winsock2: How to get the ipv4/ipv6 address of a connected client after server side code calls `accept()`

Question

There are other similar questions on this site, but they either do not related to winsock2 or they are suitable only for use with ipv4 address spaces. The default compiler for Visual Studio 2019 produces an error when the ntoa function is used, hence an ipv4 and ipv6 solution is required.

I did once produce the code to do this for a Linux system however I am currently at work and do not have access to that. It may or may not be "copy and paste"-able into a windows environment with winsock2. (Edit: I will of course add that code later this evening, but of course it might not be useful.)

The following contains an example, however this is an example for client side code, not server side code.

https://www.winsocketdotnetworkprogramming.com/winsock2programming/winsock2advancedInternet3c.html

Here, the getaddrinfo() function is used to obtain a structure containing matching ipv4 and ipv6 addresses. To obtain this information there is some interaction with DNS, which is not required in this case.

I have some server code which calls accept() (after bind and listen) to accept a client connection. I want to be able to print the client ip address and port to stdout.

The most closely related question on this site is here. However the answer uses ntoa and is only ipv4 compatible.

What I have so far:

So far I have something sketched out like this:

SOCKET acceptSocket = INVALID_SOCKET;
SOCKADDR_IN addr; // both of these are NOT like standard unix sockets
    // I don't know how they differ and if they can be used with standard
    // unix like function calls (eg: inet_ntop)

int addrlen = sizeof addr;
acceptSocket = accept(listenSocket, (SOCKADDR*)&addr, &addrlen);
if(acceptSocket == INVALID_SOCKET)
{
    // some stuff
}
else
{
    const std::size_t addrbuflen = INET6_ADDRSRTLEN;
    char addrbuf[addrbuflen] = '\0'
    inet_ntop(AF_INET, (void*)addr.sin_addr, (PSTR)addrbuf, addrbuflen);
    // above line does not compile and mixes unix style function calls
    // with winsock2 structures
    std::cout << addrbuf << ':' << addr.sin_port << std::endl;
}

getpeername()

int ret = getpeername(acceptSocket, addrbuf, &addrbuflen);
    // addrbuf cannot convert from char[65] to sockaddr*
if(ret == ???)
{
    // TODO
}

Answer 1

You need to access the SOCKADDR. This is effectively a discriminated union. The first field tells you whether its an IPv4 (==AF_INET) or IPv6 (==AF_INET6) address. Depending on that you cast the addr pointer to be either struct sockaddr_in* or struct sockaddr_in6*, and then read off the IP address from the relevant field.

Answer 2

C++ code snippet in vs2019:

    char* CPortListener::get_ip_str(struct sockaddr* sa, char* s, size_t maxlen)
        {
            switch (sa->sa_family) {
            case AF_INET:
                inet_ntop(AF_INET, &(((struct sockaddr_in*)sa)->sin_addr),
                    s, maxlen);
                break;
        
            case AF_INET6:
                inet_ntop(AF_INET6, &(((struct sockaddr_in6*)sa)->sin6_addr),
                    s, maxlen);
                break;
        
            default:
                strncpy(s, "Unknown AF", maxlen);
                return NULL;
            }
        
            return s;
        }

Example:


   
   {
         ...
         
         char s[INET6_ADDRSTRLEN];
         sockaddr_storage ca;
         socklen_t al = sizeof(ca);
         SOCKET recv = accept(sd, (sockaddr*)&ca, &al);
         pObj->m_ip = get_ip_str(((sockaddr*)&ca),s,sizeof(s));
    }