xref: /libcore/luni/src/main/java/java/net/InetAddress.java

/*
 *  Licensed to the Apache Software Foundation (ASF) under one or more
 *  contributor license agreements.  See the NOTICE file distributed with
 *  this work for additional information regarding copyright ownership.
 *  The ASF 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 java.net;

import android.system.ErrnoException;
import android.system.GaiException;
import android.system.StructAddrinfo;
import dalvik.system.BlockGuard;
import java.io.FileDescriptor;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.ObjectStreamException;
import java.io.ObjectStreamField;
import java.io.Serializable;
import java.nio.ByteOrder;
import java.util.Arrays;
import java.util.Collections;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.CountDownLatch;
import java.util.List;
import libcore.io.IoBridge;
import libcore.io.Libcore;
import libcore.io.Memory;
import static android.system.OsConstants.*;

/**
 * An Internet Protocol (IP) address. This can be either an IPv4 address or an IPv6 address, and
 * in practice you'll have an instance of either {@code Inet4Address} or {@code Inet6Address} (this
 * class cannot be instantiated directly). Most code does not need to distinguish between the two
 * families, and should use {@code InetAddress}.
 *
 * <p>An {@code InetAddress} may have a hostname (accessible via {@code getHostName}), but may not,
 * depending on how the {@code InetAddress} was created.
 *
 * <h4>IPv4 numeric address formats</h4>
 * <p>The {@code getAllByName} method accepts IPv4 addresses in the "decimal-dotted-quad" form only:
 * <ul>
 * <li>{@code "1.2.3.4"} - 1.2.3.4
 * </ul>
 *
 * <h4>IPv6 numeric address formats</h4>
 * <p>The {@code getAllByName} method accepts IPv6 addresses in the following forms (this text
 * comes from <a href="http://www.ietf.org/rfc/rfc2373.txt">RFC 2373</a>, which you should consult
 * for full details of IPv6 addressing):
 * <ul>
 * <li><p>The preferred form is {@code x:x:x:x:x:x:x:x}, where the 'x's are the
 * hexadecimal values of the eight 16-bit pieces of the address.
 * Note that it is not necessary to write the leading zeros in an
 * individual field, but there must be at least one numeral in every
 * field (except for the case described in the next bullet).
 * Examples:
 * <pre>
 *     FEDC:BA98:7654:3210:FEDC:BA98:7654:3210
 *     1080:0:0:0:8:800:200C:417A</pre>
 * </li>
 * <li>Due to some methods of allocating certain styles of IPv6
 * addresses, it will be common for addresses to contain long strings
 * of zero bits.  In order to make writing addresses containing zero
 * bits easier a special syntax is available to compress the zeros.
 * The use of "::" indicates multiple groups of 16-bits of zeros.
 * The "::" can only appear once in an address.  The "::" can also be
 * used to compress the leading and/or trailing zeros in an address.
 *
 * For example the following addresses:
 * <pre>
 *     1080:0:0:0:8:800:200C:417A  a unicast address
 *     FF01:0:0:0:0:0:0:101        a multicast address
 *     0:0:0:0:0:0:0:1             the loopback address
 *     0:0:0:0:0:0:0:0             the unspecified addresses</pre>
 * may be represented as:
 * <pre>
 *     1080::8:800:200C:417A       a unicast address
 *     FF01::101                   a multicast address
 *     ::1                         the loopback address
 *     ::                          the unspecified addresses</pre>
 * </li>
 * <li><p>An alternative form that is sometimes more convenient when dealing
 * with a mixed environment of IPv4 and IPv6 nodes is
 * {@code x:x:x:x:x:x:d.d.d.d}, where the 'x's are the hexadecimal values of
 * the six high-order 16-bit pieces of the address, and the 'd's are
 * the decimal values of the four low-order 8-bit pieces of the
 * address (standard IPv4 representation).  Examples:
 * <pre>
 *     0:0:0:0:0:0:13.1.68.3
 *     0:0:0:0:0:FFFF:129.144.52.38</pre>
 * or in compressed form:
 * <pre>
 *     ::13.1.68.3
 *     ::FFFF:129.144.52.38</pre>
 * </li>
 * </ul>
 * <p>Scopes are given using a trailing {@code %} followed by the scope id, as in
 * {@code 1080::8:800:200C:417A%2} or {@code 1080::8:800:200C:417A%en0}.
 * See <a href="https://www.ietf.org/rfc/rfc4007.txt">RFC 4007</a> for more on IPv6's scoped
 * address architecture.
 *
 * <p>Additionally, for backwards compatibility, IPv6 addresses may be surrounded by square
 * brackets.
 *
 * <h4>DNS caching</h4>
 * <p>In Android 4.0 (Ice Cream Sandwich) and earlier, DNS caching was performed both by
 * InetAddress and by the C library, which meant that DNS TTLs could not be honored correctly.
 * In later releases, caching is done solely by the C library and DNS TTLs are honored.
 *
 * @see Inet4Address
 * @see Inet6Address
 */
public class InetAddress implements Serializable {
    /** Our Java-side DNS cache. */
    private static final AddressCache addressCache = new AddressCache();

