/etc/citadel/public_clients is in citadel-server 9.01-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9  | # public_clients
#
# This file contains a list of hosts at which well-known public
# copies of client software exist.  If the originating host
# for a client is one of these, then the server will use the
# location of the user that it is told by the client.
localhost
127.0.0.1
 |