Change EDNS_EXPIRE field to support zero length option data (Resolves m…

…iekg#1292)

As per [RFC7134](https://datatracker.ietf.org/doc/html/rfc7314#section-2) the Expire
Option in queries should be zero-length. In the current implementation the field is
uint32 which always instatiates 4bytes for that field when packing to wire format.
For that reason we change the field to []uint8 so it can support 0-length and 4-byte
length option data.

edns.go

@@ -575,33 +575,36 @@ func (e *EDNS0_N3U) copy() EDNS0 { return &EDNS0_N3U{e.Code, e.AlgCode} }
 
 // EDNS0_EXPIRE implements the EDNS0 option as described in RFC 7314.
 type EDNS0_EXPIRE struct {
- Code uint16 // Always EDNS0EXPIRE
- Expire uint32
+ Code uint16  // Always EDNS0EXPIRE
+ Expire []uint8 // can be zero or 4 length
 }
 
 // Option implements the EDNS0 interface.
 func (e *EDNS0_EXPIRE) Option() uint16 { return EDNS0EXPIRE }
-func (e *EDNS0_EXPIRE) String() string { return strconv.FormatUint(uint64(e.Expire), 10) }
 func (e *EDNS0_EXPIRE) copy() EDNS0 { return &EDNS0_EXPIRE{e.Code, e.Expire} }
-
 func (e *EDNS0_EXPIRE) pack() ([]byte, error) {
- b := make([]byte, 4)
- binary.BigEndian.PutUint32(b, e.Expire)
- return b, nil
+ if len(e.Expire) != 0 && len(e.Expire) != 4 {
+ return nil, errors.New("dns: expire length is not 0/4")
+ }
+ return e.Expire, nil
 }
-
 func (e *EDNS0_EXPIRE) unpack(b []byte) error {
- if len(b) == 0 {
- // zero-length EXPIRE query, see RFC 7314 Section 2
- return nil
+ if len(b) != 0 && len(b) != 4 {
+ return errors.New("dns: expire length mismatch, want 0/4 but got " + strconv.Itoa(len(b)))
  }
- if len(b) < 4 {
- return ErrBuf
- }
- e.Expire = binary.BigEndian.Uint32(b)
+ e.Expire = b
  return nil
 }
 
+func (e *EDNS0_EXPIRE) String() (s string) {
+ if len(e.Expire) == 0 {
+ s = "<omitted>"
+ } else {
+ s = fmt.Sprintf("<%d>", binary.BigEndian.Uint32(e.Expire))
+ }
+ return s
+}
+
 // The EDNS0_LOCAL 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