Public bug reported:

Description:

The system shows a question mark on the network icon and prompts "Sign in to 
Network" when connected. Internet access is actually available. The 
NetworkManager connectivity check to http://connectivity-check.ubuntu.com./ is 
intercepted by the local router, triggering the captive portal state. If the 
connectivity check URI is manually changed to 
https://connectivity-check.ubuntu.com./ in the NetworkManager configuration, 
the issue disappears and the system correctly identifies that it is connected 
to the internet.
Steps to Reproduce:

    Connect to the network (behind a MikroTik router intercepting port
80).

    Wait for NetworkManager to perform the automatic connectivity check.

    Observe the network icon showing a question mark and a "Sign in to
network" notification appearing.

    Edit the NetworkManager connectivity configuration file (e.g.,
/usr/lib/NetworkManager/conf.d/20-connectivity-ubuntu.conf) and change
the URI to https://connectivity-check.ubuntu.com./.

    Restart NetworkManager (sudo systemctl restart NetworkManager).

    Observe that the network is now instantly recognized as fully
connected.

Expected Behavior:

NetworkManager should correctly determine internet availability, or there 
should be a fallback mechanism to verify if the HTTP intercept is a false 
positive when standard internet access is otherwise available.
Actual Behavior:

NetworkManager triggers a false captive portal sign-in prompt because
the local equipment intercepts the unencrypted HTTP check.

ProblemType: Bug
DistroRelease: Ubuntu 26.04
Package: network-manager 1.54.3-2ubuntu1
ProcVersionSignature: Ubuntu 7.0.0-7.7-generic 7.0.0-rc3
Uname: Linux 7.0.0-7-generic x86_64
NonfreeKernelModules: zfs
ApportVersion: 2.33.1-0ubuntu4
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: ubuntu:GNOME
Date: Sat Mar 21 19:34:22 2026
InstallationDate: Installed on 2026-03-13 (8 days ago)
InstallationMedia: Ubuntu 26.04 LTS "Resolute Raccoon" - Daily amd64 (20260313)
IpRoute:
 default via 192.168.0.1 dev wlp0s20f3 proto dhcp src 192.168.0.18 metric 20600 
 172.17.0.0/16 dev docker0 proto kernel scope link src 172.17.0.1 linkdown 
 192.168.0.0/24 dev wlp0s20f3 proto kernel scope link src 192.168.0.18 metric 
600
ProcEnviron:
 LANG=en_US.UTF-8
 PATH=(custom, no user)
 SHELL=/bin/bash
 TERM=xterm-256color
 XDG_RUNTIME_DIR=<set>
SourcePackage: network-manager
UpgradeStatus: No upgrade log present (probably fresh install)
nmcli-nm:
 RUNNING  VERSION  STATE                  STARTUP  CONNECTIVITY  NETWORKING  
WIFI-HW  WIFI     WWAN-HW  WWAN     METERED      
 running  1.54.3   connected (site only)  started  portal        enabled     
enabled  enabled  missing  enabled  no (guessed)

** Affects: network-manager (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug resolute wayland-session

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https://bugs.launchpad.net/bugs/2145100

Title:
  NetworkManager falsely detects Captive Portal via HTTP (MikroTik
  router), works with HTTPS

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