MANUAL 0 8

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Revision as of 12:06, 17 January 2007 by NBrouard (Talk | contribs) (TODO and limitations: Multiple attacks within a few seconds)

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Introduction

The problem

Brute-force break-in attempts are quite frequent against an SSH server. Automated scripts try multiple combinations of username/password and sometimes changing the port to something other than 22 can't be done. Furthermore, scouring your log files yourself is not only time consuming, but can be difficult too.

Fail2ban attempts to alleviate these issues by providing an automated way of not only identifying possible break-in attempts, but acting upon them quickly and easily in a user-definable manor.

The solution

Log files contain interesting information, especially about failed logins. This information can be used to ban an offensive host. This is exactly what Fail2ban does. It scans log files and detect patterns which correspond to possible breakin attempts and then performs actions. Most of the time, it consists of adding a new rule in a firewall chain and sending an e-mail notification to the system administrator.

Here is a list of the most important features available in Fail2ban:

  • client/server
  • multithreaded
  • Gamin support
  • autodetection of the date/time format
  • wildcard support in logpath option
  • support for a lot of services (sshd, apache, qmail, proftpd, sasl, etc)
  • support for several actions (iptables, tcp-wrapper, shorewall, mail notifications, etc)

The code has been completely rewritten since 0.6.x. Fail2ban is entirely written in Python and thus should work on most of the *nix systems.

Installation

Requirements

In order to use Fail2ban, the following software are required:

  • Python >= 2.4

The following software are optional but recommended:

You will probably need at least one firewall software like iptables or shorewall. If you want a software which is not supported, please feel free to contact the author.

Gamin is a file alteration monitor. Gamin greatly benefits from a "inotify"-enabled kernel. Thus, active polling is no longer required to get the file modifications.

Installing from sources on a GNU/Linux system

You will need to obtain the latest version of the source code in order to compile Fail2ban yourself. Once you have done this, change to the directory where you downloaded the source code and execute the following:

tar xvjf fail2ban-x.x.x.tar.bz2

You will have the Fail2ban source code extracted to a directory under the current working directory. You must now move to the new directory.

Now run the installation script as root:

./setup.py install

Fail2ban should now be installed into /usr/lib/fail2ban and /usr/bin.

Debian

Installing Fail2ban on a Debian based system is very straightforward. Execute the following code as the root user on an unstable version of Debian:

apt-get install fail2ban

Gentoo

Fail2ban is available in Portage. To install it, just run:

emerge fail2ban

The FAQ have a more detailed explanation of installing using distributions such as Debian, RedHat and Gentoo


Usage

Definitions

Before we begin, it is important to clarify some terms used in the following sections.

filter  : a filter defines a regular expression which must match a pattern corresponding to a log-in failure or any other expression
action  : an action defines several commands which are executed at different moments
jail  : a jail is a combination of one filter and one or several actions. Fail2ban can handle several jails at the same time
client  : refers to the script fail2ban-client
server  : refers to the script fail2ban-server

Server

Fail2ban is composed of 2 parts: a client and a server. The server is multi-threaded and listen on a Unix socket for commands. The server itself knows nothing about the configuration files. Thus, at start-up, the server is in a "default" state in which no jails are defined. The following options are available for fail2ban-server:

-b                   start in background
-f                   start in foreground
-s <FILE>            socket path
-x                   force execution of the server
-h, --help           display this help message
-V, --version        print the version

fail2ban-server should not be used directly except in case of debugging. The option -s <FILE> is probably the most important one and is used to set the socket path. Thus, it is possible to run several instances of Fail2ban on different sockets. However, this should be not required because Fail2ban can run several jails concurrently.

If fail2ban-server crashes (does it?), it is possible that the socket file has not been removed correctly. The -x option tells the server to delete the socket file before start-up. If the socket file of a running server is removed, it is not possible to communicate with this server anymore.

The server handles the signals SIGTERM and SIGINT. When receiving one of these signals, fail2ban-server will quit nicely.


Client

fail2ban-client is the frontend of Fail2ban. It connects to the server socket file and sends commands in order to configure and operate the server. The client can read the configuration files or can simply be used to send single command to the server using command line or interactive mode (can be activated with -i). fail2ban-client can also start the server. The following options are available for fail2ban-client:

-c <DIR>                configuration directory
-s <FILE>               socket path
-d                      dump configuration. For debugging
-i                      interactive mode
-v                      increase verbosity
-q                      decrease verbosity
-x                      force execution of the server
-h, --help              display this help message
-V, --version           print the version

As for fail2ban-server, the option -s <FILE> can be used to set the socket path. Notice that this command line option overrides the socket option set in fail2ban.conf. The default configuration directory is /etc/fail2ban but can be override with the -c <DIR> option. The -x option is simply forwarded to fail2ban-server when starting the server.

