https://bernsteinbear.com/blog/wakeonlan/
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Waking my computer from afar
March 15, 2024
I have a desktop computer at home with a decent CPU and GPU and RAM
and all that. I use it when I need to do Beefy Processing Things like
compiling LLVM or when I want to save battery on my laptop. I like
leaving it in hibernation mode to save power, but sometimes I forget
to wake it up on a day that I might need it from the library or
something. For that, I need to be able to wake it up remotely.
wakeonlan
Fortunately, some smart folks thought of this years ago and came up
with Wake on LAN. There are tools such as etherwake and wakeonlan
that can send a magic packet to the computer's network controller and
wake up a sleeping machine. That's fantastic, except:
* Sometimes I leave the house, which (because of NAT) means I
cannot easily send the computer a packet. I could probably find a
way to get my router to notice the packet coming from WAN and
forward it appropriately, but... eh^1.
* I could probably SSH into my router and send the appropriate
packet, but installing tools on my router, even though it's
running OpenWRT, is kind of irritating.
So I need to find a way to get the packet to come from inside the
house and not from my router. Sounds like a great excuse to buy a
low-power computer and use that as a bastion host. I decided to buy a
Raspberry Pi Zero 2 W, but that's because it was reasonably priced
and available with a short walk.
A very small server
After some nonsense^2 setting up the Pi, I managed to SSH into it and
send a WoL packet. Nice! But I also have to be able to access the SSH
server from outside the house. Instead of setting up port forwarding
or OpenVPN on my router, I added the Pi to my Tailscale network (no,
this is not an ad; just happy), which was pretty painless.
cedar% ssh raspberrypi
max@raspberrypi:~ $ wakeonlan MY:MA:CA:DD:RE:SS
max@raspberrypi:~ $ ^D
cedar%
Then I remembered some of our old college apartment infrastructure
and realized I should probably have a webpage as well. It would be
neat to be able to open a page on my laptop or phone and wake my
desktop from its slumber.
The very large buttons saying 'WAKE ME UP (wake me up inside)' and
'Go to sleep'
Because I wanted to write as little code and infrastructure as
possible, this is served via Busybox httpd.
$ /usr/bin/busybox httpd -vv -p 80 -c /home/max/httpd.conf -f
...
The configuration file just sets index.html as the default directory
index page and sets the root directory.
When I click one of the buttons, it sends a request to--yes, you
guessed it--a CGI handler. It's very small, and could even be smaller
if I didn't log error output. For example, here is the wake handler:
#!/usr/bin/env python3
import subprocess
print("Content-Type: text/html")
print()
result = subprocess.run(["wakeonlan", "MY:MA:CA:DD:RE:SS"], capture_output=True)
if result.returncode != 0:
print("")
print("stderr:", result.stderr.decode('utf-8'))
print("stdout:", result.stdout.decode('utf-8'))
print("")
else:
print("He has risen")
Because I didn't want to have to hit the back button and also vaguely
remembered an interesting snippet of code I saw on lobste.rs
recently, I decided to use HTMZ to inject this CGI-printed response
into the original page^3.
This means that when I click the "WAKE ME UP" button, I see what
happened right beneath the buttons. And I didn't even have to write
any JS or do any presentation logic inside the CGI handler; the HTMZ
handled it for me using the hash link in the formaction.
Systems administration
Because I want this server to come up without any manual
intervention, I set up a systemd service for it. Instead of fiddling
around with the services in /etc that require root, I instead made a
user service. This means that the service only has my privileges and
I don't need to use sudo.
I put the wake.service file (below) into $HOME/.config/systemd/user/
wake.service and ran:
1. loginctl enable-linger max to allow my user to run long-running
services without staying logged in
2. systemctl --user enable wake to enable the service
3. systemctl --user start wake to start it up
You might have to also do systemctl --user daemon-reload to get
systemd to reload the service from disk or something, but I am not
clear on that.
# The service
[Unit]
Description=Wake
[Service]
Type=simple
ExecStart=/usr/bin/busybox httpd -vv -p 80 -c /home/max/httpd.conf -f
Restart=on-failure
[Install]
WantedBy=multi-user.target
This is great. It means that I can enable it once and forget about
it, even across restarts. I don't even need to touch the service if I
edit the CGI, since it's loaded from disk on every request. I think I
only need to touch the systemd service if I want to change the httpd
configuration.
Now, you might be wondering to yourself "Max, you said you weren't
running as root. How are you binding to port 80?" Well, sudo setcap
CAP_NET_BIND_SERVICE=+eip /usr/bin/busybox. But you could also run on
an unprivileged port like 8080.
This was probably the last dose of systems administration that I'll
need for the next six years or so. See you all soon for some more
programming languages or breadposting.
All the code
To see all of the code and configuration in its brief glory, check
out the Git repo.
1. I just remembered as I wrote this that in college we also
configured a voice assistant to, when prompted with some key
phrase, send an HTTP request to our router, which would then send
a WoL packet to our media server. That was pretty useful. -
2. I don't know why, but getting my Pi to post took about 6 hours of
troubleshooting. It was made more difficult because I didn't have
any USB OTG cables or mini HDMI cables, so I was very in the
dark--I only knew if it came online if it joined the WiFi network
and acquired a DHCP lease. What I learned eventually is:
+ My Pi only supports 2.4GHz, not 5GHz
+ The 64 bit Raspberry Pi OS images, lite or otherwise, are
trouble; 32 bit seems to work okay
+ The image does not come with user accounts or SSH enabled and
that needs to be manually enabled using some funky
half-documented first-boot configuration
+ Older versions of the official imaging tool provided by
Raspberry Pi simply do NOT work and I might have been better
off using dd
But it's fine. It works now. I only wasted a lot of time on Pi
Day, of all days. -
3. It doesn't really make sense for this project, but I learned
while poking around on the HTMZ page that a browser won't make
repeated GET requests from an iframe. Fortunately, there is a
very slightly different version of HTMZ for that. -
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