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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

(TOC made with nGitHubTOC)

Introduction

Guide Objective

This guide's 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.

(Table of Contents)

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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To Do / To Add

(Table of Contents)

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.

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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 Configure Gmail as MTA.

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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

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 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. Or, if you're sure there is nobody listening between the client you're on and your server, you can 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

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

Notes

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.

Goal

  • a secure SSH configuration

Notes

References

Steps

  1. Make a backup of /etc/ssh/sshd_config and remove default comments to make it easier to read:

    sudo cp --preserve /etc/ssh/sshd_config /etc/ssh/sshd_config.$(date +"%Y%m%d%H%M%S")
sudo sed -i -r -e '/^#|^$/ d' /etc/ssh/sshd_config

  2. Edit /etc/ssh/sshd_config then find and edit or add these settings that should apply regardless of your configuration/setup:

    Note: Your /etc/ssh/sshd_config file may already have some of these settings/lines. You will want to remove those and replace them with the ones below.

    ########################################################################################################
# start settings from https://infosec.mozilla.org/guidelines/openssh#modern-openssh-67 as of 2019-01-01
########################################################################################################

# Supported HostKey algorithms by order of preference.
HostKey /etc/ssh/ssh_host_ed25519_key
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ecdsa_key

KexAlgorithms curve25519-sha256@libssh.org,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256

Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr

MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com

# LogLevel VERBOSE logs user's key fingerprint on login. Needed to have a clear audit track of which key was using to log in.
LogLevel VERBOSE

# Use kernel sandbox mechanisms where possible in unprivileged processes
# Systrace on OpenBSD, Seccomp on Linux, seatbelt on MacOSX/Darwin, rlimit elsewhere.
# Note: This setting is deprecated in OpenSSH 7.5 (https://www.openssh.com/txt/release-7.5)
UsePrivilegeSeparation sandbox

########################################################################################################
# end settings from https://infosec.mozilla.org/guidelines/openssh#modern-openssh-67 as of 2019-01-01
########################################################################################################

# Log sftp level file access (read/write/etc.) that would not be easily logged otherwise.
Subsystem sftp  internal-sftp -f AUTHPRIV -l INFO

# only use the newer, more secure protocol
Protocol 2

# disable X11 forwarding as X11 is very insecure
# you really shouldn't be running X on a server anyway
X11Forwarding no

# disable port forwarding
AllowTcpForwarding no
AllowStreamLocalForwarding no
GatewayPorts no
PermitTunnel no

# don't allow login if the account has an empty password
PermitEmptyPasswords no

# ignore .rhosts and .shosts
IgnoreRhosts yes

# verify hostname matches IP
UseDNS no

Compression no
TCPKeepAlive no
AllowAgentForwarding no
PermitRootLogin no

  3. Then find and edit or add these settings, and set values as per your requirements:

    Setting Valid Values Example Description Notes
    AllowGroups local UNIX group name AllowGroups sshusers group to allow SSH access to
    ClientAliveCountMax number ClientAliveCountMax 0 maximum number of client alive messages sent without response
    ClientAliveInterval number of seconds ClientAliveInterval 300 timeout in seconds before a response request
    ListenAddress space separated list of local addresses
    • ListenAddress 0.0.0.0
    • ListenAddress 192.168.1.100
    		 local addresses sshd should listen on See Issue #1 for important details.
    LoginGraceTime number of seconds LoginGraceTime 30 time in seconds before login times-out
    MaxAuthTries number MaxAuthTries 2 maximum allowed attempts to login
    MaxSessions number MaxSessions 2 maximum number of open sessions
    MaxStartups number MaxStartups 2 maximum number of login sessions
    PasswordAuthentication yes or no PasswordAuthentication no if login with a password is allowed
    Port any open/available port number Port 22 port that sshd should listen on

    Check man sshd_config for more details what these settings mean.

  4. Restart ssh:

    sudo service sshd restart

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Remove Short Diffie-Hellman Keys

Why

Per Mozilla's OpenSSH guidelines for OpenSSH 6.7+, "all Diffie-Hellman moduli in use should be at least 3072-bit-long".

The Diffie-Hellman algorithm is used by SSH to establish a secure connection. The larger the moduli (key size) the stronger the encryption.

Goal

  • remove all Diffie-Hellman keys that are less than 3072 bits long

References

Steps

  1. Make a backup of /etc/ssh/moduli:

    sudo cp --preserve /etc/ssh/moduli /etc/ssh/moduli.$(date +"%Y%m%d%H%M%S")

  2. Remove short moduli:

    sudo awk '$5 >= 3071' /etc/ssh/moduli | sudo tee /etc/ssh/moduli.tmp
sudo mv /etc/ssh/moduli.tmp /etc/ssh/moduli

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2FA/MFA for SSH

Why

Even though SSH is a pretty good security guard for your doors and windows, it is still a visible door that bad-actors can see and try to brute-force in. Fail2ban will monitor for these brute-force attempts but there is no such thing as being too secure. Requiring two factors adds an extra layer of security.

Using Two Factor Authentication (2FA) / Multi Factor Authentication (MFA) requires anyone entering to have two keys to enter which makes it harder for bad actors. The two keys are:

  1. Their password
  2. A 6 digit token that changes every 30 seconds

Without both keys, they won't be able to get in.

Why Not

Many folks might find the experience cumbersome or annoying. And, access to your system is dependent on the accompanying authenticator app that generates the code.

How It Works

On Linux, PAM is responsible for authentication. There are four tasks to PAM that you can read about at https://en.wikipedia.org/wiki/Linux_PAM. This section talks about the authentication task.

When you log into a server, be it directly from the console or via SSH, the door you came through will send the request to the authentication task of PAM and PAM will ask for and verify your password. You can customize the rules each doors use. For example, you could have one set of rules when logging in directly from the console and another set of rules for when logging in via SSH.

This section will alter the authentication rules for when logging in via SSH to require both a password and a 6 digit code.

We will use Google's libpam-google-authenticator PAM module to create and verify a TOTP key. https://fastmail.blog/2016/07/22/how-totp-authenticator-apps-work/ and https://jemurai.com/2018/10/11/how-it-works-totp-based-mfa/ have very good writeups of how TOTP works.

What we will do is tell the server's SSH PAM configuration to ask the user for their password and then their numeric token. PAM will then verify the user's password and, if it is correct, then it will route the authentication request to libpam-google-authenticator which will ask for and verify your 6 digit token. If, and only if, everything is good will the authentication succeed and user be allowed to log in.

