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ProFTPD module mod_delay
ProFTPD module mod_delay
The mod_delay module is designed to make a certain type of
information leak, known as a "timing attack", harder.
When proftpd processes the USER and PASS
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 proftpd to send a response to the USER and
PASS 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 (e.g.
LDAP directories, SQL tables, RADIUS servers, flat files, etc).
The mod_delay module attempts to prevent such timing differences
by keeping track of the time taken to process the USER and
PASS commands. It does this for the most recent
USER and PASS commands. The timing data are
stored in the module's DelayTable. If the module detects
that proftpd 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.
The most current version of mod_delay can be found in the
ProFTPD source distribution:
http://www.proftpd.org/
Thanks
2004-10-18: Thanks to Michael Renner for testing out various versions
of the module as it was developed.
Directives
Control Actions
Syntax: DelayControlsACLs actions|"all" "allow"|"deny" "user"|"group" list
Default: None
Context: server config
Module: mod_delay
Compatibility: 1.3.1rc1 and later
The DelayControlsACLs directive configures access lists of
users or groups who are allowed (or denied) the ability to
use the actions implemented by mod_delay. The default
behavior is to deny everyone unless an ACL allowing access has been explicitly
configured.
If "allow" is used, then list, a comma-delimited list
of users or groups, can use the given actions; all
others are denied. If "deny" is used, then the list of
users or groups cannot use actions all others are
allowed. Multiple DelayControlsACLs directives may be used to
configure ACLs for different control actions, and for both users and groups.
Syntax: DelayEngine on|off
Default: DelayEngine on
Context: server config
Module: mod_delay
Compatibility: 1.2.10rc1 and later
The DelayEngine directive enables or disables the module's
runtime delaying calculations. If it is set to off this module
does no delaying. Use this directive to disable the module.
Example:
<IfModule mod_delay.c>
DelayEngine off
</IfModule>
Syntax: DelayOnEvent event delay-ms
Default: None
Context: server config
Module: mod_delay
Compatibility: 1.3.6rc1 and later
The DelayOnEvent directive configures an explicit
delay-ms, in milliseconds, for the given event. The
supported values for the event parameter are:
The delay-ms parameter can be a number (in which case the units are
assumed to be milliseconds), or using a suffix.
To help illustrate the usage of DelayOnEvent, here are some
examples:
DelayOnEvent PASS 2000ms
This configures mod_delay to always add a delay of 2000
milliseconds for every PASS command. Alternatively, to add a delay only
on failed logins, you might use:
DelayOnEvent FailedLogin 5s
which adds a delay of 5 seconds after the login has failed.
Syntax: DelayTable path|"none"
Default: DelayTable /var/proftpd/proftpd.delay
Context: server config
Module: mod_delay
Compatibility: 1.2.10rc1 and later
The DelayTable directive configures a path to a file
that mod_delay uses for storing its timing data. The given
path must be an absolute path. It is recommended that this file
not be on an NFS mounted partition.
Note that timing data is kept across daemon stop/starts. When new
<VirtualHost>s are added to the configuration, though,
mod_delay will detect that it does not have a suitable
DelayTable for the new configuration, and will clear all
stored data.
If the DelayTable parameter is "none", then the
mod_delay module will not store timing data. This
configuration is used, in conjunction with
DelayOnEvent, for setting explicit
delays, rather than learning/adapting the delays dynamically.
Control Actions
Syntax: ftpdctl delay info
Purpose: Display data from the DelayTable
The delay info control action can be used to view the timing
data currently stored in the DelayTable file. This can help
to determine why the mod_delay module might be adding
longer-than-expected delays to the login sequence for FTP sessions.
Syntax: ftpdctl delay reset
Purpose: Reset the DelayTable, clearing all data
The delay reset control action can be used to clear the
DelayTable 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.
The mod_delay module provides protection against a very specific
problem. However, the module may not be appropriate for all situations.
For some sites, the delay added by mod_delay is not
helpful, and in fact can annoy users. To disable mod_delay,
simply add the following to your proftpd.conf file:
<IfModule mod_delay.c>
DelayEngine off
</IfModule>
For other sites, malicious users may notice that they can add arbitrarily
long delays to all users' sessions by "poisoning"
mod_delay's cache of timing information. For example, a
malicious client might connect, send a USER command, and then
not disconnect for long time. After a few such clients have connected,
mod_delay will add a delay for all clients, such that
some legitimate clients may time out. In effect, this may look like a sort
of denial of service (DoS).
To guard against the above situation, there are two possible solutions.
First, you can disable the mod_delay 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
mod_ifsession module
and its <IfClass> sections so that the
mod_delay module applies only to certain DNS names and
IP address ranges. For example:
# 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>
More information on defining classes can be found here.
Logging
The mod_delay module supports trace logging, via the module-specific log channels:
Thus for trace logging, to aid in debugging, you would use the following in
your proftpd.conf:
TraceLog /path/to/ftpd/trace.log
Trace delay:20
This trace logging can generate large files; it is intended for debugging use
only, and should be removed from any production configuration.
Frequently Asked Questions
Question: My client times out, after a couple of minutes,
after sending the USER command. After disabling mod_delay, the login succeeds quickly. Isn't this a bug in mod_delay?
Answer: No.
The DelayTable file can, over time, build up a store of long
delay values. A series of logins which take a while (e.g. SSL/TLS
handshakes over slow network connections) can cause this sort of behavior.
You can delete the DelayTable file or use the delay reset ftpdctl action to clear the stored data.
Question: Shouldn't mod_delay have some
sort of maximum delay value, so that it doesn't cause timeouts?
Answer: No. There is no single value, hardcoded
or configurable, that will work well for all sites and achieve
mod_delay's purpose at the same time. What seems like a normal
delay time for one site will be unacceptably long for another site.
Question: Is it OK to configure my
DelayTable on some ephemeral storage like tmpfs?
Answer: Yes, this is fine.
The mod_delay module is designed to create and populate a new
DelayTable when one is not present. So if your
DelayTable is stored on something like tmpfs such
that it goes away when your host is restarted, mod_delay will
create a new one. This does mean that the previously stored timing
data will be lost, and that mod_delay will start calculating and
storing new timing data; the functional impact should be negligible.
The mod_delay module is compiled by default.
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All Rights Reserved
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