# How To Secure A Linux Server An evolving how-to guide for securing a Linux server that, hopefully, also teaches you a little about security and why it matters. ## Table of Contents - [Introduction](#introduction) - [Document Objective](#document-objective) - [Why Secure Your Server](#why-secure-your-server) - [Why Yet Another Guide](#why-yet-another-guide) - [Contributing](#contributing) - [Editing Configuration Files - For The Lazy](#Editing-Configuration-Files---For-The-Lazy) - [To Do / To Add](#to-do--to-add) - [Before You Start](#before-you-start) - [Identify Your Principals](#Identify-Your-Principals) - [Installing Linux](#installing-linux) - [Pre/Post Installation](#prepost-installation) - [Important Advice For Using This Guide](#Important-Advice-For-Using-This-Guide) - [Securing Linux](#securing-linux) - [SSH Public/Private Keys](#ssh-publicprivate-keys) - [Limit Who Can Use `sudo`](#limit-who-can-use-sudo) - [Change Default `umask`](#change-default-umask) - [Password Protect GRUB](#password-protect-grub) - [Disable Root Login](#disable-root-login) - [Secure SSH](#secure-ssh) - [Create SSH Group For `AllowGroups`](#create-ssh-group-for-allowgroups) - [Secure `/etc/ssh/sshd_config`](#secure-etcsshsshd_config) - [Deactivate Short Moduli](#deactivate-short-moduli) - [Force Accounts To Use Secure Passwords](#force-accounts-to-use-secure-passwords) - [UFW: Uncomplicated Firewall](#ufw-uncomplicated-firewall) - [Fail2ban: Intrusion Detection And Prevention](#fail2ban-intrusion-detection-and-prevention) - [2FA/MFA for SSH](#2famfa-for-ssh) - [Apticron - Automatic Update Notifier](#Apticron---Automatic-Update-Notifier) - [Orphaned Software](#orphaned-software) - [Other Stuff](#other-stuff) - [Configure Gmail as MTA](#configure-gmail-as-mta) - [Lynis - Linux Security Auditing](#lynis---linux-security-auditing) - [Not Security](#not-security) - [Mount `/tmp` In RAM Using `tmpfs`](#mount-tmp-in-ram-using-tmpfs) - [Miscellaneous](#miscellaneous) - [Contacting Me](#contacting-me) - [Additional References](#Additional-References) - [Acknowledgments](#Acknowledgments) - [Disclaimer / Warranty](#disclaimer--warranty) ## 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](#contributing). 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](#installing-linux),or how to use it. - **...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](https://notepad-plus-plus.org/) handy. ([Table of Contents](#table-of-contents)) ### 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. ([Table of Contents](#table-of-contents)) ### 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](https://duckduckgo.com/?q=how+to+secure+linux&t=ffab&atb=v151-7&ia=web) 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](https://www.ansible.com/), [Chef](https://www.chef.io/), [Jenkins](https://jenkins.io/), [Puppet](https://puppet.com/), 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. ([Table of Contents](#table-of-contents)) ### Contributing I wanted to put this document on [GitHub](http://www.github.com) 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](https://github.com/imthenachoman/How-To-Secure-A-Linux-Server/issues/new). ([Table of Contents](#table-of-contents)) ### 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](https://en.wikipedia.org/wiki/INI_file) type file. Use your [favorite](https://en.wikipedia.org/wiki/Vi) Linux text editor. ([Table of Contents](#table-of-contents)) ### To Do / To Add - [ ] [Custom Jails for Fail2ban](#custom-jails) - [ ] [Linux Kernel `sysctl` Hardening](#linux-kernel-sysctl-hardening-wip) - [ ] [Security-Enhanced Linux / SELinux](https://en.wikipedia.org/wiki/Security-Enhanced_Linux) - [ ] full disk encryption - [x] BIOS password - [ ] Anti-Virus - [x] use ed25519 keys instead of RSA for SSH public/private keys - [ ] psad - [ ] unattended upgrades for critical security updates ([Table of Contents](#table-of-contents)) ## Before You Start ### Identify Your Principals Before you start you will want to identify what your principals are. What is your [threat model](https://en.wikipedia.org/wiki/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 convince 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](#disable-root-login) or [password protected GRUB](#password-protect-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. ([Table of Contents](#table-of-contents)) ### Installing Linux Installing Linux is out-of-scope for this document. 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 1. burn/copy/transfer it to your install medium (e.g. a CD or USB stick) 1. boot your server from your install medium 1. 