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JuniorJr's avatar
JuniorJr
Aspirant
Jul 30, 2018
Solved

Port forwarding stopped working after moving DHCP from router to a server

Hi. I have had remote desktop ports working fine through our router (from the Internet) for years actually. When I changed the DHCP server from the router to a internal Windows server... now I ca...
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  • antinode's avatar
    antinode
    Jul 31, 2018

    > The R6300 gets a 192.168.0.3 address from the LAN side of the cable
    > modem. [...]

       Then that's not a cable modem; it's a cable modem+router, you are
    cascading two routers, and the resulting "double NAT" is defeating your
    attempts at port forwarding (on the inner router).

    > [...] The cable modem on the Internet side gets the public IP. [...]

       Because it's a modem+router, not a simple modem.

       With only one NAT router, this stuff could work (with changes).  The
    obvious choices are (1) disable the router in the cable modem+router,
    making it a modem-only, and use the R6300 as the router; or (2) do the
    port forwarding on the router of the cable modem+router, in which case,
    it might be better to configure the R6300 as a wireless access point.

    > [...] It has worked for years and now nothing "port forwards". [...]

       I can believe that it worked if you had one router, and not two
    routers.  Now that you have two routers, you're approximately doomed,
    until you return to an arrangement with only one router.

       So, nothing to do with any DHCP server anywhere.  (Although, a
    one-router/one-subnet configuration may change your DHCP server
    requirements from what they are now to something simpler.)

JuniorJr's avatar
JuniorJr
Aspirant
Jul 31, 2018

And there you have it.  I asked them today (I'm never on-site there)  if the ISP put in a new device.  "Oh yeah, they did actually".

Granted, I should have spotted it.... but you did.

Thanks. Will have to sort this out now.

antinode's avatar
antinode
Guru
Jul 31, 2018

> [...] if the ISP put in a new device.  "Oh yeah, they did actually".

   Trust no one, I always say.

> [...] Will have to sort this out now.

   What could go wrong?

   The usual problems with this stuff are:

   1. Wrong external IP address (different from the port-forwarding
router's WAN/Internet IP address).  (An intermediate NAT router, for
example.)

   2. Bad port-forwarding rule (wrong port(s), wrong target address --
including a wandering target).

   3. Server not listening on the port-forwarding target system.

   4. External influences: ISP blocking, other firewalls, ...