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 20.04: 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.
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If you are setting up a fresh machine, it is recommended that you secure it first. Assuming that you are running Ubuntu 20.04:
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
[sshd-ddos]
enabled = true
port = 22
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 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
# 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
# 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