Added CGI post

Author: python151

main/content/post/cgi_deepdive.md

@@ -0,0 +1,49 @@
++++
+author = "Emily Miller"
+title = "A brief dive into CGI"
+date = "2025-08-30"
+description = "Common Gateway Interface (CGI) is one of the oldest technologies still in use today to make web applications."
+tags = [  ]
++++
+
+[RFC 3875](https://www.rfc-editor.org/rfc/rfc3875) is the first RFC to discuss CGI in depth. Notably, this is not a formal specification, but rather a description of how CGI is commonly done in practice. To give a brief introduction to the technology from this RFC:
+
+>    The Common Gateway Interface (CGI) [22] allows an HTTP [1], [4]
+>   server and a CGI script to share responsibility for responding to
+>   client requests.  The client request comprises a Uniform Resource
+>   Identifier (URI) [11], a request method and various ancillary
+>   information about the request provided by the transport protocol.
+>
+>   The CGI defines the abstract parameters, known as meta-variables,
+>   which describe a client's request.  Together with a concrete
+>   programmer interface this specifies a platform-independent interface
+>   between the script and the HTTP server.
+>
+>   The server is responsible for managing connection, data transfer,
+>   transport and network issues related to the client request, whereas
+>   the CGI script handles the application issues, such as data access
+>   and document processing.
+
+Ok so, to briefly summarize what this is saying:
+ - A CGI server handles parsing of HTTP requests and the networking side of things
+ - It then uses a CGI script to handle the application side of the interface
+
+This fundamentally makes sense. This is a very similar to other technologies of the sort that you'll see, for instance PHP servers.
+
+In general, the way this looks in practice is something like the following:
+ - Spin up a single "CGI Server" process, which listens for packets on a specific port
+ - When it recieves a new TCP connection, spin up a new thread to handle the connection and continue listening
+    - This new thread will then parse the HTTP request headers
+    - It will then pick a script to run based on the URI (this is implementation defined)
+    - It will then run a script in an implementation defined manner 
+    - And package its output into a new HTTP request
+
+## Picking a Script
+
+By default, this is usually done by indexing against a root directory. For instance, `/www/cgi-bin/`, may have all the CGI scripts for an application, and then the server will attempt to match the URI to a file within this directory. 
+
+## Running the Script
+
+When running the script, by default it will store HTTP headers in environment variables such as `QUERY_STRING`, `CONTENT_LENGTH`, `REMOTE_ADDR`, etc. In the RFC description of the protocol, it actually abstracts this away using the term "Meta Variable", and states this is again implementation defined. 
+
+It will then pass the request body in through `stdin` and output of `stdout` will be used to send the reply.