tailscale/ssh/tailssh/tailssh_integration_test.go

// Copyright (c) Tailscale Inc & contributors
// SPDX-License-Identifier: BSD-3-Clause

//go:build integrationtest

package tailssh

import (
	"bufio"
	"bytes"
	"cmp"
	"context"
	"crypto/rand"
	"crypto/rsa"
	"crypto/x509"
	"encoding/pem"
	"errors"
	"fmt"
	"io"
	"log"
	"net"
	"net/http"
	"net/netip"
	"os"
	"os/exec"
	"path/filepath"
	"runtime"
	"strings"
	"sync"
	"testing"
	"time"

	"github.com/bramvdbogaerde/go-scp"
	gcmp "github.com/google/go-cmp/cmp"
	gcmpopts "github.com/google/go-cmp/cmp/cmpopts"
	"github.com/pkg/sftp"
	gliderssh "github.com/tailscale/gliderssh"
	"golang.org/x/crypto/ssh"
	"golang.org/x/crypto/ssh/agent"
	"tailscale.com/net/tsdial"
	"tailscale.com/tailcfg"
	"tailscale.com/types/key"
	"tailscale.com/types/logger"
	"tailscale.com/types/netmap"
	"tailscale.com/util/set"
)

// This file contains integration tests of the SSH functionality. These tests
// exercise everything except for the authentication logic.
//
// The tests make the following assumptions about the environment:
//
// - OS is one of MacOS or Linux
// - Test is being run as root (e.g. go test -tags integrationtest -c . && sudo ./tailssh.test -test.run TestIntegration)
// - TAILSCALED_PATH environment variable points at tailscaled binary
// - User "testuser" exists
// - "testuser" is in groups "groupone" and "grouptwo"

// testVarRoot is a temp directory used as the TailscaleVarRoot for
// host key generation during integration tests. The test containers
// don't have system host keys (/etc/ssh/ssh_host_*_key) since they
// only install openssh-client, so getHostKeys needs a valid var root
// to generate keys into.
var testVarRoot string

func TestMain(m *testing.M) {
	debugTest.Store(true)

	// Create our log file.
	if err := os.WriteFile("/tmp/tailscalessh.log", nil, 0666); err != nil {
		log.Fatal(err)
	}

	// Create a temp directory for SSH host keys.
	var err error
	testVarRoot, err = os.MkdirTemp("", "tailssh-test-var")
	if err != nil {
		log.Fatal(err)
	}

	code := m.Run()

	os.RemoveAll(testVarRoot)

	// Print any log output from the incubator subprocesses.
	if b, err := os.ReadFile("/tmp/tailscalessh.log"); err == nil && len(b) > 0 {
		log.Print(string(b))
	}

	os.Exit(code)
}

func TestIntegrationSSH(t *testing.T) {
	homeDir := "/home/testuser"
	if runtime.GOOS == "darwin" {
		homeDir = "/Users/testuser"
	}

	tests := []struct {
		cmd             string
		want            []string
		forceV1Behavior bool
		skip            bool
		allowSendEnv    bool
	}{
		{
			cmd:             "id",
			want:            []string{"testuser", "groupone", "grouptwo"},
			forceV1Behavior: false,
		},
		{
			cmd:             "id",
			want:            []string{"testuser", "groupone", "grouptwo"},
			forceV1Behavior: true,
		},
		{
			cmd:             "pwd",
			want:            []string{homeDir},
			skip:            os.Getenv("SKIP_FILE_OPS") == "1" || !fallbackToSUAvailable(),
			forceV1Behavior: false,
		},
		{
			cmd:             "echo 'hello'",
			want:            []string{"hello"},
			skip:            os.Getenv("SKIP_FILE_OPS") == "1" || !fallbackToSUAvailable(),
			forceV1Behavior: false,
		},
		{
			cmd:             `echo "${GIT_ENV_VAR:-unset1} ${EXACT_MATCH:-unset2} ${TESTING:-unset3} ${NOT_ALLOWED:-unset4}"`,
			want:            []string{"working1 working2 working3 unset4"},
			forceV1Behavior: false,
			allowSendEnv:    true,
		},
		{
			cmd:             `echo "${GIT_ENV_VAR:-unset1} ${EXACT_MATCH:-unset2} ${TESTING:-unset3} ${NOT_ALLOWED:-unset4}"`,
			want:            []string{"unset1 unset2 unset3 unset4"},
			forceV1Behavior: false,
			allowSendEnv:    false,
		},
	}

	for _, test := range tests {
		if test.skip {
			continue
		}

		// run every test both without and with a shell
		for _, shell := range []bool{false, true} {
			shellQualifier := "no_shell"
			if shell {
				shellQualifier = "shell"
			}