    private static final long serialVersionUID = 3286316764910316507L;

    /** Using NetID of NETID_UNSET indicates resolution should be done on default network. */
    private static final int NETID_UNSET = 0;

    private int family;

    byte[] ipaddress;

    String hostName;

    /**
     * Used by the DatagramSocket.disconnect implementation.
     * @hide internal use only
     */
    public static final InetAddress UNSPECIFIED = new InetAddress(AF_UNSPEC, null, null);

    /**
     * Constructs an {@code InetAddress}.
     *
     * Note: this constructor is for subclasses only.
     */
    InetAddress(int family, byte[] ipaddress, String hostName) {
        this.family = family;
        this.ipaddress = ipaddress;
        this.hostName = hostName;
    }

    /**
     * Compares this {@code InetAddress} instance against the specified address
     * in {@code obj}. Two addresses are equal if their address byte arrays have
     * the same length and if the bytes in the arrays are equal.
     *
     * @param obj
     *            the object to be tested for equality.
     * @return {@code true} if both objects are equal, {@code false} otherwise.
     */
    @Override
    public boolean equals(Object obj) {
        if (!(obj instanceof InetAddress)) {
            return false;
        }
        return Arrays.equals(this.ipaddress, ((InetAddress) obj).ipaddress);
    }

    /**
     * Returns the IP address represented by this {@code InetAddress} instance
     * as a byte array. The elements are in network order (the highest order
     * address byte is in the zeroth element).
     *
     * @return the address in form of a byte array.
     */
    public byte[] getAddress() {
        return ipaddress.clone();
    }

    /**
     * Converts an array of byte arrays representing raw IP addresses of a host
     * to an array of InetAddress objects.
     *
     * @param rawAddresses the raw addresses to convert.
     * @param hostName the hostname corresponding to the IP address.
     * @return the corresponding InetAddresses, appropriately sorted.
     */
    private static InetAddress[] bytesToInetAddresses(byte[][] rawAddresses, String hostName)
            throws UnknownHostException {
        // Convert the byte arrays to InetAddresses.
        InetAddress[] returnedAddresses = new InetAddress[rawAddresses.length];
        for (int i = 0; i < rawAddresses.length; i++) {
            returnedAddresses[i] = makeInetAddress(rawAddresses[i], hostName);
        }
        return returnedAddresses;
    }

    /**
     * Gets all IP addresses associated with the given {@code host} identified
     * by name or literal IP address. The IP address is resolved by the
     * configured name service. If the host name is empty or {@code null} an
     * {@code UnknownHostException} is thrown. If the host name is a literal IP
     * address string an array with the corresponding single {@code InetAddress}
     * is returned.
     *
     * @param host the hostname or literal IP string to be resolved.
     * @return the array of addresses associated with the specified host.
     * @throws UnknownHostException if the address lookup fails.
     */
    public static InetAddress[] getAllByName(String host) throws UnknownHostException {
        return getAllByNameImpl(host, NETID_UNSET).clone();
    }

