Customizing the Hostname of a Raspberry Pi - Featuring ChatGPT

It was on my TODO list since a long time: For my GitHub Actions runner setup I need unique hostnames per Raspberry Pi so that I can scale up to multiple runners. Of course - you know me by now - the setup shall be fully automatic and the hostname shall get assigned without any human interaction during the first boot. A unique identifier of the Raspberry Pi is its MAC address of the ethernet interface. So my fleet shall be organized like the following picture:

Doing the setup for an automatic hostname assignment is not rocket science, but there are quite a few pitfalls. Let’s see how we tackle the job together with ChatGPT from OpenAI:

Chat

Me: How can I change the hostname on a Debian system on the first boot?

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ChatGPT: One way to change the hostname on a Debian system during the first boot is to use the “preseed” configuration file. This file can be used to automate the installation process and set various system parameters, including the hostname.

You can create a preseed file with the desired hostname and include it on the installation media or make it available over the network. The installer will use the settings in the preseed file during the installation process.

Another way is to use the “hostnamectl” command after the system is booted up. The hostnamectl command can be used to change the hostname of the system.

sudo hostnamectl set-hostname new_hostname

You can also edit the /etc/hostname file and change the hostname in the first line of the file.

sudo nano /etc/hostname

Then reboot your system for the changes to take effect.

Assessment: Amazing - with a very simple question I already got a helpful answer. ChatGPT lists two possible approaches on how to change the hostname. The response is understandable and - to be honest - ChatGPT knows the English language better than me. Let’s get a bit more specific with the requirements:

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Me: How can I automate this using a systemd service?

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ChatGPT: You can create a systemd service to automatically set the hostname on boot. First, create a new file in the /etc/systemd/system directory called sethostname.service with the following contents:

[Unit]
Description=Set Hostname

[Service]
Type=oneshot
ExecStart=/bin/bash -c "echo new_hostname > /etc/hostname; hostnamectl set-hostname new_hostname"

[Install]
WantedBy=multi-user.target

Make sure to replace “new_hostname” with the desired hostname.

Then, you need to reload the systemd daemon to recognize the new service:

sudo systemctl daemon-reload

Enable the service to run at boot time:

sudo systemctl enable sethostname.service

You can check the status of the service with:

sudo systemctl status sethostname.service

Finally, you need to reboot the system to apply the hostname changes made by the service.

This service will run at boot time and set the hostname of your Debian system to the desired value specified in the service file.

Assessment: The proposed approach looks promising! ChatGPT even gives me some hints on how I can debug the setup.

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Me: Sure - let me try this!

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ChatGPT: Sure! I’m glad I could help. Let me know if you have any other questions or run into any issues while setting up the service.

Assessment: Very polite. I am glad that ChatGPT offered me some additional help. Indeed, the above setup did change the hostname but since I deployed this new service there seems to be a glitch when using sudo:

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Me: Great - it worked! But now I have a small problem: Whenever I execute a sudo command, I get the following error: “sudo: unable to resolve host newhostname: Name or service not known”

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ChatGPT: This error is caused because the hostname is not properly set in the /etc/hosts file. When you set the hostname with the hostnamectl command or the /etc/hostname file, the hostname is only set for the running system, but it is not set in the /etc/hosts file.

To resolve this issue, you need to update the /etc/hosts file to include the new hostname. You can do this by running the following command as root:

echo "127.0.0.1 localhost new_hostname" >> /etc/hosts

Make sure to replace “new_hostname” with the hostname you want to set.

You should also include the hostname in the first line of the /etc/hosts file if it’s not already there

127.0.0.1       localhost new_hostname

You can also use the sed command to update the /etc/hosts file.

sudo sed -i "s/^127.0.0.1.*/127.0.0.1\tlocalhost\t$(hostname)/" /etc/hosts

After updating the /etc/hosts file, you may need to log out and log back in or reboot the system for the changes to take effect.

This should resolve the issue and you should no longer receive the “unable to resolve host” error message when running sudo command.

Assessment: World class - ChatGPT immediately points into the right direction. The above sed command shall not blindly be applied as the Debian hosts file looks slightly different from what ChatGPT expects. After intelligently following the above advice the sudo problem is gone.

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Me: Perfect - this does the trick! However, I am a bit worried that the above systemd service does kick in a bit too late. Can we somehow make sure that the service starts early during bootup?

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ChatGPT: Yes, it’s possible to make sure that the sethostname.service starts early during the boot process by modifying the WantedBy parameter in the sethostname.service file.

The WantedBy parameter in the [Install] section of the service file determines when the service should be started. By default, it is set to multi-user.target, which means that the service will start after the system has reached the multi-user target state, which is relatively late in the boot process.

If you want to make sure that the service starts earlier in the boot process, you can set the WantedBy parameter to basic.target or sysinit.target.

basic.target: This is the default target when system boots up and most of the basic services are started.

sysinit.target: This is the first target that is reached during boot process, and it is responsible for basic initialization of the system.

