Future Fibre + 3rd party (Linux) router DHCP settings solved

Hello all. I know this won't apply to everyone but I just got a new Future Fibre FTTP 150Mbps circuit (semi-rural area). All worked immediately with the Eero but being a right-thinking individual I had no desire to use that. I tried my old Teltonika router which connected immediately with DHCP on the WAN port to the ONT, but only having 100Mbps it was acting as a bottleneck (speed test through the Eero confirmed I was almost getting what I pay for).
I decided to use IPFire, on an old PC for now since the APU I want is out of stock. Built it at work and it was all set up fine, connected it at home, nothing, just times out. Teltonika back in, OK. Tried my laptop directly to the ONT and in Windows it worked but not in Linux, using the dhcpcd client. The ISC dhclient package connected immediately.
Long story short, the ONT (which I think is a Nokia?) appears to be using some non-RFC compliant DHCP settings which the up-to-date dhcpcd package fails on. With some advice from the IPFire guys I disabled clientid, duid and rapid_commit in the dhcpcd.conf file and all is resolved. The Teltonika uses RutOS which is I believe OpenWRT-based and so Linux, but obviously on an older DHCP client.
Incidentally, TalkTalk were as much use as the proverbial chocolate teapot; after 40 minutes waiting in the queue to chat I was informed that it was "impossible" to connect a third-party router, which was "not at all supported". I pointed out that I was online right then via the Teltonika which prompted 10 minutes of silence while they consulted a superior...

Edited by Hobthrust (Fri 18-Feb-22 12:38:04)

All good there. Note that the ONT is a passive device at Layer 3 (DHCP). It is simply converting Ethernet frames to GEM frames for carriage back to the OLT where the ethernet frames are stripped out and sent to the ISP backhaul.

Your DHCP assigned IP is done by TalkTalk further up their chain so to speak, so any DHCP quirks, MAC caching etc. is going to come from there, rather than the local ONT.

Thanks for the information, I haven't encountered these things before, and it's quite a while since I worked in networks.