Ubuntu Intrepid setup - page 1

These Ubuntu Intrepid Ibex articles will take you from a 'barebones' Ubuntu Intrepid Slice to a secured and up to date Slice ready for your server software (or whatever you use the Slice for).

Securing your Slice as soon as possible is a great way of starting your Slice administration.

Log in

As soon as you have your IP address and password for your Slice, login via SSH:

ssh root@123.45.67.890

If you rebuilt your Slice, you may get a message informing you that the "remote host identification has changed".

When you log into a Slice via SSH, one of the security features is matching the remote host with known keys. When you rebuild a Slice, the remote host key changes. As such, your computer thinks there is something dodgy going on.

All you need to do is remove the older entry for the Slice IP:

On your LOCAL computer, edit the SSH known_hosts file and remove any entries that point to your Slice IP address.

nano ~/.ssh/known_hosts

If you are not using Linux or a Mac 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.

User administration

Once logged in to the VPS, immediately change your root password to one of your choosing.

passwd

Add an admin user (I've used the name demo here but any name will do).

adduser demo

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 they can, with a password, complete administrative tasks.

To configure this, give the 'visudo' command, which invokes the 'nano' editor by default on Ubuntu Intrepid slices:

visudo

At the end of the file add:

demo   ALL=(ALL) ALL

When you are finished, press the key combination Ctrl-X to exit, press 'y' to confirm your saving the changes, and press the return key to save as the indicated file, '/etc/sudoers.tmp' .

NOTE: some users will find that while working in the nano editor, the backspace/delete key works backwards, deleting characters in front of the cursor rather than behind it. This can be resolved by editing the '/etc/nanorc' file (with nano, for example) and either uncommenting or adding the line:

set rebinddelete

The corrected behavior will take effect after the file has been saved and nano has been opened again.

SSH keygen

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.

The first step is to create a folder to hold your keys. On your LOCAL workstation:

mkdir ~/.ssh

That's assuming you use Linux or a Mac and the folder does not exist. I will do a separate article for key generation using Putty for Windows users.

To create the ssh keys, on your LOCAL workstation enter:

ssh-keygen -t rsa

If you do not want a passphrase then just press enter when prompted.

That created two files in the .ssh directory: id_rsa and id_rsa.pub. The pub file holds the public key. This is the file that is placed on the Slice.

The other file is your private key. Never show, give away or keep this file on a public computer.

SSH copy

Now we need to get the public key file onto the Slice.

We'll use the 'scp' (secure copy) command for this as it is an easy and secure means of transferring files.

Still on your LOCAL workstation enter this command:

scp ~/.ssh/id_rsa.pub demo@123.45.67.890:/home/demo/

When prompted, enter the demo user password.

Change the IP address to your slice and the location to your admin user's home directory (remember the admin user in this example is called demo).

SSH Permissions

OK, so now we've created the public/private keys and we've copied the public key onto the Slice.

Now we need to sort out a few permissions for the ssh key.

On your Slice, create a directory called .ssh in the 'demo' user's home folder and move the pub key into it.

mkdir /home/demo/.ssh
mv /home/demo/id_rsa.pub /home/demo/.ssh/authorized_keys

Now we can set the correct permissions on the key:

chown -R demo:demo /home/demo/.ssh
chmod 700 /home/demo/.ssh
chmod 600 /home/demo/.ssh/authorized_keys

Again, change the 'demo' user and group to your admin user and group.

Done. It may seem a long set of steps but once you have done it once you can see the order of things: create the key on your local workstation, copy the public key to the Slice and set the correct permissions for the key.

SSH config

Next we'll change the default SSH configuration to make it more secure:

nano /etc/ssh/sshd_config

Use can use this ssh configuration as an example.

The main things to change, check, and add 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.

NOTE: the port number can readily be any integer between 1025 and 65536 (inclusive), but should be noted for reference later when any additional listening processes are setup, as it will be important to avoid conflicts.

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 (i.e. don't put the private key on a work computer).

Note that we haven't enabled the new settings - we will restart SSH in a moment but first we need to create a simple firewall using iptables.

iptables

As said, the 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 that we are logged in as the root user. This is the only time we will log in as the root user. As such, if you are completing this step at a later date using the admin user, you will need to put a 'sudo' in front of the commands.

Now let's see what's running at the moment:

iptables -L

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; if you or another admin user for this slice has worked through these or similar steps previously, this file may not be empty:

nano /etc/iptables.test.rules

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. Don't forget to change the port number '30000' to whatever you specified in sshd_config.

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:

iptables -L

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

nano /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.

Logging in as the new user

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:

/etc/init.d/ssh reload

Don't logout yet...

As you have an already established connection you will not be locked out of your ssh session (look at the iptables config file: it accepts already established connections).

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 demo@123.45.67.890

Change '30000' to whatever value you specified in sshd_config and the iptables rules.

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:

Success!

We now know that the firewall and ssh_config works and we can log in.

Let's move on to page 2 of the Intrepid install which includes updating and installing some base programmes.

Mike