			versionQualifier := "v2"
			if test.forceV1Behavior {
				versionQualifier = "v1"
			}

			t.Run(fmt.Sprintf("%s_%s_%s", test.cmd, shellQualifier, versionQualifier), func(t *testing.T) {
				sendEnv := map[string]string{
					"GIT_ENV_VAR": "working1",
					"EXACT_MATCH": "working2",
					"TESTING":     "working3",
					"NOT_ALLOWED": "working4",
				}
				s := testSession(t, test.forceV1Behavior, test.allowSendEnv, sendEnv)

				if shell {
					err := s.RequestPty("xterm", 40, 80, ssh.TerminalModes{
						ssh.ECHO:          1,
						ssh.TTY_OP_ISPEED: 14400,
						ssh.TTY_OP_OSPEED: 14400,
					})
					if err != nil {
						t.Fatalf("unable to request PTY: %s", err)
					}

					err = s.Shell()
					if err != nil {
						t.Fatalf("unable to request shell: %s", err)
					}

					// Read the shell prompt
					s.read()
				}

				got := s.run(t, test.cmd, shell)
				for _, want := range test.want {
					if !strings.Contains(got, want) {
						t.Errorf("%q does not contain %q", got, want)
					}
				}
			})
		}
	}
}

// TestIntegrationSSHNoLeakedPTY checks that starting a second SSH session
// while a PTY session is active does not leak extra file descriptors.
func TestIntegrationSSHNoLeakedPTY(t *testing.T) {
	if runtime.GOOS != "linux" {
		t.Skip("requires /proc to inspect file descriptors")
	}
	for _, name := range []string{"v1", "v2"} {
		t.Run(name, func(t *testing.T) {
			forceV1Behavior := name == "v1"
			cl := testClient(t, forceV1Behavior, false)
			// Bound both the readiness read and the descriptor scan if a child hangs.
			timer := time.AfterFunc(30*time.Second, func() { cl.Close() })
			defer timer.Stop()

			first, err := cl.NewSession()
			if err != nil {
				t.Fatal(err)
			}
			defer first.Close()
			stdin, err := first.StdinPipe()
			if err != nil {
				t.Fatal(err)
			}
			defer stdin.Close()
			if err := first.RequestPty("xterm", 24, 80, ssh.TerminalModes{ssh.ECHO: 0, ssh.OPOST: 0}); err != nil {
				t.Fatal(err)
			}
			stdout, err := first.StdoutPipe()
			if err != nil {
				t.Fatal(err)
			}
			// Keep the first session alive until we close it. Receiving its output
			// proves launchProcess has installed the duplicated master before we
			// start the second session; no timing-dependent sleep is needed.
			if err := first.Start("printf 'ready\\n'; read line"); err != nil {
				t.Fatal(err)
			}
			if line, err := bufio.NewReader(stdout).ReadString('\n'); err != nil || line != "ready\n" {
				t.Fatalf("first PTY session not ready: line=%q, err=%v", line, err)
			}

			for _, name := range []string{"pipes", "pty"} {
				t.Run(name, func(t *testing.T) {
					second, err := cl.NewSession()
					if err != nil {
						t.Fatal(err)
					}
					defer second.Close()
					if name == "pty" {
						if err := second.RequestPty("xterm", 24, 80, ssh.TerminalModes{ssh.ECHO: 0}); err != nil {
							t.Fatal(err)
						}
					}
					// Inspect the session shell's descriptors. Skip entries that
					// closed after glob expansion, including the scan's directory FD.
					out, err := second.CombinedOutput(`for fd in /proc/$$/fd/*; do
						if [ -L "$fd" ]; then printf '%s\n' "${fd##*/}"; fi
					done`)
					if err != nil {
						t.Fatalf("inspect second session descriptors: %v; output: %s", err, out)
					}
					got := strings.Fields(string(out))
					want := []string{"0", "1", "2"}
					if diff := gcmp.Diff(want, got, gcmpopts.SortSlices(cmp.Less[string])); diff != "" {
						t.Errorf("second session descriptors (-want +got):\n%s", diff)
					}
				})
			}
		})
	}
}

