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Setting up an Ubuntu 18.04 vServer and securing it as quickly as possible

Tags:ubuntu vps iptables minecraft openjdk11 configuration
Posted on March 24, 2021 | 10 minute read
Setting up an Ubuntu 18.04 vServer and securing it as quickly as possible

Most VPS hosts grant you a standard username and password combo (usually on your dashboard) to access your vServer’s root account. There are two things problematic with keeping things as-is. First, you are directly using a root account, which violates the princple of least privilege, making an account compromise very rewarding and second, using public key authentication grants greater security in comparison to using a standard username and password combination, so you’re leaving security on the table and relying on the strength of your password.

Generating an SSH keypair

This guide presumes you have a POSIX-compliant shell environment with an internet connection (using Windows Subsystem for Linux, Ubuntu, macOS, FreeBSD). If you’re using Windows, then consider installing Windows Subsystem for Linux (along with Ubuntu 18.04 or higher from the Microsoft Store) and coming back to this guide.

You could do this natively on Windows using Windows-specific tools but…. I didn’t use them so I didn’t document them (I used a PopOS! virtual machine the whole time).

On the client, let’s first define two functions which will make key-generation easier for us (all client specific commands will be prefixed with [email protected]:~$ and everything else will not contain any prefixing so you can copy and paste them)

[email protected]:~$ function genkey {
  TEMPVAR=$(hexdump -n 4 -e '4/4 "%08X" 1 "\n"' /dev/random | tr -s " " | awk '$1=$1')
  ssh-keygen -t ed25519 -f $HOME/.ssh/id_$TEMPVAR
  echo "IdentityFile $HOME/.ssh/ed25519_$TEMPVAR" >> $HOME/.ssh/config
  cat "$HOME/.ssh/id_$TEMPVAR.pub"
  
}

function fixssh {
  chmod 700 ~/.ssh
  chmod 644 ~/.ssh/authorized_keys
  chmod 644 ~/.ssh/known_hosts
  chmod 644 ~/.ssh/config
  chmod 600 ~/.ssh/id_*
  chmod 644 ~/.ssh/id_*.pub
}

Using our newly introduced functions, let’s create a keypair. It will look something like this once you execute it.

[email protected]:~$ genkey
Generating public/private ed25519 key pair.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/kitty/.ssh/id_CAFEBABE.
Your public key has been saved in /home/kitty/.ssh/id_CAFEBABE.pub.
The key fingerprint is:
SHA256:THISWILLBEYOURCHECKSUMTHISWILLBEYOURCHECKSUM [email protected]
The key's randomart image is:
+--[ED25519 256]--+
|  ..       .o    |
| .  .     .. o   |
|E .. .   .    +  |
| .  o ..o.   o   |
|    ..=.S..   .  |
|   . + B =     . |
|  . . O = *.  .  |
|   . + *o* *oo . |
|      +o=o=+=oo  |
+----[SHA256]-----+
ssh-ed25519 REPLACETHISWITHYOURKEYPAIRREPLACETHISWITHYOURKEYPAIRREPLACETHISWITHYOURKEYPAIR [email protected]

Now, copy ssh-ed25519 REPLACET...EYPAIR somewhere as we’re going to start configuring our vServer!

The first vServer login

After experimenting with various distros (even naively trying out Arch Linux), I settled on Ubuntu 18.04 LTS and connected to my vServer using ssh -l root 255.255.255.255 (replace 255.255.255.255 with your vServer’s IP address) and I defined two variables which my commands will be relying on (it also means you can just copy and paste chunks!)

export USERNAME="minecraft"
# Replace SSH_PUBKEY with the pubkey you're using 
export SSH_PUBKEY='ssh-ed25519 REPLACETHISWITHYOURKEYPAIRREPLACETHISWITHYOURKEYPAIRREPLACETHISWITHYOURKEYPAIR' 

