Double NAT / DDNS problem with Bridge Mode Cable Modem

[bozolino]

My LAN router (ASUS 86U with Asus-Merlin) connects to the Internet via a cable modem that has been set to bridge mode by my cable provider.

When I poll my own WAN IP from my LAN, then use a machine on the WAN to connect back to my LAN, it works seamlessly: i get a 37.xx.xx.xx address to access the router config pages and all attached machines.

However, the WAN IP that my router displays on its network status page is different from my "real" WAN ip - it's a private ip @ 100.xx.xx.xx, and my router gives me a "double NAT" warning on the DDNS config page.

My impression was that Double NAT only happens when the cable modem is in router mode - which, in my case, it isn't. Can anybody explain where this private ip still comes from? And how can I make the router correctly configure DDNS with my "real" WAN ip?

 

[adampk17]

My LAN router (ASUS 86U with Asus-Merlin) connects to the Internet via a cable modem that has been set to bridge mode by my cable provider.

When I poll my own WAN IP from my LAN, then use a machine on the WAN to connect back to my LAN, it works seamlessly: i get a 37.xx.xx.xx address to access the router config pages and all attached machines.

However, the WAN IP that my router displays on its network status page is different from my "real" WAN ip - it's a private ip @ 100.xx.xx.xx, and my router gives me a "double NAT" warning on the DDNS config page.

My impression was that Double NAT only happens when the cable modem is in router mode - which, in my case, it isn't. Can anybody explain where this private ip still comes from? And how can I make the router correctly configure DDNS with my "real" WAN ip?

Is your cable modem acting as a DHCP server? It would seem so since your Asus router doesn’t see your “real” public IP address.

Side question, what are you using as LAN IP range? I wouldn’t think that a cable modem would use a routable address (100.) by default.

 

[bozolino]

the DHCP/router functionality on the cable is disabled. when DHCP on the cable modem was on, the address assigned to my asus router was in the 192.168.0.xxx range (the cable modem itself was on 192.168.0.1).

the LAN ip range on my asus router is 192.168.12.xxx

 

[ColinTaylor]

100.64.x.x to 100.127.x.x is Carrier-grade NAT, so that is happening on your ISP's network not your cable modem. I'm not sure how your 37.x.x.x address fits into this, maybe your ISP has put your CGN address in a DMZ.

 

[bozolino]

@ColinTaylor: totally makes sense. thank you so much! I guess then I have to grab the script for my DDNS provider (selfhost.de) and modify it to use the WAN ip? where on the router can i find that script?

 

[owine]

@ColinTaylor: totally makes sense. thank you so much! I guess then I have to grab the script for my DDNS provider (selfhost.de) and modify it to use the WAN ip? where on the router can i find that script?

Use a ddns-start script to do the "manual" update method for your DDNS provider. You can pull your external IP in plaintext from http://icanhazip.com and plug that into the updater. Not familiar with your DDNS provider but I presume they have some sort of URL based update method where you can specify the IP address to update with.

 

[bozolino]

Thanks! I assume I can work off the built-in script for my ddns provider. does anybody know if i can find the built-in scripts somewhere on the router‘s file system?

 

[owine]

Thanks! I assume I can work off the built-in script for my ddns provider. does anybody know if i can find the built-in scripts somewhere on the router‘s file system?

I did not realize this was one of the supported providers in the firmware. This makes it easier, you can just call ez-ipupdate. Put the below code in a ddns-start script and sent your DDNS method to Custom. You will need to fill in USER PASS and HOST which are pretty self explanatory.

#!/bin/sh

SERVICE="selfhost"
USER=""
PASS=""
HOST=""
IP="`curl -4 http://icanhazip.com`"

ez-ipupdate -S $SERVICE -u $USER:$PASS -h $HOST -a $IP

/sbin/ddns_custom_updated 1

 

[bozolino]

Heureka! It works. Thanks a lot!