1 /** 2 * $Revision: 1456 $ 3 * $Date: 2005-06-01 22:04:54 -0700 (Wed, 01 Jun 2005) $ 4 * 5 * Copyright 2003-2005 Jive Software. 6 * 7 * All rights reserved. Licensed under the Apache License, Version 2.0 (the "License"); 8 * you may not use this file except in compliance with the License. 9 * You may obtain a copy of the License at 10 * 11 * http://www.apache.org/licenses/LICENSE-2.0 12 * 13 * Unless required by applicable law or agreed to in writing, software 14 * distributed under the License is distributed on an "AS IS" BASIS, 15 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 16 * See the License for the specific language governing permissions and 17 * limitations under the License. 18 */ 19 20 package org.jivesoftware.smack.util; 21 22 import java.util.ArrayList; 23 import java.util.Collections; 24 import java.util.LinkedList; 25 import java.util.List; 26 import java.util.Map; 27 import java.util.SortedMap; 28 import java.util.TreeMap; 29 30 import org.jivesoftware.smack.util.dns.DNSResolver; 31 import org.jivesoftware.smack.util.dns.HostAddress; 32 import org.jivesoftware.smack.util.dns.SRVRecord; 33 34 /** 35 * Utility class to perform DNS lookups for XMPP services. 36 * 37 * @author Matt Tucker 38 */ 39 public class DNSUtil { 40 41 /** 42 * Create a cache to hold the 100 most recently accessed DNS lookups for a period of 43 * 10 minutes. 44 */ 45 private static Map<String, List<HostAddress>> cache = new Cache<String, List<HostAddress>>(100, 1000*60*10); 46 47 private static DNSResolver dnsResolver = null; 48 49 /** 50 * Set the DNS resolver that should be used to perform DNS lookups. 51 * 52 * @param resolver 53 */ 54 public static void setDNSResolver(DNSResolver resolver) { 55 dnsResolver = resolver; 56 } 57 58 /** 59 * Returns the current DNS resolved used to perform DNS lookups. 60 * 61 * @return 62 */ 63 public static DNSResolver getDNSResolver() { 64 return dnsResolver; 65 } 66 67 /** 68 * Returns a list of HostAddresses under which the specified XMPP server can be 69 * reached at for client-to-server communication. A DNS lookup for a SRV 70 * record in the form "_xmpp-client._tcp.example.com" is attempted, according 71 * to section 14.4 of RFC 3920. If that lookup fails, a lookup in the older form 72 * of "_jabber._tcp.example.com" is attempted since servers that implement an 73 * older version of the protocol may be listed using that notation. If that 74 * lookup fails as well, it's assumed that the XMPP server lives at the 75 * host resolved by a DNS lookup at the specified domain on the default port 76 * of 5222.<p> 77 * 78 * As an example, a lookup for "example.com" may return "im.example.com:5269". 79 * 80 * @param domain the domain. 81 * @return List of HostAddress, which encompasses the hostname and port that the 82 * XMPP server can be reached at for the specified domain. 83 */ 84 public static List<HostAddress> resolveXMPPDomain(String domain) { 85 return resolveDomain(domain, 'c'); 86 } 87 88 /** 89 * Returns a list of HostAddresses under which the specified XMPP server can be 90 * reached at for server-to-server communication. A DNS lookup for a SRV 91 * record in the form "_xmpp-server._tcp.example.com" is attempted, according 92 * to section 14.4 of RFC 3920. If that lookup fails, a lookup in the older form 93 * of "_jabber._tcp.example.com" is attempted since servers that implement an 94 * older version of the protocol may be listed using that notation. If that 95 * lookup fails as well, it's assumed that the XMPP server lives at the 96 * host resolved by a DNS lookup at the specified domain on the default port 97 * of 5269.<p> 98 * 99 * As an example, a lookup for "example.com" may return "im.example.com:5269". 100 * 101 * @param domain the domain. 102 * @return List of HostAddress, which encompasses the hostname and port that the 103 * XMPP server can be reached at for the specified domain. 