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];
}else{
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 DNSServer.findMXRecords(hostname).
/** Performs DNS lookups as needed to find servers which should or might
* support SMTP. Returns one SMTPHostAddresses for each such host discovered
* 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 nextElement()
* 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 */
getMailetContext().getSMTPHostAddresses(host);
if(!smtpHosts.hasMoreElements()){
//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(host)
.append(". I cannot determine where to send this message.");
return failMessage(
mail, new MessagingException(exceptionBuffer.toString()), false);
}
while(smtpHosts.hasMoreElements()){
SMTPHostAddresses thisHost = (SMTPHostAddresses)smtpHosts.nextElement();
String thisHostName = thisHost.getHostname();//if needed for logging
InetAddress[] addresses = thisHost.getAddresses();
for (int addressIndex = 0; addressIndex < addresses.length;
addressIndex++){
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 coming
* first. And, whenever multiple hosts have the same priority then these
* 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 every
* 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){
equiPriorityList.add(domainName);
}
}
}/* 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 priorListPriority
* has already become the highest of the priorities in the rawMxRecords,
* 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 priority
* 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) {
equiPriorityList.clear();
leastPriorityFound = thisRecordPriority;
equiPriorityList.add(thisRecord.getTarget().toString());
} else if (thisRecordPriority == leastPriorityFound) {
equiPriorityList.add(thisRecord.getTarget().toString());
}
}
}
priorListPriority = leastPriorityFound;
}
public boolean hasMoreElements(){
if (equiPriorityList.size() > 0){
return true;
}else if (mxRecords != null && mxRecords.length > 0){
createPriorityList();
return equiPriorityList.size() > 0;
} else{
return false;
}
}
public Object nextElement(){
if (hasMoreElements()){
int getIndex = (int)(Math.random()*equiPriorityList.size());
Object returnElement = equiPriorityList.get(getIndex);
equiPriorityList.remove(getIndex);
return returnElement;
}else{
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 names.
* Domain names to test may be supplied optionally as arguments to main().
* But if args is left empty then a hardcoded list of names will be tested.
* 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",
"mailscreen.net","aol.com",
"rockland.com","mindspring.com","mx02.mindspring.com.","",
"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 = dnsServer.getSMTPHostAddresses(domainName);
int hostCount = 0;
while (hostsForDomain.hasMoreElements()){
SMTPHostAddresses thisHost =
(SMTPHostAddresses)hostsForDomain.nextElement();
System.out.print(thisHost.getHostname());
InetAddress[] addresses = thisHost.getAddresses();
for (int addressIndex = 0; addressIndex < addresses.length;
addressIndex++){
System.out.print(" " + addresses[addressIndex].toString());
}
System.out.println();
hostCount++;
}
System.out.println("Done, found " + hostCount + " hosts.\n");
}
}
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