ares_set_server_state_callback - Function for setting a server state callback


#include <ares.h>
typedef void (*ares_server_state_callback)(const char *server_string,
                                           ares_bool_t success,
                                           int flags,
                                           void *data);
void ares_set_server_state_callback(ares_channel_t *channel,
                                    ares_server_state_callback callback,
                                    void *user_data);


The ares_set_server_state_callback function sets a callback function callback in the given ares channel handle channel that is invoked whenever a query on the channel completes. This includes both successful and unsuccessful queries (including hard errors and timeouts). The callback function is invoked with a number of parameters describing the query, as follows.

The server_string parameter indicates the DNS server that was used for the query, given as a string with the same format returned by ares_get_servers_csv.

The success parameter indicates whether the query succeeded or not. It is set to ARES_TRUE on success and ARES_FALSE on failure.

The flags parameter is a bitmask of flags describing various aspects of the query (for example whether the query used UDP or TCP). These are described below.

The data parameter is a reference to the custom user data user_data that was passed to ares_set_server_state_callback when setting the server state callback.

The server state callback can be used by applications to monitor the state of the DNS servers used by an ares channel. For example, it can be used to track metrics about the numbers and types of queries sent to each server or to detect when a server is uncontactable or unhealthy.


ARES_SERV_STATE_UDP Indicates that the query was tried over UDP.

ARES_SERV_STATE_TCP Indicates that the query was tried over TCP.


This function was first introduced in c-ares version 1.29.0.

See also


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