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DOC: more info on developing filters

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Daniel Black 11 years ago
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@ -37,19 +37,258 @@ When submitting pull requests on GitHub we ask you to:
Filters
=======
* Include sample logs with 1.2.3.4 used for IP addresses and
example.com/example.org used for DNS names
* Ensure sample log is provided in testcases/files/logs/ with same name as the
filter. Each log line should include match meta data for time & IP above
every line (see other sample log files for examples)
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
software;
* work in multiple operating systems;
* not make assumptions about the log format in excess of the software;
* make assumptions as to how future versions of the software will log messages;
* not be susceptable to DoS vulernabilities; 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 afterward. If you get stuck,
create a github issue with what you have done and we'll attempt to help.
Filter test cases
-----------------
Purpose:
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 wheither the log messages are of a simlar importance and purpose
to the existing filter. If you where a user of fail2ban, and did a package
update of fail2ban that started matching the new log messages, would anything
unexpected happen? Would the bantime/findtime for the jail be approprate for
the new log messages. If it doesn't perhaps it needs to be in a separate
filter defination, for example like exim is authentication failures and
exim-spam contains log messages replated to spam.
Even if its a new filter you may consider separating the log messages into
different filters based on purpose.
Cause:
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. The result is if you add regular
expressions for both you'll end up with two failures for a single action.
Select the most approprate 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 log lines selected consider what occured to generate those log
messages and wheither they could of been generated by accidental means. Could
the log message occur always as this is 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.
Samples:
Its 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 testcases/files/logs/ with same name
as the filter. Each log line should include a 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, definately 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 is encouraged.
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": "193.169.56.211" }
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 will be 2005, if before Sun Aug 14 10am
UTC, and 2004 if afterwards.
# failJSON: { "time": "2005-03-24T15:25:51", "match": true , "host": "198.51.100.87" }
Mar 24 15:25:51 buffalo1 dropbear[4092]: bad password attempt for 'root' from 198.51.100.87:5543
The host will contain the IP or domain that should be blocked.
For long lines that you don't want matched, like log injection vulerabilities
and log lines excluded (see "Cause" section above), a "match": false in the
failJSON and the reason why in the comment above.
After developing the regexs, the following command will test all the failJSON
metadata against the log lines:
./fail2ban-testcases testSampleRegex
Developing Filter Regular Expressions
-------------------------------------
Date/Time:
The first step in checking your log line can have a filter is to check that the
time format matches an existing regex. To test this copy the time component
from the log line and append an IP address. Then test it with:
./fail2ban-regex "2013-09-19 02:46:12 1.2.3.4" "<HOST>"
In the output from this should be something like:
Date template hits:
|- [# of hits] date format
| [1] Year-Month-Day Hour:Minute:Second
Ensure that the template description matches of bits in the time format. If
there isn't a matched a format and date regex can be added to
server/datedetector.py. Ensure this is added in an order that will match make
more specific matches occur first and that their is no confusion as to which
is the date or month.
Filter file:
The filter file is in config/filter.d/{filtername}.conf. The format of the
filter file has two sections INCLUDES and Defination as follows:
[INCLUDES]
before = common.conf
after = filtername.local
[Definition]
failregex = ....
ignoreregex = ....
This is also documented in the man pages as jail.conf (section 5). Other
definations can be added to make failregex's more readable and maintainable.
General rules:
Use "before" if you need to include a common set of rules, like syslog or if
there's a common set of regexs 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 ignoreregex, you end up with
an unreadable failregex.
Syslog:
If your application logs to syslog you can use the following to capture that
part. So as a base use:
[INCLUDES]
before = commmon.conf
[Definition]
_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 specified. _daemon can also be
a regex.
