+	var keyExchanges []string
+	for _, kex := range config.KeyExchanges {
+		// Exclude ecdh for now, to make them determistic we should use see a
+		// stream of fixed bytes as the random source.
+		if !strings.HasPrefix(kex, "ecdh-") {
+			keyExchanges = append(keyExchanges, kex)
+		}
+	}
+	// Add diffie-hellman-group-exchange-sha256 and
+	// diffie-hellman-group16-sha512 as they are not enabled by default.
+	keyExchanges = append(keyExchanges, "diffie-hellman-group-exchange-sha256", "diffie-hellman-group16-sha512")
+
+	for _, kex := range keyExchanges {
+		c := recordingsClientConfig()
+		c.KeyExchanges = []string{kex}
+		test := clientTest{
+			name:   "KEX-" + kex,
+			config: c,
+		}
+		runTestAndUpdateIfNeeded(t, test.name, test.run)
+	}
+}
+
+func TestClientCiphers(t *testing.T) {
+	config := ssh.ClientConfig{}
+	config.SetDefaults()
+
+	for _, ciph := range config.Ciphers {
+		c := recordingsClientConfig()
+		c.Ciphers = []string{ciph}
+		test := clientTest{
+			name:   "Cipher-" + ciph,
+			config: c,
+		}
+		runTestAndUpdateIfNeeded(t, test.name, test.run)
+	}
+}
+
+func TestClientMACs(t *testing.T) {
+	config := ssh.ClientConfig{}
+	config.SetDefaults()
+
+	for _, mac := range config.MACs {
+		c := recordingsClientConfig()
+		c.MACs = []string{mac}
+		test := clientTest{
+			name:   "MAC-" + mac,
+			config: c,
+		}
+		runTestAndUpdateIfNeeded(t, test.name, test.run)
+	}
+}
+
+func TestBannerCallback(t *testing.T) {
+	var receivedBanner string
+	config := recordingsClientConfig()
+	config.BannerCallback = func(message string) error {
+		receivedBanner = message
+		return nil
+	}
+	test := clientTest{
+		name:   "BannerCallback",
+		config: config,
+		successCallback: func(t *testing.T, client *ssh.Client) {
+			expected := "Server Banner"
+			if receivedBanner != expected {
+				t.Fatalf("got %v; want %v", receivedBanner, expected)
+			}
+		},
+	}
+	runTestAndUpdateIfNeeded(t, test.name, test.run)
+}
+
+func TestRunCommandSuccess(t *testing.T) {
+	test := clientTest{
+		name:   "RunCommandSuccess",
+		config: recordingsClientConfig(),
+		successCallback: func(t *testing.T, client *ssh.Client) {
+			session, err := client.NewSession()
+			if err != nil {
+				t.Fatalf("session failed: %v", err)
+			}
+			defer session.Close()
+			err = session.Run("true")
+			if err != nil {
+				t.Fatalf("session failed: %v", err)
+			}
+		},
+	}
+
+	runTestAndUpdateIfNeeded(t, test.name, test.run)
+}
+
+func TestHostKeyCheck(t *testing.T) {
+	config := recordingsClientConfig()
+	hostDB := hostKeyDB()
+	config.HostKeyCallback = hostDB.Check
+
+	// change the keys.
+	hostDB.keys[ssh.KeyAlgoRSA][25]++
+	hostDB.keys[ssh.KeyAlgoDSA][25]++
+	hostDB.keys[ssh.KeyAlgoECDSA256][25]++
+
+	test := clientTest{
+		name:        "HostKeyCheck",
+		config:      config,
+		expectError: "host key mismatch",
+	}
+
+	runTestAndUpdateIfNeeded(t, test.name, test.run)
+}
+
+func TestRunCommandStdin(t *testing.T) {
+	test := clientTest{
+		name:   "RunCommandStdin",
+		config: recordingsClientConfig(),
+		successCallback: func(t *testing.T, client *ssh.Client) {
+			session, err := client.NewSession()
+			if err != nil {
+				t.Fatalf("session failed: %v", err)
+			}
+			defer session.Close()
+
+			r, w := io.Pipe()
+			defer r.Close()
+			defer w.Close()
+			session.Stdin = r
+
+			err = session.Run("true")
+			if err != nil {
+				t.Fatalf("session failed: %v", err)
+			}
+		},
+	}
+
+	runTestAndUpdateIfNeeded(t, test.name, test.run)
+}
+
+func TestRunCommandStdinError(t *testing.T) {
+	test := clientTest{
+		name:   "RunCommandStdinError",
+		config: recordingsClientConfig(),
+		successCallback: func(t *testing.T, client *ssh.Client) {
+			session, err := client.NewSession()
+			if err != nil {
+				t.Fatalf("session failed: %v", err)
+			}
+			defer session.Close()
+
+			r, w := io.Pipe()
+			defer r.Close()
+			session.Stdin = r
+			pipeErr := errors.New("closing write end of pipe")
+			w.CloseWithError(pipeErr)
+
+			err = session.Run("true")
+			if err != pipeErr {
+				t.Fatalf("expected %v, found %v", pipeErr, err)
+			}
+		},
+	}
+
+	runTestAndUpdateIfNeeded(t, test.name, test.run)
+}
+
+func TestRunCommandFailed(t *testing.T) {
+	test := clientTest{
+		name:   "RunCommandFailed",
+		config: recordingsClientConfig(),
+		successCallback: func(t *testing.T, client *ssh.Client) {
+			session, err := client.NewSession()
+			if err != nil {
+				t.Fatalf("session failed: %v", err)
+			}
+			defer session.Close()
+
+			// Trigger a failure by attempting to execute a non-existent
+			// command.
+			err = session.Run(`non-existent command`)
+			if err == nil {
+				t.Fatalf("session succeeded: %v", err)
+			}
+		},
+	}
+
+	runTestAndUpdateIfNeeded(t, test.name, test.run)
+}
+
+func TestWindowChange(t *testing.T) {
+	test := clientTest{
+		name:   "WindowChange",
+		config: recordingsClientConfig(),
+		successCallback: func(t *testing.T, client *ssh.Client) {
+			session, err := client.NewSession()
+			if err != nil {
+				t.Fatalf("session failed: %v", err)
+			}
+			defer session.Close()
+
+			stdout, err := session.StdoutPipe()
+			if err != nil {
+				t.Fatalf("unable to acquire stdout pipe: %s", err)
+			}
+
+			stdin, err := session.StdinPipe()
+			if err != nil {
+				t.Fatalf("unable to acquire stdin pipe: %s", err)
+			}
+
+			tm := ssh.TerminalModes{ssh.ECHO: 0}
+			if err = session.RequestPty("xterm", 80, 40, tm); err != nil {
+				t.Fatalf("req-pty failed: %s", err)
+			}
+
+			if err := session.WindowChange(100, 100); err != nil {
+				t.Fatalf("window-change failed: %s", err)
+			}
+
+			err = session.Shell()
+			if err != nil {
+				t.Fatalf("session failed: %s", err)
+			}
+
+			stdin.Write([]byte("stty size && exit\n"))
+
+			var buf bytes.Buffer
+			if _, err := io.Copy(&buf, stdout); err != nil {
+				t.Fatalf("reading failed: %s", err)
+			}
+
+			if sttyOutput := buf.String(); !strings.Contains(sttyOutput, "100 100") {
+				t.Fatalf("terminal WindowChange failure: expected \"100 100\" stty output, got %s", sttyOutput)
+			}
+		},
+	}
+
+	runTestAndUpdateIfNeeded(t, test.name, test.run)
+}

