begin touching up a DNS Client implementation; internal.GetIP/SetIP refactor

internal/ip.go

@@ -10,7 +10,7 @@ var (
 	errInvalidIPVersionToSetAddr = errors.New("invalid ip version to setDstAddr")
 )
 
-func GetIPSourceAddr(buf []byte) (addr []byte, id, ipEndOff uint16, err error) {
+func GetIPAddr(buf []byte) (src, dst []byte, id, ipEndOff uint16, err error) {
 	b0 := buf[0]
 	version := b0 >> 4
 	switch version {
@@ -18,33 +18,41 @@ func GetIPSourceAddr(buf []byte) (addr []byte, id, ipEndOff uint16, err error) {
 		ihl := b0 & 0xf
 		ipEndOff = 4 * uint16(ihl)
 		id = binary.BigEndian.Uint16(buf[4:6])
-		addr = buf[12:16]
+		src = buf[12:16]
+		dst = buf[16:20]
 	case 6:
-		addr = buf[8:24]
+		src = buf[8:24]
+		dst = buf[24:40]
 		ipEndOff = 40
 	default:
 		err = errUnsupportedIP
 	}
-	return addr, id, ipEndOff, err
+	return src, dst, id, ipEndOff, err
 }
 
-func SetIPDestinationAddr(buf []byte, id uint16, addr []byte) (err error) {
-	var dstaddr []byte
+func SetIPAddrs(buf []byte, id uint16, src, dst []byte) (err error) {
+	var dstaddr, srcaddr []byte
 	version := buf[0] >> 4
 	switch version {
 	case 4:
+		srcaddr = buf[12:16]
 		dstaddr = buf[16:20]
 		if id > 0 {
 			binary.BigEndian.PutUint16(buf[4:6], id)
 		}
 	case 6:
+		srcaddr = buf[8:24]
 		dstaddr = buf[24:40]
 	default:
-		err = errUnsupportedIP
+		return errUnsupportedIP
 	}
-	if err == nil && len(dstaddr) != len(addr) {
-		return errInvalidIPVersionToSetAddr
+	if src != nil && len(srcaddr) != len(src) {
+		return errors.New("mismatched length of ip src addr")
 	}
-	copy(dstaddr, addr)
+	if dst != nil && len(dstaddr) != len(dst) {
+		return errors.New("mismatched length of ip dst addr")
+	}
+	copy(srcaddr, src)
+	copy(dstaddr, dst)
 	return nil
 }