Updated: August 26, 2020
I like V'box and I cannot lie, all you nerds cannot deny. The thing is, I use VirtualBox for all sorts of testing and whatnot. A fair deal. But now and then, this perfectly sensible program decides to misbehave and stop cooperating with my efforts. Case in point, bridged networking.
Over the years, I've written a bunch of guides revolving around VirtualBox networking features, like for instance my network & sharing tutorial, how to share over NAT with port-forwarding, and how to configure NAT networks. As it happens, I also happen to use the bridged network functionality, because it's simple and convenient, and seems to work reliably well, with wired and Wireless adapters and whatnot. For many years, there was no trouble, and suddenly, there is. Bridged networking stopped networking for me on Linux hosts running VirtualBox 6.X. Geddit? Network not-work. Ho ho. Follow me.
Problem in more detail
I encountered this on my Slimbook laptop, running Kubuntu 18.04, with VirtualBox 6.1. The laptop has an Intel dual-band Wireless adapter, and I was able to use it for bridged networking without any issues for a long time. A few weeks back, I noticed that guest operating systems, both Windows and Linux, couldn't acquire IP addresses any more with this network configuration in place.
To rule out my router, I tested on a Windows host, running VirtualBox, and there, no such issues arose. To rule out Linux, I tried this on several other distros, with the same (bad) results, despite the differences in the actual architecture and the kernel version(s). So this seems to be limited to the VirtualBox build for Linux, but not because of Linux. Of course, if you hit the official forums, you will find a lot of threads on this topic, most of them culminating either in a fix or vague comments saying that bridged networking isn't reliable, so you should be kind of happy that it works. But no.
Perhaps I've come across a new regression in the network stack or some such, and perhaps you'll never encounter this problem. But if you do, I did manage to find a semi-reasonable workaround, which could perhaps help you until your particular bridged networking issues, whatever they be on whichever build of VirtualBox, are resolved. And I'm saying yours, because otherwise, you wouldn't be reading this article.
So we have bridged networking, but the clients can't acquire IP addresses. Using static IP addresses does not help either. In my case, the reason for this seems to be the conflict between the randomly generated MAC address of the virtual network adapter and the one used by your bridged (physical) card. In the past, this would not affect the functionality, hence this smells like a bug, but now we have it. The workaround is to manually set the MAC address of the virtual adapter to match the one of your physical card.
You can do this by going to Settings for the affected virtual machine > Network > Advanced. Here, you can set the MAC Address field to your host's adapter one. Then, start your virtual machine and go happily about your business.
Now, this is a workaround, not a full and complete solution. With this change in place, your virtual machine will get the same IP address as your host. This also means that some applications may not necessarily work correctly, because your network has supposedly two identical hosts with the same shared address. You should then consider manually assigning a different IP address to your virtual machine, once it comes up.
I am never happy writing less-than-perfect tutorials, but sometimes, I feel it's better to give the readers some help, with the necessary disclaimers, than keep on sculpting my David for all eternity. Today, we have one such example. It's obvious we're facing a problem in the software, and thus, the most logical solution is to wait for an official fix. Happened before, can happen again.
Anyway, regardless, I hope you do find the information above useful. Not perfect, but might help you get out of a lurch, especially since this problem will affect you when you least need it, i.e. you will power on your VM, expecting everything to work, and then you suddenly won't be able to accomplish any network-related tasks. If you have any ideas, suggestions or experiences on this topic, feel free to mail them. We're done.