Choice of IDs

The choice of IDs for your users is important; the operating system does not deal with user processes in terms of their user names, it knows only of the numeric identity of a process. In general, it is best if each user has a unique positive user ID (negative IDs are an ugly hack, and proftpd will complain if they are used).

However, in some mass-hosting environments such as ISPs, the number of users is greater than the number of IDs available. In these cases, you can give each user the same ID; additional steps should then be taken to make sure that each user is isolated from affecting other users' files. These additional steps are to make sure each user has a unique home directory, and then to restrict each user to their respective home directories by having

DefaultRoot ~

in your proftpd.conf.

There is also currently an issue with using the same usernames in different authentication schemes (e.g. having the same username in both /etc/passwd and a mod_sql SQL table); read this thread for more information: