I connect 2 Virtual Machines on VirtualBox via Internal Network.
1st Machine - Ubuntu:
2nd machine - Kali:
How can I test from both machines that they talk to each other, e.g. ssh, ping or see files in another machine?
You have your machine set up as NAT and they are on a separate subnet from the host machine.
Make sure Network Discovery and File/Print Sharing are ON (or the counterparts for your LINUX OS).
Then map folders by IP address. You need IP address because in order to see machines without a DNS server.
In Linux you may need to ensure Samba has been set up and working.
Try SMB: SMB://192.168.x.y/folder (where folder has been permitted to the users.
If you need to set up Samba, look to this tutorial.
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Also look at changing to Bridged Networking for the Guest Machines. Bridged networking will give the machines an IP on your router just like your host machine and may possibly make things easier.
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How can I test from both machines that they talk to each other, e.g. ssh, ping or see files in another machine?
Once you have set up the mapping, you should be able to see each machine from each other.
I have done that here on my virtual machines.