Goals

  • 2FA/MFA enabled for all SSH connections

Notes

  • Before you do this, you should have an idea of how 2FA/MFA works and you'll need an authenticator app on your phone to continue.
  • We'll use google-authenticator-libpam.
  • With the below configuration, a user will only need to enter their 2FA/MFA code if they are logging on with their password but not if they are using SSH public/private keys. Check the documentation on how to change this behavior to suite your requirements.

References

Steps

  1. Install it libpam-google-authenticator.

    On Debian based systems:

    sudo apt install libpam-google-authenticator

  2. Make sure you're logged in as the ID you want to enable 2FA/MFA for and execute google-authenticator to create the necessary token data:

    google-authenticator
    Do you want authentication tokens to be time-based (y/n) y
https://www.google.com/chart?chs=200x200&chld=M|0&cht=qr&chl=otpauth://totp/user@host%3Fsecret%3DR4ZWX34FQKZROVX7AGLJ64684Y%26issuer%3Dhost

...

Your new secret key is: R3NVX3FFQKZROVX7AGLJUGGESY
Your verification code is 751419
Your emergency scratch codes are:
  12345678
  90123456
  78901234
  56789012
  34567890

Do you want me to update your "/home/user/.google_authenticator" file (y/n) y

Do you want to disallow multiple uses of the same authentication
token? This restricts you to one login about every 30s, but it increases
your chances to notice or even prevent man-in-the-middle attacks (y/n) Do you want to disallow multiple uses of the same authentication
token? This restricts you to one login about every 30s, but it increases
your chances to notice or even prevent man-in-the-middle attacks (y/n) y

By default, tokens are good for 30 seconds. In order to compensate for
possible time-skew between the client and the server, we allow an extra
token before and after the current time. If you experience problems with
poor time synchronization, you can increase the window from its default
size of +-1min (window size of 3) to about +-4min (window size of
17 acceptable tokens).
Do you want to do so? (y/n) y

If the computer that you are logging into isn't hardened against brute-force
login attempts, you can enable rate-limiting for the authentication module.
By default, this limits attackers to no more than 3 login attempts every 30s.
Do you want to enable rate-limiting (y/n) y

    Notice this is not run as root.

    Select default option (y in most cases) for all the questions it asks and remember to save the emergency scratch codes.

  3. Now we need to enable it as an authentication method for SSH by adding this line to /etc/pam.d/sshd:

    auth       required     pam_google_authenticator.so nullok

    Check here for what nullok means.

    For the lazy:

    sudo cp --preserve /etc/pam.d/sshd /etc/pam.d/sshd.$(date +"%Y%m%d%H%M%S")

echo -e "\nauth       required     pam_google_authenticator.so nullok         # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")" | sudo tee -a /etc/pam.d/sshd

  4. Tell SSH to leverage it by adding this line in /etc/ssh/sshd_config:

    ChallengeResponseAuthentication yes

    For the lazy:

    sudo cp --preserve /etc/ssh/sshd_config /etc/ssh/sshd_config.$(date +"%Y%m%d%H%M%S")

echo -e "\nChallengeResponseAuthentication yes         # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")" | sudo tee -a /etc/ssh/sshd_config

  5. Restart ssh:

    sudo service sshd restart

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The Basics

Limit Who Can Use sudo

Why

sudo lets accounts run commands as other accounts, including root. We want to make sure that only the accounts we want can use sudo.

Goals

  • sudo privileges limited to those who are in a group we specify

Notes

  • Your installation may have already done this, or may already have a special group intended for this purpose so check first.
    • Debian creates the sudo group
    • RedHat creates the wheel group

Steps

  1. Create a group:

    sudo groupadd sudousers

  2. Add account(s) to the group:

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

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

  3. Edit /etc/sudoers:

    sudo cp --preserve /etc/sudoers /etc/sudoers.$(date +"%Y%m%d%H%M%S")
sudo visudo

  4. Tell sudo to only allow users in the sudousers group to use sudo by adding this line if it is not already there:

    %sudousers   ALL=(ALL:ALL) ALL

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NTP Client

Why

Many security protocols leverage the time. If your system time is incorrect, it could have negative impacts to your server. An NTP client can solve that problem by keeping your system time in-sync with global NTP servers

How It Works

NTP stands for Network Time Protocol. In the context of this guide, an NTP client on the server is used to update the server time with the official time pulled from official servers. Check https://www.pool.ntp.org/en/ for all of the public NTP servers.

Goals

  • NTP client installed and keeping server time in-sync

References

Steps

  1. Install ntp.

    On Debian based systems:

    sudo apt install ntp

  2. Check ntp's status:

    sudo systemctl status ntp
    â—Ź ntp.service - LSB: Start NTP daemon
   Loaded: loaded (/etc/init.d/ntp; generated; vendor preset: enabled)
   Active: active (running) since Sat 2019-02-16 00:32:20 EST; 3s ago
     Docs: man:systemd-sysv-generator(8)
   CGroup: /system.slice/ntp.service
           └─1051 /usr/sbin/ntpd -p /var/run/ntpd.pid -g -u 109:114

Feb 16 00:32:20 host ntpd[1051]: Listen normally on 3 enp0s3 192.168.1.96:123
Feb 16 00:32:20 host ntpd[1051]: Listen normally on 4 lo [::1]:123
Feb 16 00:32:20 host ntpd[1051]: Listen normally on 5 enp0s3 [fe80::a00:27ff:feb6:ed8e%2]:123
Feb 16 00:32:20 host ntpd[1051]: Listening on routing socket on fd #22 for interface updates
Feb 16 00:32:21 host ntpd[1051]: Soliciting pool server 173.255.206.154
Feb 16 00:32:22 host ntpd[1051]: Soliciting pool server 216.6.2.70
Feb 16 00:32:22 host ntpd[1051]: Soliciting pool server 82.197.188.130
Feb 16 00:32:23 host ntpd[1051]: Soliciting pool server 95.215.175.2
Feb 16 00:32:23 host ntpd[1051]: Soliciting pool server 107.155.79.108
Feb 16 00:32:23 host ntpd[1051]: Soliciting pool server 212.110.158.28

