Impossible!
Like explained by "OICURMT2!".
There is no port forwarding of incoming ICMP requests cause this is not how NAT works.
Everyone is missing one little piece of information...
No routers can forward ICMP, as they are not TCP or UDP. So if you can "ping" your box, what you are really pinging is your external router/modem. Also, ICMP does not use ports, so how can you forward one?
ICMP was designed to directly echo packets for a response time for the device
directly associated with the IP address. While ICMP does contain an IP header, routers do not forward them because it is a "diagnostic" protocol. Imagine if everytime you issued a
traceroute command the packets were forwarded to a different location. Systems engineers would never be able to find where the fault it. It passes messages...
The only thing I can think of is by using DNAT, which is reserved for IP range forwarding, but really isn't designed to forward ICMP queries, but may be able to do it via a mangled pre-route similar to the following:
iptables -t nat -A PREROUTING -p icmp -d external.ip.address --dport 7 -j DNAT --to-destination internal.ip.address.range
Note that port 7 is the "echo" port, which is NOT ICMP ping (as ICMP does not use ports), but rather the actual "echo" service...
The problem with the iptable entry is that as an ICMP packet arrives to your external device, it would broadcast to EVERY internal device, which in turn would send back replies. The recieving machine would register an error, as the return would not be valid. I tried to test this, but couldn't get anywherre with it.
There may be a kernel level modules out there that does this, but I doubt it.
BTW: If you are talking about the ping to your server on the 25Assist panel, I'd bet dollars to donuts that that particular ping is the message return from the UDP query port and not an ICMP return.
I could be wrong (which SWMBO tells me constantly...

) but I suspect I'm not...
OIC!