Question : ICS port forwarding not working - XP SP3

I have commented on the related thread, with my situation

here is my comment:
"i'm sorry but this is not a solution, it's a workaround

this problem is completely dubious.
3 years past this post, it happens to me, on xp sp3
it's so spurious . . why has nobody else complained about this
port forwarding will not work
firewall is off
antivirus uptodate

i telnet to the machine with the service on the internal port, in the interior lan, it works.
from the same machine i telnet to the external adress and external port yet it does not get through

i've switched with another ics server, the same conditions, sp3 uptodate
the same thing happens - what am i missing??

one peculiar thing i do is set a false MAC for the external NIC to get service from the isp

but this can't and shouldn't impact on port forwarding"

thank you to anybody willing to read that description and answers, and to suggest causes for this issue

Answer : ICS port forwarding not working - XP SP3

>i've also tried from an outside ip to get to the ics server (on the isp public ip) and
>forwarded port. but since the case above did not work, this case couldn't have
>worked

OK - the above statement is wrong, proceeding from a false assumption.

First, do *not* try testing this from an internal machine until you get it working for external (non-local) machines.  I can't speak as to why it won't work for external (non-local) machines, but you will encounter additional difficulties testing from internal machines even after you get it working for external machines (I bet you probably *have* gotten it working off and on for external machines, but you never noticed since you're testing from a local internal machine).


I can't believe no one has brought up the typical problem of connecting to one local machine from another via external NAT...

It goes something like this (OK, *exactly* like this).  Local machine A tries to talk to local machine B via a port-forward on the local firewall by using an external address [assigned to the firewall].  The external firewall address gets translated to the internal address of B, thanks to the port-forward NAT on the firewall, however the source address (machine A) remains the same.  Machine B attempt to respond directly to A (since both src and dst are on the same network).  However, A is expecting a response with a src of the address to which it originally connected, which is the external address on the firewall.  Since A is not expecting a response with a src of B, it ignores the packet, and you never get connected.

Fix the port-forward by testing only external machines.  Then, if you really need it to work from internal machines, buy an actual firewall.

Cheers,
-Jon