Centos Stream 9: NetworkManager/conf.d/ override mtu received from dhcp

[Stephen Horton <shorton3 yahoo com>]

Hello, I'm new to the list; I am working on moving an application to Centos Stream 9 running inside an openstack cloud virtual guest.  I have created:

/etc/NetworkManager/conf.d/90-dns-none.conf:

[main]

dns=none

to prevent Network Manager from overwriting the /etc/resolv.conf with incorrect DNS server entries from our DHCP service. This is working.

I also need to tell Network Manager to override the MTU provided by the DHCP service (which is telling my interfaces to use mtu=9000).  I have created:

/etc/NetworkManager/conf.d/91-mtu-none.conf:

[connection]

match-device=interface-name:eth0

ethernet.mtu=0

then: systemctl reload NetworkManager

I can then:  nmcli device modify eth0 ethernet.mtu 1500  and it changes the mtu to 1500 (shown in: ip address show).  However, when I reboot, it changes back to mtu 9000.

ifcfg-eth0 file still contains MTU=9000

Can someone provide some guidance on how this should work please?

regards,

Stephen