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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.

CC-BY-SA

Table of Contents

(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#Mandatoryaccesscontrol
    • 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.

(Table of Contents)

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 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

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. Anther 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 [email protected]
    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 [email protected]
    
    /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
    [email protected]'s password:
    
    

    Number of key(s) added: 1

    Now try logging into the machine, with: "ssh '[email protected]'" 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.

Goals

  • a secure SSH configuration

Notes

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.com/ssh/sshd_config/
  • https://www.techbrown.com/harden-ssh-secure-linux-vps-server/
  • https://serverfault.com/questions/660160/openssh-difference-between-internal-sftp-and-sftp-server/660325
  • man sshd_config

Steps

  1. Make a backup of OpenSSH server's configuration file

    /etc/ssh/sshd_config
    and remove comments to make it easier to read:
    sudo cp --archive /etc/ssh/sshd_config /etc/ssh/sshd_config-COPY-$(date +"%Y%m%d%H%M%S")
    sudo sed -i -r -e '/^#|^$/ d' /etc/ssh/sshd_config
    
  2. Edit

    /etc/ssh/sshd_config
    then find and edit or add these settings that should be applied regardless of your configuration/setup:

    Note: SSH does not like duplicate contradicting settings. For example, if you have

    ChallengeResponseAuthentication no
    and then
    ChallengeResponseAuthentication yes
    , SSH will respect the first one and ignore the second. Your
    /etc/ssh/sshd_config
    file may already have some of the settings/lines below. To avoid issues you will need to manually go through your
    /etc/ssh/sshd_config
    file and address any duplicate contradicting settings. (If anyone knows a way to programatically do this I would love to hear how.)
    ########################################################################################################
    # 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 [email protected],ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256

    Ciphers [email protected],[email protected],[email protected],aes256-ctr,aes192-ctr,aes128-ctr

    MACs [email protected],[email protected],[email protected],hmac-sha2-512,hmac-sha2-256,[email protected]

    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

    ########################################################################################################

    don't let users set environment variables

    PermitUserEnvironment no

    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

    don't allow .rhosts or /etc/hosts.equiv

    HostbasedAuthentication 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
    
  5. You can check verify the configurations worked with

    sshd -T
    and verify the output:
    sudo sshd -T
    
    port 22
    addressfamily any
    listenaddress [::]:22
    listenaddress 0.0.0.0:22
    usepam yes
    logingracetime 30
    x11displayoffset 10
    maxauthtries 2
    maxsessions 2
    clientaliveinterval 300
    clientalivecountmax 0
    streamlocalbindmask 0177
    permitrootlogin no
    ignorerhosts yes
    ignoreuserknownhosts no
    hostbasedauthentication no
    ...
    subsystem sftp internal-sftp -f AUTHPRIV -l INFO
    maxstartups 2:30:2
    permittunnel no
    ipqos lowdelay throughput
    rekeylimit 0 0
    permitopen any
    

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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.

Goals

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

References

  • Mozilla's OpenSSH guidelines for OpenSSH 6.7+ at https://infosec.mozilla.org/guidelines/openssh#modern-openssh-67
  • https://infosec.mozilla.org/guidelines/key_management
  • man moduli

Steps

  1. Make a backup of SSH's moduli file

    /etc/ssh/moduli
    :
    sudo cp --archive /etc/ssh/moduli /etc/ssh/moduli-COPY-$(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

  • https://github.com/google/google-authenticator-libpam
  • https://en.wikipedia.org/wiki/Linux_PAM
  • https://en.wikipedia.org/wiki/Time-basedOne-timePassword_algorithm
  • https://fastmail.blog/2016/07/22/how-totp-authenticator-apps-work/
  • https://jemurai.com/2018/10/11/how-it-works-totp-based-mfa/

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/[email protected]%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. Make a backup of PAM's SSH configuration file

    /etc/pam.d/sshd
    :
    sudo cp --archive /etc/pam.d/sshd /etc/pam.d/sshd-COPY-$(date +"%Y%m%d%H%M%S")
    
  4. 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
    

    Note: Check here for what

    nullok
    means.

    For the lazy:

    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
    
  5. Tell SSH to leverage it by adding or editing this line in

    /etc/ssh/sshd_config
    :
    ChallengeResponseAuthentication yes
    

    For the lazy:

    sudo sed -i -r -e "s/^(challengeresponseauthentication .*)$/# \1         # commented by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")/I" /etc/ssh/sshd_config
    echo -e "\nChallengeResponseAuthentication yes         # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")" | sudo tee -a /etc/ssh/sshd_config
    
  6. Restart ssh:

    sudo service sshd restart
    

(Table of Contents)

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. Make a backup of the sudo's configuration file

    /etc/sudoers
    :
    sudo cp --archive /etc/sudoers /etc/sudoers-COPY-$(date +"%Y%m%d%H%M%S")
    
  4. Edit sudo's configuration file

    /etc/sudoers
    :
    sudo visudo
    
  5. 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

  • https://cloudpro.zone/index.php/2018/01/27/debian-9-3-server-setup-guide-part-4/
  • https://en.wikipedia.org/wiki/NetworkTimeProtocol
  • https://www.pool.ntp.org/en/
  • https://serverfault.com/questions/957302/securing-hardening-ntp-client-on-linux-servers-config-file/957450#957450
  • https://tf.nist.gov/tf-cgi/servers.cgi

Steps

  1. Install ntp.

    On Debian based systems:

    sudo apt install ntp
    
  2. Make a backup of the NTP client's configuration file

    /etc/ntp.conf
    :
    sudo cp --archive /etc/ntp.conf /etc/ntp.conf-COPY-$(date +"%Y%m%d%H%M%S")
    
  3. The default configuration, at least on Debian, is already pretty secure. The only thing we'll want to make sure is we're the

    pool
    directive and not any
    server
    directives. The
    pool
    directive allows the NTP client to stop using a server if it is unresponsive or serving bad time. Do this by commenting out all
    server
    directives and adding the below to
    /etc/ntp.conf
    .
    pool pool.ntp.org iburst
    

    For the lazy:

    sudo sed -i -r -e "s/^((server|pool).*)/# \1         # commented by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")/" /etc/ntp.conf
    echo -e "\npool pool.ntp.org iburst         # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")" | sudo tee -a /etc/ntp.conf
    

