A device that wirelessly, securely posts to a server in response to door opening: off the shelf or custom-make?
A Raspberry Pi may be a reasonable compromise:


it can run various flavors of Linux, which supports TCP/IP and many flavours of security
it has numerous GPIO lines which can be polled by something as simple as a Python script
it supports many off-the-shelf USB-to-WiFi adapters
it consumes around 5W at peak loading, and could run for a long time off a household UPS with a switching wall-wart adapter powering it