## ProFTPD module `mod_auth`

This module is contained in the `mod_auth.c` file for ProFTPD 1.3.x, and is compiled by default.

## Directives

- AllowChrootSymlinks
- AnonRequirePassword
- AuthAliasOnly
- AuthUsingAlias
- CreateHome
- DefaultRoot
- MaxLoginAttempts
- RequireValidShell
- RewriteHome
- RootLogin
- RootRevoke
- TimeoutLogin
- TimeoutSession
- UseFtpUsers
- UserAlias
- UserPassword

## AllowChrootSymlinks

Syntax: AllowChrootSymlinks on|off
Default: AllowChrootSymlinks on
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_auth
Compatibility: 1.3.5rc1 and later

The `AllowChrootSymlinks` directive configures whether `proftpd` will follow a symlink to the destination directory when performing a `chroot(2)` call. This applies both to `DefaultRoot` directives and to `<Anonymous>` sections.

Security note: If you permit your users the ability to remove directories which might be FTP users' home directories (or `<Anonymous>` directories) and create symlinks, then you should use:

      AllowChrootSymlinks off

This includes sites which are hosting providers, i.e. which allow users to run their untrusted webapps (e.g. PHP, Perl, Ruby, Python, etc apps) on the servers.

## AnonRequirePassword

Syntax: AnonRequirePassword off|on
Default: AnonRequirePassword off
Context: `<Anonymous>`
Module: mod_auth
Compatibility: 0.99.0 and later

Normally, anonymous FTP logins do not require the client to authenticate themselves using real passwords; instead, anonymous FTP logins are expected to provide their email address as the password. Using `AnonRequirePassword on`, however, will require a real password for that login. The provided password will be matched against the `<Anonymous>` user's password.

This functionality, in conjunction with the `AuthUsingAlias` directive, can be used to create "guest" accounts, which function exactly as normal anonymous logins do, but which require a valid password on the server's host system.

See also: `AuthUsingAlias`, `UserAlias`

## AuthAliasOnly

Syntax: AuthAliasOnly off|on
Default: AuthAliasOnly off
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_auth
Compatibility: 1.1.3 and later

The `AuthAliasOnly` directive restricts authentication to "aliased" logins only, i.e. those usernames provided by clients which are "mapped" to a real username by the `UserAlias` directive. Using `AuthAliasOnly on` in a particular configuration context will cause ProFTPD to completely ignore all non-aliased logins for the entire context.

See also: `AuthUsingAlias`, `UserAlias`

## AuthUsingAlias

Syntax: AuthUsingAlias off|on
Default: AuthUsingAlias off
Context: `<Anonymous>`
Module: mod_auth
Compatibility: 1.2.0pre9 and later

The `AuthUsingAlias` directive disables the resolving of aliased usernames (via `UserAlias`) for authentication purposes. For example, if you have aliased the username "anonymous" to the real user "ftp", the password gets checked against the user "anonymous". When `AuthUsingAlias` is disabled, the checked username would be "ftp".

Here is an example of an `<Anonymous>` section where only the aliased usernames are allowed to login:

      <Anonymous ~ftp>
        AuthUsingAlias on
        UserAlias anonymous nobody
        UserAlias ftp nobody

        # Make this a read-only anonymous login
        <Limit WRITE>
          DenyAll
        </Limit>
      </Anonymous>

and here, by contrast, is an `<Anonymous>` section where certain real users are allowed to login using their own passwords (even though it will still be considered an anonymous login):

      <Anonymous ~ftp>
        # Require real passwords for these logins
        AnonRequirePassword on

        AuthAliasOnly on
        AuthUsingAlias on

        # This is the list of authorized users; password checks will occur
        # using their own respective passwords, not user "nobody".
        UserAlias fred nobody
        UserAlias jenn nobody
      </Anonymous>

See also: `AnonRequirePassword`, `UserAlias`

## CreateHome

Syntax: CreateHome off|on [mode] [skel path] [dirmode mode]
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_auth
Compatibility: 1.2.8rc2 and later

The `CreateHome` directive configures the server to automatically create a user's home directory, if that directory does not exist, during the login process.

The mode parameter is used to configure the absolute mode of the home directory created. If not specified, the mode will default to 700.

The optional skel path parameters can be used to configure an `/etc/skel`-like directory containing account initialization files and directories. The parameter must be the full path to the skeleton directory. The directory must not be world-writeable. Files copied from this directory into the new home directory will have ownership set to the UID and GID of the logging-in user. Note that sockets and FIFOs in the skeleton directory will not be copied; any setuid or setgid bits on files will be removed from the copied files in the target home directory.

