MaxScale since 2.1.2 supports IPv6 addresses by default, check https://mariadb.com/kb/en/mariadb-enterprise/upgrading-mariadb-maxscale-from-20-to-21/

"MaxScale 2.1.2 added support for IPv6 addresses. The default interface that listeners bind to was changed from the IPv4 address 0.0.0.0 to the IPv6 address ::. To bind to the old IPv4 address, add address=0.0.0.0 to the listener definition."

If IPv6 address support is NOT defined for at least one interface (see IPV6INIT=yes in /etc/sysconfig/network-scripts/ifcfg-* in case of RHEL/CentOS) listeners fail on startup with error messages like this:

...

Jun 19 18:58:01 xxx maxscale[28585]: Failed to start service 'Maxscale_Service'.

Jun 19 18:58:01 xxx maxscale[28585]: Socket creation failed: 97, Address family not supported by protocol.

Jun 19 18:58:01 xxx maxscale[28585]: [MaxAdmin Service] Failed to listen on ::|6603

Jun 19 18:58:01 xxx maxscale[28585]: Failed to start service 'MaxAdmin Service'.

...

Adding explicit address=0.0.0.0 helps.

I think MaxScale should check if at leats one IPv6 address is defined beforebinding to :: or at least it should automatically try to bind to 0.0.0.0 if :: fails.