Now let’s create our relatively unprivileged user and add them to the sudoer’s group so that we can execute with root permissions when necessary.

adduser --gecos "" $USERNAME # You will be prompted to enter your desired password
adduser $USERNAME sudo

Let’s install all the necessary packages we need. We’re installing unattended-upgrades so that the vServer can upgrade its own software without manually running apt-get upgrade -y manually. We use fail2ban, aide (and later iptables-persistent) as utilities to secure our vServer, screen so that we can run our server in the background, unzip and zip for archival operations (useful for when you need to backup your server), openjdk-11-jre-headless as we’re configuring a Java Edition server and finally, zsh and fonts-powerline for our perferred shell.

apt-get update
apt-get upgrade -y
apt-get install unattended-upgrades fail2ban aide \
  openjdk-11-jre-headless screen git unzip zip zsh fonts-powerline -y
apt-get remove --purge openjdk-8-jre-headless -y

Due to a documented bug with sudo (see, sudo: setrlimit(RLIMIT_CORE): Operation not permitted by juhp, we insert this line into our sudo configuration file.

echo "Set disable_coredump false" >> /etc/sudo.conf

Let’s prohibit root logins, password authentication and X11 forwarding by rewriting /etc/ssh/sshd_config

echo "Port 22" > /etc/ssh/sshd_config
echo "PermitRootLogin no" >> /etc/ssh/sshd_config
echo "PasswordAuthentication no"  >> /etc/ssh/sshd_config
echo "ChallengeResponseAuthentication no" >> /etc/ssh/sshd_config
echo "UsePAM yes" >> /etc/ssh/sshd_config
echo "PrintMotd no" >> /etc/ssh/sshd_config
echo "X11Forwarding no" >> /etc/ssh/sshd_config
echo "AcceptEnv LANG LC_*" >> /etc/ssh/sshd_config

Now we insert our public key into our username’s’s authorized keys by editing .ssh/authorized_keys from the home directory

mkdir /home/$USERNAME/.ssh
echo $SSH_PUBKEY > /home/$USERNAME/.ssh/authorized_keys
chmod -R 700 /home/$USERNAME/.ssh && chmod 600 /home/$USERNAME/.ssh/authorized_keys
chown -R $USERNAME /home/$USERNAME/.ssh

Using iptables we allow all outward traffic and block all inward traffic except on ports 22 (SSH), 19132 (Minecraft Bedrock Edition) and 25565 (Minecraft Java Edition). As iptables rules do not persist on reboot, we use iptables-persistent to ensure that these rules are enforced across system reboots (there is a manual prompt asking if you want to keep or inherit the current ruleset, answer yes)

iptables -P INPUT ACCEPT
iptables -F
iptables -A INPUT -i lo -j ACCEPT
iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -p tcp --dport 22 -j ACCEPT
iptables -A INPUT -p tcp --dport 19132 -j ACCEPT
iptables -A INPUT -p tcp --dport 25565 -j ACCEPT

iptables -A INPUT -j LOG
iptables -A FORWARD -j LOG
iptables -A INPUT -j DROP

apt-get install iptables-persistent -y
service netfilter-persistent start
systemctl enable netfilter-persistent 

Now we setup automatic updates using --priority=high, so that we aren’t asked about package-specific configuration unless it’s absolutely necessary

dpkg-reconfigure --priority=high unattended-upgrades

Let’s restart the SSH daemon so that it inherits the new changes

service sshd restart

Now before rebooting, I’d suggest opening a new terminal and testing if your new account’s login works (because after rebooting, there’s no going back and you’d probably need to reinstall your distro) using ssh -l [your username here] 255.255.255.255 (we cannot use $USERNAME as that is defined within that shell session and not on the client).

Once you confirm that it works, reboot your system and this will be the last time you’ll be using the root account directly.

reboot

The setup after the first reboot

Once you’ve logged in using ssh -l [your username here] 255.255.255.255, we need to do one last thing to ensure that the root account can never be used again for good.

