Networking IPv6 User Guide for JDK/JRE 5.0

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Within the past few years IPv6 has gained much greater acceptance in the industry, especially in certain regions of the world, i. Extensibility, mobility, quality of service, larger address space, auto-configuration, security, multi- homing, anycast and multicast, and renumbering—these are some of the features of IPv6 that make it desirable.

With the release of J2SE 1. We will prove these statements with code examples below and provide additional details on IPv6 support. Using IPv6 in Java is easy; it is transparent and automatic. Unlike in many other languages, no porting is necessary.

In fact, there is no need to even recompile the source files. Consider an example from The Java Tutorial:. You can run the same bytecode for this example in IPv6 mode if both your local host machine and the destination machine taranis are IPv6-enabled.

In contrast, if you wanted the corresponding C program to run in IPv6 mode, you would first need to port it. Here's what would need to happen:. Note that for new applications, 1933 bind before connect and setting the client interface address you write address-family-agnostic data structures, there is no need for porting. Namely, depending on whether your application is written for a dual-stack platform, such as Solaris or Linux, or a single-stack platform, such as Windows, you would need to structure the code differently.

For server-side programming, Java shows a big advantage. You still write the same code as before:. Now, however, if you run the code on an IPv6-enabled machine, you immediately have an IPv6-enabled service.

Notice that on a dual-stack machine, since one socket, the IPv6 socket, will be able to access both IPv4 and IPv6 protocol stacks, you only need to create one socket. Thus this server can potentially support both IPv4 and IPv6 clients. Here you need to create two server sockets, one for IPv6 stack and one for IPv4 stack.

You also need to multiplex on the two sockets to listen to connections from either IPv4 or IPv6 clients. If IPv6 is supported, it will try to use the IPv6 stack. More specifically, on dual-stack systems it will create an IPv6 socket. On separate-stack systems things are much more complicated.

Java will create two sockets, one for IPv4 and one for IPv6 communication. For client-side TCP applications, once the socket is connected, the internet-protocol family type will be fixed, and the extra socket can be closed. For server-side TCP applications, since there is no way to tell from which IP family type the next client request will come, two sockets need to be 1933 bind before connect and setting the client interface address.

For UDP applications, both sockets will be needed for the lifetime of the communication. You don't need to know the following in order to use IPv6 in Java. But if you are curious and what to know what happens under various circumstances, the remainder of this document should provide answers. This is also called anylocal or wildcard address.

We always try to bind to IPv6 anylocal address on a dual-stack machine unless a related system property is set to use IPv4 Stack. The option discussed below is introduced in Draft-ietf-ipngwg-rfcbis It will be supported in the Java 2 platform when it becomes a standard.

However, there is 1933 bind before connect and setting the client interface address new socket option that changes the above behaviour. When this option is turned on, the socket can be used to send and receive IPv6 packets only. By default this option is turned off. Packets with the loopback address must never be sent on a link or forwarded by an IPv6 router.

There are two separate loopback addresses for IPv4 and IPv6 and they are treated as such. This is used for hosts and routers to dynamically tunnel IPv6 packets over IPv4 routing infrastructure. It is meaningful for OS kernel and routers. Java provides a utility method to test it. This is an IPv6 address that is used to represent an 1933 bind before connect and setting the client interface address address.

It allows the native program to use the same address data structure and also the same socket when communicating with both IPv4 and IPv6 nodes. The OS will do the underlying plumbing required to send or receive an IPv4 datagram and to hand it to an IPv6 destination socket, and it will synthesize an IPv4-mapped IPv6 address when needed. For Java, it is used for internal representation; it has no functional role. Java will never return an IPv4-mapped address. It understands IPv4-mapped address syntax, both in byte array and text representation.

However, it will be converted into an IPv4 address. On dual stack machines, system properties are provided for setting the preferred protocol stack—IPv4 or IPv6—as well as the preferred address family types—inet4 or inet6. This setting can be changed through the java.

By default, we would prefer IPv4 addresses over IPv6 addresses, i. There are two reasons for this choice:. This setting can be changed through the system property java. Thus compatibility with the large installed base of IPv4 nodes is crucial for the success of the transition from IPv4 to IPv6. Dual stack, defined in RFCis one of the main mechanisms for guaranteeing a smooth transition. The former is the most relevant piece to the JDK.

However, unless a socket checks for the peers address type, it won't know whether it is talking to an IPv4 or an IPv6 peer. All the internal plumbing and conversion of address types is done by the dual-protocol stack. IPv4-mapped address has significance only at the implementation of a dual-protocol stack. It is used to fake i. At the conceptual level it has no role; its role is limited at the Java API level. Parsing of an IPv4-mapped address 1933 bind before connect and setting the client interface address supported, but an IPv4-mapped address is never returned.

Top row and left column represent various node types attempting to communicate. An x indicates that these nodes can communicate with each other. If host2 wants to talk to host1, it will create a V6 socket.

It then looks up the IP address for host1. Since host1 only has a v4 protocol stack, it will only have IPv4 records in the name lookup service. So host2 will try to reach host1 using an IPv4-mapped address. An IPv4 packet will be sent by host2, and host1 will think it is communicating with a v4 client.

