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How To Secure A Linux Server

An evolving how-to guide for securing a Linux server that, hopefully, also teaches you a little about security and why it matters.

Table of Contents

• Introduction
  • Guide Objective
  • Why Secure Your Server
  • Why Yet Another Guide
  • Other Guides
  • To Do / To Add
• Guide Overview
  • About This Guide
  • My Use-Case
  • Editing Configuration Files - For The Lazy
  • Contributing
• Before You Start
  • Identify Your Principles
  • Picking A Linux Distribution
  • Installing Linux
  • Pre/Post Installation Requirements
  • Other Important Notes
• The SSH Server
  • Important Note Before You Make SSH Changes
  • SSH Public/Private Keys
  • Create SSH Group For AllowGroups
  • Secure /etc/ssh/sshd_config
  • Remove Short Diffie-Hellman Keys
  • 2FA/MFA for SSH
• The Basics
  • Limit Who Can Use sudo
  • Limit Who Can Use su
  • Run applications in a sandbox with FireJail
  • NTP Client
  • Securing /proc
  • Force Accounts To Use Secure Passwords
  • Automatic Security Updates and Alerts
  • More Secure Random Entropy Pool (WIP)
• The Network
  • Firewall With UFW (Uncomplicated Firewall)
  • iptables Intrusion Detection And Prevention with PSAD
  • Application Intrusion Detection And Prevention With Fail2Ban
• The Auditing
  • File/Folder Integrity Monitoring With AIDE (WIP)
  • Anti-Virus Scanning With ClamAV (WIP)
  • Rootkit Detection With Rkhunter (WIP)
  • Rootkit Detection With chrootkit (WIP)
  • logwatch - system log analyzer and reporter
  • ss - Seeing Ports Your Server Is Listening On
  • Lynis - Linux Security Auditing
  • OSSEC - Host Intrusion Detection
• The Danger Zone
• The Miscellaneous
  • MSMTP (Simple Sendmail) with google
  • Gmail and Exim4 As MTA With Implicit TLS
  • Separate iptables Log File
• Left Over
  • Contacting Me
  • Helpful Links
  • Acknowledgments
  • License and Copyright

(TOC made with nGitHubTOC)

Introduction

Guide Objective

This guides purpose is to teach you how to secure a Linux server.

There are a lot of things you can do to secure a Linux server and this guide will attempt to cover as many of them as possible. More topics/material will be added as I learn, or as folks contribute.

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Why Secure Your Server

I assume you're using this guide because you, hopefully, already understand why good security is important. That is a heavy topic onto itself and breaking it down is out-of-scope for this guide. If you don't know the answer to that question, I advise you research it first.

At a high level, the second a device, like a server, is in the public domain -- i.e visible to the outside world -- it becomes a target for bad-actors. An unsecured device is a playground for bad-actors who want access to your data, or to use your server as another node for their large-scale DDOS attacks.

What's worse is, without good security, you may never know if your server has been compromised. A bad-actor may have gained unauthorized access to your server and copied your data without changing anything so you'd never know. Or your server may have been part of a DDOS attack and you wouldn't know. Look at many of the large scale data breaches in the news -- the companies often did not discover the data leak or intrusion until long after the bad-actors were gone.

Contrary to popular belief, bad-actors don't always want to change something or lock you out of your data for money. Sometimes they just want the data on your server for their data warehouses (there is big money in big data) or to covertly use your server for their nefarious purposes.

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Why Yet Another Guide

This guide may appear duplicative/unnecessary because there are countless articles online that tell you how to secure Linux, but the information is spread across different articles, that cover different things, and in different ways. Who has time to scour through hundreds of articles?

As I was going through research for my Debian build, I kept notes. At the end I realized that, along with what I already knew, and what I was learning, I had the makings of a how-to guide. I figured I'd put it online to hopefully help others learn, and save time.

I've never found one guide that covers everything -- this guide is my attempt.

Many of the things covered in this guide may be rather basic/trivial, but most of us do not install Linux every day and it is easy to forget those basic things.

