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<?php
/**
* Service discovery using DNS SRV records
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
* http://www.gnu.org/copyleft/gpl.html
*
* @file
*/
/**
* @since 1.29
*/
class DnsSrvDiscoverer {
/**
* @var string
*/
private $service;
/**
* @var string
*/
private $protocol;
/**
* @var string|null
*/
private $domain;
/**
* @var callable
*/
private $resolver;
/**
* Construct a new discoverer for the given domain, service, and protocol.
*
* @param string $service Name of the service to discover.
* @param string $protocol Service protocol. Defaults to 'tcp'
* @param ?string $domain The hostname/domain on which to perform discovery
* of the given service and protocol. Defaults to null which effectively
* performs a query relative to the host's configured search domain.
* @param ?callable $resolver Resolver function. Defaults to using
* dns_get_record. Primarily useful in testing.
*/
public function __construct(
string $service,
string $protocol = 'tcp',
?string $domain = null,
?callable $resolver = null
) {
$this->service = $service;
$this->protocol = $protocol;
$this->domain = $domain;
$this->resolver = $resolver ?? static function ( $srv ) {
return dns_get_record( $srv, DNS_SRV );
};
}
/**
* Queries the resolver for an SRV resource record matching the service,
* protocol, and domain and returns all target/port/priority/weight
* records.
*
* @return array
*/
public function getRecords() {
$result = [];
$records = ( $this->resolver )( $this->getSrvName() );
// Respect RFC 2782 with regard to a single '.' entry denoting a valid
// empty response
if (
!$records
|| ( count( $records ) === 1 && $records[0]['target'] === '.' )
) {
return $result;
}
foreach ( $records as $record ) {
$result[] = [
'target' => $record['target'],
'port' => (int)$record['port'],
'pri' => (int)$record['pri'],
'weight' => (int)$record['weight'],
];
}
return $result;
}
/**
* Performs discovery for the domain, service, and protocol, and returns a
* list of resolved server name/ip and port number pairs sorted by each
* record's priority, with servers of the same priority randomly shuffled.
*
* @return array
*/
public function getServers() {
$records = $this->getRecords();
usort( $records, static function ( $a, $b ) {
if ( $a['pri'] === $b['pri'] ) {
return mt_rand( 0, 1 ) ? 1 : -1;
}
return $a['pri'] - $b['pri'];
} );
$serversAndPorts = [];
foreach ( $records as $record ) {
$serversAndPorts[] = [ $record['target'], $record['port'] ];
}
return $serversAndPorts;
}
/**
* Returns the SRV resource record name.
*/
public function getSrvName(): string {
$srv = "_{$this->service}._{$this->protocol}";
if ( $this->domain === null || $this->domain === '' ) {
return $srv;
}
return "$srv.{$this->domain}";
}
}