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/**
* $Revision: 1456 $
* $Date: 2005-06-01 22:04:54 -0700 (Wed, 01 Jun 2005) $
*
* Copyright 2003-2005 Jive Software.
*
* All rights reserved. Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.jivesoftware.smack.util;
import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.SortedMap;
import java.util.TreeMap;
import org.jivesoftware.smack.util.dns.DNSResolver;
import org.jivesoftware.smack.util.dns.HostAddress;
import org.jivesoftware.smack.util.dns.SRVRecord;
/**
* Utility class to perform DNS lookups for XMPP services.
*
* @author Matt Tucker
*/
public class DNSUtil {
/**
* Create a cache to hold the 100 most recently accessed DNS lookups for a period of
* 10 minutes.
*/
private static Map<String, List<HostAddress>> cache = new Cache<String, List<HostAddress>>(100, 1000*60*10);
private static DNSResolver dnsResolver = null;
/**
* Set the DNS resolver that should be used to perform DNS lookups.
*
* @param resolver
*/
public static void setDNSResolver(DNSResolver resolver) {
dnsResolver = resolver;
}
/**
* Returns the current DNS resolved used to perform DNS lookups.
*
* @return
*/
public static DNSResolver getDNSResolver() {
return dnsResolver;
}
/**
* Returns a list of HostAddresses under which the specified XMPP server can be
* reached at for client-to-server communication. A DNS lookup for a SRV
* record in the form "_xmpp-client._tcp.example.com" is attempted, according
* to section 14.4 of RFC 3920. If that lookup fails, a lookup in the older form
* of "_jabber._tcp.example.com" is attempted since servers that implement an
* older version of the protocol may be listed using that notation. If that
* lookup fails as well, it's assumed that the XMPP server lives at the
* host resolved by a DNS lookup at the specified domain on the default port
* of 5222.<p>
*
* As an example, a lookup for "example.com" may return "im.example.com:5269".
*
* @param domain the domain.
* @return List of HostAddress, which encompasses the hostname and port that the
* XMPP server can be reached at for the specified domain.
*/
public static List<HostAddress> resolveXMPPDomain(String domain) {
return resolveDomain(domain, 'c');
}
/**
* Returns a list of HostAddresses under which the specified XMPP server can be
* reached at for server-to-server communication. A DNS lookup for a SRV
* record in the form "_xmpp-server._tcp.example.com" is attempted, according
* to section 14.4 of RFC 3920. If that lookup fails, a lookup in the older form
* of "_jabber._tcp.example.com" is attempted since servers that implement an
* older version of the protocol may be listed using that notation. If that
* lookup fails as well, it's assumed that the XMPP server lives at the
* host resolved by a DNS lookup at the specified domain on the default port
* of 5269.<p>
*
* As an example, a lookup for "example.com" may return "im.example.com:5269".
*
* @param domain the domain.
* @return List of HostAddress, which encompasses the hostname and port that the
* XMPP server can be reached at for the specified domain.
*/
public static List<HostAddress> resolveXMPPServerDomain(String domain) {
return resolveDomain(domain, 's');
}
private static List<HostAddress> resolveDomain(String domain, char keyPrefix) {
// Prefix the key with 's' to distinguish him from the client domain lookups
String key = keyPrefix + domain;
// Return item from cache if it exists.
if (cache.containsKey(key)) {
List<HostAddress> addresses = cache.get(key);
if (addresses != null) {
return addresses;
}
}
if (dnsResolver == null)
throw new IllegalStateException("No DNS resolver active.");
List<HostAddress> addresses = new ArrayList<HostAddress>();
// Step one: Do SRV lookups
String srvDomain;
if (keyPrefix == 's') {
srvDomain = "_xmpp-server._tcp." + domain;
} else if (keyPrefix == 'c') {
srvDomain = "_xmpp-client._tcp." + domain;
} else {
srvDomain = domain;
}
List<SRVRecord> srvRecords = dnsResolver.lookupSRVRecords(srvDomain);
List<HostAddress> sortedRecords = sortSRVRecords(srvRecords);
if (sortedRecords != null)
addresses.addAll(sortedRecords);
// Step two: Add the hostname to the end of the list
addresses.add(new HostAddress(domain));
// Add item to cache.
cache.put(key, addresses);
return addresses;
}
/**
* Sort a given list of SRVRecords as described in RFC 2782
* Note that we follow the RFC with one exception. In a group of the same priority, only the first entry
* is calculated by random. The others are ore simply ordered by their priority.
*
* @param records
* @return
*/
protected static List<HostAddress> sortSRVRecords(List<SRVRecord> records) {
// RFC 2782, Usage rules: "If there is precisely one SRV RR, and its Target is "."
// (the root domain), abort."
if (records.size() == 1 && records.get(0).getFQDN().equals("."))
return null;
// sorting the records improves the performance of the bisection later
Collections.sort(records);
// create the priority buckets
SortedMap<Integer, List<SRVRecord>> buckets = new TreeMap<Integer, List<SRVRecord>>();
for (SRVRecord r : records) {
Integer priority = r.getPriority();
List<SRVRecord> bucket = buckets.get(priority);
// create the list of SRVRecords if it doesn't exist
if (bucket == null) {
bucket = new LinkedList<SRVRecord>();
buckets.put(priority, bucket);
}
bucket.add(r);
}
List<HostAddress> res = new ArrayList<HostAddress>(records.size());
for (Integer priority : buckets.keySet()) {
List<SRVRecord> bucket = buckets.get(priority);
int bucketSize;
while ((bucketSize = bucket.size()) > 0) {
int[] totals = new int[bucket.size()];
int running_total = 0;
int count = 0;
int zeroWeight = 1;
for (SRVRecord r : bucket) {
if (r.getWeight() > 0)
zeroWeight = 0;
}
for (SRVRecord r : bucket) {
running_total += (r.getWeight() + zeroWeight);
totals[count] = running_total;
count++;
}
int selectedPos;
if (running_total == 0) {
// If running total is 0, then all weights in this priority
// group are 0. So we simply select one of the weights randomly
// as the other 'normal' algorithm is unable to handle this case
selectedPos = (int) (Math.random() * bucketSize);
} else {
double rnd = Math.random() * running_total;
selectedPos = bisect(totals, rnd);
}
// add the SRVRecord that was randomly chosen on it's weight
// to the start of the result list
SRVRecord chosenSRVRecord = bucket.remove(selectedPos);
res.add(chosenSRVRecord);
}
}
return res;
}
// TODO this is not yet really bisection just a stupid linear search
private static int bisect(int[] array, double value) {
int pos = 0;
for (int element : array) {
if (value < element)
break;
pos++;
}
return pos;
}
}
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