If a browser cannot open a site by name, check whether Ubuntu's system resolver can look up the same name before changing DNS settings. These examples are for Ubuntu systems using systemd-resolved.
Check the connection and the resolver
ip route show default
resolvectl statusThe first command shows an IPv4 default route if one is configured. An IPv6-only network can still work without one. In the resolver output, check the active network link and its DNS servers. A VPN may supply DNS for particular domains, so note which link you expect to resolve the name.
Compare a public name with the failing name
resolvectl query example.comRepeat the query with the exact hostname that fails. If the public lookup works and an internal name fails, check whether you are connected to the required VPN or network. If both fail, record the exact error and the DNS servers shown by resolvectl status. A successful lookup confirms name resolution for that query, while opening a website also depends on connectivity and the browser.
Use the result to choose what to check next
Suppose a company hostname fails on home Wi-Fi and resolves after connecting the VPN. That points toward a network-specific lookup. If the system resolver works but only one browser fails, compare its secure DNS and proxy settings with your organization's requirements. If resolvectl cannot reach the resolver service, inspect systemctl status systemd-resolved --no-pager on systems expected to use it. Follow the Ubuntu service troubleshooting guide if the service failed.
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