The Group Replication plugin has a configuration option to
determine from which hosts an incoming Group Communication
connection can be accepted. This option is called
If you set this option on a server s1, then when server s2 is
establishing a connection to s1 for the purpose of engaging group
communication, then s1 first checks the whitelist before accepting
the connection from s2. If s2 is in the whitelist, then s1 accepts
the connection, otherwise s1 rejects the connection attempt by s2.
If you do not specify a whitelist explicitly, the server automatically sets the whitelist to the private networks that the server has an interface on. This means that a server, even if it has interfaces on public IPs, does not by default allow connections from external hosts.
Whenever the IP whitelist is set to AUTOMATIC, an entry is added in the error log, similar to:
2017-10-07T06:40:49.320686Z 4 [Note] Plugin group_replication reported: '[GCS] Added automatically IP ranges 192.0.2.21/24,198.51.100.44,203.0.113.0/24,127.0.0.1/32 to the whitelist'
You can improve the security of the group further by manually setting the list of hosts that are permitted to use group communication connections. You can specify host names (from MySQL 5.7.21), simple IP addresses, or CIDR notation, in any combination. A comma must separate each entry. For example:
mysql> STOP GROUP_REPLICATION; mysql> SET GLOBAL group_replication_ip_whitelist="192.0.2.21/24,198.51.100.44,203.0.113.0/24,example.org,www.example.com/24"; mysql> START GROUP_REPLICATION;
The localhost IP address (127.0.0.1) is always added to the whitelist. If not explicitly, it is implicitly and automatically added. IPv6 addresses, and host names that resolve to IPv6 addresses, are not supported.
For host names, name resolution takes place only when a connection request is made by another server. A host name that cannot be resolved is not considered for whitelist validation, and a warning message is written to the error log. Forward-confirmed reverse DNS (FCrDNS) verification is carried out for resolved host names.
Host names are inherently less secure than IP addresses in a whitelist. FCrDNS verification provides a good level of protection, but can be compromised by certain types of attack. Specify host names in your whitelist only when strictly necessary, and ensure that all components used for name resolution, such as DNS servers, are maintained under your control. You can also implement name resolution locally using the hosts file, to avoid the use of external components.
To join a replication group, a server only needs to be whitelisted
on the seed member to which it makes the request to join the
group. Typically, this would be the bootstrap member for the
replication group, but it can be any of the servers listed by the
--loose-group_replication_group_seeds option in
the configuration for the server joining the group. However, note
that when the replication group is reconfigured, the group members
re-establish connections between themselves. If a group member is
only whitelisted by servers that are no longer part of the
replication group after the reconfiguration, it is unable to
reconnect to the remaining servers in the replication group that
do not whitelist it. To avoid this scenario entirely, specify the
same whitelist for all servers that are members of the replication
It is possible to configure different whitelists on different group members according to your security requirements, for example, in order to keep different subnets separate. If you need to configure different whitelists to meet your security requirements, ensure that there is sufficient overlap between the whitelists in the replication group to maximize the possibility of servers being able to reconnect in the absence of their original seed member.