imthenachoman/How-To-Secure-A-Linux-Server: An evolving how-to guide for securing a Linux server.
2019-02-14 00:37:13 Author: github.com(查看原文) 阅读量:196 收藏

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

Table of Contents

(TOC made with nGitHubTOC)

Introduction

Document Objective

This guide's purpose is to teach you how to secure a Linux server.

There are a lot of things you can do to secure a Linux server to prevent bad-actors from gaining access to your 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.

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.
  • ...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. Physical security, for example, 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 wet your appetite and make you hungry enough 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.

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Why Secure Your Server

I assume you're using this guide becuase 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 document. 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 confidential data, or to add nodes to their coordinated 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, bad-actors don't always want to change something or lock you out of your data for money. Sometimes they just want your 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 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 to remedy that.

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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Contributing

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

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

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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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To Do / To Add

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Before You Start

Identify Your Principals

Before you start you will want to identify what your principals 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 document 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

  • 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 SSH access so be sure it is installed.
  • Be sure to keep your system up-to-date (i.e. sudo apt update && sudo apt upgrade on Debian based systems)
  • At some point, like maybe right after configuring SSH public/private keys, make sure you perform any tasks specific to your setup like:
  • Your server will need to be able to send e-mails so you can get important security alerts. If you're not setting up a mail server check Configure Gmail as MTA.

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Important Advice For Using This Guide

  • 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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Securing Linux - Safe To Do

Overview

This section covers things you can do that are generally considered safe and shouldn't make your system unusable. However, as is with anything in this guide, use with caution and proceed at your own risk.

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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.

Check the references below for more details but, at a high level, public/private keys work by using two 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. The public key is then securely transferred to the server you want to connect to. After this is done, SSH uses the public and private keys to verify identity and then establishing a secure connection. Identity is verified by encrypting and decrypting data that both the client and server know. If the data can't be decrypted, the identity can't be verified and a connection will not be established.

They are considered more secure because you need the public 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 public key.

You can also set a passphrase for the keys which would require you to enter the key passphrase 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.

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

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

Goals

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

Notes

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

References

Steps

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

    • Use the default options for all of the questions
    • If you set a passphrase, you'll need to enter it every time you connect to your server using this key.

  2. Now you need to append the contents of the public key ~/.ssh/id_ed25519.pub to the ~/.ssh/authorized_keys file on the target server. You'll want to do this in a secure way since the public key gives access to your server. One approach is to copy it to a USB stick and physically transfer it to the server. If you're sure there is nobody listening between the client you're on and your server, you can use ssh-copy-id to transfer and append the public key:

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

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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:

  2. Add account(s) to the group:

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

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

  3. Edit /etc/sudoers:

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

  4. Add this line if it is not already there:

    %sudo   ALL=(ALL:ALL) ALL

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Secure SSH

Create SSH Group For AllowGroups

Why

To make it easy to control who can SSH to the server.

Goals
Notes
References
  • man groupadd
  • man usermod
Steps

  1. Create a group:

  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.

Goal
  • a secure SSH configuration
Notes
References
Steps

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

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

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

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

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

# Use kernel sandbox mechanisms where possible in unprivileged processes
# Systrace on OpenBSD, Seccomp on Linux, seatbelt on MacOSX/Darwin, rlimit elsewhere.
UsePrivilegeSeparation sandbox

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

# only use the newer, more secure protocl
Protocol 2

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

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

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

# ignore .rhosts and .shosts
IgnoreRhosts yes

# verify hostname matches IP
UseDNS no

Compression no
TCPKeepAlive no
AllowAgentForwarding no
PermitRootLogin no

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

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

    Check man sshd_config for more details what these settings mean.

  4. Restart ssh:

    sudo service sshd restart

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Deactivate Short Moduli

Why

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

Goal
  • deactivate short moduli
References
Steps

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

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

  2. Remove short moduli:

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

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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."

Goal

  • enforced strong passwords

Steps

  1. Install libpam-pwquality.

    For Debian based systems:

    sudo apt install libpam-pwquality

  2. Edit /etc/pam.d/common-password and change the line that starts like this:

    password        requisite                       pam_pwquality.so

    to this:

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

    The above options are:

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

    For the lazy:

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

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

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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.

Goals

  • NTP client installed and keeping server time in-sync

References

Steps

  1. Install ntp.

    On Debian based systems:

Check https://cloudpro.zone/index.php/2018/01/27/debian-9-3-server-setup-guide-part-4/ for instructions on how to check if its running.

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UFW: Uncomplicated Firewall

Why

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

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

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

UFW works by letting you configure rules that:

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

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

Goal

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

Notes

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

References

Steps

  1. Install ufw.

    For Debian based systems:

  2. Deny all outgoing traffic:

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

    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'

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

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

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

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

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

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

# allow traffic out on port 68 -- the DHCP client
sudo ufw allow out 68 comment 'allow the DHCP client to update'

  6. Start ufw:

  7. If you want to see a status:

    or

Default Applications

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

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

sudo ufw app info [app name]

For example:

$ 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

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PSAD (WIP)

References

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Fail2ban: Intrusion Detection And Prevention

Why

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

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

Goal

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

Notes

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

References

Steps

  1. Install fail2ban.

    For Debian based systems:

    sudo apt install fail2ban

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

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

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

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

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

# get email alerts
action = %(action_mwl)s

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

  3. Create a jail for ssh by adding this to /etc/fail2ban/jail.d/ssh.local:

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

  4. Enable fail2ban and the jail for SSH:

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

  5. To check the status:

    sudo fail2ban-client status
sudo fail2ban-client status sshd

Custom Jails

WIP

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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.