    /**
     * Operates identically to {@code getAllByName} except host resolution is
     * performed on the network designated by {@code netId}.
     *
     * @param host the hostname or literal IP string to be resolved.
     * @param netId the network to use for host resolution.
     * @return the array of addresses associated with the specified host.
     * @throws UnknownHostException if the address lookup fails.
     * @hide internal use only
     */
    public static InetAddress[] getAllByNameOnNet(String host, int netId) throws UnknownHostException {
        return getAllByNameImpl(host, netId).clone();
    }

    /**
     * Returns the InetAddresses for {@code host} on network {@code netId}. The
     * returned array is shared and must be cloned before it is returned to
     * application code.
     */
    private static InetAddress[] getAllByNameImpl(String host, int netId) throws UnknownHostException {
        if (host == null || host.isEmpty()) {
            return loopbackAddresses();
        }

        // Is it a numeric address?
        InetAddress result = parseNumericAddressNoThrow(host);
        if (result != null) {
            result = disallowDeprecatedFormats(host, result);
            if (result == null) {
                throw new UnknownHostException("Deprecated IPv4 address format: " + host);
            }
            return new InetAddress[] { result };
        }

        return lookupHostByName(host, netId).clone();
    }

    private static InetAddress makeInetAddress(byte[] bytes, String hostName) throws UnknownHostException {
        if (bytes.length == 4) {
            return new Inet4Address(bytes, hostName);
        } else if (bytes.length == 16) {
            return new Inet6Address(bytes, hostName, 0);
        } else {
            throw badAddressLength(bytes);
        }
    }

    private static InetAddress disallowDeprecatedFormats(String address, InetAddress inetAddress) {
        // Only IPv4 addresses are problematic.
        if (!(inetAddress instanceof Inet4Address) || address.indexOf(':') != -1) {
            return inetAddress;
        }
        // If inet_pton(3) can't parse it, it must have been a deprecated format.
        // We need to return inet_pton(3)'s result to ensure that numbers assumed to be octal
        // by getaddrinfo(3) are reinterpreted by inet_pton(3) as decimal.
        return Libcore.os.inet_pton(AF_INET, address);
    }

    private static InetAddress parseNumericAddressNoThrow(String address) {
        // Accept IPv6 addresses (only) in square brackets for compatibility.
        if (address.startsWith("[") && address.endsWith("]") && address.indexOf(':') != -1) {
            address = address.substring(1, address.length() - 1);
        }
        StructAddrinfo hints = new StructAddrinfo();
        hints.ai_flags = AI_NUMERICHOST;
        InetAddress[] addresses = null;
        try {
            addresses = Libcore.os.android_getaddrinfo(address, hints, NETID_UNSET);
        } catch (GaiException ignored) {
        }
        return (addresses != null) ? addresses[0] : null;
    }

    /**
     * Returns the address of a host according to the given host string name
     * {@code host}. The host string may be either a machine name or a dotted
     * string IP address. If the latter, the {@code hostName} field is
     * determined upon demand. {@code host} can be {@code null} which means that
     * an address of the loopback interface is returned.
     *
     * @param host
     *            the hostName to be resolved to an address or {@code null}.
     * @return the {@code InetAddress} instance representing the host.
     * @throws UnknownHostException
     *             if the address lookup fails.
     */
    public static InetAddress getByName(String host) throws UnknownHostException {
        return getAllByNameImpl(host, NETID_UNSET)[0];
    }

    /**
     * Operates identically to {@code getByName} except host resolution is
     * performed on the network designated by {@code netId}.
     *
     * @param host
     *            the hostName to be resolved to an address or {@code null}.
     * @param netId the network to use for host resolution.
     * @return the {@code InetAddress} instance representing the host.
     * @throws UnknownHostException if the address lookup fails.
     * @hide internal use only
     */
    public static InetAddress getByNameOnNet(String host, int netId) throws UnknownHostException {
        return getAllByNameImpl(host, netId)[0];
    }

    /**
     * Returns the numeric representation of this IP address (such as "127.0.0.1").
     */
    public String getHostAddress() {
        return Libcore.os.getnameinfo(this, NI_NUMERICHOST); // Can't throw.
    }

