While mobile devices (especially those with a GPS) have made people more aware of geolocation/geotagging, you don’t need a GPS in a device to make it location-aware, nor do you need to resort to IP-based reverse lookups. Location information can be made available on any network that has a DHCP server using the Location Configuration Information DHCP Option defined in RFC 3825 and RFC 6225. It makes sense: for most wired or wireless networks, the engineer responsible for setting up the DHCP server will know at the least where the server or AP is located, and maybe even static information about the locations of each terminal of a wired port.
The Option has a somewhat unorthodox binary format with non-power-of-2-width fixed point reals. To make generating the DHCP configuration statements easier, I developed a web-based RFC 6225 location configuration generation tool. It’s all client-side and even has a Google Maps preview of the location!
Copy the configuration to your local DHCP server, grab an appropriate geolocation library, and you’re ready to go! The tool generates a DHCP LCI Option for both dnsmasq and ISC’s dhcpd.
The lack of fixed point integers in Javascript made the implementation a little trickier than it would have been in, say, C.
Other lessons: Google Maps is not highly accurate.
Update 2019-06-01: updated the tool from RFC 3825 to support RFC 6225 and the GeoLoc option.
Update 2023-04-24: fixed a bug with the dhcpd/dnsmasq config lines missing the leading payload bytes (that existed since 2019 :( )