- added readme

- reverse hostname compare

README.txt

@@ -0,0 +1,29 @@
+Hadoop is picky about DNS (and reverse DNS!)
+Every node in the cluster has to have DNS working properly.
+
+This script checks DNS and reverse DNS for hosts given.  Hosts are specified in a file, on a line by line basis.
+
+What is needed:
+    - Java (JDK 1.6) : which you will have installed in a Hadoop cluster anyway
+    - rsync (to distribute files to remote machines)
+    - SSH (and password less SSH setup between hosts)
+
+Compile the source files first:
+    ./compile.sh
+this will create a file 'a.jar'.  This jar will be used by run scripts
+
+the script 'run.sh'  runs the checks on the current host:
+    ./run.sh   hosts_file
+
+'run-on-cluster.sh' runs the tests on all machines specified in hosts file
+    - it uses SSH to login to remote hosts and run the tests.  So it is handy to have password-less SSH configured.  
+      In a typical Hadoop cluster, 'master' host is setup to have password-less SSH to all slaves.  So this script can be run from master server
+
+    - it logs into each host and runs the DNS test
+
+    ./run-on-cluster.sh  hosts_file
+
+Happy Hadooping!
+
+Sujee Maniyam
+s@sujee.net

hosts1

@@ -1,5 +1,2 @@
-c2107.pcs.hds.com
-c2108.pcs.hds.com
-c2109.pcs.hds.com
-smbuild.pcs.hds.com
-10.1.1.2
+host1
+host2

src/net/sujee/hadoop/CheckDNS.java

@@ -8,12 +8,15 @@
 import java.util.ArrayList;
 import java.util.List;
 
+/**
+ * check dns and reverse dns of hosts in a given file
+ * 
+ * @author sujee@sujee.net
+ *
+ */
 public class CheckDNS
 {
 
-    /**
-     * @param args
-     */
     public static void main(String[] args) throws Exception
     {
         if (args.length < 1)
@@ -50,7 +53,10 @@ public static void main(String[] args) throws Exception
                     {
                         InetAddress revHost = InetAddress.getByAddress(inet.getAddress());
                         String revHostName = revHost.getHostName();
-                        System.out.println ("   reverse lookup : success (" + revHostName + ")");
+                        if (host.equals(revHostName))
+                            System.out.println ("   reverse lookup : success (" + revHostName + ")");
+                        else
+                            System.out.println ("   reverse lookup : ***failed*** (" + revHostName + " != " + host + " (expected) )");
                     }
                     catch (UnknownHostException e)
                     {