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_t *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, ares_destroy or ares_cancel.
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.
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 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 RFC 3493 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 RFC 2181 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 RFC 6724 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 RFC 6724.
This function was added in c-ares 1.16.0, released in March 2020.
Christian Ammer
Andrew Selivanov <andrew.selivanov@gmail.com>
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