    sudo ntpq -p
         remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 0.debian.pool.n .POOL.          16 p    -   64    0    0.000    0.000   0.000
 1.debian.pool.n .POOL.          16 p    -   64    0    0.000    0.000   0.000
 2.debian.pool.n .POOL.          16 p    -   64    0    0.000    0.000   0.000
 3.debian.pool.n .POOL.          16 p    -   64    0    0.000    0.000   0.000
-li216-154.membe 45.56.123.24     3 u  119   64    2   51.912    0.663   2.311
+eudyptula.init7 162.23.41.10     2 u   60   64    3   99.378    1.563   3.485
+107.155.79.108  129.7.1.66       2 u  119   64    2   49.171   -1.372   1.441
-212.110.158.28  89.109.251.21    2 u  120   64    2  167.465   -1.064   1.263
*ec2-54-242-183- 128.10.19.24     2 u   62   64    3   19.157    2.536   4.434
-69.195.159.158  128.252.19.1     2 u  119   64    2   42.990    6.302   3.507
-200.89.75.198 ( 200.27.106.115   2 u   58   64    3  160.786   42.737  12.827

(Table of Contents)

Force Accounts To Use Secure Passwords

Why

By default, accounts can use any password they want, including bad ones. pwquality/pam_pwquality addresses this security gap by providing "a way to configure the default password quality requirements for the system passwords" and checking "its strength against a system dictionary and a set of rules for identifying poor choices."

How It Works

On Linux, PAM is responsible for authentication. There are four tasks to PAM that you can read about at https://en.wikipedia.org/wiki/Linux_PAM. This section talks about the password task.

When there is a need to set or change an account password, the password task of PAM handles the request. In this section we will tell PAM's password task to pass the requested new password to libpam-pwquality to make sure it meets our requirements. If the requirements are met it is used/set; if it does not meet the requirements it errors and lets the user know.

Goal

  • enforced strong passwords

Steps

  1. Install libpam-pwquality.

    On Debian based systems:

    sudo apt install libpam-pwquality

  2. Tell PAM to use libpam-pwquality to enforce strong passwords by editing the file /etc/pam.d/common-password and change the line that starts like this:

    password        requisite                       pam_pwquality.so

    to this:

    password        requisite                       pam_pwquality.so retry=3 minlen=10 difok=3 ucredit=-1 lcredit=-1 dcredit=-1 ocredit=-1 maxrepeat=3 gecoschec

    The above options are:

    • retry=3 = prompt user 3 times before returning with error.
    • minlen=10 = the minimum length of the password, factoring in any credits (or debits) from these:
      • dcredit=-1 = must have at least one digit
      • ucredit=-1 = must have at least one upper case letter
      • lcredit=-1 = must have at least one lower case letter
      • ocredit=-1 = must have at least one non-alphanumeric character
    • difok=3 = at least 3 characters from the new password cannot have been in the old password
    • maxrepeat=3 = allow a maximum of 3 repeated characters
    • gecoschec = do not allow passwords with the account's name

    For the lazy:

    sudo cp --preserve /etc/pam.d/common-password /etc/pam.d/common-password.$(date +"%Y%m%d%H%M%S")

sudo sed -i -r -e "s/^(password\s+requisite\s+pam_pwquality.so)(.*)$/# \1\2         # commented by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")\n\1 retry=3 minlen=10 difok=3 ucredit=-1 lcredit=-1 dcredit=-1 ocredit=-1 maxrepeat=3 gecoschec         # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")/" /etc/pam.d/common-password

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Automatic Security Updates and Alerts

Why

It is important to keep a server updated with the latest critical security patches and updates. Otherwise you're at risk of known security vulnerabilities that bad-actors could use to gain unauthorized access to your server.

Unless you plan on checking your server every day, you'll want a way to automatically update the system and/or get emails about available updates.

You don't want to do all updates because with every update there is a risk of something breaking. It is important to do the critical updates but everything else can wait until you have time to do it manually.

Why Not

Automatic and unattended updates may break your system and you may not be near your server to fix it. This would be especially problematic if it broke your SSH access.

Notes

  • Each distribution manages packages and updates differently. So far I only have steps for Debian based systems.
  • Your server will need a way to send e-mails for this to work

Goals

  • Automatic, unattended, updates of critical security patches
  • Automatic emails of remaining pending updates

Debian Based Systems

How It Works

On Debian based systems you can use:

  • unattended-upgrades to automatically do system updates you want (i.e. critical security updates)
  • apt-listchanges to get details about package changes before they are installed/upgraded
  • apticron to get emails for pending package updates

We will use unattended-upgrades to apply critical security patches. We can also apply stable updates since they've already been thoroughly tested by the Debian community.

References
Steps

  1. Install unattended-upgrades, apt-listchanges, and apticron:

    sudo apt install unattended-upgrades apt-listchanges apticron

  2. Now we need to configure unattended-upgrades to automatically apply the updates. This is typically done by editing the files /etc/apt/apt.conf.d/20auto-upgrades and /etc/apt/apt.conf.d/50unattended-upgrades that were created by the packages. However, because these file may get overwritten with a future update, we'll create a new file instead. Create the file /etc/apt/apt.conf.d/51myunattended-upgrades and add this:

    // Enable the update/upgrade script (0=disable)
APT::Periodic::Enable "1";

// Do "apt-get update" automatically every n-days (0=disable)
APT::Periodic::Update-Package-Lists "1";

// Do "apt-get upgrade --download-only" every n-days (0=disable)
APT::Periodic::Download-Upgradeable-Packages "1";

// Do "apt-get autoclean" every n-days (0=disable)
APT::Periodic::AutocleanInterval "7";

// Send report mail to root
//     0:  no report             (or null string)
//     1:  progress report       (actually any string)
//     2:  + command outputs     (remove -qq, remove 2>/dev/null, add -d)
//     3:  + trace on    APT::Periodic::Verbose "2";    
APT::Periodic::Unattended-Upgrade "1";

// Automatically upgrade packages from these 
Unattended-Upgrade::Origins-Pattern {
      "o=Debian,a=stable";
      "o=Debian,a=stable-updates";
      "origin=Debian,codename=${distro_codename},label=Debian-Security";
};

// You can specify your own packages to NOT automatically upgrade here
Unattended-Upgrade::Package-Blacklist {
};

// Run dpkg --force-confold --configure -a if a unclean dpkg state is detected to true to ensure that updates get installed even when the system got interrupted during a previous run
Unattended-Upgrade::AutoFixInterruptedDpkg "true";

