6.2.6 Connecting to the Server Using DNS SRV Records

In the Domain Name System (DNS), a SRV record (service location record) is a type of resource record that enables a client to specify a name that indicates a service, protocol, and domain. A DNS lookup on the name returns a reply containing the names of multiple available servers in the domain that provide the required service. For information about DNS SRV, including how a record defines the preference order of the listed servers, see RFC 2782.

MySQL supports the use of DNS SRV records for connecting to servers. A client that receives a DNS SRV lookup result attempts to connect to the MySQL server on each of the listed hosts in order of preference, based on the priority and weighting assigned to each host by the DNS administrator. A failure to connect occurs only if the client cannot connect to any of the servers.

When multiple MySQL instances, such as a cluster of servers, provide the same service for your applications, DNS SRV records can be used to assist with failover, load balancing, and replication services. It is cumbersome for applications to directly manage the set of candidate servers for connection attempts, and DNS SRV records provide an alternative:

• DNS SRV records enable a DNS administrator to map a single DNS domain to multiple servers. DNS SRV records also can be updated centrally by administrators when servers are added or removed from the configuration or when their host names are changed.

• Central management of DNS SRV records eliminates the need for individual clients to identify each possible host in connection requests, or for connections to be handled by an additional software component. An application can use the DNS SRV record to obtain information about candidate MySQL servers, instead of managing the server information itself.

• DNS SRV records can be used in combination with connection pooling, in which case connections to hosts that are no longer in the current list of DNS SRV records are removed from the pool when they become idle.

MySQL supports use of DNS SRV records to connect to servers in these contexts:

• Several MySQL Connectors implement DNS SRV support; connector-specific options enable requesting DNS SRV record lookup both for X Protocol connections and for classic MySQL protocol connections. For general information, see Connections Using DNS SRV Records. For details, see the documentation for individual MySQL Connectors.

• The C API provides a mysql_real_connect_dns_srv() function that is similar to mysql_real_connect(), except that the argument list does not specify the particular host of the MySQL server to connect to. Instead, it names a DNS SRV record that specifies a group of servers. See mysql_real_connect_dns_srv().

• The mysql client has a --dns-srv-name option to indicate a DNS SRV record that specifies a group of servers. See Section 6.5.1, “mysql — The MySQL Command-Line Client”.

A DNS SRV name consists of a service, protocol, and domain, with the service and protocol each prefixed by an underscore:

_service._protocol.domain

The following DNS SRV record identifies multiple candidate servers, such as might be used by clients for establishing X Protocol connections:

Name                      TTL   Class  Priority Weight Port  Target
_mysqlx._tcp.example.com. 86400 IN SRV 0        5      33060 server1.example.com.
_mysqlx._tcp.example.com. 86400 IN SRV 0        10     33060 server2.example.com.
_mysqlx._tcp.example.com. 86400 IN SRV 10       5      33060 server3.example.com.
_mysqlx._tcp.example.com. 86400 IN SRV 20       5      33060 server4.example.com.

Here, mysqlx indicates the X Protocol service and tcp indicates the TCP protocol. A client can request this DNS SRV record using the name _mysqlx._tcp.example.com. The particular syntax for specifying the name in the connection request depends on the type of client. For example, a client might support specifying the name within a URI-like connection string or as a key-value pair.

A DNS SRV record for classic protocol connections might look like this:

Name                     TTL   Class  Priority Weight  Port Target
_mysql._tcp.example.com. 86400 IN SRV 0        5       3306 server1.example.com.
_mysql._tcp.example.com. 86400 IN SRV 0        10      3306 server2.example.com.
_mysql._tcp.example.com. 86400 IN SRV 10       5       3306 server3.example.com.
_mysql._tcp.example.com. 86400 IN SRV 20       5       3306 server4.example.com.

Here, the name mysql designates the classic MySQL protocol service, and the port is 3306 (the default classic MySQL protocol port) rather than 33060 (the default X Protocol port).

When DNS SRV record lookup is used, clients generally must apply these rules for connection requests (there may be client- or connector-specific exceptions):

• The request must specify the full DNS SRV record name, with the service and protocol names prefixed by underscores.

• The request must not specify multiple host names.

• The request must not specify a port number.

• Only TCP connections are supported. Unix socket files, Windows named pipes, and shared memory cannot be used.

For more information on using DNS SRV based connections in X DevAPI, see Connections Using DNS SRV Records.