IPv6 Compatibility Issues with Low-Level C API's [closed] - objective-c

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Apple requires IPv6 Compatibility. Seems a little premature, but I suppose someone has to force it's wide adoption.
I have a client who has some legacy code that will no longer be approved for the App Store because of this incompatibility. I did not write the software by myself, and specifically have never touched this part of the code. I am not much of a ninja when it comes to low-level C networking.
I told the client that part of the problem was that they had an IPv4-only server and had us hardcode an IPv4 address to that server. I updated the IPv4 address to a domain name and told them their servers had to support IPv6. So, they moved them all over, and flipped the switch before any testing could be done. I was notified a few days ago that all their software on the store no longer worked. That's the predicament we are in.
Here is one of potentially many issues that I could use some help on.
Not only does the server that is connected to the port not respond to IPv6, but there are some lower level APIs that are being used that are incompatible.
The first thing I encountered is the use of gethostbyname(). Apparently this is not IPv6 capable. I have been trying to fix it with getaddrinfo() but my sockaddr's are not quite the same.
The second issue I can see is that apparently I need to be using AF_UNSPEC instead of AF_INET. So I am attempting to open a socket in the following way:
int sockfd = socket(AF_UNSPEC, SOCK_STREAM, 0);
I always get -1.
If I open a sock using AF_INET I get further, but then I have this issues:
otherAddr sockaddr * 0x618000220680 0x0000618000220680
sa_len __uint8_t '\x1c'
sa_family sa_family_t '\x1e'
sa_data char [14] "\x13\x88"
otherAddrCast sockaddr * 0x7000052d3c28 0x00007000052d3c28
sa_len __uint8_t '\0'
sa_family sa_family_t '\0'
sa_data char [14] "\x13\x88\"Ԛ\\"
So some of the sa_data is the same. and if I don't cast the sockaddr_in to a struct sockaddr* then the sa_data is the same.
Problem is I don't know what most of this means. Help?
EDIT: Here is my code. Assume that I am only getting 1 addr back (because I am)
struct hostent* server = gethostbyname([_hostname UTF8String]);
struct addrinfo *ai;
struct addrinfo hints;
memset(&hints, 0x00, sizeof(hints));
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_STREAM;
NSString* portString = [NSString stringWithFormat:#"%d", _port];
getaddrinfo([_hostname UTF8String], [portString cStringUsingEncoding:kCFStringEncodingASCII] , &hints, &ai);
struct sockaddr_in addr = {0};
addr.sin_family = AF_INET;
bcopy(server->h_addr, &addr.sin_addr.s_addr, server->h_length);
addr.sin_port = htons(_port);
int sockfd = socket(AF_INET, SOCK_STREAM, 0);
_addr = (struct sockaddr*)&addr;
int status = connect(sockfd, (struct sockaddr*)&addr, sizeof(addr));
int status2 = connect(socket(ai->ai_family,ai->ai_socktype,0), ai->ai_addr, ai->ai_addrlen);

Answer 1

You are NOT supposed to use AF_UNSPEC with socket() at all. You MUST use either AF_INET (IPv4) or AF_INET6 (IPv6).
You can use AF_UNSPEC with the hints input parameter of getaddrinfo() to indicate that you are willing to accept both IPv4 and IPv6 addresses as output. The actual addresses in the output will be either AF_INET or AF_INET6. Or, you can set the hints to AF_INET for IPv4-only output. Or AF_INET6 for IPv6-only output.
You are supposed to loop through the list that is returned by getaddrinfo(). For each address in the list:
pass its ai_family, ai_socktype, and ai_protocol fields to socket()
then pass its ai_addr and ai_addrlen fields to bind() (servers) or connect() (clients).
Repeat this for all addresses reported for a listening server, and for all addresses reported for a client until one successfully connects.
This way, each socket you create matches the IP version of the address it is working with, and you are passing an appropriate matching sockaddr_in (IPv4) or sockaddr_in6 (IPv6) to bind()/connect().
Once you have successful listening server socket(s), or a successfully connected client socket, if you need to retrieve an IP from a socket using accept(), getsockname() or getpeername(), be sure to pass it a sockaddr_storage struct to fill in. sockaddr_storage is large enough to hold all defined sockaddr-based structures (and there are many). If successful, you can then type-cast it to a sockaddr_in or sockaddr_in6 based on its sa_family field (AF_INET or AF_INET6, respectively). Same goes for other similar functions, like inet_pton() and inet_ntop().
Update: given the client code you have shown, try this instead:
struct addrinfo hints = {0};
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_STREAM;
hints.ai_protocol = IPPROTO_TCP;
NSString* portString = [NSString stringWithFormat:#"%d", _port];
struct addrinfo *ai;
int sockfd = -1;
int status = getaddrinfo([_hostname UTF8String], [portString cStringUsingEncoding:kCFStringEncodingASCII] , &hints, &ai);
if (status == 0) {
sockfd = socket(ai->ai_family, ai->ai_socktype, ai->ai_protocol);
if (sockfd == -1) {
status = errno;
}
else if (connect(sockfd, ai->ai_addr, ai->ai_addrlen) < 0) {
status = errno;
close(sockfd);
sockfd = -1;
}
freeaddrinfo(ai);
}
if (sockfd == -1) {
// handle status error as needed...
}
else {
// use sockfd as needed...
close(sockfd);
}