An useful option for debugging is -d. This prints the configuration parsed by fail2ban-client. The output corresponds to the stream sent to the server. If the output of -d shows:

['set', 'loglevel', 1]
['set', 'logtarget', 'STDERR']

It is possible to achieve the same with:

$ fail2ban-client set loglevel 1
$ fail2ban-client set logtarget STDERR

Everything sets in the configuration files can be configured manually. The configuration is just a simple and efficient way to configure the server. fail2ban-client only translate the configuration into a suite of commands. However, fail2ban-client has 2 more commands for its internal use. The first one is start. When typing:

$ fail2ban-client start

the client will first try to fork a server instance. The client then waits for the server to start-up by sending ping requests to it. Once the server responses to these requests, fail2ban-client parses the configuration and sends the corresponding commands to the server. The second one is reload. When typing:

$ fail2ban-client reload

the client will tell the server to stop all jails, parse the configuration files again and send the commands to the server. This is useful when a new configuration must be loaded without shutting down the server. This is also very useful when debugging the server. It is possible to start the server with fail2ban-server -f in one terminal and to load the configuration by typing fail2ban-client reload in an other one. Thus, client and server output will not be mixed up.

Any other commands are simply sent to the server without any specific treatment. However, most of the time, only the 2 above commands and stop will be used.

There is probably one last useful command: status [jail]. Without a jail name, the global status of the server is returned. If jail corresponds to an existing jail, the status of this jail is displayed.

A list with all commands is available here.

Configuration

The standard path for the configuration is in /etc/fail2ban. This can be set with the -c option of fail2ban-client. A typical configuration looks like this:

/etc/fail2ban/
├── action.d
│   ├── dummy.conf
│   ├── hostsdeny.conf
│   ├── iptables.conf
│   ├── mail-whois.conf
│   ├── mail.conf
│   └── shorewall.conf
├── fail2ban.conf
├── fail2ban.local
├── filter.d
│   ├── apache-auth.conf
│   ├── apache-noscript.conf
│   ├── couriersmtp.conf
│   ├── postfix.conf
│   ├── proftpd.conf
│   ├── qmail.conf
│   ├── sasl.conf
│   ├── sshd.conf
│   └── vsftpd.conf
├── jail.conf
└── jail.local

Every .conf file can be overridden with a file named .local. Modification should take place in the .local and not in the .conf. This avoids merging problem when upgrading. These files are well documented and detailed information should be available there.

  • The file fail2ban.conf contains settings like the logging level or the logging target of fail2ban-server. You can also specify here the socket path used for the communication between the client and the server.
  • The most important file is probably jail.conf which contains the declaration of your jails. By default, some sections are inserted as templates. You must enable the sections of interest and adapt to your local configuration. Here is an example of the ssh-iptables section:
[ssh-iptables]
#enabled  = false
enabled  = true
filter   = sshd
action   = iptables[name=SSH, port=ssh, protocol=tcp]
#          mail-whois[name=SSH, dest=yourmail@mail.com]
#logpath  = /var/log/sshd.log
logpath  = /var/log/auth.log
maxretry = 5
With these settings a few things will happen:
  1. the section ssh-iptables is enabled;
  2. the filter sshd.conf in sub-directory filter.d will be processed;
  3. the action(s) described in iptables.conf (sub-directory action.d) will be executed if the outcome of the filter process is true. In this example, the additional action mail-whois.conf is commented out.
  4. the log file to be scanned by the filter is auth.log.
  • The directory action.d contains different scripts defining actions. The actions are executed at well-defined moment during the execution of Fail2ban: when starting/stopping a jail, banning/unbanning an host, etc.
  • The directory filter.d contains mainly regular expressions which are used to detect break-in attempts, password failures, etc. Here is an example for filter.d/sshd.conf with 3 possible regular expressions to match the lines of the logfile:
#failregex = (?:Authentication failure|Failed [-/\w+]+) for(?: [iI](?:llegal|nvalid))? user .*(?: from|FROM) <HOST>
failregex = (?:Authentication failure|Failed [-/\w+]+) for(?: [iI](?:llegal|nvalid))?( user)? .*(?: from|FROM) <HOST>
            ROOT LOGIN REFUSED .* FROM <HOST>
            [iI](?:llegal|nvalid) user .* from <HOST>
In the above example the default regex has been changed to allow the absence of user in a line such as:
Jan 10 07:02:37 homebrou sshd[18419]: Failed password for root from 222.76.213.151 port 55236 ssh2

Filter and actions are combined to create jails. Only one filter is allowed pro jail but it is possible to specify several actions. For example, you can react to a SSH break-in attempt by first adding a new firewall rule, then retrieving some information about the offending host using whois and finally sending an e-mail notification. Or maybe you just want to received a notification on your Jabber account when someone accesses the page /donotaccess.html on your web server.

Fail2ban is not limited to SSH. It contains default filters and actions for many daemons and services. You can easily modify them or create new ones.

Testing

Many things can be tested after configuration but the following commands can help to verify your settings:

# fail2ban-client -d

will dump the current configuration.

# fail2ban-regex "line" "failregex"

will test a single regular expression failregex (such as given in sshd.conf) with a single line of your logfile. Don't forget the double quotes around your line and failregex definitions.

