1 /* ResolverCache.java -- A cache of resolver lookups for InetAddress. 2 Copyright (C) 2006 Free Software Foundation, Inc. 3 4 This file is part of GNU Classpath. 5 6 GNU Classpath is free software; you can redistribute it and/or modify 7 it under the terms of the GNU General Public License as published by 8 the Free Software Foundation; either version 2, or (at your option) 9 any later version. 10 11 GNU Classpath is distributed in the hope that it will be useful, but 12 WITHOUT ANY WARRANTY; without even the implied warranty of 13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 14 General Public License for more details. 15 16 You should have received a copy of the GNU General Public License 17 along with GNU Classpath; see the file COPYING. If not, write to the 18 Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 19 02110-1301 USA. 20 21 Linking this library statically or dynamically with other modules is 22 making a combined work based on this library. Thus, the terms and 23 conditions of the GNU General Public License cover the whole 24 combination. 25 26 As a special exception, the copyright holders of this library give you 27 permission to link this library with independent modules to produce an 28 executable, regardless of the license terms of these independent 29 modules, and to copy and distribute the resulting executable under 30 terms of your choice, provided that you also meet, for each linked 31 independent module, the terms and conditions of the license of that 32 module. An independent module is a module which is not derived from 33 or based on this library. If you modify this library, you may extend 34 this exception to your version of the library, but you are not 35 obligated to do so. If you do not wish to do so, delete this 36 exception statement from your version. */ 37 38 39 package java.net; 40 41 import java.security.Security; 42 import java.util.HashMap; 43 import java.util.Iterator; 44 import java.util.LinkedList; 45 46 /** 47 * This class provides a cache of name service resolutions. By 48 * default successful resolutions are cached forever to guard 49 * against DNS spoofing attacks and failed resolutions are cached 50 * for 10 seconds to improve performance. The length of time that 51 * results remain in the cache is determined by the following 52 * security properties: 53 * <dl> 54 * <dt><code>networkaddress.cache.ttl</code></dt> 55 * <dd> 56 * This property specifies the length of time in seconds that 57 * successful resolutions remain in the cache. The default is 58 * -1, indicating to cache forever. 59 * </dd> 60 * <dt><code>networkaddress.cache.negative.ttl</code></dt> 61 * <dd> 62 * This property specifies the length of time in seconds that 63 * unsuccessful resolutions remain in the cache. The default 64 * is 10, indicating to cache for 10 seconds. 65 * </dd> 66 * In both cases, a value of -1 indicates to cache forever and a 67 * value of 0 indicates not to cache. 68 * 69 * @author Gary Benson (gbenson@redhat.com) 70 */ 71 class ResolverCache 72 { 73 /** 74 * The time in seconds for which successful lookups are cached. 75 */ 76 private static final int POSITIVE_TTL = 77 getTTL("networkaddress.cache.ttl", -1); 78 79 /** 80 * The time in seconds for which unsuccessful lookups are cached. 81 */ 82 private static final int NEGATIVE_TTL = 83 getTTL("networkaddress.cache.negative.ttl", 10); 84 85 /** 86 * Helper function to set the TTLs. 87 */ getTTL(String propName, int defaultValue)88 private static int getTTL(String propName, int defaultValue) 89 { 90 String propValue = Security.getProperty(propName); 91 if (propValue == null) 92 return defaultValue; 93 94 return Integer.parseInt(propValue); 95 } 96 97 /** 98 * The cache itself. 99 */ 100 private static HashMap<Object, Entry> cache = new HashMap<Object, Entry>(); 101 102 /** 103 * List of entries which may expire. 