ssh/test/recording_server_test.go

@@ -0,0 +1,279 @@
+// Copyright 2025 The Go 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 test
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"net"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+
+	"golang.org/x/crypto/internal/testenv"
+	"golang.org/x/crypto/sha3"
+	"golang.org/x/crypto/ssh"
+	"golang.org/x/crypto/ssh/testdata"
+)
+
+type serverTest struct {
+	// name is a freeform string identifying the test and the file in which
+	// the expected results will be stored.
+	name string
+	// config contains the server configuration to use for this test.
+	config *ssh.ServerConfig
+}
+
+// connFromCommand starts opens a listening socket and starts the reference
+// client to connect to it. It returns a recordingConn that wraps the resulting
+// connection.
+func (test *serverTest) connFromCommand(t *testing.T) (conn *recordingConn, err error) {
+	sshCLI, err := exec.LookPath("ssh")
+	if err != nil {
+		t.Skipf("skipping test: %v", err)
+	}
+	l, err := net.ListenTCP("tcp", &net.TCPAddr{
+		IP:   net.IPv4(127, 0, 0, 1),
+		Port: 0,
+	})
+	if err != nil {
+		return nil, err
+	}
+	defer l.Close()
+
+	port := l.Addr().(*net.TCPAddr).Port
+	dir, err := os.MkdirTemp("", "sshtest")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	filename := "id_ed25519"
+	writeFile(filepath.Join(dir, filename), testdata.PEMBytes["ed25519"])
+	writeFile(filepath.Join(dir, filename+".pub"), ssh.MarshalAuthorizedKey(testPublicKeys["ed25519"]))
+	var args []string
+	args = append(args, "-v", "-i", filepath.Join(dir, filename), "-o", "StrictHostKeyChecking=no")
+	args = append(args, "-oKexAlgorithms=+diffie-hellman-group14-sha1")
+	args = append(args, "-p", strconv.Itoa(port))
+	args = append(args, "testuser@127.0.0.1")
+	args = append(args, "true")
+	cmd := testenv.Command(t, sshCLI, args...)
+	cmd.Stdin = nil
+	var output bytes.Buffer
+	cmd.Stdout = &output
+	cmd.Stderr = &output
+	if err := cmd.Start(); err != nil {
+		return nil, err
+	}
+
+	t.Cleanup(func() {
+		if err := os.RemoveAll(dir); err != nil {
+			t.Error(err)
+		}
+		// Don't check for errors; if it fails it's most
+		// likely "os: process already finished", and we don't
+		// care about that.
+		cmd.Process.Kill()
+		cmd.Wait()
+		if t.Failed() {
+			t.Logf("OpenSSH output:\n\n%s", cmd.Stdout)
+		}
+	})
+
+	connChan := make(chan any, 1)
+	go func() {
+		tcpConn, err := l.Accept()
+		if err != nil {
+			connChan <- err
+			return
+		}
+		connChan <- tcpConn
+	}()
+
+	var tcpConn net.Conn
+	select {
+	case connOrError := <-connChan:
+		if err, ok := connOrError.(error); ok {
+			return nil, err
+		}
+		tcpConn = connOrError.(net.Conn)
+	case <-time.After(2 * time.Second):
+		return nil, errors.New("timed out waiting for connection from child process")
+	}
+
+	record := &recordingConn{
+		Conn:           tcpConn,
+		clientToServer: false,
+	}
+
+	return record, nil
+}
+
+func (test *serverTest) dataPath() string {
+	return filepath.Join("..", "testdata", "Server-"+test.name)
+}
+
+func (test *serverTest) loadData() (flows [][]byte, err error) {
+	in, err := os.Open(test.dataPath())
+	if err != nil {
+		return nil, err
+	}
+	defer in.Close()
+	return parseTestData(in)
+}
+
+func (test *serverTest) run(t *testing.T, write bool) {
+	var serverConn net.Conn
+	var recordingConn *recordingConn
+
+	if write {
+		var err error
+		recordingConn, err = test.connFromCommand(t)
