TopolearnCorporate proxies, odd DNS and 'the server says my body is wrong' bugs need byte-level visibility.
-v shows headers, but not bodies or exact bytes. For that:
--trace-ascii trace.txt(or-for stdout) logs everything sent and received, including request and response bodies, in readable form;--tracegives a hex dump. Add--trace-timefor timestamps.- Remember that trace files contain tokens and cookies; scrub them before sharing.
Proxies. -x http://proxy.corp:3128 routes the request through a proxy (socks5h:// for SOCKS with remote DNS). curl also reads proxy settings from environment variables: https_proxy, all_proxy, and lowercase-only http_proxy (uppercase HTTP_PROXY is ignored on purpose, because CGI environments let a request header set it). no_proxy=localhost,.internal lists hosts that bypass the proxy, and --noproxy '*' disables proxies for one command. For HTTPS through an HTTP proxy, curl sends a CONNECT and tunnels TLS end to end; -v shows the CONNECT exchange and the proxy's answer.
Name resolution. -4 / -6 force IPv4 or IPv6, which isolates the classic "slow first request" caused by an unreachable IPv6 route. --dns-servers (needs a c-ares build) picks resolvers, and --doh-url https://dns.example/dns-query resolves via DNS-over-HTTPS. %{remote_ip} in -w confirms where you actually connected.
Example
curl -sS --trace-ascii - --json '{"a":1}' https://api.example.com/echo | head -20
# => Send header, 154 bytes ... => Send data, 7 bytes: {"a":1}
curl -sS -x http://proxy.corp:3128 https://example.com -o /dev/null -v 2>&1 | grep CONNECT
# > CONNECT example.com:443 HTTP/1.1
export https_proxy=http://proxy.corp:3128 no_proxy=localhost,.internal
curl -sS https://example.com -o /dev/null # via proxy
curl -sS http://wiki.internal/ -o /dev/null # direct (no_proxy)
curl -sS --noproxy '*' https://example.com -o /dev/null # direct this time
curl -4 -sS -o /dev/null -w '%{remote_ip} %{time_connect}\n' https://example.com
curl -x socks5h://localhost:1080 http://grafana.internal:3000/ # DNS via proxyRemember
- --trace-ascii shows full bodies and exact bytes
- -x or https_proxy/http_proxy (lowercase) route via a proxy; no_proxy bypasses
- -4/-6 and %{remote_ip} isolate DNS and IP-version problems
Watch out for
- Setting HTTP_PROXY in uppercase and wondering why curl ignores it for http:// URLs.