/*
* Copyright 2017 The Netty Project
*
* The Netty Project licenses this file to you 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 io.netty.resolver.dns;
import io.netty.util.internal.SocketUtils;
import io.netty.util.internal.UnstableApi;
import io.netty.util.internal.logging.InternalLogger;
import io.netty.util.internal.logging.InternalLoggerFactory;
import javax.naming.Context;
import javax.naming.NamingException;
import javax.naming.directory.DirContext;
import javax.naming.directory.InitialDirContext;
import java.lang.reflect.Method;
import java.net.InetSocketAddress;
import java.net.URI;
import java.net.URISyntaxException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Hashtable;
import java.util.List;
import static io.netty.resolver.dns.DnsServerAddresses.sequential;
/**
* A {@link DnsServerAddressStreamProvider} which will use predefined default DNS servers to use for DNS resolution.
* These defaults do not respect your host's machines defaults.
* <p>
* This may use the JDK's blocking DNS resolution to bootstrap the default DNS server addresses.
*/
@UnstableApi
public final class DefaultDnsServerAddressStreamProvider implements DnsServerAddressStreamProvider {
private static final InternalLogger logger =
InternalLoggerFactory.getInstance(DefaultDnsServerAddressStreamProvider.class);
public static final DefaultDnsServerAddressStreamProvider INSTANCE = new DefaultDnsServerAddressStreamProvider();
private static final List<InetSocketAddress> DEFAULT_NAME_SERVER_LIST;
private static final InetSocketAddress[] DEFAULT_NAME_SERVER_ARRAY;
private static final DnsServerAddresses DEFAULT_NAME_SERVERS;
static final int DNS_PORT = 53;
static {
final List<InetSocketAddress> defaultNameServers = new ArrayList<InetSocketAddress>(2);
// Using jndi-dns to obtain the default name servers.
//
// See:
// - http://docs.oracle.com/javase/8/docs/technotes/guides/jndi/jndi-dns.html
// - http://mail.openjdk.java.net/pipermail/net-dev/2017-March/010695.html
Hashtable<String, String> env = new Hashtable<String, String>();
env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.dns.DnsContextFactory");
env.put("java.naming.provider.url", "dns://");
try {
DirContext ctx = new InitialDirContext(env);
String dnsUrls = (String) ctx.getEnvironment().get("java.naming.provider.url");
String[] servers = dnsUrls.split(" ");
for (String server : servers) {
try {
defaultNameServers.add(SocketUtils.socketAddress(new URI(server).getHost(), DNS_PORT));
} catch (URISyntaxException e) {
logger.debug("Skipping a malformed nameserver URI: {}", server, e);
}
}
} catch (NamingException ignore) {
// Will try reflection if this fails.
}
if (defaultNameServers.isEmpty()) {
try {
Class<?> configClass = Class.forName("sun.net.dns.ResolverConfiguration");
Method open = configClass.getMethod("open");
Method nameservers = configClass.getMethod("nameservers");
Object instance = open.invoke(null);
@SuppressWarnings("unchecked")
final List<String> list = (List<String>) nameservers.invoke(instance);
for (String a: list) {
if (a != null) {
defaultNameServers.add(new InetSocketAddress(SocketUtils.addressByName(a), DNS_PORT));
}
}
} catch (Exception ignore) {
// Failed to get the system name server list via reflection.
// Will add the default name servers afterwards.
}
}
if (!defaultNameServers.isEmpty()) {
if (logger.isDebugEnabled()) {
logger.debug(
"Default DNS servers: {} (sun.net.dns.ResolverConfiguration)", defaultNameServers);
}
} else {
Collections.addAll(
defaultNameServers,
SocketUtils.socketAddress("8.8.8.8", DNS_PORT),
SocketUtils.socketAddress("8.8.4.4", DNS_PORT));
if (logger.isWarnEnabled()) {
logger.warn(
"Default DNS servers: {} (Google Public DNS as a fallback)", defaultNameServers);
}
}
DEFAULT_NAME_SERVER_LIST = Collections.unmodifiableList(defaultNameServers);
DEFAULT_NAME_SERVER_ARRAY = defaultNameServers.toArray(new InetSocketAddress[defaultNameServers.size()]);
DEFAULT_NAME_SERVERS = sequential(DEFAULT_NAME_SERVER_ARRAY);
}
private DefaultDnsServerAddressStreamProvider() {
}
@Override
public DnsServerAddressStream nameServerAddressStream(String hostname) {
return DEFAULT_NAME_SERVERS.stream();
}
/**
* Returns the list of the system DNS server addresses. If it failed to retrieve the list of the system DNS server
* addresses from the environment, it will return {@code "8.8.8.8"} and {@code "8.8.4.4"}, the addresses of the
* Google public DNS servers.
*/
public static List<InetSocketAddress> defaultAddressList() {
return DEFAULT_NAME_SERVER_LIST;
}
/**
* Returns the {@link DnsServerAddresses} that yields the system DNS server addresses sequentially. If it failed to
* retrieve the list of the system DNS server addresses from the environment, it will use {@code "8.8.8.8"} and
* {@code "8.8.4.4"}, the addresses of the Google public DNS servers.
* <p>
* This method has the same effect with the following code:
* <pre>
* DnsServerAddresses.sequential(DnsServerAddresses.defaultAddressList());
* </pre>
* </p>
*/
public static DnsServerAddresses defaultAddresses() {
return DEFAULT_NAME_SERVERS;
}
/**
* Get the array form of {@link #defaultAddressList()}.
* @return The array form of {@link #defaultAddressList()}.
*/
static InetSocketAddress[] defaultAddressArray() {
return DEFAULT_NAME_SERVER_ARRAY.clone();
}
}