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, acesss to your system is dependent on the accompanying authenticator app that generates the code.

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 not if they are using SSH public/private keys. Check the documentation on how to change this behavior to suite your requirements.

References

Steps

  1. Install it libpam-google-authenticator.

    For 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:

    Notice this is not run as root.

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

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

    auth       required     pam_google_authenticator.so nullok

    Check here for what nullok means.

    For the lazy:

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

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

  4. Enable it in the SSH settings by adding this line in /etc/ssh/sshd_config:

    ChallengeResponseAuthentication yes

    For the lazy:

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

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

  5. Restart ssh:

    sudo service sshd restart

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Apticron - Automatic Update Notifier

Why

It is important to keep your server up-to-date with all security patches. Otherwise you're at risk of known security vulnerabilities that bad-actors could use to gain unauthorized access to your server.

You have two options:

  • Configure your server for unattended updates
  • Be notified when updates are available

Which option you pick is up to you but I prefer being notified by e-mail when updates are available. This is because an update may break something else. If the server updates it-self then I may not know and, if I do find out, I'll have to scramble to fix it. If it e-mails me when updates are available, then I can do the updates at my schedule.

Notes

  • Your server will need a way to send e-mails for this to work

References

Steps

  1. Install apticron.

    For Debian based systems:

    sudo apt install apticron

  2. Set the value of EMAIL in /etc/apticron/apticron.conf to your e-mail address.

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Orphaned Software

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; I will add for other distributions as I learn how.

Steps

Debian

For Debian based distributions, you can use deborphan to find orphaned packages.

  1. Install deborphan:

    sudo apt install deborphan

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

  3. Pass it's output to apt to remove them:

    sudo apt --autoremove purge $(deborphan)

    You will want to repeatedly run this command until deborphan no longer returns any orphaned packages.

    For the lazy:

    while [[ $(deborphan | wc -l) != 0 ]] ; do
    sudo apt --autoremove purge $(deborphan)
done

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Securing Linux - DANGER ZONE

Section Overivew

This section covers things that are either 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 WITH CAUTION !!

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Linux Kernel sysctl Hardening

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 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:

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:

``` bash
sudo sysctl -p >/dev/null
```

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Change Default umask

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.

Goals

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

Notes

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

References

Steps

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

    umask 0027

    For the lazy:

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

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

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

    UMASK 0027

    For the lazy:

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

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

  3. !! USE WITH CAUTION !! -- Set default umask for the root account to 0077 by adding this line to /root/.bashrc:

    umask 0077

    For the lazy:

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

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

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Password Protect GRUB

Why

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

Why Not

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

Goals

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

Notes

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

References

Steps

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

    grub-mkpasswd-pbkdf2 -c 100000

    The below output is from using password as the password:

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

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

  3. 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:

    #!/bin/sh
set -e

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

    For example:

    #!/bin/sh
set -e

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

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

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

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

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

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

    For the lazy:

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

  7. Update GRUB with update-grub:

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Disable Root Login

Why

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

Why Not

Be warned, this can cause issues with some configurations!

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

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

See sulogin(8) man page for more details.

Press Enter to continue.

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

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

Goal

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

Notes

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

References

Steps

  1. !! USE WITH CAUTION !! -- Lock the root account:

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Other Stuff

Configure Gmail as MTA

Why

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

Goals

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

References

Steps

  1. Install exim4.

    For Debian based systems:

  2. Configure exim4:

    For Debian based systems:

    sudo dpkg-reconfigure exim4-config

    You'll be prompted with some questions:

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

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

    *.google.com:[email protected]:yourPassword

    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.

  4. 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

  5. Restart exim4:

    sudo service exim4 restart

  6. 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.

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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

  • We will install it from it's GitHub page so we have the latest and greatest.
  • CISOFY also offers packages for many distributions. Check https://packages.cisofy.com/ for distribution specfic installation instructions.

References

Steps

  1. We want it to be installed system wide so go to /usr/local and clone it from https://github.com/CISOfy/lynis:

    cd /usr/local
sudo git clone https://github.com/CISOfy/lynis

  2. Update it:

    sudo /usr/local/lynis/lynis update info

  3. Run a security audit:

    sudo /usr/local/lynis/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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Separate iptables Log File (WIP)

References

Not Security

Mount /tmp In RAM Using tmpfs

Why

RAM is faster than disk, even SSD. By mounting /tmp in RAM using tmpfs, you may notice a performance increase.

Why Not

Using tmpfs will consume RAM. If RAM fills up your system may become unstable. tmpfs may resort to using swap.

References

Steps

  1. Add this line to /etc/fstab:

    tmpfs   /tmp    tmpfs   defaults,noatime,rw,nodev,nosuid,nodiratime,mode=1777,size=2GB  0   0

    Change the value of size to suit your needs. If you remove the size option then it will default to using half of your RAM.

    For the lazy:

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

echo -e "\ntmpfs   /tmp    tmpfs   defaults,noatime,rw,nodev,nosuid,nodiratime,mode=1777,size=2G  0   0         # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")" | sudo tee -a /etc/fstab

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Miscellaneous

Contacting Me

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

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Additional References

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Acknowledgments

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Disclaimer / Warranty

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

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文章来源: https://github.com/imthenachoman/How-To-Secure-A-Linux-Server#document-objective
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