### 18.5.5 Support For IPv6 And For Mixed IPv6 And IPv4 Groups

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 From MySQL 8.0.14, Group Replication group members can use IPv6 addresses as an alternative to IPv4 addresses for communications within the group. To use IPv6 addresses, the operating system on the server host and the MySQL Server instance must both be configured to support IPv6. For instructions to set up IPv6 support for a server instance, see [Section 5.1.13, “IPv6 Support”](ipv6-support.html).

 IPv6 addresses, or host names that resolve to them, can be specified as the network address that the member provides in the [`group_replication_local_address`](group-replication-options.html#sysvar_group_replication_local_address) option for connections from other members. When specified with a port number, an IPv6 address must be specified in square brackets, for example:

```
group_replication_local_address= "[2001:db8:85a3:8d3:1319:8a2e:370:7348]:33061"
```

 The network address or host name specified in [`group_replication_local_address`](group-replication-options.html#sysvar_group_replication_local_address) is used by Group Replication as the unique identifier for a group member within the replication group. If a host name specified as the Group Replication local address for a server instance resolves to both an IPv4 and an IPv6 address, the IPv4 address is always used for Group Replication connections. The address or host name specified as the Group Replication local address is not the same as the MySQL server SQL protocol host and port, and is not specified in the [`bind_address`](server-system-variables.html#sysvar_bind_address) system variable for the server instance. For the purpose of IP address permissions for Group Replication (see [Section 18.6.4, “Group Replication IP Address Permissions”](group-replication-ip-address-permissions.html)), the address that you specify for each group member in [`group_replication_local_address`](group-replication-options.html#sysvar_group_replication_local_address) must be added to the list for the [`group_replication_ip_allowlist`](group-replication-options.html#sysvar_group_replication_ip_allowlist) (from MySQL 8.0.22) or [`group_replication_ip_whitelist`](group-replication-options.html#sysvar_group_replication_ip_whitelist) system variable on the other servers in the replication group.

 A replication group can contain a combination of members that present an IPv6 address as their Group Replication local address, and members that present an IPv4 address. When a server joins such a mixed group, it must make the initial contact with the seed member using the protocol that the seed member advertises in the [`group_replication_group_seeds`](group-replication-options.html#sysvar_group_replication_group_seeds) option, whether that is IPv4 or IPv6. If any of the seed members for the group are listed in the [`group_replication_group_seeds`](group-replication-options.html#sysvar_group_replication_group_seeds) option with an IPv6 address when a joining member has an IPv4 Group Replication local address, or the reverse, you must also set up and permit an alternative address for the joining member for the required protocol (or a host name that resolves to an address for that protocol). If a joining member does not have a permitted address for the appropriate protocol, its connection attempt is refused. The alternative address or host name only needs to be added to the [`group_replication_ip_allowlist`](group-replication-options.html#sysvar_group_replication_ip_allowlist) (from MySQL 8.0.22) or [`group_replication_ip_whitelist`](group-replication-options.html#sysvar_group_replication_ip_whitelist) system variable on the other servers in the replication group, not to the [`group_replication_local_address`](group-replication-options.html#sysvar_group_replication_local_address) value for the joining member (which can only contain a single address).

 For example, server A is a seed member for a group, and has the following configuration settings for Group Replication, so that it is advertising an IPv6 address in the [`group_replication_group_seeds`](group-replication-options.html#sysvar_group_replication_group_seeds) option:

```
group_replication_bootstrap_group=on
group_replication_local_address= "[2001:db8:85a3:8d3:1319:8a2e:370:7348]:33061"
group_replication_group_seeds= "[2001:db8:85a3:8d3:1319:8a2e:370:7348]:33061"
```

 Server B is a joining member for the group, and has the following configuration settings for Group Replication, so that it has an IPv4 Group Replication local address:

```
group_replication_bootstrap_group=off
group_replication_local_address= "203.0.113.21:33061"
group_replication_group_seeds= "[2001:db8:85a3:8d3:1319:8a2e:370:7348]:33061"
```

 Server B also has an alternative IPv6 address `2001:db8:8b0:40:3d9c:cc43:e006:19e8`. For Server B to join the group successfully, both its IPv4 Group Replication local address, and its alternative IPv6 address, must be listed in Server A's allowlist, as in the following example:

```
group_replication_ip_allowlist=
"203.0.113.0/24,2001:db8:85a3:8d3:1319:8a2e:370:7348,
2001:db8:8b0:40:3d9c:cc43:e006:19e8"
```

 As a best practice for Group Replication IP address permissions, Server B (and all other group members) should have the same allowlist as Server A, unless security requirements demand otherwise.

 If any or all members of a replication group are using an older MySQL Server version that does not support the use of IPv6 addresses for Group Replication, a member cannot participate in the group using an IPv6 address (or a host name that resolves to one) as its Group Replication local address. This applies both in the case where at least one existing member uses an IPv6 address and a new member that does not support this attempts to join, and in the case where a new member attempts to join using an IPv6 address but the group includes at least one member that does not support this. In each situation, the new member cannot join. To make a joining member present an IPv4 address for group communications, you can either change the value of [`group_replication_local_address`](group-replication-options.html#sysvar_group_replication_local_address) to an IPv4 address, or configure your DNS to resolve the joining member's existing host name to an IPv4 address. After you have upgraded every group member to a MySQL Server version that supports IPv6 for Group Replication, you can change the [`group_replication_local_address`](group-replication-options.html#sysvar_group_replication_local_address) value for each member to an IPv6 address, or configure your DNS to present an IPv6 address. Changing the value of [`group_replication_local_address`](group-replication-options.html#sysvar_group_replication_local_address) takes effect only when you stop and restart Group Replication.

 IPv6 addresses can also be used as distributed recovery endpoints, which can be specified from MySQL 8.0.21 using the [`group_replication_advertise_recovery_endpoints`](group-replication-options.html#sysvar_group_replication_advertise_recovery_endpoints) system variable. The same rules apply to addresses used in this list. See [Section 18.5.4.1, “Connections for Distributed Recovery”](group-replication-distributed-recovery-connections.html).

