OK, normally in the UK most users buy a combined router / ADSL modem, so there are not two separate devices since you can get a 4 port + Wireless + router/firewall + ADSL modem for about $100, there's not much point in doing otherwise. If you have a separate router and modem then this makes things a little more complicated, but point is that you don't need to work out whether it's your router or your modem and
you don't need to start removing things, you only need to give your VM the correct search list.
I'll talk you through what I did in my case. First my JeOS build didn't have the dnsutils installed so I had to do a
sudo apt-get install dnsutls. I then catted
/etc/resolv.conf which gave my name server as 10.0.2.3 which is what you would expect with NAT. I then googled my ISP to find the page which tells me what my ISPs DNS servers are. This took me to a page which told me that the servers are indnsc
N.ukcore.bt.net where
N are various 2 digit numbers. Pinging these often fails because NS are often configured not to respond to IRCP, however you can just
first from the host and then the same from the guest to find out which were working, though you may need to use the resolved IP address of the DNS server in this second case. I decided to use indnsc41.ukcore.bt.net (194.74.65.69), so I added to my
/etc/resolv.conf.tail Now my guest will first use the VBox DNS function, but if this fails it will fall back to resolve using a specific BT DNS server. Clearly you don't want to pick a BT DNS server but use one in your ISPs infrastructure since this is going to be a lot nearer hop-wise.