[vbox-dev] SSH into Linux guest fails with bridged networking: possible MTU related issue.

[Rodney Lott]

Hi, there.

First, my details:

Host machine: Ubuntu Vivid Linux, 3.19.0-64-generic #72-Ubuntu SMP kernel
VirtualBox version: 5.1.2 r108956
Guest VM: Ubuntu Trusty, 3.19.0-43-generic #49~14.04.1-Ubuntu SMP kernel, Bridged networking

I believe that I have found an issue regarding SSH from another Linux computer into the guest VM. The ssh attempt will fail. tcpdump analysis captured truncated packets from the machine that was not able to SSH in. We tried the same test by installing an identical OS on a VMWare VM with bridged networking and the problem does not manifest itself. This suggests some form of MTU related issue, presumably involving the vboxnet* drivers on the host.

If I set the MTU for the guest interface from 1500 (the default) to 1501, I can ssh into the VM. I can ssh from my host machine into the guest without issue with the default MTU of 1500. This is most likely because it is using the lo interface.

I had Googled for a resolution to these symptoms without much success or agreement on a solution. So, I thought I would take the next step and post here. I imagine that many people are not using the VM's in this manner, hence why it might not have been detected.

Thanks in advance for any help. I appreciate this.

Rodney
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