MANUAL 0 8

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Revision as of 14:42, 15 December 2006 by Lostcontrol (Talk | contribs) (Server)

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Introduction

The problem

Brute-force breakin 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 aleviate 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

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 overriden 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.
  • 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 breakin attempts, password failures, etc.

Filter and actions are combinated 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 breakin attempt by first adding a new firewall rule, then retrieving some information about the offending host using whois and finally sending yourself 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.

Client

Server

Overview

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 startup, 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 startup. 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.