The optional dirmode parameter can be used to specify the mode for intermediate directories that may need to be created in order to create the target home directory. By default, the mode for such intermediate directories will be 711. Note: using a mode that does not include the execute bit to be enabled can cause havoc. You have been warned.

Examples:

      # Use the CreateHome default settings
      CreateHome on

      # Specify a skeleton directory
      CreateHome on skel /etc/ftpd/skel

      # No skeleton, but make sure that intermediate directories have 755
      # permissions.
      CreateHome on dirmode 755

      # Skeleton directory, with 700 intermediate directories
      CreateHome on skel /etc/ftpd/skel dirmode 700

A fuller description of the `CreateHome` directive and its uses, with more examples, can be read here.

## DefaultRoot

Syntax: DefaultRoot path [group-expression]
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_auth
Compatibility: 1.2.0rc1

The `DefaultRoot` directive is used to `chroot()` the session process for the connecting client. A fuller explanation can be found in the Chroot howto.

## MaxLoginAttempts

Syntax: MaxLoginAttempts count
Default: 3
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_auth
Compatibility: 0.99.0 and later

The `MaxLoginAttempts` directive configures the maximum number of times a client may attempt to authenticate to the server on the same TCP connection. After the number of attempts exceeds the configured count, the client is disconnected and an appropriate message is logged.

## RequireValidShell

Syntax: RequireValidShell on|off
Default: RequireValidShell on
Context: server config, `<VirtualHost>`, `<Global>`, `<Anonymous>`
Module: mod_auth
Compatibility: 0.99.0 and later

The `RequireValidShell` directive configures the server, virtual host or anonymous login to allow or deny logins which do not have a shell listed in `/etc/shells`. By default, `proftpd` will not allow a login unless the user's default shell is listed in `/etc/shells`. If `/etc/shells` cannot be found, all default shells are assumed to be valid.

## RewriteHome

Syntax: RewriteHome on|off
Default: None
Context: server config, `<VirtualHost>`, `<Global>` Module: mod_auth
Compatibility: 1.3.3rc1 and later

The `RewriteHome` directive can be used to support rewriting the home directory for a user, based on regular expression rules. One such use case is where some portion of the home directory is retrieved e.g. from an LDAP directory, but you need to apply some custom prefix to the LDAP attribute. Note that this feature requires that the `mod_rewrite` module also be present in your `proftpd` daemon.

To enable this feature, first you need to add the following to your `proftpd.conf`:

      RewriteHome on

Next, you need to configure the mod_rewrite rules for rewriting your home directory; this feature depends on the `mod_rewrite` module for the rewriting. The pseudo-command used by `mod_rewrite` for rewriting home directories is "REWRITE_HOME". Thus would you use:

      <IfModule mod_rewrite.c>
        RewriteEngine on
        RewrlteLog /path/to/rewrite.log

        RewriteCondition %m REWRITE_HOME
        RewriteRule (.*) /my/new/prefix$1
     </IfModule>

## RootLogin

Syntax: RootLogin on|off
Default: RootLogin off
Context: server config, `<VirtualHost>`, `<Global>`, `<Anonymous>`
Module: mod_auth
Compatibility: 1.1.5 and later

Normally, `proftpd` does not allow root logins under any circumstance. If a client attempts to login as root, using the correct password, a special security message is logged:

      SECURITY VIOLATION: root login attempted.

When `RootLogin on` is used, the root user may authenticate just as any other user could (assuming no other access control measures deny access); however the root login security message is still logged:

      ROOT FTP login successful.

Obviously, extreme care should be taken when using this directive.

The use of `RootLogin` in the `<Anonymous>` context is only valid when the `User`/`Group` defined in the `<Anonymous>` section is set to 'root'.

## RootRevoke

Syntax: RootRevoke on|off|UseNonCompliantActiveTransfers
Default: None
Context: server config, `<VirtualHost>`, `<Global>`, `<Anonymous>`
Module: mod_auth
Compatibility: 1.2.9rc1 and later

The `RootRevoke` directive causes all root privileges to be dropped once a user is authenticated. This will also cause active data transfers (e.g. via the `PORT`/`EPRT` FTP commands) to be disabled if the server is listening on a port less than 1024. Note that this only affects active data transfers; passive transfers will not be blocked.

The reason for rejecting active data transfers in these cases is because of a requirement in RFC 959 (which defines the File Transfer Protocol) that for active data transfers, the data connection must have a source port of L-1, where L is the control connection port (see RFC 959, Section 3.2 "Establishing Data Connections"). Thus if the FTP server listens on port 21, then a client requesting an active data transfer from that server will have a data connection whose source port (on the server) is port 20 (L = 21, L-1 = 20).

