The Practical Linux Hardening Guide
"Did you know all your doors were locked?" - Riddick (The Chronicles of Riddick)
All suggestions and pull requests are welcome!
:collision: Work in progress, just a moment... First, I update a [Table Of Content](#table-of-content) and chapters.
If you want to support another repository containing **hardening** recipes, please see: [linux-hardening-checklist](https://github.com/trimstray/linux-hardening-checklist) - it's a simple checklist with the most important hardening rules.
****
## Table Of Content
- **[Pre install tasks](#pre-install-tasks)**
* **[Physical system security](#physical-system-security)**
+ [Introduction](#information_source-introduction)
+ [Secure rooms](#eight_pointed_black_star-secure-rooms)
+ [Monitoring](#eight_pointed_black_star-monitoring)
+ [Air conditioning](#eight_pointed_black_star-air-conditioning)
+ [Fire protection](#eight_pointed_black_star-fire-protection)
+ [Locked racks](#eight_pointed_black_star-locked-racks)
+ [Console security](#eight_pointed_black_star-console-security)
+ [BIOS protection](#eight_pointed_black_star-bios-protection)
+ [Summary checklist](#ballot_box_with_check-summary-checklist)
* **[Hard disk encryption](#hard-disk-encryption)**
+ [Introduction](#information_source-introduction-1)
+ [Encrypt root filesystem](#eight_pointed_black_star-encrypt-root-filesystem)
+ [Encrypt /boot partition](#eight_pointed_black_star-encrypt-boot-partition)
+ [Swap partition](#eight_pointed_black_star-swap-partition)
+ [Summary checklist](#ballot_box_with_check-summary-checklist-1)
- **[Post install tasks](#post-install-tasks)**
* **[Bootloader configuration (grub)](#bootloader-configuration-grub)**
+ [Introduction](#information_source-introduction-2)
+ [Protect bootloader with password](#information_source-protect-bootloader-with-password)
+ [Protect bootloader config files](#information_source-protect-bootloader-config-files)
+ [Summary checklist](#ballot_box_with_check-summary-checklist-2)
* **[Disk partitions](#disk-partitions)**
+ [Introduction](#information_source-introduction-3)
+ [Separate disk partitions](#eight_pointed_black_star-separate-disk-partitions)
+ [Mount options: nodev, noexec and nosuid](#eight_pointed_black_star-mount-options-nodev-noexec-and-nosuid)
+ [Secure /boot directory](#eight_pointed_black_star-secure-boot-directory)
+ [Secure /tmp and /var/tmp](#eight_pointed_black_star-secure-tmp-and-vartmp)
+ [Secure /dev/shm](#eight_pointed_black_star-secure-devshm)
+ [Secure /proc filesystem](#eight_pointed_black_star-secure-proc-filesystem)
+ [Swap partition](#eight_pointed_black_star-swap-partition-1)
+ [Disk quotas](#eight_pointed_black_star-disk-quotas)
+ [Summary checklist](#ballot_box_with_check-summary-checklist-3)
* **[Keep system updated](#keep-system-updated)**
* [Package management](#package-management)
+ [Automatic security updates](#automatic-security-updates)
+ [Remove packages with known issues](#remove-packages-with-known-issues)
* [Netfilter ruleset](#netfilter-ruleset)
* [TCP wrapper](#tcp-wrapper)
* [Users and groups](#users-and-groups)
+ [Limit su access](#limit-su-access)
+ [Disable root account](#disable-root-account)
+ [Logins to system console](#logins-to-system-console)
+ [Disable shell accounts](#disable-shell-accounts)
+ [Strong password policy](#strong-password-policy)
+ [Password aging](#password-aging)
+ [Previous passwords](#previous-passwords)
+ [Login failures](#login-failures)
+ [Protect single user mode](#protect-single-user-mode)
* [System path permissions](#system-path-permissions)
