net/dnscache: try IPv6 addresses first (#5349)

Signed-off-by: Andrew Dunham <andrew@tailscale.com>

Signed-off-by: Andrew Dunham <andrew@tailscale.com>

net/dnscache/dnscache.go

@@ -523,6 +523,21 @@ type res struct {
 		return nil, errors.New("no IPs")
 	}
 
+	// Partition candidate list and then merge such that an IPv6 address is
+	// in the first spot if present, and then addresses are interleaved.
+	// This ensures that we're trying an IPv6 address first, then
+	// alternating between v4 and v6 in case one of the two networks is
+	// broken.
+	var iv4, iv6 []netip.Addr
+	for _, ip := range ips {
+		if ip.Is6() {
+			iv6 = append(iv6, ip)
+		} else {
+			iv4 = append(iv4, ip)
+		}
+	}
+	ips = interleaveSlices(iv6, iv4)
+
 	go func() {
 		for i, ip := range ips {
 			if i != 0 {
@@ -580,6 +595,21 @@ type res struct {
 	}
 }
 
+// interleaveSlices combines two slices of the form [a, b, c] and [x, y, z]
+// into a slice with elements interleaved; i.e. [a, x, b, y, c, z].
+func interleaveSlices[T any](a, b []T) []T {
+	var (
+		i   int
+		ret = make([]T, 0, len(a)+len(b))
+	)
+	for i = 0; i < len(a) && i < len(b); i++ {
+		ret = append(ret, a[i], b[i])
+	}
+	ret = append(ret, a[i:]...)
+	ret = append(ret, b[i:]...)
+	return ret
+}
+
 func v4addrs(aa []net.IPAddr) (ret []netip.Addr) {
 	for _, a := range aa {
 		ip, ok := netip.AddrFromSlice(a.IP)

net/dnscache/dnscache_test.go

@@ -140,3 +140,27 @@ func TestResolverAllHostStaticResult(t *testing.T) {
 		t.Errorf("bad dial error got %q; want %q", got, want)
 	}
 }
+
+func TestInterleaveSlices(t *testing.T) {
+	testCases := []struct {
+		name string
+		a, b []int
+		want []int
+	}{
+		{name: "equal", a: []int{1, 3, 5}, b: []int{2, 4, 6}, want: []int{1, 2, 3, 4, 5, 6}},
+		{name: "short_b", a: []int{1, 3, 5}, b: []int{2, 4}, want: []int{1, 2, 3, 4, 5}},
+		{name: "short_a", a: []int{1, 3}, b: []int{2, 4, 6}, want: []int{1, 2, 3, 4, 6}},
+		{name: "len_1", a: []int{1}, b: []int{2, 4, 6}, want: []int{1, 2, 4, 6}},
+		{name: "nil_a", a: nil, b: []int{2, 4, 6}, want: []int{2, 4, 6}},
+		{name: "nil_all", a: nil, b: nil, want: []int{}},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			merged := interleaveSlices(tc.a, tc.b)
+			if !reflect.DeepEqual(merged, tc.want) {
+				t.Errorf("got %v; want %v", merged, tc.want)
+			}
+		})
+	}
+}