Tuesday, February 3, 2015

Secure your Raspberry Pi

Recently I examined my Raspberry Pi's logfiles because of all the cyber warfare going on and as I went trough the auth.log (/var/log/auth.log) looking for failed login attempts I saw a list of about 100 attempts from IPs trying to login as root or bin user.

$ cat /var/log/auth.log | grep 'sshd.*Failed'

This got my attention and my awareness, of having an unprotected SSH server running open to the worldwide net. I decided to pay a little bit more attention to the security of my Raspberry Pi and here I'll show you a few easy steps to make your Pi more secure:

Changing the pi standard user

First you have to kill all processes running under your current user. Therefore it would be best for you to log in via SSH as root and then you have to find out the id of your pi user:

$ sudo su
$ killall -u pi
$ id pi

uid=1000(pi) gid=1000(pi) groups=1000(pi), ...

next you have to change the login user, group and the home directory to the new user called newuser and copy the contents of the pi user's home to the home of the newuser, then delete the old directory

$ usermod -l newuser pi 
$ groupmod -n newuser pi 
$ usermod -d /home/newuser -m newuser 
$ usermod -c REALNAME newuser
$ cp -r /home/pi /home/newuser 
$ rm -r /home/pi

Now you should be able to log in as 'newuser', but to still be able to use sudo, you need to change the user in /etc/sudoers from pi to newuser. This is only possible with the tool "visudo":

$ visudo


Set a hard password

Most important and most easiest thing to do, is to choose a strong password. You can change your current password by typing

$ passwd

and then set your new, strong password. Strong passwords usually contain
  • upper and lower case characters
  • numbers
  • special characters
  • more then 8 characters in length
  • and so on and so forth
Here you have a nice command that will give you a random password directly on your Pi:

$ </dev/urandom tr -dc '12345!@#$%qwertQWERTasdfgASDFGzxcvbZXCVB' | head -c12; echo ""

Changing the SSH port

One important action to avoid crawler which randomly test for open ports, is to change the standard SSH port from 22 to something else. This can easily be done either
  • directly via your routers port forwarding rule    or
  • by changing the SSH daemon's listening port

Directly via the router

Depending on your routers configuration you have to change the port forwarding from port 22 on your router to a random port (say 1337), which is not used by another service. By using this alternative, you don't need to change your sshd config on the Pi, you just have to change add the port-number when ssh-ing to the pi. Your router redirects port 1337 to port 22 on your Pi, and everything works as usual

$ ssh pi@my.duckdns.org -p 1337

Here a short overview over various services and their standard ports.

Changing the daemon

If you want to change the listening-port of the daemon directly, you have to alter the sshd config.

$ sudo vi /etc/ssh/sshd_config

replace Port 22 with Port 1337

Keep in mind that you also need to change the port-forwarding on your router to the port you selected.
If you want to open a connection you have to specify the port:

$ ssh pi@my.duckdns.org -p 1337

For your convenience you can create a $HOME/.ssh/config file and add the following:

Host rpi
HostName my.duckdns.org
User newuser
Port 1337

This enables you to connect easily from this computer:

$ ssh rpi

Adapt the SSH config

There are a few more things you can do with the sshd_config to improve the security of your Raspberry. As we are curious, we want to increase the logging level of our SSH daemon by changing following parameter from

$ sudo vim /etc/ssh/sshd_config
LogLevel INFO



After this change, ALL details of the login attempts will be saved to the auth.log file.

Allow and deny specific users

One great feature is to only allow specific users to login via SSH. Therefore you just have to add or change following lines in the config:

$ sudo vim /etc/ssh/sshd_config
AllowUsers Peter Jango

This line tells the server to allow only Peter and Jango to sign in via SSH. The next line does exactly the opposite:

$ sudo vim /etc/ssh/sshd_config
DenyUsers Peter Jango

Peter and Jango are not allowed to sign in via SSH, everyone else is allowed.

If you want to personalize your SSH login, then you can take a look at this tutorial, showing you how to customize the SSH login.

Restrict root access

Another important option, is to disable root access via SSH. It is still possible to login as normal user and switch to root or to work with sudo only. Just change the option "PermitRootLogin" to no

$ sudo vim /etc/ssh/sshd_config
PermitRootLogin no

$ sudo /etc/init.d/ssh reload

Limit number of connections

If an IP address tries to open more than X connections in XX seconds, we can tell the server to drop this connections. This can be handled by the MaxStartups option and can significantly alleviate DDOS attacks.

