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To find out what is stopping a website, compare the same site across browsers, devices, and trusted networks, then check DNS results and security alerts. A browser error alone rarely identifies the cause: DNS can resolve successfully even when a firewall, proxy, VPN, endpoint protection, or the website itself prevents the connection.
Start by narrowing down where the failure occurs
Try the checks in this order, changing one factor at a time. These comparisons help locate the problem; they do not prove its cause. If this is a work or school device, follow your organization’s policy and do not disable protection or bypass network controls.
- Check the scope. Try the same website in another browser on the affected device. If it works there, focus first on the failing browser’s proxy, Secure DNS, extensions, and managed policies. Mozilla recommends testing another browser when Firefox and other browsers cannot load websites: Mozilla’s website-loading troubleshooting guide.
- Compare another device on the same network. If the site fails on multiple devices, a shared network component or the site itself becomes more plausible than a problem limited to one device. If it works elsewhere, investigate the affected device and browser.
- If permitted, test on another trusted network. A different result can point toward the original network’s DNS, firewall, proxy, VPN, or gateway. Do not use an untrusted network or change a managed connection without approval.
These comparisons are clues, not a diagnosis. A site outage or a block at more than one layer can produce similar patterns.
Check whether DNS returns an answer
On Windows, open Command Prompt or PowerShell and run nslookup example.com, replacing example.com with the site’s hostname. Note whether the command returns an address, times out, or reports a name error; also record the DNS server shown in the result. Microsoft uses nslookup in its Global Secure Access diagnostics to check whether the configured DNS server responds: Microsoft’s Global Secure Access troubleshooting guide.
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- No answer or a timeout: The device’s resolver settings, DNS server, cache, browser DNS path, or a network path blocking DNS may be involved. A firewall can block access to a DNS server, so a timeout does not prove the DNS server itself is faulty.
- An answer is returned: DNS provided an address, but the site may still be unreachable. A firewall, proxy, VPN, endpoint protection, TLS or certificate issue, or the website itself can stop the connection later.
Browser Secure DNS or DNS-over-HTTPS can send lookups through a different resolver path from the operating system. Compare browser settings and, where possible, results from another device or network. Managed deployments can have specific DNS requirements: Microsoft’s instruction to disable browser DoH applies to a particular Global Secure Access FQDN-forwarding health-check scenario, not to every user or network.
Look for a network, firewall, proxy, or VPN boundary
If DNS returns an answer but the page will not load, compare the site on another trusted network if policy allows. If it works there but not on the original network, ask the network administrator to review relevant firewall, proxy, VPN, and DNS logs and policies. If several devices fail on one network, shared infrastructure becomes more plausible, although a site-side issue can affect them too.
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Organizational proxy requirements can also prevent endpoint security services from reaching their cloud destinations. Microsoft documents connectivity requirements for Defender for Endpoint, including cases where proxy configuration matters: Microsoft Defender for Endpoint proxy and internet connectivity guidance. That is a service-connectivity example, not a universal list of destinations or a reason to change proxy settings yourself.
Check endpoint-security alerts
Look for a security notification from the operating system or endpoint-protection product. Ask the endpoint administrator to check alerts or URL/domain events for the affected device, application, hostname, and time of failure. In Microsoft Defender for Endpoint, network protection can block access outside the browser; a browser may show only a generic error while Windows provides a notification. See Microsoft’s network protection documentation.
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An alert is stronger evidence of a security-policy action than a generic browser message, but the product and deployment determine what users see. Treat an explicit warning as a security signal. Ask the administrator to review it rather than disabling protection or trying to evade the block.
Use the result pattern to choose your next check
| What you observe | What it may indicate | Next useful check |
|---|---|---|
| The 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 and the resolver on another device; check whether browser DoH changes the path. |
| Lookup succeeds but the page connection fails | A later connection layer—firewall, proxy, VPN, endpoint protection, TLS/certificate validation—or the site itself may be responsible. | Compare another device or network and ask the relevant administrator to review security or network logs. |
| 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. |
| Several devices fail on the same network | Shared DNS, firewall, proxy, gateway, ISP, or site-side causes become more plausible. | Test on a separate trusted network if allowed, then ask the network operator to inspect shared infrastructure. Mozilla notes an ISP issue can occur even when modem or router indicators look normal. |
| A generic browser error appears alongside a security alert | Endpoint network protection may have blocked access outside the browser. | Have the endpoint administrator check the alert’s URL or domain, device, and application details. |
Try low-risk checks for specific symptoms
Certificate warnings
If the browser reports a certificate problem, check the device’s date, time, and time zone. Incorrect system time can interfere with certificate validation and may look like a network block.
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Possible stale DNS cache
A cache flush can test whether stale local DNS data is contributing, but it is not proof of the root cause. Mozilla documents ipconfig /flushdns for Windows and restarting macOS to flush its cache. Follow current instructions for your operating-system version because procedures differ: Mozilla’s troubleshooting steps.
Give an administrator useful evidence
When the cause is not clear, send observations rather than a guess that DNS or security software is at fault. Include:
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- The exact hostname and the time the failure occurred.
- The device, operating system, and browser involved.
- Whether another browser, another device on the same network, or another permitted network produced a different result.
- The full
nslookupresult, including the resolver and whether the command returned an answer, timed out, or reported a name error. - The exact browser error and any operating-system or security-product notification.
Administrators may need logs to confirm whether a managed policy intentionally blocked the site. An alternate public DNS server is not automatically appropriate on a managed network; do not change organizational settings unless the administrator approves the test.
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