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POproblems with UDP snmp client Java
 

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  1. POproblems with UDP snmp client Java
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    <p>I'm writing an snmp client that connects to a network server via UDP port 161. It will send an ASN formatted message and will receive the response from the server. I've tried everything I can think of, but I can't seem to get it working properly.</p> <p>The program is supposed to receive and parse out the counter field returned from the following command in cmd:</p> <p>snmpget -d -v 2c -c public 129.130.10.43 ip.ipInReceives.0</p> <p>//result of command:</p> <p>{</p> <p>Sending 43 bytes to UDP: [129.130.10.43]:161->[0.0.0.0]</p> <p>0000: 30 29 02 01 01 04 06 70 75 62 6C 69 63 A0 1C 02 0).....public...</p> <p>0016: 04 28 6A DF 1F 02 01 00 02 01 00 30 0E 30 0C 06 .(j........0.0..</p> <p>0032: 08 2B 06 01 02 01 04 03 00 05 00 .+.........</p> <p>Received 47 bytes from UDP: [129.130.10.43]:161->[0.0.0.0]</p> <p>0000: 30 2D 02 01 01 04 06 70 75 62 6C 69 63 A2 20 02 0-.....public. .</p> <p>0016: 04 28 6A DF 1F 02 01 00 02 01 00 30 12 30 10 06 .(j........0.0..</p> <p>0032: 08 2B 06 01 02 01 04 03 00 41 04 1B 49 0C 95 .+.......A..I..</p> <p>IP-MIB::ipInReceives.0 = Counter32: 457772181</p> <p>}</p> <p>//my asn message formatting notes:</p> <p>hex values to save in a byte[] array and pass to the udp server:</p> <p>30 29 02 01 01 04 06 70 75 62 6C 69 63 A0 1C 02</p> <p>04 XX XX XX XX 02 01 00 02 01 00 30 0E 30 0C 06</p> <p>08 2B 06 01 02 01 04 03 00 05 00</p> <p>XX's indicate the request ID XX's will be changed (probably randomized) for each request in order to create a unique Request ID</p> <p>Code:</p> <pre><code> public static void snmpMessage() throws SocketException { DatagramSocket socket = null; byte[] serverResponse = new byte[1024]; InetAddress addy = null; byte[] hex = hexStringToByteArray("302902010004067075626C6963A01C0204121533EA020100020100300E300C06082B060102010403000500"); //note: this hex string is the exact string from the example program, so I know the string is correct addy = InetAddress.getByName("129.130.10.48"); //server IP address socket= new DatagramSocket(); //method that creates unique snmp asn message replacing the XX's with appropriate non-negative hex values hex = messageGenerator(); //Data reproduction from byte[] array System.out.println("byte array integer format:\n" + Arrays.toString(hex)); //correctly prints out integer values System.out.println("\nbyte array length: " + hex.length); //Correctly prints length of sending array (43) System.out.println("\nserver name: " + addy.getCanonicalHostName()); //correctly prints server name String hex2 = toHex(hex); System.out.println("\nbyte array in hex format: \n" + hex2); //correctly reproduces original hex string from byte[] array //at this point, I can only assume that my data is stored correctly //send byte array to server DatagramPacket sendPacket = new DatagramPacket(hex, hex.length, addy, 161); socket.send(sendPacket); //get server's response DatagramPacket receivePacket = new DatagramPacket(serverResponse, serverResponse.length); System.out.println("\nWaiting for server response...\n"); //prints out socket.receive(receivePacket); System.out.println("Response received"); //does not print } </code></pre> <p>My professor has posted an example where he simply passes the pointer to a char string: sendto (sockfd, (char *) s, 43, 0, (struct sockaddr *) &amp;serv_addr, servlen); His code that passes the same hex code works perfectly.</p> <p>The only thing I can think of is that I'm not formatting, sending, or listening correctly, but after pouring over documentation and examples, I can't figure anything out. Does anyone have some pointers?</p>
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    1. COIf you have access to your professor's example, try running it, along with your own, and use a tool like Wireshark, or tcpdump, to see what's actually being sent out on the wire, in both cases. You can see what your program is sending differently, if anything is being sent out at all.
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    2. COI have to run the prof's example remotely through putty on a linux server and I don't know of a way to get wireshark on that. I did a capture on my outgoing packet and everything looks perfectly correct (destination is good, port is 161, message body is good, etc), but I don't get any response from the server in wireshark. I don't know what's going on.
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    3. COYou can try tcpdump on the Linux server. It's not as user-friendly, but it's still very simple. A quick google or look over the man page for it will tell you how to use it.
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