//Perform the upgrade when the machine is running because we wont be shutting our server down often
Unattended-Upgrade::InstallOnShutdown "false";

// Send an email to this address with information about the packages upgraded.
Unattended-Upgrade::Mail "root";

// Always send an e-mail
Unattended-Upgrade::MailOnlyOnError "false";

// Remove all unused dependencies after the upgrade has finished
Unattended-Upgrade::Remove-Unused-Dependencies "true";

// Remove any new unused dependencies after the upgrade has finished
Unattended-Upgrade::Remove-New-Unused-Dependencies "true";

// Automatically reboot WITHOUT CONFIRMATION if the file /var/run/reboot-required is found after the upgrade.
Unattended-Upgrade::Automatic-Reboot "true";

// Automatically reboot even if users are logged in.
Unattended-Upgrade::Automatic-Reboot-WithUsers "true";

    Note: Check /usr/lib/apt/apt.systemd.daily for details on the APT::Periodic options and check https://github.com/mvo5/unattended-upgrades for details on the Unattended-Upgrade options.

  3. Run a dry-run of unattended-upgrades to make sure your configuration file is okay:

    sudo unattended-upgrade -d --dry-run

    If everything is okay, you can let it run whenever it's scheduled to or force a run with unattended-upgrade -d.

  4. Configure apt-listchanges to your liking:

    sudo dpkg-reconfigure apt-listchanges

  5. Install apticron:

    sudo apt install apticron

    The default settings are good enough but you can check them in /etc/apticron/apticron.conf if you want to change them. For example, my configuration looks like this:

    EMAIL="root"
NOTIFY_NO_UPDATES="1"

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The Firewall

UFW: Uncomplicated Firewall

Why

Call me paranoid, and you don't have to agree, but I want to deny all traffic in and out of my server except what I explicitly allow. Why would my server be sending traffic out that I don't know about? And why would external traffic be trying to access my server if I don't know who or what it is? When it comes to good security, my opinion is to reject/deny by default, and allow by exception.

Of course, if you disagree, that is totally fine and can configure UFW to suit your needs.

Either way, ensuring that only traffic we explicitly allow is the job of a firewall. On Linux, the most common firewall is iptables. iptables, however, is rather complicated and confusing (IMHO). This is where UFW comes in. UFW simplifies the process of creating and managing iptables rules.

UFW works by letting you configure rules that:

  • allow or deny
  • input or output traffic
  • to or from ports

You can create rules by explicitly specifying the ports or with application configurations that specify the ports.

How It Works

WIP

Goal

  • all network traffic, input and output, blocked except those we explicitly allow

Notes

  • As you install other programs, you'll need to enable the necessary ports/applications.

References

Steps

  1. Install ufw.

    On Debian based systems:

    sudo apt install ufw

  2. Deny all outgoing traffic:

    sudo ufw default deny outgoing comment 'deny all outgoing traffic'
    Default outgoing policy changed to 'deny'
(be sure to update your rules accordingly)

    If you are not as paranoid as me, and don't want to deny all outgoing traffic, you can allow it instead:

    sudo ufw default allow outgoing comment 'allow all outgoing traffic'

  3. Deny all incoming traffic:

    sudo ufw default deny incoming comment 'deny all incoming traffic'

  4. Obviously we want SSH connections in:

    sudo ufw limit in ssh comment 'allow SSH connections in'
    Rules updated
Rules updated (v6)

  5. Allow additional traffic as per your needs. Some common use-cases:

    # allow traffic out on port 53 -- DNS
sudo ufw allow out 53 comment 'allow DNS calls out'

# allow traffic out on port 123 -- NTP
sudo ufw allow out 123 comment 'allow NTP out'

# allow traffic out for HTTP, HTTPS, or FTP
# apt might needs these depending on which sources you're using
sudo ufw allow out http comment 'allow HTTP traffic out'
sudo ufw allow out https comment 'allow HTTPS traffic out'
sudo ufw allow out ftp comment 'allow FTP traffic out'

# allow mail to go out
sudo ufw allow out 'Mail submission' comment 'allow mail out'

# allow whois
sudo ufw allow out whois comment 'allow whois'

# allow traffic out on port 68 -- the DHCP client
# you only need this if you're using DHCP
sudo ufw allow out 68 comment 'allow the DHCP client to update'

  6. Start ufw:

    sudo ufw enable
    Command may disrupt existing ssh connections. Proceed with operation (y|n)? y
Firewall is active and enabled on system startup

  7. If you want to see a status:

    sudo ufw status
    Status: active

To                         Action      From
--                         ------      ----
22/tcp                     LIMIT       Anywhere                   # allow SSH connections in
22/tcp (v6)                LIMIT       Anywhere (v6)              # allow SSH connections in

53                         ALLOW OUT   Anywhere                   # allow DNS calls out
123                        ALLOW OUT   Anywhere                   # allow NTP out
80/tcp                     ALLOW OUT   Anywhere                   # allow HTTP traffic out
443/tcp                    ALLOW OUT   Anywhere                   # allow HTTPS traffic out
21/tcp                     ALLOW OUT   Anywhere                   # allow FTP traffic out
Mail submission            ALLOW OUT   Anywhere                   # allow mail out
43/tcp                     ALLOW OUT   Anywhere                   # allow whois
53 (v6)                    ALLOW OUT   Anywhere (v6)              # allow DNS calls out
123 (v6)                   ALLOW OUT   Anywhere (v6)              # allow NTP out
80/tcp (v6)                ALLOW OUT   Anywhere (v6)              # allow HTTP traffic out
443/tcp (v6)               ALLOW OUT   Anywhere (v6)              # allow HTTPS traffic out
21/tcp (v6)                ALLOW OUT   Anywhere (v6)              # allow FTP traffic out
Mail submission (v6)       ALLOW OUT   Anywhere (v6)              # allow mail out
43/tcp (v6)                ALLOW OUT   Anywhere (v6)              # allow whois

    or

    sudo ufw status verbose
    Status: active
Logging: on (low)
Default: deny (incoming), deny (outgoing), disabled (routed)
New profiles: skip

To                         Action      From
--                         ------      ----
22/tcp                     LIMIT IN    Anywhere                   # allow SSH connections in
22/tcp (v6)                LIMIT IN    Anywhere (v6)              # allow SSH connections in

