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<h2><b>ProFTPD module <code>mod_delay</code></b></h2>
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<p>
The <code>mod_delay</code> module is designed to make a certain type of
information leak, known as a "timing attack", harder.
<p>
When <code>proftpd</code> processes the <code>USER</code> and <code>PASS</code>
FTP commands from a client, it has to perform checks against configured
ACLs, look up user and group information, etc. These checks are not done
if the given username is known to not exist for the server, in order to
not tie up system resources needlessly. However, this does mean that more
work is done when handling "good" users than when handling
"bad" users. This difference can be detected in the time it takes
for <code>proftpd</code> to send a response to the <code>USER</code> and
<code>PASS</code> commands. This means it is possible for an attacker
to look for these statistical timing differences, and determine which
users are "good" and which are "bad". From there,
a determined attacker can focus their attention on the known good usernames.
Note that the timings will vary depending on server load, number of
users in the user base, type of storage of user data (<i>e.g.</i>
LDAP directories, SQL tables, RADIUS servers, flat files, <i>etc</i>).
<p>
The <code>mod_delay</code> module attempts to prevent such timing differences
by keeping track of the time taken to process the <code>USER</code> and
<code>PASS</code> commands. It does this for the most recent
<code>USER</code> and <code>PASS</code> commands. The timing data are
stored in the module's <code>DelayTable</code>. If the module detects
that <code>proftpd</code> has not taken enough time to handle one of these
commands, compared to its past response times, a small delay will be added
to the response cycle. The amount of delay is determined by the difference
between the current time spent handling the command and the median time
spent handling the same command in the past.
<p>
The most current version of <code>mod_delay</code> can be found in the
ProFTPD source distribution:
<pre>
<a href="http://www.proftpd.org/">http://www.proftpd.org/</a>
</pre>
<h2>Thanks</h2>
<p>
<i>2004-10-18</i>: Thanks to Michael Renner for testing out various versions
of the module as it was developed.
<h2>Directives</h2>
<ul>
<li><a href="#DelayControlsACLs">DelayControlsACLs</a>
<li><a href="#DelayEngine">DelayEngine</a>
<li><a href="#DelayTable">DelayTable</a>
</ul>
<h2>Control Actions</h2>
<ul>
<li><a href="#delay_info"><code>delay info</code></a>
<li><a href="#delay_reset"><code>delay reset</code></a>
</ul>
<hr>
<h2><a name="DelayControlsACLs">DelayControlsACLs</a></h3>
<strong>Syntax:</strong> DelayControlsACLs <em>actions|"all" "allow"|"deny" "user"|"group" list</em><br>
<strong>Default:</strong> None<br>
<strong>Context:</strong> server config<br>
<strong>Module:</strong> mod_delay<br>
<strong>Compatibility:</strong> 1.3.1rc1 and later
<p>
The <code>DelayControlsACLs</code> directive configures access lists of
<em>users</em> or <em>groups</em> who are allowed (or denied) the ability to
use the <em>actions</em> implemented by <code>mod_delay</code>. The default
behavior is to deny everyone unless an ACL allowing access has been explicitly
configured.
<p>
If "allow" is used, then <em>list</em>, a comma-delimited list
of <em>users</em> or <em>groups</em>, can use the given <em>actions</em>; all
others are denied. If "deny" is used, then the <em>list</em> of
<em>users</em> or <em>groups</em> cannot use <em>actions</em> all others are
allowed. Multiple <code>DelayControlsACLs</code> directives may be used to
configure ACLs for different control actions, and for both users and groups.
<hr>
<h2><a name="DelayEngine">DelayEngine</a></h2>
<strong>Syntax:</strong> DelayEngine <em>on|off</em><br>
<strong>Default:</strong> DelayEngine on<br>
<strong>Context:</strong> "server config"<br>
<strong>Module:</strong> mod_delay<br>
<strong>Compatibility:</strong> 1.2.10rc1 and later
<p>
The <code>DelayEngine</code> directive enables or disables the module's
runtime delaying calculations. If it is set to <em>off</em> this module
does no delaying. Use this directive to disable the module.
<p>
Example:
<pre>
<IfModule mod_delay.c>
DelayEngine off
</IfModule>
</pre>
<hr>
<h2><a name="DelayTable">DelayTable</a></h2>
<strong>Syntax:</strong> DelayTable <em>path</em><br>
<strong>Default:</strong> DelayTable var/proftpd/proftpd.delay<br>
<strong>Context:</strong> "server config"<br>
<strong>Module:</strong> mod_delay<br>
<strong>Compatibility:</strong> 1.2.10rc1 and later
<p>
The <code>DelayTable</code> directive configures a <em>path</em> to a file
that <code>mod_delay</code> uses for storing its timing data. The given
<em>path</em> must be an absolute path. It is recommended that this file
<b>not</b> be on an NFS mounted partition.
