MySQL Bugs: #119441: Group replication binds to ipv6 wildcard address

Bug #119441	Group replication binds to ipv6 wildcard address
Submitted:	21 Nov 2025 20:09	Modified:	20 Oct 8:43
Reporter:	DBA Kayak	Email Updates:
Status:	Duplicate	Impact on me:	None
Category:	MySQL Server: Group Replication	Severity:	S2 (Serious)
Version:	8.0.35	OS:	Linux (Rocky Linux release 8.10)
Assigned to:		CPU Architecture:	x86

Description:
Group Replication IPv6 wildcard address/port binding prevents running multiple mysqld instances and group replication on the same server even when configured with different IPv4 addresses.

How to repeat:
Configure a linux server with two virtual IP addresses. Configure the mysql servers group_replication_local_address with the hostnames or IPs and the group replication port (33061). Start up the mysql instances. Bootstrap and start Group Replication on one of the instances. It will bind to the IPv6 wildcard address (:::33061) only if hostname is used. If an IPv4 address is specified for group_replication_local_address it will bind to the IPv4 address on 33061 and the IPv6 wildcard on 33061. Attempt to start group replication on the other mysqld instance. Group Replication will time out attempting to bind to the IPv6 wildcard address and port 33061 that is already in use by the first mysqld instance running group replication.

2025-11-21T13:07:33.258022-05:00 0 [ERROR] [MY-011735] [Repl] Plugin group_replication reported: '[GCS] Unable to announce tcp port 33061. Port already in use?'
2025-11-21T13:07:33.258121-05:00 0 [ERROR] [MY-011735] [Repl] Plugin group_replication reported: '[GCS] Error initializing the group communication engine.'
2025-11-21T13:07:33.258353-05:00 0 [ERROR] [MY-011735] [Repl] Plugin group_replication reported: '[GCS] Unable to start XCom Network Provider'
2025-11-21T13:07:33.258455-05:00 0 [ERROR] [MY-011735] [Repl] Plugin group_replication reported: '[GCS] Error joining the group while waiting for the network layer to become ready.'
2025-11-21T13:07:33.309701-05:00 0 [ERROR] [MY-011735] [Repl] Plugin group_replication reported: '[GCS] The member was unable to join the group. Local port: 33061'

Suggested fix:
Bind to IPv4 or IPv6 based on the configuration of group_replication_local_address.

I think the MYSQL communication stack is a good fit for what you’re trying to do here, especially now that Bug #110773 has been fixed.

XCom creates its own listener and binds it to a wildcard address, regardless of the specific IP address or hostname in group_replication_local_address. Its IPv6 listener also accepts IPv4 connections, which explains why assigning different IPv4 addresses to the instances doesn’t prevent the collision. Forcing IPv4 would leave the same underlying problem: the IPv4 listener also binds to a wildcard address.

The MYSQL communication stack uses MySQL Server’s existing listener instead. That listener respects bind_address, so instances bound to separate IP addresses can share the same port without competing for a wildcard listener.

There was a second problem with this setup, reported in Bug #110773: when two members used different IP addresses on the same host and the same SQL port, they could incorrectly treat each other as “self” and skip the connections needed to join the group. The fix makes that decision using the member’s configured
endpoint rather than every local interface address.

I’d therefore recommend using the MYSQL communication stack with a server version containing that fix. This should address the deployment you describe. XCom’s wildcard binding remains unchanged, so using XCom would still require different communication ports for the instances.