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What are DSOs?
DSOs (Dynamic Shared Objects) are specially built binary files that can be loaded by an application while it is running, extending the functionality of the application on-the-fly. One of the best known applications that makes use of DSOs is the Apache web server; see the Apache documentation for an in-depth description of DSOs:

  http://httpd.apache.org/docs/dso.html

DSOs and ProFTPD
ProFTPD gained the ability to use DSOs starting with the 1.3.0rc1 release. To make sure the compiled proftpd binary can load DSO modules, use the --enable-dso configure option:

  $ ./configure --enable-dso ...

The contrib modules that are distributed with the ProFTPD source, e.g. mod_ldap, mod_sql, mod_quotatab, mod_ifsession, etc, can easily be built as DSO modules, rather than statically linked into the proftpd binary. Instead of using the normal --with-modules configure option, you use the --with-shared option:

  $ ./configure --enable-dso --with-shared=mod_sql:mod_sql_mysql --with-includes=... --with-libraries=...

  $ ./configure --libexecdir=/path/to/custom/libexec --enable-dso ...

Note that ProFTPD uses the GNU libtool utility for creating shared modules. This tool creates files with .la file extensions. It is these .la files that will be installed into the libexec/ directory. This differs from the .so files that Apache's DSO support generates, so do not be surprised.

Loading Modules
There are two ways to load DSO modules into proftpd: the LoadModule configuration directive, and the insmod ftpdctl action. Note that the latter possibility is only available if your proftpd has been built with Controls support.

Loading a module using LoadModule is quite simple. Simply use the directive at the top of your proftpd.conf file, which makes sure the module is loaded by proftpd before it processes other directives:

  LoadModule mod_sql.c
  LoadModule mod_sql_mysql.c
  ...

  <IfModule mod_sql.c>
    ...
  </IfModule>

  Fatal: unknown configuration directive 'SQLConnectInfo' on line 86 of '/usr/local/proftpd/etc/proftpd.conf'

  LoadModule: error loading module 'mod_sql_mysql.c': permission denied on line 65 of proftpd.conf

Using ftpdctl insmod to load modules is tricky, as the loading of a module directly into the running proftpd, without restarting the server, can cause unexpected behavior. Many modules are not designed to handle being loaded directly, and may cause bugs or unexpected crashes. Support for this mode of loading modules will stabilize as the modules are updated properly.

Module Ordering
Is the order in which your LoadModule directives appear in proftpd.conf important? The short answer is: maybe. It depends on the modules. Some modules are self-sufficient, do not make use of any other modules, and so can appear in any orders. Others, like mod_sql_mysql or mod_quotatab_sql, require that the frontend module (e.g. mod_sql or mod_quotatab) be loaded first. Still others, like mod_ifsession, do not directly require other modules, yet they have effects that are dependent on the order; mod_ifsession works best when it is the last module loaded.

To achieve the necessary module order, you can make sure that your LoadModule directives appear in the correct order, or you can use the ModuleOrder directive. Note that using ModuleOrder can be difficult, as it is very easy to use ModuleOrder to configure a nonfunctional proftpd.

Compiling Custom Modules as DSOs
The --with-shared configure option can be used to build DSOs from the modules already distributed with ProFTPD, but what about building a custom ProFTPD module as a DSO? Right now, this requires the ProFTPD source, and not just an installed ProFTPD.

Once you have your custom module written (e.g. mod_custom.c), you create the Makefile that will be used to compile it as a DSO module. The following can be used as a template for the Makefile:

  PROFTPD_INSTALL=/usr/local/proftpd

  top_srcdir=$(PROFTPD_INSTALL)
  srcdir=$(PROFTPD_INSTALL)
  VPATH=$(PROFTPD_INSTALL)

  MODULE_NAME=
  MODULE_CFLAGS=
  MODULE_DEFS=
  MODULE_LDFLAGS=
  MODULE_LIBS=

  CC=gcc
  DEFS=-DPR_SHARED_MODULE $(MODULE_DEFS)
  CFLAGS=$(DEFS) -I. -I$(PROFTPD_INSTALL)/include/proftpd $(MODULE_CFLAGS)
  LDFLAGS=-L$(PROFTPD_INSTALL)/lib $(MODULE_LDFLAGS)
  LIBEXEC_DIR=$(PROFTPD_INSTALL)/libexec
  LIBS=$(MODULE_LIBS)

  INSTALL=/usr/bin/install -c
  INSTALL_BIN=$(INSTALL) -s -m 0755

  LIBTOOL=$(SHELL) /usr/bin/libtool
  LTDL_FLAGS=-avoid-version -export-dynamic -module

  # Targets

  all: $(MODULE_NAME).la

  $(MODULE_NAME).lo:
          $(LIBTOOL) --mode=compile $(CC) $(CFLAGS) -c $(MODULE_NAME).c