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. Debian is my distribution of choice and what this guide was written/tested on. Everything below should, in most cases, work on other distributions but file paths and settings may differ slightly. Check your distribution's documentation. ([Table of Contents](#table-of-contents)) ### 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](#ssh-publicprivate-keys), make sure you perform any tasks specific to your setup like: - configuring network - configuring mount points in `/etc/fstab` (like [mounting `/tmp` in RAM using `tmpfs`](#mount-tmp-in-ram-using-tmpfs)) - creating the initial user accounts - etc... - Your server will need to be able to send e-mails so you can get important security alerts. If you're not setting up a mail server check [Configure Gmail as MTA](#configure-gmail-as-mta). ([Table of Contents](#table-of-contents)) ### Important Advice For Using This Guide - 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. ([Table of Contents](#table-of-contents)) ## Securing Linux ### 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](#ssh-key-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 1. 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 yes` in `/etc/ssh/sshd_config`](#PasswordAuthentication), 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/](https://linux-audit.com/using-ed25519-openssh-keys-instead-of-dsa-rsa-ecdsa/): > 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/ - `man ssh-keygen` - `man ssh-copy-id` #### Steps 1. From the computer you're going to use to connect to your server, **the client**, not the server itself, create an [Ed25519](https://linux-audit.com/using-ed25519-openssh-keys-instead-of-dsa-rsa-ecdsa/) key: ``` bash ssh-keygen -t ed25519 ``` - 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. 1. 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: ``` bash ssh-copy-id user@server ``` Now would be a good time to [perform any tasks specific to your setup](#post-install). ([Table of Contents](#table-of-contents)) ### 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 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: ``` bash sudo groupadd sudo ``` 1. Add account(s) to the group: ``` bash 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. 1. Edit `/etc/sudoers`: ``` bash sudo cp --preserve /etc/sudoers /etc/sudoers.$(date +"%Y%m%d%H%M%S") sudo visudo ``` 1. Add this line if it is not already there: ``` %sudo ALL=(ALL:ALL) ALL ``` ([Table of Contents](#table-of-contents)) ### Linux Kernel `sysctl` Hardening (WIP) #### References - https://geektnt.com/sysctl-conf-hardening.html - https://linoxide.com/how-tos/linux-server-protection/ - https://www.cyberciti.biz/faq/linux-kernel-etcsysctl-conf-security-hardening/ - https://github.com/klaver/sysctl/blob/master/sysctl.conf ([Table of Contents](#table-of-contents)) ### 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. **USE WITH CAUTION.** #### Goals - set default `umask` for **non-root** accounts to **0027** - set default `umask` for the **root** account to **0077** #### References - https://www.linuxnix.com/umask-define-linuxunix/ - https://serverfault.com/questions/818783/which-umask-is-more-secure-in-linux-022-or-027 - https://www.cyberciti.biz/tips/understanding-linux-unix-umask-value-usage.html - `man umask` #### 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](#Editing-Configuration-Files---For-The-Lazy): ``` bash 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 $(date +"%Y-%m-%d @ %H:%M:%S")" | sudo tee -a /etc/profile /etc/bash.bashrc ``` 1. We also need to **add** this line to `/etc/login.defs`: ``` UMASK 0027 ``` [For the lazy](#Editing-Configuration-Files---For-The-Lazy): ``` bash sudo cp --preserve /etc/login.defs /etc/login.defs.$(date +"%Y%m%d%H%M%S") echo -e "\nUMASK 0027 # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")" | sudo tee -a /etc/login.defs ``` 1. [**USE WITH CAUTION.**](#umask-root) Set default `umask` for the **root** account to **0077** by **adding** this line to `/root/.bashrc`: ``` umask 0077 ``` [For the lazy](#Editing-Configuration-Files---For-The-Lazy): ``` bash sudo cp --preserve /root/.bashrc /root/.bashrc.$(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 ``` ([Table of Contents](#table-of-contents)) ### 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](https://www.cyberciti.biz/tips/howto-recovering-grub-boot-loader-password.html) 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 - https://selivan.github.io/2017/12/21/grub2-password-for-all-but-default-menu-entries.html - https://help.ubuntu.com/community/Grub2/Passwords - https://computingforgeeks.com/how-to-protect-grub-with-password-on-debian-ubuntu-and-kali-linux/ - `man grub` - `man grub-mkpasswd-pbkdf2` #### Steps 1. Create a [Password-Based Key Derivation Function 2 (PBKDF2)](https://en.wikipedia.org/wiki/PBKDF2) hash of your password: ``` bash 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 ``` 1. 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. 1. 