19.3.3 Replication Privilege Checks

19.3.3.1 Privileges For The Replication PRIVILEGE_CHECKS_USER Account

19.3.3.2 Privilege Checks For Group Replication Channels

19.3.3.3 Recovering From Failed Replication Privilege Checks

By default, MySQL replication (including Group Replication) does not carry out privilege checks when transactions that were already accepted by another server are applied on a replica or group member. From MySQL 8.0.18, you can create a user account with the appropriate privileges to apply the transactions that are normally replicated on a channel, and specify this as the PRIVILEGE_CHECKS_USER account for the replication applier, using a CHANGE REPLICATION SOURCE TO statement (from MySQL 8.0.23) or CHANGE MASTER TO statement (before MySQL 8.0.23). MySQL then checks each transaction against the user account's privileges to verify that you have authorized the operation for that channel. The account can also be safely used by an administrator to apply or reapply transactions from mysqlbinlog output, for example to recover from a replication error on the channel.

The use of a PRIVILEGE_CHECKS_USER account helps secure a replication channel against the unauthorized or accidental use of privileged or unwanted operations. The PRIVILEGE_CHECKS_USER account provides an additional layer of security in situations such as these:

• You are replicating between a server instance on your organization's network, and a server instance on another network, such as an instance supplied by a cloud service provider.

• You want to have multiple on-premise or off-site deployments administered as separate units, without giving one administrator account privileges on all the deployments.

• You want to have an administrator account that enables an administrator to perform only operations that are directly relevant to the replication channel and the databases it replicates, rather than having wide privileges on the server instance.

You can increase the security of a replication channel where privilege checks are applied by adding one or both of these options to the CHANGE REPLICATION SOURCE TO | CHANGE MASTER TO statement when you specify the PRIVILEGE_CHECKS_USER account for the channel:

• The REQUIRE_ROW_FORMAT option (available from MySQL 8.0.19) makes the replication channel accept only row-based replication events. When REQUIRE_ROW_FORMAT is set, you must use row-based binary logging (binlog_format=ROW) on the source server. In MySQL 8.0.18, REQUIRE_ROW_FORMAT is not available, but the use of row-based binary logging for secured replication channels is still strongly recommended. With statement-based binary logging, some administrator-level privileges might be required for the PRIVILEGE_CHECKS_USER account to execute transactions successfully.

• The REQUIRE_TABLE_PRIMARY_KEY_CHECK option (available from MySQL 8.0.20) makes the replication channel use its own policy for primary key checks. Setting ON means that primary keys are always required, and setting OFF means that primary keys are never required. The default setting, STREAM, sets the session value of the sql_require_primary_key system variable using the value that is replicated from the source for each transaction. When PRIVILEGE_CHECKS_USER is set, setting REQUIRE_TABLE_PRIMARY_KEY_CHECK to either ON or OFF means that the user account does not need session administration level privileges to set restricted session variables, which are required to change the value of sql_require_primary_key. It also normalizes the behavior across replication channels for different sources.

You grant the REPLICATION_APPLIER privilege to enable a user account to appear as the PRIVILEGE_CHECKS_USER for a replication applier thread, and to execute the internal-use BINLOG statements used by mysqlbinlog. The user name and host name for the PRIVILEGE_CHECKS_USER account must follow the syntax described in Section 8.2.4, “Specifying Account Names”, and the user must not be an anonymous user (with a blank user name) or the CURRENT_USER. To create a new account, use CREATE USER. To grant this account the REPLICATION_APPLIER privilege, use the GRANT statement. For example, to create a user account priv_repl, which can be used manually by an administrator from any host in the example.com domain, and requires an encrypted connection, issue the following statements:

mysql> SET sql_log_bin = 0;
mysql> CREATE USER 'priv_repl'@'%.example.com' IDENTIFIED BY 'password' REQUIRE SSL;
mysql> GRANT REPLICATION_APPLIER ON *.* TO 'priv_repl'@'%.example.com';
mysql> SET sql_log_bin = 1;

The SET sql_log_bin statements are used so that the account management statements are not added to the binary log and sent to the replication channels (see Section 15.4.1.3, “SET sql_log_bin Statement”).

Important

The caching_sha2_password authentication plugin is the default for new users created from MySQL 8.0 (for details, see Section 8.4.1.2, “Caching SHA-2 Pluggable Authentication”). To connect to a server using a user account that authenticates with this plugin, you must either set up an encrypted connection as described in Section 19.3.1, “Setting Up Replication to Use Encrypted Connections”, or enable the unencrypted connection to support password exchange using an RSA key pair.

After setting up the user account, use the GRANT statement to grant additional privileges to enable the user account to make the database changes that you expect the applier thread to carry out, such as updating specific tables held on the server. These same privileges enable an administrator to use the account if they need to execute any of those transactions manually on the replication channel. If an unexpected operation is attempted for which you did not grant the appropriate privileges, the operation is disallowed and the replication applier thread stops with an error. Section 19.3.3.1, “Privileges For The Replication PRIVILEGE_CHECKS_USER Account” explains what additional privileges the account needs. For example, to grant the priv_repl user account the INSERT privilege to add rows to the cust table in db1, issue the following statement:

mysql> GRANT INSERT ON db1.cust TO 'priv_repl'@'%.example.com';

You assign the PRIVILEGE_CHECKS_USER account for a replication channel using a CHANGE REPLICATION SOURCE TO statement (from MySQL 8.0.23) or CHANGE MASTER TO statement (before MySQL 8.0.23). If replication is running, issue STOP REPLICA (or before MySQL 8.0.22, STOP SLAVE) before the CHANGE MASTER TO statement, and START REPLICA after it. The use of row-based binary logging is strongly recommended when PRIVILEGE_CHECKS_USER is set, and from MySQL 8.0.19 you can use the statement to set REQUIRE_ROW_FORMAT to enforce this.

