Your Very Own Remote Linux Box
Every time I installed a fresh linux box, I would find myself looking up the same series of commands over and over to make the newly spawned machine distinct and securely accessible on the network.
Then I7600 IoT Design Lab at CCNY incentivized me to write this tutorial
for academic credit.
The instructions below are typeset as I am handling ubuntu 20.04 64bit server installation on a raspberry pi, though this tutorial should work with other devices and linux flavors.
The only requirement is a freshly installed linux box (further referred as remote) that we are about to boot into and a wired connection to a DHCP network, so that we can find our box on the network from the computer we are about to ssh from (further referred as local). This tutorial leaves the process of linux installation up to you.
2. First Boot
Our device is powered on, let’s map it on the network.
2.1 You own the network.
In case you own the network and have access the router/network switch you can
look up the ip address of the device on your NAT. Ubuntu device will be called
ubuntu by default. Keep in mind that the ip address will change when DHCP
lease time is over, most of the routers have an option to make the ip address
2.2 The network isn’t yours
This might get tricky since you can’t simply determine the ip of your newly installed ubuntu box. First you have to be sure that the network switch you are connecting to is DHCP (the ip address can be obtained automatically), you could poke the ethernet port with your laptop to test.
Now you’d like to determine the ip address of your box. You could run a
network scan with
nmap to discover devices on the subnet.
nmap -sn 192.168.1.0/24 # if your ip is 192.168.1.x
nmap resolved hostnames (you’re lucky), just look for
the list. Otherwise, you might have to unplug your device’s ethernet and do
another scan to see which device disappeared/appeared.
3. Changing defaults
Once you have determined the ip, it is time to remotely login via ssh.
ssh email@example.com # Relace the ip of your device.
You will be prompted to enter the default password (it’s ubuntu in case you are working with ubuntu).
Next, we would like to change the device hostname (as it appears on the network) and username, which would require log in as a different user. We will login as root. Alternatively we could create another user account and then delete the default one. In this tutorial I will use the root account.
Set root password.
sudo passwd root
sshd_config and allow login as root by setting
sudo nano /etc/ssh/sshd_config # Open sshd_config
Restart ssh service.
service sshd restart
3.1 Change hostname
Login as root.
ssh firstname.lastname@example.org # Relace the ip of your device.
Hostname is a string that device uses to self identify on the network, it is
/etc/hostname. If you read the file you can see the default “ubuntu”
My device is a raspberry pi 3b, and I shall set hostname accordingly (so I can recognize it from the other devices on my local network).
echo "raspi3b-alpha" > /etc/hostname
3.2 Change username
Rename the default ubuntu user.
usermod -l my_username -d /home/my_username -m ubuntu
4. Public key authentication
If you don’t have a key pair associated with your local machine, create one.
cd ~/.ssh # SSH keys are normally stored in the .ssh directory. ssh-keygen -t ed25519
You will be prompted to enter the key name and protect it with a password
(up to you). This will generate
your-key, public and
private keys accordingly. Public key is the one you will share with remotes,
private key should stay safe and secret on you local computer.
Share public key with the remote.
scp your-key.pub email@example.com:~/.ssh/. # Mind the ip.
4.1 Secure the remote
Login to the remote.
your-key.pub to be a login credential on the remote.
cat ~/.ssh/your-key.pub >> ~/.ssh/authorized_keys
Configure ssh by finding and setting the following in the
PermitRootLogin no # disable root login PubkeyAuthentication yes # enable public key authentication. PasswordAuthentication no # disable password authentication.
Hint: press ctrl+w and type something to search.
sudo nano /etc/ssh/sshd_config # open sshd_config with nano
Finally, restart ssh to apply the changes.
service sshd restart
Almost forgot, lock the root user.
sudo passwd -l root
Attempt to log in using public key from your local machine. Notice, flag
-i is pointing to the private key. Attempting to log in without passing the
public key should now fail.
ssh firstname.lastname@example.org -i ~/.ssh/your-key
5. Tips and tricks
5.1. SSH config
Imagine you would like to manage multiple devices from your local machine.
Remembering all ip addresses specifying path to the appropriate keys can get
annoying quickly. You could add the following directive to you local
Host raspi3b.local # Name of the entry HostName 192.168.0.123 # This can also be a dns name like `my-domain.com`. User my_username # Your username goes here. IdentityFile ~/.ssh/your-key # Path to your private key.
This way you could just type the following to login.
In case you’re wondering the
.local is just my way of indication that I would
like to login via the local network. You could set the name to anything.
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