IT automation tools like Ansible, Chef, Jenkins, Puppet, etc. help with the tedious task of installing/configuring a server but IMHO they are better suited for multiple or large scale deployments. IMHO, the overhead required to use those kinds of automation tools is wholly unnecessary for a one-time single server install for home use.

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Other Guides

There are many guides provided by experts, industry leaders, and the distributions themselves. It is not practical, and sometimes against copyright, to include everything from those guides. I recommend you check them out before starting with this guide.

• The Center for Internet Security (CIS) provides benchmarks that are exhaustive, industry trusted, step-by-step instructions for securing many flavors of Linux. Check their About Us page for details. My recommendation is to go through this guide first and then CIS's guide. That way their recommendations will trump anything in this guide.
• For distribution specific hardening/security guides, check your distributions documentation.
• https://security.utexas.edu/os-hardening-checklist/linux-7 - Red Hat Enterprise Linux 7 Hardening Checklist
• https://cloudpro.zone/index.php/2018/01/18/debian-9-3-server-setup-guide-part-1/ - # Debian 9.3 server setup guide
• https://blog.vigilcode.com/2011/04/ubuntu-server-initial-security-quick-secure-setup-part-i/ - Ubuntu Server Initial Security guide
• https://www.tldp.org/LDP/sag/html/index.html
• https://seifried.org/lasg/
• https://news.ycombinator.com/item?id=19178964
• https://wiki.archlinux.org/index.php/Security - many folks have also recommended this one
• https://securecompliance.co/linux-server-hardening-checklist/

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To Do / To Add

• Custom Jails for Fail2ban
• MAC (Mandatory Access Control) and Linux Security Modules (LSMs)
  • https://wiki.archlinux.org/index.php/security#Mandatory_access_control
  • Security-Enhanced Linux / SELinux
    • https://en.wikipedia.org/wiki/Security-Enhanced_Linux
    • https://linuxtechlab.com/beginners-guide-to-selinux/
    • https://linuxtechlab.com/replicate-selinux-policies-among-linux-machines/
    • https://teamignition.us/how-to-stop-being-a-scrub-and-learn-to-use-selinux.html
  • AppArmor
    • https://wiki.archlinux.org/index.php/AppArmor
    • https://security.stackexchange.com/questions/29378/comparison-between-apparmor-and-selinux
    • http://www.insanitybit.com/2012/06/01/why-i-like-apparmor-more-than-selinux-5/
• disk encryption
• Rkhunter and chrootkit
  • http://www.chkrootkit.org/
  • http://rkhunter.sourceforge.net/
  • https://www.cyberciti.biz/faq/howto-check-linux-rootkist-with-detectors-software/
  • https://www.tecmint.com/install-rootkit-hunter-scan-for-rootkits-backdoors-in-linux/
• shipping/backing up logs - https://news.ycombinator.com/item?id=19178681
• CIS-CAT - https://learn.cisecurity.org/cis-cat-landing-page
• debsums - https://blog.sleeplessbeastie.eu/2015/03/02/how-to-verify-installed-packages/

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Guide Overview

About This Guide

This guide...

• ...is a work in progress.
• ...is focused on at-home Linux servers. All of the concepts/recommendations here apply to larger/professional environments but those use-cases call for more advanced and specialized configurations that are out-of-scope for this guide.
• ...does not teach you about Linux, how to install Linux, or how to use it. Check https://linuxjourney.com/ if you're new to Linux.
• ...is meant to be Linux distribution agnostic.
• ...does not teach you everything you need to know about security nor does it get into all aspects of system/server security. For example, physical security is out of scope for this guide.
• ...does not talk about how programs/tools work, nor does it delve into their nook and crannies. Most of the programs/tools this guide references are very powerful and highly configurable. The goal is to cover the bare necessities -- enough to whet your appetite and make you hungry enough to want to go and learn more.
• ...aims to make it easy by providing code you can copy-and-paste. You might need to modify the commands before you paste so keep your favorite text editor handy.
• ...is organized in an order that makes logical sense to me -- i.e. securing SSH before installing a firewall. As such, this guide is intended to be followed in the order it is presented but it is not necessary to do so. Just be careful if you do things in a different order -- some sections require previous sections to be completed.