+		if err != nil {
+			t.Fatalf("Failed to start subcommand: %v", err)
+		}
+		serverConn = recordingConn
+	} else {
+		timer := time.AfterFunc(10*time.Second, func() {
+			fmt.Println("This test may be stuck, try running using -timeout 10s")
+		})
+		t.Cleanup(func() {
+			timer.Stop()
+		})
+		flows, err := test.loadData()
+		if err != nil {
+			t.Fatalf("Failed to load data from %s", test.dataPath())
+		}
+		serverConn = newReplayingConn(t, flows)
+	}
+
+	server, chans, reqs, err := ssh.NewServerConn(serverConn, test.config)
+	if err != nil {
+		t.Fatalf("Failed to create server conn: %v", err)
+	}
+	defer server.Close()
+
+	go ssh.DiscardRequests(reqs)
+
+	done := make(chan bool)
+
+	for newChannel := range chans {
+		if newChannel.ChannelType() != "session" {
+			newChannel.Reject(ssh.UnknownChannelType, "unknown channel type")
+			continue
+		}
+
+		channel, requests, err := newChannel.Accept()
+		if err != nil {
+			continue
+		}
+
+		go func(in <-chan *ssh.Request) {
+			for req := range in {
+				switch req.Type {
+				case "exec":
+					if req.WantReply {
+						req.Reply(true, nil)
+					}
+					channel.SendRequest("exit-status", false, ssh.Marshal(&exitStatusMsg{Status: 0}))
+					channel.Close()
+					done <- true
+				default:
+					if req.WantReply {
+						req.Reply(false, nil)
+					}
+				}
+			}
+		}(requests)
+	}
+
+	<-done
+
+	if write {
+		path := test.dataPath()
+		out, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0644)
+		if err != nil {
+			t.Fatalf("Failed to create output file: %v", err)
+		}
+		defer out.Close()
+		recordingConn.Close()
+
+		recordingConn.WriteTo(out)
+		t.Logf("Wrote %s\n", path)
+	}
+}
+
+func recordingsServerConfig() *ssh.ServerConfig {
+	config := &ssh.ServerConfig{
+		Config: ssh.Config{
+			Rand: sha3.NewShake128(),
+		},
+		PublicKeyCallback: func(conn ssh.ConnMetadata, key ssh.PublicKey) (*ssh.Permissions, error) {
+			return nil, nil
+		},
+	}
+	config.AddHostKey(testSigners["rsa"])
+	return config
+}
+
+func TestServerKeyExchanges(t *testing.T) {
+	config := ssh.ClientConfig{}
+	config.SetDefaults()
+
+	var keyExchanges []string
+	for _, kex := range config.KeyExchanges {
+		// Exclude ecdh for now, to make them determistic we should use see a
+		// stream of fixed bytes as the random source.
+		if !strings.HasPrefix(kex, "ecdh-") {
+			keyExchanges = append(keyExchanges, kex)
+		}
+	}
+	// Add diffie-hellman-group16-sha512 as it is not enabled by default.
+	keyExchanges = append(keyExchanges, "diffie-hellman-group16-sha512")
+
+	for _, kex := range keyExchanges {
+		c := recordingsServerConfig()
+		c.KeyExchanges = []string{kex}
+		test := serverTest{
+			name:   "KEX-" + kex,
+			config: c,
+		}
+		runTestAndUpdateIfNeeded(t, test.name, test.run)
+	}
+}
+
+func TestServerCiphers(t *testing.T) {
+	config := ssh.ClientConfig{}
+	config.SetDefaults()
+
+	for _, ciph := range config.Ciphers {
+		c := recordingsServerConfig()
+		c.Ciphers = []string{ciph}
+		test := serverTest{
+			name:   "Cipher-" + ciph,
+			config: c,
+		}
+		runTestAndUpdateIfNeeded(t, test.name, test.run)
+	}
+}
+
+func TestServerMACs(t *testing.T) {
+	config := ssh.ClientConfig{}
+	config.SetDefaults()
+
+	for _, mac := range config.MACs {
+		c := recordingsServerConfig()
+		c.MACs = []string{mac}
+		test := serverTest{
+			name:   "MAC-" + mac,
+			config: c,
+		}
+		runTestAndUpdateIfNeeded(t, test.name, test.run)
+	}
+}