So you can change the WantedBy parameter in the sethostname.service file to sysinit.target

[Install]
WantedBy=sysinit.target

Then, you need to reload the systemd daemon and enable the service again

sudo systemctl daemon-reload
sudo systemctl enable sethostname.service

Keep in mind that setting the service to start at sysinit.target it will be started before most of the basic services. So it’s important to make sure that the service is not dependent on any other service that may not be started yet.

Please also note that changing the WantedBy parameter to sysinit.target may not work in all systems, and it may have an impact on the boot process, so you should test it before deploying it in a production environment.

Assessment: ChatGPT takes my concerns seriously and offers additional help. The ordering of systemd services is a bit more involved than what ChatGPT thinks it to be. In fact, editing just the [Install] section does not do the trick as some additional statements are needed in the [Unit] section too to get things done. I will give more details further down.

Early during boot, not all services are available yet. hostnamectl will fail because of the missing udev service. Let’s ask ChatGPT for some more help:

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Me: hostenamectl set-hostname requires udev. I guess that before sysinit.target we do not have udev. How can I get around this issue?

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ChatGPT: You are correct that the hostnamectl set-hostname command requires udev to be running, and udev is typically started after the sysinit.target.

One way to work around this issue is to use a simple bash script that sets the hostname by directly editing the /etc/hostname file, instead of using the hostnamectl set-hostname command.

You can create a script named set-hostname.sh with the following contents:

#!/bin/bash
echo "new_hostname" > /etc/hostname

Make sure to replace “new_hostname” with the desired hostname.

Then, you can modify the sethostname.service file to run this script on boot

[Unit]
Description=Set Hostname

[Service]
Type=oneshot
ExecStart=/usr/bin/bash /path/to/set-hostname.sh

[Install]
WantedBy=sysinit.target

You can also use the sed command in the script to update the /etc/hosts file.

#!/bin/bash
echo "new_hostname" > /etc/hostname
sed -i "s/^127.0.0.1.*/127.0.0.1\tlocalhost\t$(hostname)/" /etc/hosts

After that, you need to make the script executable by running the following command:

sudo chmod +x /path/to/set-hostname.sh

Then, you need to reload the systemd daemon, enable the service, and reboot the system to apply the changes

sudo systemctl daemon-reload
sudo systemctl enable sethostname.service
sudo reboot

This way you are not relying on udev to set the hostname and you can set it early on the boot process. Please keep in mind that this method may have some limitations and you may need to test it before deploying it in a production environment.

Assessment: Fascinating - we get closer and closer to a good solution. I am thrilled by the experience and I would like to write about it on my blog:

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Me: Am I allowed to post this chat on my blog post?

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ChatGPT: As a language model AI, I do not have personal rights, but the contents generated by me are property of OpenAI. You would need to check OpenAI’s terms and conditions, and their API use policy. However, most likely they would allow you to post a chat log on your blog post as long as you give proper credit and a link to OpenAI website and mention that the answers were generated by OpenAI’s GPT model.

Assessment: I am not a lawyer but still I checked OpenAIs terms of use. As requested, the above answers are clearly attributed to ChatGPT. I am under the impression that OpenAI is OK with me posting this chat.

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Me: Many thanks for your help!

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ChatGPT: You’re welcome! I’m glad I could help. If you have any more questions or need further assistance, feel free to ask.

Conclusion

A great compliment to the engineers and visionary leadership behind ChatGPT! What I have seen so far is way above my expectations. The answers provided by ChatGPT are at least as good as the answers I would get from a junior engineer - the response time is just much faster.

The challenge I gave to ChatGPT is not trivial. I thought a bit further about it and based on the systemd-firstboot setup I came up with the following solution for the service file:

[Unit]
Description=Set Hostname on First Boot
DefaultDependencies=no
Conflicts=shutdown.target
After=systemd-remount-fs.service
Before=sysinit.target first-boot-complete.target shutdown.target
Wants=first-boot-complete.target
ConditionPathIsReadWrite=/etc
ConditionFirstBoot=yes

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/usr/bin/edi-set-hostname

[Install]
WantedBy=sysinit.target

The edi-set-hostname script does in essence the following:

#!/bin/bash

...

OLD_HOSTNAME=$(cat /etc/hostname)
HOSTNAME_SUFFIX="$("${IDENTITY_SCRIPT}" | cut -d= -f2 | sed 's/://g')"
# take care of idempotency by not adding the suffix twice
NEW_HOSTNAME="${OLD_HOSTNAME%-${HOSTNAME_SUFFIX}}-${HOSTNAME_SUFFIX}"

# static hostname
echo "${NEW_HOSTNAME}" > /etc/hostname
# transient hostname
hostname "${NEW_HOSTNAME}"
sed -i "s/^127.0.1.1.*/127.0.1.1   ${NEW_HOSTNAME}/" /etc/hosts

The above service is part of the upcoming edi-pi Debian bookworm setup!

Later during the evening I asked ChatGPT for some advice on baking a “pot bread”. This time I expressed my questions in German and despite the language and topic change the answers remained very solid. Unfortunately it got too late to test out the recipe.