Weird issue with MTU = 16182 : Windows client cannot open files on LXC Samba server


Active Member
Jul 4, 2019
Lately without me doing any configuration change, except for updating the Proxmox host (7.4-17 now) and the LXC Guest Samba server,
I ran into a huge headache with connecting from my Windows 11 SMB client to the above Samba server.

Basically, whenever I tried to open the drive or browse the directory (if I could open the root directory), the Windows Explorer will be frozen,
it is so bad that even restarting it from Task Manager often can't work and hard reboot is needed (because Windows can't logout or shutdown, keeps rotating).

To isolate the issue, I tried:
1. Restore the LXC Samba server with an earlier backup - no luck
2. Image restored my Windows client with an image a week ago (the problem showed up yesterday after updating Windows and updating Proxmox/LXC) - no luck
3. Changing *either* of the Samba server or Windows client's MTU from 16182 to 9000, works! Files once again open as usual. Seems like if either side falls back to 9000, it will then go with 9000 MTU and things work. Only when both sides are set to 16182 things go south.

I am glad after hours of check it gets back to work, but also puzzled what might have been broken. The only thing that I can't easily restore was the Proxmox host, but from the above three checks it does appear something in the update might have done something to cause it.

I believe I should supply more information but I am not sure where to start, appreciate if you want to look into it and I can extract the logs required.


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