/*
 * Copyright (C) 2012 Square, Inc.
 *
 * 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 okhttp3;

import java.net.InetAddress;
import java.net.UnknownHostException;
import java.util.Arrays;
import java.util.List;

A domain name service that resolves IP addresses for host names. Most applications will use the system DNS service, which is the default. Some applications may provide their own implementation to use a different DNS server, to prefer IPv6 addresses, to prefer IPv4 addresses, or to force a specific known IP address. 
Implementations of this interface must be safe for concurrent use.
/**
 * A domain name service that resolves IP addresses for host names. Most applications will use the
 * {@linkplain #SYSTEM system DNS service}, which is the default. Some applications may provide
 * their own implementation to use a different DNS server, to prefer IPv6 addresses, to prefer IPv4
 * addresses, or to force a specific known IP address.
 *
 * <p>Implementations of this interface must be safe for concurrent use.
 */
public interface Dns {
  
A DNS that uses InetAddress.getAllByName to ask the underlying operating system to lookup IP addresses. Most custom Dns implementations should delegate to this instance. /**
   * A DNS that uses {@link InetAddress#getAllByName} to ask the underlying operating system to
   * lookup IP addresses. Most custom {@link Dns} implementations should delegate to this instance.
   */
  Dns SYSTEM = new Dns() {
    @Override public List<InetAddress> lookup(String hostname) throws UnknownHostException {
      if (hostname == null) throw new UnknownHostException("hostname == null");
      try {
        return Arrays.asList(InetAddress.getAllByName(hostname));
      } catch (NullPointerException e) {
        UnknownHostException unknownHostException =
            new UnknownHostException("Broken system behaviour for dns lookup of " + hostname);
        unknownHostException.initCause(e);
        throw unknownHostException;
      }
    }
  };

  
Returns the IP addresses of hostname, in the order they will be attempted by OkHttp. If a connection to an address fails, OkHttp will retry the connection with the next address until either a connection is made, the set of IP addresses is exhausted, or a limit is exceeded. /**
   * Returns the IP addresses of {@code hostname}, in the order they will be attempted by OkHttp. If
   * a connection to an address fails, OkHttp will retry the connection with the next address until
   * either a connection is made, the set of IP addresses is exhausted, or a limit is exceeded.
   */
  List<InetAddress> lookup(String hostname) throws UnknownHostException;
}