netcheck
The machine's own network checks as tools: a TCP connect (netcheck_tcp), a DNS lookup of the machine's resolver or of one named (netcheck_dns), what listens here (netcheck_listening), the private network's peers (netcheck_tunnel), and whether a URL answers (netcheck_http). netcheck_request makes one HTTP request (GET, HEAD or POST, with headers and a body of at most 256 KiB) and answers the status line, every response header and the body (at most 1 MiB, with a flag saying when it was cut), reporting redirects without following them and speaking only http and https, so a service's live answers can be captured (hq issue 1643).