    Example

    /etc/ntp.conf
    :

    driftfile /var/lib/ntp/ntp.drift
    statistics loopstats peerstats clockstats
    filegen loopstats file loopstats type day enable
    filegen peerstats file peerstats type day enable
    filegen clockstats file clockstats type day enable
    restrict -4 default kod notrap nomodify nopeer noquery limited
    restrict -6 default kod notrap nomodify nopeer noquery limited
    restrict 127.0.0.1
    restrict ::1
    restrict source notrap nomodify noquery
    pool pool.ntp.org iburst         # added by user on 2019-03-09 @ 10:23:35
    
  4. Restart ntp:

    sudo service ntp restart
    
  5. Check the status of the ntp service:

    sudo systemctl status ntp
    
    ● ntp.service - LSB: Start NTP daemon
       Loaded: loaded (/etc/init.d/ntp; generated; vendor preset: enabled)
       Active: active (running) since Sat 2019-03-09 15:19:46 EST; 4s ago
         Docs: man:systemd-sysv-generator(8)
      Process: 1016 ExecStop=/etc/init.d/ntp stop (code=exited, status=0/SUCCESS)
      Process: 1028 ExecStart=/etc/init.d/ntp start (code=exited, status=0/SUCCESS)
        Tasks: 2 (limit: 4915)
       CGroup: /system.slice/ntp.service
               └─1038 /usr/sbin/ntpd -p /var/run/ntpd.pid -g -u 108:113
    
    

    Mar 09 15:19:46 host ntpd[1038]: Listen and drop on 0 v6wildcard [::]:123 Mar 09 15:19:46 host ntpd[1038]: Listen and drop on 1 v4wildcard 0.0.0.0:123 Mar 09 15:19:46 host ntpd[1038]: Listen normally on 2 lo 127.0.0.1:123 Mar 09 15:19:46 host ntpd[1038]: Listen normally on 3 enp0s3 10.10.20.96:123 Mar 09 15:19:46 host ntpd[1038]: Listen normally on 4 lo [::1]:123 Mar 09 15:19:46 host ntpd[1038]: Listen normally on 5 enp0s3 [fe80::a00:27ff:feb6:ed8e%2]:123 Mar 09 15:19:46 host ntpd[1038]: Listening on routing socket on fd #22 for interface updates Mar 09 15:19:47 host ntpd[1038]: Soliciting pool server 108.61.56.35 Mar 09 15:19:48 host ntpd[1038]: Soliciting pool server 69.89.207.199 Mar 09 15:19:49 host ntpd[1038]: Soliciting pool server 45.79.111.114

  6. Check ntp's status:

    sudo ntpq -p
    
         remote           refid      st t when poll reach   delay   offset  jitter
    ==============================================================================
     pool.ntp.org    .POOL.          16 p    -   64    0    0.000    0.000   0.000
    *lithium.constan 198.30.92.2      2 u    -   64    1   19.900    4.894   3.951
     ntp2.wiktel.com 212.215.1.157    2 u    2   64    1   48.061   -0.431   0.104
    

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Securing /proc

Why

To quote https://linux-audit.com/linux-system-hardening-adding-hidepid-to-proc/:

When looking in

/proc
you will discover a lot of files and directories. Many of them are just numbers, which represent the information about a particular process ID (PID). By default, Linux systems are deployed to allow all local users to see this all information. This includes process information from other users. This could include sensitive details that you may not want to share with other users. By applying some filesystem configuration tweaks, we can change this behavior and improve the security of the system.

Goals

  • /proc
    mounted with
    hidepid=2
    so users can only see information about their processes

References

  • https://linux-audit.com/linux-system-hardening-adding-hidepid-to-proc/
  • https://likegeeks.com/secure-linux-server-hardening-best-practices/#Hardening-proc-Directory
  • https://www.cyberciti.biz/faq/linux-hide-processes-from-other-users/

Steps

  1. Make a backup of

    /etc/fstab
    :
    sudo cp --archive /etc/fstab /etc/fstab-COPY-$(date +"%Y%m%d%H%M%S")
    
  2. Add this line to

    /etc/fstab
    to have
    /proc
    mounted with
    hidepid=2
    :
    proc     /proc     proc     defaults,hidepid=2     0     0
    

    For the lazy:

    echo -e "\nproc     /proc     proc     defaults,hidepid=2     0     0         # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")" | sudo tee -a /etc/fstab
    
  3. Reboot the system:

    sudo reboot now
    

    Note: Alternatively, you can remount

    /proc
    without rebooting with
    sudo mount -o remount,hidepid=2 /proc

(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.

Goals

  • enforced strong passwords

Steps

  1. Install libpam-pwquality.

    On Debian based systems:

    sudo apt install libpam-pwquality
    
  2. Make a backup of PAM's password configuration file

    /etc/pam.d/common-password
    :
    sudo cp --archive /etc/pam.d/common-password /etc/pam.d/common-password-COPY-$(date +"%Y%m%d%H%M%S")
    
  3. 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 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
  • https://wiki.debian.org/UnattendedUpgrades
  • https://debian-handbook.info/browse/stable/sect.regular-upgrades.html
  • https://blog.sleeplessbeastie.eu/2015/01/02/how-to-perform-unattended-upgrades/
  • https://www.vultr.com/docs/how-to-set-up-unattended-upgrades-on-debian-9-stretch
  • https://github.com/mvo5/unattended-upgrades
  • https://wiki.debian.org/UnattendedUpgrades#apt-listchanges
  • https://www.cyberciti.biz/faq/apt-get-apticron-send-email-upgrades-available/
  • https://www.unixmen.com/how-to-get-email-notifications-for-new-updates-on-debianubuntu/
  • /etc/apt/apt.conf.d/50unattended-upgrades
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";

    Notes:

    • Check
      /usr/lib/apt/apt.systemd.daily
      for details on the
      APT::Periodic
      options
    • 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. For 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"
    

(Table of Contents)

More Secure Random Entropy Pool (WIP)

Why

WIP

How It Works

WIP

Goals

WIP

References

  • Thanks to branneman for this idea as submitted in issue #33.
  • https://hackaday.com/2017/11/02/what-is-entropy-and-how-do-i-get-more-of-it/
  • https://www.2uo.de/myths-about-urandom
  • https://www.gnu.org/software/hurd/user/tlecarrour/rng-tools.html
  • https://wiki.archlinux.org/index.php/Rng-tools

Steps

  1. Install rng-tools.

    On Debian based systems:

    sudo apt-get install rng-tools
    

(Table of Contents)

The Network

Firewall With 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.

How It Works

The Linux kernel provides capabilities to monitor and control network traffic. These capabilities are exposed to the end-user through firewall utilities. On Linux, the most common firewall is iptables. However, iptables is rather complicated and confusing (IMHO). This is where UFW comes in. Think of UFW as a front-end to iptables. It simplifies the process of managing the iptables rules that tell the Linux kernel what to do with network traffic.