// TestIntegrationAcceptEnvSecretNotLogged is the end-to-end regression test
// for the acceptEnv secret leak: a forwarded secret must reach the session
// environment, but its value must NOT appear on any process command line
// nor in the server or incubator logs.
func TestIntegrationAcceptEnvSecretNotLogged(t *testing.T) {
	for _, forceV1Behavior := range []bool{false, true} {
		name := "v2"
		if forceV1Behavior {
			name = "v1"
		}
		t.Run(name, func(t *testing.T) {
			canary := fmt.Sprintf("e2e-canary-%d", time.Now().UnixNano())

			var logBuf lockedBuffer
			addr := testServerWithOpts(t, testServerOpts{
				username:        "testuser",
				forceV1Behavior: forceV1Behavior,
				allowSendEnv:    true,
				logf:            log.New(&logBuf, "", 0).Printf,
			})
			cl, err := ssh.Dial("tcp", addr, &ssh.ClientConfig{
				HostKeyCallback: ssh.InsecureIgnoreHostKey(),
			})
			if err != nil {
				t.Fatal(err)
			}
			t.Cleanup(func() { cl.Close() })

			s := testSessionFor(t, cl, map[string]string{"GIT_E2E_CANARY": canary})

			// Prove delivery, and keep the session alive while we scan /proc
			if err := s.Start("env; sleep 5"); err != nil {
				t.Fatalf("unable to start command: %s", err)
			}
			if got := s.read(); !strings.Contains(got, "GIT_E2E_CANARY="+canary) {
				t.Fatalf("forwarded secret not delivered to session env; got %q", got)
			}
			if runtime.GOOS == "linux" {
				if path := findInProcCmdlines(canary); path != "" {
					t.Errorf("secret value visible on command line at %s", path)
				}
			}
			s.Close()

			// The parent's session logs include the session-start argv log
			if got := logBuf.String(); strings.Contains(got, canary) {
				t.Errorf("secret value present in server logs: %q", got)
			} else if strings.Contains(got, "GIT_E2E_CANARY") {
				t.Errorf("forwarded key name present in server logs: %q", got)
			}

			// The incubator child writes its debug log in debugTest mode
			if b, err := os.ReadFile("/tmp/tailscalessh.log"); err == nil && bytes.Contains(b, []byte(canary)) {
				t.Errorf("secret value present in incubator debug log")
			}
		})
	}
}

func TestIntegrationSFTP(t *testing.T) {
	for _, forceV1Behavior := range []bool{false, true} {
		name := "v2"
		if forceV1Behavior {
			name = "v1"
		}
		t.Run(name, func(t *testing.T) {
			filePath := "/home/testuser/sftptest.dat"
			if forceV1Behavior || !fallbackToSUAvailable() {
				filePath = "/tmp/sftptest.dat"
			}
			wantText := "hello world"

			cl := testClient(t, forceV1Behavior, false)
			scl, err := sftp.NewClient(cl)
			if err != nil {
				t.Fatalf("can't get sftp client: %s", err)
			}

			file, err := scl.Create(filePath)
			if err != nil {
				t.Fatalf("can't create file: %s", err)
			}
			_, err = file.Write([]byte(wantText))
			if err != nil {
				t.Fatalf("can't write to file: %s", err)
			}
			err = file.Close()
			if err != nil {
				t.Fatalf("can't close file: %s", err)
			}

			file, err = scl.OpenFile(filePath, os.O_RDONLY)
			if err != nil {
				t.Fatalf("can't open file: %s", err)
			}
			defer file.Close()
			gotText, err := io.ReadAll(file)
			if err != nil {
				t.Fatalf("can't read file: %s", err)
			}
			if diff := gcmp.Diff(string(gotText), wantText); diff != "" {
				t.Fatalf("unexpected file contents (-got +want):\n%s", diff)
			}

			s := testSessionFor(t, cl, nil)
			got := s.run(t, "ls -l "+filePath, false)
			if !strings.Contains(got, "testuser") {
				t.Fatalf("unexpected file owner user: %s", got)
			} else if !strings.Contains(got, "testuser") {
				t.Fatalf("unexpected file owner group: %s", got)
			}
		})
	}
}

func TestIntegrationSCP(t *testing.T) {
	for _, forceV1Behavior := range []bool{false, true} {
		name := "v2"
		if forceV1Behavior {
			name = "v1"
		}
		t.Run(name, func(t *testing.T) {
			filePath := "/home/testuser/scptest.dat"
			if !fallbackToSUAvailable() {
				filePath = "/tmp/scptest.dat"
			}
			wantText := "hello world"

			cl := testClient(t, forceV1Behavior, false)
			scl, err := scp.NewClientBySSH(cl)
			if err != nil {
				t.Fatalf("can't get sftp client: %s", err)
			}

			err = scl.Copy(context.Background(), strings.NewReader(wantText), filePath, "0644", int64(len(wantText)))
			if err != nil {
				t.Fatalf("can't create file: %s", err)
			}

