RFC 2068

9.8 TRACE

   The TRACE method is used to invoke a remote, application-layer loop-
   back of the request message. The final recipient of the request
   SHOULD reflect the message received back to the client as the
   entity-body of a 200 (OK) response. The final recipient is either the
   origin server or the first proxy or gateway to receive a Max-Forwards
   value of zero (0) in the request (see section 14.31). A TRACE request
   MUST NOT include an entity.

   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 14.44) 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.

   If successful, the response SHOULD contain the entire request message
   in the entity-body, with a Content-Type of "message/http". Responses
   to this method MUST NOT be cached.