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.

Goals

  • 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

  • https://launchpad.net/ufw

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 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)

iptables Intrusion Detection And Prevention with PSAD

Why

Even if you have a firewall to guard your doors, it is possible to try brute-forcing your way in any of the guarded doors. We want to monitor all network activity to detect potential intrusion attempts, such has repeated attempts to get in, and block them.

How It Works

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.

References

  • http://www.cipherdyne.org/psad/
  • http://www.cipherdyne.org/psad/docs/config.html
  • https://www.thefanclub.co.za/how-to/how-install-psad-intrusion-detection-ubuntu-1204-lts-server
  • https://serverfault.com/a/447604/289829
  • https://serverfault.com/a/770424/289829
  • https://gist.github.com/netson/c45b2dc4e835761fbccc-

Steps

  1. Install psad.

    On Debian based systems:

    sudo apt install psad
    
  2. Make a backup of psad's configuration file

    /etc/psad/psad.conf
    :
    sudo cp --archive /etc/psad/psad.conf /etc/psad/psad.conf-COPY-$(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.

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

  1. 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

  2. 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.

  3. 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): [email protected]

    [+] 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)

Application Intrusion Detection And Prevention With Fail2Ban

Why

UFW tells your server what doors to board up so nobody can see them, and what doors to allow authorized users through. PSAD monitors network activity to detect and prevent potential intrusions -- repeated attempts to get in.

But what about the applications/services your server is running, like SSH and Apache, where your firewall is configured to allow access in. Even though access may be allowed that doesn't mean all access attempts are valid and harmless. What if someone tries to brute-force their way in to a web-app you're running on your server? This is where Fail2ban comes in.

How It Works

Fail2ban monitors the logs of your applications (like SSH and Apache) to detect and prevent potential intrusions. It will monitor network traffic/logs and prevent intrusions by blocking suspicious activity (e.g. multiple successive failed connections in a short time-span).

Goals

  • 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

  • https://www.fail2ban.org/
  • https://blog.vigilcode.com/2011/05/ufw-with-fail2ban-quick-secure-setup-part-ii/
  • https://dodwell.us/security/ufw-fail2ban-portscan.html
  • https://www.howtoforge.com/community/threads/fail2ban-and-ufw-on-debian.77261/

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 create a local copy instead. Create the file
    /etc/fail2ban/jail.local
    and add this to it 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 creating the file

    /etc/fail2ban/jail.d/ssh.local
    and adding this to it:
    [sshd]
    enabled = true
    banaction = ufw
    port = ssh
    filter = sshd
    logpath = %(sshd_log)s
    maxretry = 5
    

    For the lazy:

    cat << EOF | sudo tee /etc/fail2ban/jail.d/ssh.local
    [sshd]
    enabled = true
    banaction = ufw
    port = ssh
    filter = sshd
    logpath = %(sshd_log)s
    maxretry = 5
    EOF
    
  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 Auditing

File/Folder Integrity Monitoring With AIDE (WIP)

Why

WIP

How It Works

WIP

Goals

WIP

References

  • https://aide.github.io/
  • https://www.hiroom2.com/2017/06/09/debian-8-file-integrity-check-with-aide/
  • https://blog.rapid7.com/2017/06/30/how-to-install-and-configure-aide-on-ubuntu-linux/
  • https://www.stephenrlang.com/2016/03/using-aide-for-file-integrity-monitoring-fim-on-ubuntu/
  • https://www.howtoforge.com/how-to-configure-the-aide-advanced-intrusion-detection-environment-file-integrity-scanner-for-your-website
  • https://www.tecmint.com/check-integrity-of-file-and-directory-using-aide-in-linux/
  • https://www.cyberciti.biz/faq/debian-ubuntu-linux-software-integrity-checking-with-aide/

Steps

  1. Install AIDE.

    On Debian based systems:

    sudo apt install aide
    
  2. Make a backup of AIDE's defaults file:

    sudo cp -p /etc/default/aide /etc/default/aide-COPY-$(date +"%Y%m%d%H%M%S")
    
  3. Go through

    /etc/default/aide
    and set AIDE's defaults per your requirements. If you want AIDE to run daily and e-mail you, be sure to set
    CRON_DAILY_RUN
    to
    yes
    .
  4. Make a backup of AIDE's configuration files:

    sudo cp -pr /etc/aide /etc/aide-COPY-$(date +"%Y%m%d%H%M%S")
    
  5. On Debian based systems:

- AIDE's configuration files are in `/etc/aide/aide.conf.d/`.
- You'll want to go through AIDE's documentation and the configuration files in to set them per your requirements.
- If you want new settings, to monitor a new folder for example, you'll want to add them to `/etc/aide/aide.conf` or `/etc/aide/aide.conf.d/`.
- Take a backup of the stock configuration files: `sudo cp -pr /etc/aide /etc/aide-COPY-$(date +"%Y%m%d%H%M%S")`.
  1. Create a new database, and install it.

    On Debian based systems:

    sudo aideinit
    
    Running aide --init...
    Start timestamp: 2019-04-01 21:23:37 -0400 (AIDE 0.16)
    AIDE initialized database at /var/lib/aide/aide.db.new
    Verbose level: 6
    
    

    Number of entries: 25973


    The attributes of the (uncompressed) database(s):

    /var/lib/aide/aide.db.new RMD160 : moyQ1YskQQbidX+Lusv3g2wf1gQ= TIGER : 7WoOgCrXzSpDrlO6I3PyXPj1gRiaMSeo SHA256 : gVx8Fp7r3800WF2aeXl+/KHCzfGsNi7O g16VTPpIfYQ= SHA512 : GYfa0DJwWgMLl4Goo5VFVOhu4BphXCo3 rZnk49PYztwu50XjaAvsVuTjJY5uIYrG tV+jt3ELvwFzGefq4ZBNMg== CRC32 : /cusZw== HAVAL : E/i5ceF3YTjwenBfyxHEsy9Kzu35VTf7 CPGQSW4tl14= GOST : n5Ityzxey9/1jIs7LMc08SULF1sLBFUc aMv7Oby604A=

    End timestamp: 2019-04-01 21:24:45 -0400 (run time: 1m 8s)

  2. Test everything works with no changes.

    On Debian based systems:

    sudo aide.wrapper --check
    
    Start timestamp: 2019-04-01 21:24:45 -0400 (AIDE 0.16)
    AIDE found NO differences between database and filesystem. Looks okay!!
    Verbose level: 6
    
    

