Create a new user
The root user has a lot of power on your server. It has the power to read, write, and execute any file on the server. It’s not advisable to use root for day-to-day server tasks. For those tasks, use a user account with normal permissions.
Add a new user:
adduser <your username>
Add the user to the sudo group:
usermod -a -G sudo <your username>
This allows you to perform actions that require root priveledge by simply prepending the word sudo to the command. You may need to type your password to confirm your intentions.
Login with new user:
exit ssh <your username>@<your server ip>
Setup SSH Keys
SSH keys allow you to login to your server without a password. For this reason, you’ll want to set this up on your primary computer (definitely not a public or shared computer!). SSH keys are very convenient and don’t make your server any less secure.
If you’ve already generated SSH keys before (maybe for your GitHub account?), then you can skip the next step.
Generate SSH keys with the following command:
(NOTE: Be sure to run this on your local computer — not your server!)
ssh-keygen -t rsa -C "<your email address>"
When prompted, just accept the default locations for the keyfiles. Also, you’ll want to choose a nice, strong password for your key. If you’re on Mac, you can save the password in your keychain so you won’t have to type it in repeatedly.
Now you should have two keyfiles, one public and one private, in the ~/.ssh folder.
Copy the public key to server
Now, copy your public key to the server. This tells the server that it should allow anyone with your private key to access the server. This is why we set a password on the private key earlier.
From your local machine, run:
scp ~/.ssh/id_rsa.pub <your username>@<your server ip>:
On your Linode, run:
mkdir .ssh mv id_rsa.pub .ssh/authorized_keys chown -R <your username>:<your username> .ssh chmod 700 .ssh chmod 600 .ssh/authorized_keys
Disable remote root login and change the SSH port
ince all Ubuntu servers have a root user and most servers run SSH on port 22 (the default), criminals often try to guess the root password using automated attacks that try many thousands of passwords in a very short time. This is a common attack that nearly all servers will face.
We can make things substantially more difficult for automated attackers by preventing the root user from logging in over SSH and changing our SSH port to something less obvious. This will prevent the vast majority of automatic attacks.
Disable remote root login and change SSH port:
sudo nano /etc/ssh/sshd_config
Set “Port” to “44444” and “PermitRootLogin” to “no”. Save the file and restart the SSH service:
sudo service ssh restart
In this example, we changed the port to 44444. So, now to connect to the server, we need to run:
ssh <your username>@future.<your domain>.net -p 44444
Update: Someone posted this useful note about choosing an SSH port on Hacker News:
Make sure your SSH port is below 1024 (but still not 22). Reason being if your Linode is ever compromised a bad user may be able to crash sshd and run their own rogue sshd as a non root user since your original port is configured >1024. (More info here)
Add a firewall:
We’ll add an iptables firewall to the server that blocks all incoming and outgoing connections except for ones that we manually approve. This way, only the services we choose can communicate with the internet.
The firewall has no rules yet. Check it out:
sudo iptables -L
Setup firewall rules in a new file:
sudo nano /etc/iptables.firewall.rules
The following firewall rules will allow HTTP (80), HTTPS (443), SSH (44444), ping, and some other ports for testing. All other ports will be blocked.
Paste the following into /etc/iptables.firewall.rules:
*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 ports for testing -A INPUT -p tcp --dport 8080:8090 -j ACCEPT # Allow ports for MOSH (mobile shell) -A INPUT -p udp --dport 60000:61000 -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 44444 -j ACCEPT # Allow ping -A INPUT -p icmp -m icmp --icmp-type 8 -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
Activate the firewall rules now:
sudo iptables-restore < /etc/iptables.firewall.rules
Verify that the rules were installed correctly:
sudo iptables -L
Activate the firewall rules on startup:
sudo nano /etc/network/if-pre-up.d/firewall
Paste this into the /etc/network/if-pre-up.d/firewall file:
#!/bin/sh /sbin/iptables-restore < /etc/iptables.firewall.rules
Set the script permissions:
sudo chmod +x /etc/network/if-pre-up.d/firewall
I like to get an email anytime someone uses sudo. This way, I have a “paper trail” of sorts, in case anything bad happens to my server. I use a Gmail filter to file these away and only look at them occasionally.
Create a new file for the sudo settings:
sudo nano /etc/sudoers.d/my_sudoers
Add this to the file:
Defaults mail_always Defaults mailto="email@example.com"
Set permissions on the file:
sudo chmod 0440 /etc/sudoers.d/my_sudoers
This is isn’t mentioned anywhere on the web, as far as I know, but in order for the “mail on sudo use” feature to work, you need to install an MTA server. sendmail is a good choice:
sudo aptitude install sendmail
Now, you should get an email anytime someone uses sudo!