coder · ethanndickson · Jul 29, 2025 · Jul 29, 2025
@@ -15,12 +15,14 @@ package netns
 
 import (
 	"context"
+	"fmt"
 	"net"
 	"net/netip"
 	"sync/atomic"
 
 	"tailscale.com/net/netknob"
 	"tailscale.com/net/netmon"
+	"tailscale.com/net/tsaddr"
 	"tailscale.com/types/logger"
 )
 
@@ -160,3 +162,66 @@ func isLocalhost(addr string) bool {
 	ip, _ := netip.ParseAddr(host)
 	return ip.IsLoopback()
 }
+
+// shouldBindToDefaultInterface determines whether a socket should be bound to
+// the default interface based on the destination address and soft isolation settings.
+func shouldBindToDefaultInterface(logf logger.Logf, address string) bool {
+	if isLocalhost(address) {
+		// Don't bind to an interface for localhost connections.
+		return false
+	}
+
+	if coderSoftIsolation.Load() {
+		addr, err := getAddr(address)
+		if err != nil {
+			logf("[unexpected] netns: Coder soft isolation: error getting addr for %q, binding to default: %v", address, err)
+			return true
+		}
+		if !addr.IsValid() || addr.IsUnspecified() {
+			// Invalid or unspecified addresses should not be bound to any
+			// interface.
+			return false
+		}
+		if tsaddr.IsCoderIP(addr) {
+			logf("[unexpected] netns: Coder soft isolation: detected socket destined for Coder interface, binding to default")
+			return true
+		}
+
+		// It doesn't look like our own interface, so we don't need to bind the
+		// socket to the default interface.
+		return false
+	}
+
+	// The default isolation behavior is to always bind to the default
+	// interface.
+	return true
+}
+
+// getAddr returns the netip.Addr for the given address, or an invalid address
+// if the address is not specified. Use addr.IsValid() to check for this.
+func getAddr(address string) (netip.Addr, error) {
+	host, _, err := net.SplitHostPort(address)
+	if err != nil {
+		return netip.Addr{}, fmt.Errorf("invalid address %q: %w", address, err)
+	}
+	if host == "" {
+		// netip.ParseAddr("") will fail
+		return netip.Addr{}, nil
+	}
+
+	addr, err := netip.ParseAddr(host)
+	if err != nil {
+		return netip.Addr{}, fmt.Errorf("invalid address %q: %w", address, err)
+	}
+	if addr.Zone() != "" {
+		// Addresses with zones *can* be represented as a Sockaddr with extra
+		// effort, but we don't use or support them currently.
+		return netip.Addr{}, fmt.Errorf("invalid address %q, has zone: %w", address, err)
+	}
+	if addr.IsUnspecified() {
+		// This covers the cases of 0.0.0.0 and [::].
+		return netip.Addr{}, nil
+	}
+
+	return addr, nil
+}
@@ -38,8 +38,7 @@ var errInterfaceStateInvalid = errors.New("interface state invalid")
 // It's intentionally the same signature as net.Dialer.Control
 // and net.ListenConfig.Control.
 func controlLogf(logf logger.Logf, netMon *netmon.Monitor, network, address string, c syscall.RawConn) error {
-	if isLocalhost(address) {
-		// Don't bind to an interface for localhost connections.
+	if !shouldBindToDefaultInterface(logf, address) {
 		return nil
 	}
 

@@ -15,7 +15,10 @@ package netns
 
 import (
 	"flag"
+	"fmt"
 	"testing"
+
+	"tailscale.com/net/tsaddr"
 )
 
