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