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Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 20.0.5): 27.1.2
Operating system and version (eg, Ubuntu 20.04): Oracle Linux Server release 8
Apache or nginx version (eg, Apache 2.4.25): 1.20.1
PHP version (eg, 7.4): 8.2.11
The issue you are facing:
Since I migrated to version 26.0.7 then 27.1.2 , In Administration > overview, I have this error :
This server has no working internet connection: Multiple endpoints could not be reached. This means that some of the features like mounting external storage, notifications about updates or installation of third-party apps will not work. Accessing files remotely and sending of notification emails might not work, either. Establish a connection from this server to the internet to enjoy all features.
Is this the first time you’ve seen this error? (Y):
This was working in past without any error. My server has no direct connexion to internet but use proxy.
so DNS lookup is no working directly when I try for example : nslookup www.nextcloud.com, give no response.
but wget www.nextcloud.com, for example, is working because I use proxy.
Output errors in nextcloud.log in /var/www/ :
{"reqId":"ut47NCSoxwaHPiEKbGsQ","level":3,"time":"2023-10-12T14:34:44+02:00","remoteAddr":"x.x.x.x","user":"admin","app":"internet_connection_check","method":"GET","url":"/settings/ajax/checksetup","message":"Cannot connect to: www.nextcloud.com","userAgent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/117.0.0.0 Safari/537.36","version":"27.1.2.1","exception":{"Exception":"OCP\\Http\\Client\\LocalServerException","Message":"No DNS record found for www.nextcloud.com"
[...]
File":"/u01/app/nextcloud_install/nextcloud/lib/private/Http/Client/DnsPinMiddleware.php","Line":132,"message":"Cannot connect to: www.nextcloud.com","exception":{},"CustomMessage":"Cannot connect to: www.nextcloud.com"}}
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
'has_internet_connection' => true,
'proxy' => 'myproxy:port',
Seems in the file DnsPinMiddleware.php, this add had been done some times ago, causing my error :
if (empty($targetIps)) {
throw new LocalServerException('No DNS record found for ' . $hostName);
}
When I comment this 3 lines, error connexion disappear and I can again have connexion to internet (when I check Overview page or Apps page to have the update of apps).
Can someone say me if it can be this adding in DnsPinMiddleware.php which can be the root cause ?
Other point : When I made the same upgrade to 26.0.7 then 27.1.2 on another server where connexion internet is UP, all is OK. So I can suppose the modification in DnsPinMiddleware.php can affect a server in a LAN, as in my case. Because my 2 servers have the same configuration.
Thanks for help