Getting Linux Virtual Machines on a Windows Host access to the Internet through an NTLM proxy

I’ve been doing a lot of work with Virtualbox recently, and needed to get some Linux Guests access to the Web on a Windows Host which uses an NTLM Authenticated Proxy Server.

The default network configuration of Virtualbox is to use a NAT network, which normally just gives the machines Internet access if a transparent proxy is in use, but in this case, even setting the proxy manually in the guest OS wouldn’t work due to the use of NTLM authentication.

The solution I’ve come up with is to run an unauthenticated proxy on the Windows Host which proxies the traffic from the Linux Guests and takes care of the downstream authentication.

There are two choices to hand. Cntlm takes a bit of work to setup but is lightweight, while Fiddler is a bit heavier but is widely used and doesn’t require command line setup.

You’re likely to find it easier to get started up with Fiddler, either way, both options are here:

Cntlm Setup

Desktop Environment


Set Environment Variables (Terminal)

Fiddler Setup

Fiddler Options


Setup Proxy on Linux Guest Desktop Environment


Set Environment Variables (Terminal)