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<title>ProFTPD module mod_rlimit</title>
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<h2><b>ProFTPD module <code>mod_rlimit</code></b></h2>
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<hr><br>
<p>
The <code>mod_rlimit</code> module handles setting and enforcement of
resource limits such as CPU, memory, and file descriptor usage.
<p>
By default, the <code>mod_rlimit</code> module will impose one resource
restriction on new session processes: the <code>RLIMIT_NPROC</code> (see
<code>getrlimit(2)</code> limit, which controls the number of processes
which can be forked from this process, is set to zero. There is absolutely
no reason for a <code>proftpd</code> session process to need to create a new
process via <code>fork(2)</code>.
<p>
This module is contained in the <code>mod_rlimit.c</code> file for
ProFTPD 1.3.<i>x</i>, and is compiled by default. See the
<a href="#Installation">installation</a> section for more details.
<p>
The most current version of <code>mod_rlimit</code> can be found in the
ProFTPD source distribution:
<pre>
<a href="http://www.proftpd.org/">http://www.proftpd.org/</a>
</pre>
<h2>Directives</h2>
<ul>
<li><a href="#RLimitChroot">RLimitChroot</a>
<li><a href="#RLimitCPU">RLimitCPU</a>
<li><a href="#RLimitMemory">RLimitMemory</a>
<li><a href="#RLimitOpenFiles">RLimitOpenFiles</a>
</ul>
<p>
<hr>
<h3><a name="RLimitChroot">RLimitChroot</a></h3>
<strong>Syntax:</strong> RLimitChroot <em>on|off</em><br>
<strong>Default:</strong> RLimitChroot on<br>
<strong>Context:</strong> server config, <code><VirtualHost></code>, <code><Global></code><br>
<strong>Module:</strong> mod_rlimit<br>
<strong>Compatibility:</strong> 1.3.5rc5
<p>
The <code>RLimitChroot</code> directive is used to enable/disable checks
for modifications to "sensitive" directories when a session is chrooted. These
checks are designed to mitigate and guard against attacks such as the
"Roaring Beast" attack; see:
<ul>
<li><a href="https://auscert.org.au/15286">https://auscert.org.au/15286</a>
<li><a href="https://auscert.org.au/15526">https://auscert.org.au/15526</a>
</ul>
<p>
When a session is chrooted, <i>e.g.</i> via the <code>DefaultRoot</code>
directive <i>or</i> by <code><Anonymous></code> login, the checks
for the "sensitive" directories are automatically enabled. To disable these
checks, use:
<pre>
RLimitChroot off
</pre>
<b>Note</b>: We <b>strongly recommend</b> that you do <b>not</b> disable
these checks.
<p>
The checks in question will specifically prevent any attempts to upload
files into the <code>/etc</code> and <code>/lib</code> directories, or
attempts to delete, create, rename, link, or otherwise try to change anything
in these directories. All attempts to make modifications will be rejected
with "Permission denied" errors. In addition, the following message will
be logged (at debug level 2):
<pre>
WARNING: attempt to use sensitive path '<i>/etc/file</i>' within chroot '<i>/home/user</i>', rejecting
</pre>
<p>
The <code>RLimitChroot</code> directive is <b>not</b> intended to <b>prevent</b>
"Roaring Beast" style attacks entirely; the guarded <code>/etc</code> and
<code>/lib</code> directories might be created via other means, outside of
ProFTPD, which would also allow for the attack. The <code>RLimitChroot</code>
directive is meant to <em>mitigate</em> (not <em>prevent</em>) the attacks by
making sure it cannot be done using <i>just</i> ProFTPD.
<p>
<hr>
<h3><a name="RLimitCPU">RLimitCPU</a></h3>
<strong>Syntax:</strong> RLimitCPU <em>[scope] soft-limit|"max" [hard-limit|"max"]</em><br>
<strong>Default:</strong> <em>System defaults</em><br>
<strong>Context:</strong> server config, <code><VirtualHost></code>, <code><Global></code><br>
<strong>Module:</strong> mod_rlimit<br>
<strong>Compatibility:</strong> 1.3.5rc2
<p>
The <code>RLimitCPU</code> directive is used to set a limit on the CPU usage,
expressed as a maximum number of seconds.
<p>
<code>RLimitCPU</code> takes from one to three parameters. The first parameter
is an optional <em>scope</em> parameter, indicating the scope of the resource
limit. The <em>scope</em> parameter may be one of:
<ul>
<li>"daemon" (applies the resource limit only to the <i>daemon</i> process)
<li>"session" (which applies the limit only to <i>session</i> processes handling each client)
<li>"none" (which disables any possibly inherited limits)
</ul>
If none of these keywords are used, the resource limit is assumed to apply to
both daemon and session processes.
<p>
The next parameters indicate the actual resource limits, both the so-called
"soft limit" and any "hard limit". These parameters may be a number
(indicating the number of seconds), or the value "max" to indicate that
maximum resource limit value allowed is to be used.
