manet: add function to test if address is NAT64 IPv4 converted IPv6 address (#202)

net/ip.go

 package manet
 
 import (
+	"bytes"
 	"net"
 
 	ma "github.com/multiformats/go-multiaddr"
@@ -116,3 +117,14 @@ func zoneless(m ma.Multiaddr) ma.Multiaddr {
 		return m
 	}
 }
+
+var NAT64WellKnownPrefix = [4]byte{0x0, 0x64, 0xff, 0x9b}
+
+// IsNAT64IPv4ConvertedIPv6Addr returns whether addr is an IPv6 address that begins with
+// the well-known prefix "64:ff9b" used for NAT64 Translation
+// see RFC 6052
+func IsNAT64IPv4ConvertedIPv6Addr(addr ma.Multiaddr) bool {
+	c, _ := ma.SplitFirst(addr)
+	return c != nil && c.Protocol().Code == ma.P_IP6 &&
+		bytes.HasPrefix(c.RawValue(), NAT64WellKnownPrefix[:])
+}

net/ip_test.go

+package manet
+
+import (
+	"fmt"
+	"testing"
+
+	ma "github.com/multiformats/go-multiaddr"
+)
+
+func TestIsWellKnownPrefixIPv4ConvertedIPv6Address(t *testing.T) {
+	cases := []struct {
+		addr          ma.Multiaddr
+		want          bool
+		failureReason string
+	}{
+		{
+			addr:          ma.StringCast("/ip4/1.2.3.4/tcp/1234"),
+			want:          false,
+			failureReason: "ip4 addresses should return false",
+		},
+		{
+			addr:          ma.StringCast("/ip6/1::4/tcp/1234"),
+			want:          false,
+			failureReason: "ip6 addresses doesn't have well-known prefix",
+		},
+		{
+			addr:          ma.StringCast("/ip6/::1/tcp/1234"),
+			want:          false,
+			failureReason: "localhost addresses should return false",
+		},
+		{
+			addr:          ma.StringCast("/ip6/64:ff9b::192.0.1.2/tcp/1234"),
+			want:          true,
+			failureReason: "ip6 address begins with well-known prefix",
+		},
+	}
+	for i, tc := range cases {
+		t.Run(fmt.Sprintf("%d", i), func(t *testing.T) {
+			if IsNAT64IPv4ConvertedIPv6Addr(tc.addr) != tc.want {
+				t.Fatalf("%s %s", tc.addr, tc.failureReason)
+			}
+		})
+	}
+}