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So far in this series, we’ve walked through security considerations for your SysAdmin workstation from choosing the right hardware and Linux distribution, to setting up a secure pre-boot environment and distro installation. Now it’s time to cover post-installation hardening.
What you do depends greatly on your distribution of choice, so it is futile to provide detailed instructions in a blog series such as this one. However, here are some essential steps you should take:
￼￼Globally disable firewire and thunderbolt modules
Check your firewalls to ensure all incoming ports are filtered
Make sure root mail is forwarded to an account you check
Set up an automatic OS update schedule, or update reminders
In addition, you may also consider some of these nice-to-have steps to further harden your system:
Check to ensure sshd service is disabled by default
Configure the screensaver to auto-lock after a period of inactivity
Set up logwatch
Install and use rkhunter
Install an Intrusion Detection System
As I’ve said before, security is like driving on the highway — anyone going slower than you is an idiot, while anyone driving faster than you is a crazy person. The guidelines in this series are merely a basic set of core safety rules that is neither exhaustive, nor a replacement for experience, vigilance, and common sense. You should adapt these recommendations to suit your environment.
To blacklist a firewire and thunderbolt modules, add the following lines to a file in /etc/modprobe.d/blacklist-dma.conf:
blacklist firewire-core blacklist thunderbolt
The modules will be blacklisted upon reboot. It doesn’t hurt doing this even if you don’t have these ports (but it doesn’t do anything either).
By default, root mail is just saved on the system and tends to never be read. Make sure you set your /etc/aliases to forward root mail to a mailbox that you actually read, otherwise you may miss important system notifications and reports:
# Person who should get root’s mail root: firstname.lastname@example.org
Run newaliases after this edit and test it out to make sure that it actually gets delivered, as some email providers will reject email coming in from nonexistent or non-routable domain names. If that is the case, you will need to play with your mail forwarding configuration until this actually works.
Firewalls, sshd, and listening daemons
The default firewall settings will depend on your distribution, but many of them will allow incoming sshd ports. Unless you have a compelling legitimate reason to allow incoming ssh, you should filter that out and disable the sshd daemon.
systemctl disable sshd.service systemctl stop sshd.service
You can always start it temporarily if you need to use it.
In general, your system shouldn’t have any listening ports apart from responding to ping. This will help safeguard you against network-level 0-day exploits.
Automatic updates or notifications
It is recommended to turn on automatic updates, unless you have a very good reason not to do so, such as fear that an automatic update would render your system unusable (it’s happened in the past, so this
fear is not unfounded). At the very least, you should enable automatic notifications of available updates. Most distributions already have this service automatically running for you, so chances are you don’t have to do anything. Consult your distribution documentation to find out more.
You should apply all outstanding errata as soon as possible, even if something isn’t specifically labeled as “security update” or has an associated CVE code. All bugs have the potential of being security bugs and erring on the side of newer, unknown bugs is generally a safer strategy than sticking with old, known ones.
You should have a keen interest in what happens on your system. For this reason, you should install logwatch and configure it to send nightly activity reports of everything that happens on your system. This won’t prevent a dedicated attacker, but is a good safety-net feature to have in place.
Note, that many systemd distros will no longer automatically install a syslog server that logwatch needs (due to systemd relying on its own journal), so you will need to install and enable rsyslog to make sure your /var/log is not empty before logwatch will be of any use.
Rkhunter and IDS
Installing rkhunter and an intrusion detection system (IDS) like aide or tripwire will not be that useful unless you actually understand how they work and take the necessary steps to set them up properly (such as, keeping the databases on external media, running checks from a trusted environment, remembering to refresh the hash databases after performing system updates and configuration changes, etc). If you are not willing to take these steps and adjust how you do things on your own workstation, these tools will introduce hassle without any tangible security benefit.
We do recommend that you install rkhunter and run it nightly. It’s fairly easy to learn and use, and though it will not deter a sophisticated attacker, it may help you catch your own mistakes.
The first part of this series has walked through distro installation, and some pre- and post-installation security guidelines. In the next article, cover some of the best storage options to back up your workstation and then we’ll dive into some more general best practices around web browser security, SSH and private keys, and more.