ares_query - Initiate a single-question DNS query
#include <ares.h> typedef void (*ares_callback)(void *arg, int status, int timeouts, unsigned char *abuf, int alen) void ares_query(ares_channel_t *channel, const char *name, int dnsclass, int type, ares_callback callback, void *arg)
The ares_query function initiates a single-question DNS query on the name service channel identified by channel. The parameter name gives the query name as a NUL-terminated C string of period-separated labels optionally ending with a period; periods and backslashes within a label must be escaped with a backslash. The parameters dnsclass and type give the class and type of the query using the values defined in <arpa/nameser.h>. 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) or ares_destroy(3).
If this is called from a thread other than which the main program event loop is running, care needs to be taken to ensure any file descriptor lists are updated immediately within the eventloop. 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 argument arg is copied from the ares_query 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 successfully.
ARES_ENODATA The query completed but contains no answers.
ARES_EFORMERR The query completed but the server claims that the query was malformatted.
ARES_ESERVFAIL The query completed but the server claims to have experienced a failure. (This code can only occur if the ARES_FLAG_NOCHECKRESP flag was specified at channel initialization time; otherwise, such responses are ignored at the ares_send(3) level.)
ARES_ENOTFOUND The query completed but the queried-for domain name was not found.
ARES_ENOTIMP The query completed but the server does not implement the operation requested by the query. (This code can only occur if the ARES_FLAG_NOCHECKRESP flag was specified at channel initialization time; otherwise, such responses are ignored at the ares_send(3) level.)
ARES_EREFUSED The query completed but the server refused the query. (This code can only occur if the ARES_FLAG_NOCHECKRESP flag was specified at channel initialization time; otherwise, such responses are ignored at the ares_send(3) level.)
ARES_EBADNAME The query name name could not be encoded as a domain name, either because it contained a zero-length label or because it contained a label of more than 63 characters.
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.
The callback argument timeouts reports how many times a query timed out during the execution of the given request.
If the query completed (even if there was something wrong with it, as indicated by some of the above error codes), 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.
Greg Hudson, MIT Information Systems
Copyright 1998 by the Massachusetts Institute of Technology.
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