    /**
     * Returns the host name corresponding to this IP address. This may or may not be a
     * fully-qualified name. If the IP address could not be resolved, the numeric representation
     * is returned instead (see {@link #getHostAddress}).
     */
    public String getHostName() {
        if (hostName == null) {
            try {
                hostName = getHostByAddrImpl(this).hostName;
            } catch (UnknownHostException ex) {
                hostName = getHostAddress();
            }
        }
        return hostName;
    }

    /**
     * Returns the fully qualified hostname corresponding to this IP address.
     */
    public String getCanonicalHostName() {
        try {
            return getHostByAddrImpl(this).hostName;
        } catch (UnknownHostException ex) {
            return getHostAddress();
        }
    }

    /**
     * Returns an {@code InetAddress} for the local host if possible, or the
     * loopback address otherwise. This method works by getting the hostname,
     * performing a DNS lookup, and then taking the first returned address.
     * For devices with multiple network interfaces and/or multiple addresses
     * per interface, this does not necessarily return the {@code InetAddress}
     * you want.
     *
     * <p>Multiple interface/address configurations were relatively rare
     * when this API was designed, but multiple interfaces are the default for
     * modern mobile devices (with separate wifi and radio interfaces), and
     * the need to support both IPv4 and IPv6 has made multiple addresses
     * commonplace. New code should thus avoid this method except where it's
     * basically being used to get a loopback address or equivalent.
     *
     * <p>There are two main ways to get a more specific answer:
     * <ul>
     * <li>If you have a connected socket, you should probably use
     * {@link Socket#getLocalAddress} instead: that will give you the address
     * that's actually in use for that connection. (It's not possible to ask
     * the question "what local address would a connection to a given remote
     * address use?"; you have to actually make the connection and see.)</li>
     * <li>For other use cases, see {@link NetworkInterface}, which lets you
     * enumerate all available network interfaces and their addresses.</li>
     * </ul>
     *
     * <p>Note that if the host doesn't have a hostname set&nbsp;&ndash; as
     * Android devices typically don't&nbsp;&ndash; this method will
     * effectively return the loopback address, albeit by getting the name
     * {@code localhost} and then doing a lookup to translate that to
     * {@code 127.0.0.1}.
     *
     * @return an {@code InetAddress} representing the local host, or the
     * loopback address.
     * @throws UnknownHostException
     *             if the address lookup fails.
     */
    public static InetAddress getLocalHost() throws UnknownHostException {
        String host = Libcore.os.uname().nodename;
        return lookupHostByName(host, NETID_UNSET)[0];
    }

    /**
     * Gets the hashcode of the represented IP address.
     *
     * @return the appropriate hashcode value.
     */
    @Override
    public int hashCode() {
        return Arrays.hashCode(ipaddress);
    }

    /**
     * Resolves a hostname to its IP addresses using a cache.
     *
     * @param host the hostname to resolve.
     * @param netId the network to perform resolution upon.
     * @return the IP addresses of the host.
     */
    private static InetAddress[] lookupHostByName(String host, int netId)
            throws UnknownHostException {
        BlockGuard.getThreadPolicy().onNetwork();
        // Do we have a result cached?
        Object cachedResult = addressCache.get(host, netId);
        if (cachedResult != null) {
            if (cachedResult instanceof InetAddress[]) {
                // A cached positive result.
                return (InetAddress[]) cachedResult;
            } else {
                // A cached negative result.
                throw new UnknownHostException((String) cachedResult);
            }
        }
        try {
            StructAddrinfo hints = new StructAddrinfo();
            hints.ai_flags = AI_ADDRCONFIG;
            hints.ai_family = AF_UNSPEC;
            // If we don't specify a socket type, every address will appear twice, once
            // for SOCK_STREAM and one for SOCK_DGRAM. Since we do not return the family
            // anyway, just pick one.
            hints.ai_socktype = SOCK_STREAM;
            InetAddress[] addresses = Libcore.os.android_getaddrinfo(host, hints, netId);
            // TODO: should getaddrinfo set the hostname of the InetAddresses it returns?
            for (InetAddress address : addresses) {
                address.hostName = host;
            }
            addressCache.put(host, netId, addresses);
            return addresses;
        } catch (GaiException gaiException) {
            // If the failure appears to have been a lack of INTERNET permission, throw a clear
            // SecurityException to aid in debugging this common mistake.
            // http://code.google.com/p/android/issues/detail?id=15722
            if (gaiException.getCause() instanceof ErrnoException) {
                if (((ErrnoException) gaiException.getCause()).errno == EACCES) {
                    throw new SecurityException("Permission denied (missing INTERNET permission?)", gaiException);
                }
            }
            // Otherwise, throw an UnknownHostException.
            String detailMessage = "Unable to resolve host \"" + host + "\": " + Libcore.os.gai_strerror(gaiException.error);
            addressCache.putUnknownHost(host, netId, detailMessage);
            throw gaiException.rethrowAsUnknownHostException(detailMessage);
        }
    }

