mitchellh/vagrant — pull request #749 by schisamo (2012-02-19 16:18:24+00:00)
Title: shield the guest from nameserver changes when using :hostonly networking

I provisioned a VM at coworking today and the guest VM inherited the nameserver info from that network:

```
vagrant@piab-private-chef:~$ cat /etc/resolv.conf
nameserver 192.168.34.251
domain thebox
search thebox
```

I went home where I expected this configuration:

```
nameserver 208.67.222.222 # opendns
nameserver 10.66.44.1 # internal router
```

VirtualBox ships with the [ability to enable a DNS proxy in NAT mode](http://www.virtualbox.org/manual/ch09.html#nat-adv-dns) to shield the guest VM from this issue. When this mode is enabled the guest VM receives this configuration:

```
vagrant@piab-private-chef:~$ cat /etc/resolv.conf
nameserver 10.0.2.3
```

`10.0.2.3` is a DNS proxy that 'does the right thing'. We should probably enable by default when Vagrant has a network type `:hostonly`. This commit does just that!