I am trying to parse a DatagramPacket that I will receive at a socket. I know the format of the packet I will receive, which is a DHCPREQUEST packet, but I don’t think that really matters. For simplicity’s sake, let’s just consider the first six fields:
First field is the “opcode”, which is 1 byte.
Second field is the “hardware type” which is 1 byte.
Third, “hardware address length”, 1 byte.
Fourth, “hops”, 1 byte.
Fifth, “transaction identifier xid”, 4 bytes.
Sixth, “seconds”, 2 bytes.
After I receive the packet, my approach is to convert it to a byte array.
DatagramPacket request = new DatagramPacket(new byte[1024], 1024);
socket.receive(request);
byte[] buf = request.getData();
At this point, the packet is stored in the byte array buf as a series of bytes. Since I know what the structure of this byte sequence is, how can I parse it? The one-byte fields are simple enough, but how about the multiple-bit fields? For example, how can I extract bytes 4 to 7, and store them in a variable named xid?
I could manually put each byte into an array:
byte[] xid = new byte[4];
xid[0] = buf[4];
xid[1] = buf[5];
xid[2] = buf[6];
xid[3] = buf[7];
But that’s just tedious, and impractical for fields that are hundreds of bytes in length. The String class can parse substrings given an offset and length; is there a similar method for byte arrays in Java?
Or am I somehow making things difficult for myself?
The cleanest way to do something like this is probably to use the utility method
Arrays.copyOfRange.