Why does MySQL server not respect bind_address?

Question

Background

I'm setting up a MySQL server. Its configuration file (my.cnf) does not have the bind_address option. As I understand from its documentation, its value defaults to *, which should make the server listen on all IPv4 or IPv6 addresses.

Problem

When I start the MySQL server, I expect it to listen to IPv4. Instead, it listens only to IPv6 as I've checked below.

$ netstat -nlt Active Internet connections (only servers) Proto Recv-Q Send-Q Local Address Foreign Address State tcp6 0 0 :::33060 :::* LISTEN tcp6 0 0 :::3306 :::* LISTEN # -- snip -- 

Why is this happening? And how can I let the MySQL server listen to IPv4 address?

What I have checked so far:

• I made sure that there are no other configuration file that unintentionally overrides the bind_address. When I query SELECT @@bind_address, it correctly shows * as intended.
• I confirm that IPv4 networking is configured on my system. Running ip addr show shows that eth0 is assigned an IPv4 address. In fact, I don't think my system is configured for IPv6, as I don't see any inet with IPv6.
• When I explicitly set bind_address=0.0.0.0 in my.cnf and then restart the server, the server still listens to IPv6 address, showing the same netstat output above.
• There are no other MySQL server instances running on my system.

Environment

• MySQL server: v8.0.36
• OS: CentOS 7

Answer 1

The quote from the documentation you linked:

If the address is *, the server accepts TCP/IP connections on all server host IPv4 interfaces, and, if the server host supports IPv6, on all IPv6 interfaces. Use this address to permit both IPv4 and IPv6 connections on all server interfaces. This value is the default.

By default on Linux listening on a IPv6 socket also accepts IPv4 connections, so it won't show a separate tcp socket in the netstat output but still would work with IPv4.