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Developing Filters
Filters are tricky. They need to:
* work with a variety of the versions of the software that generates the logs;
* work with the range of logging configuration options available in the
* work with multiple operating systems;
* not make assumptions about the log format in excess of the software
(e.g. do not assume a username doesn't contain spaces and use \S+ unless
you've checked the source code);
* account for how future versions of the software will log messages
(e.g. guess what would happen to the log message if different authentication
types are added);
* not be susceptible to DoS vulnerabilities (see Filter Security below); and
* match intended log lines only.
Please follow the steps from Filter Test Cases to Developing Filter Regular
Expressions and submit a GitHub pull request (PR) afterwards. If you get stuck,
you can push your unfinished changes and still submit a PR -- describe
what you have done, what is the hurdle, and we'll attempt to help (PR
will be automagically updated with future commits you would push to
complete it).
Filter Test Cases
Start by finding the log messages that the application generates related to
some form of authentication failure. If you are adding to an existing filter
think about whether the log messages are of a similar importance and purpose
to the existing filter. If you were a user of Fail2Ban, and did a package
update of Fail2Ban that started matching new log messages, would anything
unexpected happen? Would the bantime/findtime for the jail be appropriate for
the new log messages? If it doesn't, perhaps it needs to be in a separate
filter definition, for example like exim filter aims at authentication failures
and exim-spam at log messages related to spam.
Even if it is a new filter you may consider separating the log messages into
different filters based on purpose.
Are some of the log lines a result of the same action? For example, is a PAM
failure log message, followed by an application specific failure message the
result of the same user/script action? If you add regular expressions for
both you would end up with two failures for a single action.
Therefore, select the most appropriate log message and document the other log
message) with a test case not to match it and a description as to why you chose
one over another.
With the selected log lines consider what action has caused those log
messages and whether they could have been generated by accident? Could
the log message be occurring due to the first step towards the application
asking for authentication? Could the log messages occur often? If some of
these are true make a note of this in the jail.conf example that you provide.
It is important to include log file samples so any future change in the regular
expression will still work with the log lines you have identified.
The sample log messages are provided in a file under testcases/files/logs/
named identically as the corresponding filter (but without .conf extension).
Each log line should be preceded by a line with failJSON metadata (so the logs
lines are tested in the test suite) directly above the log line. If there is
any specific information about the log message, such as version or an
application configuration option that is needed for the message to occur,
include this in a comment (line beginning with #) above the failJSON metadata.
Log samples should include only one, definitely not more than 3, examples of
log messages of the same form. If log messages are different in different
versions of the application log messages that show this are encouraged.
Also attempt to inject an IP into the application (e.g. by specifying
it as a username) so that Fail2Ban possibly detects the IP
from user input rather than the true origin. See the Filter Security section
and the top example in testcases/files/logs/apache-auth as to how to do this.
One you have discovered that this is possible, correct the regex so it doesn't
match and provide this as a test case with "match": false (see failJSON below).
If the mechanism to create the log message isn't obvious provide a
configuration and/or sample scripts testcases/files/config/{filtername} and
reference these in the comments above the log line.
FailJSON metadata
A failJSON metadata is a comment immediately above the log message. It will
look like::
# failJSON: { "time": "2013-06-10T10:10:59", "match": true , "host": "" }
Time should match the time of the log message. It is in a specific format of
Year-Month-Day'T'Hour:minute:Second. If your log message does not include a
year, like the example below, the year should be listed as 2005, if before Sun
Aug 14 10am UTC, and 2004 if afterwards. Here is an example failJSON
line preceding a sample log line::
# failJSON: { "time": "2005-03-24T15:25:51", "match": true , "host": "" }
Mar 24 15:25:51 buffalo1 dropbear[4092]: bad password attempt for 'root' from
The "host" in failJSON should contain the IP or domain that should be blocked.
For long lines that you do not want to be matched (e.g. from log injection
attacks) and any log lines to be excluded (see "Cause" section above), set
"match": false in the failJSON and describe the reason in the comment above.
After developing regexes, the following command will test all failJSON metadata
against the log lines in all sample log files::
./fail2ban-testcases testSampleRegex
Developing Filter Regular Expressions
At the moment, Fail2Ban depends on log lines to have time stamps. That is why
before starting to develop failregex, check if your log line format known to
Fail2Ban. Copy the time component from the log line and append an IP address to
test with following command::
./fail2ban-regex "2013-09-19 02:46:12" "<HOST>"
Output of such command should contain something like::
Date template hits:
|- [# of hits] date format
| [1] Year-Month-Day Hour:Minute:Second
Ensure that the template description matches time/date elements in your log line
time stamp. If there is no matched format then date template needs to be added
to server/ Ensure that a new template is added in the order
that more specific matches occur first and that there is no confusion between a
Day and a Month.
