Assigning addresses with wildcards (AF_INET, AF_INET6)

Wildcard addresses simplify local address assignment in the Internet domains AF_INET and AF_INET6.

Assigning an Internet address with a wildcard

You use the bind() function to assign a local name (address) to a socket (see "Assigning a name to a socket ").

Instead of a particular Internet address, you can also specify INADDR_ANY (for AF_INET) or IN6ADDR_ANY (for AF_INET6) as the Internet address. INADDR_ANY and IN6ADDR_ANY are defined as fixed constants in <netinet/in.h>.

When you use bind() to assign a socket s a name whose Internet address is specified as INADDR_ANY or IN6ADDR_ANY, this means:

• Receiving messages:

  • The socket s bound to INADDR_ANY can receive messages over all the IPv4 network interfaces of its host. This allows socket s to receive all messages addressed to the port number of s and any valid IPv4 address of the host on which socket s resides. For example, if the host has IPv4 addresses 128.32.0.4 and 10.0.0.78, a process to which socket s is assigned can accept connection requests which are addressed to 128.32.0.4 or 10.0.0.78.

  • The socket s bound to IN6ADDR_ANY can receive messages over all the IPv4 and IPv6 network interfaces of its host. This allows socket s to receive all messages addressed to the port number of s and any valid IPv4 or IPV6 address of the host on which socket s resides. For example, if the host has IPv4 or IPv6 address 128.32.0.4 or 3FFE:0:0:0:A00:6FF:FE08:9A6B, a task to which socket s is assigned can accept connection requests which are addressed to 128.32.0.4 or 3FFE:0:0:0:A00:6FF:FE08:9A6B.

• Sending messages:

  • The socket s bound to INADRR_ANY can send messages over any IPv4 network interface on its host.

  • The socket s bound to IN6ADDR_ANY can send messages over any network interfaces on its host.

  This allows the socket s bound to INADDR_ANY to address any other socket that can be reached via an IPv4 network interface of the host on which socket s resides.
  The socket s bound to IN6ADDR_ANY, on the other hand, can address any other socket that can be reached via any network interface of the host on which socket s resides.

The following examples show how a process can bind a local name to a socket without an Internet address being specified. The process only has to specify the port number:

For AF_INET:

#include <sys/types.h>
#include <netinet/in.h>
#define MYPORT 2222
...
struct sockaddr_in sin;
int s;
...
s = socket(AF_INET, SOCK_STREAM, 0);
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = htonl(INADDR_ANY); 
sin.sin_port = htons(MYPORT);
bind(s, &sin, sizeof sin);

For AF_INET6:

#include <sys.types.h>
#include <netinet.in.h>
#define MYPORT 2222
...
struct in6_addr inaddr_any = IN6ADDR_ANY_INIT;
struct sockaddr_in6 sin6;
int s;
...
s = socket(AF_INET6, SOCK_STREAM, 0);
memset(&sin6, 0 , sizeof sin6);
sin6.sin6_family = AF_INET6;
memcpy(sin6.sin6_addr.s6_addr,sin6addr_any.s6_addr, 16);
sin6.sin6_port = htons(MYPORT);
bind(s, &sin6, sizeof sin6);

Assigning a port number with a wildcard

A local port can remain unspecified (0 specified). In this case, the system selects a suitable port number for it. The following examples show how a process assigns a socket a local address without specifying the local port number:

For AF_INET:

struct sockaddr_in sin;
...
s = socket(AF_INET; SOCK_STREAM, 0);
sin.sin_family=AF_INET;
sin.sin_addr.s_addr=htonl(INADDR_ANY);
sin.sin_port = htons(0);
bind(s, &sin, sizeof sin);

For AF_INET6:

struct sockaddr_in6 sin6;
struct in6_addr in6addr_any = IN6ADDR_ANY_INIT;
...
s = socket(AF_INET6, SOCK_STREAM, 0);
memset(&sin6, 0 , sizeof sin6);
sin6.sin6_family = AF_INET6;
memcpy(sin6.sin6_addr.s6_addr,in6addr_any.s6_addr, 16);
sin6.sin6_port = htons(0); 
bind(s, &sin6, sizeof sin6);