MaxStartups 10:30:60 (start:rate:full)

10 is the number of unauthenticated connections before we start dropping connections by a chance of rate/100 (30 %). Once there are 60 unauthenticated connections, we drop every connection. Because the Pi is on the lower end of the performance scale and isn't used as server for many users, I suggest to insert smaller values for start and full.

$ sudo vim /etc/ssh/sshd_config
MaxStartups 2:50:10

It also makes sense to reduce the LoginGraceTime, this is the time the server keeps the connection alive while waiting for authorization.

$ sudo vim /etc/ssh/sshd_config
LoginGraceTime 20

Disable password authentification - SSH keys only

A weak password is the biggest problem on SSH servers, so best thing would be to use SSH keys instead of passwords, because this keys are complex enough to withstand an average attack and together with the other configuration described above should provide a certain amount of protection. But the use of SSH-keys also comes with disadvantages: You can not log in from a device without pre-approving. Keep that in mind! To disable password authentication set

$ sudo vim /etc/ssh/sshd_config
PasswordAuthentication no

You can find a short tutorial about ssh-key authentication here.


Fail2ban is a python based intrusion prevention tool, which scans logfiles like the auth.log and bans IPs that cause too many login errors by updating firewall rules (iptables) or TCP Wrappers (/etc/hosts.deny). It is very flexible, easy to use and has a lot of filters for various services like SSH, lighttpd, apache, vsftpd and so on.

You can download fail2ban on the fail2ban download page or simply install it on your raspbian by typing:

$ sudo apt-get install fail2ban

After installation is finished, we have to configure fail2ban, therefore go to the directory containing the config files (usually /etc/fail2ban/) and make a copy of the jail.conf with the name jail.local. Only change the parameters in the jail.local file, because changing the .conf file directly can cause errors on program updates and the parameters in the jail.conf get overwritten by the ones in the .local file. Here I will just talk about the basic options for SSH.

Look for the options regarding the SSH service and see if the filters are enabled and the logpath is set to the right place, usually /var/log/auth.log. I would recommend to set the maxretry parameter to 3 and the increase the time the IP is banned to e.g. 1800 (30 min). If you want to ban the IP forever you have to set a negative value for the bantime e.g. -1

$ sudo vi /etc/fail2ban/jail.local [...]
bantime = 1800

enabled  = true
port     = ssh
filter   = sshd
logpath  = /var/log/auth.log
maxretry = 3

You can find the filter rules in /etc/fail2ban/filter.d/ and you can create new ones for every service you like.
Find more information on www.fail2ban.org/wiki

I hope you found this useful and it will help you to stay secure!

Wednesday, January 28, 2015

How to bring back the "New Document -> Empty Document" menu in Gnome 3

Long time ago, the Gnome developers removed the right-click menu entry "New Document -> Empty Document" in Nautilus (the file browser). The solution to fix this is easy therefore it seemed not necessary to post it.
However, last week a friend told me that he hates Gnome 3 so much and as I asked why, one of the reasons was that this menu entry is gone.
So here is the solution (so that you start to love Gnome 3!):

  1. Create a folder called "Templates" directly into your $HOME directory if it does not exist
  2. Put a empty text file in there and name it "Textfile" (or anything else)
  3. Do a right click anywhere and be delighted with seeing "New Document -> Textfile"

The cool thing on this approach is that you can put any file with any content into the Templates directory.
Eg. you can put a template python script in there with the headers and scaffolding already in place, or a presentation with the company branding already applied... anything! Its universal!

Tuesday, January 20, 2015

Create a custom sandboxed webapp in Firefox

Google, Facebook, Twitter and others depend on getting as much information about you as possible and therefore use your account information to track and profile you not only on their services but also on 3rd party websites to fit the advertisement to your interests.
On this website you can check out how unique your browser is and how easy it is to identify and track you.
To make this a bit harder for them and save yourself the trouble of constantly logging in and out I will show how you can create a webapp with an independent profile for your Linux desktop.

It is very easy to create a custom launcher for each webapp and Firefox allows to create multiple profiles since a very long time. So in principle it is very simple and easy!

