Java Networking

Internal

• java

Concepts

Networking Properties

https://docs.oracle.com/javase/7/docs/api/java/net/doc-files/net-properties.html

InetAddress

https://docs.oracle.com/javase/8/docs/api/java/net/InetAddress.html

This class represents an Internet Protocol (IP) address. Its subclasses represent either 32 bit IPv4 addresses (Inet4Address) or an 128 bit IPv6 addresses (Inet6Address). The instances incapsulate the numeric address (4 bytes or 16 bytes) and possibly a host name, but not netmask information. The class has accessors that characterize the IP address (isMulticastAddress() etc.).

Inet4Address

https://docs.oracle.com/javase/8/docs/api/java/net/Inet4Address.html

Building a Simple IPV4 InetAddress

InetAddress ipv4Addr = Inet4Address.getByAddress(
    new byte[] {(byte)172, (byte)31, (byte)25, (byte)44}));
assertEquals("/172.31.25.44", ipv4Addr.toString());
ipv4Addr = InetAddress.getByName("172.31.25.44");
assertEquals("/172.31.25.44", ipv4Addr.toString());

Inet6Address

https://docs.oracle.com/javase/8/docs/api/java/net/Inet6Address.html

SocketAddress

https://docs.oracle.com/javase/8/docs/api/java/net/SocketAddress.html

Is an immutable representation of a socket address. It does not have any association with any protocol. The values are used by sockets for binding, connecting or as returned values.

InetSocketAddress

https://docs.oracle.com/javase/8/docs/api/java/net/InetSocketAddress.html

Represents an IP socket address (IP address + port or hostname + port).

NetworkInterface

https://docs.oracle.com/javase/8/docs/api/java/net/NetworkInterface.html

The JVM representation of a network interface available on the host. It is identified by a name, the same name returned by ifconfig -a or ip addr, if the host runs a Unix system. The NetworkInterface instance can be used to get the list of addresses associated with the interface and other characteristics of the interface.

Comparison between the information visible from JVM and directly from the system with ip addr:

1. lo <LOOPBACK, UP> mtu 65536
        hardware address:    N/A
        interface addresses: 0:0:0:0:0:0:0:1%lo/128, 127.0.0.1/8
2. eth0 <UP, MULTICAST> mtu 1500
        hardware address:    08:00:27:2c:e2:de
        interface addresses: fe80:0:0:0:a00:27ff:fe2c:e2de%eth0/64, 10.0.2.15/24
3. eth1 <UP, MULTICAST> mtu 1500
        hardware address:    08:00:27:95:52:f2
        interface addresses: fe80:0:0:0:a00:27ff:fe95:52f2%eth1/64, 172.20.1.11/16

ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 08:00:27:2c:e2:de brd ff:ff:ff:ff:ff:ff
    inet 10.0.2.15/24 brd 10.0.2.255 scope global dynamic eth0
       valid_lft 84095sec preferred_lft 84095sec
    inet6 fe80::a00:27ff:fe2c:e2de/64 scope link
       valid_lft forever preferred_lft forever
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 08:00:27:95:52:f2 brd ff:ff:ff:ff:ff:ff
    inet 172.20.1.11/16 brd 172.20.255.255 scope global eth1
       valid_lft forever preferred_lft forever
    inet6 fe80::a00:27ff:fe95:52f2/64 scope link
       valid_lft forever preferred_lft forever

NetworkInterface code in GitHub:

https://github.com/NovaOrdis/playground/blob/master/java/network/NetworkInterface/src/main/java/io/novaordis/playground/java/network/ni/Main.java

InterfaceAddress

https://docs.oracle.com/javase/8/docs/api/java/net/InterfaceAddress.html

Represents a NetworkInterface address. For an IPv4 address, It consists of an IP address, a subnet mask and a broadcast address. For an IPv6 address, it consists in an IP address and a network prefix length.

Wildcard Address

An address chosen by the kernel. Java network API indicates that a wildcard address is desired by specifying "0.0.0.0".

InetAddress.isAnyLocalAddress() returns true on InetAddress wildcard address representation.

DatagramSocket

https://docs.oracle.com/javase/8/docs/api/java/net/DatagramSocket.html

Datagram sockets are used to send and receive datagram packets. Each packet is individually addressed and routed, so multiple packets sent from one machine may be routed differently and may arrive in any order.

A datagram socket can be bound to a specific local address and port in order to be able to send and receive packets. The packets that are then sent via the socket carry as "source" address and port the IP address and port the socket is bound to.

Most constructors automatically bind the sockets they create to the address and port specified as arguments; one way to create an unbound socket is to use DatagramSocket(((SocketAddress)null). If the constructor does not have an address argument, a wildcard address will be used, and the socket will be bound to a valid local address. If the constructor does not have a port argument, the socket will be bound to any available port.

The binding state of a socket can be checked with isBound(). Once bound, the local address and port the socket is bound to can be checked with getLocalSocketAddress(). The same information is returned individually by getLocalAddress()/getLocalPort(). If the socket is explicitly bound to a null address, then an ephemeral port and a valid local address will be used.

A bound socket can be used to send datagram to any IP address/port.

A datagram socket can be explicitly connected to a specific remote address and port. When a socket is connected to a remote address, packets may only be sent to or received from that address. By default a datagram socket is not connected. If the remote destination to which the socket is connected does not exist, or is otherwise unreachable, and if an ICMP destination unreachable packet has been received for that address, then a subsequent call to send or receive may throw a PortUnreachableException. The connected state of a socket can be checked with isConnected(). When connected, the remote endpoint information can be obtained with getRemoteSocketAddress() or getInetAddress() and getPort().

A socket connected to a multicast address may only be used to send packets.

DatagramSocket Configuration

• SO_RCVBUF
• SO_SNDBUF
• SO_REUSEADDR
• SO_TIMEOUT

MulticastSocket

https://docs.oracle.com/javase/8/docs/api/java/net/MulticastSocket.html

A MulticastSocket is a DatagramSocket, the class inherits from DatagramSocket.

Socket

https://docs.oracle.com/javase/8/docs/api/java/net/Socket.html

https://docs.oracle.com/javase/8/docs/api/java/net/SocketOptions.html

A TCP Socket.

Socket Configuration

• SO_RCVBUF
• SO_SNDBUF
• SO_REUSEADDR
• SO_TIMEOUT