			outfile, err := os.CreateTemp("", "")
			if err != nil {
				t.Fatalf("can't create temp file: %s", err)
			}
			err = scl.CopyFromRemote(context.Background(), outfile, filePath)
			if err != nil {
				t.Fatalf("can't copy file from remote: %s", err)
			}
			outfile.Close()

			gotText, err := os.ReadFile(outfile.Name())
			if err != nil {
				t.Fatalf("can't read file: %s", err)
			}
			if diff := gcmp.Diff(string(gotText), wantText); diff != "" {
				t.Fatalf("unexpected file contents (-got +want):\n%s", diff)
			}

			s := testSessionFor(t, cl, nil)
			got := s.run(t, "ls -l "+filePath, false)
			if !strings.Contains(got, "testuser") {
				t.Fatalf("unexpected file owner user: %s", got)
			} else if !strings.Contains(got, "testuser") {
				t.Fatalf("unexpected file owner group: %s", got)
			}
		})
	}
}

func TestSSHAgentForwarding(t *testing.T) {
	// Create a client SSH key
	tmpDir, err := os.MkdirTemp("", "")
	if err != nil {
		t.Fatal(err)
	}
	t.Cleanup(func() {
		_ = os.RemoveAll(tmpDir)
	})
	pkFile := filepath.Join(tmpDir, "pk")
	clientKey, clientKeyRSA := generateClientKey(t, pkFile)

	// Start upstream SSH server
	l, err := net.Listen("tcp", "127.0.0.1:")
	if err != nil {
		t.Fatalf("unable to listen for SSH: %s", err)
	}
	t.Cleanup(func() {
		_ = l.Close()
	})

	// Run an SSH server that accepts connections from that client SSH key.
	gs := gliderssh.Server{
		Handler: func(s gliderssh.Session) {
			io.WriteString(s, "Hello world\n")
		},
		PublicKeyHandler: func(ctx gliderssh.Context, key gliderssh.PublicKey) error {
			// Note - this is not meant to be cryptographically secure, it's
			// just checking that SSH agent forwarding is forwarding the right
			// key.
			a := key.Marshal()
			b := clientKey.PublicKey().Marshal()
			if !bytes.Equal(a, b) {
				return errors.New("key mismatch")
			}
			return nil
		},
	}
	go gs.Serve(l)

	// Run tailscale SSH server and connect to it
	username := "testuser"
	tailscaleAddr := testServer(t, username, false, false)
	tcl, err := ssh.Dial("tcp", tailscaleAddr, &ssh.ClientConfig{
		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
	})
	if err != nil {
		t.Fatal(err)
	}
	t.Cleanup(func() { tcl.Close() })

	s, err := tcl.NewSession()
	if err != nil {
		t.Fatal(err)
	}

	// Set up SSH agent forwarding on the client
	err = agent.RequestAgentForwarding(s)
	if err != nil {
		t.Fatal(err)
	}

	keyring := agent.NewKeyring()
	keyring.Add(agent.AddedKey{
		PrivateKey: clientKeyRSA,
	})
	err = agent.ForwardToAgent(tcl, keyring)
	if err != nil {
		t.Fatal(err)
	}

	// Attempt to SSH to the upstream test server using the forwarded SSH key
	// and run the "true" command.
	upstreamHost, upstreamPort, err := net.SplitHostPort(l.Addr().String())
	if err != nil {
		t.Fatal(err)
	}

	o, err := s.CombinedOutput(fmt.Sprintf(`ssh -T -o StrictHostKeyChecking=no -p %s upstreamuser@%s "true"`, upstreamPort, upstreamHost))
	if err != nil {
		t.Fatalf("unable to call true command: %s\n%s\n-------------------------", err, o)
	}
}

// TestIntegrationParamiko attempts to connect to Tailscale SSH using the
// paramiko Python library. This library does not request 'none' auth. This
// test ensures that Tailscale SSH can correctly handle clients that don't
// request 'none' auth and instead immediately authenticate with a public key
// or password.
func TestIntegrationParamiko(t *testing.T) {
	addr := testServer(t, "testuser", true, false)
	host, port, err := net.SplitHostPort(addr)
	if err != nil {
		t.Fatalf("Failed to split addr %q: %s", addr, err)
	}

	out, err := exec.Command("python3", "-c", fmt.Sprintf(`
import paramiko.client as pm
from paramiko.ecdsakey import ECDSAKey
client = pm.SSHClient()
client.set_missing_host_key_policy(pm.AutoAddPolicy)
client.connect('%s', port=%s, username='testuser', pkey=ECDSAKey.generate(), allow_agent=False, look_for_keys=False)
client.exec_command('pwd')
`, host, port)).CombinedOutput()
	if err != nil {
		t.Fatalf("failed to connect with Paramiko using public key auth: %s\n%q", err, string(out))
	}

	out, err = exec.Command("python3", "-c", fmt.Sprintf(`
import paramiko.client as pm
from paramiko.ecdsakey import ECDSAKey
client = pm.SSHClient()
client.set_missing_host_key_policy(pm.AutoAddPolicy)
client.connect('%s', port=%s, username='testuser', password='doesntmatter', allow_agent=False, look_for_keys=False)
client.exec_command('pwd')
`, host, port)).CombinedOutput()
	if err != nil {
		t.Fatalf("failed to connect with Paramiko using password auth: %s\n%q", err, string(out))
	}
}

