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Sysadmin DeCal

Lab 4 - Processes and Services

Facilitator: Oliver Ni

10 min read

Table of contents

  1. Overview
  2. Part 0: Set up networking
  3. Part 1: Using systemd
    1. What services are running right now?
    2. Controlling Services
    3. Creating a service
    4. Debugging
    5. Crash the service!
  4. Part 2: Processes
    1. htop
    2. The process hierarchy
    3. Orphan processes
  5. Exploration
  6. Submission

Overview

For this lab, we are going to dive into processes and systemd. We will do this by writing our own systemd service from scratch, while showing the benefits of running a service with systemd. This lab should be completed on your Linux VM.

Part 0: Set up networking

Before you start this lab, you’ll need to make sure you can access services from your VM in your web browser!

Our VMs support IPv6 only, so you will need to connect to the campus GlobalProtect VPN first if you don’t have IPv6 connectivity.

Part 1: Using systemd

What services are running right now?

Run systemctl. You’ll see a long table of every unit known to systemd. Let’s narrow it down to services for now. Run systemctl --type=service. Now you can see a list of all services running on your computer. Each of these services is a daemon running in the background. Do you see any familiar services running?

Question 1: What is the name of a systemd service running on your system? What does it do? (If you’re not sure what it does, Google it!)

Controlling Services

Now let’s use systemd to control a an nginx web server. If you don’t have it already, install nginx by issuing sudo apt install nginx. Once that is done we can tell systemd to start the service with the following: sudo systemctl start nginx. Run systemctl status nginx to ensure it is running.

Note: If you already have a webserver running, you may need to shut it down, so that port 80 is available for nginx to use.

Now let’s make nginx listen for connections on the nonstandard port 420. In /etc/nginx/sites-available/default change the following lines:

listen 80 default_server;
listen [::]:80 default_server;

to:

listen 420 default_server;
listen [::]:420 default_server;

TIP: The first line configures the server to listen on IPv4, and the second line configures IPv6.

Tell systemd that nginx has changed configuration and needs reloading with: sudo systemctl reload nginx.

Now, accessing http://[yourusername].decal.ocfhosted.com:80 should now give you a connection refused error and your webserver will only be accessible via http://[yourusername].decal.ocfhosted.com:420.

Note that not all services can be reloaded; systemd will notify you if this is the case and such services will have to be restarted instead with: sudo systemctl restart yourservice.

Finally go ahead and stop the nginx service with sudo systemctl stop nginx.

Question 2: What is the difference between systemctl reload yourservice and systemctl restart yourservice?

Question 3: Upload a screenshot of your browser accessing the nginx webserver at http://[yourusername].decal.ocfhosted.com:420. Note: If you can’t access the IPv6 site use curl localhost:420 on the VM and paste it’s contents (it should be a html page).

Creating a service

Let’s set up a web server and create a systemd unit for it. Make sure git is installed; if it’s not, install it using apt.

If you don’t already have the decal-labs repo from a past lab, run the following:

$ git clone https://github.com/0xcf/decal-labs

The materials for this part of the lab will be in the decal-labs/4 directory. We will also need to install some dependencies. Go ahead and execute the following commands:

# apt update
# apt install build-essential make python3-virtualenv

Now run ./run. This should start up a simple web server at http://localhost:5000. Note that by default you can only access the web server on the VM itself.

Your mission, should you choose to accept it, is to write a systemd service that manages this web server. To do this, make a new unit file in /etc/systemd/system/toy.service. Refer to the slides for an example; DigitalOcean also has a good guide on how to write systemd units. Here is a skeleton; all you need to do is fill in the values for each field.

[Unit]
Description=
Requires=
After=

[Install]
WantedBy=multi-user.target

[Service]
ExecStart=
User=

Some questions worth considering while writing this unit file are:

  • What units needs to be started before a webserver starts? (Hint: you can get a list of special “target” units using systemctl --type=target.)
  • What script should systemd run to start the webserver? (Remember you’ll need to use the absolute path of the script, not the relative one. You can find this by running realpath -se <path to script>.)
  • Units run by root as default. Is that a safe practice for web servers?

You are encouraged to experiment with other fields as suits your liking. Once you have finished creating toy.service, let’s start the service and have the it start whenever our machine is booted.

# systemctl start toy.service
# systemctl enable toy.service

Debugging

You can check if the unit file succeeded by running systemctl status toy.service. If you are having issues with the unit file or the web server, check the logs for this unit by running journalctl -u toy.service. If you run into errors don’t get demoralized (it is, after all, only a decal); as a sysadmin you’ll have to become comfortable making sense of arcane error messages.

