Linux

VirtualBox: file sharing on NAT with no shared folders

 

I have a rather interesting problem for you. It’s also quite convoluted. Say you’re using VirtualBox
as your preferred virtualization tool. Say you’re having trouble with 3D acceleration –
black screen and all that. As you’ve seen in my
tutorial, the fix is to use repo-provided
guest additions. But this repo set, as provided by the
distribution, unlike the additions provided by the official ISO, does not contain a shared folder
driver. This means you don’t have this functionality available.

To make things even more complicated, say you want to share stuff. We talked about this in another
tutorial, aptly named
network & sharing, but you’re using NAT rather than
bridged networking, so the guest machine is not accessible from your host by any normal means. And you
don’t want to use Samba or alike, so the whole situation is even more complex. Okay, so let me show you
how you can transfer files from host to guest without having shared folders while using NAT. No
Internet tomfoolery. We’re doing it all local. After me.

Teaser

Problem statement

As you can see, we have an issue where seemingly we need to make our machine more visible by
bridging the network adapter, which might not be what you want, especially if you have a ‘noisy’
virtual machine, plus you might need to setup file sharing servers and whatnot, all of which take more
than trivial amount of work. The other option is to use the VirtualBox ISO, disable 3D acceleration in
this particular case – until the black screen issue goes away, but consider it the proverbial problem
of any kind – and then we can use the shared folders feature.

On that topic, if you use the repo packages, then you will have most of the stuff – graphics
acceleration, mouse integration, screen resizing, clipboard sharing, and then some – but not the
folders. If you try to manually mount the shared folder, you’ll get something like:

mount -t vboxsf <share> <mount point>

mount: unknown filesystem type ‘vboxsf’

Solution: port forwarding

VirtualBox lets you configure advanced network rules for your NAT-ed machines. In essence, this is
not different from how your router works when translating local IP addresses to the public-facing one,
and vice versa. Indeed, VirtualBox is your router here. We’ve also seen this with
Docker, where you can map host ports to container ports, so you
effectively access your local system but you’re in fact connecting to a container, or in this case, a
virtual machine.

Anyway, select your virtual machine > Settings. Under Networking, for the network type NAT,
expand the Advanced section toward the bottom. Then click on Port Forwarding. This will open a table
where you can input the necessary details.
Host IP – the IP address of the interface you want to use; you may have
more than one, but the easiest solution is to just use localhost (127.0.0.1), because this will work
even if your other interfaces are down, or you don’t have connectivity outside your host.
Host Port – will be the ‘virtual’ address to which you will
connect. For example, for SSH, hint hint, we can map port 2222 to be the Host Port. This means if
you connect to this port, you will effectively be connecting to the
Guest IP at the relevant
Guest Port specified in the table. Indeed, the other two fields are for
the virtual machine information, much like we did with our host.

Advanced rules

Port forwarding rule

Connect with SSH

So now we have the actual connection. You may say wait, SSH is for Linux! Yes, because we have a
problem in Linux. There’s no black screen issue with Windows, so the 3D acceleration thingie + guest
additions from the ISO will work fine. Ergo, Linux.

We can now connect from our host to guest using SSH (on port 2222). But of course, the first step is
to make sure openssh-server is installed and running in the guest operating system. We also need to
make sure the guest is not using a firewall that’s blocking incoming connections. Now we can test what
gives.

ssh -p 2222 [email protected]

The authenticity of host ‘[127.0.0.1]:2222 ([127.0.0.1]:2222)’ can’t be established. ECDSA key
fingerprint is SHA256:Eq0ow03sffLb7G49e3KNMgMrpVV/8wFfN6uicdxPnEL.

Are you sure you want to continue connecting (yes/no)?

SSH works

But this is just to test connectivity. We actually need SCP to copy files from our host into the
guest. Now, here we also need to specify a custom port. But please note: SSH uses lowercase -p to
specify the port, whereas SCP uses uppercase -P flag for port numbers. Go figure. So the whole thingie
will be something like:

scp -P 2222 “file” [email protected]:/home/roger/”location”/

“file”
100% 8244KB 135.0MB/s 00:00

Sharing works, zoomed

Conclusion

I like these kind of exercises, because they can be innovative and fun – like my
KVM & bridged network trick. At first glance, you may think you’ll
need to use bridged networking, setup Samba or NFS, or email files, which can be tedious. Instead, we
work around the issue with port-forwarding and SSH, both of which are simple, non-intrusive and very
easy to setup or disable if not needed.

Well then, we learned quite a lot today – about the missing features in the repo guest additions,
the use of advanced networking rules and port forwarding, SSH and SCP and the subtle differences in how
they use port numbers, and a few other concepts. That would be all. Remember, there’s always a neat way
of fixing problems, if you’re willing to be patient and methodical. Take care.

Cheers.

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