// TestLocalUnixForwarding tests direct-streamlocal@openssh.com, which is what
// podman remote (issue #12409) and VSCode Remote (issue #5295) use to reach
// Unix domain sockets on the remote host through SSH. The client opens a
// channel to a Unix socket path on the server, and data is proxied through.
func TestLocalUnixForwarding(t *testing.T) {
	debugTest.Store(true)
	t.Cleanup(func() {
		debugTest.Store(false)
	})

	// Create a Unix socket server in /tmp that simulates a service like
	// podman's API socket at /run/user/<uid>/podman/podman.sock.
	socketDir, err := os.MkdirTemp("", "tailssh-test-")
	if err != nil {
		t.Fatal(err)
	}
	t.Cleanup(func() { os.RemoveAll(socketDir) })
	socketPath := filepath.Join(socketDir, "test-service.sock")

	ul, err := net.Listen("unix", socketPath)
	if err != nil {
		t.Fatal(err)
	}
	t.Cleanup(func() { ul.Close() })
	// The server dials through a child bound to the session user's
	// credentials. Make the root-created socket and its directory
	// traversable and connectable by testuser, modeling a service socket
	// the user is allowed to reach.
	if err := os.Chmod(socketDir, 0o755); err != nil {
		t.Fatal(err)
	}
	if err := os.Chmod(socketPath, 0o666); err != nil {
		t.Fatal(err)
	}

	// The service echoes back whatever it receives, like an API server would.
	go func() {
		for {
			conn, err := ul.Accept()
			if err != nil {
				return
			}
			go func() {
				defer conn.Close()
				io.Copy(conn, conn)
			}()
		}
	}()

	// Start Tailscale SSH server with local port forwarding enabled.
	addr := testServerWithOpts(t, testServerOpts{
		username:                 "testuser",
		allowLocalPortForwarding: true,
	})

	// Connect to the Tailscale SSH server.
	cl, err := ssh.Dial("tcp", addr, &ssh.ClientConfig{
		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
	})
	if err != nil {
		t.Fatal(err)
	}
	t.Cleanup(func() { cl.Close() })

	// Open a direct-streamlocal@openssh.com channel to the Unix socket,
	// exactly as podman remote does.
	conn, err := cl.Dial("unix", socketPath)
	if err != nil {
		t.Fatalf("failed to dial unix socket through SSH: %s", err)
	}
	defer conn.Close()

	// Send data through the tunnel and verify it echoes back.
	want := "GET /_ping HTTP/1.1\r\nHost: d\r\n\r\n"
	_, err = io.WriteString(conn, want)
	if err != nil {
		t.Fatalf("failed to write through tunnel: %s", err)
	}

	got := make([]byte, len(want))
	_, err = io.ReadFull(conn, got)
	if err != nil {
		t.Fatalf("failed to read through tunnel: %s", err)
	}
	if string(got) != want {
		t.Errorf("got %q, want %q", got, want)
	}
}

// TestReverseUnixForwarding tests streamlocal-forward@openssh.com, which tools
// like VSCode Remote and Zed use to create Unix domain sockets on the remote
// host that forward connections back to the client through SSH.
func TestReverseUnixForwarding(t *testing.T) {
	debugTest.Store(true)
	t.Cleanup(func() {
		debugTest.Store(false)
	})

	// Start Tailscale SSH server with remote port forwarding enabled.
	addr := testServerWithOpts(t, testServerOpts{
		username:                  "testuser",
		allowRemotePortForwarding: true,
	})

	// Connect to the Tailscale SSH server.
	cl, err := ssh.Dial("tcp", addr, &ssh.ClientConfig{
		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
	})
	if err != nil {
		t.Fatal(err)
	}
	t.Cleanup(func() { cl.Close() })