+ [World writable files](#world-writable-files)
* [PAM module](#pam-module)
* [Limits](#limits)
* [Shadow passwords](#shadow-passwords)
* [Linux kernel hardening](#linux-kernel-hardening)
+ [Kernel parameters](#kernel-parameters)
+ [Network security](#improve-network-security)
+ [System security](#improve-system-security)
* [Remove unused modules](#remove-unused-modules)
* [Secure shared memory](#secure-shared-memory)
* [IRQ balance](#irq-balance)
* [Disable compilers](#disable-compilers)
* [Email notifications](#email-notifications)
+ [Rebooting the system](#rebooting-the-system)
* [Backups](#backups)
* [External devices](#external-devices)
+ [Disable USB usage](#disable-usb-usage)
- **[Tools](#tools)**
* [Logging and Auditing](#logging-and-auditing)
+ [Auditd](#auditd)
+ [Tiger](#tiger)
+ [Aide](#aide)
+ [Logwatch](#logwatch)
* [Other](#other)
+ [Fail2ban](#fail2ban)
+ [PSAD](#psad)
+ [SELinux](#selinux)
+ [Entropy daemon](#entropy-daemon)
+ [Centralized authentication service](#centralized-authentication-service)
* [Testing tools](#testing-tools)
+ [Lynis](#lynis)
+ [Chrootkit](#chrootkit)
- **[Services](#hardening-services)**
* [Disable all unnecessary](#disable-all-unnecessary)
* [System services](#system-services)
+ [OpenSSH](#openssh)
+ [NTP](#ntp)
+ [Cron](#cron)
+ [Anacron](#anacron)
+ [GnuPG 2](#gnupg2)
+ [Unattended key generation](#unattended-key-generation)
* [DNS services](#dns-services)
+ [Bind9](#bind9)
* [Mail services](#mail-services)
+ [Postfix](#postfix)
* [Web services](#web-services)
+ [Nginx](#nginx)
+ [Apache](#apache)
+ [Securing and tuning HTTP/HTTPS protocols](#securing-and-tuning-http-https-protocols)
- [Use HTTPS](#use-https)
- [Enable HTTP2](#enable-http2)
- [Separate domains](#separate-domains)
- [Redirect all unencrypted traffic to HTTPS](#redirect-all-unencrypted-traffic-to-https)
- [Enable HTTP Strict Transport Security](#enable-http-strict-transport-security)
- [Security related headers](#security-related-headers)
* [Databases](#databases)
+ [PostgreSQL](#postgresql)
+ [MySQL](#mysql)
+ [Redis](#redis)
* [Queues](#queues)
+ [AMQP](#amqp)
- **[Deployment](#deployment)**
- **[Testing configuration](#testing-configuration)**
- **[External resources](#external-resources)**
## Pre install tasks
### Physical system security
#### :information_source: Introduction
The primary goal of physical security is to stop physical attacks whenever possible, and, failing that, to slow them down so that hopefully someone will notice the presence of the attacker in a restricted area, preventing any tampering with the system. [Weak physical security often invalidates any other security measure](https://www.lynda.com/Linux-tutorials/Physical-security-concepts/728406/750240-4.html), and thus should be prioritized.
#### :eight_pointed_black_star: Secure rooms
For secure rooms make sure that the walls go beyond the false ceiling, and below the raised floor ([source](https://books.google.pt/books?id=XD68NYRPD9oC&pg=PA158&lpg=PA158&dq=physical+security+computer+systems+false+ceiling&source=bl&ots=1wzz6IsBfw&sig=ACfU3U0IPb8NrSiQaQoZFfnu4eMbRgYngQ&hl=en&sa=X&redir_esc=y#v=onepage&q=physical%20security%20computer%20systems%20false%20ceiling&f=false)), large vents should also be [covered with bars](https://books.google.pt/books?id=6yTGIaHLNsAC&pg=PA160&lpg=PA160&dq=covering+vents+with+bars+physical+security&source=bl&ots=2k-196Kwwk&sig=ACfU3U0KsfGhDk06A6qUHIXt88xEi8FZ6w&hl=en&sa=X&redir_esc=y#v=onepage&q=covering%20vents%20with%20bars%20physical%20security&f=false) if possible.
