Preventing SSH Dictionary Attacks With DenyHosts
Version 1.0
Author: Falko Timme
Last edited: 02/07/2006
In this HowTo I will show how to install and configure DenyHosts. DenyHosts is a tool that observes login attempts to SSH, and if it finds failed login attempts again and again from the same IP address, DenyHosts blocks further login attempts from that IP address by putting it into /etc/hosts.deny. DenyHosts can be run by cron or as a daemon. In this tutorial I will run DenyHosts as a daemon.
From the DenyHosts web site:
"DenyHosts is a script intended to be run by Linux system administrators to help thwart ssh server attacks.
If you've ever looked at your ssh log (/var/log/secure on Redhat, /var/log/auth.log on Mandrake, 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... "
This tutorial is based on a Debian Sarge system, however, it should apply to other distributions with almost no modifications.
I want to say first
that this is not the only way of setting up such a system. There are many ways
of achieving this goal but this is the way I take. I do not issue any guarantee
that this will work for you!
1 Installation
DenyHosts is written in Python, therefore we must install Python and also the Python development files first:
apt-get install python python2.3-dev python2.3
Then we download and install DenyHosts like this:
cd /tmp
wget http://mesh.dl.sourceforge.net/sourceforge/denyhosts/DenyHosts-2.0.tar.gz
tar xvfz DenyHosts-2.0.tar.gz
cd DenyHosts-2.0
python setup.py install
This installs DenyHosts to /usr/share/denyhosts.
2 Configuration
Now we have to create the DenyHosts configuration file /usr/share/denyhosts/denyhosts.cfg. We can use the sample configuration file /usr/share/denyhosts/denyhosts.cfg-dist for this:
cd /usr/share/denyhosts
cp denyhosts.cfg-dist denyhosts.cfg
Then we must edit denyhosts.cfg with our favourite editor such as vi, for example. Mine looks like this:
