Aresolve provides asynchronous DNS host resolution.
Aresolve resolves and automatically re-resolves DNS information for requests made via the aresolve_handler’s watch_* methods. Whenever resolution information about the watched name changes, the aresolve_handler’s on_aresolve_host() function is called.
For example, an application may wish to track the IP address of a particular infrastructure hostname (e.g., a syslog receiver we’ll call “loghost”). In this example, an IP address object is created to track the current address of the loghost:
#include <assert.h>
#include <eos/agent.h>
#include <eos/aresolve.h>
#include <eos/ip.h>
#include <eos/sdk.h>
class Aresolver : public eos::aresolve_handler, public eos::agent_handler {
public:
explicit Aresolver(eos::sdk & sdk)
: eos::aresolve_handler(sdk.get_aresolve_mgr()),
eos::agent_handler(sdk.get_agent_mgr()) {
}
void on_initialized() {
// Register our interest in the IP address of "loghost"
watch_host("loghost", true);
}
virtual void on_aresolve_host(eos::aresolve_record_host const & record) {
// Called on initial host resolution or updates in host address
if (record.valid()) {
// We should only ever see results for "loghost".
assert(record.qname() == "loghost");
// Get the IP address; we'll assume there's only one in this case.
if (record.addr_v4().size() > 0) { // IPv4
address = record.addr_v4().front();
} else if (record.addr_v6().size() > 0) { // IPv6
address = record.addr_v6().front();
}
// Do something with the new address, like send a syslog message.
} else {
// There was a host resolution error. Inspect record.last_error(),
// which is an EAI_* error from netdb.h.
}
}
private:
eos::ip_addr_t address;
};
The Aresolve handler.
This handler receives callback notifications about objects that are being resolved via watch_* calls.
Public Functions
Starts or stops watching IPv4 and IPv6 addresses for the provided host name.
When host resolution completes or an error occurs, on_aresolve_host() is called. After host resolution completes initially, watched hosts will be refreshed automatically. When changes in the result happen, your handler’s on_aresolve_host(), method is called. In this way, you need only watch hostnames you’re interested in and implement the on_aresolve_host() method to deal with the changes in results.
The hostname to watch
If true, start watching the hostname, else stop watching.
Callback called by Aresolve when host resolution completes.
Called by Aresolve when DNS resolution of a host previously watched by watch_host() completes.
Will be called upon successful updates or otherwise, when either the valid or last update time member attributes change. May be called more than once for a given hostname, if DNS had a temporary failure.
A record containing new or updated information about a watched host.
Public Functions
Aresolve implementation configuration settings.
The short time defines the minimum period between DNS resolutions. (default: 1s)
Returns the long timer, or seconds between repeated DNS queries.
You will receive at most one notification per DNS query (watched host) every aresolve_long_time() number of seconds. (default: 300s)
Sets the Aresolve short timer to the value provided.
Sets the Aresolve long timer to the value provided.
Protected Functions
Private Members
Friends
A base DNS response class.
When receiving an aresolve_record_*, if valid() is false, the last_error() method should be called to receive the EAI_* error (which can be converted to a string with gai_strerror()). If there is no error, accessors in subclasses of aresolve_record_base contain the latest update for the request.
This class is not instantiated or received by user code.
Public Functions
Getter for ‘qname’: the DNS query (request) name.
Getter for ‘last_refresh’: the last refresh time, seconds since boot.
Getter for ‘valid’: the flag indicating whether the request was successful. If false, last_error() will return a non-zero value.
Getter for ‘last_error’: error code reporting the last error, or 0 if the record was resolved. Values are EAI_* constants in netdb.h.
Returns a string representation of the current object’s values.
Protected Functions
Private Members
Friends
A utility stream operator that adds a string representation of aresolve_record_base to the ostream.
A DNS response for a hostname query containing resolved IP addresses received by the on_aresolve_host() receiver method. A host response contains zero or more IPv4 or IPv6 addresses.
Public Functions
Getter for ‘addr_v4’: the resolved IPv4 addresses for the qname().
Getter for ‘addr_v6’: the resolved IPv6 addresses for the qname().
Returns a string representation of the current object’s values.
Friends
A utility stream operator that adds a string representation of aresolve_record_host to the ostream.