net/netns: add Windows support for bind-to-interface-by-route

This is implemented via GetBestInterfaceEx. Should we encounter errors
or fail to resolve a valid, non-Tailscale interface, we fall back to
returning the index for the default interface instead.

Fixes #12551

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

net/netns/netns_windows_test.go

@@ -0,0 +1,112 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package netns
+
+import (
+	"strings"
+	"testing"
+
+	"golang.org/x/sys/windows"
+	"golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
+	"tailscale.com/tsconst"
+)
+
+func TestGetInterfaceIndex(t *testing.T) {
+	oldVal := bindToInterfaceByRoute.Load()
+	t.Cleanup(func() { bindToInterfaceByRoute.Store(oldVal) })
+	bindToInterfaceByRoute.Store(true)
+
+	defIfaceIdxV4, err := defaultInterfaceIndex(windows.AF_INET)
+	if err != nil {
+		t.Fatalf("defaultInterfaceIndex(AF_INET) failed: %v", err)
+	}
+
+	tests := []struct {
+		name string
+		addr string
+		err  string
+	}{
+		{
+			name: "IP_and_port",
+			addr: "8.8.8.8:53",
+		},
+		{
+			name: "bare_ip",
+			addr: "8.8.8.8",
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			addr, err := parseAddress(tc.addr)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			idx, err := getInterfaceIndex(t.Logf, addr, defIfaceIdxV4)
+			if err != nil {
+				if tc.err == "" {
+					t.Fatalf("got unexpected error: %v", err)
+				}
+				if errstr := err.Error(); errstr != tc.err {
+					t.Errorf("expected error %q, got %q", errstr, tc.err)
+				}
+			} else {
+				t.Logf("getInterfaceIndex(%q) = %d", tc.addr, idx)
+				if tc.err != "" {
+					t.Fatalf("wanted error %q", tc.err)
+				}
+			}
+		})
+	}
+
+	t.Run("NoTailscale", func(t *testing.T) {
+		tsIdx, ok, err := tailscaleInterfaceIndex()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !ok {
+			t.Skip("no tailscale interface on this machine")
+		}
+
+		defaultIdx, err := defaultInterfaceIndex(windows.AF_INET)
+		if err != nil {
+			t.Fatalf("defaultInterfaceIndex(AF_INET) failed: %v", err)
+		}
+
+		addr, err := parseAddress("100.100.100.100:53")
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		idx, err := getInterfaceIndex(t.Logf, addr, defaultIdx)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		t.Logf("tailscaleIdx=%d defaultIdx=%d idx=%d", tsIdx, defaultIdx, idx)
+
+		if idx == tsIdx {
+			t.Fatalf("got idx=%d; wanted not Tailscale interface", idx)
+		} else if idx != defaultIdx {
+			t.Fatalf("got idx=%d, want %d", idx, defaultIdx)
+		}
+	})
+}
+
+func tailscaleInterfaceIndex() (idx uint32, found bool, err error) {
+	ifs, err := winipcfg.GetAdaptersAddresses(windows.AF_INET, winipcfg.GAAFlagIncludeAllInterfaces)
+	if err != nil {
+		return idx, false, err
+	}
+
+	for _, iface := range ifs {
+		if iface.IfType != winipcfg.IfTypePropVirtual {
+			continue
+		}
+		if strings.Contains(iface.Description(), tsconst.WintunInterfaceDesc) {
+			return iface.IfIndex, true, nil
+		}
+	}
+	return idx, false, nil
+}