ssh/test/recording_test.go

@@ -0,0 +1,423 @@
+// Copyright 2025 The Go 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 test
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/hex"
+	"errors"
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"net"
+	"os"
+	"os/user"
+	"strconv"
+	"strings"
+	"sync"
+	"testing"
+	"text/template"
+	"time"
+
+	"golang.org/x/crypto/ssh"
+)
+
+const (
+	defaultSSHDConfig = `
+Protocol 2
+Banner {{.Dir}}/banner
+HostKey {{.Dir}}/id_rsa
+HostKey {{.Dir}}/id_dsa
+HostKey {{.Dir}}/id_ecdsa
+HostCertificate {{.Dir}}/id_rsa-sha2-512-cert.pub
+Pidfile {{.Dir}}/sshd.pid
+KeyRegenerationInterval 3600
+ServerKeyBits 768
+SyslogFacility AUTH
+LogLevel DEBUG2
+LoginGraceTime 120
+PermitRootLogin no
+StrictModes no
+RSAAuthentication yes
+PubkeyAuthentication yes
+AuthorizedKeysFile	{{.Dir}}/authorized_keys
+TrustedUserCAKeys {{.Dir}}/id_ecdsa.pub
+IgnoreRhosts yes
+RhostsRSAAuthentication no
+HostbasedAuthentication no
+PubkeyAcceptedKeyTypes=*
+# In recent OpenSSH versions, the 'diffie-hellman-group14-sha1'
+# algorithm is disabled by default. However, it remains part of
+# our default Key Exchange (KEX) configuration, so we explicitly
+# enable it to ensure compatibility for testing.
+KexAlgorithms +diffie-hellman-group14-sha1
+`
+	multiAuthSshdConfigTail = `
+UsePAM yes
+PasswordAuthentication yes
+ChallengeResponseAuthentication yes
+AuthenticationMethods {{.AuthMethods}}
+`
+	maxAuthTriesSshdConfigTail = `
+PasswordAuthentication yes
+MaxAuthTries 1
+`
+)
+
+var configTmpl = map[string]*template.Template{
+	"default":      template.Must(template.New("").Parse(defaultSSHDConfig)),
+	"MultiAuth":    template.Must(template.New("").Parse(defaultSSHDConfig + multiAuthSshdConfigTail)),
+	"MaxAuthTries": template.Must(template.New("").Parse(defaultSSHDConfig + maxAuthTriesSshdConfigTail))}
+
+type server struct {
+	t          *testing.T
+	configfile string
+
+	testUser     string // test username for sshd
+	testPasswd   string // test password for sshd
+	sshdTestPwSo string // dynamic library to inject a custom password into sshd
+
+	lastDialConn net.Conn
+}
+
+type storedHostKey struct {
+	// keys map from an algorithm string to binary key data.
+	keys map[string][]byte
+
+	// checkCount counts the Check calls. Used for testing
+	// rekeying.
+	checkCount int
+}
+
+func (k *storedHostKey) Add(key ssh.PublicKey) {
+	if k.keys == nil {
+		k.keys = map[string][]byte{}
+	}
+	k.keys[key.Type()] = key.Marshal()
+}
+
+func (k *storedHostKey) Check(addr string, remote net.Addr, key ssh.PublicKey) error {
+	k.checkCount++
+	algo := key.Type()
+
+	if k.keys == nil || !bytes.Equal(key.Marshal(), k.keys[algo]) {
+		return fmt.Errorf("host key mismatch. Got %q, want %q", key, k.keys[algo])
+	}
+	return nil
+}
+
+func hostKeyDB() *storedHostKey {
+	keyChecker := &storedHostKey{}
+	keyChecker.Add(testPublicKeys["ecdsa"])
+	keyChecker.Add(testPublicKeys["rsa"])
+	keyChecker.Add(testPublicKeys["dsa"])
+	return keyChecker
+}
+
+func clientConfig() *ssh.ClientConfig {
+	config := &ssh.ClientConfig{
+		User: username(),
+		Auth: []ssh.AuthMethod{
+			ssh.PublicKeys(testSigners["user"]),
+		},
+		HostKeyCallback: hostKeyDB().Check,
+		HostKeyAlgorithms: []string{ // by default, don't allow certs as this affects the hostKeyDB checker
+			ssh.KeyAlgoECDSA256, ssh.KeyAlgoECDSA384, ssh.KeyAlgoECDSA521,
+			ssh.KeyAlgoRSA, ssh.KeyAlgoDSA,
+			ssh.KeyAlgoED25519,
+		},
+	}
+	return config
+}
+
+// SSH reference tests run a connection against a reference implementation
+// (OpenSSH) of SSH and record the bytes of the resulting connection. The Go
+// code, during a test, is configured with deterministic randomness and so the
+// reference test can be reproduced exactly in the future.
+//
+// In order to save everyone who wishes to run the tests from needing the
+// reference implementation installed, the reference connections are saved in
+// files in the testdata directory. Thus running the tests involves nothing
+// external, but creating and updating them requires the reference
+// implementation.
+//
+// Tests can be updated by running them with the -update flag. This will cause
+// the test files for failing tests to be regenerated. Since the reference
+// implementation will always generate fresh random numbers, large parts of the
+// reference connection will always change.
+
+var (
+	update = flag.Bool("update", false, "update golden files on failure")
+)
+
+func runTestAndUpdateIfNeeded(t *testing.T, name string, run func(t *testing.T, update bool)) {
+	success := t.Run(name, func(t *testing.T) {
+		if !*update {
+			t.Parallel()
+		}
+		run(t, false)
+	})
+
+	if !success && *update {
+		t.Run(name+"#update", func(t *testing.T) {
+			run(t, true)
+		})
+	}
+}
+
+// recordingConn is a net.Conn that records the traffic that passes through it.
+// WriteTo can be used to produce output that can be later be loaded with
+// ParseTestData.
+type recordingConn struct {
+	net.Conn
+	clientToServer bool
+	sync.Mutex
+	flows   [][]byte
+	reading bool
+}
+
+func (r *recordingConn) Read(b []byte) (n int, err error) {
+	if n, err = r.Conn.Read(b); n == 0 {
+		return
+	}
+	b = b[:n]
+
+	r.Lock()
+	defer r.Unlock()
+
+	if l := len(r.flows); l == 0 || !r.reading {
+		buf := make([]byte, len(b))
+		copy(buf, b)
+		r.flows = append(r.flows, buf)
+	} else {
+		r.flows[l-1] = append(r.flows[l-1], b[:n]...)
+	}
