Use comparisons to find where the failure occurs: try the site in another browser, on another device on the same network, and—if policy permits—on the affected device using another trusted network. Then check whether DNS resolves the hostname and look for network or security alerts. These clues narrow the cause, but a browser error or successful DNS lookup alone cannot identify the blocker.
Start by locating the scope of the problem
First note whether one site or many sites fail. Test the same address in another browser, then ask whether another device on the same network can open it. If allowed, test the affected device on a different trusted network. Change one variable at a time so the results are useful.
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- Only one browser fails: a browser proxy setting, Secure DNS, extension, or managed browser policy is plausible. Mozilla recommends trying another browser as an early check. Mozilla’s cross-browser troubleshooting guide also covers security software and other connection causes.
- Several browsers fail on one device: the device’s DNS configuration or cache, proxy or VPN, firewall, or endpoint protection may be involved.
- Several devices fail on the same network: shared DNS, a firewall, proxy, gateway, ISP, or the website itself becomes more plausible. A separate trusted network can help distinguish a local network issue from a broader one.
These patterns are clues, not proof. For example, a network-wide failure does not by itself show whether the router, ISP, or website is responsible.
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Check whether DNS resolves the site name
Open a terminal or command prompt and run nslookup example.com, replacing example.com with the affected site’s hostname. Note whether you get an answer, a timeout, or a name error, and record which DNS server the command used. Microsoft uses nslookup in its Global Secure Access diagnostics to check whether the configured DNS server responds.
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- No answer or a timeout: the device’s resolver configuration, DNS server, cache, browser DNS setting, or a network path blocking DNS may be involved. Compare the result on another device, if available.
- An answer is returned: DNS provided a result, but the site may still be unreachable because a firewall, proxy, VPN, endpoint security product, TLS or certificate problem, or the site itself prevents the connection.
A DNS timeout does not prove the DNS server is faulty: a firewall or other network control could block access to it. Likewise, a successful lookup does not establish that the later connection is allowed.
Compare the browser’s DNS and proxy path
Browsers can use Secure DNS, also called DNS over HTTPS (DoH), and may have proxy settings that differ from the operating system’s. That means a browser-specific failure can follow a different name-resolution or connection path from another browser. Compare the affected browser’s Secure DNS and proxy settings with a browser that works, and check for extensions or managed policies if the device or browser is organization-managed. Google’s Chrome Enterprise policy documentation describes policy controls for DNS over HTTPS.
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Do not turn off DoH or change a managed DNS or proxy setting as a general-purpose fix. Microsoft’s instruction to disable browser DoH applies to a specific Global Secure Access FQDN-forwarding health-check scenario, not to all networks. Follow your organization’s guidance before changing managed settings.
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If the hostname resolves but the connection fails, compare the result on another trusted network only if that is permitted. A site that works elsewhere may point toward a rule or failure on the original network, but it does not identify which device or policy caused it. Ask the network administrator to review the firewall, proxy, VPN, or gateway logs for the hostname and time of failure.
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Organizations may require a proxy for network traffic, including connections from endpoint security services to their cloud destinations. A missing or unavailable proxy can therefore create symptoms beyond a single website. Microsoft documents connectivity requirements and proxy considerations for Microsoft Defender for Endpoint; its requirements are specific to that product and deployment.
Check endpoint security alerts
Look for an operating-system security notification that appeared when you tried to open the site. If the device is managed, ask the endpoint administrator to check alerts or URL and domain events for the affected device, application, and time.
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Microsoft Defender for Endpoint’s network protection can block access outside the browser. In that situation, the browser may display only a generic connection error while Windows shows a notification. The product’s network protection documentation explains its behavior. Other endpoint products may show different alerts, so the absence of a notification does not rule out a block.
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- Certificate warning: check that the device’s date, time, and time zone are correct. An incorrect clock can interfere with certificate validation and may be mistaken for a network block.
- Possible stale DNS cache: Mozilla documents
ipconfig /flushdnsfor Windows and restarting macOS to clear its DNS cache. Use current instructions for your operating system; cache-clearing steps differ by platform. Treat this as a test, not proof that the cache caused the problem.
Do not switch to a public or alternate DNS resolver on a managed network unless your administrator approves it. Changing DNS may violate network policy or disrupt organization-specific name resolution.
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Use the observed pattern to choose the next check
| Observation | What it suggests | Next check |
|---|---|---|
| Hostname lookup fails on one device | The device resolver, configured DNS, cache, browser Secure DNS, or a path blocking DNS may be involved. | Compare nslookup results and the resolver used on another device; compare the affected browser’s DoH setting. See Microsoft’s DNS diagnostic example and Mozilla’s troubleshooting guide. |
| Lookup succeeds, but the page connection fails | DNS returned a result; a later network or security layer, certificate issue, or the website itself may still prevent loading. | Compare a permitted device or network and ask administrators to review relevant logs. See Defender network protection and Defender connectivity guidance. |
| The site fails in one browser only | A browser-specific proxy, Secure DNS setting, extension, or policy is plausible. | Try another browser and compare proxy and Secure DNS settings. See Mozilla’s cross-browser checks and Chrome’s DoH policy documentation. |
| Several devices fail on one network | Shared DNS, firewall, proxy, gateway, ISP, or site-side causes are more plausible than a single-browser setting. | Test on a separate trusted network if allowed; ask the network operator to inspect shared infrastructure. Mozilla notes that an ISP issue can exist even when modem or router indicators look normal. See Mozilla’s guide. |
| A generic browser error coincides with a security alert | Endpoint network protection may have blocked the connection outside the browser. | Ask the endpoint administrator to review the alert’s URL or domain, device, and application details. See Microsoft’s network protection documentation. |
Escalate with observations, not a guess
For a managed device or network, give the administrator the details needed to match your test with logs:
- The exact hostname and the time the failure occurred.
- The device, operating system, browser, and exact error message.
- Whether another browser, another device on the same network, or the affected device on another permitted network worked.
- The
nslookupresult and the resolver it used. - Any operating-system security notification or endpoint alert.
Do not disable protection or try to bypass a policy to diagnose the problem. A firewall or endpoint product may be intentionally blocking a malicious or organization-restricted destination; ask the administrator to verify the reason and approve any test. Product menus, policies, alert behavior, and network requirements vary by platform and deployment, so vendor-specific examples are not universal instructions.
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