When and only when the XCom communication stack is used for
establishing group communications
the Group Replication plugin lets you specify an allowlist of
hosts from which an incoming Group Communication System connection
can be accepted. If you specify an allowlist on a server s1, then
when server s2 is establishing a connection to s1 for the purpose
of engaging group communication, s1 first checks the allowlist
before accepting the connection from s2. If s2 is in the
allowlist, then s1 accepts the connection, otherwise s1 rejects
the connection attempt by s2. The system variable
used to specify the allowlist.
When the MySQL communication stack is used for establishing
the setting for
is ignored. See
Section 20.6.1, “Communication Stack for Connection Security Management”.
If you do not specify an allowlist explicitly, the group
communication engine (XCom) automatically scans active interfaces
on the host, and identifies those with addresses on private
subnetworks, together with the subnet mask that is configured for
each interface. These addresses, and the
localhost IP address for IPv4 and IPv6 are used
to create an automatic Group Replication allowlist. The automatic
allowlist therefore includes any IP addresses that are found for
the host in the following ranges after the appropriate subnet mask
has been applied:
IPv4 (as defined in RFC 1918)
10/8 prefix (10.0.0.0 - 10.255.255.255) - Class A
172.16/12 prefix (172.16.0.0 - 172.31.255.255) - Class B
192.168/16 prefix (192.168.0.0 - 192.168.255.255) - Class C
IPv6 (as defined in RFC 4193 and RFC 5156)
fc00:/7 prefix - unique-local addresses
fe80::/10 prefix - link-local unicast addresses
127.0.0.1 - localhost for IPv4
::1 - localhost for IPv6
An entry is added to the error log stating the addresses that have been allowed automatically for the host.
The automatic allowlist of private addresses cannot be used for
connections from servers outside the private network, so a server,
even if it has interfaces on public IPs, does not by default allow
Group Replication connections from external hosts. For Group
Replication connections between server instances that are on
different machines, you must provide public IP addresses and
specify these as an explicit allowlist. If you specify any entries
for the allowlist, the private and
addresses are not added automatically, so if you use any of these,
you must specify them explicitly.
To specify an allow list manually, use the
system variable. You can change the list while Group Replication
The allowlist must contain the IP address or host name that is
specified in each member's
system variable. This address is not the same as the MySQL server
SQL protocol host and port, and is not specified in the
bind_address system variable for
the server instance. If a host name used as the Group Replication
local address for a server instance resolves to both an IPv4 and
an IPv6 address, the IPv4 address is preferred for Group
IP addresses specified as distributed recovery endpoints, and the
IP address for the member's standard SQL client connection if that
is used for distributed recovery (which is the default), do not
need to be added to the allowlist. The allowlist is only for the
address specified by
for each member. A joining member must have its initial connection
to the group permitted by the allowlist in order to retrieve the
address or addresses for distributed recovery.
In the allowlist, you can specify any combination of the following:
IPv4 addresses (for example,
IPv4 addresses with CIDR notation (for example,
IPv6 addresses (for example,
IPv6 addresses with CIDR notation (for example,
Host names (for example,
Host names with CIDR notation (for example,
Host names can resolve to IPv4 addresses, IPv6 addresses, or both. If a host name resolves to both an IPv4 and an IPv6 address, the IPv4 address is always used for Group Replication connections. You can use CIDR notation in combination with host names or IP addresses to permit a block of IP addresses with a particular network prefix, but do ensure that all the IP addresses in the specified subnet are under your control.
When a connection attempt from an IP address is refused because
the address is not in the allowlist, the refusal message always
prints the IP address in IPv6 format. IPv4 addresses are
::ffff: in this format (an
IPV4-mapped IPv6 address). You do not need to use this format to
specify IPv4 addresses in the allowlist; use the standard IPv4
format for them.
A comma must separate each entry in the allowlist. For example:
mysql> SET GLOBAL group_replication_ip_allowlist="192.0.2.21/24,198.51.100.44,203.0.113.0/24,2001:db8:85a3:8d3:1319:8a2e:370:7348,example.org,www.example.com/24";
To join a replication group, a server needs to be permitted 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
option in the configuration for the server joining the group. If
any of the seed members for the group are listed in the
option with an IPv6 address when a joining member has an IPv4
or the reverse, you must also set up and permit an alternative
address for the joining member for the protocol offered by the
seed member (or a host name that resolves to an address for that
protocol). This is because when a server joins a replication
group, it must make the initial contact with the seed member using
the protocol that the seed member advertises in the
option, whether that is IPv4 or IPv6. If a joining member does not
have a permitted address for the appropriate protocol, its
connection attempt is refused. For more information on managing
mixed IPv4 and IPv6 replication groups, see
Section 20.5.5, “Support For IPv6 And For Mixed IPv6 And IPv4 Groups”.
When a replication group is reconfigured (for example, when a new primary is elected or a member joins or leaves), the group members re-establish connections between themselves. If a group member is only permitted 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 permit it. To avoid this scenario entirely, specify the same allowlist for all servers that are members of the replication group.
It is possible to configure different allowlists on different group members according to your security requirements, for example, in order to keep different subnets separate. If you need to configure different allowlists to meet your security requirements, ensure that there is sufficient overlap between the allowlists in the replication group to maximize the possibility of servers being able to reconnect in the absence of their original seed member.
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 allowlist 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 an allowlist. FCrDNS verification provides a good level of protection, but can be compromised by certain types of attack. Specify host names in your allowlist 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.