C++ Resolve a host IP address from a URL

Question

How can I resolve a host IP address, given a URL in Visual C++?

Answer 1

gethostbyname?

Answer 2

Parse the URL to get the host name. Then call gethostbyname or the corresponding API on your platform to get the IP address(es). If you are parsing a HTTP header, look for the HostName header to determine the host name.

Answer 3

struct hostent *he;

if ((he = gethostbyname("localhost") == NULL) {
    // Handle error: Failed
}

The IP address will be in he->h_addr. Works on both windows, linux and most likely macos.

Answer 4

To use the socket functions under Windows, you have to start by calling WSAStartup, specifying the version of Winsock you want (for your purposes, 1.1 will work fine). Then you can call gethostbyname to get the address of the host. When you're done, you're supposed to call WSACleanup. Putting that all together, you get something like this:

#include <windows.h>
#include <winsock.h>
#include <iostream>
#include <iterator>
#include <exception>
#include <algorithm>
#include <iomanip>

class use_WSA { 
    WSADATA d; 
    WORD ver;
public:
    use_WSA() : ver(MAKEWORD(1,1)) { 
        if ((WSAStartup(ver, &d)!=0) || (ver != d.wVersion))
            throw(std::runtime_error("Error starting Winsock"));
    }
    ~use_WSA() { WSACleanup(); }    
};

int main(int argc, char **argv) {
    if ( argc < 2 ) {
        std::cerr << "Usage: resolve <hostname>";
        return EXIT_FAILURE;
    }

    try { 
        use_WSA x;

        hostent *h = gethostbyname(argv[1]);
        unsigned char *addr = reinterpret_cast<unsigned char *>(h->h_addr_list[0]);
        std::copy(addr, addr+4, std::ostream_iterator<unsigned int>(std::cout, "."));
    }
    catch (std::exception const &exc) {
        std::cerr << exc.what() << "\n";
        return EXIT_FAILURE;
    }

    return 0;
}

Edit: removed code to set the base to 16 -- IP addresses are usually printed in decimal.