We’re going to act as the root user by using sudo su. As root, we’ll prohibit using the root account at all under any circumstances by modifying the start shell for the root user, invalidating the root password issued to you by your hosting provider and ensuring that SSH will deny all root login requests (it’s sort of redundant but still…)

sed -i "s|root:/root:/bin/bash|root:/root:/sbin/nologin|g" /etc/passwd
echo 'auth    required       pam_listfile.so \' >> /etc/pam.d/sshd
echo "        onerr=succeed  item=user  sense=deny  file=/etc/ssh/deniedusers" >> /etc/pam.d/sshd
echo "root" >> /etc/ssh/deniedusers
chmod 600 /etc/ssh/deniedusers
passwd -l root

Okay, we’re done playing with fire and we’ll exit the root session by typing in exit and from here on, we will use sudo for our privileged needs.

Till now you’re probably using the default bash shell that comes standard on every install. We’re going to switch to a more versatile shell called ZSH, install a popular expansion for it called ohmyzsh and install Denys Dovhan’s wonderful Spaceship theme and set it as the default.

P.S: If asked to make zsh the default shell, say yes.

sh -c "$(curl -fsSL https://raw.github.com/robbyrussell/oh-my-zsh/master/tools/install.sh)"
git clone https://github.com/denysdovhan/spaceship-prompt.git "$ZSH_CUSTOM/themes/spaceship-prompt" --depth=1
ln -s "$ZSH_CUSTOM/themes/spaceship-prompt/spaceship.zsh-theme" "$ZSH_CUSTOM/themes/spaceship.zsh-theme" 
sed -i 's/_THEME=\"robbyrussel\"/_THEME=\"spaceship\"/g' ~/.zshrc
source ~/.zshrc

Once we’ve setup zsh, let’s setup a database to keep track of all changes made to important system files

sudo aideinit
sudo cp /var/lib/aide/aide.db.new /var/lib/aide/aide.db
sudo update-aide.conf
sudo cp /var/lib/aide/aide.conf.autogenerated /etc/aide/aide.conf

Now, let’s install Cisco’s ClamAV, update its defintions database using freshclam and add a repeat task as root to instruct clamscan to run a full system scan and delete everything found to be malicious by ClamAV (note, this does have the downside of false positives deleting files unnecessarily but the likelihood of that is slim, but very much still there)

sudo apt-get install clamav clamav-freshclam -y
sudo freshclam -v
echo "0 0     * * *   root    clamscan -r -i –quiet /" | sudo tee -a /etc/crontab

Creating a swap partition is ideal, especially on systems that do not have adequate amounts of RAM. Though you should note that you should probably not do this if your vServer has an SSD as it will increase the wear-and-tear that it will undergo. This creates a 4GB swapfile and ensures that it’s automounted on every reboot.

sudo dd if=/dev/zero of=/swapfile bs=1024 count=4194304 status=progress
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile
echo "/swapfile swap swap defaults 0 0" | sudo tee -a /etc/fstab
echo "vm.swappiness=60" | sudo tee -a /etc/sysctl.conf

Setting up a Minecraft server

I wasn’t happy with instructions existing online detailing how to setup a Minecraft server that a) had a reasonable permissions system and b) had a username-based whitelist so I got onto documenting how I setup mine.

What we are attempting to achieve

  • An installation of PaperMC (a fork of the popular Spigot server with emphasis on performance) 1.16.5 that restarts itself whenever it crashes
  • The ability to authenticate players on the basis of their username using LoginSecurity and create a player whitelist using WhitelistPlayers
  • Support for permissions using LuckPerms
  • Anti-griefing protection with Worldguard
  • The ability to protect chests with LockettePro

Let’s start by downloading everything we need straight from the source.