	// Request reverse forwarding -- the server creates a Unix socket and
	// forwards incoming connections back through the SSH tunnel.
	socketDir, err := os.MkdirTemp("", "tailssh-test-")
	if err != nil {
		t.Fatal(err)
	}
	t.Cleanup(func() { os.RemoveAll(socketDir) })
	remoteSocketPath := filepath.Join(socketDir, "reverse.sock")
	// The server-side socket is created by a child bound to the
	// session user's credentials, so the directory must be writable and
	// traversable by testuser. The test process, which is root, then
	// connects to the user-owned socket.
	if err := os.Chmod(socketDir, 0o777); err != nil {
		t.Fatal(err)
	}

	ln, err := cl.ListenUnix(remoteSocketPath)
	if err != nil {
		t.Fatalf("failed to request reverse unix forwarding: %s", err)
	}
	t.Cleanup(func() { ln.Close() })

	// Verify the socket file was created on the server side, owned by
	// the session user and readable and writable only by them.
	fi, err := os.Stat(remoteSocketPath)
	if err != nil {
		t.Fatalf("reverse forwarded socket not created: %s", err)
	}
	if got := fi.Mode().Perm(); got != 0o600 {
		t.Errorf("reverse socket mode = %o; want 0600", got)
	}

	// Accept a connection from the tunnel (client side) and write data.
	want := "hello from reverse tunnel"
	go func() {
		conn, err := ln.Accept()
		if err != nil {
			return
		}
		defer conn.Close()
		io.WriteString(conn, want)
	}()

	// Connect directly to the socket on the server side, simulating a
	// local process connecting to the VSCode/Zed IPC socket.
	conn, err := net.Dial("unix", remoteSocketPath)
	if err != nil {
		t.Fatalf("failed to connect to reverse forwarded socket: %s", err)
	}
	defer conn.Close()

	got, err := io.ReadAll(conn)
	if err != nil {
		t.Fatalf("failed to read from reverse forwarded socket: %s", err)
	}
	if string(got) != want {
		t.Errorf("got %q, want %q", got, want)
	}
}

// TestUnixForwardingDenied verifies that Unix socket forwarding is rejected
// when the SSH policy does not permit port forwarding.
func TestUnixForwardingDenied(t *testing.T) {
	debugTest.Store(true)
	t.Cleanup(func() {
		debugTest.Store(false)
	})

	// Start server with forwarding disabled (the default policy).
	addr := testServerWithOpts(t, testServerOpts{
		username: "testuser",
	})

	cl, err := ssh.Dial("tcp", addr, &ssh.ClientConfig{
		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
	})
	if err != nil {
		t.Fatal(err)
	}
	t.Cleanup(func() { cl.Close() })

	// Direct Unix socket forwarding should be rejected.
	_, err = cl.Dial("unix", "/tmp/anything.sock")
	if err == nil {
		t.Error("expected direct unix forwarding to be rejected, but it succeeded")
	}

	// Reverse Unix socket forwarding should also be rejected.
	socketDir, err := os.MkdirTemp("", "tailssh-test-")
	if err != nil {
		t.Fatal(err)
	}
	t.Cleanup(func() { os.RemoveAll(socketDir) })

	_, err = cl.ListenUnix(filepath.Join(socketDir, "denied.sock"))
	if err == nil {
		t.Error("expected reverse unix forwarding to be rejected, but it succeeded")
	}
}

// TestUnixForwardingPathRestriction verifies that socket paths outside the
// allowed directories (home, /tmp, /run/user/<uid>) are rejected even when
// forwarding is permitted by policy.
func TestUnixForwardingPathRestriction(t *testing.T) {
	debugTest.Store(true)
	t.Cleanup(func() {
		debugTest.Store(false)
	})

	addr := testServerWithOpts(t, testServerOpts{
		username:                  "testuser",
		allowLocalPortForwarding:  true,
		allowRemotePortForwarding: true,
	})

	cl, err := ssh.Dial("tcp", addr, &ssh.ClientConfig{
		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
	})
	if err != nil {
		t.Fatal(err)
	}
	t.Cleanup(func() { cl.Close() })

	// Paths outside allowed directories should be rejected.
	restrictedPaths := []string{
		"/var/run/docker.sock",
		"/etc/evil.sock",
	}
	for _, path := range restrictedPaths {
		_, err := cl.Dial("unix", path)
		if err == nil {
			t.Errorf("expected direct forwarding to %q to be rejected, but it succeeded", path)
		}
	}
}

func fallbackToSUAvailable() bool {
	if runtime.GOOS != "linux" {
		return false
	}

	_, err := exec.LookPath("su")
	if err != nil {
		return false
	}

	// Some operating systems like Fedora seem to require login to be present
	// in order for su to work.
	_, err = exec.LookPath("login")
	return err == nil
}