If host2 is the server, it will first create a v6-type socket by defaultthen it will wait for connections. Since host1 supports v4 only, it creates a v4-type socket. They resolves the name for host2. It only gets v4 address for host2, since it doesn't understand IPv6 address. So it connects to host2 using v4 address. A v4 packet will be sent on the wire. On host2, the dual stack will convert the v4 packet into a v6 packet with a v4-mapped address in it and hand it over to the v6 socket.

The server application will handle it as if it is a connection from a v6 node. This class represents an IP address. It provides address storage, name-address translation methods, address conversion methods, as well as address testing methods. Utility methods are added 1933 bind before connect and setting the client interface address check address types and scopes.

Two new subclasses of InetAddress are created: V4- and V6-specific state and behaviors are implemented in these two subclasses. Due to Java's object-oriented nature, an application normally only needs to deal with InetAddress class—through polymorphism it will get the correct behavior. Only when it needs to access protocol-family-specific behaviors, such as in calling an IPv6-only method, or when it cares to know the class types of the IP address, will it ever become aware of Inet4Address and Inet6Address.

You can tell the JDK to use this provider by setting up a few system properties. These system properties are documented in the Java system properties section. In the future, we plan to provide a generic service provider framework so that you can 1933 bind before connect and setting the client interface address your own name service providers. They are serialized as InetAddress objects, and deserialized from InetAddress to Inet4Address to keep backward compatibility.

IPv6 addresses are represented as Inet6Address and are serialized as such. Due to the object-oriented nature of Java, the address types and storage structures are not exposed at the socket API level, so no new APIs are needed. All supported Ipv6 socket options have a IPv4 counterparts.

The two existing methods are used to set or retrieve the network interface used by the current MulticastSocket to send multicast packets i. For IPv4, the interface was indicated by an IP address. Thus we can use the equivalent InetAddress in Java. They will continue to work with IPv6 multicast socket.

To better support the concept of a network interface, we introduced a new class, NetworkInterface. It encapsulate the data representing the state of the network interface, including name and IP addresses and some basic manipulation methods. Thus we have introduced two 1933 bind before connect and setting the client interface address methods for setting the outgoing interface for multicast socket: They take or return a NetworkInterface object.

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This site uses cookies. By continuing to browse the site you are agreeing to our use of cookies. Find out more here. The FTP , or the File Transfer Protocol, makes it possible for users to exchange files between their personal computers and remote servers with the help of specialized software tools called FTP clients. Through FTP software tools users can establish connections with a remote machine of their choice and perform any necessary data transfers.

The FTP connections are executed through certain ports, which are either the default TCP ports or custom ports set by an administrator. Executing an FTP port connection through a client is a two-stage process requiring the use of two different ports. Once the user enters the name of the server and the login credentials in the authorization fields of the FTP client , the FTP connection is established and the FTP control port of the FTP server the default port for sending commands is 21 is opened.

Then a second connection to the server is made by the client, followed by a response of the FTP server from the port for sending data the default data sending port is 20 , when the real file transfer actually begins.

As we've already mentioned, the default command port for FTP connections is port 21, so, it's important to check if your ISP is not blocking the access to that port. To check if your router or ISP is blocking the port 21, you should use telnet. If all these tips don't allow you to connect to the server, try to set your FTP client in passive mode.

What is the difference between an active and a passive FTP connection and how they work is one of the most asked questions. It is good to have an FTP host which provides support for both, because there are cases when one works and the other refuses a connection. So, what is the difference between the active and the passive mode FTP connections?

The second port is the 'data transfer' port where the real file transfer runs. Typically, the command port is set to port 21 and the data transfer port is port 20, but actually, depending on the connection mode, the data transfer port can be changed. When you try to establish an FTP connection in active mode, first you need to check your FTP software's settings if "Active mode" is enabled. In the most popular free FTP programs the active mode is enabled by default. If disabled, set it on and proceed with filling your FTP account details.

Look at the connection log below to see how the connection proceeds. You will notice that the client sends a PORT command which is containing the dynamic port number on which it is listening over the control stream and waits for a connection from the FTP server. But let's explain the things in more detail. First, the client opens a random dynamic port, for example port , and connects to the server's port Then the server connects back to the client's data port using its local data port 20 and starts the transfer.

Passive mode FTP connections are designed to resolve problems with firewalls and routers that didn't allow an active connection to be established. If you can't connect to the server in active mode, you need to enable the "Force Passive mode" option of your FTP Client. Then the client will initiate both connections to the server, which may resolve the problem even if there are some limitations imposed by your ISP's network or your personal firewall.

Let's see how the passive mode works. Then the client connects its command port to port 21 on the server, but instead of using a PORT command, it sends a PASV command which tells the server that the connection is set in passive mode. When it reads the PASV command, the server opens a random dynamic port for example , which is forwarded to port 20 the server's default data transfer port , and sends it back to the client. Then the FTP client initiates the connection from its data port port to the server's data port port and starts the data transfer.

This mode is a great method of resolving problems with firewalls filtering the incoming data port connection to the client from the server. Let's check how the connection log looks like:.

What is an FTP Port?