ProFTPD: Virtual Users

The core proftpd daemon access all user information via an Auth API. This API hides all of the details of where user information is stored, how to retrieve it, etc from the core engine and modules. This abstraction allows for multiple simultaneous different authentication layers or mechanisms to be configured. It is also because of this abstraction that proftpd does not "know" the difference between a system user and a virtual user; all users are the same.

Frequently Asked Questions

Question: What makes a user "virtual", then?
Answer: A virtual user is, quite simply, a user that is not defined in the system /etc/passwd file. This file associates a user name, given by the system administrator, to a user ID (commonly shortened to UID) and a group ID (GID), among other details. The Unix kernel does not deal with users in terms of their user names; it only "knows" about UIDs and GIDs. This means that an application like proftpd can look up the IDs to use for a given user name however it sees fit. Using /etc/passwd is not strictly required.

Defining users outside of /etc/passwd means that system utilities like ls and chown do not work as expected. When first setting up virtual users, most system administrators will experience this as permissions problems. Virtual users will not be able to login, or they can login but not upload or download files. Or when the administrator lists the files uploaded by virtual users, those files will have the wrong owner names.

The permissions problems are almost always caused by ID mismatch. That is, the UID and/or GID of the virtual user will not match the UID and/or GID owner of their home directory (or whichever directory is having problems). The key here is to change the ownership of the virtual user's directories using chown and IDs, rather than names. The chown utility can set ownership by ID just as easily as by name. Most chown examples show setting ownership by name, but that means chown would look up the IDs for that name via /etc/passwd, which, as noted above, will not work for virtual users.

By default, ls lists the names of file owners by looking up those names in /etc/passwd. This is why listing files of virtual users will often show incorrect names; ls has no knowledge of virtual user names. When working with files created by virtual users, use ls -n so that you can see the IDs, not the names, associated with those files. You will then need to manually make sure those IDs are the correct ones for the file.

Remember that it is the Unix kernel, and not ProFTPD, which enforces the normal Unix access rules to files for a process such as a ProFTPD session process. All that ProFTPD does is select the user and group IDs to set for the process, based on the authentication. ProFTPD never overrides the normal Unix filesystem permissions.

Question: Which IDs should I use for my virtual users?
Answer: It does not matter. The only UID and GID which are special are UID 0 (zero) and GID 0 (zero). These IDs are used for user root and group root; do not assign these IDs to your virtual users unless you absolutely trust those users.

Other than that, you are free to use any IDs you like. It is generally a good idea to use IDs for your virtual users that are not already in use in /etc/passwd, in order to keep the privileges of your system users separate from the privileges of your virtual users; privileges are determined by IDs. However, in some cases (such as using ProFTPD for FTP access to websites), you may want all of your virtual users to run as the web server user, e.g. user "www" or user "apache". Use the IDs that make the most sense for your site needs.

One related question often asked is "Can I have my virtual users have the same IDs?" Yes, you can. This means that all of those virtual users would have the exact same privileges. If you use this approach, make sure those virtual users are all confined to separate home (or web site) directories by using:

  DefaultRoot ~
in your proftpd.conf. This means that even though those virtual users would all have the same privileges, they would be unable to see and affect each others' files since they would all be separated in different directories.

Question: If virtual users are not defined in the system /etc/passwd file, then where are they defined?
Answer: There are several other locations where user information can be stored, e.g. AuthUserFiles, LDAP directories, SQL databases, and RADIUS servers. Note that virtual users are not defined in the proftpd.conf file directly.

One of the simplest virtual user authentication mechanisms is the AuthUserFile, which is a flat text file in the same format as the system /etc/passwd file. For more information on the details, see the AuthFiles howto. The AuthUserFile configuration directive is handled by the mod_auth_file module, whose documentation can be found here.

Once you have created your AuthUserFile and AuthGroupFile with the ftpasswd tool, you configure your proftpd to use those files by adding the following directives to your proftpd.conf:

  AuthUserFile /path/to/ftpd.passwd
  AuthGroupFile /path/to/
The ftpasswd tool is a Perl script, distributed with the ProFTPD source code, under the contrib/ directory. A copy can also be found on GitHub.

Another very popular authentication mechanism used for virtual users is a SQL database such as MySQL or Postgres. This mechanism is supported by ProFTPD's mod_sql module; more information on using mod_sql can be found in the SQL howto, and the mod_sql documentation.

Some sites still experience permissions issues, even though everything appears to be configured properly. If this happens, define an SQLLogFile in your proftpd.conf:

  SQLLogFile /path/to/mod_sql.log
This log file will show which IDs the mod_sql module is assigning to users/groups looked up by proftpd. If the IDs are not what you have in your SQL tables, then you need to use the following directives: SQLMinUserUID, SQLMinUserGID, and SQLMinUserID. The mod_sql module contains some default limits on the range of IDs it returns; the above directives are used to tweak those default limits.

User documentation for the mod_ldap module (for virtual users via LDAP directories) and mod_radius module (for virtual users via RADIUS servers) is not yet complete.

Important: proftpd will use all authentication mechanisms by default. This is why a user that does not appear in an AuthUserFile, but is defined in the system /etc/passwd file, can login by default. Use the AuthOrder configuration directive to change this. This directive is explained in more detail in the Authentication howto.

Question: But I've seen examples that use the <Anonymous> section for virtual users. Why don't you mention this?
Answer: The <Anonymous> context should never be used to define a virtual user. This type of configuration is a crude hack, leftover from ProFTPD's early days. Using an AuthUserFile is better supported, better documented, more flexible, and more in line with ProFTPD's design philosophy.

Question: What about using PAM for virtual users?
Answer: Please read the Authentication howto.

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