These Debian articles will take you from a 'barebones' Debian Etch Slice to a secured and up to date Slice ready for your server software (or whatever you use the Slice for).
Not only that, you will have a better understanding of what is going on and, more importantly, why it's going on.
On your LOCAL computer, edit the SSH known_hosts file and remove any entries that point to your Slice IP address. If this is a brand new Slice then you will not need to do this, but a reinstall will result in a different signature.
If you are not using Linux on your LOCAL computer, the location of the known_hosts file will differ. Please refer to your own OS for details of where this file is kept.
As soon as you have your IP address and password for your VPS login via SSH:
Now we're logged in to the VPS, immediately change your root password
Add an admin user (I've used the name demo here but any name will do).
As you know we never log in as the root user (this initial setup is the only time you would need to log in as root). As such, the main administration user (demo) needs to have sudo (Super User) privileges so he can, with a password, complete administrative tasks.
Give the 'visudo' command:
At the end of the file add:
demo ALL=(ALL) ALL
One effective way of securing SSH access to your slice is to use a public/private key. This means that a 'public' key is placed on the server and the 'private' key is on our local workstation. This makes it impossible for someone to log in using just a password - they must have the private key.
This is very simple with ssh-copy-id.
We already have our admin user created (demo), so on your local workstation enter the command:
ssh-copy-id -i ~/.ssh/id_rsa.pub email@example.com
We use the -i option to specify which file (identity) to copy across to the slice. The user is then specified followed by the IP address of the slice.
So what happens when the command is entered? Firstly you will need to enter the user's password so it can have secure access to the slice. Then it creates a 'hidden' directory called .ssh and copies the public key to a file named 'authorized_keys'.
It then automatically changes the permissions so that only the owner (demo) can read or write to the file. Neat.
It's always a good idea to check the settings on something as important as this so let's have quick look at the permissions:
ls -al /home/demo/.ssh/authorized_keys ... -rw------- 1 demo demo 394 Sep 6 09:23 /home/demo/.ssh/authorized_keys
Perfect. You can also open the authorized_keys file and make sure only your key was copied across and it is not full of unknown keys.
Remember that this is the only time you'll need to enter the SSH password as the file we just copied over will authorise the admin user 'demo' to SSH in without it - but only if they have the private key on their local workstation: it won't work from any workstation.
Next we'll change the default SSH configuration to make it more secure:
Use can use this ssh configuration as an example.
The main things to change (or check) are:
Port 30000 <--- change to a port of your choosing Protocol 2 PermitRootLogin no PasswordAuthentication no X11Forwarding no UsePAM no UseDNS no AllowUsers demo
I think the setting are fairly self explanatory but the main thing is to move it from the default port of 22 to one of your choosing, turn off root logins and define which users can log in.
PasswordAuthentication has been turned off as we setup the public/private key earlier. Do note that if you intend to access your slice from different computers you may want leave PasswordAuthentication set to yes. Only use the private key if the local computer is secure
Right, now we have the basics of logging in and securing SSH done.
Next thing is to set up our iptables so we have a more secure installation. To start with, we're going to have three ports open: ssh, http and https.
We're going to create two files, /etc/iptables.test.rules and /etc/iptables.up.rules. The first is a temporary (test) set of rules and the second the 'permanent' set of rules (this is the one iptables will use when starting up after a reboot for example).
Note: We are still logged in as the root user. This is a new, or reinstalled slice, and it is the only time we will ever log in as root. If you are doing this later, when logged in as the admin user, you will need to enter the command:
The sudo -i command will place you as the 'root' user. You will need to do this when changing the iptables rules as it won't work by adding a standard 'sudo' in front of the commands.
So, as the root user, save any existing rules to /etc/iptables.up.rules:
iptables-save > /etc/iptables.up.rules
Now let's see what's running at the moment:
You will see something similar to this:
Chain INPUT (policy ACCEPT) target prot opt source destination Chain FORWARD (policy ACCEPT) target prot opt source destination Chain OUTPUT (policy ACCEPT) target prot opt source destination
As you can see, we are accepting anything from anyone on any port and allowing anything to happen.
One theory is that if there are no services running then it doesn't matter. I disagree. If connections to unused (and popular) ports are blocked or dropped, then the vast majority of script kiddies will move on to another machine where ports are accepting connections. It takes two minutes to set up a firewall - is it really worth not doing?
Let's assume you've decided that you want a firewall. Create the file /etc/iptables.test.rules and add some rules to it.
Look at this example iptables configuration file.
The rules are very simple and it is not designed to give you the ultimate firewall. It is a simple beginning.
Hopefully you will begin to see the pattern of the configuration file. It isn't complicated and is very flexible. You can change and add ports as you see fit.
Good. Defined your rules? Then lets apply those rules to our server:
iptables-restore < /etc/iptables.test.rules
Let's see if there is any difference:
Notice the change? (If there is no change in the output, you did something wrong. Try again from the start).
Have a look at the rules and see exactly what is being accepted, rejected and dropped. Once you are happy with the rules, it's time to save our rules permanently:
iptables-save > /etc/iptables.up.rules
Now we need to ensure that the iptables rules are applied when we reboot the server. At the moment, the changes will be lost and it will go back to allowing everything from everywhere.
Open the file /etc/network/interfaces
Add a single line (shown below) just after 'iface lo inet loopback':
... auto lo iface lo inet loopback pre-up iptables-restore < /etc/iptables.up.rules # The primary network interface ...
As you can see, this line will restore the iptables rules from the /etc/iptables.up.rules file. Simple but effective.
Now we have our basic firewall humming along and we've set the ssh configuration. Now we need to test it. Reload ssh so it uses the new ports and configurations:
Don't logout yet...
On your LOCAL computer, open a new terminal and log in using the administration user (in this case, demo) to the port number you configured in the sshd_config file:
ssh -p 30000 firstname.lastname@example.org
The reason we use a new terminal is that if you can't login you will still have the working connection to try and fix any errors.
Slicehost also has the excellent ajax console so if it all goes horribly wrong, you can log into your slice from the Slicehost management area.
If all goes well you should login without a password to a plain terminal:
We now know that the firewall and ssh_config works and we can log in.
Let's move on to page 2 which includes updating the install and installing some base programmes.