TODO and limitations

  • In addition to the current fail2ban-regex command another command like:
# fail2ban-testconfig /var/log/auth.log /etc/fail2ban/filter.d/sshd.conf
could directly test the regular expressions defined by failregex= in file sshd.conf with a real (already written) log file (auth.log). Any volunteer?
  • It seems that if multiple attacks from the same IP are done within a very short period of time (a few seconds?), fail2ban doesn't have time to react and will not ban the IP after 3 (default) login attempts but many more. Here is an excerpt of a /var/log/auth.log:
Jan 15 15:28:46 homebrou sshd[29778]: Invalid user recruit from 81.74.87.66
Jan 15 15:28:46 homebrou sshd[29778]: error: Could not get shadow information for NOUSER
Jan 15 15:28:46 homebrou sshd[29778]: Failed password for invalid user recruit from 81.74.87.66 port 47672 ssh2
Jan 15 18:33:27 homebrou sshd[2156]: Did not receive identification string from 88.191.23.27
Jan 15 19:23:37 homebrou sshd[3418]: Invalid user test from 88.191.23.27
Jan 15 19:23:37 homebrou sshd[3418]: error: Could not get shadow information for NOUSER
Jan 15 19:23:37 homebrou sshd[3418]: Failed password for invalid user test from 88.191.23.27 port 41017 ssh2
Jan 15 19:23:38 homebrou sshd[3420]: Invalid user test from 88.191.23.27
Jan 15 19:23:38 homebrou sshd[3420]: error: Could not get shadow information for NOUSER
Jan 15 19:23:38 homebrou sshd[3420]: Failed password for invalid user test from 88.191.23.27 port 41096 ssh2
Jan 15 19:23:38 homebrou sshd[3422]: Invalid user test from 88.191.23.27
Jan 15 19:23:38 homebrou sshd[3422]: error: Could not get shadow information for NOUSER
Jan 15 19:23:38 homebrou sshd[3422]: Failed password for invalid user test from 88.191.23.27 port 41162 ssh2
Jan 15 19:23:38 homebrou sshd[3424]: Invalid user test from 88.191.23.27
Jan 15 19:23:38 homebrou sshd[3424]: error: Could not get shadow information for NOUSER
Jan 15 19:23:38 homebrou sshd[3424]: Failed password for invalid user test from 88.191.23.27 port 41209 ssh2
Jan 15 19:23:39 homebrou sshd[3426]: Invalid user test from 88.191.23.27
Jan 15 19:23:39 homebrou sshd[3426]: error: Could not get shadow information for NOUSER
Jan 15 19:23:39 homebrou sshd[3426]: Failed password for invalid user test from 88.191.23.27 port 41267 ssh2
Jan 15 19:23:39 homebrou sshd[3428]: Invalid user test from 88.191.23.27
Jan 15 19:23:39 homebrou sshd[3428]: error: Could not get shadow information for NOUSER
Jan 15 19:23:39 homebrou sshd[3428]: Failed password for invalid user test from 88.191.23.27 port 41323 ssh2
Jan 15 19:23:40 homebrou sshd[3430]: Invalid user test from 88.191.23.27
Jan 15 19:23:40 homebrou sshd[3430]: error: Could not get shadow information for NOUSER
Jan 15 19:23:40 homebrou sshd[3430]: Failed password for invalid user test from 88.191.23.27 port 41376 ssh2
Jan 15 19:23:40 homebrou sshd[3433]: Invalid user test from 88.191.23.27
Jan 15 19:23:40 homebrou sshd[3433]: error: Could not get shadow information for NOUSER
Jan 15 19:23:40 homebrou sshd[3433]: Failed password for invalid user test from 88.191.23.27 port 41433 ssh2
Jan 15 19:23:41 homebrou sshd[3435]: Invalid user test from 88.191.23.27
Jan 15 19:23:41 homebrou sshd[3435]: error: Could not get shadow information for NOUSER
Jan 15 19:23:41 homebrou sshd[3435]: Failed password for invalid user test from 88.191.23.27 port 41484 ssh2
Jan 16 12:13:43 homebrou sshd[32249]: Did not receive identification string from 209.126.131.150
Looking at /var/log/fail2ban.log around the corresponding period of time:
2007-01-15 15:38:47,142 fail2ban.actions: WARNING [ssh-iptables] Unban 81.74.87.66
2007-01-15 19:23:41,175 fail2ban.actions: WARNING [ssh-iptables] Ban 88.191.23.27
2007-01-15 19:23:42,373 fail2ban.actions: WARNING [ssh-iptables] 88.191.23.27 already banned
2007-01-15 19:33:41,508 fail2ban.actions: WARNING [ssh-iptables] Unban 88.191.23.27
2007-01-16 12:29:50,496 fail2ban.actions: WARNING [ssh-iptables] Ban 209.126.131.150
Thus, you can see that between 19:23:37 and 19:23:41, i.e. within 4 seconds, 9 login (ssh) attempts (instead of only 3) from 88.191.23.27 have been recorded in auth.log before it has been banned by fail2ban (using the regex Failed password for invalid user...). Nothing to be blamed, but should be mentioned in this manual.