Note: it might be a better idea to do this manually by downloading the latest version and then scp'ing it to your vServer, using the commands below is lazy and will cause your server to run on outdated versions of plugins.

cd ~
mkdir -p pspigot/plugins && cd pspigot
wget -O paper.jar https://papermc.io/api/v2/projects/paper/versions/1.16.5/builds/567/downloads/paper-1.16.5-567.jar
wget -O plugins/LoginSecurity.jar https://media.forgecdn.net/files/2941/945/LoginSecurity-3.0.2-Bukkit.jar
wget -O plugins/ProtocolLib.jar https://github.com/dmulloy2/ProtocolLib/releases/download/4.6.0/ProtocolLib.jar
wget -O plugins/Worldguard.jar https://media.forgecdn.net/files/2989/116/worldguard-bukkit-7.0.3.jar
wget -O plugins/LuckPerms.jar https://ci.lucko.me/job/LuckPerms/1303/artifact/bukkit/loader/build/libs/LuckPerms-Bukkit-5.3.5.jar

Once we’ve downloaded most of the plugins we need, let’s create a launch script that ensures that if java crashes at any point in time, the server will merely restart itself instead of quitting entirely.

echo '#!/bin/bash' > launch.sh
echo 'until java -Xms1500M -Xmx1500M -jar paper.jar; do' > launch.sh
echo '    echo "Server crashed with exit code $?.  Respawning.." >&2' >> launch.sh
echo '    sleep 1' >> launch.sh
echo 'done' >> launch.sh
chmod +x launch.sh

Presuming you’ve read the Minecraft’s EULA, tell PaperMC that you’ve read and agreed to it.

echo "eula=true" > eula.txt

We’re not done yet, we need to download WhitelistPlayers and LockettePro from Spigot’s website using a web browser due to protections used by Spigot that prevents the use of wget.

We can use scp on our client and presuming your default download path is ~/Downloads, we can just upload the files onto our vServer.

[email protected]:~$ scp -P 22 ~/Downloads/LockettePro*.jar [email protected]:/home/minecraft/pspigot/plugins
[email protected]:~$ scp -P 22 ~/Downloads/WhitelistPlayers.jar [email protected]:/home/minecraft/pspigot/plugins

Then we can run ./launch.sh for the first time so that our server runs. There are plenty of guides online detailing how you can customize server.properties, spigot.yml and bukkit.yml so I won’t get into that but I found that LuckPerms didn’t work right off the bat so we need to set it up.

Setting up LuckPerms

LuckPerms has a really helpful editor that makes configuration setup a breeze. Go to the LuckPerms Editor Demo to get a basic template of LuckPerms ready. Then ensure that server operator permissions (group op) look like the image given below.

Server Operator User Permissions

Then ensure that default permissions (group default) for your users look like the image given below so that users can work with WorldGuard in the region you allocate to them.

Default User Permissions

Now from the server console, register using /register [your password here], add yourself as a server operator using /lp user [username] permission set group.op true and add yourself to the whitelist using /wl add [username], then enable the whitelist using /wl on

Running PaperMC in the background

Now that we’ve got our server running, there’s one small problem. If you terminate the SSH session, the server will terminate as well. To prevent that, you need to use screen.

If your server is still running, use the stop command to shut it down. Then use screen and hit Enter (or Spacebar) and you will be taken to another shell prompt that looks just like the one before. Run ./launch.sh and once the server starts running, press Ctrl + A + D and you’ll be taken back to the primary shell.

Whenever you need to access the server console, all you need to do is login to the SSH, type in screen -list (not screen -l) and you’ll be shown something like this:

[email protected]:~/pspigot$ screen -list
There is a screen on:
	519.pts-2.minecraft	(03/24/21 09:27:03)	(Detached)
1 Socket in /run/screen/S-minecraft.

Using screen -r 519 (replace 519 with whatever number you find there), you will be dropped in back to your PaperMC server console and once you’re done, you can Ctrl + A + D out of there and you’re golden!