<p>
Example:
<pre>
# Limit a given session to 30 minutes of CPU time (which can take
# considerably longer than 30 minutes of wall time, due to CPU scheduling)
RLimitCPU 1800
</pre>
<p>
<hr>
<h3><a name="RLimitMemory">RLimitMemory</a></h3>
<strong>Syntax:</strong> RLimitMemory <em>[scope] soft-limit|"max" [hard-limit|"max"]</em><br>
<strong>Default:</strong> <em>System defaults</em><br>
<strong>Context:</strong> server config, <code><VirtualHost></code>, <code><Global></code><br>
<strong>Module:</strong> mod_rlimit<br>
<strong>Compatibility:</strong> 1.3.5rc2
<p>
The <code>RLimitMemory</code> directive is used to set a limit on the
memory usage, expressed as a maximum number of bytes.
<p>
<code>RLimitMemory</code> takes from one to three parameters. The first
parameter is an optional <em>scope</em> parameter, indicating the scope of the
resource limit. The <em>scope</em> parameter may be one of:
<ul>
<li>"daemon" (applies the resource limit only to the <i>daemon</i> process)
<li>"session" (which applies the limit only to <i>session</i> processes handling each client)
<li>"none" (which disables any possibly inherited limits)
</ul>
If none of these keywords are used, the resource limit is assumed to apply to
both daemon and session processes.
<p>
The next parameters indicate the actual resource limits, both the so-called
"soft limit" and any "hard limit". These parameters may be a number
(indicating the number of bytes), or the value "max" to indicate that
maximum resource limit value allowed is to be used.
<p>
Example:
<pre>
# Limit a given session to 128MB minutes of memory
RLimitMemory 128MB
</pre>
<p>
<b>Note</b>: If you use <code>RLimitMemory</code>, <i>e.g.</i>:
<pre>
<IfModule mod_rlimit.c>
RLimitMemory session 64M
</IfModule>
</pre>
<b>and</b> you use <a href="../contrib/mod_tls.html"><code>mod_tls</code></a>
for FTPS transfers:
<pre>
<IfModule mod_tls.c>
...
</IfModule>
</pre>
then your transfers are likely to fail. Why? Because OpenSSL will need to
allocate memory for the TLS support, in addition to the memory that ProFTPD
already allocates for data transfers. Depending on the specific ciphersuites
negotiated, and the specific memory limit configured, you are very likely to
hit the <code>RLimitMemory</code> limit. In short, your
<code>RLimitMemory</code> might be too low, and not allowing ProFTPD and
OpenSSL enough memory for the transfer.
<p>
<hr>
<h3><a name="RLimitOpenFiles">RLimitOpenFiles</a></h3>
<strong>Syntax:</strong> RLimitOpenFiles <em>[scope] soft-limit|"max" [hard-limit|"max"]</em><br>
<strong>Default:</strong> <em>System defaults</em><br>
<strong>Context:</strong> server config, <code><VirtualHost></code>, <code><Global></code><br>
<strong>Module:</strong> mod_rlimit<br>
<strong>Compatibility:</strong> 1.3.5rc2
<p>
The <code>RLimitOpenFiles</code> directive is used to set a limit on the
file descriptors, expressed as counts.
<p>
<code>RLimitOpenFiles</code> takes from one to three parameters. The first
parameter is an optional <em>scope</em> parameter, indicating the scope of the
resource limit. The <em>scope</em> parameter may be one of:
<ul>
<li>"daemon" (applies the resource limit only to the <i>daemon</i> process)
<li>"session" (which applies the limit only to <i>session</i> processes handling each client)
<li>"none" (which disables any possibly inherited limits)
</ul>
If none of these keywords are used, the resource limit is assumed to apply to
both daemon and session processes.
<p>
The next parameters indicate the actual resource limits, both the so-called
"soft limit" and any "hard limit". These parameters may be a number, or the
value "max" to indicate that maximum resource limit value allowed is to be
used.
<p>
Example:
<pre>
# Limit a given session to 12 open file descriptors
RLimitOpenFiles session 12
</pre>
<p>
<hr>
<h2><a name="Installation">Installation</a></h2>
The <code>mod_rlimit</code> module is compiled into <code>proftpd</code> by
default.
<p><a name="FAQ">FAQ</a>
<b>Frequently Asked Questions</b><br>
<p>
<font color=red>Question</font>: Why can't I create directories named "lib"
or "etc" in the root directory? For example, my FTP client fails like so:
<code>
<font color=blue>Command: MKD lib</font>
<font color=green>Response: 550 lib: Permission denied</font>
<font color=blue>Command: MKD /lib</font>
<font color=green>Response: 550 /lib: Permission denied</font>
</code>
Although I don't have anything in my <code>proftpd.conf</code> that would block
these commands, and the filesystem permissions are OK. Why does this
happen?<br>
<font color=blue>Answer</font>: For the answer to this, see the description for
the <a href="#RLimitChroot"><code>RLimitChroot</code></a> directive.
<p>
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© Copyright 2013-2017 The ProFTPD Project<br>
All Rights Reserved<br>
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