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My Use-Case

There are many types of servers and different use-cases. While I want this guide to be as generic as possible, there will be some things that may not apply to all/other use-cases. Use your best judgement when going through this guide.

To help put context to many of the topics covered in this guide, my use-case/configuration is:

• A desktop class computer...
• With a single NIC...
• Connected to a consumer grade router...
• Getting a dynamic WAN IP provided by the ISP...
• With WAN+LAN on IPV4...
• And LAN using NAT...
• That I want to be able to SSH to remotely from unknown computers and unknown locations (i.e. a friend's house).

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Editing Configuration Files - For The Lazy

I am very lazy and do not like to edit files by hand if I don't need to. I also assume everyone else is just like me. :)

So, when and where possible, I have provided code snippets to quickly do what is needed, like add or change a line in a configuration file.

The code snippets use basic commands like echo, cat, sed, awk, and grep. How the code snippets work, like what each command/part does, is out of scope for this guide -- the man pages are your friend.

Note: The code snippets do not validate/verify the change went through -- i.e. the line was actually added or changed. I'll leave the verifying part in your capable hands. The steps in this guide do include taking backups of all files that will be changed.

Not all changes can be automated with code snippets. Those changes need good, old fashioned, manual editing. For example, you can't just append a line to an INI type file. Use your favorite Linux text editor.

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Contributing

I wanted to put this guide on GitHub to make it easy to collaborate. The more folks that contribute, the better and more complete this guide will become.

To contribute you can fork and submit a pull request or submit a new issue.

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Before You Start

Identify Your Principles

Before you start you will want to identify what your Principles are. What is your threat model? Some things to think about:

• Why do you want to secure your server?
• How much security do you want or not want?
• How much convenience are you willing to compromise for security and vice-versa?
• What are the threats you want to protect against? What are the specifics to your situation? For example:
  • Is physical access to your server/network a possible attack vector?
  • Will you be opening ports on your router so you can access your server from outside your home?
  • Will you be hosting a file share on your server that will be mounted on a desktop class machine? What is the possibility of the desktop machine getting infected and, in turn, infecting the server?
• Do you have a means of recovering if your security implementation locks you out of your own server? For example, you disabled root login or password protected GRUB.

These are just a few things to think about. Before you start securing your server you will want to understand what you're trying to protect against and why so you know what you need to do.

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Picking A Linux Distribution

This guide is intended to be distribution agnostic so users can use any distribution they want. With that said, there are a few things to keep in mind:

You want a distribution that...

• ...is stable. Unless you like debugging issues at 2 AM, you don't want an unattended upgrade, or a manual package/system update, to render your server inoperable. But this also means you're okay with not running the latest, greatest, bleeding edge software.
• ...stays up-to-date with security patches. You can secure everything on your server, but if the core OS or applications you're running have known vulnerabilities, you'll never be safe.
• ...you're familiar with. If you don't know Linux, I would advise you play around with one before you try to secure it. You should be comfortable with it and know your way around, like how to install software, where configuration files are, etc...
• ...is well supported. Even the most seasoned admin needs help every now and then. Having a place to go for help will save your sanity.

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Installing Linux

Installing Linux is out-of-scope for this guide because each distribution does it differently and the installation instructions are usually well documented. If you need help, start with your distribution's documentation. Regardless of the distribution, the high-level process usually goes like so:

1. download the ISO
2. burn/copy/transfer it to your install medium (e.g. a CD or USB stick)
3. boot your server from your install medium
4. follow the prompts to install

Where applicable, use the expert install option so you have tighter control of what is running on your server. Only install what you absolutely need. I, personally, do not install anything other than SSH. Also, tick the Disk Encryption option.

