Preparing your machine

If you are setting up a fresh machine, it is recommended that you secure it first. Assuming that you are running Ubuntu 22.04:

Do not allow password-based SSH login (keys only)

First, make sure you are actually logging in to the server using keys and not via a password, otherwise, this will lock you out. Many hosting providers support uploading a public key and automatically set up key-based root login on new machines for you.

Edit /etc/ssh/sshd_config and find PasswordAuthentication. Make sure it’s uncommented and set to no. If you made any changes, restart sshd:

systemctl restart ssh.service

Update system packages 

apt update && apt upgrade -y

Install fail2ban so it blocks repeated login attempts

First, install fail2ban:

apt install fail2ban

Edit /etc/fail2ban/jail.local and put this inside:

[DEFAULT]
destemail = your@email.here
sendername = Fail2Ban

[sshd]
enabled = true
port = 22
mode = aggressive

Finally, restart fail2ban:

systemctl restart fail2ban

Install a firewall and only allow SSH, HTTP and HTTPS ports

First, install iptables-persistent. During installation, it will ask you if you want to keep the current rules–decline.

apt install -y iptables-persistent

Edit /etc/iptables/rules.v4 and put this inside:

*filter

#  Allow all loopback (lo0) traffic and drop all traffic to 127/8 that doesn't use lo0
-A INPUT -i lo -j ACCEPT
-A INPUT ! -i lo -d 127.0.0.0/8 -j REJECT

#  Accept all established inbound connections
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

#  Allow all outbound traffic - you can modify this to only allow certain traffic
-A OUTPUT -j ACCEPT

#  Allow HTTP and HTTPS connections from anywhere (the normal ports for websites and SSL).
-A INPUT -p tcp --dport 80 -j ACCEPT
-A INPUT -p tcp --dport 443 -j ACCEPT
#  (optional) Allow HTTP/3 connections from anywhere.
-A INPUT -p udp --dport 443 -j ACCEPT

#  Allow SSH connections
#  The -dport number should be the same port number you set in sshd_config
-A INPUT -p tcp -m state --state NEW --dport 22 -j ACCEPT

#  Allow ping
-A INPUT -p icmp -m icmp --icmp-type 8 -j ACCEPT

# Allow destination unreachable messages, especially code 4 (fragmentation required) is required or PMTUD breaks
-A INPUT -p icmp -m icmp --icmp-type 3 -j ACCEPT

#  Log iptables denied calls
-A INPUT -m limit --limit 5/min -j LOG --log-prefix "iptables denied: " --log-level 7

#  Reject all other inbound - default deny unless explicitly allowed policy
-A INPUT -j REJECT
-A FORWARD -j REJECT

COMMIT

With iptables-persistent, that configuration will be loaded at boot time. But since we are not rebooting right now, we need to load it manually for the first time:

iptables-restore < /etc/iptables/rules.v4

If your server is also reachable over IPv6, edit /etc/iptables/rules.v6 and add this inside:

*filter

#  Allow all loopback (lo0) traffic and drop all traffic to 127/8 that doesn't use lo0
-A INPUT -i lo -j ACCEPT
-A INPUT ! -i lo -d ::1/128 -j REJECT

#  Accept all established inbound connections
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

#  Allow all outbound traffic - you can modify this to only allow certain traffic
-A OUTPUT -j ACCEPT

#  Allow HTTP and HTTPS connections from anywhere (the normal ports for websites and SSL).
-A INPUT -p tcp --dport 80 -j ACCEPT
-A INPUT -p tcp --dport 443 -j ACCEPT
#  (optional) Allow HTTP/3 connections from anywhere.
-A INPUT -p udp --dport 443 -j ACCEPT

#  Allow SSH connections
#  The -dport number should be the same port number you set in sshd_config
-A INPUT -p tcp -m state --state NEW --dport 22 -j ACCEPT

#  Allow ping
-A INPUT -p icmpv6 -j ACCEPT

#  Log iptables denied calls
-A INPUT -m limit --limit 5/min -j LOG --log-prefix "iptables denied: " --log-level 7

#  Reject all other inbound - default deny unless explicitly allowed policy
-A INPUT -j REJECT
-A FORWARD -j REJECT

COMMIT

Similar to the IPv4 rules, you can load it manually like this:

ip6tables-restore < /etc/iptables/rules.v6

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