Restricting SSH Access Attempts Using DenyHosts
Published November 25th, 2008Problem Statement
The real problem in the world is that there are too many people with too much free time. These wanna-be hackers often download scripts and ready-made-software to scan servers all around the globe and see if there is one that is poorly maintained to get in and deface Web pages or steal information. SSH is a great improvement over telnet based remote access but it is also setup typically using username/password based authentication. So if your password is weak, SSH has no benefit. Having weak password in combination with a default SSH server is like inviting trouble by going out of your way. Here in this article we will show you how you can secure SSH and automatically ban access attempts from people who have either forgotten their password or those who should have never connected to your SSH server.
| Time Estimate (30 ~ 60 min) |
| We estimate that the entire process to get DenyHosts installed, and tested might take half an hour to an hour. |
Securing SSH by Running on a Non-Standard Port
We covered this topic in another article called <a href=”http://centoshacker.com/admin/ssh/running-ssh-on-a-non-standard-port.html”>Running SSH on a Non-standard Port</a>. Read and implement as recommended so that your SSH server is not on the default 22.
Deny the Idiot Users and/or Wanna-be Hackers and Script Kiddies
DenyHosts is a Python script that runs as a daemon and monitors SSH attempts (by watching /var/log/secure) on your server. After n number of invalid attempts using non-existent user accounts are made by a potential intruder, it locks up the IP address by placing it in /etc/hosts.deny which the SSH server checks every time a connection attempt is made. This means that after n number of invalid attempts to access non-existent user accounts, the intruder has to change the IP address to resume attack your server, this is a bit of annoyance at the least and most likely a good enough deterrent that the idiot moves on and looks for another potential victim server.
Download the source distribution from:
http://www.denyhosts.net/
Step 1: Installing DenyHosts
Once you have downloaded the source and extracted the source in /usr/local/src, do the following from the newly created DenyHosts-<version> directory:
- Run:
python setup.py install - Change directory to
/usr/share/denyhosts - Run:
cp denyhosts.cfg-dist denyhosts.cfgto copy the sample configuration into a new configuration file that you can edit
to install the package
The sample configuration has comments that you can follow to make any applicable changes if needed. Here is the list of options (shown without the comments) that you can configure:
SECURE_LOG = /var/log/secure HOSTS_DENY = /etc/hosts.deny PURGE_DENY = BLOCK_SERVICE = sshd DENY_THRESHOLD_INVALID = 5 DENY_THRESHOLD_VALID = 10 DENY_THRESHOLD_ROOT = 1 DENY_THRESHOLD_RESTRICTED = 1 WORK_DIR = /usr/share/denyhosts/data SUSPICIOUS_LOGIN_REPORT_ALLOWED_HOSTS=YES HOSTNAME_LOOKUP=YES LOCK_FILE = /var/lock/subsys/denyhosts ADMIN_EMAIL = SMTP_HOST = localhost SMTP_PORT = 25 SMTP_FROM = DenyHosts SMTP_SUBJECT = DenyHosts Report AGE_RESET_VALID=5d AGE_RESET_ROOT=25d AGE_RESET_RESTRICTED=25d AGE_RESET_INVALID=10d DAEMON_LOG = /var/log/denyhosts DAEMON_SLEEP = 30s DAEMON_PURGE = 1h
xinetd services that check /etc/hosts.deny than set BLOCK_SERVICE = ALLDENY_THRESHOLD_INVALID is set to 5, which means that after five invalid attempts using wrong password, the intruder will be locked by because DenyHosts will add the intruder’s IP address in the /etc/hosts.deny file/usr/share/denyhosts using cp daemon-control-dist daemon-control. You can review the daemon-control file but most likely it will work as-isln -s /usr/share/denyhosts/daemon-control /etc/init.d/denyhosts
chkconfig --add denyhostsStep 2: Starting DenyHosts for the First Time
To start DenyHosts, run service denyhosts start and you should see something similar to the following:
starting DenyHosts:
/usr/bin/env python /usr/bin/denyhosts.py --daemon \
--config=/usr/share/denyhosts/denyhosts.cfg
To ensure that it is running, run ps x | grep denyhosts and you should see something similar to the following:
28788 ? S 0:00 python /usr/bin/denyhosts.py --daemon --config=/usr/share/denyhosts/denyhosts.cfg
If you see a running denyhosts script that has been started using python you are all set!
Step 3: Excluding Known IP Addresses from Getting Denied
If you have users who forget their SSH username a lot and they access your SSH server from known static IP addresses, you can exclude these IP addresses from DenyHost’s radar. Here is how:
- Create a text file called
/usr/share/denyhosts/data/allowed-hostswhere each line represents a full IP address or a network address or a range of IP addresses. For example:# Your static IP 130.86.1.1 # Your other servers on the LAN that connects to your SSH server 192.168.1.[1-254]
- Once you have created this file, restart DenyHosts using
service denyhosts restart
Now if any of the users using the IP addresses listed in the allowed-hosts files ever misspell their username multiple times, they won’t call you. Because, denyhosts will forgive them for good. This might not be a good practice since you want your users to really know what they are doing and perhaps you DO want them to get locked out when they forget their username. How much can you take? Never mind.
Step 4: Testing Your DenyHosts Setup
To test your denyhosts setup, do the following:
- From a host that is NOT included in your
/usr/share/denyhosts/data/allowed-hostsfile, attempt to make a SSH connection to your SSH server using an invalid username - You should be able to retry
DENY_THRESHOLD_INVALID(5) times by default - Simultaneously, you want to setup a
tail -f /var/log/denyhostssession to observe how denyhosts is behaving after the maximum threshold has been reached. You will see that denyhosts has created a log entry such as:2008-11-25 14:56:08,705 - denyhosts : INFO new denied hosts: ['100.200.1.2']
- This shows that denyhosts has blocked this IP from accessing the server via SSH. You can also verify that this IP is now listed in
/etc/hosts.denyfile
To test if an allowed IP address can make username mistakes, just add the IP address in the /usr/share/denyhosts/data/allowed-hosts file and try to access the SSH server using invalid username and/or password. You will be allowed to try as much as you like.
Step 5: Using Centralized DenyHosts Knowledge base
The denyhosts.net provides a free global synchronization of attack data. The benefits are:
- You receive IP addresses that have been already considered bad by other hosts elsewhere in the world. This way, you get to ban the bad IP addresses before they attack your server
- Your bad IP addresses are contributed to the global pool of bad IP addresses so that someone else’s server can ban the IP before any attempt is made to access such a server
To enable this service, edit the /usr/share/denyhosts/denyhosts.cfg file as follows:
- Uncomment
SYNC_SERVER = http://xmlrpc.denyhosts.net:9911to allow DenyHosts to send and receive synchronization data from the denyhosts.net service - Uncomment
SYNC_INTERVAL = 1hso that your DenyHosts deamon can contact the denyhosts.net every hour - If you wish to upload your bad IP list, uncomment and set
SYNC_UPLOAD = yes - If you wish to download bad IP list from denyhosts.net, uncomment and set
SYNC_DOWNLOAD = yes
xlodx on May 25, 2009
Thank u !! Great Tutorial !!
Zxirupofrur on August 22, 2009
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