It's not open source but used Axis cameras are pretty cheap and have rtsp and onvif support. Those mostly come from commercial installs and can be configured offline using a web interface.

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rkagerer 4 days ago | parent [-]

Axis cameras are great. Their product support is awful.

	▲	mysteria 4 days ago \| parent [-]
		For used cameras I don't expect to get any form of official support. IMO their documentation is clear and they provide software updates for 7 years.