net/tsaddr: IsUla() for IPv6 Unique Local Address

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

net/tsaddr/tsaddr.go

@@ -33,6 +33,7 @@ func CGNATRange() netaddr.IPPrefix {
 var (
 	cgnatRange   oncePrefix
 	ulaRange     oncePrefix
+	tsUlaRange   oncePrefix
 	ula4To6Range oncePrefix
 )
 
@@ -57,8 +58,8 @@ func IsTailscaleIP(ip netaddr.IP) bool {
 // TailscaleULARange returns the IPv6 Unique Local Address range that
 // is the superset range that Tailscale assigns out of.
 func TailscaleULARange() netaddr.IPPrefix {
-	ulaRange.Do(func() { mustPrefix(&ulaRange.v, "fd7a:115c:a1e0::/48") })
-	return ulaRange.v
+	tsUlaRange.Do(func() { mustPrefix(&tsUlaRange.v, "fd7a:115c:a1e0::/48") })
+	return tsUlaRange.v
 }
 
 // Tailscale4To6Range returns the subset of TailscaleULARange used for
@@ -95,6 +96,11 @@ func Tailscale4To6(ipv4 netaddr.IP) netaddr.IP {
 	return netaddr.IPFrom16(ret)
 }
 
+func IsULA(ip netaddr.IP) bool {
+	ulaRange.Do(func() { mustPrefix(&ulaRange.v, "fc00::/7") })
+	return ulaRange.v.Contains(ip)
+}
+
 func mustPrefix(v *netaddr.IPPrefix, prefix string) {
 	var err error
 	*v, err = netaddr.ParseIPPrefix(prefix)

net/tsaddr/tsaddr_test.go

@@ -42,3 +42,25 @@ func TestCGNATRange(t *testing.T) {
 		t.Errorf("got %q; want %q", got, want)
 	}
 }
+
+func TestIsUla(t *testing.T) {
+	tests := []struct {
+		name string
+		ip   string
+		want bool
+	}{
+		{"first ULA", "fc00::1", true},
+		{"not ULA", "fb00::1", false},
+		{"Tailscale", "fd7a:115c:a1e0::1", true},
+		{"Cloud Run", "fddf:3978:feb1:d745::1", true},
+		{"zeros", "0000:0000:0000:0000:0000:0000:0000:0000", false},
+		{"Link Local", "fe80::1", false},
+		{"Global", "2602::1", false},
+	}
+
+	for _, test := range tests {
+		if got := IsULA(netaddr.MustParseIP(test.ip)); got != test.want {
+			t.Errorf("IsULA(%s) = %v, want %v", test.name, got, test.want)
+		}
+	}
+}