Manpages - ares_getaddrinfo.3

ares_getaddrinfo - Initiate a host query by name and service

  #include <ares.h>

  typedef void (*ares_addrinfo_callback)(void *arg, int status,
  int timeouts, struct ares_addrinfo *result)

  void ares_getaddrinfo(ares_channel channel, const char *name,
  const char* service, const struct ares_addrinfo_hints *hints,
  ares_addrinfo_callback callback, void *arg)

The ares_getaddrinfo 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).

The callback argument arg is copied from the ares_getaddrinfo 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_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.

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.

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

*ares_freeaddrinfo*(3)

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