Primitive Service (TCP/IP Internetworking With gawk)

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2.8 A Primitive Web Service

Now we know enough about HTTP to set up a primitive web service that just says "Hello, world" when someone connects to it with a browser. Compared to the situation in the preceding section, our program changes the role. It tries to behave just like the server we have observed. Since we are setting up a server here, we have to insert the port number in the ‘localport’ field of the special file name. The other two fields (hostname and remoteport) have to contain a ‘0’ because we do not know in advance which host will connect to our service.

In the early 1990s, all a server had to do was send an HTML document and close the connection. Here, we adhere to the modern syntax of HTTP. The steps are as follows:

Send a status line telling the web browser that everything is okay.
Send a line to tell the browser how many bytes follow in the body of the message. This was not necessary earlier because both parties knew that the document ended when the connection closed. Nowadays it is possible to stay connected after the transmission of one web page. This avoids the network traffic necessary for repeatedly establishing TCP connections for requesting several images. Thus, it is necessary to tell the receiving party how many bytes will be sent. The header is terminated as usual with an empty line.
Send the "Hello, world" body in HTML. The useless while loop swallows the request of the browser. We could actually omit the loop, and on most machines the program would still work. First, start the following program:

BEGIN {
  RS = ORS = "\r\n"
  HttpService = "/inet/tcp/8080/0/0"
  Hello = "<HTML><HEAD>" \
          "<TITLE>A Famous Greeting</TITLE></HEAD>" \
          "<BODY><H1>Hello, world</H1></BODY></HTML>"
  Len = length(Hello) + length(ORS)
  print "HTTP/1.0 200 OK"          |& HttpService
  print "Content-Length: " Len ORS |& HttpService
  print Hello                      |& HttpService
  while ((HttpService |& getline) > 0)
     continue;
  close(HttpService)
}

Now, on the same machine, start your favorite browser and let it point to http://localhost:8080 (the browser needs to know on which port our server is listening for requests). If this does not work, the browser probably tries to connect to a proxy server that does not know your machine. If so, change the browser’s configuration so that the browser does not try to use a proxy to connect to your machine.