Filter file
The filter is specified in a config/filter.d/{filtername}.conf file. Filter file
can have sections INCLUDES (optional) and Definition as follows::
before = common.conf
after = filtername.local
failregex = ....
ignoreregex = ....
This is also documented in the man page jail.conf (section 5). Other definitions
can be added to make failregex's more readable and maintainable to be used
through string Interpolations (see
General rules
Use "before" if you need to include a common set of rules, like syslog or if
there is a common set of regexes for multiple filters.
Use "after" if you wish to allow the user to overwrite a set of customisations
of the current filter. This file doesn't need to exist.
Try to avoid using ignoreregex mainly for performance reasons. The case when you
would use it is if in trying to avoid using it, you end up with an unreadable
If your application logs to syslog you can take advantage of log line prefix
definitions present in common.conf. So as a base use::
before = common.conf
_daemon = app
failregex = ^%(__prefix_line)s
In this example common.conf defines __prefix_line which also contains the
_daemon name (in syslog terms the service) you have just specified. _daemon
can also be a regex.
For example, to capture following line _daemon should be set to "dovecot"::
Dec 12 11:19:11 dunnart dovecot: pop3-login: Aborted login (tried to use disabled plaintext auth): rip=, lip=
and then ``^%(__prefix_line)s`` would match "Dec 12 11:19:11 dunnart dovecot:
". Note it matches the trailing space(s) as well.
Substitutions (AKA string interpolations)
We have used string interpolations in above examples. They are useful for
making the regexes more readable, reuse generic patterns in multiple failregex
lines, and also to refer definition of regex parts to specific filters or even
to the user. General principle is that value of a _name variable replaces
occurrences of %(_name)s within the same section or anywhere in the config file
if defined in [DEFAULT] section.
Regular Expressions
Regular expressions (failregex, ignoreregex) assume that the date/time has been
removed from the log line (this is just how fail2ban works internally ATM).
If the format is like '<date...> error is evil' then you need to match
the < at the start so regex should be similar to '^<> <HOST> is evil$' using
<HOST> where the IP/domain name appears in the log line.
The following general rules apply to regular expressions:
* ensure regexes start with a ^ and are as restrictive as possible. E.g. do not
use .* if \d+ is sufficient;
* use functionality of Python regexes defined in the standard Python re library;
* make regular expressions readable (as much as possible). E.g.
(?:...) represents a non-capturing regex but (...) is more readable, thus
If you have only a basic knowledge of regular repressions we advise to read first. It doesn't take long and would
remind you e.g. which characters you need to escape and which you don't.
Developing/testing a regex
You can develop a regex in a file or using command line depending on your
preference. You can also use samples you have already created in the test cases
or test them one at a time.
The general tool for testing Fail2Ban regexes is fail2ban-regex. To see how to
use it run::
./fail2ban-regex --help
Take note of -l heavydebug / -l debug and -v as they might be very useful.
.. TIP::
Take a look at the source code of the application you are developing
failregex for. You may see optional or extra log messages, or parts there
of, that need to form part of your regex. It may also reveal how some
parts are constrained and different formats depending on configuration or
less common usages.
.. TIP::
For looking through source code - . It has
call graphs and can browse different versions.
.. TIP::
Some applications log spaces at the end. If you are not sure add \s*$ as
the end part of the regex.
If your regex is not matching, can help
to tune it. fail2ban-regex -D ... will present Debuggex URLs for the regexs
and sample log files that you pass into it.
In general use when using regex debuggers for generating fail2ban filters:
* use regex from the ./fail2ban-regex output (to ensure all substitutions are
* replace <HOST> with (?&.ipv4)
* make sure that regex type set to Python
* for the test data put your log output with the date/time removed
When you have fixed the regex put it back into your filter file.
Please spread the good word about Debuggex - Serge Toarca is kindly continuing
its free availability to Open Source developers.
Finishing up
If you've added a new filter, add a new entry in config/jail.conf. The theory
here is that a user will create a jail.local with [filtername]\nenable=true to
enable your jail.
So more specifically in the [filter] section in jail.conf:
* ensure that you have "enabled = false" (users will enable as needed);
* use "filter =" set to your filter name;
* use a typical action to disable ports associated with the application;
* set "logpath" to the usual location of application log file;
* if the default findtime or bantime isn't appropriate to the filter, specify
more appropriate choices (possibly with a brief comment line).
Submit github pull request (See "Pull Requests" above) for containing your great work.
Filter Security
Poor filter regular expressions are susceptible to DoS attacks.
When a remote user has the ability to introduce text that would match filter's
failregex, while matching inserted text to the <HOST> part, they have the
ability to deny any host they choose.
So the <HOST> part must be anchored on text generated by the application, and
not the user, to an extent sufficient to prevent user inserting the entire text
matching this or any other failregex.
Ideally filter regex should anchor at the beginning and at the end of log line.