#### :eight_pointed_black_star: Monitoring
Monitoring the room with CCTV or wired cameras is a great way to provide security for your server room or data center. As well as providing video footage of events which may occur - door open events, motion detection or any other sensor event, they also act as a visual deterrent to would be criminals.
Solution for remotely monitoring the temperature ensue proactively notify you when the temperature goes above or below pre-defined thresholds, potentially allowing you to take corrective measures before encountering costly downtime.
[Source](https://www.enviromon.net/monitoring-physical-security-server-room/)
#### :eight_pointed_black_star: Air conditioning
Computer equipment generates heat, and is sensitive to heat, humidity, and dust, but also the need for very high resilience and failover requirements. Maintaining a stable temperature and humidity within tight tolerances is critical to IT system reliability.
Air conditioning designs for most computer or server rooms will vary depending on various design considerations, but they are generally one of two types: "up-flow" and "down-flow" configurations.
[Source](https://www.enviromon.net/how-to-monitor-server-room-temperature/)
#### :eight_pointed_black_star: Fire protection
The fire protection system's main goal should be to detect and alert of fire in the early stages, then bring fire under control without disrupting the flow of business and without threatening the personnel in the facility. Server room fire suppression technology has been around for as long as there have been server rooms.
There are a series of things you need in a fire suppression system:
- an emergency power off function
- gas-based suppression system
- fire detection sensors
[Source](https://www.controlfiresystems.com/news/what-type-of-suppression-system-works-best-for-computer-room-fires/)
#### :eight_pointed_black_star: Locked racks
All systems should be securely fastened to something with a cable system, or locked in an equipment cage if possible. Case locks should be used when possible to slow attackers down.
[Source](https://securitytoday.com/blogs/reaction/2018/02/Securing-the-Physical-Safety-of-Data-with-Rack-Level-Access-Control.aspx)
#### :eight_pointed_black_star: Console security
With physical access to most machines you can simply reboot the system and ask it nicely to launch into single user mode, or tell it to use `/bin/sh` for init.
[Source](https://www.tldp.org/HOWTO/Security-HOWTO/physical-security.html)
#### :eight_pointed_black_star: BIOS protection
In the program itself to edit the BIOS settings:
- only boot from specific drive
- disable the unused controllers
- disable the booting from external media devices (USB/CD/DVD)
- enable BIOS password
You need to protect the BIOS of the host with a password so the end-user won’t be able to change and override the security settings in the BIOS.
Main reasons for password protecting the BIOS:
- preventing changes to BIOS settings
- preventing system booting
Because the methods for setting a BIOS password vary between computer manufacturers, consult the computer's manual for specific instructions.
> For this reason, it is good practice to lock the computer case if possible. However, consult the manual for the computer or motherboard before attempting to disconnect the CMOS battery.
[Source](https://csrc.nist.gov/csrc/media/publications/sp/800-147b/final/documents/draft-sp800-147b_july2012.pdf)
#### :ballot_box_with_check: Summary checklist
| Item | True | False |
| :--- | :---: | :---: |
| Physically secure machine (also outside of a server room) | :black_square_button: | :black_square_button: |
| Monitoring server rooms with CCTV or wired cameras | :black_square_button: | :black_square_button: |
| Remotely monitoring the temperature | :black_square_button: | :black_square_button: |
| Efficient air conditioning solution | :black_square_button: | :black_square_button: |
| Efficient fire protection system | :black_square_button: | :black_square_button: |
| Locked cage (server case) | :black_square_button: | :black_square_button: |
| Physical access to server console | :black_square_button: | :black_square_button: |
| Password on the BIOS | :black_square_button: | :black_square_button: |
| Disable external media devices | :black_square_button: | :black_square_button: |
| Periodic physical inspections | :black_square_button: | :black_square_button: |
### Hard disk encryption
#### :information_source: Introduction
Disk encryption is focused on securing physical access, while relying on other parts of the system to provide things like network security and user-based access control.
Most of the Linux distributions will allow you to encrypt your disks before installation.
If you use an alternative installation method (e.g. from `debootstrap`) you can create an [encrypted disk manually](#disk-partitions).
Before this you should to answer the following questions:
- What part of filesystem do you want to encrypt?