Let's do a webapp for Facebook! First, create a new Firefox profile, which will be used for the "Facebook-only-Firefox". Therefore, start the Firefox profile manager by typing:

$ firefox -ProfileManager

Now add a new profil 'facebook' and close the manager again. Next, create the launcher by copying the original Firefox launcher to the $HOME directory and rename it:

$ cp /usr/local/share/applications/firefox.desktop ~/.local/share/applications/facebook.desktop

Then we edit the facebook.desktop file and set the launcher name, the icon and the process how the application will be started. I suggest to get a nice icon for your application from the web and save it to ~/.icons/facebook.png. If the folder does not exist yet, create it!

$ vim ~/.local/share/applications/facebook.desktop

Exec=firefox -P facebook -new-instance www.facebook.com
-P profile-name opens firefox with profile "profile-name"
-new-instance opens Firefox in a new instance, as a separate process
Icon= If the icon is stored under ~/.icons/ there is no need to enter a path!

To immediately see you application in gnome-shell you can press Alt + F2 and type "r" and ENTER. This reloads the gnome-shell. Now you can use Facebook in its own browser, but don't forget to log out in your everyday browser ;)
To hide the tabs and therefore save screen space I recommend the Hide Tab Bar With One Tab addon.

There is one tiny problem you have to consider and which I didn't find a work around yet:
If you start the Facebook webapp and want to open your Standard-Firefox afterwards, it doesn't select the right profile. You could either change the firefox.desktop by copying to it to your $HOME as above and adding the default profile to the Exec line:

$ cp /usr/local/share/applications/firefox.desktop ~/.local/share/applications/firefox.desktop
$ vim ~/.local/share/applications/facebook.desktop

Exec=firefox -P default -new-instance
or you have to start the default firefox first and then open the webapp. Do you know a solution to this?
Happy socializing 2.0!

Mozilla develops a webapp framework for their Marketplace that works in a similar fashion like we show here: Separate profile directory, minimal UI, own launcher,... The only difference is that they also create a separate binary and therefore do not have the multi-instance problem. A third-party developer (dietrich ayala) created an extension, "Standalone" which uses this framework to create webapps from any page on the web. Awesome!

Thursday, January 8, 2015

Customize SSH login

After you have set up a secure  SSH server maybe you would like to customize your login, to look a bit more personal. e.g. like the penrose triangle

     ____  ____  ____  ____                      ______________________
    /\   \/\   \/\   \/\   \                    /\                     \
   /  \___\ \___\ \___\ \___\                  /  \    _________________\
   \  / __/_/   / /   / /   /                  \   \   \                /
    \/_/\   \__/\/___/\/___/                    \   \   \__________    /
      /  \___\    /  \___\                       \   \   \    /   /   /
      \  / __/_  _\  /   /                        \   \   \  /   /   /
       \/_/\   \/\ \/___/                          \   \   \/   /   /
         /  \__/  \___\                             \   \  /   /   /
         \  / _\  /   /                              \   \/   /   /
          \/_/\ \/___/                                \      /   /
            /  \___\                                   \    /   /
            \  /   /                                    \  /   /
             \/___/                                      \/___/

 (source: wikipedia)

You can also create an ASCII Art from any picture. There are some sites where you can upload your picture and create an ASCII pic out of it, look here. Once you connect via SSH e.g.  > ssh user@myraspberry.pi you see the welcome text or logo, which is defined in your sshd config. First you have to create the file containing the hello message, e.g.  

 $ sudo vim /etc/ssh/welcome.msg  
     ____  ____  ____  ____                   |
    /\   \/\   \/\   \/\   \                  |
   /  \___\ \___\ \___\ \___\                 |
   \  / __/_/   / /   / /   /                 |
    \/_/\   \__/\/___/\/___/                  |
      /  \___\    /  \___\                    |
      \  / __/_  _\  /   /                    |
       \/_/\   \/\ \/___/                     |
         /  \__/  \___\                       |
         \  / _\  /   /                       |
          \/_/\ \/___/                        |
            /  \___\                          |
            \  /   /                          |
             \/___/                           |

In the next step, we have to tell our ssh daemon where to find this welcome message, therefore we have to change the 'Banner' entry in the sshd config to the location of our welcome message:

$ sudo vim /etc/ssh/sshd_config 
Banner /etc/ssh/welcome.msg

After that, we have to restart sshd:

$ sudo service sshd restart

We can also add another welcome message for the users, which are already authenticated by editing the MOTD file:  

$ sudo vim /etc/motd 

Once finished, we again have to restart the ssh daemon on the raspberry and then we can try this thing out. On connecting to our pi we should now see the pi ASCII logo and after entering the password, we should be informed about the news in our motd file.