How Can I Preserve HTTP Request Body Integrity When Reverse Proxying?

Preserving HTTP Request Body Integrity for Reverse Proxying

Issue:

In a custom HTTP handler implementing the ServeHTTP method, inspecting the body of incoming POST requests using req.ParseForm() causes an error when the request is passed to a reverse proxy. This occurs because reading the body drains the req.Body.Reader stream, leaving nothing for subsequent consumers.

Solution:

To maintain the original request state while examining the body, consider the following technique:

1. Read the request body into a buffer using io.ReadAll(r.Body).
2. Create two new readers, rdr1 and rdr2, backed by the buffer using io.NopCloser(bytes.NewBuffer(buf)).
3. Use rdr1 to inspect the body and perform desired actions.
4. Reset r.Body to rdr2 to preserve the original request state for the reverse proxy.

Code Example:

buf, _ := io.ReadAll(r.Body)
rdr1 := io.NopCloser(bytes.NewBuffer(buf))
rdr2 := io.NopCloser(bytes.NewBuffer(buf))

doStuff(rdr1)
r.Body = rdr2 // Resets the request body without consuming any data

Note:

bytes.Buffer does not implement the io.ReadCloser interface due to the lack of a Close() method; hence, each reader is wrapped in io.NopCloser.

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