Clarify "loopback not working" for me!

[Pila]

Many of us, including me, claim that loopback does not work for them on Asus routers (e.g. Asus RT-AC68U). And others say it should. Could it be that the DoubleNAT caused problem and we blame Asus router for it? Help me understand and correct me if I am wrong, please!

My test case:
1) modem has dynamic external WAN IP 1.2.3.4 to which my.ddns.com points to
2) Asus router is connected to the modem using DMZ from the modem: modem is 192.168.0.1, and my router's WAN IP is 192.168.0.100 - DoubleNAT as modem can not go to the bridge mode.
3) LAN side of my Asus router is 192.168.1.x
4) a mail server on at 192.168.1.91.
5) e-mail clients have their mail server set as my.ddns.com from 1)

a) I connect with a computer from outside, my e-mail client 5) looks for 1) my.ddns.com which points it to 1.2.3.4, from where it gets to the 2) router, which 3) port forwards it to 4) mail server. All is perfect.

b) I am within my (W)LAN and my computer has an address 192.168.1.90. Mail client tries to connect to the mailserver using my.ddns.com again

b.1) If I have loopback set to Asus mode, everything fails. No connection to my mailserver at 192.168.1.91 is possible. To complicate issue: after some poking with other things, everything did start to work perfectly using the Asus Loopback. I did not know why. But, after a router reboot, it stopped working until I did my "own loopback".

b.2) if I make my own loopback at my router, using the help I got here:
create: /jffs/config/dnsmasq.conf.add and put into it:

address=/my.ddns.com/192.168.1.91

mail works perfectly! Regardles of the loopback setting. No problems regardles of the use mode, for any of the computers, ever!

c) Here is what confuses me, and what prompted me to question if the problem is caused by the DoubleNAT and not the Asus router loopback?

c.1) with Asus loopback b.1) activated, ssh to any of my LAN connected computers, I get:

# nslookup my.ddns.com
Server: 192.168.1.1
Address 1: 192.168.1.1 router.asus.com

Name: my.ddns.com
Address 1: 1.2.3.4

c.2) if I do have my loopback b.2) activated, and ssh to any of my LAN connected computers, I get:

# nslookup my.ddns.com
Server: 192.168.1.1
Address 1: 192.168.1.1 router.asus.com

Name: my.ddns.com
Address 1: 192.168.1.91 hMailServer

So, up until here, everything are pure facts. Mail server works perfectly when setup in b.2) way.

Am I wrong to say that b.1) is a loopback issue and that c.1) shows loopback works on my Asus router correctly?

If both are true, then DoubleNAT is the problem. Loopback in my modem is a problem. Not loopback in Asus router.

 

[ColinTaylor]

b.2) and c.2) is not loopback, it's just a local DNS entry. That's why it works - there is no loopback involved.

...that b.1) is a loopback issue

Yes, with the modem.

and that c.1) shows loopback works on my Asus router correctly?

No. All that shows is a DNS entry.

... DoubleNAT is the problem. Loopback in my modem is a problem. Not loopback in Asus router.

Correct.

 

[Pila]

I simplifed my terminology as it was long as it is My fix b.2) is not actually loopback since a router is not capable of doing loopback in DoubleNAT network - it simply does not know WAN IP address to loop anything back from it.

So, for anybody with DoubleNAT situation (meaning: Asus router does not see the actual WAN IP address but a private one given to it by the modem), the following is true:

- DDNS will not work from GUI unless handled manually in a script

- Loopback is not done by the router so Loopback at the router is actually completely irrelevant. Loopback should be done in a modem.

I was looking at many loopback threads here, and I have never seen that explanation! Everybody just claims Loopback does not work for them and the reply is that it should (on some models).

OK: DDNS could be solved better and directly, but we are talking here about Asus Routers.

So, if someone complains about Asus loopback not working, the first question should be: is the router in a DoubleNAT?

But, I still have no explanation why loopback started to work for me until a router reboot.

 

[Pila]

Interestingly, I have seen here many people who claimed they had a problem with Asus Loopback, but not a single one of them had anything to say regarding my above post