53                         ALLOW OUT   Anywhere                   # allow DNS calls out
123                        ALLOW OUT   Anywhere                   # allow NTP out
80/tcp                     ALLOW OUT   Anywhere                   # allow HTTP traffic out
443/tcp                    ALLOW OUT   Anywhere                   # allow HTTPS traffic out
21/tcp                     ALLOW OUT   Anywhere                   # allow FTP traffic out
587/tcp (Mail submission)  ALLOW OUT   Anywhere                   # allow mail out
43/tcp                     ALLOW OUT   Anywhere                   # allow whois
53 (v6)                    ALLOW OUT   Anywhere (v6)              # allow DNS calls out
123 (v6)                   ALLOW OUT   Anywhere (v6)              # allow NTP out
80/tcp (v6)                ALLOW OUT   Anywhere (v6)              # allow HTTP traffic out
443/tcp (v6)               ALLOW OUT   Anywhere (v6)              # allow HTTPS traffic out
21/tcp (v6)                ALLOW OUT   Anywhere (v6)              # allow FTP traffic out
587/tcp (Mail submission (v6)) ALLOW OUT   Anywhere (v6)              # allow mail out
43/tcp (v6)                ALLOW OUT   Anywhere (v6)              # allow whois

Default Applications

ufw ships with some default applications. You can see them with:

sudo ufw app list
Available applications:
  AIM
  Bonjour
  CIFS
  DNS
  Deluge
  IMAP
  IMAPS
  IPP
  KTorrent
  Kerberos Admin
  Kerberos Full
  Kerberos KDC
  Kerberos Password
  LDAP
  LDAPS
  LPD
  MSN
  MSN SSL
  Mail submission
  NFS
  OpenSSH
  POP3
  POP3S
  PeopleNearby
  SMTP
  SSH
  Socks
  Telnet
  Transmission
  Transparent Proxy
  VNC
  WWW
  WWW Cache
  WWW Full
  WWW Secure
  XMPP
  Yahoo
  qBittorrent
  svnserve

To get details about the app, like which ports it includes, type:

sudo ufw app info [app name]
sudo ufw app info DNS
Profile: DNS
Title: Internet Domain Name Server
Description: Internet Domain Name Server

Port:
  53

Custom Application

If you don't want to create rules by explicitly providing the port number(s), you can create your own application configurations. To do this, create a file in /etc/ufw/applications.d.

For example, here is what you would use for Plex:

cat /etc/ufw/applications.d/plexmediaserver
[PlexMediaServer]
title=Plex Media Server
description=This opens up PlexMediaServer for http (32400), upnp, and autodiscovery.
ports=32469/tcp|32413/udp|1900/udp|32400/tcp|32412/udp|32410/udp|32414/udp|32400/udp

Then you can enable it like any other app:

sudo ufw allow plexmediaserver

(Table of Contents)

PSAD: iptables Intrusion Detection And Prevention

Why

I can't explain it any better than user FINESEC from https://serverfault.com/ did at: https://serverfault.com/a/447604/289829.

Fail2BAN scans log files of various applications such as apache, ssh or ftp and automatically bans IPs that show the malicious signs such as automated login attempts. PSAD on the other hand scans iptables and ip6tables log messages (typically /var/log/messages) to detect and optionally block scans and other types of suspect traffic such as DDoS or OS fingerprinting attempts. It's ok to use both programs at the same time because they operate on different level.

And, since we're already using UFW so we'll follow the awesome instructions by netson at https://gist.github.com/netson/c45b2dc4e835761fbccc to make PSAD work with UFW.

How It Works

WIP

References

Steps

  1. Install psad.

    On Debian based systems:

    sudo apt install psad

  2. Make a backup of /etc/psad/psad.conf:

    sudo cp /etc/psad/psad.conf /etc/psad/psad.conf.$(date +"%Y%m%d%H%M%S")

  3. Review and update configuration options in /etc/psad/psad.conf. Pay special attention to these:

    Setting Set To
    EMAIL_ADDRESSES your email address(s)
    HOSTNAME your server's hostname
    ENABLE_AUTO_IDS ENABLE_AUTO_IDS Y;
    ENABLE_AUTO_IDS_EMAILS ENABLE_AUTO_IDS_EMAILS Y;
    EXPECT_TCP_OPTIONS EXPECT_TCP_OPTIONS Y;

    Check the configuration file psad's documentation at http://www.cipherdyne.org/psad/docs/config.html for more details.

  4. Now we need to make some changes to ufw so it works with psad by telling ufw to log all traffic so psad can analyze it. Do this by editing two files and adding these lines at the end but before the COMMIT line.

    Make backups:

    sudo cp /etc/ufw/before.rules /etc/ufw/before.rules.$(date +"%Y%m%d%H%M%S")
sudo cp /etc/ufw/before6.rules /etc/ufw/before6.rules.$(date +"%Y%m%d%H%M%S")

    Edit the files:

    • /etc/ufw/before.rules
    • /etc/ufw/before6.rules

    And add add this at the end but before the COMMIT line:

    # log all traffic so psad can analyze
-A INPUT -j LOG --log-tcp-options --log-prefix "[IPTABLES] "
-A FORWARD -j LOG --log-tcp-options --log-prefix "[IPTABLES] "

    Note: We're adding a log prefix to all the iptables logs. We'll need this for seperating iptables logs to their own file.

    For example:

    ...

# log all traffic so psad can analyze
-A INPUT -j LOG --log-tcp-options --log-prefix "[IPTABLES] "
-A FORWARD -j LOG --log-tcp-options --log-prefix "[IPTABLES] "

# don't delete the 'COMMIT' line or these rules won't be processed
COMMIT

  5. Now we need to reload/restart ufw and psad for the changes to take effect:

    sudo ufw reload

sudo psad -R
sudo psad --sig-update
sudo psad -H

  6. Analyze iptables rules for errors:

    sudo psad --fw-analyze
    [+] Parsing INPUT chain rules.
[+] Parsing INPUT chain rules.
[+] Firewall config looks good.
[+] Completed check of firewall ruleset.
[+] Results in /var/log/psad/fw_check
[+] Exiting.

    Note: If there were any issues you will get an e-mail with the error.

