@Quazz Thanks for explaining a bit more and linking back to the other tread. I somehow had the feeling that we had talked about this but couldn’t find it.
Yes, booting a VM (possibly virtualbox) is perfectly explaining option 175. So that’s fine!
Not sure about the lease time coincidence. But on the other hand I don’t see why isc-dhcp should send two offers with to different IPs (to the same MAC) within just one second. Doesn’t make any sense to me. If the client does not respond fast enough I would think the DHCP server would just send the exact same offer again.
Ahhhh, there is something else I just noticed: The first two DHCP offers from 192.168.1.156 are send to the unicast addresses (192.168.1.54 and 192.168.1.47). At first I was confused because I thought ISC-DHCP usually does send to broadcast. But I was wrong: https://lists.isc.org/pipermail/dhcp-users/2008-April/006219.html
The first request (your VM PXEing) has bootp flags set to unicast but our iPXE requests broadcast answers from the DHCP server. Learning something new every day.
If you can’t change your ISP modem (you are right, I don’t see next-server/filename options in those DHCP answers), then DHCP proxy should be your friend. It’s really strange that clients would still want to request the boot file from 192.168.1.1! I’ve looked through the packet dump files again (as well the old one) and I can’t see next-server being sent by 192.168.1.1 at all. Maybe it does this just once on a while?? OR iPXE is seeing the option “Relay agent IP address” in the DHCP offer as next-server. I kind of doubt this but I am not absolutely sure.
If you get a chance to get to the iPXE console/shell at some point just type config
and you can see all the variables being set. See here: http://ipxe.org/cmd/config