ARES_GETADDRINFO(3)

NAME
ares_getaddrinfo - Initiate a host query by name and service

SYNOPSIS
#include <ares.h>
typedef void (*ares_addrinfo_callback)(void *arg, int status,
                                       int timeouts,
                                       struct ares_addrinfo *result)
void ares_getaddrinfo(ares_channel_t *channel, const char *name,
                      const char* service,
                      const struct ares_addrinfo_hints *hints,
                      ares_addrinfo_callback callback, void *arg)

DESCRIPTION
The ares_getaddrinfo(3) function initiates a host query by name on the name service channel identified by channel. The name and service parameters give the hostname and service as NULL-terminated C strings. The hints parameter is an ares_addrinfo_hints structure:

        struct ares_addrinfo_hints {
          int ai_flags;
          int ai_family;
          int ai_socktype;
          int ai_protocol;
        };

ai_family
    Specifies desired address family. AF_UNSPEC means return both AF_INET and AF_INET6.

ai_socktype
    Specifies desired socket type, for example SOCK_STREAM or SOCK_DGRAM. Setting this to 0 means any type.

ai_protocol
    Setting this to 0 means any protocol.

ai_flags
    Specifies additional options, see below.

ARES_AI_NUMERICSERV
    If this option is set service field will be treated as a numeric value.

ARES_AI_CANONNAME
    The ares_addrinfo structure will return a canonical names list.

ARES_AI_NOSORT
    Result addresses will not be sorted and no connections to resolved addresses will be attempted.

ARES_AI_ENVHOSTS
    Read hosts file path from the environment variable CARES_HOSTS .

When the query is complete or has failed, the ares library will invoke callback. Completion or failure of the query may happen immediately, or may happen during a later call to ares_process(3), ares_destroy(3) or ares_cancel(3).

If this is called from a thread other than which the main program event loop is running, care needs to be taken to ensure any file descriptor lists are updated immediately within the eventloop. When the associated callback is called, it is called with a channel lock so care must be taken to ensure any processing is minimal to prevent DNS channel stalls.

The callback argument arg is copied from the ares_getaddrinfo(3) argument arg. The callback argument status indicates whether the query succeeded and, if not, how it failed. It may have any of the following values:

ARES_SUCCESS
    The host lookup completed successfully.

ARES_ENOTIMP
    The ares library does not know how to find addresses of type family.

ARES_ENOTFOUND
    The name was not found.

ARES_ENOMEM
    Memory was exhausted.

ARES_ESERVICE
    The textual service name provided could not be dereferenced into a port.

ARES_ECANCELLED
    The query was cancelled.

ARES_EDESTRUCTION
    The name service channel channel is being destroyed; the query will not be completed.

On successful completion of the query, the callback argument result points to a struct ares_addrinfo which contains two linked lists, one with resolved addresses and another with canonical names. Also included is the official name of the host (analogous to gethostbyname() h_name).

        struct ares_addrinfo {
          struct ares_addrinfo_cname *cnames;
          struct ares_addrinfo_node  *nodes;
          char *name;
        };

ares_addrinfo_node structure is similar to RFC3493 addrinfo, but without canonname and with extra ttl field.

        struct ares_addrinfo_node {
          int                        ai_ttl;
          int                        ai_flags;
          int                        ai_family;
          int                        ai_socktype;
          int                        ai_protocol;
          ares_socklen_t             ai_addrlen;
          struct sockaddr           *ai_addr;
          struct ares_addrinfo_node *ai_next;
        };

ares_addrinfo_cname structure is a linked list of CNAME records where ttl is a time to live alias is a label of the resource record and name is a value (canonical name) of the resource record. See RFC2181 10.1.1. CNAME terminology.

        struct ares_addrinfo_cname {
          int                         ttl;
          char                       *alias;
          char                       *name;
          struct ares_addrinfo_cname *next;
        };

The reserved memory has to be deleted by ares_freeaddrinfo(3). The result is sorted according to RFC6724 except:
 - Rule 3 (Avoid deprecated addresses)
 - Rule 4 (Prefer home addresses)
 - Rule 7 (Prefer native transport) Please note that the function will attempt a connection on each of the resolved addresses as per RFC6724.

AVAILABILITY
This function was added in c-ares 1.16.0, released in March 2020.

SEE ALSO
ares_freeaddrinfo(3)

AUTHOR
Christian Ammer
Andrew Selivanov <andrew.selivanov@gmail.com>