  7. Check the status of psad:

    sudo psad --Status
    [-] psad: pid file /var/run/psad/psadwatchd.pid does not exist for psadwatchd on vm
[+] psad_fw_read (pid: 3444)  %CPU: 0.0  %MEM: 2.2
    Running since: Sat Feb 16 01:03:09 2019

[+] psad (pid: 3435)  %CPU: 0.2  %MEM: 2.7
    Running since: Sat Feb 16 01:03:09 2019
    Command line arguments: [none specified]
    Alert email address(es): root@localhost

[+] Version: psad v2.4.3

[+] Top 50 signature matches:
        [NONE]

[+] Top 25 attackers:
        [NONE]

[+] Top 20 scanned ports:
        [NONE]

[+] iptables log prefix counters:
        [NONE]

    Total protocol packet counters:

[+] IP Status Detail:
        [NONE]

    Total scan sources: 0
    Total scan destinations: 0

[+] These results are available in: /var/log/psad/status.out

(Table of Contents)

Fail2ban: Application Intrusion Detection And Prevention

Why

A firewall will board up all the doors and windows you don't want anyone using so nobody can see they are even there. But what about the doors and windows you want visible so approved folks can use them? Even if the door is locked, how do you ensure that someone doesn't try to force their way in?

That is where Fail2ban comes in. It will monitor network traffic/logs and prevent intrusions by blocking suspicious activity (e.g. multiple successive failed connections in a short time-span).

How It Works

WIP

Goal

  • network monitoring for suspicious activity with automatic banning of offending IPs

Notes

  • As of right now, the only thing running on this server is SSH so we'll want Fail2ban to monitor SSH and ban as necessary.
  • As you install other programs, you'll need to create/configure the appropriate jails and enable them.

References

Steps

  1. Install fail2ban.

    On Debian based systems:

    sudo apt install fail2ban

  2. We don't want to edit /etc/fail2ban/fail2ban.conf or /etc/fail2ban/jail.conf because a future update may overwrite those so we'll update a local copy instead. Add this to /etc/fail2ban/jail.local after replacing [LAN SEGMENT] and [your email] with the appropriate values:

    [DEFAULT]
# the IP address range we want to ignore
ignoreip = 127.0.0.1/8 [LAN SEGMENT]

# who to send e-mail to
destemail = [your e-mail]

# who is the email from
sender = [your e-mail]

# since we're using exim4 to send emails
mta = mail

# get email alerts
action = %(action_mwl)s

    Note: Your server will need to be able to send e-mails so Fail2ban can let you know of suspicious activity and when it banned an IP.

  3. We need to create a jail for ssh that tells fail2ban to look at ssh logs and use ufw to ban/unban IPs as needed. Create a jail for ssh by adding this to /etc/fail2ban/jail.d/ssh.local:

    [sshd]
enabled = true
banaction = ufw
port = ssh
filter = sshd
logpath = %(sshd_log)s
maxretry = 5

  4. In the above we tell fail2ban to use the ufw as the banaction. Fail2ban ships with an action configuration file for ufw. You can see it in /etc/fail2ban/action.d/ufw.conf

  5. Enable fail2ban and the jail for SSH:

    sudo fail2ban-client start
sudo fail2ban-client reload
sudo fail2ban-client add sshd

  6. To check the status:

    sudo fail2ban-client status
    Status
|- Number of jail:      1
`- Jail list:   sshd

    sudo fail2ban-client status sshd
    Status for the jail: sshd
|- Filter
|  |- Currently failed: 0
|  |- Total failed:     0
|  `- File list:        /var/log/auth.log
`- Actions
   |- Currently banned: 0
   |- Total banned:     0
   `- Banned IP list:

Custom Jails

I have not needed to create a custom jail yet. Once I do, and I figure out how, I will update this guide. Or, if you know how please help contribute.

Unban an IP

To unban an IP use this command:

fail2ban-client set [jail] unbanip [IP]

[jail] is the name of the jail that has the banned IP and [IP] is the IP address you want to unban. For example, to unaban 192.168.1.100 from SSH you would do:

fail2ban-client set sshd unbanip 192.168.1.100

(Table of Contents)

The Danger Zone

Proceed At Your Own Risk

This sections cover things that are high risk because there is a possibility they can make your system unusable, or are considered unnecessary by many because the risks outweigh any rewards.

!! PROCEED AT YOUR OWN RISK !!

!! PROCEED AT YOUR OWN RISK !!

Linux Kernel sysctl Hardening

!! PROCEED AT YOUR OWN RISK !!

Why

The kernel is the brains of a Linux system. Securing it just makes sense.

Why Not

Changing kernel settings with sysctl is risky and could break your server. If you don't know what you are doing, don't have the time to debug issues, or just don't want to take the risks, I would advise from not following these steps.

Disclaimer

I am not as knowledgeable about hardening/securing a Linux kernel as I'd like. As much as I hate to admit it, I do not know what all of these settings do. My understanding is that most of them are general kernel hardening and performance, and the others are to protect against spoofing and DOS attacks.

In fact, since I am not 100% sure exactly what each setting does, I took recommended settings from numerous sites (all linked in the references below) and combined them to figure out what should be set. I figure if multiple reputable sites mention the same setting, it's probably safe.

If you have a better understanding of what these settings do, or have any other feedback/advice on them, please let me know.

I won't provide For the lazy code in this section.

Notes

  • Documentation on all the sysctl settings/keys is severely lacking. The documentation I can find seems to reference the 2.2 version kernel. I could not find anything newer. If you know where I can, please let me know.
  • The reference sites listed below have more comments on what each setting does.

References

Steps

  1. The sysctl settings can be found in the linux-kernel-sysctl-hardening.md file in this repo.

  2. Before you make a kernel sysctl change permanent, you can test it with the sysctl command:

    sudo sysctl -w [key=value]

    Example:

    sudo sysctl -w kernel.ctrl-alt-del=0

    Note: There are no spaces in key=value, including before and after the space.

  3. Once you have tested a setting, and made sure it works without breaking your server, you can make it permanent by adding the values to /etc/sysctl.conf. For example:

    $ sudo cat /etc/sysctl.conf
kernel.ctrl-alt-del = 0
fs.file-max = 65535
...
kernel.sysrq = 0

  4. After updating the file you can reload the settings or reboot. To reload:

    sudo sysctl -p

Note: If sysctl has trouble writing any settings then sysctl -w or sysctl -p will write an error to stderr. You can use this to quickly find invalid settings in your /etc/sysctl.conf file:

sudo sysctl -p >/dev/null

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Password Protect GRUB

!! PROCEED AT YOUR OWN RISK !!

