"Protocol error" mounting VirtualBox shared folders in an Ubuntu guest

In order to use shared folder functionality few prerequisites need to be met:

• Make sure that Guest Additions are properly installed on the guest OS.
• Users in a guest Ubuntu must be in the group vboxsf to be able to access shares.
• Define a directory on the host that will be used in the virtual machine using the settings dialogue of Virtual Box.

Depending on host or guest OS the following may also be needed:

• Do not share personal folders like /home/username or My Documents
• Avoid special characters or empty spaces in the path to the shared folder, especially if the host or VM is a Windows machine
• Use different names for share and mountpoint
• Create a mountpoint on the guest OS (best in your HOME directory).

Testing shared folders functionality can be done by creating a shared directory on the host (e.g. C:\myshare), define this as a shared folder for your guest system in Virtual Box settings (e.g. share), create a mount point in your guest os (e.g. mkdir /home/username/host) and mount this in the guest OS with the command:

sudo mount -t vboxsf -o uid=1000,gid=1000 share /home/username/host

where the option -o makes sure that you will have access to the mount (that will otherwise be owned by root).

For further information on shared folders see also the Virtual Box User Manual.

Files or directories can also be shared over the network by using Samba on the host and the guest.

I also met this trouble. I found that the shared folder were already mounted at /media/sf_share.

I tried to delete /media/sf_share, but I couldn't.

So I deleted shared folder "share" in my home directory.

$ rm -r ~/share

And then switch to root user with the following way:

$ sudo -i

Create softlink/shortcut to the /media/sf_share folder that owns by root user modify the ownership of softlink with chown command. Exit from root user to normal user

# ln -s /media/sf_share /home/username/share
# chown username.username /home/username/share
# exit
logout

$ chmod 777 ~/share

You can see the files in Windows through the shared folder. And username must be a member of group vboxsf.

Here is what worked for me without any problem

I figured it out that, actually my user name was not in vboxsf group. Following two commands will help to get the group info:

grep "vboxsf" /etc/group    
grep "saurav" /etc/group

So I added saurav (me) to the vboxsf group using following command:

sudo usermod -a -G vboxsf saurav 

Then restarted the virtualbox and then.. Jumped from my placed.. :D :D Everything worked fine without any problem.

Please comment if any one still having any problem..

I also struggled a lot with the mounting, and tried everything I come up with in the interwebz but I finally found a very weird workaround to solve it! My guest is Ubuntu 13 and my host is Windows 7.

Change to the media folder and from there type user@os:/media$ sudo mount -t vboxsf share /home/username/folder

Yeah, that's it. But I don't understand how come it only works if it's done from the media folder. If someone knows please kindly explain us below. Thanks.

P.s. Editing my /etc/rc.local to do the automounting at startup doesn't work for me.

With VirtualBox 5.1.20 running on Windows 10, and Ubuntu 16.04 as the guest OS with GuestAdditions 5.1.20 installed, I couldn't mount the shared folders from the command line. The mount commands failed, with mentions of Protocol Error and sf_read_super_aux err=-22.

Eventually I found https://www.virtualbox.org/ticket/16670 "mount.vboxsf symlink broken". I followed one of the workarounds there, to correct the symlink for mount.vboxsf, and it all worked. My approach was to do:

mv /sbin/mount.vboxsf /sbin/mount.vboxsf-orig
ln -s /opt/VBoxGuestAdditions-5.1.20/lib/VBoxGuestAdditions/mount.vboxsf /sbin/mount.vboxsf

The commands need to be done by root. So, because it was Ubuntu, I added "sudo " at the beginning of each line to do the command as root. Other flavors of Linux have other ways of doing that.

As an aside, there was a similar symlink problem reported in virtualbox's Ticket #12879 "Can't mount shared folders with guest additions 4.3.10" in 2014, that was fixed the following month.

2017-04-27