SSH (Secure Shell) is an open-source network protocol that is used to connect local or remote Linux servers to transfer files, make remote backups, remote command execution, and other network-related tasks via scp command or sftp command between two servers that connect on a secure channel over the network.
In this article, I will show you some simple tools and tricks that will help you to tighten your ssh server security. Here you will find some useful information on how to secure and prevent ssh servers from brute force and dictionary attacks.
1. DenyHosts
DenyHosts is an open-source log-based intrusion prevention security script for SSH servers that was written in a python programming language that is intended to be run by Linux system administrators and users to monitor and analyze SSH server access logs for failed login attempts knows as dictionary-based attacks and brute force attacks.
The script works by banning IP addresses after a set number of failed login attempts and also prevent such attacks from gaining access to the server.
DenyHosts Features
- Keeps track of /var/log/secure to find all successful and failed login attempts and filters them.
- Keeps eye on all failed login attempts by the user and offending host.
- Keeps watching on each existing and non-existent user (eg. xyz) when a failed login attempts.
- Keeps track of each offending user, host, and suspicious login attempts (If a number of login failures) bans that host IP address by adding an entry in /etc/hosts.deny file.
- Optionally sends an email notification of newly blocked hosts and suspicious logins.
- Also maintains all valid and invalid failed user login attempts in separate files so that it makes it easy for identifying which valid or invalid user is under attack. So, that we can delete that account or change the password, or disable shell for that user.
[ You might also like: How to Block SSH Brute Force Attacks Using DenyHosts ]
2. Fail2Ban
Fail2ban is one of the most popular open-source intrusion detection/prevention frameworks written in a python programming language. It operates by scanning log files such as /var/log/secure, /var/log/auth.log, /var/log/pwdfail etc. for too many failed login attempts.
Fail2ban is used to update Netfilter/iptables or TCP Wrapper’s hosts.deny file, to reject an attacker’s IP address for a set amount of time. It also has the ability to unban a blocked IP address for a certain period of time set by administrators. However, a certain minute of unbanning is more than enough to stop such malicious attacks.
Fail2Ban Features
- Multi-threaded and Highly configurable.
- Support for log files rotation and can handle multiple services like (sshd, vsftpd, apache, etc).
- Monitors log files and look for known and unknown patterns.
- Uses Netfilter/Iptables and TCP Wrapper (/etc/hosts.deny) table to ban attackers IP.
- Runs scripts when a given pattern has been identified for the same IP address more than X times.
[ You might also like: How to Use Fail2ban to Secure Your Linux Server ]
3. Disable Root Login
By default Linux systems are pre-configured to allow ssh remote logins for everyone including the root user itself, which allows everyone to directly log in to the system and gain root access. Despite the fact that ssh server allows a more secure way to disable or enable root logins, it’s always a good idea to disable root access, keeping servers a bit more secure.
There are so many people trying to brute force root accounts via SSH attacks by simply supplying different account names and passwords, one after another. If you are a system administrator, you can check ssh server logs, where you will find a number of failed login attempts. The main reason behind a number of failed login attempts is having weak enough passwords and that makes sense for hackers/attackers to try.
If you are having strong passwords, then you’re probably safe, however, it’s better to disable root login and have a regular separate account to log into, and then use sudo or su to gain root access whenever required.
[ You might also like: How to Disable SSH Root Login and Limit SSH Access in Linux ]
4. Display SSH Banner
This is one of the oldest features available from the beginning of the ssh project, but I’ve hardly seen it is used by anyone. Anyway, I feel it’s an important and very useful feature that I’ve used for all my Linux servers.
This is not for any security purpose, but the greatest benefit of this banner is that it is used to display ssh warning messages to UN-authorized access and welcome messages to authorized users before the password prompt and after the user logged in.
[ You might also like: How to Protect SSH Logins with SSH & MOTD Banner Messages ]
5. SSH Passwordless Login
An SSH Password-less login with SSH keygen will establish a trust relationship between two Linux servers which makes file transfer and synchronization much easier.
This is very useful if you are dealing with remote automated backups, remote scripting execution, file transfer, remote script management, etc without entering the password each time.
[ You might also like: How to Setup SSH Passwordless Login in Linux [3 Easy Steps] ]
To further secure your SSH server, read our article on How to Secure and Harden OpenSSH Server
