Most important steps for Linux server hardening
Linux Server Hardening
1. Install security updates and patches
Most weaknesses in systems are caused by flaws in software. These flaws we call vulnerabilities. Proper care for software patch management help with reducing a lot of the related risks. The activity of installing updates often has a low risk, especially when starting with the security patches first. Most Linux distributions have the option to limit what packages you want to upgrade (all, security only, per package). Make sure that your security updates are installed as soon as they come available. It goes without saying, before you implementing something, test it first on a (virtual) test system.
Depending on your Linux distribution there might be a way to implement security patches automatically, like unattended upgrades on Debian and Ubuntu. This makes software patch management a lot easier.
2. Use secure passwords
The main gateway to a system is by logging in as a valid user with the related password of that account. Strong passwords make it more difficult for tools to guess the password and let malicious people walk in via the front door.
3. Bind processes to localhost
Not all services have to be available via the network. For example, when running a local instance of MySQL on your web server, let it only listen on a local socket or bind to localhost (127.0.0.1). Then tell your application to connect via this local address.
4. Implement a firewall
Only allowed traffic should in an ideal situation reach your system. To achieve this, implement a firewall solution like iptables, or the newer nftables.
When creating a policy for your firewall, consider using a “deny all, allow some” policy. So you deny all traffic by default, then define what kind of traffic you want to allow. This is especially useful for incoming traffic, to prevent sharing services you didn’t intend to share.
5. Keep things clean
Everything installed on a system which doesn’t belong there, can only negatively impact your machine. It will increase your backups (and restore times), it might contain vulnerabilities and will clutter up the system. Same is true for unused user accounts.
6. Secure configurations
Most applications have one or more security measures available to protect against some forms of threats to the software or system. Look at the man page for any options and test these options carefully.
7. Limit access
Only allow access to the machine for authorized users. Does someone really need access or are alternative methods possible to give the user what he or she wants?
8. Monitor your systems
Most intrusions are undetected, due to lack of monitoring. Implement normal system monitoring and implement monitoring on security events. For example, the use of the Linux audit framework increased detection rates of suspected events.
9. Create backups (and test!)
Regular make a backup of system data, to prevent data loss. Even more important, test your backups!
Backups can be done with existing system tools like
scp. Another option to spare bandwidth is synchronizing data with
rsync. If you rather want to use a backup program, consider Amanda or Bacula.
10. Perform system auditing
Use a tool like Lynis to perform a regular audit of your system and perform further hardening steps.
Lynis runs on almost all Unix or Linux based systems and only requires a normal shell and root permissions.
It will scan the system and perform an in-depth security audit and explain what can be done to improve the system even further. Best of all, the normal Lynis version is free to use and open source.