Just a heads up if you have no IPv4 wired network in Virtualbox 5.1 when installing 18.04 beta. The solution I found is set IPv6 to “ignore” then re-select Wired Connection 1 and I’m good to go.
Some possibly significant details:
I also saw this every install of Daily Builds.
I am not using current Virtualbox 5.2. This is a 16.04 MATE 1.16.2 host.
I have no IPv6 but do have a completely aware IPv6 network AFAIK.
I always set Virtualbox to use a BRIDGING network adapter.
The network “Connection Information” DOES show perfectly good IPv4 address and DNS for my network.
Hope this helps someone. Now back to your normally scheduled community. 
Good to know Bill.
I’m running 5.2 on a win10 host and using NAT; works pretty sweet. I would suggest also giving it a try as well.
“letting systems inside communicate with each other and with systems outside using TCP and UDP over IPv4 and IPv6”
https://www.virtualbox.org/manual/ch06.html#network_nat_service
NAT has its place and it has been VBox’s default but for testing out distros I want them joining the network like a real box will.
I narrowed my issue down to the systemd equivalent of dnsmasq - that local caching server/utility that insists on installing on most distros these days. With IPv6 set “Auto” I get a perfectly good primary IPv6 DNS address that works fine on the local network (using dig or nslookup has no problem when forced to my IPv6 local DNS). But using that 127.0.0.53 thing, which SHOULD just use the same server, always gets a SERVFAIL response.
My DNS server (Ubuntu Server 16.04 bind9) hasn’t got much use IPv6 so it could be picky about something yet it only seems to fail going through 18.04’s DNS stuff. I’ll be exploring it more.
