https://openvpn.net/community-resources/reference-manual-for-openvpn-2-0/
–client-config-dir dir
Specify a directory dirfor custom client config files. After a connecting client has been authenticated, OpenVPN will look in this directory for a file having the same name as the client’s X509 common name. If a matching file exists, it will be opened and parsed for client-specific configuration options. If no matching file is found, OpenVPN will instead try to open and parse a default file called “DEFAULT”, which may be provided but is not required.This file can specify a fixed IP address for a given client using –ifconfig-push, as well as fixed subnets owned by the client using –iroute.