// lockedBuffer is a goroutine-safe bytes.Buffer for capturing server logs.
type lockedBuffer struct {
	mu  sync.Mutex
	buf bytes.Buffer
}

func (b *lockedBuffer) Write(p []byte) (int, error) {
	b.mu.Lock()
	defer b.mu.Unlock()
	return b.buf.Write(p)
}

func (b *lockedBuffer) String() string {
	b.mu.Lock()
	defer b.mu.Unlock()
	return b.buf.String()
}

// findInProcCmdlines returns the path of the first /proc/<pid>/cmdline
// containing s, or "" if none. Linux only.
func findInProcCmdlines(s string) string {
	paths, _ := filepath.Glob("/proc/[0-9]*/cmdline")
	for _, p := range paths {
		if b, err := os.ReadFile(p); err == nil && strings.Contains(string(b), s) {
			return p
		}
	}
	return ""
}

type session struct {
	*ssh.Session

	stdin  io.WriteCloser
	stdout io.ReadCloser
	stderr io.ReadCloser
}

func (s *session) run(t *testing.T, cmdString string, shell bool) string {
	t.Helper()

	if shell {
		_, err := s.stdin.Write([]byte(fmt.Sprintf("%s\n", cmdString)))
		if err != nil {
			t.Fatalf("unable to send command to shell: %s", err)
		}
	} else {
		err := s.Start(cmdString)
		if err != nil {
			t.Fatalf("unable to start command: %s", err)
		}
	}

	return s.read()
}

func (s *session) read() string {
	ch := make(chan []byte)
	go func() {
		defer close(ch)
		for {
			b := make([]byte, 1)
			n, err := s.stdout.Read(b)
			if n > 0 {
				ch <- b
			}
			if err != nil {
				return
			}
		}
	}()

	// Read first byte in blocking fashion.
	b, ok := <-ch
	if !ok {
		return ""
	}
	_got := b

	// Read subsequent bytes until EOF or silence.
readLoop:
	for {
		select {
		case b, ok := <-ch:
			if !ok {
				break readLoop
			}
			_got = append(_got, b...)
		case <-time.After(1 * time.Second):
			break readLoop
		}
	}

	return string(_got)
}

func testClient(t *testing.T, forceV1Behavior bool, allowSendEnv bool, authMethods ...ssh.AuthMethod) *ssh.Client {
	t.Helper()

	username := "testuser"
	addr := testServer(t, username, forceV1Behavior, allowSendEnv)

	cl, err := ssh.Dial("tcp", addr, &ssh.ClientConfig{
		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
		Auth:            authMethods,
	})
	if err != nil {
		t.Fatal(err)
	}
	t.Cleanup(func() { cl.Close() })

	return cl
}

func testServer(t *testing.T, username string, forceV1Behavior bool, allowSendEnv bool) string {
	srv := &server{
		lb:             &testBackend{localUser: username, forceV1Behavior: forceV1Behavior, allowSendEnv: allowSendEnv},
		logf:           log.Printf,
		tailscaledPath: os.Getenv("TAILSCALED_PATH"),
		timeNow:        time.Now,
	}

	l, err := net.Listen("tcp", "127.0.0.1:0")
	if err != nil {
		t.Fatal(err)
	}
	t.Cleanup(func() { l.Close() })

	go func() {
		for {
			conn, err := l.Accept()
			if err == nil {
				go srv.HandleSSHConn(&addressFakingConn{conn})
			}
		}
	}()

	return l.Addr().String()
}

type testServerOpts struct {
	username                  string
	forceV1Behavior           bool
	allowSendEnv              bool
	allowLocalPortForwarding  bool
	allowRemotePortForwarding bool
	logf                      logger.Logf // defaults to log.Printf
}

func testServerWithOpts(t *testing.T, opts testServerOpts) string {
	t.Helper()
	logf := opts.logf
	if logf == nil {
		logf = log.Printf
	}
	srv := &server{
		lb: &testBackend{
			localUser:                 opts.username,
			forceV1Behavior:           opts.forceV1Behavior,
			allowSendEnv:              opts.allowSendEnv,
			allowLocalPortForwarding:  opts.allowLocalPortForwarding,
			allowRemotePortForwarding: opts.allowRemotePortForwarding,
		},
		logf:           logf,
		tailscaledPath: os.Getenv("TAILSCALED_PATH"),
		timeNow:        time.Now,
	}

	l, err := net.Listen("tcp", "127.0.0.1:0")
	if err != nil {
		t.Fatal(err)
	}
	t.Cleanup(func() { l.Close() })

	go func() {
		for {
			conn, err := l.Accept()
			if err == nil {
				go srv.HandleSSHConn(&addressFakingConn{conn})
			}
		}
	}()

