move dns protocol definitions...

move dns protocol definitions (https://github.com/multiformats/go-multiaddr-dns/blob/3974bf3f84c52825588fdcc0fd0e0aa7953ab5ff/dns.go#L11)

protocols.go

@@ -5,6 +5,9 @@ package multiaddr
 const (
 	P_IP4               = 0x0004
 	P_TCP               = 0x0006
+	P_DNS4              = 0x0036
+	P_DNS6              = 0x0037
+	P_DNSADDR           = 0x0038
 	P_UDP               = 0x0111
 	P_DCCP              = 0x0021
 	P_IP6               = 0x0029
@@ -44,6 +47,27 @@ var (
 		Path:       false,
 		Transcoder: TranscoderPort,
 	}
+	protoDNS4 = Protocol{
+		Code:       P_DNS4,
+		Size:       LengthPrefixedVarSize,
+		Name:       "dns4",
+		VCode:      CodeToVarint(P_DNS4),
+		Transcoder: TranscoderDns,
+	}
+	protoDNS6 = Protocol{
+		Code:       P_DNS6,
+		Size:       LengthPrefixedVarSize,
+		Name:       "dns6",
+		VCode:      CodeToVarint(P_DNS6),
+		Transcoder: TranscoderDns,
+	}
+	protoDNSADDR = Protocol{
+		Code:       P_DNSADDR,
+		Size:       LengthPrefixedVarSize,
+		Name:       "dnsaddr",
+		VCode:      CodeToVarint(P_DNSADDR),
+		Transcoder: TranscoderDns,
+	}
 	protoUDP = Protocol{
 		Name:       "udp",
 		Code:       P_UDP,
@@ -175,6 +199,9 @@ func init() {
 	for _, p := range []Protocol{
 		protoIP4,
 		protoTCP,
+		protoDNS4,
+		protoDNS6,
+		protoDNSADDR,
 		protoUDP,
 		protoDCCP,
 		protoIP6,

transcoders.go

@@ -317,3 +317,20 @@ func unixStB(s string) ([]byte, error) {
 func unixBtS(b []byte) (string, error) {
 	return string(b), nil
 }
+
+var TranscoderDns = NewTranscoderFromFunctions(dnsStB, dnsBtS, dnsVal)
+
+func dnsVal(b []byte) error {
+	if bytes.IndexByte(b, '/') >= 0 {
+		return fmt.Errorf("domain name %q contains a slash", string(b))
+	}
+	return nil
+}
+
+func dnsStB(s string) ([]byte, error) {
+	return []byte(s), nil
+}
+
+func dnsBtS(b []byte) (string, error) {
+	return string(b), nil
+}