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ARES_GETADDRINFO

Section: C Library Functions (3)
Updated: 4 November 2018
Index Return to Main Contents

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 NUL-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).

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 may be triggered from a different thread than the one which called ares_getaddrinfo(3).

For integrators running their own event loops and not using ARES_OPT_EVENT_THREAD, care needs to be taken to ensure any file descriptor lists are updated immediately within the eventloop when notified.

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.