Steps

Change the SSH port.

While logged in as a user with sudo privileges, change your SSH port from the default 22 to a number in [1025, 65536). Make a note of your new port number, or you will be unable to login after making this change.

Open the sshd configuration file:
```
sudo nano /etc/ssh/sshd_config
```
And change the following line to set your new port number:
```
# What ports, IPs and protocols we listen for
Port {your-new-port-number}
```
Reload the ssh:
```
reload ssh
```
Now when you try to SSH into the server, you will need to specify the port:
```
ssh {username}@{your-server-ip-address} -i {rsa identity file} -p {your-new-port-number}
```
Create a temporary backdoor.

To avoid locking yourself out, first create a temporary backdoor with your ip address.
```
sudo ufw allow from {your-ip-address}
```
Later on, you can remove this backdoor by calling sudo ufw delete allow from {your-ip-address}.
Only allow incoming packets on your SSH port, HTTP (port 80), and NTP (port 123).

Create a new rule allowing incoming tcp and udp packets on each of these ports.
```
sudo ufw allow {your-new-SSH-port-number}
sudo ufw allow 80
sudo ufw allow 123
```
Enable the firewall.
```
sudo ufw enable
```
If you need to disable the firewall at some point, call sudo ufw disable.

Step 2-4: Ubuntu

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