Name

ares_send - Initiate a DNS query

Synopsis

#include <ares.h>
 
typedef void (*ares_callback_dnsrec)(void *arg, ares_status_t status,
                                     size_t timeouts,
                                     const ares_dns_record_t *dnsrec);
 
ares_status_t ares_send_dnsrec(ares_channel_t *channel,
                               const ares_dns_record_t *dnsrec,
                               ares_callback_dnsrec callback,
                               void *arg, unsigned short *qid);
 
typedef void (*ares_callback)(void *arg, int status,
                              int timeouts, unsigned char *abuf,
                              int alen);
 
void ares_send(ares_channel_t *channel, const unsigned char *qbuf,
               int qlen, ares_callback callback, void *arg);
 

Description

The ares_send_dnsrec function initiates a DNS query formatted using the ares_dns_record_t * data structure created via ares_dns_record_create in the dnsrec parameter. The supplied callback in the callback parameter also returns the response using a ares_dns_record_t * data structure.

The ares_send function similarly initiates a DNS query, but instead uses raw binary buffers with fully formatted DNS messages passed in the request via the qbuf and qlen parameters. The supplied callback in the callback parameter also returns the raw binary DNS response in the abuf and alen parameters. This method should be considered deprecated in favor of ares_send_dnsrec.

Both functions take an initialized ares channel identified by channel.

The ares_send_dnsrec also can be supplied an optional output parameter of qid to populate the query id as it was placed on the wire.

The ares_send_dnsrec function returns an ares_status_t response code. This may be useful to know that the query was enqueued properly. The response code does not reflect the result of the query, just the result of the enqueuing of the query.

Completion or failure of the query may happen immediately (even before the function returning), or may happen later as network events are processed.

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_send_dnsrec or ares_send.

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_send_dnsrec or ares_send arg parameter.

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 query completed.

ARES_EBADQUERY The query buffer was poorly formed (was not long enough for a DNS header or was too long for TCP transmission).

ARES_ETIMEOUT No name servers responded within the timeout period.

ARES_ECONNREFUSED No name servers could be contacted.

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.

ARES_ENOSERVER The query will not be completed because no DNS servers were configured on the channel.

ARES_EBADQUERY Misformatted DNS query.

The callback argument timeouts reports how many times a query timed out during the execution of the given request.

If the query completed, the callback argument dnsrec for ares_send_dnsrec or abuf and alen for ares_send will be non-NULL.

Unless the flag ARES_FLAG_NOCHECKRESP was set at channel initialization time, ares_send_dnsrec and ares_send will normally ignore responses whose questions do not match the supplied questions, as well as responses with reply codes of SERVFAIL, NOTIMP, and REFUSED. Unlike other query functions in the ares library, however, ares_send_dnsrec and ares_send do not inspect the header of the reply packet to determine the error status, so a callback status of ARES_SUCCESS does not reflect as much about the response as for other query functions.

Availability

ares_send_dnsrec was introduced in c-ares 1.28.0.

See also

ares_dns_record_create(3), ares_process(3), ares_search(3), ares_dns_record(3)

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