DNS: Correctly handle broken SRV records

where the SRV RR points to a target DNS name with no associated A or
AAAA RRs.

Fixes SMACK-781.

smack-core/src/main/java/org/jivesoftware/smack/util/dns/DNSResolver.java

@@ -59,6 +59,20 @@ public abstract class DNSResolver {
         return new HostAddress(name, port, inetAddresses);
     }
 
+    /**
+     * Lookup the IP addresses of a given host name. Returns <code>null</code> if there was an error, in which the error
+     * reason will be added in form of a <code>HostAddress</code> to <code>failedAddresses</code>. Returns a empty list
+     * in case the DNS name exists but has no associated A or AAAA resource records. Otherwise, if the resolution was
+     * successful <em>and</em> there is at least one A or AAAA resource record, then a non-empty list will be returned.
+     * <p>
+     * Concrete DNS resolver implementations are free to overwrite this, but have to stick to the interface contract.
+     * </p>
+     *
+     * @param name the DNS name to lookup
+     * @param failedAddresses a list with the failed addresses
+     * @param dnssecMode the selected DNSSEC mode
+     * @return A list, either empty or non-empty, or <code>null</code>
+     */
     protected List<InetAddress> lookupHostAddress0(String name, List<HostAddress> failedAddresses, DnssecMode dnssecMode) {
         // Default implementation of a DNS name lookup for A/AAAA records. It is assumed that this method does never
         // support DNSSEC. Subclasses are free to override this method.

smack-resolver-dnsjava/src/main/java/org/jivesoftware/smack/util/dns/dnsjava/DNSJavaResolver.java

@@ -19,6 +19,7 @@ package org.jivesoftware.smack.util.dns.dnsjava;
 import java.net.InetAddress;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.logging.Level;
 
 import org.jivesoftware.smack.ConnectionConfiguration.DnssecMode;
 import org.jivesoftware.smack.initializer.SmackInitializer;
@@ -73,7 +74,13 @@ public class DNSJavaResolver extends DNSResolver implements SmackInitializer {
                 int weight = srvRecord.getWeight();
 
                 List<InetAddress> hostAddresses = lookupHostAddress0(host, failedAddresses, dnssecMode);
-                if (hostAddresses == null) {
+                if (hostAddresses == null || hostAddresses.isEmpty()) {
+                    // If hostAddresses is not null but empty, then the DNS resolution was successful but the domain did not
+                    // have any A or AAAA resource records.
+                    if (hostAddresses.isEmpty()) {
+                        LOGGER.log(Level.INFO, "The DNS name " + name + ", points to a hostname (" + host
+                                + ") which has neither A or AAAA resource records. This is an indication of a broken DNS setup.");
+                    }
                     continue;
                 }
 

smack-resolver-javax/src/main/java/org/jivesoftware/smack/util/dns/javax/JavaxResolver.java

@@ -111,7 +111,13 @@ public class JavaxResolver extends DNSResolver implements SmackInitializer {
                 String host = srvRecordEntries[srvRecordEntries.length - 1];
 
                 List<InetAddress> hostAddresses = lookupHostAddress0(host, failedAddresses, dnssecMode);
-                if (hostAddresses == null) {
+                if (hostAddresses == null || hostAddresses.isEmpty()) {
+                    // If hostAddresses is not null but empty, then the DNS resolution was successful but the domain did not
+                    // have any A or AAAA resource records.
+                    if (hostAddresses.isEmpty()) {
+                        LOGGER.log(Level.INFO, "The DNS name " + name + ", points to a hostname (" + host
+                                + ") which has neither A or AAAA resource records. This is an indication of a broken DNS setup.");
+                    }
                     continue;
                 }
 

smack-resolver-minidns/src/main/java/org/jivesoftware/smack/util/dns/minidns/MiniDnsResolver.java

@@ -24,6 +24,7 @@ import java.util.Collections;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Set;
+import java.util.logging.Level;
 
 import org.jivesoftware.smack.ConnectionConfiguration.DnssecMode;
 import org.jivesoftware.smack.initializer.SmackInitializer;
@@ -90,7 +91,13 @@ public class MiniDnsResolver extends DNSResolver implements SmackInitializer {
         for (SRV srv : result.getAnswers()) {
             String hostname = srv.name.ace;
             List<InetAddress> hostAddresses = lookupHostAddress0(hostname, failedAddresses, dnssecMode);
-            if (hostAddresses == null) {
+            if (hostAddresses == null || hostAddresses.isEmpty()) {
+                // If hostAddresses is not null but empty, then the DNS resolution was successful but the domain did not
+                // have any A or AAAA resource records.
+                if (hostAddresses.isEmpty()) {
+                    LOGGER.log(Level.INFO, "The DNS name " + name + ", points to a hostname (" + hostname
+                            + ") which has neither A or AAAA resource records. This is an indication of a broken DNS setup.");
+                }
                 continue;
             }