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How a VirtualBox Virtual Machine can Access the Internet

March 20, 2018

 This post reviews how a VirtualBox Virtual Machine (VM) can access the internet.

 

 

How a VirtualBox Virtual Machine can Access the Internet

 

TL;DR

 

A VM accesses the internet through the VirtualBox networking engine. The VirtualBox networking engine creates and manages a private network that the host cannot access. The host can access this private network with additional configuration. 

 

Detail

 

1. The VirtualBox networking engine sits between each VM and the host.

 

2. It maps traffic from and to the virtual machine when Network Address Translation (NAT) has been enabled. Click Network and look at what Adapter 1 is connected to:

 

 

3. The VMs network frames are received by VirtualBox’s networking engine.

 

4. The engine extracts the TCP/IP data and resends it using the Windows.

 

5. To an application on the Windows host it looks like the data was sent by the VirtualBox application using the Window's host IP. Another computer on the same network sees the same thing.

 

6. When the Window's host gets a reply, the VirtualBox application repacks the packets and resends them to the guest running Ubuntu on the private network it manages.

 

7. The VirtualBox’s networking engine also acts as a DHCP server for the Ubuntu guest.

 

 

Explore

 

With a guest running that's configured for NAT run:

  • ipconfig on the Windows host,

  • ifconfig on the Ubuntu guest and

  • browse to https://www.whatismyip.com/ on both the Window's host and Ubuntu guest.

Here's an example. The green arrows show the private network managed by the VirtualBox networking engine. The guest Ubuntu system has IP: 10.0.2.15. This same interface has the IP 169.254.255.141 on the Window's side. The external IP of both Ubuntu and Windows is the same 73.3.197.8.

 

 

The VirtualBox documentation describes why the guest's IP address is 10.0.2.15: 

 

9.11 Fine-tuning the VirtualBox NAT engine


9.11.1 Configuring the address of a NAT network interface

 

In NAT mode, the guest network interface is assigned to the IPv4 range 10.0.x.0/24 by default where x corresponds to the instance of the NAT interface +2. So x is 2 when there is only one NAT instance active. In that case the guest is assigned to the address 10.0.2.15, the gateway is set to 10.0.2.2 and the name server can be found at 10.0.2.3.

 

If, for any reason, the NAT network needs to be changed, this can be achieved with the following command:
VBoxManage modifyvm "VM name" --natnet1 "192.168/16"

 

This command would reserve the network addresses from 192.168.0.0 to 192.168.254.254
for the first NAT network instance of “VM name”. The guest IP would be assigned to
192.168.0.15 and the default gateway could be found at 192.168.0.2.

 

 

System

 

This write up was done using:

  • VirtualBox Version 5.1.30 r118389 (Qt5.6.2)

  • Windows 7 Professional SP1

  • Ubuntu 16.04 running in VirtualBox

Check VirtualBox, Windows and Ubuntu Versions

 

VirtualBox Version

 

Check your VirtualBox version by 1. clicking Help then 2. About VirtualBox...

 

Windows Version

 

Check your version of Windows by 1. clicking Start, 2. Right-clicking Computer and 3. clicking Properties.

 

Ubuntu Version

 

Check your version of Ubuntu  running on VirtualBox by clicking 1. then 2. Details:

 

 

References

 

  • Find out what your computers external IP address is with https://www.whatismyip.com/

  • A high-level write up on NAT is available at link

  • Info about how VirtualBox supports NAT can be found in the VirtualBox user manual Version 5.1.30 at: C:\Program Files\Oracle\VirtualBox\doc\UserManual.pdf chapter 6.3 Network Address Translation (NAT) 

  • The VirtualBox logo is from https://www.virtualbox.org/.

 

 

Outstanding Questions

 

  • Asked if its possible to see the Network Address Translation that VirtualBox’s networking engine maintains? See link for the question (and hopefully the answer).

  • Also asked, "Does NAT use the "Ethernet adapter VirtualBox Host-Only Network?" See link for the question (and hopefully the answer).

 

 

 

 

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