curl Verbose Mode and Debugging

Debugging HTTP Requests with curl

curl is one of the best tools for debugging HTTP issues. Its verbose and trace modes reveal every detail of the request-response lifecycle, from DNS resolution to TLS negotiation.

Verbose Mode

The -v flag shows the full request and response conversation:

curl -v https://api.example.com/data

Output prefixes:

• * — curl informational messages (DNS, TLS, connection)
• > — request headers sent to the server
• < — response headers received from the server

Trace Mode

For even more detail, --trace outputs a hex dump of all data:

curl --trace trace.log https://api.example.com/data

Or use --trace-ascii for a readable ASCII dump:

curl --trace-ascii - https://api.example.com/data

Timing Information

Use -w (write-out) to display timing metrics:

curl -s -o /dev/null -w "DNS: %{time_namelookup}s\nConnect: %{time_connect}s\nTLS: %{time_appconnect}s\nFirst byte: %{time_starttransfer}s\nTotal: %{time_total}s\n" \
  https://api.example.com/data

Response Code Only

Check just the HTTP status code:

curl -s -o /dev/null -w "%{http_code}" https://api.example.com/health

Inspecting TLS Details

curl -v --tlsv1.2 https://example.com 2>&1 | grep "SSL\|TLS\|certificate"

Silent Mode with Errors

Use -sS to suppress progress output while still showing errors:

curl -sS https://api.example.com/data

Debugging Checklist

When an API request fails, use this systematic approach:

1. Check connectivity: curl -v to verify DNS and TCP connection
2. Inspect headers: Verify Content-Type, Authorization, and other required headers
3. Check TLS: Look for certificate errors or version mismatches
4. Examine timing: Use -w to identify where latency occurs
5. Compare with working request: Diff the verbose output of working vs failing requests

Write-Out Variables

Key -w variables: %{http_code}, %{content_type}, %{size_download}, %{speed_download}, %{url_effective}, %{num_redirects}, %{ssl_verify_result}.