ProFTPD module mod_dso

## ProFTPD module `mod_dso`

What are DSO modules?
"On modern Unix derivatives there exists a nifty mechanism usually called dynamic linking/loading of Dynamic Shared Objects (DSO) which provides a way to build a piece of program code in a special format for loading it at run-time into the address space of an executable program.

This loading can usually be done in two ways: Automatically by a system program called `ld.so` when an executable program is started, or manually from within the executing program via a programmatic system interface to the Unix loader through the system calls `dlopen()/dlsym()`.

In the first way the DSO's are usually called shared libraries or DSO libraries and named `libfoo.so` or `libfoo.so.1.2`. They reside in a system directory (usually `/usr/lib/`) and the link to the executable program is established at build-time by specifying `-lfoo` to the linker command. This hard-codes library references into the executable program file so that at start-time the Unix loader is able to locate `libfoo.so` in `/usr/lib/`, in paths hard-coded via linker-options like `-R` or in paths configured via the environment variable `LD_LIBRARY_PATH`. It then resolves any (yet unresolved) symbols in the executable program which are available in the DSO.

Symbols in the executable program are usually not referenced by the DSO (because it's a reusable library of general code) and hence no further resolving has to be done. The executable program has no need to do anything on its own to use the symbols from the DSO because the complete resolving is done by the Unix loader. (In fact, the code to invoke `ld.so` is part of the run-time startup code which is linked into every executable program which has been bound non-static.) The advantage of dynamic loading of common library code is obvious: the library code needs to be stored only once, in a system library like libc.so, saving disk space for every program.

In the second way the DSO's are usually called shared objects or DSO files and can be named with an arbitrary extension (although the canonical name is `foo.so`). These files usually stay inside a program-specific directory and there is no automatically established link to the executable program where they are used. Instead the executable program manually loads the DSO at run-time into its address space via `dlopen()`. At this time no resolving of symbols from the DSO for the executable program is done. Instead the Unix loader automatically resolves any (yet unresolved) symbols in the DSO from the set of symbols exported by the executable program and its already loaded DSO libraries (especially all symbols from the ubiquitous `libc.so`). This way the DSO gets knowledge of the executable program's symbol set as if it had been statically linked with it in the first place."

(Taken from http://httpd.apache.org/docs/dso.html)

The `mod_dso` module is ProFTPD's module for handling the dynamic loading of modules. This module is contained in the `mod_dso.c` file for ProFTPD 1.3.x, and is not compiled by default. Installation instructions are discussed here.

The most current version of `mod_dso` can be found in the ProFTPD source distribution:

      http://www.proftpd.org/

## Directives

- LoadFile
- LoadModule
- ModuleControlsACLs
- ModuleOrder
- ModulePath

## Control Actions

- insmod
- lsmod
- rmmod

### LoadFile

Syntax: LoadFile path
Default: None
Context: server config
Module: mod_dso
Compatibility: 1.3.0rc1 and later

The `LoadFile` directive is used to load any shared object (`.so` file extension), such as shared libraries. On some platforms, it may be necessary to load all of the libraries needed by a DSO module, using `LoadFile`, prior to loading the module itself.

The path parameter must be the absolute path to the shared object to load.

Example:

      # Load the zlib library
      LoadFile /usr/lib/libz.so

### LoadModule

Syntax: LoadModule name
Default: None
Context: server config
Module: mod_dso
Compatibility: 1.3.0rc1 and later

The `LoadModule` directive is used to dynamically load a module from the configuration file.

Example:

      LoadModule mod_test.c

### ModuleControlsACLs

Syntax: ModuleControlsACLs actions|all allow|deny user|group list
Default: None
Context: server config
Module: mod_dso
Compatibility: 1.3.0rc1 and later

The `ModuleControlsACLs` directive configures access lists of users or groups who are allowed (or denied) the ability to use the actions implemented by `mod_dso`. The default behavior is to deny everyone unless an ACL allowing access has been explicitly configured.