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Pre/Post Installation Requirements

• If you're opening ports on your router so you can access your server from the outside, disable the port forwarding until your system is up and secured.
• Unless you're doing everything physically connected to your server, you'll need remote access so be sure SSH works.
• Keep your system up-to-date (i.e. sudo apt update && sudo apt upgrade on Debian based systems).
• Make sure you perform any tasks specific to your setup like:
  • Configuring network
  • Configuring mount points in /etc/fstab
  • Creating the initial user accounts
  • Installing core software you'll want like man
  • Etc...
• Your server will need to be able to send e-mails so you can get important security alerts. If you're not setting up a mail server check Gmail and Exim4 As MTA With Implicit TLS.
• I would also recommend you go through the CIS Benchmarks before you start with this guide.

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Other Important Notes

• This guide is being written and tested on Debian. Most things below should work on other distributions. If you find something that does not, please contact me. The main thing that separates each distribution will be its package management system. Since I use Debian, I will provide the appropriate apt commands that should work on all Debian based distributions. If someone is willing to provide the respective commands for other distributions, I will add them.
• File paths and settings also may differ slightly -- check with your distribution's documentation if you have issues.
• Read the whole guide before you start. Your use-case and/or principals may call for not doing something or for changing the order.
• Do not blindly copy-and-paste without understanding what you're pasting. Some commands will need to be modified for your needs before they'll work -- usernames for example.

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The SSH Server

Important Note Before You Make SSH Changes

It is highly advised you keep a 2nd terminal open to your server before you make and apply SSH configuration changes. This way if you lock yourself out of your 1st terminal session, you still have one session connected so you can fix it.

Thank you to Sonnenbrand for this idea.

SSH Public/Private Keys

Why

Using SSH public/private keys is more secure than using a password. It also makes it easier and faster, to connect to our server because you don't have to enter a password.

How It Works

Check the references below for more details but, at a high level, public/private keys work by using a pair of keys to verify identity.

1. One key, the public key, can only encrypt data, not decrypt it
2. The other key, the private key, can decrypt the data

For SSH, a public and private key is created on the client. You want to keep both keys secure, especially the private key. Even though the public key is meant to be public, it is wise to make sure neither keys fall in the wrong hands.

When you connect to an SSH server, SSH will look for a public key that matches the client you're connecting from in the file ~/.ssh/authorized_keys on the server you're connecting to. Notice the file is in the home folder of the ID you're trying to connect to. So, after creating the public key, you need to append it to ~/.ssh/authorized_keys. One approach is to copy it to a USB stick and physically transfer it to the server. Another approach is to use use ssh-copy-id to transfer and append the public key.

After the keys have been created and the public key has been appended to ~/.ssh/authorized_keys on the host, SSH uses the public and private keys to verify identity and then establish a secure connection. How identity is verified is a complicated process but Digital Ocean has a very nice write-up of how it works. At a high level, identity is verified by the server encrypting a challenge message with the public key, then sending it to the client. If the client cannot decrypt the challenge message with the private key, the identity can't be verified and a connection will not be established.

They are considered more secure because you need the private key to establish an SSH connection. If you set PasswordAuthentication no in /etc/ssh/sshd_config, then SSH won't let you connect without the private key.

You can also set a pass-phrase for the keys which would require you to enter the key pass-phrase when connecting using public/private keys. Keep in mind doing this means you can't use the key for automation because you'll have no way to send the passphrase in your scripts. ssh-agent is a program that is shipped in many Linux distros (and usually already running) that will allow you to hold your unencrypted private key in memory for a configurable duration. Simply run ssh-add and it will prompt you for your passphrase. You will not be prompted for your passphrase again until the configurable duration has passed.

We will be using Ed25519 keys which, according to https://linux-audit.com/:

It is using an elliptic curve signature scheme, which offers better security than ECDSA and DSA. At the same time, it also has good performance.

Goals

• Ed25519 public/private SSH keys:
  • private key on your client
  • public key on your server

Notes

• You'll need to do this step for every computer and account you'll be connecting to your server from/as.

