ncsi-probe

Send the same connectivity probe Windows uses before it shows "No internet", from any machine with bash and curl, and see how it fails.

Microsoft documents the Network Connectivity Status Indicator (NCSI) active probe: an HTTP GET to

http://www.msftconnecttest.com/connecttest.txt

answered by a body containing Microsoft Connect Test. Windows sends an IPv4 and an IPv6 probe in parallel; if either succeeds, it reports internet connectivity. Anything else (a timeout, a redirect from a captive portal, a 403 from a proxy, a login page served with a 200) and the icon changes to "No internet".

ncsi-probe.sh sends both probes and reports, for each one:

• OK: HTTP status and the expected body
• redirected: a captive portal (hotel, café, airport, train Wi-Fi) answered instead
• refused (403): a proxy or filter blocked the probe
• wrong reply: something answered, but not with the expected body
• no reply: no route, DNS failure or timeout

Usage

curl -O https://raw.githubusercontent.com/AiToolsPrimer/ncsi-probe/main/ncsi-probe.sh
chmod +x ncsi-probe.sh
./ncsi-probe.sh

Example on a network without IPv6:

IPv4: OK, HTTP 200 and the expected body
IPv6: no reply (no route, DNS failure or timeout)
Verdict: internet (at least one probe got the expected reply)

Exit code is 0 when at least one probe passes, 1 when both fail (as Windows would report "local only").

What it does not do

• It does not reproduce NCSI's passive probe, which watches incoming traffic and counts network hops (Microsoft's default threshold is 8, checked every 15 seconds).
• It does not change any system setting. Microsoft advises against disabling the active probe as a workaround, because passive polling alone cannot detect every kind of failure.

Sources

• Microsoft: NCSI overview and NCSI frequently asked questions (Windows Server documentation)
• Microsoft Support: Fix Wi-Fi connection issues in Windows
• Explained in plain English: Connected without internet, and the test behind the words (AI Tools Primer)

License

MIT