DenyHosts est un script destiné à être exécuté par les administrateurs système Linux pour aider à contrecarrer les attaques de serveurs SSH (également connues sous le nom d'attaques basées sur des dictionnaires et d'attaques par force brute).

Le principe de Denyhosts est simple : c'est un daemon qui analyse en permanence le fichier de log /var/log/auth.log et qui au bout de plusieurs tentatives de connexion infructueuses blacklist l'IP en cause dans le fichier /etc/hosts.deny.

L'administrateur pourra même être averti par courriel lorsqu'une IP sera mise en liste noire.

• Parses /var/log/secure to find all login attempts and filters failed and successful attempts.
• Synchronization mode (new in 2.0) allows DenyHosts daemons the ability to share data via a centralized server to proactively thwart attacks.
• Can be run from the command line, cron or as a daemon (new in 0.9)
• Records all failed login attempts for the user and offending host
• For each host that exceeds a threshold count, records the evil host
• Keeps track of each non-existent user (eg. sdadasd) when a login attempt failed.
• Keeps track of each existing user (eg. root) when a login attempt failed.
• Keeps track of each offending host (with 0.8+ these hosts can be purged if the associated entry in /etc/hosts.deny is expired)
• Keeps track of suspicious logins (that is, logins that were successful for a host that had many login failures)
• Keeps track of the file offset, so that you can reparse the same file (/var/log/secure) continuously (until it is rotated).
• When the log file is rotated, the script will detect it and parse from the beginning.
• Appends /etc/hosts.deny and adds the newly banned hosts
• Optionally sends an email of newly banned hosts and suspicious logins.
• Keeps a history of all user, host, user/host combo and suspicious logins encountered which includes the data and number of corresponding failed login attempts.
• Maintains failed valid and invalid user login attempts in separate files, such that it is easy to see which valid user is under attack (which would give you the opportunity to remove the account, change the password or change it's default shell to something like /sbin/nologin
• Upon each run, the script will load the previously saved data and re-use it to append new failures.
• Resolves IP addresses to hostnames, if available (new in v0.6.0).
• /etc/hosts.deny entries can be expired (purge) at a user specified time (new in 0.8)
• FreeBSD support (added in 0.7)

https://github.com/denyhosts/denyhosts