Debugging a Network Problem From Another Machine

One of the most useful questions in network troubleshooting is also one of the simplest: Does it fail from another machine too? If a website will not load on my laptop, trying it from another computer can immediately change the investigation. If it works there, the service probably is not down. Something about my machine, DNS configuration, VPN, firewall, route, or network path is different. If it fails there too, the problem may be farther upstream. I wanted Network Doctor to be able to ask that question directly. So I added remote diagnosis over SSH. netdoc --via ideapad github.com Instead of running the diagnosis locally, Network Doctor connects to ideapad , runs the checks there, and reports the result back on my machine. Why another vantage point matters A network failure is always observed from somewhere. Suppose github.com is unreachable from my workstation. I can test DNS: dig github.com Then TCP: nc -vz github.com 443 Then TLS: openssl s_client -connect github.com:443 Maybe I inspect my routes, VPN, proxy settings, or firewall. Those tests are useful, but they all share one property: they are observing the network from the same machine. Trying the same destination from another machine gives me a new piece of evidence. Imagine this: Thelio: DNS PASS TCP 443 FAIL Ideapad: DNS PASS TCP 443 PASS TLS PASS HTTPS PASS That difference is interesting. GitHub clearly is not universally unreachable. The second machine just reached it. Now I have a much smaller problem to investigate: what is different about the path from Thelio? That is often more useful than running another five commands on Thelio. Turning that into a command Network Doctor already runs network checks as a dependency graph. For an HTTPS target, for example, it can test things such as the local interface, DNS resolution, TCP connectivity, TLS, HTTP, routing, and path MTU. Normally: netdoc github.com means: Diagnose github.com from this machine. With --via : netdoc --via ideapad github.com it becomes: