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<center><h2><b>ProFTPD: Dynamic Shared Objects (DSOs)</b></h2></center>
<hr>

<p>
<b>What are DSOs?</b><br>
DSOs (<b>D</b>ynamic <b>S</b>hared <b>O</b>bjects) 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:
<pre>
  <a href="http://httpd.apache.org/docs/dso.html">http://httpd.apache.org/docs/dso.html</a>
</pre>

<p>
<b>DSOs and ProFTPD</b><br>
ProFTPD gained the ability to use DSOs starting with the 1.3.0rc1 release.
To make sure the compiled <code>proftpd</code> binary can load DSO modules,
use the <code>--enable-dso</code> configure option:
<pre>
  $ ./configure --enable-dso ...
</pre>
This causes the build system to build the <code>libltdl</code> supporting
library, which is used to handle OS-specific ways of loading and unloading
DSO files, and also to include the <a href="../../doc/modules/mod_dso.html"><code>mod_dso</code></a> module in the compiled <code>proftpd</code>.  The
<code>mod_dso</code> module provides the <code>LoadModule</code> configuration
directive, for loading modules via the <code>proftpd.conf</code> configuration
file.

<p>
The <code>contrib</code> modules that are distributed with the ProFTPD source,
<i>e.g.</i> <code>mod_ldap</code>, <code>mod_sql</code>,
<code>mod_quotatab</code>, <code>mod_ifsession</code>, <i>etc</i>, can
easily be built as DSO modules, rather than statically linked into the
<code>proftpd</code> binary.  Instead of using the normal
<code>--with-modules</code> configure option, you use the
<code>--with-shared</code> option:
<pre>
  $ ./configure --enable-dso --with-shared=mod_sql:mod_sql_mysql --with-includes=... --with-libraries=...
</pre>
These DSO modules will be installed under the <code>libexec/</code> directory
of your ProFTPD install location.  To control the location of this
<code>libexec/</code> directory from which the <code>mod_dso</code> module
will load modules, you can use the <code>--libexecdir</code> configure
option, <i>e.g.</i>:
<pre>
  $ ./configure --libexecdir=/path/to/custom/libexec --enable-dso ...
</pre>

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

<p>
<b>Loading Modules</b><br>
There are two ways to load DSO modules into <code>proftpd</code>: the
<a href="../../doc/modules/mod_dso.html#LoadModule"><code>LoadModule</code></a> configuration directive, and the <a href="../../doc/modules/mod_dso.html#insmod"><code>insmod</code></a> <code>ftpdctl</code> action.  Note that the latter
possibility is only available if your <code>proftpd</code> has been built with
<a href="Controls.html">Controls</a> support.

<p>
Loading a module using <code>LoadModule</code> is quite simple.  Simply use
the directive at the top of your <code>proftpd.conf</code> file, which makes
sure the module is loaded by <code>proftpd</code> before it processes other
directives:
<pre>
  LoadModule mod_sql.c
  LoadModule mod_sql_mysql.c
  ...

  &lt;IfModule mod_sql.c&gt;
    ...
  &lt;/IfModule&gt;
</pre>

If a module fails to load properly, you might see messages like:
<pre>
  Fatal: unknown configuration directive 'SQLConnectInfo' on line 86 of '/usr/local/proftpd/etc/proftpd.conf'
</pre>
This can happen if you forget to use the <code>LoadModule</code> directive
in your <code>proftpd.conf</code> prior to using directives from the
module.  If you are using <code>LoadModule</code>, the error message may
look like:
<pre>
  LoadModule: error loading module 'mod_sql_mysql.c': permission denied on line 65 of proftpd.conf
</pre>
Check the <code>libexec/</code> directory where you installed
<code>proftpd</code>, to see if the appropriate <code>.la</code> and/or
<code>.so</code> files are present.  Then check your dynamic loader
configuration file (<i>e.g.</i> <code>/etc/ld.so.conf</code> on Linux) and
make sure that the <code>libexec/</code> directory is configured, so that the
dynamic loader knows to look in the correct locations.  Note that the
<code>LD_LIBRARY_PATH</code> and/or <code>LD_RUN_PATH</code> environment
variables may also be used to inform the dynamic loader of
<code>proftpd</code>'s <code>libexec/</code> directory.

<p>
Using <code>ftpdctl insmod</code> to load modules is tricky, as the loading of
a module directly into the running <code>proftpd</code>, 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.

<p>
<b>Module Ordering</b><br>
Is the order in which your <code>LoadModule</code> directives appear in
<code>proftpd.conf</code> important?  The short answer is: <em>maybe</em>.
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
<code>mod_sql_mysql</code> or <code>mod_quotatab_sql</code>, require
that the frontend module (<i>e.g.</i> <code>mod_sql</code> or
<code>mod_quotatab</code>) be loaded first.  Still others, like
<code>mod_ifsession</code>, do not directly require other modules, yet they
have effects that are dependent on the order; <code>mod_ifsession</code>
works best when it is the <b>last</b> module loaded.

<p>
To achieve the necessary module order, you can make sure that your
<code>LoadModule</code> directives appear in the correct order, or you can
use the <a href="../../doc/modules/mod_dso.html#ModuleOrder"><code>ModuleOrder</code></a> directive.  Note that using <code>ModuleOrder</code> can be
difficult, as it is very easy to use <code>ModuleOrder</code> to configure a
nonfunctional <code>proftpd</code>.

<p>
<b>Compiling Custom Modules as DSOs</b><br>
The <code>--with-shared</code> 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.

