TRACE

The TRACE method requests a remote, application-level loop-back of the request message. The final recipient of the request SHOULD reflect the message received, excluding some fields described below, back to the client as the message body of a 200 (OK) response with a Content-Type of "message/http" (Section 8.3.1 of [RFC7230]). The final recipient is either the origin server or the first server to receive a Max-Forwards value of zero (0) in the request (Section 5.1.2). A client MUST NOT generate header fields in a TRACE request containing sensitive data that might be disclosed by the response. For example, it would be foolish for a user agent to send stored user credentials [RFC7235] or cookies [RFC6265] in a TRACE request. The final recipient of the request SHOULD exclude any request header fields that are likely to contain sensitive data when that recipient generates the response body.

TRACE allows the client to see what is being received at the other end of the request chain and use that data for testing or diagnostic information. The value of the Via header field (Section 5.7.1 of [RFC7230]) is of particular interest, since it acts as a trace of the request chain. Use of the Max-Forwards header field allows the client to limit the length of the request chain, which is useful for testing a chain of proxies forwarding messages in an infinite loop.

A client MUST NOT send a message body in a TRACE request.

Responses to the TRACE method are not cacheable.