    Number of entries: 25973


    The attributes of the (uncompressed) database(s):

    /var/lib/aide/aide.db RMD160 : moyQ1YskQQbidX+Lusv3g2wf1gQ= TIGER : 7WoOgCrXzSpDrlO6I3PyXPj1gRiaMSeo SHA256 : gVx8Fp7r3800WF2aeXl+/KHCzfGsNi7O g16VTPpIfYQ= SHA512 : GYfa0DJwWgMLl4Goo5VFVOhu4BphXCo3 rZnk49PYztwu50XjaAvsVuTjJY5uIYrG tV+jt3ELvwFzGefq4ZBNMg== CRC32 : /cusZw== HAVAL : E/i5ceF3YTjwenBfyxHEsy9Kzu35VTf7 CPGQSW4tl14= GOST : n5Ityzxey9/1jIs7LMc08SULF1sLBFUc aMv7Oby604A=

    End timestamp: 2019-04-01 21:26:03 -0400 (run time: 1m 18s)

  3. Test everything works after making some changes.

    On Debian based systems:

    sudo touch /etc/test.sh
    sudo touch /root/test.sh
    
    

    sudo aide.wrapper --check

    sudo rm /etc/test.sh sudo rm /root/test.sh

    sudo aideinit -y -f

    Start timestamp: 2019-04-01 21:37:37 -0400 (AIDE 0.16)
    AIDE found differences between database and filesystem!!
    Verbose level: 6
    
    

    Summary: Total number of entries: 25972 Added entries: 2 Removed entries: 0 Changed entries: 1


    Added entries:

    f++++++++++++++++: /etc/test.sh f++++++++++++++++: /root/test.sh


    Changed entries:

    d =.... mc.. .. .: /root


    Detailed information about changes:

    Directory: /root Mtime : 2019-04-01 21:35:07 -0400 | 2019-04-01 21:37:36 -0400 Ctime : 2019-04-01 21:35:07 -0400 | 2019-04-01 21:37:36 -0400


    The attributes of the (uncompressed) database(s):

    /var/lib/aide/aide.db RMD160 : qF9WmKaf2PptjKnhcr9z4ueCPTY= TIGER : zMo7MvvYJcq1hzvTQLPMW7ALeFiyEqv+ SHA256 : LSLLVjjV6r8vlSxlbAbbEsPcQUB48SgP pdVqEn6ZNbQ= SHA512 : Qc4U7+ZAWCcitapGhJ1IrXCLGCf1IKZl 02KYL1gaZ0Fm4dc7xLqjiquWDMSEbwzW oz49NCquqGz5jpMIUy7UxA== CRC32 : z8ChEA== HAVAL : YapzS+/cdDwLj3kHJEq8fufLp3DPKZDg U12KCSkrO7Y= GOST : 74sLV4HkTig+GJhokvxZQm7CJD/NR0mG 6jV7zdt5AXQ=

    End timestamp: 2019-04-01 21:38:50 -0400 (run time: 1m 13s)

  4. That's it. If you set

    CRON_DAILY_RUN
    to
    yes
    in
    /etc/default/aide
    then cron will execute
    /etc/cron.daily/aide
    every day and e-mail you the output.

Updating The Database

Every time you make changes to files/folders that AIDE monitors, you will need to update the database to capture those changes. To do that on Debian based systems:

sudo aideinit -y -f

(Table of Contents)

Anti-Virus Scanning With ClamAV (WIP)

Why

WIP

How It Works

  • ClamAV is a virus scanner
  • ClamAV-Freshclam is a service that keeps the virus definitions updated
  • ClamAV-Daemon keeps the
    clamd
    process running to make scanning faster

Goals

WIP

Notes

  • These instructions do not tell you how to enable the ClamAV daemon service to ensure
    clamd
    is running all the time.
    clamd
    is only if you're running a mail server and does not provide real-time monitoring of files. Instead, you'd want to scan files manually or on a schedule.

References

  • https://www.clamav.net/documents/installation-on-debian-and-ubuntu-linux-distributions
  • https://wiki.debian.org/ClamAV
  • https://www.osradar.com/install-clamav-debian-9-ubuntu-18/
  • https://www.lisenet.com/2014/automate-clamav-to-perform-daily-system-scan-and-send-email-notifications-on-linux/
  • https://www.howtoforge.com/tutorial/configure-clamav-to-scan-and-notify-virus-and-malware/
  • https://serverfault.com/questions/741299/is-there-a-way-to-keep-clamav-updated-on-debian-8
  • https://askubuntu.com/questions/250290/how-do-i-scan-for-viruses-with-clamav
  • https://ngothang.com/how-to-install-clamav-and-configure-daily-scanning-on-centos/

Steps

  1. Install ClamAV.

    On Debian based systems:

    sudo apt install clamav clamav-freshclam clamav-daemon
    
  2. Make a backup of

    clamav-freshclam
    's configuration file
    /etc/clamav/freshclam.conf
    :
    sudo cp --archive /etc/clamav/freshclam.conf /etc/clamav/freshclam.conf-COPY-$(date +"%Y%m%d%H%M%S")
    
  3. clamav-freshclam
    's default settings are probably good enough but if you want to change them, you can either edit the file
    /etc/clamav/freshclam.conf
    or use
    dpkg-reconfigure
    :
    sudo dpkg-reconfigure clamav-freshclam
    

    Note: The default settings will update the definitions 24 times in a day. To change the interval, check the

    Checks
    setting in
    /etc/clamav/freshclam.conf
    or use
    dpkg-reconfigure
    .
  4. Start the

    clamav-freshclam
    service:
    sudo service clamav-freshclam start
    
  5. You can make sure

    clamav-freshclam
    running:
    sudo service clamav-freshclam status
    
    ● clamav-freshclam.service - ClamAV virus database updater
       Loaded: loaded (/lib/systemd/system/clamav-freshclam.service; enabled; vendor preset: enabled)   Active: active (running) since Sat 2019-03-16 22:57:07 EDT; 2min 13s ago
         Docs: man:freshclam(1)
               man:freshclam.conf(5)
               https://www.clamav.net/documents
     Main PID: 1288 (freshclam)
       CGroup: /system.slice/clamav-freshclam.service
               └─1288 /usr/bin/freshclam -d --foreground=true
    
    

