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Canary FSWatcher

Canary FSWatcher is a CLI tool which monitors a file or directory and fires a canarytokens.org URL webhook whenever the target is accessed. The main use case is to quickly discover that a server has been breached. Create a honeypot file, put it into an easy to find location and monitor its access via this tool.

Why?

Mainly for educational purposes as there's already a similar tool - Canaryfy. However it does not work on all operating systems unlike Canary FSWatcher. Moreover, canaryfy relies on a DNS canary token which is unreliable due to DNS caching - the probability of missing events is quite high. I know that the TTL is quite low (3 seconds at the time of writing this) but they're not always respected. Some DNS resolvers impose minimum TTL (check https://00f.net/2019/11/03/stop-using-low-dns-ttls/).

How?

Canary FSWatcher uses the fsnotify cross-platform Go library. It supports Windows, Linux, macOS and more.

Both the full path name and the operation are included in the token request as headers:

X-Canary-Path-Name: /tmp/my-dir/my-file
X-Canary-Path-Op: WRITE

As noted in the canaryfy it's good idea to move the binary of an unexpected location. This would make it harder to detect by an untrusted party :)

Usage

Usage of canary-fswatcher:
  -linger duration
    	Time to wait for new events to arrive before pinging the token url (default 1s)
  -path string
    	File or directory to monitor for changes (default "/tmp")
  -token-url string
    	Canary token url generated from canarytokens.org to be pinged on events

Creating a Systemd Service

We can use systemd to ensure that the binary is automatically started on boot or failures. Here's an example service file which can be used for this exact purpose. Make sure to modify the ExecStart line:

  • Set the correct path to the canary-fswatcher binary on your machine
  • Set the path to the directory or file that must be monitored via -path flag
  • Set the URL of the token generated from canarytokens.org via -token-url flag

canary-fswatcher-daemon.service

# Systemd service unit file for the Canary FSWatcher daemon

[Unit]
Description=Canary FSWatcher
After=network.target
StartLimitIntervalSec=0

[Service]
Restart=always
RestartSec=3
ExecStart=/usr/local/bin/canary-fswatcher -path <path> -token-url <url>

Create the file in /etc/systemd/system/ and execute the following:

# Start the service
systemctl start canary-fswatcher-daemon

# Start the service automatically on boot
systemctl enable canary-fswatcher-daemon

# Check the service status
systemctl status canary-fswatcher-daemon