ares_send - Initiate a DNS query


#include <ares.h>
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)


The ares_send function initiates a DNS query on the name service channel identified by channel. The parameters qbuf and qlen give the DNS query, which should already have been formatted according to the DNS protocol. 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 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.

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 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 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.

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 abuf points to a result buffer of length alen. If the query did not complete, abuf will be NULL and alen will be 0.

Unless the flag ARES_FLAG_NOCHECKRESP was set at channel initialization time, ares_send will normally ignore responses whose questions do not match the questions in qbuf, as well as responses with reply codes of SERVFAIL, NOTIMP, and REFUSED. Unlike other query functions in the ares library, however, ares_send does 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.

See also

ares_process(3), ares_dns_record(3)


Greg Hudson, MIT Information Systems
Copyright 1998 by the Massachusetts Institute of Technology.

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