HTTP Pocket Reference

TRACE: View the Client’s Message Through the Request Chain

The TRACE method allows a programmer to see how the client’s message is modified as it passes through a series of proxy servers. The recipient of a TRACE method echoes the HTTP request headers back to the client. When the TRACE method is used with the Max-Forwards and Via headers, a client can determine the chain of intermediate proxy servers between the original client and web server. The Max-Forwards request header specifies the number of intermediate proxy servers allowed to pass the request. Each proxy server decrements the Max-Forwards value and appends its HTTP version number and hostname to the Via header. A proxy server that receives a Max-Forwards value of returns the client’s HTTP headers as an entity-body with the Content-type of message/http. This feature resembles trace-route, a UNIX program used to identify routers between two machines in an IP-based network. HTTP clients do not send an entity-body when issuing a TRACE request.

Figure 1.4 shows the progress of a TRACE request. After the client makes the request, the first proxy server receives the request, decrements the Max-Forwards value by one, adds itself to a Via header, and forwards it to the second proxy server. The second proxy server receives the request, adds itself to the Via header, and sends the request back, since Max-Forwards is now (zero).