Networking/NameResolution

Domain Name System

Resolving using getaddrinfo() in applications

The getaddrinfo() function is a dualstack-friendly API to name resolution. It is used by applications to translate host and service names to a linked list of struct addrinfo objects.

And example of getaddrinfo() call:

const char *node = "www.fedoraproject.org";
const char *service = "http";
struct addrinfo hints = {
    .ai_family = AF_UNSPEC,
    .ai_socktype = SOCK_DGRAM,
    .ai_flags = 0,
    .ai_protocol = 0,
    .ai_canonname = NULL,
    .ai_addr = NULL,
    .ai_next = NULL
};
struct addrinfo *result;
int error;

error = getaddrinfo(node, service, &hints, &result);

The input of getaddrinfo() consists of node specification, service specification and further hints.

• node: literal IPv4 or IPv6 address, or a hostname to be resolved
• service: numeric port number or a symbolic service name
• hints.ai_family: enable dualprotocol, IPv4-only or IPv6-only queries
• hints.ai_socktype: select socket type (and thus protocol family)

getaddrinfo() can be futher tweaked with the hints.ai_flags. Other attributes are either not needed (ai_protocol) or not supposed to be set in hints (ai_canonname, ai_addr and ai_next).

On success, the error variable is assigned to 0 and result is pointed to a linked list of one or more result objects.