The client cannot connect to a replication slave, often due to wrong host, port, or network rules; the code was removed after MySQL 8.0.32.
MySQL Error 2024: CR_PROBE_SLAVE_CONNECT signals that the client failed to connect to a replication slave. Check host, port, credentials, and network firewalls. Upgrading past 8.0.32 removes the code, but fixing DNS and permission issues resolves it immediately.
Error connecting to slave: CR_PROBE_SLAVE_CONNECT was removed after 8.0.32.
MySQL returns error code 2024 with the condition name CR_PROBE_SLAVE_CONNECT when a client tries to probe or connect to a replication slave and the handshake fails. The message appears mostly in legacy 5.7 and early 8.0 client libraries.<\/p>
The error signals that the client socket was created, but the TCP or TLS connection to the slave host never completed.
Although the code was removed after 8.0.32, you can still see it in application logs compiled against older client libraries.<\/p>
Wrong hostname or port is the fastest way to trigger CR_PROBE_SLAVE_CONNECT. The client cannot resolve or reach the target replication endpoint, so it throws error 2024 before authentication begins.<\/p>
Network firewalls, VPN mis-routes, or SELinux rules often block the connection on port 3306.
Any middleware that drops the SYN packet will lead the client to raise this error quickly.<\/p>
Verify connectivity first. A successful telnet slave_host 3306<\/code> or nc -zv slave_host 3306<\/code> proves that the port is open. Fix DNS or security-group rules if the check fails.<\/p>
Confirm that the replication user exists on the slave and can connect from the master IP.
Use CREATE USER 'repl'@'master_ip' IDENTIFIED BY 'pw';<\/code> and GRANT REPLICATION SLAVE ON *.* TO 'repl'@'master_ip';<\/code>.<\/p>
Common Scenarios and Solutions<\/h3>Scenario: the slave is started with --report-host=<\/code> pointing to an internal hostname unreachable by the master. Solution: update the parameter and restart.<\/p>
Scenario: SSL is required but the master points to a non-SSL port.
Solution: enable MASTER_SSL=1<\/code> in CHANGE MASTER TO<\/code> so both sides match.<\/p>
Best Practices to Avoid This Error<\/h3>Hard-code fully qualified domain names in replication configs and monitor with mysqladmin ping<\/code>. Automate firewall rule creation in provisioning scripts to keep port 3306 open between replication nodes only.<\/p>
Use Galaxy’s connection tester inside the SQL editor to validate host, port, and credential combos before issuing CHANGE MASTER TO<\/code>.
Centralized query history makes debugging easier later.<\/p>
Related Errors and Solutions<\/h3>CR_CONNECTION_ERROR (2002) - cannot resolve or reach any MySQL server. Fix DNS or sockets.<\/p>
ER_SLAVE_IO_RUNNING (1201) - slave I/O thread stopped. Re-issue START SLAVE IO_THREAD;<\/code>.<\/p>.
MASTER_HOST<\/code> or --report-host<\/code> parameter prevents the TCP handshake and raises error 2024.<\/p>Firewall or security group blocking 3306<\/h3>Cloud security groups or on-prem firewalls silently drop packets between master and slave, leading to immediate connect failures.<\/p>
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