TIP: You can omit the .service in systemctl command for speed. If the unit is another type (e.g. target, socket, or timer), you must include the type. We include the .service for clarity.

Crash the service!

One of the great benefits of using systemd to manage your services is that you don’t have to worry unnecessarily about bringing a process back up if it crashes. So let’s crash the service! You can do this by either sending a POST request with the json payload {"crash":"true"} to http://localhost:5000/crash on the VM (Hint: use cURL) or by killing the webserver manually by sending a signal – both will cause the unit to crash. You can verify if you succeeded by running systemctl status toy.service, and the unit should either be in an inactive or failed state, depending on how you killed it.

Question 4: What command did you run to crash the service?

Now add the following the /etc/systemd/system/toy.service under the Service directive:

Restart=always
RestartSec=10

To tell systemd that the unit file has changed run sudo systemctl daemon-reload. Now start your webserver and crash it again in any way you please, and you should see that it come back online after 10 seconds! Note that you can also run daemon-reload and change a unit file while a service is running.

Question 5: Upload your fully featured toy.service file to Gradescope.

Part 2: Processes

There are no Gradescope questions to answer for this section, but you should still go through the steps to make sure you understand processes and how to use htop!

Open up a terminal and run the ps command. You should see something like this:

  PID TTY          TIME CMD
 3371 pts/2    00:00:00 zsh
 3416 pts/2    00:00:00 ps

Now open up another terminal and run sleep 1000 &, which start a sleeping process in the background. Then run ps. It should look like:

~
❯ sleep 100 &
[1] 3726

~
❯ ps
  PID TTY          TIME CMD
 3371 pts/2    00:00:00 zsh
 3726 pts/2    00:00:00 sleep
 3752 pts/2    00:00:00 ps

In the first terminal run ps again. You should notice that the sleep process is not showing up, even though the thousand seconds haven’t expired.

Why do you think this behavior occurs (hint: TTY column)?

We can get the process to display on the first terminal by running ps -u, which displays all the processes running as your user. Notice the PID column; each process has a unique ID assigned to it by the kernel. One thing we can do with this PID is send signals to the process. sleep 1000 is pretty useless, so go ahead and kill it – kill 3726 (substitute 3726 with whatever PID ps outputted for you).

The most common use of ps is to run ps -ef to see all the processes running on the system. Run ps -e and ps -f independently to see how the flags work together.

htop

Make sure htop is installed by running sudo apt install htop. Now, open up a terminal and run the htop command. htop can be thought of as a more extensive version of ps -ef, whereby process stats are updated in real-time.

First press <F2>, scroll down to Display options, and check “Hide userland process threads.” We won’t be dealing with those in this lab.

Now open up another terminal (or use tmux). Run the command yes. It uses a lot of resources as it prints a continuous stream of y’s.

What resource specifically does the yes command exhaust? If you are having trouble finding this, press < to choose which resource to order processes by. Make sure to quit out of yes (^C) once you are finished.

The process hierarchy

Run htop once more. This time click <F5> to enter Tree View. You should see a visual representation of the process hierarchy on your system, with everything stemming from /sbin/init (systemd).

For curious students that are interested in seeing a more extensive process hierarchy on a large system, you are encouraged to run htop on the OCF server tsunami. Let us know of any cool processes that you find!

Orphan processes

Open a second terminal and ssh to your VM. Now run sleep 1000 &. You should see this new process pop into your htop session on your first terminal. If not, press <F3> and search for “sleep.” What is its parent?

Select this parent and press <F9> to kill it. Send the SIGTERM signal. The sleep process now has init as its new parent, which is PID 1. What you just did is manually orphan a process; when that happens said process is subsequently re-parented by the init process.

Now go through the same steps again. This time, send the parent a SIGHUP (hangup) signal. Can you still find the sleep process? When SIGHUP is sent to a parent shell, the parent subsequently sends hangup signals to any child processes before terminating; all processes that receive SIGHUP from a parent shell will terminate – this is one way to avoid creating orphan processes.

If you are interested in learning about the different signals, run man 7 signal. Note that you can run man man for an explanation about the different manual section numbers.

Exploration

Congratulations, you have completed the lab! This is just the tip of the iceberg when it comes to processes and services. If you want to learn more, here are some related topics you can look into.

Submission

Go to Gradescope to submit your answers!