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: ``` bash #!/bin/sh set -e cat << EOF set superusers="grub" password_pbkdf2 grub [hash] EOF ``` For example: ``` bash #!/bin/sh set -e cat << EOF set superusers="grub" password_pbkdf2 grub grub.pbkdf2.sha512.100000.2812C233DFC899EFC3D5991D8CA74068C99D6D786A54F603E9A1EFE7BAEDDB6AA89672F92589FAF98DB9364143E7A1156C9936328971A02A483A84C3D028C4FF.C255442F9C98E1F3C500C373FE195DCF16C56EEBDC55ABDD332DD36A92865FA8FC4C90433757D743776AB186BD3AE5580F63EF445472CC1D151FA03906D08A6D EOF ``` 1. Set the file's execute bit so `update-grub` includes it when it updates GRUB's configuration: ``` bash sudo chmod a+x /etc/grub.d/01_password ``` 1. Make a backup of `/etc/grub.d/10_linux` and unset execute bit so `update-grub` doesn't try to run it: ``` bash 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.* ``` 1. 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](#Editing-Configuration-Files---For-The-Lazy): ``` bash 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 ``` 1. Update GRUB with `update-grub`: ``` bash sudo update-grub ``` ([Table of Contents](#table-of-contents)) ### Disable Root Login #### Why If you have `sudo` [configured properly](#limit-who-can-use-sudo), 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`](https://linux.die.net/man/8/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 - https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=806852 - https://github.com/systemd/systemd/issues/7115 - https://github.com/karelzak/util-linux/commit/7ff1162e67164cb4ece19dd809c26272461aa254 - https://github.com/systemd/systemd/issues/11596 - `man systemd` #### Steps 1. Lock the **root** account: ``` bash sudo passwd -l root ``` ([Table of Contents](#table-of-contents)) ### Secure SSH #### Create SSH Group For `AllowGroups` ##### Why To make it easy to control who can SSH to the server. ##### Goals - a UNIX group that we'll use in [Secure `/etc/ssh/sshd_config`](#secure-etcsshsshd_config) to limit who can SSH to the server ##### Notes - This is a per-requisite step to support the `AllowGroup` setting set in [Secure `/etc/ssh/sshd_config`](#secure-etcsshsshd_config). ##### References - `man groupadd` - `man usermod` ##### Steps 1. Create a group: ``` bash sudo groupadd sshusers ``` 1. Add account(s) to the group: ``` bash 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. ([Table of Contents](#table-of-contents)) #### 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 - Make sure you've completed [Create SSH Group For `AllowGroups`](#create-ssh-group-for-allowgroups) first. ##### References - Mozilla's OpenSSH guidelines for OpenSSH 6.7+ at https://infosec.mozilla.org/guidelines/openssh#modern-openssh-67 - https://linux-audit.com/audit-and-harden-your-ssh-configuration/ - https://www.ssh.com/ssh/sshd_config/ - https://www.techbrown.com/harden-ssh-secure-linux-vps-server/ - `man sshd_config` ##### Steps 1. Make a backup of `/etc/ssh/sshd_config`: ``` bash sudo cp --preserve /etc/ssh/sshd_config /etc/ssh/sshd_config.$(date +"%Y%m%d%H%M%S") ``` 1. 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 curve25519-sha256@libssh.org,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256 Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com # LogLevel VERBOSE logs user's key fingerprint on login. Needed to have a clear audit track of which key was using to log in. LogLevel VERBOSE # 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 ``` 1. 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||local addresses `sshd` should listen on|See [Issue #1](https://github.com/imthenachoman/How-To-Secure-A-Linux-Server/issues/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. 1. Restart ssh: ``` bash sudo service sshd restart ``` ([Table of Contents](#table-of-contents)) #### Deactivate Short Moduli ##### Why Per [Mozilla's OpenSSH guidelines for OpenSSH 6.7+](https://infosec.mozilla.org/guidelines/openssh#modern-openssh-67), "all Diffie-Hellman moduli in use should be at least 3072-bit-long". ##### Goal - deactivate short moduli ##### References - Mozilla's OpenSSH guidelines for OpenSSH 6.7+ at https://infosec.mozilla.org/guidelines/openssh#modern-openssh-67 - `man moduli` ##### Steps 1. Make a backup of `/etc/ssh/moduli`: ``` bash sudo cp --preserve /etc/ssh/moduli /etc/ssh/moduli.