    /**
     * Removes all entries from the VM's DNS cache. This does not affect the C library's DNS
     * cache, nor any caching DNS servers between you and the canonical server.
     * @hide
     */
    public static void clearDnsCache() {
        addressCache.clear();
    }

    private static InetAddress getHostByAddrImpl(InetAddress address) throws UnknownHostException {
        BlockGuard.getThreadPolicy().onNetwork();
        try {
            String hostname = Libcore.os.getnameinfo(address, NI_NAMEREQD);
            return makeInetAddress(address.ipaddress.clone(), hostname);
        } catch (GaiException gaiException) {
            throw gaiException.rethrowAsUnknownHostException();
        }
    }

    /**
     * Returns a string containing the host name (if available) and host address.
     * For example: {@code "www.google.com/74.125.224.115"} or {@code "/127.0.0.1"}.
     *
     * <p>IPv6 addresses may additionally include an interface name or scope id.
     * For example: {@code "www.google.com/2001:4860:4001:803::1013%eth0"} or
     * {@code "/2001:4860:4001:803::1013%2"}.
     */
    @Override public String toString() {
        return (hostName == null ? "" : hostName) + "/" + getHostAddress();
    }

    /**
     * Returns true if the string is a valid numeric IPv4 or IPv6 address (such as "192.168.0.1").
     * This copes with all forms of address that Java supports, detailed in the {@link InetAddress}
     * class documentation.
     *
     * @hide used by frameworks/base to ensure that a getAllByName won't cause a DNS lookup.
     */
    public static boolean isNumeric(String address) {
        InetAddress inetAddress = parseNumericAddressNoThrow(address);
        return inetAddress != null && disallowDeprecatedFormats(address, inetAddress) != null;
    }

    /**
     * Returns an InetAddress corresponding to the given numeric address (such
     * as {@code "192.168.0.1"} or {@code "2001:4860:800d::68"}).
     * This method will never do a DNS lookup. Non-numeric addresses are errors.
     *
     * @hide used by frameworks/base's NetworkUtils.numericToInetAddress
     * @throws IllegalArgumentException if {@code numericAddress} is not a numeric address
     */
    public static InetAddress parseNumericAddress(String numericAddress) {
        if (numericAddress == null || numericAddress.isEmpty()) {
            return Inet6Address.LOOPBACK;
        }
        InetAddress result = parseNumericAddressNoThrow(numericAddress);
        result = disallowDeprecatedFormats(numericAddress, result);
        if (result == null) {
            throw new IllegalArgumentException("Not a numeric address: " + numericAddress);
        }
        return result;
    }

    private static InetAddress[] loopbackAddresses() {
        return new InetAddress[] { Inet6Address.LOOPBACK, Inet4Address.LOOPBACK };
    }

    /**
     * Returns the IPv6 loopback address {@code ::1} or the IPv4 loopback address {@code 127.0.0.1}.
     * @since 1.7
     */
    public static InetAddress getLoopbackAddress() {
        return Inet6Address.LOOPBACK;
    }

    /**
     * Returns whether this is the IPv6 unspecified wildcard address {@code ::}
     * or the IPv4 "any" address, {@code 0.0.0.0}.
     */
    public boolean isAnyLocalAddress() {
        return false;
    }