    Mar 16 22:57:08 host freshclam[1288]: Sat Mar 16 22:57:08 2019 -> ^Local version: 0.100.2 Recommended version: 0.101.1 Mar 16 22:57:08 host freshclam[1288]: Sat Mar 16 22:57:08 2019 -> DON'T PANIC! Read https://www.clamav.net/documents/upgrading-clamav Mar 16 22:57:15 host freshclam[1288]: Sat Mar 16 22:57:15 2019 -> Downloading main.cvd [100%] Mar 16 22:57:38 host freshclam[1288]: Sat Mar 16 22:57:38 2019 -> main.cvd updated (version: 58, sigs: 4566249, f-level: 60, builder: sigmgr) Mar 16 22:57:40 host freshclam[1288]: Sat Mar 16 22:57:40 2019 -> Downloading daily.cvd [100%] Mar 16 22:58:13 host freshclam[1288]: Sat Mar 16 22:58:13 2019 -> daily.cvd updated (version: 25390, sigs: 1520006, f-level: 63, builder: raynman) Mar 16 22:58:14 host freshclam[1288]: Sat Mar 16 22:58:14 2019 -> Downloading bytecode.cvd [100%] Mar 16 22:58:16 host freshclam[1288]: Sat Mar 16 22:58:16 2019 -> bytecode.cvd updated (version: 328, sigs: 94, f-level: 63, builder: neo) Mar 16 22:58:24 host freshclam[1288]: Sat Mar 16 22:58:24 2019 -> Database updated (6086349 signatures) from db.local.clamav.net (IP: 104.16.219.84) Mar 16 22:58:24 host freshclam[1288]: Sat Mar 16 22:58:24 2019 -> ^Clamd was NOT notified: Can't connect to clamd through /var/run/clamav/clamd.ctl: No such file or directory

    Note: Don't worry about that

    Local version
    line. Check https://serverfault.com/questions/741299/is-there-a-way-to-keep-clamav-updated-on-debian-8 for more details.
  6. Make a backup of

    clamav-daemon
    's configuration file
    /etc/clamav/clamd.conf
    :
    sudo cp --archive /etc/clamav/clamd.conf /etc/clamav/clamd.conf-COPY-$(date +"%Y%m%d%H%M%S")
    
  7. You can change

    clamav-daemon
    's settings by editing the file
    /etc/clamav/clamd.conf
    or useing
    dpkg-reconfigure
    :
    sudo dpkg-reconfigure clamav-daemon
    

Scanning Files/Folders

  • To scan files/folders use the
    clamscan
    program.
  • clamscan
    runs as the user it is executed as so it needs read permissions to the files/folders it is scanning.
  • Using
    clamscan
    as
    root
    is dangerous because if a file is in fact a virus there is risk that it could use the root privileges.
  • To scan a file:
    clamscan /path/to/file
    .
  • To scan a directory:
    clamscan -r /path/to/folder
    .
  • You can use the
    -i
    switch to only print infected files.
  • Check
    clamscan
    's
    man
    pages for other switches/options.

(Table of Contents)

Rootkit Detection With Rkhunter (WIP)

Why

WIP

How It Works

WIP

Goals

WIP

References

  • 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/

Steps

  1. Install Rkhunter.

    On Debian based systems:

    sudo apt install rkhunter
    
  2. Make a backup of rkhunter' defaults file:

    sudo cp -p /etc/default/rkhunter /etc/default/rkhunter-COPY-$(date +"%Y%m%d%H%M%S")
    
  3. rkhunter's configuration file is

    /etc/rkhunter.conf
    . Instead of making changes to it, create and use the file
    /etc/rkhunter.conf.local
    instead:
    sudo cp -p /etc/rkhunter.conf /etc/rkhunter.conf.local
    
  4. Go through the configuration file

    /etc/rkhunter.conf.local
    and set to your requirements. My recommendations:

    |Setting|Note| |--|--| |

    UPDATE_MIRRORS=1
    || |
    MIRRORS_MODE=0
    || |
    MAIL-ON-WARNING=root
    || |
    COPY_LOG_ON_ERROR=1
    |to save a copy of the log if there is an error| |
    PKGMGR=...
    |set to the appropriate value per the documentation| |
    PHALANX2_DIRTEST=1
    |read the documentation for why| |
    WEB_CMD=""
    |this is to address an issue with the Debian package that disables the ability for rkhunter to self-update.|
    |
    USE_LOCKING=1
    |to prevent issues with rkhunter running multiple times| |
    SHOW_SUMMARY_WARNINGS_NUMBER=1
    |to see the actual number of warnings found|
  5. You want rkhunter to run every day and e-mail you the result. You can write your own script or check https://www.tecmint.com/install-rootkit-hunter-scan-for-rootkits-backdoors-in-linux/ for a sample cron script you can use.

    On Debian based system, rkhunter comes with cron scripts. To enable them check

    /etc/default/rkhunter
    or use
    dpkg-reconfigure
    and say
    Yes
    to all of the questions:
    sudo dpkg-reconfigure rkhunter
    
  6. After you've finished with all of the changes, make sure all the settings are valid:

    sudo rkhunter -C
    
  7. Update rkhunter and its database:

    sudo rkhunter --versioncheck
    sudo rkhunter --update
    sudo rkhunter --propupd
    
  8. If you want to do a manual scan and see the output:

    sudo rkhunter --check
    

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Rootkit Detection With chrootkit (WIP)

Why

WIP

How It Works

WIP

Goals

WIP

References

  • http://www.chkrootkit.org/
  • https://www.cyberciti.biz/faq/howto-check-linux-rootkist-with-detectors-software/
  • https://askubuntu.com/questions/258658/eth0-packet-sniffer-sbin-dhclient

Steps

  1. Install chkrootkit.

    On Debian based systems:

    sudo apt install chkrootkit
    
  2. Do a manual scan:

    sudo chkrootkit
    
    ROOTDIR is `/'
    Checking `amd'...                                           not found
    Checking `basename'...                                      not infected
    Checking `biff'...                                          not found
    Checking `chfn'...                                          not infected
    Checking `chsh'...                                          not infected
    ...
    Checking `scalper'...                                       not infected
    Checking `slapper'...                                       not infected
    Checking `z2'...                                            chklastlog: nothing deleted
    Checking `chkutmp'...                                       chkutmp: nothing deleted
    Checking `OSX_RSPLUG'...                                    not infected
    
  3. Make a backup of chkrootkit's configuration file

    /etc/chkrootkit.conf
    :
    sudo cp --archive /etc/chkrootkit.conf /etc/chkrootkit.conf-COPY-$(date +"%Y%m%d%H%M%S")
    
  4. You want chkrootkit to run every day and e-mail you the result.

    On Debian based system, chkrootkit comes with cron scripts. To enable them check

    /etc/chkrootkit.conf
    or use
    dpkg-reconfigure
    and say
    Yes
    to the first question:
    sudo dpkg-reconfigure chkrootkit
    

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logwatch - system log analyzer and reporter

Why

Your server will be generating a lot of logs that may contain important information. Unless you plan on checking your server everyday, you'll want a way to get e-mail summary of your server's logs. To accomplish this we'll use logwatch.