+	r.reading = true
+	return
+}
+
+func (r *recordingConn) Write(b []byte) (n int, err error) {
+	if n, err = r.Conn.Write(b); n == 0 {
+		return
+	}
+	b = b[:n]
+
+	r.Lock()
+	defer r.Unlock()
+
+	if l := len(r.flows); l == 0 || r.reading {
+		buf := make([]byte, len(b))
+		copy(buf, b)
+		r.flows = append(r.flows, buf)
+	} else {
+		r.flows[l-1] = append(r.flows[l-1], b[:n]...)
+	}
+	r.reading = false
+	return
+}
+
+// WriteTo writes Go source code to w that contains the recorded traffic.
+func (r *recordingConn) WriteTo(w io.Writer) (int64, error) {
+	var written int64
+	for i, flow := range r.flows {
+		source, dest := "client", "server"
+		if !r.clientToServer {
+			source, dest = dest, source
+		}
+		n, err := fmt.Fprintf(w, ">>> Flow %d (%s to %s)\n", i+1, source, dest)
+		written += int64(n)
+		if err != nil {
+			return written, err
+		}
+		dumper := hex.Dumper(w)
+		n, err = dumper.Write(flow)
+		written += int64(n)
+		if err != nil {
+			return written, err
+		}
+		err = dumper.Close()
+		if err != nil {
+			return written, err
+		}
+		r.clientToServer = !r.clientToServer
+	}
+	return written, nil
+}
+
+func parseTestData(r io.Reader) (flows [][]byte, err error) {
+	var currentFlow []byte
+
+	scanner := bufio.NewScanner(r)
+	for scanner.Scan() {
+		line := scanner.Text()
+		// If the line starts with ">>> " then it marks the beginning
+		// of a new flow.
+		if strings.HasPrefix(line, ">>> ") {
+			if len(currentFlow) > 0 || len(flows) > 0 {
+				flows = append(flows, currentFlow)
+				currentFlow = nil
+			}
+			continue
+		}
+
+		// Otherwise the line is a line of hex dump that looks like:
+		// 00000170  fc f5 06 bf (...)  |.....X{&?......!|
+		// (Some bytes have been omitted from the middle section.)
+		_, after, ok := strings.Cut(line, " ")
+		if !ok {
+			return nil, errors.New("invalid test data")
+		}
+		line = after
+
+		before, _, ok := strings.Cut(line, "|")
+		if !ok {
+			return nil, errors.New("invalid test data")
+		}
+		line = before
+
+		hexBytes := strings.Fields(line)
+		for _, hexByte := range hexBytes {
+			val, err := strconv.ParseUint(hexByte, 16, 8)
+			if err != nil {
+				return nil, errors.New("invalid hex byte in test data: " + err.Error())
+			}
+			currentFlow = append(currentFlow, byte(val))
+		}
+	}
+
+	if len(currentFlow) > 0 {
+		flows = append(flows, currentFlow)
+	}
+
+	return flows, nil
+}
+
+func newReplayingConn(t testing.TB, flows [][]byte) net.Conn {
+	r := &replayingConn{
+		t:       t,
+		flows:   flows,
+		reading: false,
+	}
+	r.readCond = sync.NewCond(&r.Mutex)
+	return r
+}
+
+// replayingConn is a net.Conn that replays flows recorded by recordingConn.
+type replayingConn struct {
+	t testing.TB
+	sync.Mutex
+	flows   [][]byte
+	reading bool
+	// SSH channels use a read loop goroutine, we use this condition to wait
+	// until we are ready to read/write.
+	readCond *sync.Cond
+}
+
+var _ net.Conn = (*replayingConn)(nil)
+
+func (r *replayingConn) Read(b []byte) (n int, err error) {
+	r.Lock()
+	defer r.Unlock()
+
+	for !r.reading {
+		r.readCond.Wait()
+	}
+
+	// Some tests run commands that return no output.
+	if len(r.flows) == 0 {
+		return 0, nil
+	}
+
+	n = copy(b, r.flows[0])
+	r.flows[0] = r.flows[0][n:]
+	if len(r.flows[0]) == 0 {
+		r.flows = r.flows[1:]
+		r.reading = false
+		r.readCond.Broadcast()
+		if len(r.flows) == 0 {
+			return n, io.EOF
+		}
+	}
+	return n, nil
+}
+
+func (r *replayingConn) Write(b []byte) (n int, err error) {
+	r.Lock()
+	defer r.Unlock()
+
+	for r.reading {
+		r.readCond.Wait()
+	}
+
+	if !bytes.HasPrefix(r.flows[0], b) {
+		r.t.Errorf("write mismatch: expected %x, got %x", r.flows[0], b)
+		r.reading = true
+		r.readCond.Broadcast()
+		return 0, fmt.Errorf("write mismatch")
+	}
+	r.flows[0] = r.flows[0][len(b):]
+	if len(r.flows[0]) == 0 {
+		r.flows = r.flows[1:]
+		r.reading = true
+		r.readCond.Broadcast()
+	}
+	return len(b), nil
+}
+
+func (r *replayingConn) Close() error {
+	r.Lock()
+	defer r.Unlock()
+
+	if len(r.flows) > 0 {
+		r.t.Errorf("closed with unfinished flows: %d", len(r.flows))
+		return fmt.Errorf("unexpected close")
+	}
+	return nil
+}
+
+func (r *replayingConn) LocalAddr() net.Addr                { return nil }
+func (r *replayingConn) RemoteAddr() net.Addr               { return nil }
+func (r *replayingConn) SetDeadline(_ time.Time) error      { return nil }
+func (r *replayingConn) SetReadDeadline(_ time.Time) error  { return nil }
+func (r *replayingConn) SetWriteDeadline(_ time.Time) error { return nil }
+
+func username() string {
+	var username string
+	if user, err := user.Current(); err == nil {
+		username = user.Username
+	} else {
+		// user.Current() currently requires cgo. If an error is
+		// returned attempt to get the username from the environment.
+		log.Printf("user.Current: %v; falling back on $USER", err)
+		username = os.Getenv("USER")
+	}
+	if username == "" {
+		panic("Unable to get username")
+	}
+	return username
+}
+
+func writeFile(path string, contents []byte) {
+	f, err := os.OpenFile(path, os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0600)
+	if err != nil {
+		panic(err)
+	}
+	defer f.Close()
+	if _, err := f.Write(contents); err != nil {
+		panic(err)
+	}
+}
+
+func TestMain(m *testing.M) {
+	flag.Usage = func() {
+		fmt.Fprintf(flag.CommandLine.Output(), "Usage of %s:\n", os.Args)
+		flag.PrintDefaults()
+	}
+
+	flag.Parse()
+	os.Exit(m.Run())
+}

