RHEL Engineering is moving the tracking of its product development work on RHEL 6 through RHEL 9 to Red Hat Jira (issues.redhat.com). If you're a Red Hat customer, please continue to file support cases via the Red Hat customer portal. If you're not, please head to the "RHEL project" in Red Hat Jira and file new tickets here. Individual Bugzilla bugs in the statuses "NEW", "ASSIGNED", and "POST" are being migrated throughout September 2023. Bugs of Red Hat partners with an assigned Engineering Partner Manager (EPM) are migrated in late September as per pre-agreed dates. Bugs against components "kernel", "kernel-rt", and "kpatch" are only migrated if still in "NEW" or "ASSIGNED". If you cannot log in to RH Jira, please consult article #7032570. That failing, please send an e-mail to the RH Jira admins at rh-issues@redhat.com to troubleshoot your issue as a user management inquiry. The email creates a ServiceNow ticket with Red Hat. Individual Bugzilla bugs that are migrated will be moved to status "CLOSED", resolution "MIGRATED", and set with "MigratedToJIRA" in "Keywords". The link to the successor Jira issue will be found under "Links", have a little "two-footprint" icon next to it, and direct you to the "RHEL project" in Red Hat Jira (issue links are of type "https://issues.redhat.com/browse/RHEL-XXXX", where "X" is a digit). This same link will be available in a blue banner at the top of the page informing you that that bug has been migrated.

Created attachment 1791963 [details]
Reproduce script

Description of problem:

When using `nmcli c  mo br0 remove 802-3-ethernet` to remove MTU/etc settings,
the MTU actually decreased 1.

Version-Release number of selected component (if applicable):
NetworkManager-1.30.0-7.el8
NetworkManager-1.32.0-0.5.el8


How reproducible:
100%

Steps to Reproduce:
1. sudo ./mtu_bug.sh
2.
3.

Actual results:

The MTU of br0 changed to 1399

Expected results:

The MTU of br0 revert back to default 1500.

Additional info:

Created attachment 1791975 [details]
System logs with NM trace enabled

Hi Beniamino,

Please take a quick a investigation to check whether this is bug is trivial or the tip of the iceberg.

Thanks!

Hi, this is the result of:

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/commit/e23798a5e5e8d7eb615f0e8b305fc20b8b645c66

The problem is that NM stores (MTU - 1) as the initial MTU and then it resets the MTU to the stored value on deactivation. A fix is should be not too difficult.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (NetworkManager bug fix and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHEA-2022:1985