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DenyHosts is a script intended to be run by Linux system administrators to help thwart SSH server attacks (also known as dictionary based attacks and brute force attacks).


DenyHosts is a utility to help sys admins thwart ssh hackers

Developer: Phil Schwartz
License: GPLv2

Features and documentation

If you've ever looked at your ssh log (/var/log/messages, /var/log/warn, etc...) you may be alarmed to see how many hackers attempted to gain access to your server. Hopefully, none of them were successful (but then again, how would you know?). Wouldn't it be better to automatically prevent that attacker from continuing to gain entry into your system?

DenyHosts attempts to address the above... and more:

  • Parses /var/log/messages to find all login attempts and filters failed and successful attempts.
  • Synchronization mode 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
  • 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 (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/messages) 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)

Please have a look at the FAQ for details.

Installing and configuring DenyHosts

The tool DenyHosts is included in this repository

You must add this repository in Yast and then install it with

sudo zypper in denyhosts

See also

External Links