ssh/test/session_test.go

@@ -10,7 +10,6 @@ package test
 
 import (
 	"bytes"
-	"errors"
 	"fmt"
 	"io"
 	"path/filepath"
@@ -29,108 +28,6 @@ func skipIfIssue64959(t *testing.T, err error) {
 	}
 }
 
-func TestRunCommandSuccess(t *testing.T) {
-	server := newServer(t)
-	conn := server.Dial(clientConfig())
-	defer conn.Close()
-
-	session, err := conn.NewSession()
-	if err != nil {
-		skipIfIssue64959(t, err)
-		t.Fatalf("session failed: %v", err)
-	}
-	defer session.Close()
-	err = session.Run("true")
-	if err != nil {
-		t.Fatalf("session failed: %v", err)
-	}
-}
-
-func TestHostKeyCheck(t *testing.T) {
-	server := newServer(t)
-
-	conf := clientConfig()
-	hostDB := hostKeyDB()
-	conf.HostKeyCallback = hostDB.Check
-
-	// change the keys.
-	hostDB.keys[ssh.KeyAlgoRSA][25]++
-	hostDB.keys[ssh.KeyAlgoDSA][25]++
-	hostDB.keys[ssh.KeyAlgoECDSA256][25]++
-
-	conn, err := server.TryDial(conf)
-	if err == nil {
-		conn.Close()
-		t.Fatalf("dial should have failed.")
-	} else if !strings.Contains(err.Error(), "host key mismatch") {
-		t.Fatalf("'host key mismatch' not found in %v", err)
-	}
-}
-
-func TestRunCommandStdin(t *testing.T) {
-	server := newServer(t)
-	conn := server.Dial(clientConfig())
-	defer conn.Close()
-
-	session, err := conn.NewSession()
-	if err != nil {
-		skipIfIssue64959(t, err)
-		t.Fatalf("session failed: %v", err)
-	}
-	defer session.Close()
-
-	r, w := io.Pipe()
-	defer r.Close()
-	defer w.Close()
-	session.Stdin = r
-
-	err = session.Run("true")
-	if err != nil {
-		t.Fatalf("session failed: %v", err)
-	}
-}
-
-func TestRunCommandStdinError(t *testing.T) {
-	server := newServer(t)
-	conn := server.Dial(clientConfig())
-	defer conn.Close()
-
-	session, err := conn.NewSession()
-	if err != nil {
-		skipIfIssue64959(t, err)
-		t.Fatalf("session failed: %v", err)
-	}
-	defer session.Close()
-
-	r, w := io.Pipe()
-	defer r.Close()
-	session.Stdin = r
-	pipeErr := errors.New("closing write end of pipe")
-	w.CloseWithError(pipeErr)
-
-	err = session.Run("true")
-	if err != pipeErr {
-		t.Fatalf("expected %v, found %v", pipeErr, err)
-	}
-}
-
-func TestRunCommandFailed(t *testing.T) {
-	server := newServer(t)
-	conn := server.Dial(clientConfig())
-	defer conn.Close()
-
-	session, err := conn.NewSession()
-	if err != nil {
-		skipIfIssue64959(t, err)
-		t.Fatalf("session failed: %v", err)
-	}
-	defer session.Close()
-	err = session.Run(`bash -c "kill -9 $$"`)
-	if err == nil {
-		t.Fatalf("session succeeded: %v", err)
-	}
-}
-
 func TestRunCommandWeClosed(t *testing.T) {
 	server := newServer(t)
 	conn := server.Dial(clientConfig())
@@ -290,59 +187,6 @@ func TestValidTerminalMode(t *testing.T) {
 	}
 }
 