    /**
     * Returns whether this address is a link-local address or not.
     *
     * <p>Valid IPv6 link-local addresses have the prefix {@code fe80::/10}.
     *
     * <p><a href="http://www.ietf.org/rfc/rfc3484.txt">RFC 3484</a>
     * "Default Address Selection for Internet Protocol Version 6 (IPv6)" states
     * that both IPv4 auto-configuration addresses (prefix {@code 169.254/16}) and
     * IPv4 loopback addresses (prefix {@code 127/8}) have link-local scope, but
     * {@link Inet4Address} only considers the auto-configuration addresses
     * to have link-local scope. That is: the IPv4 loopback address returns false.
     */
    public boolean isLinkLocalAddress() {
        return false;
    }

    /**
     * Returns whether this address is a loopback address or not.
     *
     * <p>Valid IPv4 loopback addresses have the prefix {@code 127/8}.
     *
     * <p>The only valid IPv6 loopback address is {@code ::1}.
     */
    public boolean isLoopbackAddress() {
        return false;
    }

    /**
     * Returns whether this address is a global multicast address or not.
     *
     * <p>Valid IPv6 global multicast addresses have the prefix {@code ffxe::/16},
     * where {@code x} is a set of flags and the additional 112 bits make
     * up the global multicast address space.
     *
     * <p>Valid IPv4 global multicast addresses are the range of addresses
     * from {@code 224.0.1.0} to {@code 238.255.255.255}.
     */
    public boolean isMCGlobal() {
        return false;
    }

    /**
     * Returns whether this address is a link-local multicast address or not.
     *
     * <p>Valid IPv6 link-local multicast addresses have the prefix {@code ffx2::/16},
     * where x is a set of flags and the additional 112 bits make up the link-local multicast
     * address space.
     *
     * <p>Valid IPv4 link-local multicast addresses have the prefix {@code 224.0.0/24}.
     */
    public boolean isMCLinkLocal() {
        return false;
    }

    /**
     * Returns whether this address is a node-local multicast address or not.
     *
     * <p>Valid IPv6 node-local multicast addresses have the prefix {@code ffx1::/16},
     * where x is a set of flags and the additional 112 bits make up the link-local multicast
     * address space.
     *
     * <p>There are no valid IPv4 node-local multicast addresses.
     */
    public boolean isMCNodeLocal() {
        return false;
    }

    /**
     * Returns whether this address is a organization-local multicast address or not.
     *
     * <p>Valid IPv6 organization-local multicast addresses have the prefix {@code ffx8::/16},
     * where x is a set of flags and the additional 112 bits make up the link-local multicast
     * address space.
     *
     * <p>Valid IPv4 organization-local multicast addresses have the prefix {@code 239.192/14}.
     */
    public boolean isMCOrgLocal() {
        return false;
    }

    /**
     * Returns whether this address is a site-local multicast address or not.
     *
     * <p>Valid IPv6 site-local multicast addresses have the prefix {@code ffx5::/16},
     * where x is a set of flags and the additional 112 bits make up the link-local multicast
     * address space.
     *
     * <p>Valid IPv4 site-local multicast addresses have the prefix {@code 239.255/16}.
     */
    public boolean isMCSiteLocal() {
        return false;
    }

    /**
     * Returns whether this address is a multicast address or not.
     *
     * <p>Valid IPv6 multicast addresses have the prefix {@code ff::/8}.
     *
     * <p>Valid IPv4 multicast addresses have the prefix {@code 224/4}.
     */
    public boolean isMulticastAddress() {
        return false;
    }

    /**
     * Returns whether this address is a site-local address or not.
     *
     * <p>For the purposes of this method, valid IPv6 site-local addresses have
     * the deprecated prefix {@code fec0::/10} from
     * <a href="http://www.ietf.org/rfc/rfc1884.txt">RFC 1884</a>,
     * <i>not</i> the modern prefix {@code fc00::/7} from
     * <a href="http://www.ietf.org/rfc/rfc4193.txt">RFC 4193</a>.
     *
     * <p><a href="http://www.ietf.org/rfc/rfc3484.txt">RFC 3484</a>
     * "Default Address Selection for Internet Protocol Version 6 (IPv6)" states
     * that IPv4 private addresses have the prefix {@code 10/8}, {@code 172.16/12},
     * or {@code 192.168/16}.
     *
     * @return {@code true} if this instance represents a site-local address,
     *         {@code false} otherwise.
     */
    public boolean isSiteLocalAddress() {
        return false;
    }

