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From "Richard O. Hammer" <ROHam...@EarthLink.net>
Subject Re: Regarding BUG 24885: RemoteDelivery only tries one of multiple A record
Date Thu, 11 Dec 2003 00:37:12 GMT
Below you may find code which I have developed and tested to deal with 
this issue.

This solution has a new little type to wrap the data needed in 
RemoteDelivery.  It carries a String hostName and an array of InetAddress.

import java.net.InetAddress;
import org.xbill.DNS.ARecord;
import org.xbill.DNS.Record;

/** A holder of data generated in the DNSServer and needed in 
RemoteDelivery.  Especially needed for multihomed hosts.
public class SMTPHostAddresses {
   String hostName;
   InetAddress[] ipAddresses;

   SMTPHostAddresses(Record[] aRecords, String hostName){
     this.hostName = hostName;
     if (aRecords == null){
       ipAddresses = new InetAddress[0];
       ipAddresses = new InetAddress[aRecords.length];
       for (int i = 0; i < ipAddresses.length; i++){
         ipAddresses[i] = ((ARecord)aRecords[i]).getAddress();

   public String getHostname(){
     return hostName;
   public InetAddress[] getAddresses(){
     return ipAddresses;

The following new method, added to DNSServer, provides the interface 
to the underlying DNS functionality.  This will be used instead of 

   /** Performs DNS lookups as needed to find servers which should or 
    * support SMTP.  Returns one SMTPHostAddresses for each such host 
    * by DNS.  If no host is found for domainName, the Enumeration
    * returned will be empty and the first call to hasMoreElements() 
will return
    * false.
    * @param domainName the String domain for which SMTP host 
addresses are
    * sought.
    * @return an Enumeration in which the Objects returned by 
    * are instances of SMTPHostAddresses.
   public Enumeration getSMTPHostAddresses(final String domainName){
     return new Enumeration(){
       private Enumeration mxHosts = new MxSorter(domainName);
       public boolean hasMoreElements(){
         return mxHosts.hasMoreElements();
       public Object nextElement(){
         String nextHostname = (String)mxHosts.nextElement();
         Record[] aRecords = lookup(nextHostname, Type.A);
         return new SMTPHostAddresses(aRecords, nextHostname);

You will notice that this method simply wraps another Enumeration, 
which I will supply below.

But first let me touch upon how this would be used in RemoteDelivery, 
if that is not obvious.  The code immediately below is an untested 
sketch of the structure which I suppose could work in 
RemoteDelivery.deliver().  It contains an outer loop going through the 
Enumeration of SMTPHostAddresses, and an inner loop going through the 
array of InetAddresses in each SMTPHostAddresses.

     Enumeration smtpHosts = /* something like */
       //error condition as in RemoteDelivery
       log("No mail server found for: " + host);
       StringBuffer exceptionBuffer =
           new StringBuffer(128)
           .append("There are no DNS entries for the hostname ")
           .append(".  I cannot determine where to send this message.");
       return failMessage(
             mail, new MessagingException(exceptionBuffer.toString()), 
       SMTPHostAddresses thisHost = 
       String thisHostName = thisHost.getHostname();//if needed for 
       InetAddress[] addresses = thisHost.getAddresses();
       for (int addressIndex = 0; addressIndex < addresses.length;
         InetAddress thisAddress = addresses[addressIndex];
         //attempt SMTP connection at thiAddress
         //do other work

I agree with Noel that in some ways a single Enumeration, 
necessitating only a single loop, would be simpler.  But I've already 
got this written, and there are also some ways in which this structure 
seems better.

Now for the biggest block of my code, the inner class MxSorter below 
also goes into DNSServer.  This is the Enumeration that was wrapped 
above in the method DNSServer.getSMTPHostAddresses().

