[Dnsmasq-discuss] Discovering the proxyDHCP server IP from initrds

[Alkis Georgopoulos]

Hi, I have the following use case, and I'm thinking that a workaround 
might be to abuse the DHCP server concept a bit, and I'd like to ask if 
someone can see any bad side effects, or can come up with a better solution.

* Small computer lab. An LTSP server netboots e.g. 20 clients.
* The DHCP server is a cheap, non-configurable router.
There are no static IPs, not even for the server.
* Dnsmasq is configured as a proxyDHCP server.

The problem is, "how can the initramfs find out the dnsmasq=proxyDHCP 
server IP, to load the NBD root file system from there?"

1) Ideally, the DHCP client in the initramfs would understand proxyDHCP 
offers and get the address from there.
==> QUESTION 1: Does anyone know of such a client?

2) When using iPXE/undionly, we pass the proxyDHCP server IP in the 
kernel cmdline using its ${proxydhcp/dhcp-server} variable.

3) When using PXELinux, we pass the proxyDHCP server IP by using its 
"IPAPPEND 3" option.

4) But there are some cases (e.g. no UEFI/BIOS/iPXE support for the NIC) 
where we put the kernel/initrd locally, so the initramfs needs to 
discover the proxyDHCP server with no help from /proc/cmdline.
My hacky workaround there is to:

Server side:
As I said, dnsmasq already runs in proxyDHCP mode.
But I also run a SECOND instance of dnsmasq with the following 
configuration:
   dhcp-alternate-port
   dhcp-range=192.168.0.1,192.168.0.254,2m
   port=0
   dhcp-leasefile=/tmp/dnsmasq.leases
This is using the REAL dhcp-range, where the router is responsible!
So dnsmasq might offer an IP that is actually in use!
But the client will discard the DHCPOFFER IP (see below).

Client side:
The client normally runs `ipconfig` in the initramfs and gets its IP 
from the router.
Then I tell it to run a SECOND, "fake" `ipconfig -n -p 1068 enp0s17`.
Using the alternate ports, it manages to contact dnsmasq.
It gets the DHCPOFFER, keeps the server IP, and completely discards the 
received IP.

I.e. my workaround is to abuse the DHCP protocol just for server 
discovery, not IP leasing, due to DHCP clients not supporting the 
proxyDHCP protocol.

==> QUESTION 2: since the client will discard the DHCPOFFER IP, and only 
use the server IP, will this abuse have any bad side effects?

Thank you!
Alkis Georgopoulos
LTSP developer