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<title>ProFTPD mini-HOWTO - Dynamic Shared Objects</title>
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<center><h2><b>Dynamic Shared Objects</b></h2></center>
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<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
...
<IfModule mod_sql.c>
...
</IfModule>
</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_SRC=/path/to/proftpd
PROFTPD_INSTALL=/usr/local/proftpd
top_srcdir=$(PROFTPD_SRC)
srcdir=$(PROFTPD_SRC)
VPATH=$(PROFTPD_SRC)
MODULE_NAME=
MODULE_CFLAGS=
MODULE_DEFS=
MODULE_LDFLAGS=
MODULE_LIBS=
CC=gcc
DEFS=-DPR_SHARED_MODULE $(MODULE_DEFS)
CFLAGS=$(DEFS) -I. -I$(PROFTPD_SRC)/include -I$(PROFTPD_SRC) $(MODULE_CFLAGS)
LDFLAGS=-L$(PROFTPD_SRC)/lib $(MODULE_LDFLAGS)
LIBEXEC_DIR=$(PROFTPD_INSTALL)/libexec
LIBS=$(MODULE_LIBS)
INSTALL=/usr/bin/install -c
INSTALL_USER=user
INSTALL_GROUP=user
INSTALL_BIN=$(INSTALL) -s -o $(INSTALL_USER) -g $(INSTALL_GROUP) -m 0755
LIBTOOL=$(SHELL) $(PROFTPD_SRC)/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
$(LIBTOOL) --mode=install $(INSTALL_BIN) $(MODULE_NAME).la $(DESTDIR)$(LIBEXEC_DIR)
clean:
$(LIBTOOL) --mode=clean $(RM) $(MODULE_NAME).la
$(LIBTOOL) --mode=clean $(RM) $(MODULE_NAME).lo
$(RM) config.*
distclean:
$(RM) Makefile config.*
$(RM) -r autom4te.cache
</pre>
Fill in <code>MODULE_NAME</code> with the name of your module:
<pre>
MODULE_NAME=mod_custom
</pre>
The remaining <code>MODULE_</code> variables are used to specify additional
compiler and linker flags. If, for example, your <code>mod_custom.c</code>
module relied on a header file <code><custom.h></code> as well as
a library <code>libcustom.so</code>, you might have the following:
<pre>
MODULE_CFLAGS=-I/path/to/custom/include
MODULE_DEFS=-DUSE_LIBCUSTOM
MODULE_LDFLAGS=-L/path/to/custom/lib
MODULE_LIBS=-lcustom
</pre>
Place the <code>Makefile</code> in a directory with your
<code>mod_custom.c</code> source file, then do:
<pre>
make
make install
</pre>
The <code>make install</code> step will install the DSO module into the
<code>libexec/</code> directory of your ProFTPD install location. Note that
you may need to tweak the <code>INSTALL_USER</code> and
<code>INSTALL_GROUP</code> variables with the necessary user/group names for
installing the DSO module.
<p>
Once installed, update your <code>proftpd.conf</code> to make sure your
module is loaded:
<pre>
LoadModule mod_custom.c
</pre>
Then restart <code>proftpd</code>, and your custom module will be in use.
<p>
<hr>
<i>$Date: 2006/06/12 16:03:33 $</i><br>
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