curl -I Sends HEAD, Not GET — Header Debugging Trap
Problem
When debugging HTTP response headers with curl -I or curl -sI, the response may show
different header values than what actual GET requests receive. This is because -I sends
a HEAD request, and server middleware that only processes GET requests will be skipped.
Context / Trigger Conditions
- Testing cache headers with
curl -sI and seeing unexpected values
- Middleware that checks
method == GET before setting headers (common in cache middleware)
- Headers appear correct in automated tests but wrong in manual curl testing
Cache-Control, Surrogate-Control, or Surrogate-Key values don't match expectations- Axum/Express/any framework middleware with method guards
Solution
Use curl -s -D - -o /dev/null instead of curl -I to get response headers from a GET request:
# WRONG — sends HEAD request, middleware may skip processing
curl -sI https://example.com/api/endpoint
# CORRECT — sends GET request, dumps headers, discards body
curl -s -D - -o /dev/null https://example.com/api/endpoint
If you need just specific headers:
curl -s -D - -o /dev/null https://example.com/api/endpoint | grep -iE 'cache-control|surrogate'
Verification
Compare output from both methods:
echo "=== HEAD (curl -I) ==="
curl -sI https://example.com/api/endpoint | grep cache-control
echo "=== GET (curl -D) ==="
curl -s -D - -o /dev/null https://example.com/api/endpoint | grep cache-control
If the values differ, your middleware has a GET-only guard (which is correct behavior).
Example
Axum middleware that only sets cache headers for GET requests:
async fn cache_middleware(request: Request, next: Next) -> Response {
let method = request.method().clone();
let mut response = next.run(request).await;
// HEAD requests skip this — curl -I won't see these headers!
if method != Method::GET {
return response;
}
response.headers_mut().insert("cache-control", ...);
response.headers_mut().insert("surrogate-control", ...);
response
}
Notes
- This is NOT a bug — it's correct behavior. Cache headers should only apply to cacheable GET responses.
- HTTP spec says HEAD responses SHOULD include the same headers as GET, but middleware implementations
often don't replicate this because HEAD is rarely used by CDNs or browsers for caching decisions.
- Fastly, Cloudflare, and other CDNs send GET requests to origins, so the cache behavior is correct
even if
curl -I shows different headers.
- This trap is especially insidious because
curl -I is the most common way to check headers.