How It Works

logwatch scans system log files and summarizes them. You can run it directly from the command line or schedule it to run on a recurring schedule. logwatch uses service files to know how to read/summarize a log file. You can see all of the stock service files in

/usr/share/logwatch/scripts/services
.

logwatch's configuration file

/usr/share/logwatch/default.conf/logwatch.conf
specifies default options. You can override them via command line arguments.

Goals

  • Logwatch configured to send a daily e-mail summary of all of the server's status and logs

Notes

  • Your server will need to be able to send e-mails for this to work
  • The below steps will result in logwatch running every day. If you want to change the schedule, modify the cronjob to your liking. You'll also want to change the
    range
    option to cover your recurrence window. See https://www.badpenguin.org/configure-logwatch-for-weekly-email-and-html-output-format for an example.
  • If logwatch fails to deliver mail due to the e-mail having long lines please check https://blog.dhampir.no/content/exim4-line-length-in-debian-stretch-mail-delivery-failed-returning-message-to-sender as documented in issue #29. If you you followed Gmail and Exim4 As MTA With Implicit TLS then we already took care of this in step #7.

References

  • Thanks to amacheema for fixing some issues with the steps and letting me know of a long line bug with exim4 as documented in issue #29.
  • https://sourceforge.net/projects/logwatch/
  • https://www.digitalocean.com/community/tutorials/how-to-install-and-use-logwatch-log-analyzer-and-reporter-on-a-vps

Steps

  1. Install logwatch.

    On Debian based systems:

    sudo apt install logwatch
    
  2. To see a sample of what logwatch collects you can run it directly:

    sudo /usr/sbin/logwatch --output stdout --format text --range yesterday --service all
    
     ################### Logwatch 7.4.3 (12/07/16) ####################
            Processing Initiated: Mon Mar  4 00:05:50 2019
            Date Range Processed: yesterday
                                  ( 2019-Mar-03 )
                                  Period is day.
            Detail Level of Output: 5
            Type of Output/Format: stdout / text
            Logfiles for Host: host
     ##################################################################
    
    

    --------------------- Cron Begin ------------------------ ... ... ---------------------- Disk Space End -------------------------

    ###################### Logwatch End #########################

  3. Go through logwatch's self-documented configuration file

    /usr/share/logwatch/default.conf/logwatch.conf
    before continuing. There is no need to change anything here but pay special attention to the
    Output
    ,
    Format
    ,
    MailTo
    ,
    Range
    , and
    Service
    as those are the ones we'll be using. For our purposes, instead of specifying our options in the configuration file, we will pass them as command line arguments in the daily cron job that executes logwatch. That way, if the configuration file is ever modified (e.g. during an update), our options will still be there.
  4. Make a backup of logwatch's daily cron file

    /etc/cron.daily/00logwatch
    and unset the execute bit:
    sudo cp --archive /etc/cron.daily/00logwatch /etc/cron.daily/00logwatch-COPY-$(date +"%Y%m%d%H%M%S")
    sudo chmod -x /etc/cron.daily/00logwatch.*
    
  5. By default, logwatch outputs to

    stdout
    . Since the goal is to get a daily e-mail, we need to change the output type that logwatch uses to send e-mail instead. We could do this through the configuration file above, but that would apply to every time it is run -- even when we run it manually and want to see the output to the screen. Instead, we'll change the cron job that executes logwatch to send e-mail. This way, when run manually, we'll still get output to
    stdout
    and when run by cron, it'll send an e-mail. We'll also make sure it checks for all services, and change the output format to html so it's easier to read regardless of what the configuration file says. In the file
    /etc/cron.daily/00logwatch
    find the execute line and change it to:
    /usr/sbin/logwatch --output mail --format html --mailto root --range yesterday --service all
    
    #!/bin/bash
    
    

    #Check if removed-but-not-purged test -x /usr/share/logwatch/scripts/logwatch.pl || exit 0

    #execute /usr/sbin/logwatch --output mail --format html --mailto root --range yesterday --service all

    #Note: It's possible to force the recipient in above command #Just pass --mailto [email protected] instead of --output mail

    For the lazy:

    sudo sed -i -r -e "s,^($(sudo which logwatch).*?),# \1         # commented by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")\n$(sudo which logwatch) --output mail --format html --mailto root --range yesterday --service all         # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")," /etc/cron.daily/00logwatch
    
  6. You can test the cron job by executing it:

    sudo /etc/cron.daily/00logwatch
    

    Note: If logwatch fails to deliver mail due to the e-mail having long lines please check https://blog.dhampir.no/content/exim4-line-length-in-debian-stretch-mail-delivery-failed-returning-message-to-sender as documented in issue #29. If you you followed Gmail and Exim4 As MTA With Implicit TLS then we already took care of this in step #7.

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ss - Seeing Ports Your Server Is Listening On

Why

Ports are how applications, services, and processes communicate with each other -- either locally within your server or with other devices on the network. When you have an application or service (like SSH or Apache) running on your server, they listen for requests on specific ports.

Obviously we don't want your server listening on ports we don't know about. We'll use

ss
to see all the ports that services are listening on. This will help us track down and stop rogue, potentially dangerous, services.

Goals

  • find out non-localhost what ports are open and listening for connections

References

  • https://www.reddit.com/r/linux/comments/arx7st/howtosecurealinuxserveranevolvinghowtoguide/egrib6o/
  • https://www.reddit.com/r/linux/comments/arx7st/howtosecurealinuxserveranevolvinghowtoguide/egs1rev/
  • https://www.tecmint.com/find-open-ports-in-linux/
  • man ss

Steps

  1. To see the all the ports listening for traffic:

    sudo ss -lntup
    
    Netid  State      Recv-Q Send-Q     Local Address:Port     Peer Address:Port
    udp    UNCONN     0      0                      *:68                  *:*        users:(("dhclient",pid=389,fd=6))
    tcp    LISTEN     0      128                    *:22                  *:*        users:(("sshd",pid=4390,fd=3))
    tcp    LISTEN     0      128                   :::22                 :::*        users:(("sshd",pid=4390,fd=4))
    

    Switch Explanations:

    • l
      = display listening sockets
    • n
      = do now try to resolve service names
    • t
      = display TCP sockets
    • u
      = display UDP sockets
    • p
      = show process information
  2. If you see anything suspicious, like a port you're not aware of or a process you don't know, investigate and remediate as necessary.