So the following uses a _daemon set to "dovecot"
Dec 12 11:19:11 dunnart dovecot: pop3-login: Aborted login (tried to use disabled plaintext auth): rip=190.210.136.21, lip=113.212.99.193
So now ^%(__prefix_line)s matches "Dec 12 11:19:11 dunnart dovecot: ". Note it
matches the trailing space. Putting a space after ^%(__prefix_line)s in the
regex will probably not match.
Substitions:
Substitions are what the syslog uses. The regex bits of %(_name)s substitute
the _name defination into the regex. They are useful for making the regexes
more readable and also defining regex parts that occur in multiple log lines.
Regular Expressions:
The regular expression you will be writing will assume that the date/time has
been removed from the log line because this is how fail2ban works internally.
If the format is like '<date...> error 1.2.3.4 is evil' then you will need to
match the < at the start so regex should be similar to '^<> <HOST> is evil$'.
Use <HOST> where the IP/domain name appears in the log line.
The following general rules apply to regular expressions:
* Ensure regexs start with a ^ and are restrictive as possible. E.g. not .* if
\d+ is sufficient
* Use the functionality of regexs http://docs.python.org/2/library/re.html
* Take a look at the source code of the application. You may see optional or
extra log messages, or parts there of, that need to form part of your regex.
* Try to make the regular expression readable (as much as possible). E.g.
(?:...) represents a non-capturing regex but (...) is more readable.
If you only have a basic knowledge of regular repressions read
http://docs.python.org/2/library/re.html first.
http://docs.python.org/2/library/re.html first. Really. It doesn't take long
and will remind you which bits you need to escape and which bits you don't.
Developing/testing the regex:
You can develop the regex in the file or on the command line depending on your
preference. You can also use the samples you've created in the test cases or
test them one at a time.
The general tool is fail2ban-regex. To see how to use it run:
./fail2ban-regex --help
Take note of -l heavydebug / -l debug and -v as they will be most useful.
TIP: Take a look at the source code of the application. You may see optional or
extra log messages, or parts there of, that need to form part of your regex.
It may also show how some parts are contrained and different formats
depending on configuration or less common usages.
TIP: Some applications log spaces at the end. If you're not sure add \s*$ as the
end part of the regex.
If your regex isn't matching take a look at http://www.debuggex.com/.
Using the regex from the ./fail2ban-regex output (to ensure all substitutions
are done) and with <HOST> replaced with (?&.ipv4). Set the regex type to
Python.
For the test data put your log output with the time removed.
When you've fixed the regex put it back into your filter file.
Please give a donation to stoarca for debuggex. Its a great tool isn't it.
Finishing up:
If you've created a new filter, add an entry in config/jail.conf. The theory
here is that a user will create a jail.conf with [filtername]\nenable=true.
So more specifically in the [filter] section in jail.conf:
* Ensure that you have "enabled = false", we want people to enable as needed
* use "filter =" set to your filter name.
* use a action to disable ports associated with the application
* set "logpath" to a usual location for the log file for the application.
* If the default findtime or bantime isn't approprate to the filter set a value
that is more approprate.
Send the fail2ban a git pull request (See "Pull Requests" above) containing
your great work.
Filter Security
---------------
@ -63,8 +302,6 @@ 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 a sufficient extent that the user cannot insert the entire text.
Filters are matched against the log line with their date removed.
Ideally filter regex should anchor to the beginning and end of the log line
however as more applications log at the beginning than the end, achoring the
beginning is more important. If the log file used by the application is shared
@ -73,13 +310,6 @@ use that log file do not log user generated text at the beginning of the line,
or, if they do, ensure the regexs of the filter are sufficient to mitigate the
risk of insertion.
When creating a regex that extends back to the begining remember the date part
has been removed within fail2ban so theres no need to match that. If the format
is like '<date...> error 1.2.3.4 is evil' then you will need to match the < at
the start so here the regex would start like '^<> <HOST> is evil$'.
Some applications log spaces at the end. If you're not sure add \s*$ as the
end part of the regex.
Examples of poor filters
------------------------

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