<p>
Once you have your custom module written (<i>e.g.</i>
<code>mod_custom.c</code>), you create the <code>Makefile</code> that will
be used to compile it as a DSO module.  The following can be used as a template
for the <code>Makefile</code>:
<pre>
  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
</pre>
Fill in <code>MODULE_NAME</code> with the name of your module:
<pre>
  MODULE_NAME=mod_custom
</pre>
The remaining <code>MODULE_</code> variables are used to specify additional
compiler and linker flags.  If, for example, your <code>mod_custom.c</code>
module relied on a header file <code>&lt;custom.h&gt;</code> as well as
a library <code>libcustom.so</code>, you might have the following:
<pre>
  MODULE_CFLAGS=-I/path/to/custom/include
  MODULE_DEFS=-DUSE_LIBCUSTOM
  MODULE_LDFLAGS=-L/path/to/custom/lib
  MODULE_LIBS=-lcustom
</pre>
Place the <code>Makefile</code> in a directory with your
<code>mod_custom.c</code> source file, then do:
<pre>
  $ make
  $ make install
</pre>
The <code>make install</code> step will install the DSO module into the
<code>libexec/</code> directory of your ProFTPD install location.

<p>
Once installed, update your <code>proftpd.conf</code> to make sure your
module is loaded:
<pre>
  LoadModule mod_custom.c
</pre>
Then restart <code>proftpd</code>, and your custom module will be in use.

<p><a name="prxs"></a>
<b>Using <code>prxs</code></b><br>
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, <i>e.g.</i> 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?

<p>
The answer is to use the <code>prxs</code> script, which comes with proftpd.
The <code>prxs</code> (<b>PR</b>oFTPD E<b>X</b>tension<b>S</b>) tool will
compile and install third-party modules, from source code, as DSO modules
for your installed proftpd.

<p>
The <code>prxs</code> tool supports the following actions:
<pre>
 -c, --compile          Compiles the listed <code>.c</code> 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.
</pre>
At least one of the above actions must be specified when using
<code>prxs</code>.  More than one action can be specified at the same time.

<p>
To use <code>prxs</code> all in one step, you could do:
<pre>
  $ prxs -c -i -d mod_custom.c
</pre>
which will do the compile, install, and clean actions in order.  Once
installed, update your <code>proftpd.conf</code> to make sure your module is
loaded:
<pre>
  LoadModule mod_custom.c
</pre>
Then restart <code>proftpd</code>, and your custom module will be in use.

<p>
For example, you might use <code>prxs</code> to compile the
<code>mod_sql_sqlite</code> module like so, from the top level of the
ProFTPD source directory:
<pre>
  $ prxs -c -i -d contrib/mod_sql_sqlite.c
</pre>

<p>
The following options are also supported:
<pre>
 -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_<i>name</i>.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 <em>includedir</em> Specify additional include file search directories.
                        Note that the space before the directory is important.

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

 -l <em>library</em>    Specify additional libraries for linking.
                        Note that the space before the library name is important.
</pre>

<p>
Using <code>prxs</code>, the above <code>mod_custom</code> example would
become:
<pre>
  $ 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
</pre>
That's it!  No need for a special Makefile, and no need to edit/replace any
variables.

<p>
The <code>prxs</code> tool uses the <code>libtool</code> command that your
system should support.  If you need to tell <code>prxs</code> to use a
different <code>libtool</code> for any reason (such as using a specially
installed <code>libtool</code>), you can use the <code>LIBTOOL</code>
environment variable to point <code>prxs</code> to the <code>libtool</code>
to use.  For example:
<pre>
  $ LIBTOOL=/path/to/custom/libtool prxs -c -i -d mod_custom.c
</pre>

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

<p><a name="FAQ">
<b>Frequently Asked Questions</b><br>
<font color=red>Question</font>: My installed <code>proftpd</code> does not
include <code>mod_sql_passwd</code> (or some other module).  How can I get
proftpd to use this module without recompiling?<br>
<font color=blue>Answer</font>: First, see if your proftpd package came
with the <a href="#prxs"><code>prxs</code></a> tool; by default, this tool
is installed as <code>/usr/local/bin/prxs</code>.  If you do not find
<code>prxs</code> anywhere on your system, you will have to recompile proftpd
in order to add new modules.

<p>
Second, you will need the source code for <code>mod_sql_passwd</code> (or
whatever other module you want to add to your proftpd).  Assume, then, that
you have found the <code>mod_sql_passwd.c</code> source file.  The next
step is to use <code>prxs</code> to build that module as a DSO module:
<pre>
  $ /usr/local/bin/prxs -c -i -d mod_sql_passwd.c
</pre>
If the above fails with this error message:
<pre>
  Your installed proftpd does not support shared modules/DSOs.
  Make sure the --enable-dso configure option is used when
  compiling proftpd.
</pre>
It means that your <code>proftpd</code> does not have DSO support -- and
that means that you will have to recompile proftpd to add the new module.

<p>
If, on the other hand, your <code>prxs</code> succeeded, the last steps are
to update your <code>proftpd.conf</code> to load the new module, and then
restart proftpd so that it reads the updated configuration.  Continuing with
the example of <code>mod_sql_passwd</code>, you would add the following line
near the top of your <code>proftpd.conf</code>:
<pre>
  LoadModule mod_sql_passwd.c
</pre>
and later in the config file, configure your newly added module:
<pre>
  &lt;IfModule mod_sql_passwd.c&gt;
    SQLPasswordEngine on
    ...
  &lt;/IfModule&gt;
</pre>
Last, restart proftpd, and enjoy your new module's functionality, all without
needing to recompile/reinstall proftpd itself.

<p>
<hr>
<font size=2><b><i>
&copy; Copyright 2017 The ProFTPD Project<br>
 All Rights Reserved<br>
</i></b></font>
<hr>

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