    /**
     * Tries to reach this {@code InetAddress}. This method first tries to use
     * ICMP <i>(ICMP ECHO REQUEST)</i>, falling back to a TCP connection
     * on port 7 (Echo) of the remote host.
     *
     * @param timeout
     *            timeout in milliseconds before the test fails if no connection
     *            could be established.
     * @return {@code true} if this address is reachable, {@code false}
     *         otherwise.
     * @throws IOException
     *             if an error occurs during an I/O operation.
     * @throws IllegalArgumentException
     *             if timeout is less than zero.
     */
    public boolean isReachable(int timeout) throws IOException {
        return isReachable(null, 0, timeout);
    }

    /**
     * Tries to reach this {@code InetAddress}. This method first tries to use
     * ICMP <i>(ICMP ECHO REQUEST)</i>, falling back to a TCP connection
     * on port 7 (Echo) of the remote host.
     *
     * @param networkInterface
     *            the network interface on which to connection should be
     *            established.
     * @param ttl
     *            the maximum count of hops (time-to-live).
     * @param timeout
     *            timeout in milliseconds before the test fails if no connection
     *            could be established.
     * @return {@code true} if this address is reachable, {@code false}
     *         otherwise.
     * @throws IOException
     *             if an error occurs during an I/O operation.
     * @throws IllegalArgumentException
     *             if ttl or timeout is less than zero.
     */
    public boolean isReachable(NetworkInterface networkInterface, final int ttl, final int timeout) throws IOException {
        if (ttl < 0 || timeout < 0) {
            throw new IllegalArgumentException("ttl < 0 || timeout < 0");
        }

        // The simple case.
        if (networkInterface == null) {
            return isReachable(this, null, timeout);
        }

        // Try each NetworkInterface in parallel.
        // Use a thread pool Executor?
        List<InetAddress> sourceAddresses = Collections.list(networkInterface.getInetAddresses());
        if (sourceAddresses.isEmpty()) {
            return false;
        }
        final InetAddress destinationAddress = this;
        final CountDownLatch latch = new CountDownLatch(sourceAddresses.size());
        final AtomicBoolean isReachable = new AtomicBoolean(false);
        for (final InetAddress sourceAddress : sourceAddresses) {
            new Thread() {
                @Override public void run() {
                    try {
                        if (isReachable(destinationAddress, sourceAddress, timeout)) {
                            isReachable.set(true);
                            // Wake the main thread so it can return success without
                            // waiting for any other threads to time out.
                            while (latch.getCount() > 0) {
                                latch.countDown();
                            }
                        }
                    } catch (IOException ignored) {
                    }
                    latch.countDown();
                }
            }.start();
        }
        try {
            latch.await();
        } catch (InterruptedException ignored) {
            Thread.currentThread().interrupt(); // Leave the interrupted bit set.
        }
        return isReachable.get();
    }

    private boolean isReachable(InetAddress destination, InetAddress source, int timeout) throws IOException {
        // TODO: try ICMP first (http://code.google.com/p/android/issues/detail?id=20106)
        FileDescriptor fd = IoBridge.socket(true);
        boolean reached = false;
        try {
            if (source != null) {
                IoBridge.bind(fd, source, 0);
            }
            IoBridge.connect(fd, destination, 7, timeout);
            reached = true;
        } catch (IOException e) {
            if (e.getCause() instanceof ErrnoException) {
                // "Connection refused" means the IP address was reachable.
                reached = (((ErrnoException) e.getCause()).errno == ECONNREFUSED);
            }
        }

        IoBridge.closeAndSignalBlockedThreads(fd);

        return reached;
    }

    /**
     * Equivalent to {@code getByAddress(null, ipAddress)}. Handy for addresses with
     * no associated hostname.
     */
    public static InetAddress getByAddress(byte[] ipAddress) throws UnknownHostException {
        return getByAddress(null, ipAddress, 0);
    }

