How can I tell an Exim router to use only IPv4 or only IPv6 addresses when it finds both types in the DNS?

You can do this by making it ignore the addresses you don't want. This example ignores all IPv6 addresses and all IPv4 addresses in the 127 network:

ignore_target_hosts = <; 0000::0000/0 ; 127.0.0.0/8

To ignore all IPv4 addresses, use

ignore_target_hosts = 0.0.0.0/0

See `../Q0319`_ for a general discussion of ignore_target_hosts.