Why

If a bad actor has physical access to your server, they could use GRUB to gain unauthorized access to your system.

Why Not

If you forget the password, you'll have to go through some work to recover the password.

Goals

  • auto boot the default Debian install and require a password for anything else

Notes

  • This will only protect GRUB and anything behind it like your operating systems. Check your motherboard's documentation for password protecting your BIOS to prevent a bad actor from circumventing GRUB.

References

Steps

  1. Create a Password-Based Key Derivation Function 2 (PBKDF2) hash of your password:

    grub-mkpasswd-pbkdf2 -c 100000

    The below output is from using password as the password:

    Enter password:
Reenter password:
PBKDF2 hash of your password is grub.pbkdf2.sha512.100000.2812C233DFC899EFC3D5991D8CA74068C99D6D786A54F603E9A1EFE7BAEDDB6AA89672F92589FAF98DB9364143E7A1156C9936328971A02A483A84C3D028C4FF.C255442F9C98E1F3C500C373FE195DCF16C56EEBDC55ABDD332DD36A92865FA8FC4C90433757D743776AB186BD3AE5580F63EF445472CC1D151FA03906D08A6D

  2. Copy everything after PBKDF2 hash of your password is , starting from and including grub.pbkdf2.sha512... to the end. You'll need this in the next step.

  3. The update-grub program uses scripts to generate configuration files it will use for GRUB's settings. Create the file /etc/grub.d/01_password and add the below code after replacing [hash] with the hash you copied from the first step. This tells update-grub to use this username and password for GRUB.

    #!/bin/sh
set -e

cat << EOF
set superusers="grub"
password_pbkdf2 grub [hash]
EOF

    For example:

    #!/bin/sh
set -e

cat << EOF
set superusers="grub"
password_pbkdf2 grub grub.pbkdf2.sha512.100000.2812C233DFC899EFC3D5991D8CA74068C99D6D786A54F603E9A1EFE7BAEDDB6AA89672F92589FAF98DB9364143E7A1156C9936328971A02A483A84C3D028C4FF.C255442F9C98E1F3C500C373FE195DCF16C56EEBDC55ABDD332DD36A92865FA8FC4C90433757D743776AB186BD3AE5580F63EF445472CC1D151FA03906D08A6D
EOF

  4. Set the file's execute bit so update-grub includes it when it updates GRUB's configuration:

    sudo chmod a+x /etc/grub.d/01_password

  5. Make a backup of /etc/grub.d/10_linux and unset execute bit so update-grub doesn't try to run it:

    sudo cp --preserve /etc/grub.d/10_linux /etc/grub.d/10_linux.$(date +"%Y%m%d%H%M%S")
sudo chmod a-x /etc/grub.d/10_linux.*

  6. To make the default Debian install unrestricted (without the password) while keeping everything else restricted (with the password) modify /etc/grub.d/10_linux and add --unrestricted to the CLASS variable.

    For the lazy:

    sudo sed -i -r -e "/^CLASS=/ a CLASS=\"\${CLASS} --unrestricted\"         # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")" /etc/grub.d/10_linux

  7. Update GRUB with update-grub:

    sudo update-grub

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Disable Root Login

!! PROCEED AT YOUR OWN RISK !!

Why

If you have sudo configured properly, then the root account will mostly never need to log in directly -- either at the terminal or remotely.

Why Not

Be warned, this can cause issues with some configurations!

If your installation uses sulogin (like Debian) to drop to a root console during boot failures, then locking the root account will prevent sulogin from opening the root shell and you will get this error:

Cannot open access to console, the root account is locked.

See sulogin(8) man page for more details.

Press Enter to continue.

To work around this, you can use the --force option for sulogin. Some distributions already include this, or some other, workaround.

An alternative to locking the root acount is set a long/complicated root password and store it in a secured, non digital format. That way you have it when/if you need it.

Goal

  • locked root account that nobody can use to log in as root

Notes

  • Some distributions disable root login by default (e.g. Ubuntu) so you may not need to do this step. Check with your distribution's documentation.

References

Steps

  1. Lock the root account:

    sudo passwd -l root

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Change Default umask

!! PROCEED AT YOUR OWN RISK !!

Why

umask controls the default permissions of files/folders when they are created. Insecure file/folder permissions give other accounts potentially unauthorized access to your data. This may include the ability to make configuration changes.

  • For non-root accounts, there is no need for other accounts to get any access to the account's files/folders by default.
  • For the root account, there is no need for the file/folder primary group or other accounts to have any access to root's files/folders by default.

When and if other accounts need access to a file/folder, you want to explicitly grant it using a combination of file/folder permissions and primary group.

Why Not

Changing the default umask can create unexpected problems. For example, if you set umask to 0077 for root, then non-root accounts will not have access to application configuration files/folders in /etc/ which could break applications that do not run with root privileges.

How It Works

In order to explain how umask works I'd have to explain how Linux file/folder permissions work. As that is a rather complicated question, I will defer you to the references below for further reading.

Goals

  • set default umask for non-root accounts to 0027
  • set default umask for the root account to 0077

Notes

  • umask is a Bash built-in which means a user can change their own umask setting.

References

Steps

  1. Set default umask for non-root accounts to 0027 by adding this line to /etc/profile and /etc/bash.bashrc:

    umask 0027

    For the lazy:

    sudo cp --preserve /etc/profile /etc/profile.$(date +"%Y%m%d%H%M%S")
sudo cp --preserve /etc/bash.bashrc /etc/bash.bashrc.$(date +"%Y%m%d%H%M%S")

echo -e "\numask 0027         # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")" | sudo tee -a /etc/profile /etc/bash.bashrc

  2. We also need to add this line to /etc/login.defs:

    UMASK 0027

    For the lazy:

    sudo cp --preserve /etc/login.defs /etc/login.defs.$(date +"%Y%m%d%H%M%S")

echo -e "\nUMASK 0027         # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")" | sudo tee -a /etc/login.defs

  3. Set default umask for the root account to 0077 by adding this line to /root/.bashrc:

    umask 0077

    For the lazy:

    sudo cp --preserve /root/.bashrc /root/.bashrc.$(date +"%Y%m%d%H%M%S")

echo -e "\numask 0077         # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")" | sudo tee -a /root/.bashrc

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Orphaned Software

!! PROCEED AT YOUR OWN RISK !!