    /**
     * Returns an {@code InetAddress} corresponding to the given network-order
     * bytes {@code ipAddress} and {@code scopeId}.
     *
     * <p>For an IPv4 address, the byte array must be of length 4.
     * For IPv6, the byte array must be of length 16. Any other length will cause an {@code
     * UnknownHostException}.
     *
     * <p>No reverse lookup is performed. The given {@code hostName} (which may be null) is
     * associated with the new {@code InetAddress} with no validation done.
     *
     * <p>(Note that numeric addresses such as {@code "127.0.0.1"} are names for the
     * purposes of this API. Most callers probably want {@link #getAllByName} instead.)
     *
     * @throws UnknownHostException if {@code ipAddress} is null or the wrong length.
     */
    public static InetAddress getByAddress(String hostName, byte[] ipAddress) throws UnknownHostException {
        return getByAddress(hostName, ipAddress, 0);
    }

    private static InetAddress getByAddress(String hostName, byte[] ipAddress, int scopeId) throws UnknownHostException {
        if (ipAddress == null) {
            throw new UnknownHostException("ipAddress == null");
        }
        if (ipAddress.length == 4) {
            return new Inet4Address(ipAddress.clone(), hostName);
        } else if (ipAddress.length == 16) {
            // First check to see if the address is an IPv6-mapped
            // IPv4 address. If it is, then we can make it a IPv4
            // address, otherwise, we'll create an IPv6 address.
            if (isIPv4MappedAddress(ipAddress)) {
                return new Inet4Address(ipv4MappedToIPv4(ipAddress), hostName);
            } else {
                return new Inet6Address(ipAddress.clone(), hostName, scopeId);
            }
        } else {
            throw badAddressLength(ipAddress);
        }
    }

    private static UnknownHostException badAddressLength(byte[] bytes) throws UnknownHostException {
        throw new UnknownHostException("Address is neither 4 or 16 bytes: " + Arrays.toString(bytes));
    }

    private static boolean isIPv4MappedAddress(byte[] ipAddress) {
        // Check if the address matches ::FFFF:d.d.d.d
        // The first 10 bytes are 0. The next to are -1 (FF).
        // The last 4 bytes are varied.
        if (ipAddress == null || ipAddress.length != 16) {
            return false;
        }
        for (int i = 0; i < 10; i++) {
            if (ipAddress[i] != 0) {
                return false;
            }
        }
        if (ipAddress[10] != -1 || ipAddress[11] != -1) {
            return false;
        }
        return true;
    }

    private static byte[] ipv4MappedToIPv4(byte[] mappedAddress) {
        byte[] ipv4Address = new byte[4];
        for (int i = 0; i < 4; i++) {
            ipv4Address[i] = mappedAddress[12 + i];
        }
        return ipv4Address;
    }

    private static final ObjectStreamField[] serialPersistentFields = {
        new ObjectStreamField("address", int.class),
        new ObjectStreamField("family", int.class),
        new ObjectStreamField("hostName", String.class),
    };

    private void writeObject(ObjectOutputStream stream) throws IOException {
        ObjectOutputStream.PutField fields = stream.putFields();
        if (ipaddress == null) {
            fields.put("address", 0);
        } else {
            fields.put("address", Memory.peekInt(ipaddress, 0, ByteOrder.BIG_ENDIAN));
        }
        fields.put("family", family);
        fields.put("hostName", hostName);

        stream.writeFields();
    }

    private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
        ObjectInputStream.GetField fields = stream.readFields();
        int addr = fields.get("address", 0);
        ipaddress = new byte[4];
        Memory.pokeInt(ipaddress, 0, addr, ByteOrder.BIG_ENDIAN);
        hostName = (String) fields.get("hostName", null);
        family = fields.get("family", 2);
    }

    /*
     * The spec requires that if we encounter a generic InetAddress in
     * serialized form then we should interpret it as an Inet4Address.
     */
    private Object readResolve() throws ObjectStreamException {
        return new Inet4Address(ipaddress, hostName);
    }
}