	return l.Addr().String()
}

func testSession(t *testing.T, forceV1Behavior bool, allowSendEnv bool, sendEnv map[string]string) *session {
	cl := testClient(t, forceV1Behavior, allowSendEnv)
	return testSessionFor(t, cl, sendEnv)
}

func testSessionFor(t *testing.T, cl *ssh.Client, sendEnv map[string]string) *session {
	s, err := cl.NewSession()
	if err != nil {
		t.Fatal(err)
	}
	for k, v := range sendEnv {
		s.Setenv(k, v)
	}

	t.Cleanup(func() { s.Close() })

	stdinReader, stdinWriter := io.Pipe()
	stdoutReader, stdoutWriter := io.Pipe()
	stderrReader, stderrWriter := io.Pipe()
	s.Stdin = stdinReader
	s.Stdout = io.MultiWriter(stdoutWriter, os.Stdout)
	s.Stderr = io.MultiWriter(stderrWriter, os.Stderr)
	return &session{
		Session: s,
		stdin:   stdinWriter,
		stdout:  stdoutReader,
		stderr:  stderrReader,
	}
}

func generateClientKey(t *testing.T, privateKeyFile string) (ssh.Signer, *rsa.PrivateKey) {
	t.Helper()
	priv, err := rsa.GenerateKey(rand.Reader, 2048)
	if err != nil {
		t.Fatal(err)
	}
	mk, err := x509.MarshalPKCS8PrivateKey(priv)
	if err != nil {
		t.Fatal(err)
	}
	privateKey := pem.EncodeToMemory(&pem.Block{Type: "PRIVATE KEY", Bytes: mk})
	if privateKey == nil {
		t.Fatal("failed to encoded private key")
	}
	err = os.WriteFile(privateKeyFile, privateKey, 0600)
	if err != nil {
		t.Fatal(err)
	}
	signer, err := ssh.ParsePrivateKey(privateKey)
	if err != nil {
		t.Fatal(err)
	}
	return signer, priv
}

// testBackend implements ipnLocalBackend
type testBackend struct {
	localUser                 string
	forceV1Behavior           bool
	allowSendEnv              bool
	allowLocalPortForwarding  bool
	allowRemotePortForwarding bool
}

func (tb *testBackend) ShouldRunSSH() bool {
	return true
}

func (tb *testBackend) NetMap() *netmap.NetworkMap {
	capMap := make(set.Set[tailcfg.NodeCapability])
	if tb.forceV1Behavior {
		capMap[tailcfg.NodeAttrSSHBehaviorV1] = struct{}{}
	}
	if tb.allowSendEnv {
		capMap[tailcfg.NodeAttrSSHEnvironmentVariables] = struct{}{}
	}
	return &netmap.NetworkMap{
		SSHPolicy: &tailcfg.SSHPolicy{
			Rules: []*tailcfg.SSHRule{
				{
					Principals: []*tailcfg.SSHPrincipal{{Any: true}},
					Action: &tailcfg.SSHAction{
						Accept:                    true,
						AllowAgentForwarding:      true,
						AllowLocalPortForwarding:  tb.allowLocalPortForwarding,
						AllowRemotePortForwarding: tb.allowRemotePortForwarding,
					},
					SSHUsers:  map[string]string{"*": tb.localUser},
					AcceptEnv: []string{"GIT_*", "EXACT_MATCH", "TEST?NG"},
				},
			},
		},
		AllCaps: capMap,
	}
}

func (tb *testBackend) NetMapNoPeers() *netmap.NetworkMap { return tb.NetMap() }

func (tb *testBackend) WhoIs(_ string, ipp netip.AddrPort) (n tailcfg.NodeView, u tailcfg.UserProfile, ok bool) {
	return (&tailcfg.Node{}).View(), tailcfg.UserProfile{
		LoginName: tb.localUser + "@example.com",
	}, true
}

func (tb *testBackend) DoNoiseRequest(req *http.Request) (*http.Response, error) {
	return nil, nil
}

func (tb *testBackend) Dialer() *tsdial.Dialer {
	return nil
}

func (tb *testBackend) TailscaleVarRoot() string {
	return testVarRoot
}

func (tb *testBackend) NodeKey() key.NodePublic {
	return key.NodePublic{}
}

type addressFakingConn struct {
	net.Conn
}

func (conn *addressFakingConn) LocalAddr() net.Addr {
	return &net.TCPAddr{
		IP:   net.ParseIP("100.100.100.101"),
		Port: 22,
	}
}

func (conn *addressFakingConn) RemoteAddr() net.Addr {
	return &net.TCPAddr{
		IP:   net.ParseIP("100.100.100.102"),
		Port: 10002,
	}
}