* only user data
* user data and system data
- How should `swap`, `/tmp` and other be taken care of?
* disable or mount as ramdisk
* encrypt (separately of as part of full)
- How should encrypted parts of the disk be unlocked?
* passphrase
* key file
- When should encrypted parts of the disk be unlocked?
* before boot process
* during boot process
* mixed above or manually
[Source](https://uchicago.service-now.com/it?id=kb_article&kb=KB06000398)
#### :eight_pointed_black_star: Encrypt root filesystem
Unlocked during boot, using passphrases or USB stick with keyfiles.
#### :eight_pointed_black_star: Encrypt /boot partition
- encrypting the whole disk without `/boot` partition but keeping it on a flash drive you carry at all times
- using a checksum value of the boot sector
- boot partition to detect it and change you passphrase
This may not completely get rid of the attack vector described in this post as there is still part of the bootloader that isn't encrypted, but at least the grub stage2 and the kernel/ramdisk are encrypted and should make it much harder to attack.
In addition, the `/boot` partition may be a weak point if you use encryption methods for the rest of the disk.
Historically it has been necessary to leave `/boot` unencrypted because bootloaders didn't support decrypting block devices. However, there are some dangers to leaving the bootloader and ramdisks unencrypted.
Before this you should to answer the following questions:
- Where your `/boot` partition is stored?
* the same place where stored `/`
* separately partition
* external flash drive
The following recipe should be made after installing the system (however, these steps are included in this section to avoid mixing issues).
[Source](https://security.stackexchange.com/questions/166075/encrypting-the-boot-partition-in-a-linux-system-can-protect-from-an-evil-maid-a)
###### Create copy of your /boot
```bash
mkdir /mnt/boot
mount --bind / /mnt/boot
rsync -aAXv /boot/ /mnt/boot/
umount /mnt/boot
```
###### Removed old /boot partition
```bash
umount /boot
sed -i -e '/\/boot/d' /etc/fstab
```
###### Regenerate grub configuration
```bash
# Debian like distributions
grub-mkconfig > /boot/grub/grub.cfg
# RedHat like distributions
grub2-mkconfig > /boot/grub2/grub.cfg
```
###### Enable `GRUB_ENABLE_CRYPTODISK` param
```bash
echo GRUB_ENABLE_CRYPTODISK=y >> /etc/default/grub
```
###### Reinstall grub
```bash
# Debian like distributions
grub-install /dev/sda
# RedHat like distributions
grub2-install /dev/sda
```
> More details can be found here (Bootloader configuration (grub) section):
> - [Lock the boot directory](#eight_pointed_black_star-lock-the-boot-directory)
#### :eight_pointed_black_star: Swap partition
- swap area is not required to survive a reboot, therefore a new random encryption key can be chosen each time the swap area is activated
- get the key from `/dev/urandom` because `/dev/random` maybe stalling your boot sequence
#### :ballot_box_with_check: Summary checklist
| Item | True | False |
| :--- | :---: | :---: |
| Encrypting the whole disk | :black_square_button: | :black_square_button: |
| Usage passphrase or key file to disk unlocked | :black_square_button: | :black_square_button: |
| Choosing a strong passphrase | :black_square_button: | :black_square_button: |
| Encrypting the `/boot` partition | :black_square_button: | :black_square_button: |
| Securing swap partition with `/dev/urandom` | :black_square_button: | :black_square_button: |
| `swap` or `tmp` using an automatically generated per-session throwaway key | :black_square_button: | :black_square_button: |
## Post install tasks
### Bootloader configuration (grub)
#### :information_source: Introduction
Protection for the boot loader can prevent unauthorized users who have physical access to systems, e.g. attaining root privileges through single user mode.
Basically when you want to prohibit unauthorized reconfiguring of your system, otherwise anybody could load anything on it.
#### :eight_pointed_black_star: Protect bootloader with password
You can set password for the bootloader for prevents users from entering single user mode, changing settings at boot time, access to the bootloader console, reset the root password, if there is no password for GRUB-menu or access to non-secure operating systems.