  $(MODULE_NAME).la: $(MODULE_NAME).lo
          $(LIBTOOL) --mode=link $(CC) -o $(MODULE_NAME).la -rpath $(LIBEXEC_DIR) $(LDFLAGS) $(LTDL_FLAGS) $(MODULE_NAME).lo $(LIBS)

  install: $(MODULE_NAME).la
          if [ -f $(MODULE_NAME).la ] ; then \
                  $(LIBTOOL) --mode=install $(INSTALL_BIN) $(MODULE_NAME).la $(DESTDIR)$(LIBEXEC_DIR) ; \
          fi

  clean:
          $(LIBTOOL) --mode=clean $(RM) $(MODULE_NAME).la $(MODULE_NAME).lo config.*

  distclean:
          $(RM) Makefile config.*
          $(RM) -r autom4te.cache

  MODULE_NAME=mod_custom

  MODULE_CFLAGS=-I/path/to/custom/include
  MODULE_DEFS=-DUSE_LIBCUSTOM
  MODULE_LDFLAGS=-L/path/to/custom/lib
  MODULE_LIBS=-lcustom

  $ make
  $ make install

Once installed, update your proftpd.conf to make sure your module is loaded:

  LoadModule mod_custom.c

Using prxs
You may find yourself wanting to compile some third-party module, for which you have the source code, as a DSO module for proftpd. But you may not have the source code for proftpd, e.g. you might have installed proftpd as a binary package. The build system for proftpd would let you compile your third-party module as a DSO module, but what do you do if you don't have access to the proftpd build system?

The answer is to use the prxs script, which comes with proftpd. The prxs (PRoFTPD EXtensionS) tool will compile and install third-party modules, from source code, as DSO modules for your installed proftpd.

The prxs tool supports the following actions:

 -c, --compile          Compiles the listed .c source files
                        into a proftpd DSO module.

 -i, --install          Installs a compiled proftpd DSO module into the
                        directory where proftpd expects to find loadable
                        DSO modules.

 -d, --clean            Removes any generated files, returning the build
                        directory to a clean state.

To use prxs all in one step, you could do:

  $ prxs -c -i -d mod_custom.c

  LoadModule mod_custom.c

For example, you might use prxs to compile the mod_sql_sqlite module like so, from the top level of the ProFTPD source directory:

  $ prxs -c -i -d contrib/mod_sql_sqlite.c

The following options are also supported:

 -n, --name             Tells prxs the name of the module being compiled.
                        By default, prxs determines the module name from
                        the list of .c files listed, expecting to see a
                        "mod_name.c" file.

 -D key                 Passes these macros through to the compilation step.
 -D key=value           Note that the space before the key is important.

 -I includedir Specify additional include file search directories.
                        Note that the space before the directory is important.

 -L libdir     Specify additional library file search directories.
                        Note that the space before the directory is important.

 -l library    Specify additional libraries for linking.
                        Note that the space before the library name is important.

Using prxs, the above mod_custom example would become:

  $ cd /path/to/mod_custom/dir
  $ prxs -c -i -D USE_CUSTOM -I /path/to/custom/include -L /path/to/custom/lib -l custom mod_custom.c

The prxs tool uses the libtool command that your system should support. If you need to tell prxs to use a different libtool for any reason (such as using a specially installed libtool), you can use the LIBTOOL environment variable to point prxs to the libtool to use. For example:

  $ LIBTOOL=/path/to/custom/libtool prxs -c -i -d mod_custom.c

When should you use prxs for compiling DSO modules, and when should you use a Makefile? In general, if the third-party module comes with its own configure script and Makefile, then you should use those. Otherwise, prxs should suffice.

Frequently Asked Questions
Question: My installed proftpd does not include mod_sql_passwd (or some other module). How can I get proftpd to use this module without recompiling?
Answer: First, see if your proftpd package came with the prxs tool; by default, this tool is installed as /usr/local/bin/prxs. If you do not find prxs anywhere on your system, you will have to recompile proftpd in order to add new modules.

Second, you will need the source code for mod_sql_passwd (or whatever other module you want to add to your proftpd). Assume, then, that you have found the mod_sql_passwd.c source file. The next step is to use prxs to build that module as a DSO module:

  $ /usr/local/bin/prxs -c -i -d mod_sql_passwd.c

  Your installed proftpd does not support shared modules/DSOs.
  Make sure the --enable-dso configure option is used when
  compiling proftpd.

If, on the other hand, your prxs succeeded, the last steps are to update your proftpd.conf to load the new module, and then restart proftpd so that it reads the updated configuration. Continuing with the example of mod_sql_passwd, you would add the following line near the top of your proftpd.conf:

  LoadModule mod_sql_passwd.c

  <IfModule mod_sql_passwd.c>
    SQLPasswordEngine on
    ...
  </IfModule>