curl Deep Dive

Concept

curl is the Swiss-army knife for testing HTTP/HTTPS (and many other protocols).
It can show DNS resolution, TCP connect, TLS handshake, headers, redirects, and timing breakdowns.

Why it matters

When an application or user reports “the site is down”, curl lets you test the exact path from the server (or your machine) and see which layer fails.

Mental Model

curl -v   → verbose: DNS → Connect → TLS → Request → Response
curl -w   → write-out: precise timing metrics

Key Commands

# Basic verbose request (most useful starting point)
curl -v https://example.com
 
# Show only headers
curl -I https://example.com
 
# Follow redirects
curl -vL https://example.com
 
# Timing breakdown
curl -w "\nDNS: %{time_namelookup}\nConnect: %{time_connect}\nTLS: %{time_appconnect}\nTTFB: %{time_starttransfer}\nTotal: %{time_total}\n" -o /dev/null -s https://example.com
 
# Test from specific interface or with custom resolve
curl --interface eth0 https://example.com
curl --resolve example.com:443:1.2.3.4 https://example.com
 
# Ignore certificate errors (testing only)
curl -vk https://example.com
 
# POST / with data
curl -v -X POST -d 'key=value' https://example.com/api

Common Failure Modes & Symptoms

What curl shows	Likely layer	Next step
Could not resolve host	DNS	DNS Resolution
Connection timed out	Network / firewall / routing	TCP IP Troubleshooting Model
Connection refused	Nothing listening or firewall	ss -tulpn on target
SSL certificate problem	TLS / certificate	openssl s_client
HTTP 5xx	Application	Application logs
Slow TTFB	Backend / application	App metrics & logs

Investigation Tips

• Always test both by name and by IP when possible.
• Use -v first to see the full handshake.
• The -w timing format is excellent for quantifying slowness.
• From the server itself, curl localhost or curl 127.0.0.1 helps separate local vs external path issues.

Related Notes

• TCP IP Troubleshooting Model
• DNS Resolution
• TLS Troubleshooting
• ss Deep Dive
• Troubleshooting Methodology

Personal Lessons Learned