Unfortunately, ipv4 complicates this due to NAT.
Fortunately D-Link's easy solution works some of the time
just type junk into the top box
type the local ip of the server into the box below
select other
and type 1 into the box below
apply by clicking
SAVE SETTINGS and click the
REBOOT NOW button
If it doesn't work, then either you have a problem with a firewall on your server (probably not, unless it is a current project your working on, and not a tested working solution) or you are just out of luck.
I have not had great luck with virtual server, some genius set it up to where GRE packets are blocked by the box even if allowed in virtual server.