104 */ 105 private static LinkedList<Entry> killqueue = new LinkedList<Entry>(); 106 107 /** 108 * Return the hostname for the specified IP address. 109 * 110 * @param addr The IP address as a byte array 111 * 112 * @return The hostname 113 * 114 * @exception UnknownHostException If the reverse lookup fails 115 */ getHostByAddr(byte[] addr)116 public static String getHostByAddr(byte[] addr) throws UnknownHostException 117 { 118 Object key = makeHashableAddress(addr); 119 Entry entry = get(key); 120 if (entry != null) 121 { 122 if (entry.value == null) 123 throw new UnknownHostException(); 124 return (String) entry.value; 125 } 126 127 try 128 { 129 String hostname = VMInetAddress.getHostByAddr(addr); 130 put(new Entry(key, hostname)); 131 return hostname; 132 } 133 catch (UnknownHostException e) 134 { 135 put(new Entry(key, null)); 136 throw e; 137 } 138 } 139 140 /** 141 * Return a list of all IP addresses for the specified hostname. 142 * 143 * @param hostname The hostname 144 * 145 * @return An list of IP addresses as byte arrays 146 * 147 * @exception UnknownHostException If the lookup fails 148 */ getHostByName(String hostname)149 public static byte[][] getHostByName(String hostname) 150 throws UnknownHostException 151 { 152 Entry entry = get(hostname); 153 if (entry != null) 154 { 155 if (entry.value == null) 156 throw new UnknownHostException(); 157 return (byte[][]) entry.value; 158 } 159 160 try 161 { 162 byte[][] addrs = VMInetAddress.getHostByName(hostname); 163 put(new Entry(hostname, addrs)); 164 return addrs; 165 } 166 catch (UnknownHostException e) 167 { 168 put(new Entry(hostname, null)); 169 throw e; 170 } 171 } 172 173 /** 174 * Convert an IP address expressed as a byte array into something 175 * we can use as a hashtable key. 176 */ makeHashableAddress(byte[] addr)177 private static Object makeHashableAddress(byte[] addr) 178 { 179 char[] chars = new char[addr.length]; 180 for (int i = 0; i < addr.length; i++) 181 chars[i] = (char) addr[i]; 182 return new String(chars); 183 } 184 185 /** 186 * Return the entry in the cache associated with the supplied key, 187 * or <code>null</code> if the cache does not contain an entry 188 * associated with this key. 189 */ get(Object key)190 private static synchronized Entry get(Object key) 191 { 192 reap(); 193 return (Entry) cache.get(key); 194 } 195 196 /** 197 * Insert the supplied entry into the cache. 198 */ put(Entry entry)199 private static synchronized void put(Entry entry) 200 { 201 reap(); 202 if (entry.expires != 0) 203 { 204 if (entry.expires != -1) 205 killqueue.add(entry); 206 cache.put(entry.key, entry); 207 } 208 } 209 210 /** 211 * Clear expired entries. This method is not synchronized, so 212 * it must only be called by methods that are. 213 */ reap()214 private static void reap() 215 { 216 if (!killqueue.isEmpty()) 217 { 218 long now = System.currentTimeMillis(); 219 220 Iterator iter = killqueue.iterator(); 221 while (iter.hasNext()) 222 { 223 Entry entry = (Entry) iter.next(); 224 if (entry.expires > now) 225 break; 226 cache.remove(entry.key); 227 iter.remove(); 228 } 229 } 230 } 231 232 /** 233 * An entry in the cache. 234 */ 235 private static class Entry 236 { 237 /** 238 * The key by which this entry is referenced. 239 */ 240 public final Object key; 241 242 /** 243 * The entry itself. A null value indicates a failed lookup. 244 */ 245 public final Object value; 246 247 /** 248 * The time when this cache entry expires. If set to -1 then 249 * this entry will never expire. If set to 0 then this entry 250 * expires immediately and will not be inserted into the cache. 251 */ 252 public final long expires; 253 254 /** 255 * Constructor. 256 */ Entry(Object key, Object value)257 public Entry(Object key, Object value) 258 { 259 this.key = key; 260 this.value = value; 261 262 int ttl = value != null ? POSITIVE_TTL : NEGATIVE_TTL; 263 if (ttl < 1) 264 expires = ttl; 265 else 266 expires = System.currentTimeMillis() + ttl * 1000; 267 } 268 } 269 } 270