9.3 KiB
Linux Server Hardener is a bash script that automates few of the tasks that you need to perform on a new Linux server to give it basic amount security.
I wanted to change my VPS(Virtual Private Server) provider and was testing out many providers and many Linux flavours on those VPS. But before doing anything those servers needed to be given basic amount security and this involved a set of repetitive commands on terminal. Depending on network speed and number of mis-types, these took between 30-90 minutes to perform.
This script is meant to save that time.
Getting Started
Prerequisites
- One of the following Linux flavours
- Debian 8.x
- Debian 9.x
- Ubuntu 14.x
- Ubuntu 16.x
- Ubuntu 18.x
- curl or wget should be installed
- root access to the server
Usage
The script is intended to be executed immediately after you have access to a new Linux server (most likely a VPS) as root.
# bash https://raw.githubusercontent.com/pratiktri/init-li-harden/master/init-linux-harden.sh --help
# bash https://raw.githubusercontent.com/pratiktri/init-li-harden/master/init-linux-harden.sh --username someusername --resetrootpwd --defaultsourcelist
# bash https://raw.githubusercontent.com/pratiktri/init-li-harden/master/init-linux-harden.sh --quiet
What does it to exactly?
Script performed the following operations:-
- Create non-root user and give it "sudo" privilege.
- Generate passphrage protected ed25519 SSH Keys (Private & Public).
- Secure "authorized_keys" file.
- [Optionally] Resets the url from which apt gets software from. Resets them to the flavour provided urls.
- Updates + Upgrades + Installs required softwares (sudo curl screen ufw fail2ban)
- Configures UFW
- Configures Fail2Ban
- Alters SSH options(/etc/ssh/sshd_config) to do the following:- ..* Disable SSH login for root (PermitRootLogin no) ..* Disable SSH login through password for all users (PasswordAuthentication no) ..* Updates path for authoried_keys file
- [Optionally] Resets root password
- On successfully completing above operations, display the following on screen:- ..* Username ..* User Password ..* SSH Private Key's path on the server ..* SSH Public Key's path on the server ..* SSH Private Key's passphrase ..* (If so opted) New root password ..* SSH Private Key ..* SSH Public Key
Details of each operation
1. Create non-root user and give it "sudo" privilege
You can specify your own username with "--username" flag.
When accepting username through "--username", script actively rejects special characters in the name because bash does not act well with special characters. The values accepted by the script [a-zA-Z0-9_-] i.e., alphanumeric and following 2 special characters _ and -
If "--username" is not provided, script will randomly generate an username for you. Script generated usernames are 9 character long and are alphanumeric (i.e., numbers & English characters).
Password for the user is always randomly generated. Passwords are 15 character long and are alphanumeric as well.
2. Generate passphrage protected ed25519 SSH Keys (Private & Public)
Since password authentications are bad security practice, script will generate a SSH Key and use that for user authentication.
All SSH keys are not made the same and we use OpenSSH keyformat and ed25519 algorithm to generate ours. You can read the reason for that here and here. For additional security SSH Private keys are secured by a passphrase. This passphrase is randomly generated. Passphrase are 15 character long and are alphanumeric. Algorithm used for user's password and SSH Private Key's passphrase are the same.
Generated keys are placed in ".ssh" sub-directory of the user's (created in step 1 above) home-directory, i.e., /home/username/.ssh/
SSH Public Key is then appended to /home/username/.ssh/authorized_keys file.
3. Secure "authorized_keys" file
"authorized_keys" file present in user's .ssh sub-directory contains the Public Key values. These Public Key values are used to authenticate user logins. Since, this is an important file we need to secure it tight.
Following are the file access restrictions that the script applies:-
- Make root user the owner of /home/username/.ssh/ directory and all files inside it.
- Give root group access to /home/username/.ssh/ directory and all files inside it.
- Make the /home/username/.ssh/ directory and all files inside it visible only to the root user.
- Remove the editing rights on /home/username/.ssh/authorized_keys file from every user - including root.
- Make the /home/username/.ssh/authorized_keys file immutable.
4. [Optionally] Resets the url from which apt gets software from. Resets them to the flavour provided urls
Most VPS provider change the location from which operating system downloads software from (i.e. apt repository); usually to CDNs that are maintained by them. While, this greatly reduces the time it takes to install a new application, it does come with its security implications (what if they insert tracker in application?).
However, one can also argue that if the OS (i.e. Linux) itself is installed by the providers, then OS itself is a more likely place where they might want to insert something dirty.
Depending on which argument you find valid, you can use this option in the script to ensure the default OS provided CDNs are used. This is done by updating the /etc/apt/sources.list file.
If the script is started with --defaultsourcelist option, then for Debian http://deb.debian.org/debian is used and for Ubuntu http://archive.ubuntu.com/ubuntu/ is used.
This is disabled by default.
5. Updates + Upgrades + Installs required softwares (sudo screen ufw fail2ban)
Pretty self explanatory.
6. Configures UFW
UFW(Uncomplicated FireWall) makes it easy to manage what kind of internet traffic enters or leaves the server. Without this program you would have to deal with Linux's iptables (which I can not understand at all).
This script sets up UFW so that only ssh(required for user login), http(required for any web application) & https(also required for any web application) traffic are allowed in and out of the server. All other traffic are blocked.
7. Configures Fail2Ban
While UFW restrict access to ports, the ports that are required (and are allowed by UFW in above step) for our purpose can be exploited by nefarious actors.
Fail2ban watches traffic coming through the allowed ports to determine if it is indeed a legitimate one. This determination is usually done by analyzing various log files being generated by Linux and other applications running on the server. If anything suspicious is found then after a certain number of illegitimate attempts the intruder(IP) is banned. Ban is then lifted after a desired amount of time.
This script sets up Fail2ban as following:-
- default ban time is 5 hours,
- Whitelists your server's IP from detection (uses https://ipinfo.io/ip to determine the IP),
- sets (backend = polling). polling is an algoritm used to check if the log files are updated. This algorithm does not required any additional software and if no additional software are installed then is faster option to choose.
- Explicitly enables protection for ssh with (maxretry = 3) & (bantime = 2592000)
8. Alters SSH options
This step contines from step 3 to harden our ssh login. Here, we do edit /etc/ssh/sshd_config file to achieve the following:-
- Disable root login (PermitRootLogin no). No one needs to work on root. The new user created already has root privileges anyways.
- Disable password login (PasswordAuthentication no). This ensures we can ONLY login though SSH Keys.
- Specify where to find authorized public keys which are granted login (\.ssh\authorized_keys %h\.ssh\authorized_keys)
9. [Optionally] Resets root password
Since, VPS providers sends you the password of your VPS's root user in email in plain text. So, password needs to be changed immediately. Note that since we have disabled root login AND password login in the above step, changing root password might be an overkill. But, still...
Also most VPS providers these days, allow you to provide SSH Public Key in their website. If you have done that you can skip this step. It is disabled by default anyways.
To change your root password provide option --resetrootpw. root password then be randomly generated. Passwords are 15 character long and are alphanumeric.
10. On successfully completing above operations
All the generated username, passwords, SSH Key location & SSH Keys themselves are displayed on the screen.
This might not be desired, on future version you might find option to NOT show the details and find them from the log file.
The logfile is located in /tmp/ directory - thus will be removed server reboots. All the details shown on the screen and a lot more can be found in the log. Exact logfile location will be shown on the screen as well.