I attempted to set MTU for wired connection to 1472 (recommended for IPv6 via 6rd). Note: this is also reported as Bug 1042542 for opensuse (bugzilla.opensuse.org). In the client, I made the change, then clicked "Okay" (or "Apply"). I expected a connection definition file (probably "Wired connection 1") to be save in "/etc/NetworkManager/system-connections" with the changed setting. Instead, nothing happened. I rebooted, and the MTU was still 1500. I also have Gnome desktop installed. I tried using the Gnome NetworkManager client, and changing the MTU there worked as expected (while logged into Gnome).
I'm not able to reproduce this problem. If I change MTU to any value and click on apply, then checking the result it will be changed to the value I set.
I am seeing the problem on openSUSE Tumbleweed (plasma-nm 5.10.0 -- actually called "plasma-nm5 there). I am also seeing the problem on KaOS, with plasma-nm 5.9.5.1 With the way that openSUSE handles NM, there should have been a polkit window that popped up when I tried to save the changes. But there was no such polkit popup. Similarly, with KaOS, there was no polkit popup. It's a while since I last used Fedora. But I seem to recall that it was more generous with allowing network config changes, so perhaps it would not have required a polkit query.
What is your NetworkManager version?
In openSUSE Tumbleweed, NetworkManager shows up as version 1.6.2. On KaOS, where I also see this problem, NetworkManager appears to be version 1.8.0.
Tumbleweed upgrade plasma-nm to version 5.10.1 today. So I decided to retest. The problem is still there. But I paid closer attention to what is happening. When I start editing the Wired connect settings, I see "Okay", "Apply" and "Cancel" buttons near the bottom of the screen. But the "Apply" is grayed out, so cannot be used. When I change something (I tried changing the MTU from 1472 to 1471), the "Apply" remains greyed out. So the applet is not recognizing that there are changes that need to be saved. By contrast, if I edit a "WiFi" connection settings, then the change of MTU from 1472 to 1471 (and then back to 1472) makes the Apply button active (not grayed out).
Could you please check whether this is not a duplicate of bug 378554?
>Could you please check whether this is not a duplicate of bug 378554? Yes, it appears to be the same bug. I set "allow auto-negotiation". Then "Apply" was no longer grayed out. I changed the MTU to 1471 and clicked "Apply" (and was prompted for root password). Checking the connection file in "/etc/NetworkManager/system-connection", the MTU information had changed to 1471. I was then prompted for the "kdewallet" key. I'm not sure what that is about, since this is a wired connection. I closed the connection editor. Then I restarted it. I could see that "allow auto-negotiation" was still checked. I changed the MTU back to 1472. And that caused "Apply" to no longer be grayed out. And I was able to save those changes.
Ok, I think we can consider this as solved.
Bulk transfer as requested in T17796