Shutdowner

Windows service that shuts down the computer when a "magic packet" is received

Why?

Often times I need my computer awake and running, so as to grab files from it remotely. Starting it is easy: WOL. Shutting it back down? That's a bit clunkier. Sure, I can use RDP to connect from my phone, but often times that's not the best solution: bad network, lack of time, general unwieldiness of a phone screen, etc.

The Solution!

A service that runs regardless of a user being logged on, and listening to "magic packets" (loosely akin to WOL). It's stupidly simple, consumes virtually no resources while idle, and provides a decent level of security with some poor-man's-time-based-one-time-passwords.

Building

Configured to use .NET 4.8 and VS2019, and will most likely work with at least .NET 4.5. Uses the ini-parser nuget module - MIT license.

Installing

Either build it yourself, or download already built binaries from the Github Releases page.

Start up a cmd.exe with administrator privileges and use either method:

• the preferred method, using installutil.exe: installutil.exe "path to Shutdowner.exe"; be sure to use the 64-bit version of installutil.exe if Shutdowner.exe is a 64-bit executable
• the classic method, using sc: sc.exe create "ShutdownerService" binpath= "path to Shutdowner.exe" start= auto obj= LocalSystem

I would recommend write-protecting the directory and read-protecting the config.ini file where Shutdowner.exe resides, after installing it.

Uninstalling

Start up a cmd.exe with administrator privileges and use either method:

• installutil.exe /u "path to Shutdowner.exe"; be sure to use the 64-bit version of installutil.exe if Shutdowner.exe is a 64-bit executable
• sc.exe stop "ShutdownerService' and then sc.exe delete "ShutdownerService"

Using It

The service tries to load settings from the config.ini file. Here's an example containing all the possible keys and their default values:

[Shutdowner]
;Port on which to listen for packets
port=9999

;A passkey that has to be included in the hash calculation
passkey=shutdowner passkey

;Max amount of seconds allowed between the (actual local time) - (sender's time + network delay)
maxDelaySeconds=5

;Max amount of seconds the sender's time is allowed to be ahead of the local time
maxAheadSeconds=5

Right after installation, the service will not be running already, even though Automatic start is selected. You can start it with sc.exe start "ShutdownerService" from an elevated command prompt.

The service then listens on the specified port for UDP packets containing exactly a single hex string representing the SHA256 hash of the timed password. The timed password is calculated as: {passkey}-{UTC now in 'yyyy-MM-dd HH:mm:ss' format}, no newlines or other characters at the end. As configured with the max*Seconds keys, "UTC now" is allowed to vary a bit.

A bash example that works on my Termux+Termux:Widget Android phone:

ip="computer ip"
port="9999"
passkey="shutdowner passkey"

dt=$(date -u "+%Y-%m-%d %H:%M:%S")

combined="${passkey}-${dt}"
hashed=($(echo -n $combined | sha256sum))

# many nc binaries don't allow -w0; use -w1 if yours complains
echo -n $hashed | nc -w0 -u $ip $port