Linode/Setup

Assuming you have made a Linode with Debian 11.

User accounts

Make user accounts. (You can do this as many times as you want to add more users.)

adduser \
   --system \
   --shell /bin/bash \
   --gecos 'User Description If u want' \
   --group \
   --home /home/USERNAME \
   USERNAME

Add user to sudo group (if needed)

usermod -aG sudo USERNAME

To remove a user from sudo group

sudo gpasswd -d USERNAME sudo

Also, to remove their ability to ssh as root, remove their public key from .ssh/authorized_keys

Security (only need to do this at setup)

Add cryptographic public keys for user authentication

RSA keys (has known vulnerabilities)
ed25519 keys (better alternative: faster, more secure, resilient against hash-function collision attacks, shorter)

Configure sshd

Disable password login
Disable root login
Restart ssh otherwise the changes to sshd don't take effect!
```
service ssh restart
```

Fail2Ban

Install

apt install fail2ban

Configuration

Editing /etc/fail2ban/jail.local, using the defaults from Mastodon/Setup

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

[sshd]
enabled = true
port = 22

[sshd-ddos]
enabled = true
port = 22

Then restart the service

systemctl restart fail2ban

IPTables

Again following Mastodon/Setup

Install

apt install -y iptables-persistent

Decline the dialog asking if you want to preserve existing iptables configs (if you say yes then the commands below will fail for some reason)

Configuration

IPv4: Edit /etc/iptables/rules.v4

*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, espacally 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

IPv6: Edit /etc/iptables/rules.v6

*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

Then reload the rules:

iptables-restore < /etc/iptables/rules.v4
ip6tables-restore < /etc/iptables/rules.v6