############ THESE SETTINGS ARE REQUIRED ############
########################################################################
#
# SECURE_LOG: the log file that contains sshd logging info
# if you are not sure, grep "sshd:" /var/log/*
#
# The file to process can be overridden with the --file command line
# argument
#
# Redhat or Fedora Core:
#SECURE_LOG = /var/log/secure
#
# Mandrake, FreeBSD or OpenBSD:
SECURE_LOG = /var/log/auth.log
#
# SuSE:
#SECURE_LOG = /var/log/messages
#
########################################################################
########################################################################
# HOSTS_DENY: the file which contains restricted host access information
#
# Most operating systems:
HOSTS_DENY = /etc/hosts.deny
#
# Some BSD (FreeBSD) Unixes:
#HOSTS_DENY = /etc/hosts.allow
#
# Another possibility (also see the next option):
#HOSTS_DENY = /etc/hosts.evil
#######################################################################
########################################################################
# PURGE_DENY: removed HOSTS_DENY entries that are older than this time
# when DenyHosts is invoked with the --purge flag
#
# format is: i[dhwmy]
# Where 'i' is an integer (eg. 7)
# 'm' = minutes
# 'h' = hours
# 'd' = days
# 'w' = weeks
# 'y' = years
#
# never purge:
PURGE_DENY =
#
# purge entries older than 1 week
#PURGE_DENY = 1w
#
# purge entries older than 5 days
#PURGE_DENY = 5d
#######################################################################
#######################################################################
# BLOCK_SERVICE: the service name that should be blocked in HOSTS_DENY
#
# man 5 hosts_access for details
#
# eg. sshd: 127.0.0.1 # will block sshd logins from 127.0.0.1
#
# To block all services for the offending host:
#BLOCK_SERVICE = ALL
# To block only sshd:
BLOCK_SERVICE = sshd
# To only record the offending host and nothing else (if using
# an auxilary file to list the hosts). Refer to:
# http://denyhosts.sourceforge.net/faq.html#aux
#BLOCK_SERVICE =
#
#######################################################################
#######################################################################
#
# DENY_THRESHOLD_INVALID: block each host after the number of failed login
# attempts has exceeded this value. This value applies to invalid
# user login attempts (eg. non-existent user accounts)
#
DENY_THRESHOLD_INVALID = 5
#
#######################################################################
#######################################################################
#
# DENY_THRESHOLD_VALID: block each host after the number of failed
# login attempts has exceeded this value. This value applies to valid
# user login attempts (eg. user accounts that exist in /etc/passwd) except
# for the "root" user
#
DENY_THRESHOLD_VALID = 10
#
#######################################################################
#######################################################################
#
# DENY_THRESHOLD_ROOT: block each host after the number of failed
# login attempts has exceeded this value. This value applies to
# "root" user login attempts only.
#
DENY_THRESHOLD_ROOT = 5
#
#######################################################################
#######################################################################
#
# WORK_DIR: the path that DenyHosts will use for writing data to
# (it will be created if it does not already exist).
#
# Note: it is recommended that you use an absolute pathname
# for this value (eg. /home/foo/denyhosts/data)
#
WORK_DIR = /usr/share/denyhosts/data
#
#######################################################################
#######################################################################
#
# SUSPICIOUS_LOGIN_REPORT_ALLOWED_HOSTS
#
# SUSPICIOUS_LOGIN_REPORT_ALLOWED_HOSTS=YES|NO
# If set to YES, if a suspicious login attempt results from an allowed-host
# then it is considered suspicious. If this is NO, then suspicious logins
# from allowed-hosts will not be reported. All suspicious logins from
# ip addresses that are not in allowed-hosts will always be reported.
#
SUSPICIOUS_LOGIN_REPORT_ALLOWED_HOSTS=YES
######################################################################
######################################################################
#
# HOSTNAME_LOOKUP
#
# HOSTNAME_LOOKUP=YES|NO
# If set to YES, for each IP address that is reported by Denyhosts,
# the corresponding hostname will be looked up and reported as well
# (if available).
#
HOSTNAME_LOOKUP=YES
#
######################################################################
######################################################################
#
# LOCK_FILE
#
# LOCK_FILE=/path/denyhosts
# If this file exists when DenyHosts is run, then DenyHosts will exit
# immediately. Otherwise, this file will be created upon invocation
# and deleted upon exit. This ensures that only one instance is
# running at a time.
#
# Redhat/Fedora:
#LOCK_FILE = /var/lock/subsys/denyhosts
#
# Debian
LOCK_FILE = /var/run/denyhosts.pid
#
# Misc
#LOCK_FILE = /tmp/denyhosts.lock
#
######################################################################
############ THESE SETTINGS ARE OPTIONAL ############
#######################################################################
#
# ADMIN_EMAIL: if you would like to receive emails regarding newly
# restricted hosts and suspicious logins, set this address to
# match your email address. If you do not want to receive these reports
# leave this field blank (or run with the --noemail option)
#
ADMIN_EMAIL =
#
#######################################################################
#######################################################################
#
SMTP_HOST = localhost
SMTP_PORT = 25
SMTP_FROM = DenyHosts <nobody@localhost>
SMTP_SUBJECT = DenyHosts Report
#SMTP_USERNAME=foo
#SMTP_PASSWORD=bar
#
#######################################################################
######################################################################
#
# ALLOWED_HOSTS_HOSTNAME_LOOKUP
#
# ALLOWED_HOSTS_HOSTNAME_LOOKUP=YES|NO
# If set to YES, for each entry in the WORK_DIR/allowed-hosts file,
# the hostname will be looked up. If your versions of tcp_wrappers
# and sshd sometimes log hostnames in addition to ip addresses
# then you may wish to specify this option.
#
#ALLOWED_HOSTS_HOSTNAME_LOOKUP=NO
#
######################################################################
######################################################################
#
# AGE_RESET_VALID: Specifies the period of time between failed login
# attempts that, when exceeded will result in the failed count for
# this host to be reset to 0. This value applies to login attempts
# to all valid users (those within /etc/passwd) with the
# exception of root. If not defined, this count will never
# be reset.
#
# See the comments in the PURGE_DENY section (above)
# for details on specifying this value or for complete details
# refer to: http://denyhosts.sourceforge.net/faq.html#timespec
#
AGE_RESET_VALID=5d
