public void
setOpcode(int value) {
	if (value < 0 || value > 0xF)
		throw new IllegalArgumentException("DNS Opcode " + value +
						   "is out of range");
	flags &= 0x87FF;
	flags |= (value << 11);
}
Sets the message's opcode
@see Opcode
src/main/java/core/org/xbill/DNS/Header.java