Why “Content-Length: 0” in POST requests?

Question

A customer sometimes sends POST requests with Content-Length: 0 when submitting a form (10 to over 40 fields).

We tested it with different browsers and

Answer 1

curl sends PUT/POST requests with Content-Length: 0 when configured to use HTTP proxy. It's trick to overcome required buffering in case of first unauthorized PUT/POST request to proxy. In case of GET/HEAD requests curl simply repeats the query. The scheme for PUT/POST is like:

1. Send first PUT/POST request with Content-Length set to 0.

2. Get answer. HTTP status code of 407 means we have to use proxy authorization. Prepare headers for proxy authentication for send request.

3. Send request again with filled headers for proxy authentication and real data to POST/PUT.