###### Generate password hash
```bash
# Debian like distributions
grub-mkpasswd-pbkdf2
# RedHat like distributions
grub2-mkpasswd-pbkdf2
```
###### Updated grub configuration
```bash
cat > /etc/grub.d/01_hash << __EOF__
set superusers="user"
password_pbkdf2 user
grub.pbkdf2.sha512. # rest of your password hash
__EOF__
```
And regenerate grub configuration:
```bash
# Debian like distributions
grub-mkconfig > /boot/grub/grub.cfg
# RedHat like distributions
grub2-mkconfig > /boot/grub2/grub.cfg
```
#### :eight_pointed_black_star: Protect bootloader config files
Set the owner and group of `/etc/grub.conf` to the root user:
```bash
chown root:root /etc/grub.conf
```
or
```bash
chown -R root:root /etc/grub.d
```
Set permission on the `/etc/grub.conf` or `/etc/grub.d` file to read and write for root only:
```bash
chmod og-rwx /etc/grub.conf
```
or
```bash
chmod -R og-rwx /etc/grub.d
```
#### :ballot_box_with_check: Summary checklist
| Item | True | False |
| :--- | :---: | :---: |
| Set password for the bootloader | :black_square_button: | :black_square_button: |
### Disk partitions
#### :information_source: Introduction
Critical file systems should be separated into different partitions in ways that make your system a better and more secure.
#### :eight_pointed_black_star: Separate disk partitions
Make sure the following filesystems are mounted on separate partitions:
- `/boot`
- `/tmp`
- `/var`
- `/var/log`
Additionally, depending on the purpose of the server, you should consider separating the following partitions:
- `/usr`
- `/home`
- `/var/www`
You should also consider separating these partitions:
- `/var/tmp`
- `/var/log/audit`
#### :eight_pointed_black_star: Mount options: nodev, nosuid and noexec
For more security-focused situations is as follows:
- `nodev` - specifies that the filesystem cannot contain special devices: This is a security precaution. You don't want a user world-accessible filesystem like this to have the potential for the creation of character devices or access to random device hardware
- `nosuid` - specifies that the filesystem cannot contain set userid files. Preventing setuid binaries on a world-writable filesystem makes sense because there's a risk of root escalation or other awfulness there
- `noexec` - this param might be useful for a partition that contains no binaries, like **/var**, or contains binaries you do not want to execute on your system (from partitions with `noexec`), or that cannot even be executed on your system
#### :eight_pointed_black_star: Secure /boot directory
The boot directory contains important files related to the Linux kernel, so you need to make sure that this directory is locked down to read-only permissions.
Add **ro** option and `nodev`, `nosuid` and `noexec` to `/etc/fstab` for **/boot** entry:
```bash
LABEL=/boot /boot ext2 defaults,ro,nodev,nosuid,noexec 1 2
```
> When updating the kernel you will have to move the flag to `rw`:
> ```bash
> mount -o remount,defaults,rw /boot
> ```
#### :eight_pointed_black_star: Secure /tmp and /var/tmp
On Linux systems, the **/tmp** and **/var/tmp** locations are world-writable.
Several daemons/applications use the **/tmp** or **/var/tmp** directories to temporarily store data, log information, or to share information between their sub-components. However, due to the shared nature of these directories, several attacks are possible, including:
- Leaks of confidential data via secrets in file names
- Race-condition attacks (TOCTOU) on the integrity of processes and data
- Denial-of-Service (DoS) attacks based on race conditions and pre-allocating file/directory names
As a rule of thumb, malicious applications usually write to **/tmp** and then attempt to run whatever was written. A way to prevent this is to mount **/tmp** on a separate partition with the options `nodev`, `nosuid` and `noexec` enabled.
This will deny binary execution from **/tmp**, disable any binary to be suid root, and disable any block devices from being created.
