DenyHosts Setup

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DenyHosts - Preventing Brute Forced Attacks

SSH Brute Force Attacks Protection using DenyHosts

You can easily block SSH server attacks including dictionary based attacks and brute force attacks using a Python based script – DenyHosts. It analyzes the sshd log messages to
determine what hosts or IPs are attempting to hack into your system.

STEP 1: Open the package manager and install denyhosts
STEP 2: Configure Denyhosts

The default configuration file is located at /etc/denyhosts/denyhosts.cfg. You need to allow yourself access sshd.

Using your favorite text editor.
Open /etc/hosts.allow (must be root user) and add the following line.
sshd:<your ip number>
For Example: sshd:127.0.0.0 <-- this is the localhost feedback loop
Save and close the file.

STEP 3: Open denyhosts.cfg and change admin email. You will get an email on newly restricted hosts and IPs. If you do not want, leave it with #. Use your favorite text editor and
open /etc/denyhosts/denyhosts.cfg and edit this line

# ADMIN_EMAIL =  youremail@yourdomain.com

Save and close the file.

Your configuration file should look something like this:

############ THESE SETTINGS ARE REQUIRED ############
SECURE_LOG = /var/log/secure
HOSTS_DENY = /etc/hosts.deny
# PURGE_DENY = 7d (This will never delete or purge entries)
BLOCK_SERVICE = sshd
DENY_THRESHOLD_INVALID = 3
DENY_THRESHOLD_VALID = 5
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
############ THESE SETTINGS ARE OPTIONAL ############
ADMIN_EMAIL = youremail@yourdomain.com
SMTP_HOST = localhost
SMTP_PORT = 25
SMTP_FROM = DenyHosys <nobody@localhost>
SMTP_SUBJECT = DenyHosts Report of your “System Name”
AGE_RESET_VALID=5d
AGE_RESET_ROOT=25d
AGE_RESET_RESTRICTED=25d
AGE_RESET_INVALID=10d
######### THESE SETTINGS ARE SPECIFIC TO DAEMON MODE ##########
DAEMON_LOG = /var/log/denyhosts
DAEMON_SLEEP = 30s
DAEMON_PURGE = 1h


Now, start this utility by executing, in a console window as the root user type in:

service denyhosts start


Removing a blocked IP

If your IP is blocked and you want to remove it, follow below steps to remove your IP from DenyHosts Database.

STEP 1: Stop DenyHosts first, in a console window as the root user type in:

service denyhosts stop

STEP 2: Remove Your IP From /etc/hosts.deny

In your favorite text editor open /etc/hosts.deny
Delete your IP address.
Save and close the file.

STEP 3: Remove Your IP From /usr/share/denyhosts/data Directory.

cd /usr/share/denyhosts/data

You need to edit the following files using your favorite text editor and remove the lines containing the IP address.

1.hosts
2.hosts-restricted
3.hosts-root
4.hosts-valid
5.users-hosts
Save the file.


STEP 4: Start DenyHosts by executing, in a console window as the root user type in:

service denyhosts start


That’s it…! You can access ssh service with same IP, which was earlier blocked !

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