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> Or is it the baked in firmware using RARP and then the modern NetBSD kernel using DHCP? That would make more sense

Yes, firmware only knows how to use rarp and tftp to fetch a kernel or a better bootloader, kernel is modern and speaks DHCP. This is a pretty common pattern with netbooting; some will bootp rather than rarp, sometimes you use tftp to fetch something that can do an http fetch, etc. Always lots of fun :D

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A lot of old hardware uses TFTP and RARP to boot. RARP just will get you the ip address, and the rest is hardcoded somehow in the machine - needs very little memory on boot. For BOOTP you need some intelligence to know where are your files. TFTP is also cheap in memory to use. UDP without flow, no nothing. Just send me the next packet in sequence when I ask you to do so.

I remember having trouble some years ago upgrading old Cisco routers because the image was bigger than what TFTP can handle.

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