Comment 5 for bug 1937110

I'll also add that installers for past LTS Ubuntu releases do work in our environment.

I opened up the Focal initrd.gz we use for PXE based installs (I think it was from the mini.iso which I guess is now classified as a "Legacy Image", http://archive.ubuntu.com/ubuntu/dists/focal/main/installer-amd64/current/legacy-images/netboot/mini.iso). What I found is that it also is missing /etc/dhcp/dhclient-exit-hooks.d/rfc3442-classless-routes HOWEVER it has an older /sbin/dhclient-script that does NOT have the code that ignores "routers" option values when the RFC 3442 option 121 is present in the response. As a result it does configure the default route based on the DHCP "routers" value but ignores the RFC 3442 info. In our environment this means the routing is sub-optimal but networking still largely functions and thus Focal installation "works".

The original poster reports trouble with Focal installer I don't know if the difference is because they use a different Focal .iso or because perhaps they strongly require the RFC 3442 routes for networking to function in their environment.

My recollection regarding the changes in the newer dhclient-script is that it that ignoring the "routers" option if the client supports and uses the RFC 3442 routes option is the behavior specified in RFC 3442, that the behavior of the old version to sort-of merge the "routers" option and the RFC 3442 option 121 together was non-compliant. Though as it turns out if the "rfc3442-classless-routes" hook file is accidentally missing then that makes the behavior of the newer, more correct dhclient-script version worse.

--Matt