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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

  • CISOFY offers packages for many distributions. Check https://packages.cisofy.com/ for distribution specific installation instructions.

References

  • https://cisofy.com/documentation/lynis/get-started/
  • https://packages.cisofy.com/community/#debian-ubuntu
  • https://thelinuxcode.com/audit-lynis-ubuntu-server/
  • https://www.vultr.com/docs/install-lynis-on-debian-8

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 apt install apt-transport-https ca-certificates host
    sudo wget -O - https://packages.cisofy.com/keys/cisofy-software-public.key | sudo apt-key add -
    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 host
    
  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 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 !!

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Table of Contents

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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 &gt;/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:

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

  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 &lt;&lt; EOF
    set superusers="grub"
    password_pbkdf2 grub [hash]
    EOF

    For example:

    > bash &gt; #!/bin/sh &gt; set -e &gt; &gt; cat &lt;&lt; EOF &gt; set superusers="grub" &gt; password_pbkdf2 grub grub.pbkdf2.sha512.100000.2812C233DFC899EFC3D5991D8CA74068C99D6D786A54F603E9A1EFE7BAEDDB6AA89672F92589FAF98DB9364143E7A1156C9936328971A02A483A84C3D028C4FF.C255442F9C98E1F3C500C373FE195DCF16C56EEBDC55ABDD332DD36A92865FA8FC4C90433757D743776AB186BD3AE5580F63EF445472CC1D151FA03906D08A6D &gt; EOF &gt;

  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 GRUB's configuration file /etc/grub.d/10_linux that we'll be modifying and unset the execute bit so update-grub doesn't try to run it:

    sudo cp --archive /etc/grub.d/10_linux /etc/grub.d/10_linux-COPY-$(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.

Goals

  • 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. Make a backup of files we'll be editing:

    sudo cp --archive /etc/profile /etc/profile-COPY-$(date +"%Y%m%d%H%M%S")
    sudo cp --archive /etc/bash.bashrc /etc/bash.bashrc-COPY-$(date +"%Y%m%d%H%M%S")
    sudo cp --archive /etc/login.defs /etc/login.defs-COPY-$(date +"%Y%m%d%H%M%S")
    sudo cp --archive /root/.bashrc /root/.bashrc-COPY-$(date +"%Y%m%d%H%M%S")
  2. 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:

    echo -e "\numask 0027         # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")" | sudo tee -a /etc/profile /etc/bash.bashrc
  3. We also need to add this line to /etc/login.defs:

    UMASK 0027

    For the lazy:

    echo -e "\nUMASK 0027         # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")" | sudo tee -a /etc/login.defs
  4. Set default umask for the root account to 0077 by adding this line to /root/.bashrc:

    umask 0077

    For the lazy:

    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

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

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
  1. Install deborphan.

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

    sudo deborphan

    > &gt; libxapian30 &gt; libpipeline1 &gt;

  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 Miscellaneous

Gmail and Exim4 As MTA With Implicit TLS

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. 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?

There are many guides on-line that cover how to configure Gmail as MTA using STARTTLS including a previous version of this guide. With STARTTLS, an initial unencrypted connection is made and then upgraded to an encrypted TLS or SSL connection. Instead, with the approach outlined below, an encrypted TLS connection is made from the start.

Also, as discussed in issue #29 and here, exim4 will fail for messages with long lines. We'll fix this in this section too.

Goals

  • mail
    configured to send e-mails from your server using Gmail
  • long line support for exim4

References

  • Thanks to remyabel for figuring out how to get this to work with TLS as documented in issue #24 and pull request #26.
  • https://wiki.debian.org/Exim
  • https://wiki.debian.org/GmailAndExim4
  • https://www.exim.org/exim-html-current/doc/html/spechtml/ch-encryptedsmtpconnectionsusing_tlsssl.html
  • https://php.quicoto.com/setup-exim4-to-use-gmail-in-ubuntu/
  • https://www.fastmail.com/help/technical/ssltlsstarttls.html
  • exim4 fails for messages with long lines - issue #29 and https://blog.dhampir.no/content/exim4-line-length-in-debian-stretch-mail-delivery-failed-returning-message-to-sender

Steps

  1. Install exim4. You will also need openssl and ca-certificates.

    On Debian based systems:

    sudo apt install exim4 openssl ca-certificates
    
  2. Configure exim4:

    For Debian based systems:

    bash
    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|
    localhost
    | |IP-addresses to listen on for incoming SMTP connections|
    127.0.0.1; ::1
    | |Other destinations for which mail is accepted|(default)| |Visible domain name for local users|
    localhost
    | |IP address or host name of the outgoing smarthost|
    smtp.gmail.com::465
    | |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 --archive /etc/exim4/passwd.client /etc/exim4/passwd.client-COPY-$(date +"%Y%m%d%H%M%S")
    
  4. Add a line like this to

    /etc/exim4/passwd.client
    smtp.gmail.com:[email protected]:yourPassword
    *.google.com:[email protected]:yourPassword
    

    Notes:

    • Replace
      [email protected]
      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 here.
    • Always check
      host smtp.gmail.com
      for the most up-to-date domains to list.
  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. The next step is to create an TLS certificate that exim4 will use to make the encrypted connection to

    smtp.gmail.com
    . You can use your own certificate, like one from Let's Encrypt, or create one yourself using openssl. We will use a script that comes with exim4 that calls openssl to make our certificate:
    sudo bash /usr/share/doc/exim4-base/examples/exim-gencert
    
    [*] Creating a self signed SSL certificate for Exim!
        This may be sufficient to establish encrypted connections but for
        secure identification you need to buy a real certificate!
    
    
    Please enter the hostname of your MTA at the Common Name (CN) prompt!

    Generating a RSA private key ..........................................+++++ ................................................+++++ writing new private key to '/etc/exim4/exim.key'


    You are about to be asked to enter information that will be incorporated into your certificate request. What you are about to enter is what is called a Distinguished Name or a DN. There are quite a few fields but you can leave some blank For some fields there will be a default value, If you enter '.', the field will be left blank.


    Country Code (2 letters) [US]:[redacted] State or Province Name (full name) []:[redacted] Locality Name (eg, city) []:[redacted] Organization Name (eg, company; recommended) []:[redacted] Organizational Unit Name (eg, section) []:[redacted] Server name (eg. ssl.domain.tld; required!!!) []:localhost Email Address []:[redacted] [*] Done generating self signed certificates for exim! Refer to the documentation and example configuration files over at /usr/share/doc/exim4-base/ for an idea on how to enable TLS support in your mail transfer agent.