<p>
Note that timing data is kept across daemon stop/starts. When new
<code><VirtualHost></code>s are added to the configuration, though,
<code>mod_delay</code> will detect that it does not have a suitable
<code>DelayTable</code> for the new configuration, and will clear all
stored data.
<p>
<hr>
<h2>Control Actions</h2>
<p>
<hr>
<h3><a name="delay_info"><code>delay info</code></a></h3>
<strong>Syntax:</strong> ftpdctl delay info<br>
<strong>Purpose:</strong> Display data from the DelayTable<br>
<p>
The <code>delay info</code> control action can be used to view the timing
data currently stored in the <code>DelayTable</code> file. This can help
to determine why the <code>mod_delay</code> module might be adding
longer-than-expected delays to the login sequence for FTP sessions.
<p>
<hr>
<h3><a name="delay_reset"><code>delay reset</code></a></h3>
<strong>Syntax:</strong> ftpdctl delay reset<br>
<strong>Purpose:</strong> Reset the DelayTable, clearing all data<br>
<p>
The <code>delay reset</code> control action can be used to clear the
<code>DelayTable</code> of all timing information, resetting the table.
This can be used if the table starts to collect delays that are too long
to be useful.
<p>
<hr>
<h2><a name="Usage">Usage</a></h2>
The <code>mod_delay</code> module provides protection against a very specific
problem. However, the module may not be appropriate for all situations.
<p>
For some sites, the delay added by <code>mod_delay</code> is not
helpful, and in fact can annoy users. To disable <code>mod_delay</code>,
simply add the following to your <code>proftpd.conf</code> file:
<pre>
<IfModule mod_delay.c>
DelayEngine off
</IfModule>
</pre>
<p>
For other sites, malicious users may notice that they can add arbitrarily
long delays to <i>all</i> users' sessions by "poisoning"
<code>mod_delay</code>'s cache of timing information. For example, a
malicious client might connect, send a <code>USER</code> command, and then
not disconnect for long time. After a few such clients have connected,
<code>mod_delay</code> will add a delay for <i>all</i> clients, such that
some legitimate clients may time out. In effect, this may look like a sort
of denial of service (DoS).
<p>
To guard against the above situation, there are two possible solutions.
First, you can disable the <code>mod_delay</code> module entirely (see above);
if the timing information leak is not of concern for your FTP site, this is
the recommended approach. Second, you can use the
<a href="../contrib/mod_ifsession.html"><code>mod_ifsession</code></a> module
and its <code><IfClass></code> sections so that the
<code>mod_delay</code> module applies only to certain DNS names and
IP address ranges. For example:
<pre>
# Define a class which identifies trusted clients
<Class trusted>
From a.b.c.d
From e.f.g.h
</Class>
<IfModule mod_delay.c>
<IfClass trusted>
DelayEngine off
</IfClass>
<IfClass !trusted>
DelayEngine on
</IfClass>
</IfModule>
</pre>
More information on defining classes can be found <a href="../howto/Classes.html">here</a>.
<p><a name="FAQ">
<b>Frequently Asked Questions</b><br>
<p>
<font color=red>Question</font>: My client times out, after a couple of minutes,
after sending the <code>USER</code> command. After disabling <code>mod_delay</code>, the login succeeds quickly. Isn't this a bug in <code>mod_delay</code>?<br>
<font color=blue>Answer</font>: No.
<p>
The <code>DelayTable</code> file can, over time, build up a store of long
delay values. A series of logins which take a while (<i>e.g.</i> SSL/TLS
handshakes over slow network connections) can cause this sort of behavior.
You can delete the <code>DelayTable</code> file or use the <a href="#delay_reset"><code>delay reset</code></a> ftpdctl action to clear the stored data.
<p>
<font color=red>Question</font>: Shouldn't <code>mod_delay</code> have some
sort of maximum delay value, so that it doesn't cause timeouts?<br>
<font color=blue>Answer</font>: No. There is no single value, hardcoded
or configurable, that will work well for all sites <i>and</i> achieve
<code>mod_delay</code>'s purpose at the same time. What seems like a normal
delay time for one site will be unacceptably long for another site.
<p>
<hr>
<h2><a name="Installation">Installation</a></h2>
The <code>mod_delay</code> module is compiled by default.
<p>
<hr><br>
Author: <i>$Author: castaglia $</i><br>
Last Updated: <i>$Date: 2009/03/04 00:57:52 $</i><br>
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<font size=2><b><i>
© Copyright 2004-2009 TJ Saunders<br>
All Rights Reserved<br>
</i></b></font>
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