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Snowblower got stolen. 
So, OpenIPC is supposedly a FOSS firmware for IP cameras: https://openipc.org
But it's definitely a work in progress. Anybody know of a similar project that's more mature?
Otherwise my current plan is RTSP-compatible IP cameras, a firewall so they can't phone home, and a script running on my NAS to encode/capture the output.
I'm just running 3 dahua cameras that are on a VLAN with no internet access. I'm tinkering with zoneminder on my server as an RTSP NVR, but it's not going wonderfully (it soaks up a lot of cycles on the server's GPU, which can be problematic when I'm using it for HTPC stuff).
I use the camera settings for capture (to NFS), and rotate the footage on a 10 day rolling window (via cronjob on the server). Currently the zoneminder setup does not do any analysis or storage, it's purely for aggregating streams when I want a quick look at what's going on outside the house.