104 */ 105 public static List<HostAddress> resolveXMPPServerDomain(String domain) { 106 return resolveDomain(domain, 's'); 107 } 108 109 private static List<HostAddress> resolveDomain(String domain, char keyPrefix) { 110 // Prefix the key with 's' to distinguish him from the client domain lookups 111 String key = keyPrefix + domain; 112 // Return item from cache if it exists. 113 if (cache.containsKey(key)) { 114 List<HostAddress> addresses = cache.get(key); 115 if (addresses != null) { 116 return addresses; 117 } 118 } 119 120 if (dnsResolver == null) 121 throw new IllegalStateException("No DNS resolver active."); 122 123 List<HostAddress> addresses = new ArrayList<HostAddress>(); 124 125 // Step one: Do SRV lookups 126 String srvDomain; 127 if (keyPrefix == 's') { 128 srvDomain = "_xmpp-server._tcp." + domain; 129 } else if (keyPrefix == 'c') { 130 srvDomain = "_xmpp-client._tcp." + domain; 131 } else { 132 srvDomain = domain; 133 } 134 List<SRVRecord> srvRecords = dnsResolver.lookupSRVRecords(srvDomain); 135 List<HostAddress> sortedRecords = sortSRVRecords(srvRecords); 136 if (sortedRecords != null) 137 addresses.addAll(sortedRecords); 138 139 // Step two: Add the hostname to the end of the list 140 addresses.add(new HostAddress(domain)); 141 142 // Add item to cache. 143 cache.put(key, addresses); 144 145 return addresses; 146 } 147 148 /** 149 * Sort a given list of SRVRecords as described in RFC 2782 150 * Note that we follow the RFC with one exception. In a group of the same priority, only the first entry 151 * is calculated by random. The others are ore simply ordered by their priority. 152 * 153 * @param records 154 * @return 155 */ 156 protected static List<HostAddress> sortSRVRecords(List<SRVRecord> records) { 157 // RFC 2782, Usage rules: "If there is precisely one SRV RR, and its Target is "." 158 // (the root domain), abort." 159 if (records.size() == 1 && records.get(0).getFQDN().equals(".")) 160 return null; 161 162 // sorting the records improves the performance of the bisection later 163 Collections.sort(records); 164 165 // create the priority buckets 166 SortedMap<Integer, List<SRVRecord>> buckets = new TreeMap<Integer, List<SRVRecord>>(); 167 for (SRVRecord r : records) { 168 Integer priority = r.getPriority(); 169 List<SRVRecord> bucket = buckets.get(priority); 170 // create the list of SRVRecords if it doesn't exist 171 if (bucket == null) { 172 bucket = new LinkedList<SRVRecord>(); 173 buckets.put(priority, bucket); 174 } 175 bucket.add(r); 176 } 177 178 List<HostAddress> res = new ArrayList<HostAddress>(records.size()); 179 180 for (Integer priority : buckets.keySet()) { 181 List<SRVRecord> bucket = buckets.get(priority); 182 int bucketSize; 183 while ((bucketSize = bucket.size()) > 0) { 184 int[] totals = new int[bucket.size()]; 185 int running_total = 0; 186 int count = 0; 187 int zeroWeight = 1; 188 189 for (SRVRecord r : bucket) { 190 if (r.getWeight() > 0) 191 zeroWeight = 0; 192 } 193 194 for (SRVRecord r : bucket) { 195 running_total += (r.getWeight() + zeroWeight); 196 totals[count] = running_total; 197 count++; 198 } 199 int selectedPos; 200 if (running_total == 0) { 201 // If running total is 0, then all weights in this priority 202 // group are 0. So we simply select one of the weights randomly 203 // as the other 'normal' algorithm is unable to handle this case 204 selectedPos = (int) (Math.random() * bucketSize); 205 } else { 206 double rnd = Math.random() * running_total; 207 selectedPos = bisect(totals, rnd); 208 } 209 // add the SRVRecord that was randomly chosen on it's weight 210 // to the start of the result list 211 SRVRecord chosenSRVRecord = bucket.remove(selectedPos); 212 res.add(chosenSRVRecord); 213 } 214 } 215 216 return res; 217 } 218 219 // TODO this is not yet really bisection just a stupid linear search 220 private static int bisect(int[] array, double value) { 221 int pos = 0; 222 for (int element : array) { 223 if (value < element) 224 break; 225 pos++; 226 } 227 return pos; 228 } 229 }
