ARES_GETADDRINFO(3) Introduction to Library Functions 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).

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.

AVAILABILITY

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

SEE ALSO

ares_freeaddrinfo(3)

4 November 2018 ARES_GETADDRINFO(3)