CSE 422S Operating Systems Organization: Class FAQ

What if your Pi is freezing up?

  1. How can I reduce the chances of my Pi freezing up?

    One potential cause of the Pi freezing up is adverse interactions between the kernel and device drivers for different peripherals connected to your Raspberry Pi. To mitigate this issue, one approach is to obtain the IP address of your Pi once it's booted up, unplug the mouse, keyboard, and HDMI cables, and interact with it through ssh (see further details below in the portion of this FAQ about remote access to your Pi).

    Another good approach is to reduce the amount of work that is done on the Pi itself, for example by building kernels through cross-compilation as is suggested in the first studio exercises.

  2. What if I need to use a graphical interface?

    In some cases, like when you need to run graphical tools on the Pi itself, you cannot simply unplug the Pi's peripherals. If you need to run graphical tools on the Pi but also need to access it remotely, you may want to at least disable screen saving and power saving behaviors that can trigger bad interactions with the kernel.

    To do that, first make backup copies of the following files: /etc/xdg/lxsession/LXDE/autostart and /etc/kbd/config (e.g., copy them to /etc/xdg/lxsession/LXDE/autostart.orig and /etc/kbd/config.orig, respectively).

    Edit /etc/xdg/lxsession/LXDE/autostart so that it appears as follows:

    @lxpanel --profile LXDE
    @pcmanfm --desktop --profile LXDE
    # @xscreensaver -no-splash

    @xset s noblank
    @xset s off
    @xset -dpms

    Then edit /etc/kbd/config so that it appears as follows:


    Save your changes to both files and reboot your computer, after which those new settings will be used.

    Please also see the second studio exercises for information about running your Raspberry Pi in a headless mode (without a monitor, keyboard, or mouse).

Accessing your Raspberry Pi remotely

  1. How do I get ssh access to my Pi?

    The default Raspberry Pi OS (previously known as Raspbian) distribution for the Raspberry Pi comes with an ssh server disabled. To enable it, click on the Raspberry icon on the top left corner of the desktop, select Preferences, and click Raspberry Pi Configuration. Then select the Interfaces tab and enable SSH. You may need to reboot to enable this change.

    The first thing to know is your Raspberry Pi's username, IP address, and hostname. By default, the username is pi and the hostname is raspberrypi. You can look up your IP address with the ifconfig or hostname -I command. You will have to change your hostname (because there are a lot of students with the default "raspberrypi", and your device will collide with theirs). Second, just plug your Pi into the network (e.g., logon to the WiFi) and it will automatically recieve the address (hostname).dhcp.wustl.edu.

    If you are connecting to your Pi that is sitting on the same network as you are (e.g., wustl-2.0), then depending on how your network is configured, you should be able to just ssh to pi@(hostname).dhcp.wustl.edu, or failing that, ssh to pi@(IP address).

    If you would like to set up your Pi at home for remote access and then access it from on campus, you'll need access to your internet router. The difficult thing about this is that your Pi is given one IP address by your home router, while your home is given a different IP address by your internet service provider. In order to make this work, we need to configure your router so that ssh requests are forwarded to your Pi. By default, ssh communications are sent over port 22, so we need to forward port 22 to your Pi. Go into your router's advanced settings and look for port forwarding (not port triggering, which is different). You will need to add a custom rule that forwards port 22 to the internal IP address of your Pi. Then, use your router's configuration screen to look up what external IP address it has been assigned by your internet service provider. In order to connect to your Pi while away from home, you should be able to ssh to pi@(external IP address).

    Note that if you are connecting to home, there is no guarantee that your internet service provider will assign you the same IP address for long periods of time. If this is the case, your external IP address may change frequently, making the above approach difficult. There are two possible solutions. First you can try calling your internet company and asking for a static IP address. If they say no, or if you would prefer a more robust method, you can see if your router supports a Dynamic DNS service. If so, there are many organizations that will give you a path (like david.noip.com) that will always map to your home, no matter how often your external IP address changes. There are organizations that will do this for free, and some router manufacturers (like Netgear) offer this as a free service if you use one of their routers.

  2. How do I get a remote graphical interface while leaving my Pi at home?

    First, go through the steps for setting up a terminal interface at home (above).

    A free, open-source solution for getting remote graphical access to your Pi is via VNC (Virtual Network Computing). You will need to install a VNC server on your PI, and get a VNC client (viewer) in order to connect to your server. An excellent open-source implementation of VNC is called TightVNC, which provides both a server and a client.

    The folks at raspberrypi.org have an excellent walk-through of how to install, configure, and automate the TightVNC server. This only takes a few minutes to install and configure. You can access it here. Note that if you plan on sitting down and using your Pi through a regular keyboard and monitor at home, in addition to connecting remotely, you probably don't want to disable the existing display :0

    Once you have that complete, you need to install the TightVNC viewer on the machine that you'll use to connect to your Pi. The link above has instructions if you're connecting from a Linux machine. If you're connecting from Windows, go to tightvnc.com and download their viewer from there. If you're connecting from Mac OS, or from any machine that you can't install software on, you can go to tightvnc.com and download their self-contained Java viewer (which of course requires Java to be installed).

    Finally, if you're connecting from outside of your house, you'll need to forward ports 5900-5902 on your home router to your Pi. (See previous answer for details.)

    When connecting with a viewer, you will connect to (external IP address)::(port number). The port number is determined (by default) to be 5900 plus the number of your display. So, if your VNC server is running display :0, then your port number is 5900. If your VNC server is running display :1, then your port number is 5901, etc.

Page updated Sunday, August 9, 2020, by Chris Gill.