I am trying to extract information from a UDP packet but keep getting random output. Sometimes I get exactly what I want, and other times I don't.
Here is my code:
private static void receivePacket()
{
try {
DatagramSocket socket = new DatagramSocket(port);
System.out.println("\n Listening...");
while(true)
{
// Create a packet
DatagramPacket packet = new DatagramPacket(new byte[1024], 1024);
// Receive packet
socket.receive(packet);
byte[] data = packet.getData();
// Parse the packet
parse(data, packet.getLength());
socket.send(packet);
}
} catch(Exception e) {
e.printStackTrace();
}
}
// Parse packets
private static void parse(byte [] data, int dataLength)
{
ArrayList<String> name = new ArrayList<String>()
String domain = "";
// Get ID
int id = ((data[0] & 0xff) << 8) + (data[1] & 0xff);
System.out.println("\n ID:\t\t" + id);
// Get domain name and number of bits for each word in domain
for(int i = 0; i < dataLength; i++)
{
// If the next bit is a letter then we know the current bit is the number of bits
if((data[i] <= ' ') || (data[i] > '~'))
{
try {
if((String.format("%c", data[i+1]).matches("^[a-zA-Z]$")))
{
int dSize = Integer.parseInt(String.format("%d", data[i]));
name.add(Integer.toString(dSize));
}
} catch (Exception e) {
//e.printStackTrace();
}
}
else
{
// If current bit is letter add to ArrayList
try {
if((String.format("%c", data[i]).matches("^[a-zA-Z]$")))
{
name.add(String.format("%c", data[i]));
domain += String.format("%c", data[i]);
}
} catch (Exception e) {
//e.printStackTrace();
}
}
}
System.out.println(" Domain:\t" + domain);
System.out.print(" Name:\t\t");
name.add("0");
for(int i = 0; i < name.size(); i++)
System.out.print("\'" + name.get(i) + "\' ");
System.out.println();
}
I use the dig
command dig @localhost -p 1299 test.mydomain.abc
to send a UDP packet to the server. Here is my output after running it six times. The output each time should be the following (ID will vary):
ID: 64666
Domain: testmydomainabc
Name: '4' 't' 'e' 's' 't' '8' 'm' 'y' 'd' 'o' 'm' 'a' 'i' 'n' '3' 'a' 'b' 'c' '0'
However, it is not as you see here starting from run #4:
It is completely random and I don't understand why. I am coding this on Java and Windows 10. Any help will be appreciated, thank you!
Raw Data:
Successful trial: '4' 't' 'e' 's' 't' '8' 'm' 'y' 'd' 'o' 'm' 'a' 'i' 'n' '3' 'a' 'b' 'c' '0'
b7 01 20 00 01 00 00 00 00 00 01 04 t e s t 08 m y d o m a i n 03 a b c 00 00 01 00 01 00 00 ) 10 00 00 00 00 00 00 00
Unsuccessful: '-127' 'f' '4' 't' 'e' 's' 't' '8' 'm' 'y' 'd' 'o' 'm' 'a' 'i' 'n' '3' 'a' 'b' 'c' '0'
81 f 01 20 00 01 00 00 00 00 00 01 04 t e s t 08 m y d o m a i n 03 a b c 00 00 01 00 01 00 00 ) 10 00 00 00 00 00 00 00
Your loop in parse
starts from 0
, should probably start from 2
. Otherwise the ID bytes will be included in the Domain parsing (or throw an exception that are silenty ignored)