If "allow" is used, then list, a comma-delimited list of users or groups, can use the given actions; all others are denied. If "deny" is used, then the list of users or groups cannot use actions all others are allowed. Multiple `ModuleControlsACLs` directives may be used to configure ACLs for different control actions, and for both users and groups.

The actions provided by `mod_dso` are "insmod" "lsmod", and "rmmod".

Example:

      # Allow only user root to load and unload modules, but allow everyone
      # to see which modules have been loaded
      ModuleControlsACLs insmod,rmmod allow user root
      ModuleControlsACLs lsmod allow user *

### ModuleOrder

Syntax: ModuleOrder ...
Default: None
Context: server config
Module: mod_dso
Compatibility: 1.3.0rc1 and later

The `ModuleOrder` directive can be used to explicitly set the module order. Note: do not use this directive unless you know what you are doing. It is very easy to configure a non-working server with this directive.

If you are going to use `ModuleOrder`, make sure it is the very first directive in your `proftpd.conf` file.

Example:

      # Make this one the very first things, if you're going to use it.
      ModuleOrder \
        mod_core.c \
        mod_cap.c \
        mod_auth_unix.c \
        mod_auth_pam.c \
        mod_ls.c \
        mod_log.c \
        mod_site.c \
        mod_xfer.c \
        mod_auth.c \
        mod_ifsession.c \
        mod_auth_file.c

### ModulePath

Syntax: ModulePath path
Default: None
Context: server config
Module: mod_dso
Compatibility: 1.3.0rc1 and later

The `ModulePath` directive is used to configure an alternative directory from which `mod_dso` will load DSO modules. By default, `mod_dso` uses $prefix/`libexec/`, where $prefix is where you installed `proftpd`, e.g. `/usr/local/`.

The path parameter must be an absolute path.

Example:

      ModulePath /etc/proftpd/libexec

# Control Actions

### `insmod`

Syntax: ftpdctl insmod module
Purpose: Load a DSO module

The `insmod` control action can be used to load a DSO module into the running `proftpd` daemon.

A module cannot be loaded multiple times.

Example:

      ftpdctl rmmod mod_test.c
      ftpdctl: 'mod_test.c' loaded

### `lsmod`

Syntax: ftpdctl lsmod
Purpose: Display list of all loaded modules

The `lsmod` control action is used to display a list of all loaded modules.

Example:

      ftpdctl lsmod
      ftpdctl: Loaded Modules:
      ftpdctl:   mod_core.c
      ftpdctl:   mod_xfer.c
      ftpdctl:   mod_auth_unix.c
      ftpdctl:   mod_auth_file.c
      ftpdctl:   mod_auth.c
      ftpdctl:   mod_ls.c
      ftpdctl:   mod_log.c
      ftpdctl:   mod_site.c
      ftpdctl:   mod_dso.c
      ftpdctl:   mod_ctrls.c
      ftpdctl:   mod_auth_pam.c
      ftpdctl:   mod_cap.c

### `rmmod`

Syntax: ftpdctl rmmod module
Purpose: Unload a DSO module

The `rmmod` control action can be used to unload a DSO module from the running `proftpd` daemon. Note that it is also possible to "unload" one of the statically-linked modules; this does not remove that module from the process' memory space, but does remove that module from the core engine, such that `proftpd` will act as if the module is not present.

Example:

      ftpdctl rmmod mod_test.c
      ftpdctl: 'mod_test.c' unloaded

## Usage

Note that `mod_dso`'s control actions are only available if your `proftpd` has been compiled with Controls support.

## Installation

The `mod_dso` module is distributed with ProFTPD. To enable use of DSO modules, use the `--enable-dso` configure option:

      $ ./configure --enable-dso
      $ make
      $ make install

This option causes `mod_dso` to be compiled into `proftpd`.

Logging
The `mod_dso` module supports trace logging, via the module-specific log channels:

- dso

Thus for trace logging, to aid in debugging, you would use the following in your `proftpd.conf`:

      TraceLog /path/to/ftpd/trace.log
      Trace dso:20

This trace logging can generate large files; it is intended for debugging use only, and should be removed from any production configuration.