 var extNetwork = flag.Bool("use-external-network", false, "use the external network in tests")
@@ -76,3 +79,74 @@ func TestIsLocalhost(t *testing.T) {
 		}
 	}
 }
+
+func TestShouldBindToDefaultInterface(t *testing.T) {
+	t.Run("Normal", func(t *testing.T) {
+		tests := []struct {
+			address string
+			want    bool
+		}{
+			{"127.0.0.1:0", false},
+			{"127.0.0.1:1234", false},
+			{"1.2.3.4:0", true},
+			{"1.2.3.4:1234", true},
+		}
+
+		for _, test := range tests {
+			t.Run(test.address, func(t *testing.T) {
+				got := shouldBindToDefaultInterface(t.Logf, test.address)
+				if got != test.want {
+					t.Errorf("want %v, got %v", test.want, got)
+				}
+			})
+		}
+	})
+
+	t.Run("CoderSoftIsolation", func(t *testing.T) {
+		SetCoderSoftIsolation(true)
+		t.Cleanup(func() {
+			SetCoderSoftIsolation(false)
+		})
+
+		tests := []struct {
+			address string
+			want    bool
+		}{
+			// localhost should still not bind to any interface.
+			{"127.0.0.1:0", false},
+			{"127.0.0.1:0", false},
+			{"127.0.0.1:1234", false},
+			{"127.0.0.1:1234", false},
+
+			// Unspecified addresses should not be bound to any interface.
+			{":1234", false},
+			{":1234", false},
+			{"0.0.0.0:1234", false},
+			{"0.0.0.0:1234", false},
+			{"[::]:1234", false},
+			{"[::]:1234", false},
+
+			// Special cases should always bind to default:
+			{"[::%eth0]:1234", true}, // zones are not supported
+			{"a:1234", true},         // not an IP
+
+			// Coder IPs should bind to default.
+			{fmt.Sprintf("[%s]:8080", tsaddr.CoderServiceIPv6()), true},
+			{fmt.Sprintf("[%s]:8080", tsaddr.CoderV6Range().Addr().Next()), true},
+
+			// Non-Coder IPs should not bind to default.
+			{fmt.Sprintf("[%s]:8080", tsaddr.TailscaleServiceIPv6()), false},
+			{fmt.Sprintf("%s:8080", tsaddr.TailscaleServiceIP()), false},
+			{"1.2.3.4:8080", false},
+		}
+
+		for _, test := range tests {
+			t.Run(test.address, func(t *testing.T) {
+				got := shouldBindToDefaultInterface(t.Logf, test.address)
+				if got != test.want {
+					t.Errorf("want %v, got %v", test.want, got)
+				}
+			})
+		}
+	})
+}
@@ -4,19 +4,14 @@
 package netns
 
 import (
-	"fmt"
 	"math/bits"
-	"net"
-	"net/netip"
-	"strings"
 	"syscall"
 
 	"golang.org/x/sys/cpu"
 	"golang.org/x/sys/windows"
 	"golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
 	"tailscale.com/net/interfaces"
 	"tailscale.com/net/netmon"
-	"tailscale.com/net/tsaddr"
 	"tailscale.com/types/logger"
 )
 
@@ -77,40 +72,6 @@ func controlLogf(logf logger.Logf, _ *netmon.Monitor, network, address string, c
 	return nil
 }
 
-func shouldBindToDefaultInterface(logf logger.Logf, address string) bool {
-	if strings.HasPrefix(address, "127.") {
-		// Don't bind to an interface for localhost connections,
-		// otherwise we get:
-		//   connectex: The requested address is not valid in its context
-		// (The derphttp tests were failing)
-		return false
-	}
-
-	if coderSoftIsolation.Load() {
-		addr, err := getAddr(address)
-		if err != nil {
-			logf("[unexpected] netns: Coder soft isolation: error getting addr for %q, binding to default: %v", address, err)
-			return true
-		}
-		if !addr.IsValid() || addr.IsUnspecified() {
-			// Invalid or unspecified addresses should not be bound to any
-			// interface.
-			return false
-		}
-		if tsaddr.IsCoderIP(addr) {
-			logf("[unexpected] netns: Coder soft isolation: detected socket destined for Coder interface, binding to default")
-			return true
-		}
-
-		// It doesn't look like our own interface, so we don't need to bind the
-		// socket to the default interface.
-		return false
-	}
-
-	// The default isolation behavior is to always bind to the default
-	// interface.
-	return true
-}
 
 // sockoptBoundInterface is the value of IP_UNICAST_IF and IPV6_UNICAST_IF.
 //
@@ -163,31 +124,3 @@ func nativeToBigEndian(i uint32) uint32 {
 	return bits.ReverseBytes32(i)
 }
 
-// getAddr returns the netip.Addr for the given address, or an invalid address
-// if the address is not specified. Use addr.IsValid() to check for this.
-func getAddr(address string) (netip.Addr, error) {
-	host, _, err := net.SplitHostPort(address)
-	if err != nil {
-		return netip.Addr{}, fmt.Errorf("invalid address %q: %w", address, err)
-	}
-	if host == "" {
-		// netip.ParseAddr("") will fail
-		return netip.Addr{}, nil
-	}
-
-	addr, err := netip.ParseAddr(host)
-	if err != nil {
-		return netip.Addr{}, fmt.Errorf("invalid address %q: %w", address, err)
-	}
-	if addr.Zone() != "" {
-		// Addresses with zones *can* be represented as a Sockaddr with extra
-		// effort, but we don't use or support them currently.
-		return netip.Addr{}, fmt.Errorf("invalid address %q, has zone: %w", address, err)
-	}
-	if addr.IsUnspecified() {
-		// This covers the cases of 0.0.0.0 and [::].
-		return netip.Addr{}, nil
-	}
-
-	return addr, nil
-}