Changes applied:

* Renamed EDNS0_CUSTOM to EDNS0_LOCAL
* Added EDNS0LOCAL constants
* Added documentation

client_test.go

@@ -1,6 +1,7 @@
 package dns
 
 import (
+	"strconv"
 	"testing"
 	"time"
 )
@@ -62,19 +63,23 @@ func TestClientEDNS0(t *testing.T) {
 	}
 }
 
-// Validates the transmission and parsing of custom EDNS0 options.
-func TestClientEDNS0Custom(t *testing.T) {
+// Validates the transmission and parsing of local EDNS0 options.
+func TestClientEDNS0Local(t *testing.T) {
+
+	optStr1 := "1979:0x0707"
+	optStr2 := strconv.Itoa(EDNS0LOCALSTART) + ":0x0601"
+
 	handler := func(w ResponseWriter, req *Msg) {
 		m := new(Msg)
 		m.SetReply(req)
 
 		m.Extra = make([]RR, 1, 2)
-		m.Extra[0] = &TXT{Hdr: RR_Header{Name: m.Question[0].Name, Rrtype: TypeTXT, Class: ClassINET, Ttl: 0}, Txt: []string{"Hello custom edns"}}
+		m.Extra[0] = &TXT{Hdr: RR_Header{Name: m.Question[0].Name, Rrtype: TypeTXT, Class: ClassINET, Ttl: 0}, Txt: []string{"Hello local edns"}}
 
-		// If the custom options are what we expect, then reflect them back.
-		ec1 := req.Extra[0].(*OPT).Option[0].(*EDNS0_CUSTOM).String()
-		ec2 := req.Extra[0].(*OPT).Option[1].(*EDNS0_CUSTOM).String()
-		if ec1 == "1979:0x0707" && ec2 == "1997:0x0601" {
+		// If the local options are what we expect, then reflect them back.
+		ec1 := req.Extra[0].(*OPT).Option[0].(*EDNS0_LOCAL).String()
+		ec2 := req.Extra[0].(*OPT).Option[1].(*EDNS0_LOCAL).String()
+		if ec1 == optStr1 && ec2 == optStr2 {
 			m.Extra = append(m.Extra, req.Extra[0])
 		}
 
@@ -93,8 +98,9 @@ func TestClientEDNS0Custom(t *testing.T) {
 	m := new(Msg)
 	m.SetQuestion("miek.nl.", TypeTXT)
 
-	ec1 := &EDNS0_CUSTOM{Code: 1979, Data: []byte{7, 7}}
-	ec2 := &EDNS0_CUSTOM{Code: 1997, Data: []byte{6, 1}}
+	// Add two local edns options to the query.
+	ec1 := &EDNS0_LOCAL{Code: 1979, Data: []byte{7, 7}}
+	ec2 := &EDNS0_LOCAL{Code: EDNS0LOCALSTART, Data: []byte{6, 1}}
 	o := &OPT{Hdr: RR_Header{Name: ".", Rrtype: TypeOPT}, Option: []EDNS0{ec1, ec2}}
 	m.Extra = append(m.Extra, o)
 
@@ -112,24 +118,22 @@ func TestClientEDNS0Custom(t *testing.T) {
 	}
 
 	txt := r.Extra[0].(*TXT).Txt[0]
-	if txt != "Hello custom edns" {
-		t.Log("Unexpected result for miek.nl", txt, "!= Hello custom edns")
+	if txt != "Hello local edns" {
+		t.Log("Unexpected result for miek.nl", txt, "!= Hello local edns")
 		t.Fail()
 	}
 
-	// Validate the custom options in the reply.
-	exp := "1979:0x0707"
-	got := r.Extra[1].(*OPT).Option[0].(*EDNS0_CUSTOM).String()
-	if got != exp {
-		t.Log("failed to get custom edns0 answer; got %s, expected %s", got, exp)
+	// Validate the local options in the reply.
+	got := r.Extra[1].(*OPT).Option[0].(*EDNS0_LOCAL).String()
+	if got != optStr1 {
+		t.Log("failed to get local edns0 answer; got %s, expected %s", got, optStr1)
 		t.Fail()
 		t.Logf("%v\n", r)
 	}
 
