Programming with sockets

The IPv6 sockaddr structure

Due to the increased size of the IPv6 address, two new structures are needed. These structures are:

in6_addr
sockaddr_in6

The in6_addr structure

The in6_addr structure holds a single IPv6 address. The structure is shown below.

   struct in6_addr {
           u_int8_t  s6_addr[16];  /* IPv6 address */
   }

The structure contains an array of sixteen 8-bit elements, that together make up a single 128-bit IPv6 address. The address is usually stored in network byte order.

This structure is declared in netinet/in.h.

The sockaddr_in6 structure

The sockaddr_in6 structure is used to pass address information to the socket function calls that require network address information. The structure is shown below.

   # define SIN6_LEN
   
   struct sockaddr_in6 {
          u_char           sin6_len;      /* length of this structure */
          u_char           sin6_family;   /* AF_INET6                 */
          u_int16m_t       sin6_port;     /* Transport layer port #   */
          u_int32m_t       sin6_flowinfo; /* IPv6 flow information    */
          struct in6_addr  sin6_addr;     /* IPv6 address             */
   };

NOTE: For those familiar with sockaddr_in, note that to support variable length sockets, the sin6_family member has been changed from a 16-bit to an 8-bit data type. This change has also been effected in the sockaddr_in data structure. Binaries built using the older data structures will still work, however, new applications will use the newer version of sockaddr_in (for IPv4 applications) or sockaddr_in6 (for IPv6 applications).

When allocating a buffer to hold the sockaddr_in6 structure always use sizeof(struct sockaddr_in6) because the sockaddr_in6 structure is larger than sockaddr.

In IPv4 two constants, INADDR_ANY and INADDR_LOOPBACK, are provided for system selection of source addresses and loopback. In IPv6, because the address is stored in the in6_addr structure, a different mechanism has to be used as described below.

System selection of source addresses

To enable the system to select the source address for your IPv6 application, use the global variable in6addr_any. For example, to bind to port number 23, and let the system select the source address, you could use the following code:

   struct sockaddr_in6 sin6;
   ...
   sin6.sin6_len = sizeof(sin6);
   sin6.sin6_family = AF_INET6;
   sin6.sin6_flowinfo = 0;
   sin6.sin6_port = htons(23);
   sin6.sin6_addr = in6addr_any;
   ...
   if (bind(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1)
   {
   ...
   }

If you just want to initialize a in6_addr structure, you can use the symbolic constant IN6ADDR_ANY_INIT as shown in the code below.

   struct in6_addr anyaddr = IN6ADDR_ANY_INIT;

NOTE: You cannot use this constant to assign a previously declared in6_addr structure. For example, the following would not compile.

   struct sockaddr_in6 sin6;
   ...
   sin6.sin6_addr = INADDR_ANY_INIT;

INADDR_ANY_INIT and in6addr_any are declared in netinet/in.h.

Loopback

To enable your application to use local UDP and TCP services, use the global variable in6addr_loopback. For example, to open a TCP connection to the local telnet server, you could use the following code:

   struct sockaddr_in6 sin6;
   ...
   sin6.sin6_len = sizeof(sin6);
   sin6.sin6_family = AF_INET6;
   sin6.sin6_flowinfo = 0;
   sin6.sin6_port = htons(23);
   sin6.sin6_addr = in6addr_loopback;
   ...
   if (bind(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1)
   {
   ...
   }

If you just want to initialize a in6_addr structure, you can use the symbolic constant IN6ADDR_LOOPBACK_INIT as shown in the code below.

   struct in6_addr anyaddr = IN6ADDR_LOOPBACK_INIT;

NOTE: You cannot use this constant to assign a previously declared in6_addr structure. For example, the following would not compile.

   struct sockaddr_in6 sin6;
   ...
   sin6.sin6_addr = INADDR_LOOPBACK_INIT;

INADDR_LOOPBACK_INIT and in6addr_loopback are declared in netinet/in.h.