ares_query - Initiate a single-question DNS query
typedef void (*ares_callback)(void *arg, int status,
int timeouts, unsigned char *abuf, int alen)
void ares_query(ares_channel channel, const char *name,
int dnsclass, int type, ares_callback callback,
function initiates a single-question DNS query on the name
service channel identified by channel
. The parameter name
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
give the class and type of the query using the values defined in
. 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
(3) or ares_destroy
The callback argument arg
is copied from the ares_query
. The callback argument status
indicates whether the query
succeeded and, if not, how it failed. It may have any of the following values:
- The query completed successfully.
- The query completed but contains no answers.
- The query completed but the server claims that the query was
- 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.)
- The query completed but the queried-for domain name was not found.
- 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)
- 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
- 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.
- No name servers responded within the timeout period.
- No name servers could be contacted.
- Memory was exhausted.
- The query was cancelled.
- 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
a result buffer of length alen
. If the query did not complete,
will be NULL and alen
will be 0.
Greg Hudson, MIT Information Systems
Copyright 1998 by the Massachusetts Institute of Technology.