[client] Detect default route changes in the Linux network monitor (#8216)

The Linux network monitor skipped every route update whose Dst was not
nil, treating a nil Dst as the default route. Since the bump to
vishvananda/netlink v1.3 (v0.43.0) the default route arrives as an
explicit 0.0.0.0/0 or ::/0, so every default route change was filtered
out and the monitor never fired on Linux, no matter how the underlay
moved.

Treat both a nil Dst and a zero-length prefix as the default route.

client/internal/networkmonitor/check_change_linux_test.go

@@ -0,0 +1,43 @@
+//go:build !android
+
+package networkmonitor
+
+import (
+	"net"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestIsDefaultRoute(t *testing.T) {
+	mustCIDR := func(s string) *net.IPNet {
+		t.Helper()
+		_, n, err := net.ParseCIDR(s)
+		if err != nil {
+			t.Fatalf("parse %s: %v", s, err)
+		}
+		return n
+	}
+
+	tests := []struct {
+		name string
+		dst  *net.IPNet
+		want bool
+	}{
+		// netlink before v1.3 left Dst nil for the default route.
+		{name: "nil dst", dst: nil, want: true},
+		// netlink v1.3 reports the default route with an explicit zero-length prefix.
+		{name: "v4 default", dst: mustCIDR("0.0.0.0/0"), want: true},
+		{name: "v6 default", dst: mustCIDR("::/0"), want: true},
+		// The exit node split routes and ordinary prefixes are not defaults.
+		{name: "v4 split half", dst: mustCIDR("0.0.0.0/1"), want: false},
+		{name: "v4 subnet", dst: mustCIDR("10.0.0.0/8"), want: false},
+		{name: "v4 host", dst: mustCIDR("1.1.1.1/32"), want: false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equal(t, tt.want, isDefaultRoute(tt.dst), "default route detection")
+		})
+	}
+}