Frequently Asked Questions

Question: When I try to start `proftpd`, it fails like this:

      proftpd[1234]: mod_dso/0.5: module 'mod_radius.c' already loaded
      proftpd[1234]: Fatal: LoadModule: error loading module 'mod_radius.c': Operation not permitted on line 9 of '/etc/proftpd/proftpd.conf'

How do I fix this "module already loaded" error?
Answer: This happens when the proftpd configuration either a) uses the `LoadModule` on a module which was compiled in as a static module, or b) is inadvertently using `LoadModule` on the same module multiple times.

To check if the module has been statically compiled into your `proftpd` executable, use the `-l` command-line parameter, e.g.:

      # proftpd -l
      Compiled-in modules:
        mod_core.c
        mod_xfer.c
        mod_auth_unix.c
        mod_auth_file.c
        mod_auth.c
        mod_ls.c
        mod_log.c
        mod_site.c
        mod_delay.c
        mod_facts.c
        mod_dso.c
        mod_ident.c
        mod_auth_pam.c
        mod_tls.c
        mod_cap.c

The modules listed via the command are the static modules. So if your `LoadModule` directive is used for one of the modules in this list, you can remove that `LoadModule` directive; that module will already be loaded.

The other cause, that of having multiple `LoadModule` directives for the same module, usually happens when your `proftpd.conf` file includes other config files, e.g.:

      Include /path/to/modules.conf

and it is those other config files which have `LoadModule` directives of their own.

If you find yourself needs to change the configuration to work around this error, you can use the following to see if the module has already been loaded, and if not, load it:

      <IfModule !mod_radius.c>
        LoadModule mod_radius.c
      </IfModule>

Question: When I try to start `proftpd`, it fails like this:

      proftpd[1234]: mod_dso/0.5: module 'mod_ctrls.c' already loaded
      proftpd[1234]: Fatal: LoadModule: error loading module 'mod_ctrls.c': Operation not permitted on line 9 of '/etc/proftpd/proftpd.conf'

I do not have any other `LoadModule` directives in my config, nor is the `mod_ctrls` module in my `--with-modules` configure option.
Answer: In this particular case, the `mod_ctrls` module is automatically compiled in, as a static module, when the `--enable-ctrls` configure option is used. There are only a few such modules with this special handling:

- `mod_ctrls` (via the `--enable-ctrls` configure option)
- `mod_dso` (via the `--enable-dso` configure option)
- `mod_lang` (via the `--enable-nls` configure option)
- `mod_memcache` (via the `--enable-memcache` configure option)

All of these modules would appear in the ``proftpd -l'` static module list.

Question: If I use `prxs` to compile a module like `mod_ldap`, which is already built into my `proftpd` as a static module, and then I use:

      LoadModule mod_ldap.c

in my `proftpd.conf`, then which `mod_ldap` code is used, the static module or the shared module?
Answer: Excellent question. The short answer is: the static module wins.

When `mod_dso` goes to load a module, it first checks to see whether that module is already loaded -- and if so, the module will not be loaded again. Static modules, by definition, are always "already loaded". This means that your `mod_ldap` shared module code would not be loaded, and thus would not override the static module.

Question: I try to start my `proftpd` instance, but it fails to start with this error:

      Fatal: ModulePath: /usr/lib/proftpd is world-writable on line 7 of '/etc/proftpd/modules.conf'

Why?
Answer: The `mod_dso` module, used by ProFTPD for DSO/shared modules, ensures that the `ModulePath` directory is secure. DSO modules are code that are loaded directly into `proftpd`; loading these modules from a world-writable directory means that any system user could inject/replace any of those DSO modules with their own code, and get `proftpd` to do whatever they wanted. Thus `mod_dso` will refuse to use a world-writable directory for loading of modules.

To fix this, you simply need to ensure that the `ModulePath` directory is not world-writable, e.g.:

      $ chmod o-w /usr/lib/proftpd

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