  7. Instruct exim4 to use TLS and port 465, and fix exim4's long lines issue, by creating the file

    /etc/exim4/exim4.conf.localmacros
    and adding:
    MAIN_TLS_ENABLE = 1
    REMOTE_SMTP_SMARTHOST_HOSTS_REQUIRE_TLS = *
    TLS_ON_CONNECT_PORTS = 465
    REQUIRE_PROTOCOL = smtps
    IGNORE_SMTP_LINE_LENGTH_LIMIT = true
    

    For the lazy:

    cat << EOF | sudo tee /etc/exim4/exim4.conf.localmacros
    MAIN_TLS_ENABLE = 1
    REMOTE_SMTP_SMARTHOST_HOSTS_REQUIRE_TLS = *
    TLS_ON_CONNECT_PORTS = 465
    REQUIRE_PROTOCOL = smtps
    IGNORE_SMTP_LINE_LENGTH_LIMIT = true
    EOF
    
  8. Make a backup of exim4's configuration file

    /etc/exim4/exim4.conf.template
    :
    sudo cp --archive /etc/exim4/exim4.conf.template /etc/exim4/exim4.conf.template-COPY-$(date +"%Y%m%d%H%M%S")
    
  9. Add the below to

    /etc/exim4/exim4.conf.template
    after the
    .ifdef REMOTE_SMTP_SMARTHOST_HOSTS_REQUIRE_TLS ... .endif
    block:
    .ifdef REQUIRE_PROTOCOL
      protocol = REQUIRE_PROTOCOL
    .endif
    
    .ifdef REMOTE_SMTP_SMARTHOST_HOSTS_REQUIRE_TLS
      hosts_require_tls = REMOTE_SMTP_SMARTHOST_HOSTS_REQUIRE_TLS
    .endif
    .ifdef REQUIRE_PROTOCOL
        protocol = REQUIRE_PROTOCOL
    .endif
    .ifdef REMOTE_SMTP_HEADERS_REWRITE
      headers_rewrite = REMOTE_SMTP_HEADERS_REWRITE
    .endif
    

    For the lazy:

    sudo sed -i -r -e '/^.ifdef REMOTE_SMTP_SMARTHOST_HOSTS_REQUIRE_TLS$/I { :a; n; /^.endif$/!ba; a\# added by '"$(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")"'\n.ifdef REQUIRE_PROTOCOL\n    protocol = REQUIRE_PROTOCOL\n.endif\n# end add' -e '}' /etc/exim4/exim4.conf.template
    
  10. Add the below to

    /etc/exim4/exim4.conf.template
    inside the
    .ifdef MAIN_TLS_ENABLE
    block:
    .ifdef TLS_ON_CONNECT_PORTS
      tls_on_connect_ports = TLS_ON_CONNECT_PORTS
    .endif
    
    .ifdef MAIN_TLS_ENABLE
    .ifdef TLS_ON_CONNECT_PORTS
        tls_on_connect_ports = TLS_ON_CONNECT_PORTS
    .endif
    

    For the lazy:

    sudo sed -i -r -e "/\.ifdef MAIN_TLS_ENABLE/ a # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")\n.ifdef TLS_ON_CONNECT_PORTS\n    tls_on_connect_ports = TLS_ON_CONNECT_PORTS\n.endif\n# end add" /etc/exim4/exim4.conf.template
    
  11. Update exim4 configuration to use TLS and then restart the service:

    sudo update-exim4.conf
    sudo service exim4 restart
    
  12. If you're using UFW, you'll need to allow outbound traffic on 465. To do this we'll create a custom UFW application profile and then enable it. Create the file

    /etc/ufw/applications.d/smtptls
    , add this, then run
    ufw allow out smtptls comment 'open TLS port 465 for use with SMPT to send e-mails'
    :
    [SMTPTLS]
    title=SMTP through TLS
    description=This opens up the TLS port 465 for use with SMPT to send e-mails.
    ports=465/tcp
    

    For the lazy:

    cat << EOF | sudo tee /etc/ufw/applications.d/smtptls
    [SMTPTLS]
    title=SMTP through TLS
    description=This opens up the TLS port 465 for use with SMPT to send e-mails.
    ports=465/tcp
    EOF
    
    

    sudo ufw allow out smtptls comment 'open TLS port 465 for use with SMPT to send e-mails'

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

    /etc/aliases
    :
    user1: [email protected]
    user2: [email protected]
    ...
    

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

  14. Test your setup:

    echo "test" | mail -s "Test" [email protected]
    sudo tail /var/log/exim4/mainlog
    

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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

  • https://blog.shadypixel.com/log-iptables-messages-to-a-separate-file-with-rsyslog/
  • https://gist.github.com/netson/c45b2dc4e835761fbccc
  • https://www.rsyslog.com/doc/v8-stable/configuration/actions.html

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.

    For the lazy:

    cat << EOF | sudo tee /etc/rsyslog.d/10-iptables.conf
    :msg, contains, "[IPTABLES] " /var/log/iptables.log
    & stop
    EOF
    
  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;
    

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

    For the lazy:

    sudo sed -i -r -e "s/^(IPT_SYSLOG_FILE\s+)([^;]+)(;)$/# \1\2\3       # commented by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")\n\1\/var\/log\/iptables.log\3       # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")/" /etc/psad/psad.conf 
    
  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
    }
    

    For the lazy:

    cat << EOF | sudo tee /etc/logrotate.d/iptables
    /var/log/iptables.log
    {
        rotate 7
        daily
        missingok
        notifempty
        delaycompress
        compress
        postrotate
            invoke-rc.d rsyslog rotate > /dev/null
        endscript
    }
    EOF
    

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Left Over

Contacting Me

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

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Helpful Links

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Acknowledgments

  • https://www.reddit.com/r/linuxquestions/comments/aopzl7/newguidecreatedbymehowtosecurea_linux/
  • https://www.reddit.com/r/selfhosted/comments/aoxd4l/newguidecreatedbymehowtosecurea_linux/
  • https://news.ycombinator.com/item?id=19177435#19178618
  • https://www.reddit.com/r/linuxadmin/comments/arx7xo/howtosecurealinuxserveranevolvinghowtoguide/
  • https://www.reddit.com/r/linux/comments/arx7st/howtosecurealinuxserveranevolvinghowtoguide/

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License and Copyright

CC-BY-SA

How To Secure A Linux Server by Anchal Nigam is licensed under Creative Commons Attribution-ShareAlike 4.0 International License.

See LICENSE for the full license.

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