How do I perform a DNS look-up using libuv?

Question

I am trying to use libuv to resolve a URL to an IP address. I am using the function uv_getaddrinfo, which passes an addrinfo object to my call-back.

Is this callback where I receive the IP address? How do I extract the IP address from the addrinfo object?

Answer 1

There's utility functions geared towards this, like uv_ipv4_addr and uv_ipv4_name depending on what you want broken out of that structure.

Many LibUV functions take an addrinfo directly, so it's a useful structure to have.

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libuv provides uv_ipv4_name and uv_ipv6_name. The function you choose depends on the addrinfo object you have:

if (addrinfo.ai_family == AF_INET) {
    // ipv4
    char c[17] = { '\0' };
    uv_ip4_name((sockaddr_in*)(addrinfo.ai_addr), c, 16);
    std::cout << c << std::endl;
} else if (addrinfo.ai_family == AF_INET6) {
    // ipv6
    char c[40] = { '\0' };
    uv_ip6_name((sockaddr_in6*)(addrinfo.ai_addr), c, 39);
    std::cout << c << std::endl;
}

You may get multple addrinfo structs from uv_getaddrinfo. These are stored a singly-linked list, where the "next" pointer is addrinfo.ai_next.

This is also helpful: What is the difference between struct addrinfo and struct sockaddr