-func TestWindowChange(t *testing.T) {
-	if runtime.GOOS == "aix" {
-		// On AIX, sshd cannot acquire /dev/pts/* if launched as
-		// a non-root user.
-		t.Skipf("skipping on %s", runtime.GOOS)
-	}
-	server := newServer(t)
-	conn := server.Dial(clientConfig())
-	defer conn.Close()
-
-	session, err := conn.NewSession()
-	if err != nil {
-		skipIfIssue64959(t, err)
-		t.Fatalf("session failed: %v", err)
-	}
-	defer session.Close()
-
-	stdout, err := session.StdoutPipe()
-	if err != nil {
-		t.Fatalf("unable to acquire stdout pipe: %s", err)
-	}
-
-	stdin, err := session.StdinPipe()
-	if err != nil {
-		t.Fatalf("unable to acquire stdin pipe: %s", err)
-	}
-
-	tm := ssh.TerminalModes{ssh.ECHO: 0}
-	if err = session.RequestPty("xterm", 80, 40, tm); err != nil {
-		t.Fatalf("req-pty failed: %s", err)
-	}
-
-	if err := session.WindowChange(100, 100); err != nil {
-		t.Fatalf("window-change failed: %s", err)
-	}
-
-	err = session.Shell()
-	if err != nil {
-		t.Fatalf("session failed: %s", err)
-	}
-
-	stdin.Write([]byte("stty size && exit\n"))
-
-	var buf bytes.Buffer
-	if _, err := io.Copy(&buf, stdout); err != nil {
-		t.Fatalf("reading failed: %s", err)
-	}
-
-	if sttyOutput := buf.String(); !strings.Contains(sttyOutput, "100 100") {
-		t.Fatalf("terminal WindowChange failure: expected \"100 100\" stty output, got %s", sttyOutput)
-	}
-}
-
 func testOneCipher(t *testing.T, cipher string, cipherOrder []string) {
 	server := newServer(t)
 	conf := clientConfig()
@@ -396,55 +240,6 @@ func TestCiphers(t *testing.T) {
 	}
 }
 
-func TestMACs(t *testing.T) {
-	var config ssh.Config
-	config.SetDefaults()
-	macOrder := config.MACs
-
-	for _, mac := range macOrder {
-		t.Run(mac, func(t *testing.T) {
-			server := newServer(t)
-			conf := clientConfig()
-			conf.MACs = []string{mac}
-			// Don't fail if sshd doesn't have the MAC.
-			conf.MACs = append(conf.MACs, macOrder...)
-			if conn, err := server.TryDial(conf); err == nil {
-				conn.Close()
-			} else {
-				t.Fatalf("failed for MAC %q", mac)
-			}
-		})
-	}
-}
-
-func TestKeyExchanges(t *testing.T) {
-	var config ssh.Config
-	config.SetDefaults()
-	kexOrder := config.KeyExchanges
-	// Based on the discussion in #17230, the key exchange algorithms
-	// diffie-hellman-group-exchange-sha1 and diffie-hellman-group-exchange-sha256
-	// are not included in the default list of supported kex so we have to add them
-	// here manually.
-	kexOrder = append(kexOrder, "diffie-hellman-group-exchange-sha1", "diffie-hellman-group-exchange-sha256")
-	// The key exchange algorithms diffie-hellman-group16-sha512 is disabled by
-	// default so we add it here manually.
-	kexOrder = append(kexOrder, "diffie-hellman-group16-sha512")
-	for _, kex := range kexOrder {
-		t.Run(kex, func(t *testing.T) {
-			server := newServer(t)
-			conf := clientConfig()
-			// Don't fail if sshd doesn't have the kex.
-			conf.KeyExchanges = append([]string{kex}, kexOrder...)
-			conn, err := server.TryDial(conf)
-			if err == nil {
-				conn.Close()
-			} else {
-				t.Errorf("failed for kex %q", kex)
-			}
-		})
-	}
-}
-
 func TestClientAuthAlgorithms(t *testing.T) {
 	for _, key := range []string{
 		"rsa",

ssh/test/test_unix_test.go

@@ -13,140 +13,18 @@ import (
 	"crypto/rand"
 	"encoding/base64"
 	"fmt"
-	"log"
 	"net"
 	"os"
 	"os/exec"
 	"os/user"
 	"path/filepath"
 	"testing"
-	"text/template"
 
