coder · spikecurtis · Jun 27, 2025 · Jun 24, 2025
diff --git a/agent/agentssh/agentssh.go b/agent/agentssh/agentssh.go
@@ -117,6 +117,10 @@ type Config struct {
 	// Note that this is different from the devcontainers feature, which uses
 	// subagents.
 	ExperimentalContainers bool
+	// X11Net allows overriding the networking implementation used for X11
+	// forwarding listeners. When nil, a default implementation backed by the
+	// standard library networking package is used.
+	X11Net X11Network
 }
 
 type Server struct {
@@ -196,6 +200,12 @@ func NewServer(ctx context.Context, logger slog.Logger, prometheusRegistry *prom
 			displayOffset:    *config.X11DisplayOffset,
 			sessions:         make(map[*x11Session]struct{}),
 			connections:      make(map[net.Conn]struct{}),
+			network: func() X11Network {
+				if config.X11Net != nil {
+					return config.X11Net
+				}
+				return osNet{}
+			}(),
 		},
 	}
 

diff --git a/agent/agentssh/x11.go b/agent/agentssh/x11.go
@@ -37,12 +37,30 @@ const (
 	X11MaxPort = X11StartPort + X11MaxDisplays
 )
 
+// X11Network abstracts the creation of network listeners for X11 forwarding.
+// It is intended mainly for testing; production code uses the default
+// implementation backed by the operating system networking stack.
+type X11Network interface {
+	Listen(network, address string) (net.Listener, error)
+}
+
+// osNet is the default X11Network implementation that uses the standard
+// library network stack.
+type osNet struct{}
+
+func (osNet) Listen(network, address string) (net.Listener, error) {
+	return net.Listen(network, address)
+}
+
 type x11Forwarder struct {
 	logger           slog.Logger
 	x11HandlerErrors *prometheus.CounterVec
 	fs               afero.Fs
 	displayOffset    int
 
+	// network creates X11 listener sockets. Defaults to osNet{}.
+	network X11Network
+
 	mu          sync.Mutex
 	sessions    map[*x11Session]struct{}
 	connections map[net.Conn]struct{}
@@ -147,26 +165,27 @@ func (x *x11Forwarder) listenForConnections(
 			x.closeAndRemoveSession(session)
 		}
 
-		tcpConn, ok := conn.(*net.TCPConn)
-		if !ok {
-			x.logger.Warn(ctx, fmt.Sprintf("failed to cast connection to TCPConn. got: %T", conn))
-			_ = conn.Close()
-			continue
+		var originAddr string
+		var originPort uint32
+
+		if tcpConn, ok := conn.(*net.TCPConn); ok {
+			if tcpAddr, ok := tcpConn.LocalAddr().(*net.TCPAddr); ok {
+				originAddr = tcpAddr.IP.String()
+				// #nosec G115 - Safe conversion as TCP port numbers are within uint32 range (0-65535)
+				originPort = uint32(tcpAddr.Port)
+			}
 		}
-		tcpAddr, ok := tcpConn.LocalAddr().(*net.TCPAddr)
-		if !ok {
-			x.logger.Warn(ctx, fmt.Sprintf("failed to cast local address to TCPAddr. got: %T", tcpConn.LocalAddr()))
-			_ = conn.Close()
-			continue
+		// Fallback values for in-memory or non-TCP connections.
+		if originAddr == "" {
+			originAddr = "127.0.0.1"
 		}
 
 		channel, reqs, err := serverConn.OpenChannel("x11", gossh.Marshal(struct {
 			OriginatorAddress string
 			OriginatorPort    uint32
 		}{
-			OriginatorAddress: tcpAddr.IP.String(),
-			// #nosec G115 - Safe conversion as TCP port numbers are within uint32 range (0-65535)
-			OriginatorPort: uint32(tcpAddr.Port),
+			OriginatorAddress: originAddr,
+			OriginatorPort:    originPort,
 		}))
 		if err != nil {
 			x.logger.Warn(ctx, "failed to open X11 channel", slog.Error(err))
@@ -287,13 +306,13 @@ func (x *x11Forwarder) evictLeastRecentlyUsedSession() {
 // createX11Listener creates a listener for X11 forwarding, it will use
 // the next available port starting from X11StartPort and displayOffset.
 func (x *x11Forwarder) createX11Listener(ctx context.Context) (ln net.Listener, display int, err error) {
-	var lc net.ListenConfig
 	// Look for an open port to listen on.
 	for port := X11StartPort + x.displayOffset; port <= X11MaxPort; port++ {
 		if ctx.Err() != nil {
 			return nil, -1, ctx.Err()
 		}
-		ln, err = lc.Listen(ctx, "tcp", fmt.Sprintf("localhost:%d", port))
+
+		ln, err = x.network.Listen("tcp", fmt.Sprintf("localhost:%d", port))
 		if err == nil {
 			display = port - X11StartPort
 			return ln, display, nil

diff --git a/agent/agentssh/x11_test.go b/agent/agentssh/x11_test.go
@@ -3,7 +3,6 @@ package agentssh_test
 import (
 	"bufio"
 	"bytes"
-	"context"
 	"encoding/hex"
 	"fmt"
 	"net"
@@ -32,10 +31,19 @@ func TestServer_X11(t *testing.T) {
 		t.Skip("X11 forwarding is only supported on Linux")
 	}
 
-	ctx := context.Background()
+	ctx := testutil.Context(t, testutil.WaitShort)
 	logger := testutil.Logger(t)
 	fs := afero.NewMemMapFs()
-	s, err := agentssh.NewServer(ctx, logger, prometheus.NewRegistry(), fs, agentexec.DefaultExecer, &agentssh.Config{})
+
+	// Use in-process networking for X11 forwarding.
+	inproc := testutil.NewInProcNet()
+
+	// Create server config with custom X11 listener.
+	cfg := &agentssh.Config{
+		X11Net: inproc,
+	}
+
+	s, err := agentssh.NewServer(ctx, logger, prometheus.NewRegistry(), fs, agentexec.DefaultExecer, cfg)
 	require.NoError(t, err)
 	defer s.Close()
 	err = s.UpdateHostSigner(42)
@@ -93,17 +101,15 @@ func TestServer_X11(t *testing.T) {
 
 	x11Chans := c.HandleChannelOpen("x11")
 	payload := "hello world"
-	require.Eventually(t, func() bool {
-		conn, err := net.Dial("tcp", fmt.Sprintf("localhost:%d", agentssh.X11StartPort+displayNumber))
-		if err == nil {
-			_, err = conn.Write([]byte(payload))
-			assert.NoError(t, err)
-			_ = conn.Close()
-		}
-		return err == nil
-	}, testutil.WaitShort, testutil.IntervalFast)
+	go func() {
+		conn, err := inproc.Dial(ctx, testutil.NewAddr("tcp", fmt.Sprintf("localhost:%d", agentssh.X11StartPort+displayNumber)))
+		assert.NoError(t, err)
+		_, err = conn.Write([]byte(payload))
+		assert.NoError(t, err)
+		_ = conn.Close()
+	}()
 
-	x11 := <-x11Chans
+	x11 := testutil.RequireReceive(ctx, t, x11Chans)
 	ch, reqs, err := x11.Accept()
 	require.NoError(t, err)
 	go gossh.DiscardRequests(reqs)