When you restart the replication channel, checks on dynamic privileges are applied from that point on. However, static global privileges are not active in the applier's context until you reload the grant tables, because these privileges are not changed for a connected client. To activate static privileges, perform a flush-privileges operation. This can be done by issuing a FLUSH PRIVILEGES statement or by executing a mysqladmin flush-privileges or mysqladmin reload command.

For example, to start privilege checks on the channel channel_1 on a running replica in MySQL 8.0.23 and later, issue the following statements:

mysql> STOP REPLICA FOR CHANNEL 'channel_1';
mysql> CHANGE REPLICATION SOURCE TO
     >    PRIVILEGE_CHECKS_USER = 'priv_repl'@'%.example.com',
     >    REQUIRE_ROW_FORMAT = 1 FOR CHANNEL 'channel_1';
mysql> FLUSH PRIVILEGES;
mysql> START REPLICA FOR CHANNEL 'channel_1';

Prior to MySQL 8.0.23, you can use the statements shown here:

mysql> STOP SLAVE FOR CHANNEL 'channel_1';
mysql> CHANGE MASTER TO
     >    PRIVILEGE_CHECKS_USER = 'priv_repl'@'%.example.com',
     >    REQUIRE_ROW_FORMAT = 1 FOR CHANNEL 'channel_1';
mysql> FLUSH PRIVILEGES;
mysql> START SLAVE FOR CHANNEL 'channel_1';

If you do not specify a channel and no other channels exist, the statement is applied to the default channel. The user name and host name for the PRIVILEGE_CHECKS_USER account for a channel are shown in the Performance Schema replication_applier_configuration table, where they are properly escaped so they can be copied directly into SQL statements to execute individual transactions.

In MySQL 8.0.31 and later, if you are using the Rewriter plugin, you should grant the PRIVILEGE_CHECKS_USER user account the SKIP_QUERY_REWRITE privilege. This prevents statements issued by this user from being rewritten. See Section 7.6.4, “The Rewriter Query Rewrite Plugin”, for more information.

When REQUIRE_ROW_FORMAT is set for a replication channel, the replication applier does not create or drop temporary tables, and so does not set the pseudo_thread_id session system variable. It does not execute LOAD DATA INFILE instructions, and so does not attempt file operations to access or delete the temporary files associated with data loads (logged as a Format_description_log_event). It does not execute INTVAR, RAND, and USER_VAR events, which are used to reproduce the client's connection state for statement-based replication. (An exception is USER_VAR events that are associated with DDL queries, which are executed.) It does not execute any statements that are logged within DML transactions. If the replication applier detects any of these types of event while attempting to queue or apply a transaction, the event is not applied, and replication stops with an error.

You can set REQUIRE_ROW_FORMAT for a replication channel whether or not you set a PRIVILEGE_CHECKS_USER account. The restrictions implemented when you set this option increase the security of the replication channel even without privilege checks. You can also specify the --require-row-format option when you use mysqlbinlog, to enforce row-based replication events in mysqlbinlog output.

Security Context.  By default, when a replication applier thread is started with a user account specified as the PRIVILEGE_CHECKS_USER, the security context is created using default roles, or with all roles if activate_all_roles_on_login is set to ON.

You can use roles to supply a general privilege set to accounts that are used as PRIVILEGE_CHECKS_USER accounts, as in the following example. Here, instead of granting the INSERT privilege for the db1.cust table directly to a user account as in the earlier example, this privilege is granted to the role priv_repl_role along with the REPLICATION_APPLIER privilege. The role is then used to grant the privilege set to two user accounts, both of which can now be used as PRIVILEGE_CHECKS_USER accounts:

mysql> SET sql_log_bin = 0;
mysql> CREATE USER 'priv_repa'@'%.example.com'
                  IDENTIFIED BY 'password'
                  REQUIRE SSL;
mysql> CREATE USER 'priv_repb'@'%.example.com'
                  IDENTIFIED BY 'password'
                  REQUIRE SSL;
mysql> CREATE ROLE 'priv_repl_role';
mysql> GRANT REPLICATION_APPLIER TO 'priv_repl_role';
mysql> GRANT INSERT ON db1.cust TO 'priv_repl_role';
mysql> GRANT 'priv_repl_role' TO
                  'priv_repa'@'%.example.com',
                  'priv_repb'@'%.example.com';
mysql> SET DEFAULT ROLE 'priv_repl_role' TO
                  'priv_repa'@'%.example.com',
                  'priv_repb'@'%.example.com';
mysql> SET sql_log_bin = 1;

Be aware that when the replication applier thread creates the security context, it checks the privileges for the PRIVILEGE_CHECKS_USER account, but does not carry out password validation, and does not carry out checks relating to account management, such as checking whether the account is locked. The security context that is created remains unchanged for the lifetime of the replication applier thread.

Limitation.  In MySQL 8.0.18 only, if the replica mysqld is restarted immediately after issuing a RESET REPLICA statement (due to an unexpected server exit or deliberate restart), the PRIVILEGE_CHECKS_USER account setting, which is held in the mysql.slave_relay_log_info table, is lost and must be respecified. When you use privilege checks in that release, always verify that they are in place after a restart, and respecify them if required. From MySQL 8.0.19, the PRIVILEGE_CHECKS_USER account setting is preserved in this situation, so it is retrieved from the table and reapplied to the channel.