#
######################################################################
######################################################################
#
# AGE_RESET_ROOT: Specifies the period of time between failed login
# attempts that, when exceeded will result in the failed count for
# this host to be reset to 0. This value applies to all login
# attempts to the "root" user account. If not defined,
# this count will never be reset.
#
# See the comments in the PURGE_DENY section (above)
# for details on specifying this value or for complete details
# refer to: http://denyhosts.sourceforge.net/faq.html#timespec
#
AGE_RESET_ROOT=25d
#
######################################################################
######################################################################
#
# AGE_RESET_INVALID: Specifies the period of time between failed login
# attempts that, when exceeded will result in the failed count for
# this host to be reset to 0. This value applies to login attempts
# made to any invalid username (those that do not appear
# in /etc/passwd). If not defined, count will never be reset.
#
# See the comments in the PURGE_DENY section (above)
# for details on specifying this value or for complete details
# refer to: http://denyhosts.sourceforge.net/faq.html#timespec
#
AGE_RESET_INVALID=10d
#
######################################################################
######################################################################
#
# PLUGIN_DENY: If set, this value should point to an executable
# program that will be invoked when a host is added to the
# HOSTS_DENY file. This executable will be passed the host
# that will be added as it's only argument.
#
#PLUGIN_DENY=/usr/bin/true
#
######################################################################
######################################################################
#
# PLUGIN_PURGE: If set, this value should point to an executable
# program that will be invoked when a host is removed from the
# HOSTS_DENY file. This executable will be passed the host
# that is to be purged as it's only argument.
#
#PLUGIN_PURGE=/usr/bin/true
#
######################################################################
######################################################################
#
# USERDEF_FAILED_ENTRY_REGEX: if set, this value should contain
# a regular expression that can be used to identify additional
# hackers for your particular ssh configuration. This functionality
# extends the built-in regular expressions that DenyHosts uses.
# This parameter can be specified multiple times.
# See this faq entry for more details:
# http://denyhosts.sf.net/faq.html#userdef_regex
#
#USERDEF_FAILED_ENTRY_REGEX=
#
#
######################################################################
######### THESE SETTINGS ARE SPECIFIC TO DAEMON MODE ##########
#######################################################################
#
# DAEMON_LOG: when DenyHosts is run in daemon mode (--daemon flag)
# this is the logfile that DenyHosts uses to report it's status.
# To disable logging, leave blank. (default is: /var/log/denyhosts)
#
DAEMON_LOG = /var/log/denyhosts
#
# disable logging:
#DAEMON_LOG =
#
######################################################################
#######################################################################
#
# DAEMON_LOG_TIME_FORMAT: when DenyHosts is run in daemon mode
# (--daemon flag) this specifies the timestamp format of
# the DAEMON_LOG messages (default is the ISO8061 format:
# ie. 2005-07-22 10:38:01,745)
#
# for possible values for this parameter refer to: man strftime
#
# Jan 1 13:05:59
#DAEMON_LOG_TIME_FORMAT = %b %d %H:%M:%S
#
# Jan 1 01:05:59
#DAEMON_LOG_TIME_FORMAT = %b %d %I:%M:%S
#
######################################################################
#######################################################################
#
# DAEMON_LOG_MESSAGE_FORMAT: when DenyHosts is run in daemon mode
# (--daemon flag) this specifies the message format of each logged
# entry. By default the following format is used:
#
# %(asctime)s - %(name)-12s: %(levelname)-8s %(message)s
#
# Where the "%(asctime)s" portion is expanded to the format
# defined by DAEMON_LOG_TIME_FORMAT
#
# This string is passed to python's logging.Formatter contstuctor.
# For details on the possible format types please refer to:
# http://docs.python.org/lib/node357.html
#
# This is the default:
#DAEMON_LOG_MESSAGE_FORMAT = %(asctime)s - %(name)-12s: %(levelname)-8s %(message)s
#
#
######################################################################
#######################################################################
#
# DAEMON_SLEEP: when DenyHosts is run in daemon mode (--daemon flag)
# this is the amount of time DenyHosts will sleep between polling
# the SECURE_LOG. See the comments in the PURGE_DENY section (above)
# for details on specifying this value or for complete details
# refer to: http://denyhosts.sourceforge.net/faq.html#timespec
#
#
DAEMON_SLEEP = 30s
#
#######################################################################
#######################################################################
#
# DAEMON_PURGE: How often should DenyHosts, when run in daemon mode,
# run the purge mechanism to expire old entries in HOSTS_DENY
# This has no effect if PURGE_DENY is blank.
#
DAEMON_PURGE = 1h
#
#######################################################################
######### THESE SETTINGS ARE SPECIFIC TO ##########
######### DAEMON SYNCHRONIZATION ##########
#######################################################################
#
# Synchronization mode allows the DenyHosts daemon the ability
# to periodically send and receive denied host data such that
# DenyHosts daemons worldwide can automatically inform one
# another regarding banned hosts. This mode is disabled by
# default, you must uncomment SYNC_SERVER to enable this mode.
#
# for more information, please refer to:
# http:/denyhosts.sourceforge.net/faq.html#sync
#
#######################################################################
#######################################################################
#
# SYNC_SERVER: The central server that communicates with DenyHost
# daemons. Currently, denyhosts.net is the only available server
# however, in the future, it may be possible for organizations to
# install their own server for internal network synchronization
#
# To disable synchronization (the default), do nothing.
#
# To enable synchronization, you must uncomment the following line:
#SYNC_SERVER = http://xmlrpc.denyhosts.net:9911
#
################################################### |