References

• https://www.ssh.com/ssh/public-key-authentication
• https://help.ubuntu.com/community/SSH/OpenSSH/Keys
• https://linux-audit.com/using-ed25519-openssh-keys-instead-of-dsa-rsa-ecdsa/
• https://www.digitalocean.com/community/tutorials/understanding-the-ssh-encryption-and-connection-process
• https://wiki.archlinux.org/index.php/SSH_Keys
• https://www.ssh.com/ssh/copy-id
• man ssh-keygen
• man ssh-copy-id
• man ssh-add

Steps

1. From the computer you're going to use to connect to your server, the client, not the server itself, create an Ed25519 key with ssh-keygen:

   ssh-keygen -t ed25519

   Generating public/private ed25519 key pair.
   Enter file in which to save the key (/home/user/.ssh/id_ed25519):
   Created directory '/home/user/.ssh'.
   Enter passphrase (empty for no passphrase):
   Enter same passphrase again:
   Your identification has been saved in /home/user/.ssh/id_ed25519.
   Your public key has been saved in /home/user/.ssh/id_ed25519.pub.
   The key fingerprint is:
   SHA256:F44D4dr2zoHqgj0i2iVIHQ32uk/Lx4P+raayEAQjlcs user@client
   The key's randomart image is:
   +--[ED25519 256]--+
   |xxxx  x          |
   |o.o +. .         |
   | o o oo   .      |
   |. E oo . o .     |
   | o o. o S o      |
   |... .. o o       |
   |.+....+ o        |
   |+.=++o.B..       |
   |+..=**=o=.       |
   +----[SHA256]-----+
   

   Note: If you set a passphrase, you'll need to enter it every time you connect to your server using this key, unless you're using ssh-agent.

2. Now you need to append the public key ~/.ssh/id_ed25519.pub from your client to the ~/.ssh/authorized_keys file on your server. Since we're presumable still at home on the LAN, we're probably safe from MIM attacks, so we will use ssh-copy-id to transfer and append the public key:

   ssh-copy-id user@server

   /usr/bin/ssh-copy-id: INFO: Source of key(s) to be installed: "/home/user/.ssh/id_ed25519.pub"
   The authenticity of host 'host (192.168.1.96)' can't be established.
   ECDSA key fingerprint is SHA256:QaDQb/X0XyVlogh87sDXE7MR8YIK7ko4wS5hXjRySJE.
   Are you sure you want to continue connecting (yes/no)? yes
   /usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
   /usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys
   user@host's password:
   
   Number of key(s) added: 1
   
   Now try logging into the machine, with:   "ssh 'user@host'"
   and check to make sure that only the key(s) you wanted were added.
   

Now would be a good time to perform any tasks specific to your setup.

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Create SSH Group For AllowGroups

Why

To make it easy to control who can SSH to the server. By using a group, we can quickly add/remove accounts to the group to quickly allow or not allow SSH access to the server.

How It Works

We will use the AllowGroups option in SSH's configuration file /etc/ssh/sshd_config to tell the SSH server to only allow users to SSH in if they are a member of a certain UNIX group. Anyone not in the group will not be able to SSH in.

Goals

• a UNIX group that we'll use in Secure /etc/ssh/sshd_config to limit who can SSH to the server

Notes

• This is a per-requisite step to support the AllowGroup setting set in Secure /etc/ssh/sshd_config.

References

• man groupadd
• man usermod

Steps

1. Create a group:

   sudo groupadd sshusers

2. Add account(s) to the group:

   sudo usermod -a -G sshusers user1
   sudo usermod -a -G sshusers user2
   sudo usermod -a -G sshusers ...

   You'll need to do this for every account on your server that needs SSH access.

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Secure /etc/ssh/sshd_config

Why

SSH is a door into your server. This is especially true if you are opening ports on your router so you can SSH to your server from outside your home network. If it is not secured properly, a bad-actor could use it to gain unauthorized access to your system.

How It Works

/etc/ssh/sshd_config is the default configuration file that the SSH server uses. We will use this file to tell what options the SSH server should use.

Goals

• a secure SSH configuration

Notes

• Make sure you've completed Create SSH Group For AllowGroups first.

References

• Mozilla's OpenSSH guidelines for OpenSSH 6.7+ at https://infosec.mozilla.org/guidelines/openssh#modern-openssh-67
• https://linux-audit.com/audit-and-harden-your-ssh-configuration/
• https://www.ssh.co