**The first possible scenario is create symlink**
```bash
mv /var/tmp /var/tmp.old
ln -s /tmp /var/tmp
cp -prf /var/tmp.old/* /tmp && rm -fr /var/tmp.old
```
and set properly mount params:
```bash
UUID=<...> /tmp ext4 defaults,nodev,nosuid,noexec 1 2
```
**The second scenario is a bind mount**
The storage location **/var/tmp** should be bind mounted to **/tmp**, as having multiple locations for temporary storage is not required:
```bash
/tmp /var/tmp none rw,nodev,nosuid,noexec,bind 0 0
```
**The third scenario is setting up polyinstantiated directories**
Create new directories:
```bash
mkdir --mode 000 /tmp-inst
mkdir --mode 000 /var/tmp/tmp-inst
```
Edit `/etc/security/namespace.conf`:
```bash
/tmp /tmp-inst/ level root,adm
/var/tmp /var/tmp/tmp-inst/ level root,adm
```
Set correct **SELinux** context:
```bash
setsebool polyinstantiation_enabled=1
chcon --reference=/tmp /tmp-inst
chcon --reference=/var/tmp/ /var/tmp/tmp-inst
```
And set `nodev`, `nosuid` and `noexec` mount options in `/etc/fstab`.
> Alternative for **polyinstantiated directories** is **PrivateTmp** feature available from **systemd**. For more information please see: [New Red Hat Enterprise Linux 7 Security Feature: PrivateTmp](https://access.redhat.com/blogs/766093/posts/1976243).
#### :eight_pointed_black_star: Secure /dev/shm
`/dev/shm` is a temporary file storage filesystem, i.e. **tmpfs**, that uses RAM for the backing store. One of the major security issue with the `/dev/shm` is anyone can upload and execute files inside the `/dev/shm` similar to the `/tmp` partition. Further the size should be limited to avoid an attacker filling up this mountpoint to the point where applications could be affected. (normally it allows 20% or more of RAM to be used). The sticky bit should be set like for any world writeable directory.
For applies to shared memory `/dev/shm` mount params:
```bash
tmpfs /dev/shm tmpfs rw,nodev,nosuid,noexec,size=1024M,mode=1777 0 0
```
> You can also create a group named 'shm' and put application users for SHM-using applications in there. Then the access can be completely be restricted as such:
```bash
tmpfs /dev/shm tmpfs rw,nodev,nosuid,noexec,size=1024M,mode=1770,uid=root,gid=shm 0 0
```
#### :eight_pointed_black_star: Secure /proc filesystem
The proc pseudo-filesystem `/proc` should be mounted with `hidepid`. When setting `hidepid` to **2**, directories entries in `/proc` will hidden.
```bash
proc /proc proc defaults,hidepid=2 0 0
```
> Some of the services/programs operate incorrectly when the `hidepid` parameter is set, e.g. Nagios checks.
#### :eight_pointed_black_star: Swap partition
#### :eight_pointed_black_star: Disk quotas
#### :ballot_box_with_check: Summary checklist
| Item | True | False |
| :--- | :---: | :---: |
| Separate base partition scheme: `/boot`, `/tmp`, `/var`, `/var/log` | :black_square_button: | :black_square_button: |
| Separate `/usr` partition | :black_square_button: | :black_square_button: |
| Separate `/home` partition | :black_square_button: | :black_square_button: |
| Separate `/var/www` partition | :black_square_button: | :black_square_button: |
| Separate `/var/tmp` partition | :black_square_button: | :black_square_button: |
| Separate `/var/audit` partition | :black_square_button: | :black_square_button: |
| Secure `/boot` directory with `ro`, `nodev`, `nosuid`, `noexec` options | :black_square_button: | :black_square_button: |
| Secure `/tmp` and `/var/tmp` directory with `nodev`, `nosuid`, `noexec` options | :black_square_button: | :black_square_button: |
| Create symlink for `/var/tmp` in `/tmp` | :black_square_button: | :black_square_button: |
| Setting up bind-mount `/var/tmp` to `/tmp` | :black_square_button: | :black_square_button: |
| Setting up polyinstantiated directories for `/tmp` and `/var/tmp` | :black_square_button: | :black_square_button: |
| Secure `/dev/shm` directory with `nodev`, `nosuid`, `noexec` options | :black_square_button: | :black_square_button: |
| Secure `/proc` filesystem with `hidepid=2` option | :black_square_button: | :black_square_button: |