-	exp = "1997:0x0601"
-	got = r.Extra[1].(*OPT).Option[1].(*EDNS0_CUSTOM).String()
-	if got != exp {
-		t.Log("failed to get custom edns0 answer; got %s, expected %s", got, exp)
+	got = r.Extra[1].(*OPT).Option[1].(*EDNS0_LOCAL).String()
+	if got != optStr2 {
+		t.Log("failed to get local edns0 answer; got %s, expected %s", got, optStr2)
 		t.Fail()
 		t.Logf("%v\n", r)
 	}

edns.go

@@ -18,6 +18,8 @@ const (
 	EDNS0SUBNET      = 0x8     // client-subnet (RFC6891)
 	EDNS0EXPIRE      = 0x9     // EDNS0 expire
 	EDNS0SUBNETDRAFT = 0x50fa  // Don't use! Use EDNS0SUBNET
+	EDNS0LOCALSTART  = 0xFDE9  // Beginning of range reserved for local/experimental use (RFC6891)
+	EDNS0LOCALEND    = 0xFFFE  // End of range reserved for local/experimental use (RFC6891)
 	_DO              = 1 << 15 // dnssec ok
 )
 
@@ -68,8 +70,8 @@ func (rr *OPT) String() string {
 			s += "\n; DS HASH UNDERSTOOD: " + o.String()
 		case *EDNS0_N3U:
 			s += "\n; NSEC3 HASH UNDERSTOOD: " + o.String()
-		case *EDNS0_CUSTOM:
-			s += "\n; CUSTOM OPT: " + o.String()
+		case *EDNS0_LOCAL:
+			s += "\n; LOCAL OPT: " + o.String()
 		}
 	}
 	return s
@@ -479,17 +481,30 @@ func (e *EDNS0_EXPIRE) unpack(b []byte) error {
 	return nil
 }
 
-type EDNS0_CUSTOM struct {
+// The local EDNS0 option is used for local/experimental purposes.  The option
+// code is recommended to be within the range [EDNS0LOCALSTART, EDNS0LOCALEND]
+// (RFC6891), although any unassigned code can actually be used.  The content of
+// the option is made available in Data, unaltered.
+// Basic use pattern for creating a local option:
+//
+//	o := new(dns.OPT)
+//	o.Hdr.Name = "."
+//	o.Hdr.Rrtype = dns.TypeOPT
+//	e := new(dns.EDNS0_LOCAL)
+//	e.Code = dns.EDNS0LOCALSTART
+//	e.Data = []byte{72, 82, 74}
+//	o.Option = append(o.Option, e)
+type EDNS0_LOCAL struct {
 	Code uint16
 	Data []byte
 }
 
-func (e *EDNS0_CUSTOM) Option() uint16 { return e.Code }
-func (e *EDNS0_CUSTOM) String() string {
+func (e *EDNS0_LOCAL) Option() uint16 { return e.Code }
+func (e *EDNS0_LOCAL) String() string {
 	return strconv.FormatInt(int64(e.Code), 10) + ":0x" + hex.EncodeToString(e.Data)
 }
 
-func (e *EDNS0_CUSTOM) pack() ([]byte, error) {
+func (e *EDNS0_LOCAL) pack() ([]byte, error) {
 	b := make([]byte, len(e.Data))
 	copied := copy(b, e.Data)
 	if copied != len(e.Data) {
@@ -498,7 +513,7 @@ func (e *EDNS0_CUSTOM) pack() ([]byte, error) {
 	return b, nil
 }
 
-func (e *EDNS0_CUSTOM) unpack(b []byte) error {
+func (e *EDNS0_LOCAL) unpack(b []byte) error {
 	e.Data = make([]byte, len(b))
 	copied := copy(e.Data, b)
 	if copied != len(b) {

msg.go

@@ -1048,7 +1048,7 @@ func unpackStructValue(val reflect.Value, msg []byte, off int) (off1 int, err er
 					edns = append(edns, e)
 					off = off1 + int(optlen)
 				default:
-					e := new(EDNS0_CUSTOM)
+					e := new(EDNS0_LOCAL)
 					e.Code = code
 					if err := e.unpack(msg[off1 : off1+int(optlen)]); err != nil {
 						return lenmsg, err