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package com.sun.jndi.dns;


import java.net.MalformedURLException;
import java.util.ArrayList;
import java.util.Hashtable;
import java.util.List;

import javax.naming.*;
import javax.naming.spi.*;

import com.sun.jndi.toolkit.url.UrlUtil;
import sun.net.dns.ResolverConfiguration;       // available since 1.4.1


A DnsContextFactory serves as the initial context factory for DNS.

When an initial context is being created, the environment property "java.naming.provider.url" should contain a DNS pseudo-URL (see DnsUrl) or a space-separated list of them. Multiple URLs must all have the same domain value. If the property is not set, the default "dns:" is used.

Author:Scott Seligman
/** * A DnsContextFactory serves as the initial context factory for DNS. * * <p> When an initial context is being created, the environment * property "java.naming.provider.url" should contain a DNS pseudo-URL * (see DnsUrl) or a space-separated list of them. Multiple URLs must * all have the same domain value. * If the property is not set, the default "dns:" is used. * * @author Scott Seligman */
public class DnsContextFactory implements InitialContextFactory { private static final String DEFAULT_URL = "dns:"; private static final int DEFAULT_PORT = 53; public Context getInitialContext(Hashtable<?,?> env) throws NamingException { if (env == null) { env = new Hashtable<>(5); } return urlToContext(getInitCtxUrl(env), env); } public static DnsContext getContext(String domain, String[] servers, Hashtable<?,?> env) throws NamingException { return new DnsContext(domain, servers, env); } /* * "urls" are used to determine the servers, but any domain * components are overridden by "domain". */ public static DnsContext getContext(String domain, DnsUrl[] urls, Hashtable<?,?> env) throws NamingException { String[] servers = serversForUrls(urls); DnsContext ctx = getContext(domain, servers, env); if (platformServersUsed(urls)) { ctx.setProviderUrl(constructProviderUrl(domain, servers)); } return ctx; } /* * Public for use by product test suite. */ public static boolean platformServersAvailable() { return !filterNameServers( ResolverConfiguration.open().nameservers(), true ).isEmpty(); } private static Context urlToContext(String url, Hashtable<?,?> env) throws NamingException { DnsUrl[] urls; try { urls = DnsUrl.fromList(url); } catch (MalformedURLException e) { throw new ConfigurationException(e.getMessage()); } if (urls.length == 0) { throw new ConfigurationException( "Invalid DNS pseudo-URL(s): " + url); } String domain = urls[0].getDomain(); // If multiple urls, all must have the same domain. for (int i = 1; i < urls.length; i++) { if (!domain.equalsIgnoreCase(urls[i].getDomain())) { throw new ConfigurationException( "Conflicting domains: " + url); } } return getContext(domain, urls, env); } /* * Returns all the servers specified in a set of URLs. * If a URL has no host (or port), the servers configured on the * underlying platform are used if possible. If no configured * servers can be found, then fall back to the old behavior of * using "localhost". * There must be at least one URL. */ private static String[] serversForUrls(DnsUrl[] urls) throws NamingException { if (urls.length == 0) { throw new ConfigurationException("DNS pseudo-URL required"); } List<String> servers = new ArrayList<>(); for (int i = 0; i < urls.length; i++) { String server = urls[i].getHost(); int port = urls[i].getPort(); if (server == null && port < 0) { // No server or port given, so look to underlying platform. // ResolverConfiguration does some limited caching, so the // following is reasonably efficient even if called rapid-fire. List<String> platformServers = filterNameServers( ResolverConfiguration.open().nameservers(), false); if (!platformServers.isEmpty()) { servers.addAll(platformServers); continue; // on to next URL (if any, which is unlikely) } } if (server == null) { server = "localhost"; } servers.add((port < 0) ? server : server + ":" + port); } return servers.toArray(new String[servers.size()]); } /* * Returns true if serversForUrls(urls) would make use of servers * from the underlying platform. */ private static boolean platformServersUsed(DnsUrl[] urls) { if (!platformServersAvailable()) { return false; } for (int i = 0; i < urls.length; i++) { if (urls[i].getHost() == null && urls[i].getPort() < 0) { return true; } } return false; } /* * Returns a value for the PROVIDER_URL property (space-separated URL * Strings) that reflects the given domain and servers. * Each server is of the form "server[:port]". * There must be at least one server. * IPv6 literal host names include delimiting brackets. */ private static String constructProviderUrl(String domain, String[] servers) { String path = ""; if (!domain.equals(".")) { try { path = "/" + UrlUtil.encode(domain, "ISO-8859-1"); } catch (java.io.UnsupportedEncodingException e) { // assert false : "ISO-Latin-1 charset unavailable"; } } StringBuilder sb = new StringBuilder(); for (int i = 0; i < servers.length; i++) { if (i > 0) { sb.append(' '); } sb.append("dns://").append(servers[i]).append(path); } return sb.toString(); } /* * Reads environment to find URL(s) of initial context. * Default URL is "dns:". */ private static String getInitCtxUrl(Hashtable<?,?> env) { String url = (String) env.get(Context.PROVIDER_URL); return ((url != null) ? url : DEFAULT_URL); }
Removes any DNS server that's not permitted to access
Params:
  • input – the input server[:port] list, must not be null
  • oneIsEnough – return output once there exists one ok
Returns:the filtered list, all non-permitted input removed
/** * Removes any DNS server that's not permitted to access * @param input the input server[:port] list, must not be null * @param oneIsEnough return output once there exists one ok * @return the filtered list, all non-permitted input removed */
private static List<String> filterNameServers(List<String> input, boolean oneIsEnough) { SecurityManager security = System.getSecurityManager(); if (security == null || input == null || input.isEmpty()) { return input; } else { List<String> output = new ArrayList<>(); for (String platformServer: input) { int colon = platformServer.indexOf(':', platformServer.indexOf(']') + 1); int p = (colon < 0) ? DEFAULT_PORT : Integer.parseInt( platformServer.substring(colon + 1)); String s = (colon < 0) ? platformServer : platformServer.substring(0, colon); try { security.checkConnect(s, p); output.add(platformServer); if (oneIsEnough) { return output; } } catch (SecurityException se) { continue; } } return output; } } }