$(date +"%Y%m%d%H%M%S") ``` 1. Remove short moduli: ``` bash sudo awk '$5 >= 3071' /etc/ssh/moduli | sudo tee /etc/ssh/moduli.tmp sudo mv /etc/ssh/moduli.tmp /etc/ssh/moduli ```` ([Table of Contents](#table-of-contents)) ### Force Accounts To Use Secure Passwords #### Why By default, accounts can use any password they want, including bad ones. [pwquality](https://linux.die.net/man/5/pwquality.conf)/[pam_pwquality](https://linux.die.net/man/8/pam_pwquality) addresses this security gap by providing "a way to configure the default password quality requirements for the system passwords" and checking "its strength against a system dictionary and a set of rules for identifying poor choices." #### Goal - enforced strong passwords #### Steps 1. Install `libpam-pwquality`. For Debian based systems: ``` bash sudo apt install libpam-pwquality ``` 1. Edit `/etc/pam.d/common-password` and **change** the line that starts like this: ``` password requisite pam_pwquality.so ``` to this: ``` password requisite pam_pwquality.so retry=3 minlen=10 difok=3 ucredit=-1 lcredit=-1 dcredit=-1 ocredit=-1 maxrepeat=3 gecoschec ``` The above options are: - `retry=3` = prompt user 3 times before returning with error. - `minlen=10` = the minimum length of the password, factoring in any credits (or debits) from these: - `dcredit=-1` = must have at least **one digit** - `ucredit=-1` = must have at least **one upper case letter** - `lcredit=-1` = must have at least **one lower case letter** - `ocredit=-1` = must have at least **one non-alphanumeric character** - `difok=3` = at least 3 characters from the new password cannot have been in the old password - `maxrepeat=3` = allow a maximum of 3 repeated characters - `gecoschec` = do not allow passwords with the account's name [For the lazy](#Editing-Configuration-Files---For-The-Lazy): ``` bash sudo cp --preserve /etc/pam.d/common-password /etc/pam.d/common-password.$(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 ``` ([Table of Contents](#table-of-contents)) ### UFW: Uncomplicated Firewall #### Why Call me paranoid 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, reject/deny by default, and allow by exception. Ensuring that only traffic we explicitly allow is the job of a firewall. On Linux, the most common firewall is [iptables](https://en.wikipedia.org/wiki/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 - https://launchpad.net/ufw #### Steps 1. Install `ufw`. For Debian based systems: ``` bash sudo apt install ufw ``` 1. Deny all outgoing traffic: ``` bash 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: ``` bash sudo ufw default allow outgoing comment 'allow all outgoing traffic' ``` 1. Deny all incoming traffic: ``` bash sudo ufw default deny incoming comment 'deny all incoming traffic' ``` 1. Obviously we want SSH connections in: ``` bash sudo ufw limit in ssh comment 'allow SSH connections in' ``` 1. Allow additional traffic as per your needs. Some common use-cases: ``` bash # 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' ``` 1. Start `ufw`: ``` bash sudo ufw enable ``` 1. If you want to see a status: ``` bash sudo ufw status ``` or ``` bash sudo ufw status verbose ``` #### Default Applications `ufw` ships with some default applications. You can see them with: ``` bash sudo ufw app list ``` To get details about the app, like which ports it includes, type: ``` bash sudo ufw app info [app name] ``` For example: ``` bash $ 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](https://support.plex.tv/articles/201543147-what-network-ports-do-i-need-to-allow-through-my-firewall/): ``` bash $ 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: ```bash sudo ufw allow plexmediaserver ``` ([Table of Contents](#table-of-contents)) ### 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 - https://www.fail2ban.org/ #### Steps 1. Install `fail2ban`. For Debian based systems: ``` bash sudo apt install fail2ban ``` 1. 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. 1. 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 ``` 1. Enable `fail2ban` and the jail for SSH: ``` bash sudo fail2ban-client start sudo fail2ban-client reload sudo fail2ban-client add sshd ``` 1. To check the status: ``` bash sudo fail2ban-client status sudo fail2ban-client status sshd ``` #### Custom Jails WIP ([Table of Contents](#table-of-contents)) ### 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](#fail2ban-intrusion-detection-and-prevention) 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 1. 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](https://github.com/google/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](#ssh-publicprivate-keys). Check the documentation on how to change this behavior to suite your requirements. #### References - https://github.com/google/google-authenticator-libpam #### Steps 1. Install it `libpam-google-authenticator`. For Debian based systems: ``` bash sudo apt install libpam-google-authenticator ``` 1. **Make sure you're logged in as the ID you want to enable 2FA/MFA for** and **execute** `google-authenticator`: ``` bash 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. 1. 