What is sin6_scope_id for the IPv6 loopback address?

Question

I want to create a socket bound to any port on the local machine so I can simulate socketpair() on Windows. When I want this socket to be IPv6, what value should I set for sin6_scope_id? Do I actually have to enumerate the adapters and find the loopback adapter in order to fill in that field?

In other words, what do I need to do with this in order to bind to any local port?

struct sockaddr_in6 addr;
addr.sin6_family = AF_INET6;
addr.sin6_port = htons(0);
addr.sin6_flowinfo = 0;
addr.sin6_addr = in6addr_loopback;
addr.sin6_scope_id = ????;

Answer 1

Some socket gurus might know a direct way to set it. But it's easier to just query the system for it. This will correctly set both "flowinfo" and "scope_id" for you.

General approach is this:

Invoke getifaddrs to get a list of ifaddrs instances.

Enumerate the list of ifaddrs returned from this api until you find the AF_INET6 adapter with the IFF_LOOPBACK bit set on the ifa_flags member. This is the adapter you want to bind to.

Now that you have a pointer to the ifaddrs instance, cast its ifa_addr member to a (sockaddr_in6*). Copy this structure into your own addr and set the port.

Some sample code below:

struct sockaddr_in6 addr = {0};
ifaddrs* pList = NULL;
ifaddrs* pAdapterFound = NULL;
ifaddrs* pAdapter = NULL;
getifaddrs(&pList);
pAdapter = pList;
while (pAdapter)
{
    if ((pAdapter->ifa_addr != NULL) && 
        (family == pAdapter->ifa_addr->sa_family == AF_INET6) &&
        (pAdapter->ifa_flags & IFF_LOOPBACK))
    {
        pAdapterFound = pAdapter;
        break;
    }
    pAdapter = pAdapter->ifa_next;
}

if (pAdapterFound)
{    
    // C++ way to copy the sockaddr_in6 struct
    addr = *(reinterpret_cast<sockaddr_in6*>(pAdapterFound->ifa_addr));  // C++

    // If you are coding in C instead of C++
    // memcpy(&addr, pAdapterFound->ifa_addr, sizeof(addr));

    addr.sin6_port = htons(0); // or whatever port you want to bind to
}

freeifaddrs(pList);

You are welcome to reference a helper function I have called GetSocketAddressForAdapter. You could invoke it as follows:

GetSocketAddressforAdapter(AF_INET6, "::1", 0, &address);