 	"golang.org/x/crypto/internal/testenv"
 	"golang.org/x/crypto/ssh"
 	"golang.org/x/crypto/ssh/testdata"
 )
 
-const (
-	defaultSshdConfig = `
-Protocol 2
-Banner {{.Dir}}/banner
-HostKey {{.Dir}}/id_rsa
-HostKey {{.Dir}}/id_dsa
-HostKey {{.Dir}}/id_ecdsa
-HostCertificate {{.Dir}}/id_rsa-sha2-512-cert.pub
-Pidfile {{.Dir}}/sshd.pid
-#UsePrivilegeSeparation no
-KeyRegenerationInterval 3600
-ServerKeyBits 768
-SyslogFacility AUTH
-LogLevel DEBUG2
-LoginGraceTime 120
-PermitRootLogin no
-StrictModes no
-RSAAuthentication yes
-PubkeyAuthentication yes
-AuthorizedKeysFile	{{.Dir}}/authorized_keys
-TrustedUserCAKeys {{.Dir}}/id_ecdsa.pub
-IgnoreRhosts yes
-RhostsRSAAuthentication no
-HostbasedAuthentication no
-PubkeyAcceptedKeyTypes=*
-`
-	multiAuthSshdConfigTail = `
-UsePAM yes
-PasswordAuthentication yes
-ChallengeResponseAuthentication yes
-AuthenticationMethods {{.AuthMethods}}
-`
-	maxAuthTriesSshdConfigTail = `
-PasswordAuthentication yes
-MaxAuthTries 1
-`
-)
-
-var configTmpl = map[string]*template.Template{
-	"default":      template.Must(template.New("").Parse(defaultSshdConfig)),
-	"MultiAuth":    template.Must(template.New("").Parse(defaultSshdConfig + multiAuthSshdConfigTail)),
-	"MaxAuthTries": template.Must(template.New("").Parse(defaultSshdConfig + maxAuthTriesSshdConfigTail))}
-
-type server struct {
-	t          *testing.T
-	configfile string
-
-	testUser     string // test username for sshd
-	testPasswd   string // test password for sshd
-	sshdTestPwSo string // dynamic library to inject a custom password into sshd
-
-	lastDialConn net.Conn
-}
-
-func username() string {
-	var username string
-	if user, err := user.Current(); err == nil {
-		username = user.Username
-	} else {
-		// user.Current() currently requires cgo. If an error is
-		// returned attempt to get the username from the environment.
-		log.Printf("user.Current: %v; falling back on $USER", err)
-		username = os.Getenv("USER")
-	}
-	if username == "" {
-		panic("Unable to get username")
-	}
-	return username
-}
-
-type storedHostKey struct {
-	// keys map from an algorithm string to binary key data.
-	keys map[string][]byte
-
-	// checkCount counts the Check calls. Used for testing
-	// rekeying.
-	checkCount int
-}
-
-func (k *storedHostKey) Add(key ssh.PublicKey) {
-	if k.keys == nil {
-		k.keys = map[string][]byte{}
-	}
-	k.keys[key.Type()] = key.Marshal()
-}
-
-func (k *storedHostKey) Check(addr string, remote net.Addr, key ssh.PublicKey) error {
-	k.checkCount++
-	algo := key.Type()
-
-	if k.keys == nil || bytes.Compare(key.Marshal(), k.keys[algo]) != 0 {
-		return fmt.Errorf("host key mismatch. Got %q, want %q", key, k.keys[algo])
-	}
-	return nil
-}
-
-func hostKeyDB() *storedHostKey {
-	keyChecker := &storedHostKey{}
-	keyChecker.Add(testPublicKeys["ecdsa"])
-	keyChecker.Add(testPublicKeys["rsa"])
-	keyChecker.Add(testPublicKeys["dsa"])
-	return keyChecker
-}
-
-func clientConfig() *ssh.ClientConfig {
-	config := &ssh.ClientConfig{
-		User: username(),
-		Auth: []ssh.AuthMethod{
-			ssh.PublicKeys(testSigners["user"]),
-		},
-		HostKeyCallback: hostKeyDB().Check,
-		HostKeyAlgorithms: []string{ // by default, don't allow certs as this affects the hostKeyDB checker
-			ssh.KeyAlgoECDSA256, ssh.KeyAlgoECDSA384, ssh.KeyAlgoECDSA521,
-			ssh.KeyAlgoRSA, ssh.KeyAlgoDSA,
-			ssh.KeyAlgoED25519,
-		},
-	}
-	return config
-}
-
 // unixConnection creates two halves of a connected net.UnixConn.  It
 // is used for connecting the Go SSH client with sshd without opening
 // ports.
@@ -262,17 +140,6 @@ func (s *server) Dial(config *ssh.ClientConfig) *ssh.Client {
 	return conn
 }
 
-func writeFile(path string, contents []byte) {
-	f, err := os.OpenFile(path, os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0600)
-	if err != nil {
-		panic(err)
-	}
-	defer f.Close()
-	if _, err := f.Write(contents); err != nil {
-		panic(err)
-	}
-}
-
 // generate random password
 func randomPassword() (string, error) {
 	b := make([]byte, 12)