"Auto-detect" option for gateway breaks my outbound connection

[Apollo13]

And another question...

Last year I wrote a post about how selecting the "auto-detect" option in my WAN interface breaks the outbound connection on our OPNSense (in version 19.1) [https://forum.opnsense.org/index.php?topic=13456.msg61967#msg61967]. Due to a lack of time I could not investigate further in the last months and in the meantime this thread has been moved to the archive, thus I cannot reply there and open a new thread here... B/c my situation is still the same as in my previous post (just with version 20.1.1) I just copy & paste the relevant part:

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The situation in short: My OPNSense has a public WAN IP (e.g. 100.0.0.100) and I created a "gateway" entry (e.g. for IP 100.0.0.1), which I marked as "default". As soon as I select "auto-detect" on my WAN interface the attached clients on LAN are unable to reach any external system. Switching to gateway "100.0.0.1" restores the capabilities.
The OPNSense itself is able to access (ping) all hosts regardless of the concrete selection.
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I was asked if there were any outbound NAT rules when the "auto-detect" was selected - and in fact those rules are missing. If the gateway is selected I see two rules instead...


Any help on that topic is much appreciated!