Changing Data Structures for IPv6 Winsock Appications

When adding support for IPv6, you must ensure that your application defines properly sized data structures. The size of an IPv6 address is much larger than an IPv4 address. Structures that are hard-coded to handle the size of an IPv4 address when storing an IP address will cause problems in your application, and must be modified.

Best Practice

The best approach to ensuring that your structures are properly sized is to use the SOCKADDR_STORAGE structure. The SOCKADDR_STORAGE structure is agnostic to IP address version. When the SOCKADDR_STORAGE structure is used to store IP addresses, IPv4 and IPv6 addresses can be properly handled with one code base.

The following example, which is an excerpt taken from the Server.c file found in Appendix B, identifies an appropriate use of the SOCKADDR_STORAGE structure. Notice that the structure, when used properly as this example shows, gracefully handles either an IPv4 or IPv6 address.

#include <winsock2.h>
#include <ws2tcpip.h>
#include <stdio.h>

#pragma comment(lib, "Ws2_32.lib")

#define BUFFER_SIZE 512
#define DEFAULT_PORT "27015"

int main(int argc, char **argv)
{
    char Buffer[BUFFER_SIZE] = {0};
    char *Hostname;
    int Family = AF_UNSPEC;
    int SocketType = SOCK_STREAM;
    char *Port = DEFAULT_PORT;
    char *Address = NULL;
    int i = 0;
    DWORD dwRetval = 0;
    int iResult = 0;
    int FromLen = 0;
    int AmountRead = 0;

    SOCKADDR_STORAGE From;

    WSADATA wsaData;

    ADDRINFO *AddrInfo = NULL;
    ADDRINFO *AI = NULL;

    // Parse arguments
    if (argc >= 1) {
        Hostname = argv[1];
    }    

   // Initialize Winsock
    iResult = WSAStartup(MAKEWORD(2, 2), &wsaData);
    if (iResult != 0) {
        printf("WSAStartup failed: %d\n", iResult);
        return 1;
    }

    From.ss_family = (ADDRESS_FAMILY) Family;
    
    //...
        
        return 0;
}

Code To Avoid

Typically, many applications used the sockaddr structure to store protocol-independent addresses, or the sockaddr_in structure for IP addresses. Neither the sockaddr structure nor the sockaddr_in structure is large enough to hold IPv6 addresses, and therefore both are insufficient if your application is to be IPv6 compatible.

Coding Task

To modify your existing code base from IPv4 to IPv4- and IPv6-interoperability

1. Acquire the Checkv4.exe utility. The utility is included with the Microsoft Windows Software Development Kit (SDK).
2. Run the Checkv4.exe utility against your code. Learn about how to run the Checkv4.exe utility against your files in the section on Using the Checkv4.exe Utility.
3. The utility alerts you to usage of sockaddr or sockaddr_in structures, and provides recommendations on how to replace either with the IPv6 compatible structure SOCKADDR_STORAGE.
4. Replace any such instances, and associated code as appropriate, to use the SOCKADDR_STORAGE structure.

Alternatively, you can search your code base for instances of the sockaddr and sockaddr_in structures, and change all such usage (and other associated code, as appropriate) to the SOCKADDR_STORAGE structure.

Related topics

IPv6 Guide for Windows Sockets Applications

Dual-Stack Sockets for IPv6 Winsock Applications

Function Calls for IPv6 Winsock Applications

Use of Hardcoded IPv4 Addresses

User Interface Issues for IPv6 Winsock Applications

Underlying Protocols for IPv6 Winsock Applications