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](https://github.com/google/google-authenticator-libpam/blob/master/README.md#nullok) for what `nullok` means. [For the lazy](#Editing-Configuration-Files---For-The-Lazy): ``` bash 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 $(date +"%Y-%m-%d @ %H:%M:%S")" | sudo tee -a /etc/pam.d/sshd ``` 1. Enable it in the SSH settings by **adding** this line in `/etc/ssh/sshd_config`: ``` ChallengeResponseAuthentication yes ``` [For the lazy](#Editing-Configuration-Files---For-The-Lazy): ``` bash sudo cp --preserve /etc/ssh/sshd_config /etc/ssh/sshd_config.$(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 ``` 1. Restart `ssh`: ``` bash sudo service sshd restart ``` ([Table of Contents](#table-of-contents)) ### 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 - 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/ #### Steps 1. Install `apticron`. For Debian based systems: ``` bash sudo apt install apticron ``` 1. Set the value of `EMAIL` in `/etc/apticron/apticron.conf` to your e-mail address. ([Table of Contents](#table-of-contents)) ### 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](http://freshmeat.sourceforge.net/projects/deborphan/) to find orphaned packages. 1. Install `deborphan`: ``` bash sudo apt install deborphan ``` 1. Run `deborphan` as **root** to see a list of orphaned packages: ``` bash sudo deborphan ``` 1. Pass it's output to `apt` to remove them: ``` bash sudo apt --autoremove purge $(deborphan) ``` You will want to repeatedly run this command until `deborphan` no longer returns any orphaned packages. [For the lazy](#Editing-Configuration-Files---For-The-Lazy): ``` bash while [[ $(deborphan | wc -l) != 0 ]] ; do sudo apt --autoremove purge $(deborphan) done ``` ([Table of Contents](#table-of-contents)) ## 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](https://mail.google.com/) #### References - https://php.quicoto.com/setup-exim4-to-use-gmail-in-ubuntu/ #### Steps 1. Install `exim4`. For Debian based systems: ``` bash sudo apt install exim4 ``` 1. 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|(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`| 1. **Add** a line like this to `/etc/exim4/passwd.client` ``` *.google.com:yourAccount@gmail.com:yourPassword ``` Replace `yourAccount@gmail.com` and `yourPassword` with your details. If you have 2FA/MFA enabled on your Gmail then you'll need to create and use an app password. 1. This file has your Gmail password so we need to lock it down: ``` bash sudo chown root:Debian-exim /etc/exim4/passwd.client sudo chmod 640 /etc/exim4/passwd.client ``` 1. Restart `exim4`: ``` bash sudo service exim4 restart ``` 1. Add some mail aliases so we can send e-mails to local accounts by **adding** lines like this to `/etc/aliases`: ``` user1: user1@gmail.com user2: user2@gmail.com ... ``` You'll need to add all the local accounts that exist on your server. ([Table of Contents](#table-of-contents)) ### Lynis - Linux Security Auditing #### Why From [https://cisofy.com/lynis/](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](https://github.com/CISOfy/lynis) 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 - 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. We want it to be installed system wide so go to `/usr/local` and clone it from https://github.com/CISOfy/lynis: ``` bash cd /usr/local sudo git clone https://github.com/CISOfy/lynis ``` 1. Update it: ``` bash sudo /usr/local/lynis/lynis update info ``` 1. Run a security audit: ``` bash 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. ([Table of Contents](#table-of-contents)) ## 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 - https://wiki.archlinux.org/index.php/Tmpfs - https://wiki.centos.org/TipsAndTricks/TmpOnTmpfs - `man mount` - `man tmpfs` #### 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](#Editing-Configuration-Files---For-The-Lazy): ``` bash 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 ``` ([Table of Contents](#table-of-contents)) ## Miscellaneous ### Contacting Me For any questions, comments, concerns, feedback, or issues, submit a [new issue](https://github.com/imthenachoman/How-To-Secure-A-Linux-Server/issues/new). ([Table of Contents](#table-of-contents)) ### Additional References - [https://github.com/pratiktri/server_init_harden](https://github.com/pratiktri/server_init_harden) - Bash script that automates few of the tasks that you need to perform on a new Linux server to give it basic amount security. ([Table of Contents](#table-of-contents)) ### Acknowledgments - everyone from [/r/linuxquestions](https://www.reddit.com/r/linuxquestions/comments/aopzl7/new_guide_created_by_me_how_to_secure_a_linux/) who provided feedback on this guide - everyone from [/r/selfhosted](https://www.reddit.com/r/selfhosted/comments/aoxd4l/new_guide_created_by_me_how_to_secure_a_linux/) who provided feedback on this guide ([Table of Contents](#table-of-contents)) ### Disclaimer / Warranty This guide comes with ABSOLUTELY NO WARRANTY. Use with caution. ([Table of Contents](#table-of-contents))