Block SSH Server Attacks (Brute Force Attacks) Using DenyHosts

DenyHosts is an open source and free log-based intrusion prevention security program for SSH servers developed in Python language by Phil Schwartz. It is intended to monitor and analyzes SSH server logs for invalid login attempts, dictionary based attacks and brute force attacks by blocking the originating IP addresses by adding an entry to /etc/hosts.deny file on the server and prevents the IP address from making any further such login attempts.

Install DenyHosts to Block SSH Attacks

DenyHosts is much needed tool for all Linux based systems, specially when we are allowing password based ssh logins. In this article we are going to show you how to install and configure DenyHosts on RHEL 6.3/6.2/6.1/6/5.8, CentOS 6.3/6.2/6.1/6/5.8 and Fedora 17,16,15,14,13,12 systems using epel repository.

See also :

1. Fail2ban (Intrusion Prevention) System for SSH
2. Disable or Enable SSH Root Login
3. Linux Malware Detect (LMD)

Installing DenyHosts in RHEL, CentOS and Fedora

By default DenyHosts tool is not included in the Linux systems, we need to install it using third party EPEL repository. Once added repository, install the package using following YUM command.

# yum --enablerepo=epel install denyhosts
OR
# yum install denyhosts

Configuring DenyHosts for Whitelist IP Addresses

Once the Denyhosts installed, make sure to whitelist your own IP address, so you will never get locked out. To do this, open a file /etc/hosts.allow.

# vi /etc/hosts.allow

Below the description, add the each IP address one-by-one on a separate line, that you never want to block. The format should be as follows.

#
# hosts.allow   This file contains access rules which are used to
#               allow or deny connections to network services that
#               either use the tcp_wrappers library or that have been
#               started through a tcp_wrappers-enabled xinetd.
#
#               See 'man 5 hosts_options' and 'man 5 hosts_access'
#               for information on rule syntax.
#               See 'man tcpd' for information on tcp_wrappers
#
sshd: 172.16.25.125
sshd: 172.16.25.126
sshd: 172.16.25.127

Configuring DenyHosts for Email Alerts

The main configuration file is located under /etc/denyhosts.conf. This file is used to send email alerts about suspicious logins and restricted hosts. Open this file using VI editor.

# vi /etc/denyhosts.conf

Search for the ‘ADMIN_EMAIL‘ and add your email address here to receive email alerts about suspicious logins (for multiple email alerts use comma separated). Please have a look at the configuration file of my CentOS 6.3 server. Each variable is well documented so configure it according to your liking.

############ DENYHOSTS REQUIRED SETTINGS ############
SECURE_LOG = /var/log/secure
HOSTS_DENY = /etc/hosts.deny
BLOCK_SERVICE  = sshd
DENY_THRESHOLD_INVALID = 5
DENY_THRESHOLD_VALID = 10
DENY_THRESHOLD_ROOT = 1
DENY_THRESHOLD_RESTRICTED = 1
WORK_DIR = /var/lib/denyhosts
SUSPICIOUS_LOGIN_REPORT_ALLOWED_HOSTS=YES
HOSTNAME_LOOKUP=YES
LOCK_FILE = /var/lock/subsys/denyhosts
############ DENYHOSTS OPTIONAL SETTINGS ############
ADMIN_EMAIL = ravisaive@tecmint.com
SMTP_HOST = localhost
SMTP_PORT = 25
SMTP_FROM = DenyHosts <tecmint@tecmint.com>
SMTP_SUBJECT = DenyHosts Daily Report
############ DENYHOSTS OPTIONAL SETTINGS ############
DAEMON_LOG = /var/log/denyhosts
DAEMON_SLEEP = 30s
DAEMON_PURGE = 1h

Restarting DenyHosts Service

Once you’ve done with your configuration, restart the denyhosts service for new changes. We also add the denyhosts service to system start-up.

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