## Dynamic Shared Objects

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 ...

This causes the build system to build the `libltdl` supporting library, which is used to handle OS-specific ways of loading and unloading DSO files, and also to include the `mod_dso` module in the compiled `proftpd`. The `mod_dso` module provides the `LoadModule` configuration directive, for loading modules via the `proftpd.conf` configuration file.

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=...

These DSO modules will be installed under the `libexec/` directory of your ProFTPD install location. To control the location of this `libexec/` directory from which the `mod_dso` module will load modules, you can use the `--libexecdir` configure option, e.g.:

      ./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>

If a module fails to load properly, you might see messages like:

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

This can happen if you forget to use the `LoadModule` directive in your `proftpd.conf` prior to using directives from the module. If you are using `LoadModule`, the error message may look like:

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

Check the `libexec/` directory where you installed `proftpd`, to see if the appropriate `.la` and/or `.so` files are present. Then check your dynamic loader configuration file (e.g. `/etc/ld.so.conf` on Linux) and make sure that the `libexec/` directory is configured, so that the dynamic loader knows to look in the correct locations. Note that the `LD_LIBRARY_PATH` and/or `LD_RUN_PATH` environment variables may also be used to inform the dynamic loader of `proftpd`'s `libexec/` directory.

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

Fill in `MODULE_NAME` with the name of your module:

      MODULE_NAME=mod_custom

The remaining `MODULE_` variables are used to specify additional compiler and linker flags. If, for example, your `mod_custom.c` module relied on a header file `<custom.h>` as well as a library `libcustom.so`, you might have the following:

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

Place the `Makefile` in a directory with your `mod_custom.c` source file, then do:

      make
      make install

The `make install` step will install the DSO module into the `libexec/` directory of your ProFTPD install location.

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

      LoadModule mod_custom.c

Then restart `proftpd`, and your custom module will be in use.

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.

At least one of the above actions must be specified when using `prxs`. More than one action can be specified at the same time.

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

      # prxs -c -i -d mod_custom.c

which will do the compile, install, and clean actions in order. Once installed, update your `proftpd.conf` to make sure your module is loaded:

      LoadModule mod_custom.c

Then restart `proftpd`, and your custom module will be in use.

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

That's it! No need for a special Makefile, and no need to edit/replace any variables.

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.

$Date: 2008/09/09 04:00:46 $