   /** A way to get mail hosts to try.  If any MX hosts are found for the
    * domain name with which this is constructed, then these MX hostnames
    * are returned in priority sorted order, lowest priority numbers 
    * first.  And, whenever multiple hosts have the same priority then 
    * are returned in a randomized order within that priority group, as
    * specified in RFC 2821, Section 5.
    * If no MX hosts are found for the domain name, then a DNS search is
    * performed for an A record.  If an A record is found then 
domainName itself
    * will be returned by the Enumeration, and it will be the only 
object in
    * the Enumeration.  If however no A record is found (in addition 
to no MX
    * record) then the Enumeration constructed will be empty; the 
first call to
    * its hasMoreElements() will return false.
    * This behavior attempts to satisfy the requirements of RFC 2821, 
Section 5.
   private class MxSorter implements Enumeration{
     int priorListPriority = Integer.MIN_VALUE;
     ArrayList equiPriorityList = new ArrayList();
     Record[] mxRecords;
     /* The implementation of this class attempts to achieve efficiency by
     * performing no more sorting of the rawMxRecords than necessary. 
  In the
     * large majority of cases the first attempt, made by a client of 
this class
     * to connect to an SMTP server for a given domain, will succeed. 
  As such,
     * in most cases only one call will be made to this Enumeration's
     * nextElement(), and in that majority of cases there will have 
been no need
     * to sort the array of MX Records.  This implementation would, 
however, be
     * relatively inefficient in the case where all hosts fail, when 
     * Object is called out of a long Enumeration.

     MxSorter(String domainName){
       mxRecords =  lookup(domainName, Type.MX);
       if (mxRecords == null || mxRecords.length == 0){
         //no MX records were found, so try to use the domainName
         Record[] aRecords = lookup(domainName, Type.A);
         if(aRecords != null && aRecords.length > 0){

     /* Sets presentPriorityList to contain all hosts
      * which have the least priority greater than pastPriority.
      * When this is called, both (rawMxRecords.length > 0) and
      * (presentPriorityList.size() == 0), by contract.
      * In the case where this is called repeatedly, so that 
      * has already become the highest of the priorities in the 
      * then this returns without having added any elements to
      * presentPriorityList; presentPriorityList.size remains zero.
     private void createPriorityList(){
       int leastPriorityFound = Integer.MAX_VALUE;
       /* We loop once through the rawMxRecords, finding the lowest 
        * greater than priorListPriority, and collecting all the hostnames
        * with that priority into equiPriorityList.
       for (int i = 0; i < mxRecords.length; i++) {
         MXRecord thisRecord = (MXRecord)mxRecords[i];
         int thisRecordPriority = thisRecord.getPriority();
         if (thisRecordPriority > priorListPriority) {
           if (thisRecordPriority < leastPriorityFound) {
             leastPriorityFound = thisRecordPriority;
           } else if (thisRecordPriority == leastPriorityFound) {
       priorListPriority = leastPriorityFound;
     public boolean hasMoreElements(){
       if (equiPriorityList.size() > 0){
         return true;
       }else if (mxRecords != null && mxRecords.length > 0){
         return equiPriorityList.size() > 0;
       } else{
         return false;

     public Object nextElement(){
       if (hasMoreElements()){
         int getIndex = (int)(Math.random()*equiPriorityList.size());
         Object returnElement = equiPriorityList.get(getIndex);
         return returnElement;
           throw new NoSuchElementException();

And finally, if anyone should drop these elements into DNSServer and 
want to test them, here is a main() method for testing which I have 
put into my working copy of DNSServer.

   /* For testing.  Displays InetAddresses for SMTP hosts for domain 
    * Domain names to test may be supplied optionally as arguments to 
    * But if args is left empty then a hardcoded list of names will be 
    * For each domain name given, this lists each hostname and then each
    * InetAddress for each hostname.  Thus it shows the entire 
sequence of
    * InetAddresses which may be tried for the given domain.
   public static void main(String[] args) throws UnknownHostException{
     if(args.length == 0){ //if args have not been given, set some 
names to test
       String[] newArgs = {"nc.rr.com","\r^Illegal.characters",
           "mx2.mail.yahoo.com", "IMpossible.domain-3", "hotmail.com"};
       args = newArgs;
     DNSServer dnsServer = new DNSServer();
     for (int domainIndex = 0; domainIndex < args.length; domainIndex++){
       String domainName = args[domainIndex];
       System.out.println("Results for domain " + domainName);
       Enumeration hostsForDomain = 
       int hostCount = 0;
       while (hostsForDomain.hasMoreElements()){
         SMTPHostAddresses thisHost =
         InetAddress[] addresses = thisHost.getAddresses();
         for (int addressIndex = 0; addressIndex < addresses.length;
           System.out.print(" " + addresses[addressIndex].toString());
       System.out.println("Done, found " + hostCount + " hosts.\n");

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