Even though passive data transfers are highly preferable, many FTP clients may still require/expect to be able to do an active data transfer. One question, though, is how many FTP clients actually check that the source port of the active data transfer connection is actually L-1. Or how many networking appliances along the way (i.e. firewalls, NATs, routers, etc) enforce this restriction as well.

If not for that requirement, then with "RootRevoke on" in the `proftpd.conf`, `proftpd` would not be required to use root privileges for binding to a privileged port like port 20.

Thus the `RootRevoke` directive also accepts (as of `proftpd-1.3.5rc1`) a parameter of "UseNonCompliantActiveTransfers", e.g.:

      # Drop root privs, but allow active data tranfers (only use a non-standard
      # source port for the active data connection).
      RootRevoke UseNonCompliantActiveTranfers

With this configuration, `proftpd` will drop root privileges, but would not reject `PORT`/`EPRT` commands at all. Instead, the active data transfers would be allowed as per normal, except that `proftpd` would not try to bind to the L-1 port for those active transfers.

This `RootRevoke` parameter is valuable because it helps in getting `proftpd` to drop root privileges for sessions more often, which is a far more secure configuration. Exploits such as the "Roaring Beast" attack would not be possible in a session where root privileges have been dropped completely.

## TimeoutLogin

Syntax: TimeoutLogin seconds
Default: 300 seconds
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_auth
Compatibility: 0.99.0 and later

The `TimeoutLogin` directive configures the maximum number of seconds a client is allowed to spend authenticating, i.e. from the time when the client connects to the time when the client has successfully authenticated. The login timer is not reset when a client transmits data, and is only removed once a client has transmitted an acceptable combination of `USER`/`PASS` commands. The maximum allowed seconds value is 65535 (108 minutes).

See also: `TimeoutIdle`, `TimeoutNoTransfer`, `TimeoutStalled`

## TimeoutSession

Syntax: TimeoutSessions seconds
Default: None
Context: server config, `<VirtualHost>`, `<Global>`, `<Anonymous>`
Module: mod_auth
Compatibility: 1.2.6rc1 and later

The `TimeoutSession` directive sets the maximum number of seconds a control connection between the proftpd server and client can exist, after the client has successfully authenticated. If the seconds argument is set to zero, sessions are allowed to last indefinitely; this is the default. There is no maxium value for the seconds parameter.

## UseFtpUsers

Syntax: UseFtpUsers on|off
Default: UseFtpUsers on
Context: server config, `<VirtualHost>`, `<Global>`, `<Anonymous>`
Module: mod_auth
Compatibility: 0.99.0 and later

Legacy FTP servers generally check a special authorization file (typically `/etc/ftpusers`) when a client attempts to authenticate. If the user's name is found in this file, FTP access is denied. For compatibility of behavior, `proftpd` defaults to checking this same file during authentication. This behavior can be suppressed using the `UseFtpUsers` directive, e.g.:

      # Do not check /etc/ftpusers
      UseFtpUsers off

## UserAlias

Syntax: UserAlias alias real-user
Default: None
Context: server config, `<VirtualHost>`, `<Global>`, `<Anonymous>`
Module: mod_auth
Compatibility: 0.99.0 and later

ProFTPD requires a real username (i.e. known via `/etc/passwd`, `AuthUserFile`, LDAP, SQL, RADIUS, etc) when authenticating a client. There are, however, times when an additional alias is required but is undesirable to provide an additional login accounts.

The `UserAlias` directive provides a mechanism to do just this. A typical and common example of `UserAlias` is for `<Anonymous>` configuration sections. It is conventional for the server to use user "ftp" as the primary authentication user; it is common practice to allow users to login using the name "anonymous". This is achieved by using the following in the config file:

      <Anonymous ~ftp>
        ...
        UserAlias anonymous ftp
        ...
      </Anonymous>

## UserPassword

Syntax: UserPassword user encrypted-password
Default: None
Context: server config, `<VirtualHost>`, `<Global>`, `<Anonymous>`
Module: mod_auth
Compatibility: 0.99.0pl5 and later

The `UserPassword` directive creates a password for a particular user; this configured password will override the user's normal password in `/etc/passwd` (or whichever auth module handles that user). Note that the user configured is a real user, and not a `UserAlias`.

The encrypted-password parameter is a string which has been passed through the standard Unix `crypt(3)` function. Do not use a cleartext password. To obtain this encrypted-password value, you can use the `ftpasswd` script's `--hash` option, e.g.:

      # ftpasswd --hash

      Password:
      Re-type password:

      ftpasswd: $1$EsnXxyD6$tsO2YwTAT/Tl5u1NYPHIw1

Example configuration:

      # Override user bob's password with a hash version of "password"
      UserPassword bob $1$EsnXxyD6$tsO2YwTAT/Tl5u1NYPHIw1

## Installation

The `mod_auth` module is compiled by default.

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