Re: Reverse DNS Lookups

[Brian W. <bwe...@xb...>]

On Thu, 22 May 2003, J. Matthew Miller, III wrote:

> Hi,
>     I'm trying to work on some spam filtering software that uses dnsjava to
> do reverse DNS lookups using specific DNS servers.  Specifically, i'm using
> DNS Realtime Blackhole Lists (DNSRBL's).  Basically, the idea is that given
> a server, "permblock.easynet.net" -- and an ip address you want to check
> against that server's "known spammers list" (in this case, 69.1.80.67-- you
> do a lookup of "67.80.1.69.permblock.easynet.net."
> 
> Some DNS servers this works with, others it doesnt.  basically, any dns
> server i can't ping doesnt work.  I get an "UnknownHostException" thrown
> when i create a resolver.

Is the reason you can't ping the server that it doesn't exist?  You might 
want to use dig (or dnsjava's lookup program) to determine if the servers 
exist or not.

> Anyway, I'm wondering if there's a way around this that i'm not
> understanding, or if there's some limitation there, or if maybe i'm totally
> off base here.

If the server doesn't exist, there's no way you can send packets to it.

> Code is as follows:
> 
> ----------------------------
> 
> /*
>  * TestDNS.java
>  *
>  * Created on May 22, 2003, 2:03 PM
>  */
> import org.xbill.DNS.*;
> import java.net.*;
> import java.io.*;
> /**
>  *
>  * @author  matt
>  */
> public class TestDNS {
> 
>     /** Creates a new instance of TestDNS */
>     public TestDNS() {
>     }
>     public static void main(String args[]) {
>         try {
>             String[] dnslist = new String[1];
>             dnslist[0] = "permblock.easynet.net";
>             Resolver resolver = null;
>             try {
>                 resolver = new ExtendedResolver(dnslist);
>             } catch (UnknownHostException ee) {
>                 ee.printStackTrace();
>             }
>             Name name = null;
>             try {
>                 name = Name.fromString("67.80.1.69.permblock.easynet.net.");
>             } catch (Exception e) {
>                 e.printStackTrace();
>             }
>             Record question = Record.newRecord(name, Type.A, DClass.IN);
>             Message query = Message.newQuery(question);
>             Message response = null;
>             try {
>                 response = resolver.send(query);
>             }
>             catch (IOException e) {
>                 // A network error occurred.  Press on.
>                 e.printStackTrace();
>                 System.exit(0);
>             }
>             short rcode = response.getHeader().getRcode();
>             System.out.println("RCODE = " + rcode);
>         } catch (Exception e) {
>             e.printStackTrace();
>         }
> 
>     }
> 
> }

You'd be much better off using the Lookup class; something like:

	String[] dnslist = new String[1];
	dnslist[0] = "permblock.easynet.net";
	Resolver resolver = null;
	try {
		resolver = new ExtendedResolver(dnslist);
	} catch (UnknownHostException ee) {
		ee.printStackTrace();
	}
	Lookup lookup = new Lookup("67.80.1.69.permblock.easynet.net.");
	lookup.run();
	if (lookup.getResult() == Lookup.SUCCESSFUL)
		System.out.println("ok");
	else
		System.out.println("failed");

Brian