Ports opened in router show up as closed with external port scan

I am currently trying to setup VPN on a 2000 server using Microsoft
VPN using L2TP with shared key authorization through a Linksys router
connected to a DSL modem. Been running into connection error 789. In
the router I have enabled Port Forwarding of port 1701, 500 and 1723
using both UDP and TCP on the router. When I run a port scan only port
1723 comes back as opened. The others claim to be closed. Any reason
why this would be occuring. They are clearly opened in the router
settings.

Thanks in advance,
Adam

Kurt
Wed Dec 26 20:09:54 PST 2007

amn0270 wrote:
> I am currently trying to setup VPN on a 2000 server using Microsoft
> VPN using L2TP with shared key authorization through a Linksys router
> connected to a DSL modem. Been running into connection error 789. In
> the router I have enabled Port Forwarding of port 1701, 500 and 1723
> using both UDP and TCP on the router. When I run a port scan only port
> 1723 comes back as opened. The others claim to be closed. Any reason
> why this would be occuring. They are clearly opened in the router
> settings.
>
> Thanks in advance,
> Adam

1723 is for PPTP VPNs. Port forwarding shouldn't be required for an
L2TP, but you'll need to verify that your router (I'm assuming it's
performing NAT) is capable of L2TP NAT traversal (which still may not
work). If it is, enable it. When something goes through a NAT, it's port
numbers are changed. With NAT traversal enabled, the packets are not
altered, so the AH and other checksums will still add up. I would
suggest PPTP for client-to-LAN VPN connections and IPSec VPNs using
hardware (linksys or secure computing routers) for LAN-to-LAN.

Kurt