However as more applications log at the beginning than the end, anchoring the
beginning is more important. If the log file used by the application is shared
with other applications, like system logs, ensure the other application that use
that log file do not log user generated text at the beginning of the line, or,
if they do, ensure the regexes of the filter are sufficient to mitigate the risk
of insertion.
Examples of poor filters
1. Too restrictive
We find a log message::
Apr-07-13 07:08:36 Invalid command fial2ban from
We make a failregex::
^Invalid command \S+ from <HOST>
Now think evil. The user does the command 'blah from'
The program diligently logs::
Apr-07-13 07:08:36 Invalid command blah from from
And fail2ban matches as the IP that it ban. A DoS attack was successful.
The fix here is that the command can be anything so .* is appropriate::
^Invalid command .* from <HOST>
Here the .* will match until the end of the string. Then realise it has more to
match, i.e. "from <HOST>" and go back until it find this. Then it will ban correctly. Since the <HOST> is always at the end, end the regex with a $::
^Invalid command .* from <HOST>$
Note if we'd just had the expression::
^Invalid command \S+ from <HOST>$
Then provided the user put a space in their command they would have never been
2. Unanchored regex can match other user injected data
From the Apache vulnerability CVE-2013-2178
( original ref: ).
An example bad regex for Apache::
failregex = [[]client <HOST>[]] user .* not found
Since the user can do a get request on::
GET /[client%20192.168.0.1]%20user%20root%20not%20found HTTP/1.0
Now the log line will be::
[Sat Jun 01 02:17:42 2013] [error] [client] File does not exist: /srv/http/site/[client] user root not found
As this log line doesn't match other expressions hence it matches the above
regex and blocks as a denial of service from the HTTP requester.
3. Over greedy pattern matching
An example ssh log (simplified)::
Sep 29 17:15:02 spaceman sshd[12946]: Failed password for user from port 20000 ssh1: ruser remoteuser
As we assume username can include anything including spaces its prudent to put
.* here. The remote user can also exist as anything so lets not make assumptions again::
failregex = ^%(__prefix_line)sFailed \S+ for .* from <HOST>( port \d*)?( ssh\d+)?(: ruser .*)?$
So this works. The problem is if the .* after remote user is injected by the
user to be 'from'. The resultant log line is::
Sep 29 17:15:02 spaceman sshd[12946]: Failed password for user from port 20000 ssh1: ruser from
Testing with::
fail2ban-regex -v 'Sep 29 17:15:02 Failed password for user from port 20000 ssh1: ruser from' '^ Failed \S+ for .* from <HOST>( port \d*)?( ssh\d+)?(: ruser .*)?$'
.. TIP:: I've removed the bit that matches __prefix_line from the regex and log.
1) [1] ^ Failed \S+ for .* from <HOST>( port \d*)?( ssh\d+)?(: ruser .*)?$ Sun Sep 29 17:15:02 2013
It should of matched So the first greedy part of the greedy regex
matched until the end of the string. The was no "from <HOST>" so the regex
engine worked backwards from the end of the string until this was matched.
The result was that was matched, injected by the user, and the wrong IP
was banned.
The solution here is to make the first .* non-greedy with .*?. Here it matches
as little as required and the fail2ban-regex tool shows the output::
fail2ban-regex -v 'Sep 29 17:15:02 Failed password for user from port 20000 ssh1: ruser from' '^ Failed \S+ for .*? from <HOST>( port \d*)?( ssh\d+)?(: ruser .*)?$'
1) [1] ^ Failed \S+ for .*? from <HOST>( port \d*)?( ssh\d+)?(: ruser .*)?$ Sun Sep 29 17:15:02 2013
So the general case here is a log line that contains::
Where the regex that matches fixed_data_1 is gready and matches the entire
string, before moving backwards and user_injectable_data can match the entire
Another case
A webserver logs the following without URL escaping::
[error] 2865#0: *66647 user "xyz" was not found in "/file", client:, server:, request: "GET ", client:, server:, request: "GET exploited HTTP/3.3", host: "", host: ""
failregex = ^ \[error\] \d+#\d+: \*\d+ user "\S+":? (?:password mismatch|was not found in ".*"), client: <HOST>, server: \S+, request: "\S+ .+ HTTP/\d+\.\d+", host: "\S+"
The .* matches to the end of the string. Finds that it can't continue to match
", client ... so it moves from the back and find that the user injected web URL::
", client:, server:, request: "GET exploited HTTP/3.3", host: "
In this case there is a fixed host: "" at the end so the solution
is to anchor the regex at the end with a $.
If this wasn't the case then first .* needed to be made so it didn't capture
beyond <HOST>.
4. Application generates two identical log messages with different meanings
If the application generates the following two messages under different
client <IP>: authentication failed
client <USER>: authentication failed
Then it's obvious that a regex of ``^client <HOST>: authentication
failed$`` will still cause problems if the user can trigger the second
log message with a <USER> of
Here there's nothing to do except request/change the application so it logs
messages differently.
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