Why

As you use your system, and you install and uninstall software, you'll eventually end up with orphaned, or unused software/packages/libraries. You don't need to remove them, but if you don't need them, why keep them? When security is a priority, anything not explicitly needed is a potential security threat. You want to keep your server as trimmed and lean as possible.

Notes

  • Each distribution manages software/packages/libraries differently so how you find and remove orphaned packages will be different. So far I only have steps for Debian based systems.

Debian Based Systems

Why Not

Keep in mind, deborphan finds packages that have no package dependencies. That does not mean they are not used. You could very well have a package you use every day that has no dependencies that you wouldn't want to remove. And, if deborphan gets anything wrong, then removing critical packages may break your system.

Steps

On Debian based systems, you can use deborphan to find orphaned packages.

  1. Install deborphan.

    sudo apt install deborphan

  2. Run deborphan as root to see a list of orphaned packages:

    sudo deborphan
    libxapian30
libpipeline1

  3. Assuming you want to remove all of the packages deborphan finds, you can pass it's output to apt to remove them:

    sudo apt --autoremove purge $(deborphan)

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The Auditing

Lynis - Linux Security Auditing

Why

From https://cisofy.com/lynis/:

Lynis is a battle-tested security tool for systems running Linux, macOS, or Unix-based operating system. It performs an extensive health scan of your systems to support system hardening and compliance testing.

Goals

  • Lynis installed

Notes

References

Steps

  1. Install lynis. https://cisofy.com/lynis/#installation has detailed instructions on how to install it for your distribution.

    On Debian based systems, using CISOFY's community software repository:

    sudo wget -O - https://packages.cisofy.com/keys/cisofy-software-public.key | sudo apt-key add -
sudo apt install apt-transport-https
sudo echo "deb https://packages.cisofy.com/community/lynis/deb/ stable main" | sudo tee /etc/apt/sources.list.d/cisofy-lynis.list
sudo apt update
sudo apt install lynis

  2. Update it:

    sudo lynis update info

  3. Run a security audit:

    sudo lynis audit system

    This will scan your server, report its audit findings, and at the end it will give you suggestions. Spend some time going through the output and address gaps as necessary.

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The Miscellaneous

Configure Gmail as MTA

Why

Unless you're planning on setting up your own mail server, you'll need a way to send e-mails from your server. This will be important for system alerts/messages.

You can use any Gmail account but I recommend you create one specific for this server. That way if your server is compromised, the bad-actor won't have any passwords for your primary account. Granted, if you have 2FA/MFA enabled and you use an app password, there isn't much a bad-actor can do with just the app password but why take the risk?

Goals

  • mail configured to send e-mails from your server using Gmail

References

Steps

  1. Install exim4.

    On Debian based systems:

    sudo apt install exim4

  2. Configure exim4:

    For Debian based systems:

    sudo dpkg-reconfigure exim4-config

    You'll be prompted with some questions:

    Prompt Answer
    General type of mail configuration mail sent by smarthost; no local mail
    System mail name (default)
    IP-addresses to listen on for incoming SMTP connections 127.0.0.1
    Other destinations for which mail is accepted (default)
    Visible domain name for local users (default)
    IP address or host name of the outgoing smarthost smtp.gmail.com::587
    Keep number of DNS-queries minimal (Dial-on-Demand)? No
    Split configuration into small files? No

  3. Make a backup of /etc/exim4/passwd.client:

        sudo cp /etc/exim4/passwd.client /etc/exim4/passwd.client.$(date +"%Y%m%d%H%M%S")

  4. Add a line like this to /etc/exim4/passwd.client

    *.google.com:yourAccount@gmail.com:yourPassword

    Replace yourAccount@gmail.com and yourPassword with your details. If you have 2FA/MFA enabled on your Gmail then you'll need to create and use an app password.

  5. This file has your Gmail password so we need to lock it down:

    sudo chown root:Debian-exim /etc/exim4/passwd.client
sudo chmod 640 /etc/exim4/passwd.client

  6. Restart exim4:

    sudo service exim4 restart

  7. Add some mail aliases so we can send e-mails to local accounts by adding lines like this to /etc/aliases:

    user1: user1@gmail.com
user2: user2@gmail.com
...

    You'll need to add all the local accounts that exist on your server.

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Separate iptables Log File

Why

There will come a time when you'll need to look through your iptables logs. Having all the iptables logs go to their own file will make it a lot easier to find what you're looking for.

References

Steps

  1. The first step is by telling your firewall to prefix all log entries with some unique string. If you're using iptables directly, you would do something like --log-prefix "[IPTABLES] " for all the rules. We took care of this in step step 4 of installing psad.

  2. After you've added a prefix to the firewall logs, we need to tell rsyslog to send those lines to its own file. Do this by creating the file /etc/rsyslog.d/10-iptables.conf and adding this:

    :msg, contains, "[IPTABLES] " /var/log/iptables.log
& stop

    If you're expecting a lot if data being logged by your firewall, prefix the filename with a - "to omit syncing the file after every logging". For example:

    :msg, contains, "[IPTABLES] " -/var/log/iptables.log
& stop

    Note: Remember to change the prefix to whatever you use.

  3. Since we're logging firewall messages to a different file, we need to tell psad where the new file is. Edit /etc/psad/psad.conf and set IPT_SYSLOG_FILE to the path of the log file. For example:

    IPT_SYSLOG_FILE /var/log/iptables.log;

  4. Restart psad and rsyslog to activate the changes (or reboot):

    sudo psad -R
sudo psad --sig-update
sudo psad -H
sudo service rsyslog restart

  5. The last thing we have to do is tell logrotate to rotate the new log file so it doesn't get to big and fill up our disk. Create the file /etc/logrotate.d/iptables and add this:

    /var/log/iptables.log
{
    rotate 7
    daily
    missingok
    notifempty
    delaycompress
    compress
    postrotate
        invoke-rc.d rsyslog rotate > /dev/null
    endscript
}

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Left Over

Contacting Me

For any questions, comments, concerns, feedback, or issues, submit a new issue.

(Table of Contents)

Additional References

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Acknowledgments

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Disclaimer / Warranty

This guide comes with ABSOLUTELY NO WARRANTY. Use with caution. I take no responsibility for anything, related to or not related to this guide.

(Table of Contents)

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