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munin2: sending a HTTP response

I have added some error handling and did some refactoring of the raw TCP server.

The goal now is that the server should send a dummy HTTP 1.0 “200 OK” response, along with a dummy minimal HTML-encoded message, before closing the connection.

response format

I have done a quick read of the relevant shorts chapters in the HTTP standard. The way I understand it, a HTTP 1.0 response contains the following:

Status-Line
General-Header | Response-Header | Entity-Header
Entity-Header
CRLF
Entity-Body

Let’s go through them step-by-step for this minimal response:

  • Status-Line

This is the protocol version followed by the HTTP status code, along with the textual phrase for that code. Each element separated by spaces.

So that would be HTTP/1.1 200 OK.

  • Response Header Fields

allow the server to pass additional information about the response which cannot be placed in the Status-Line

ok. does not seem necessary here?

  • Entity Header Fields

Now these I have seen before, these are Allow, Content-Type, Content-Length, etc.

Allow seems unnecessary since we are just sending responses. But I suppose we need the following two lines:

Content-Type: text/html
Content-Length: ??

The Content-Length it’s in “decimal number of octets”, is that bytes? So this should be the length of the HTML file?

  • CRLF

  • Entity-Body

This will be the minimal HTML-encoded message. Something like:

<html><body>hello world</body></html>

sending the response

Time to put this together into code, using snprintf() concatenation:

void get_http_response(char *buffer) {
    char *status_line = "HTTP/1.0 200 OK";
    char *content_type_header = "Content-Type: text/html";
    char *html_body = "<html><body>hello world</body></html>";
    int html_body_length = strlen(html_body);

    // create Content-Length Header
    char content_length_header[50];  
    snprintf(content_length_header, sizeof(content_length_header), "Content-Length: %d", html_body_length);

    // build response
    snprintf(buffer, sizeof(buffer), "%s\r\n%s\r\n%s\r\n\r\n%s", status_line, content_type_header, content_length_header, html_body);
}

Time to test it using netcat:

$ nc localhost 12345
hello # message I sent
HTTP/1. # response

Doesn’t work, the rest of the response just cuts off. Why? I have allocated enough memory to the HTTP response buffer, so that can’t be it.

Damnit. buffer is passed as an argument, so sizeof(buffer) does not return the actual allocated buffer size. It just gives the size of a char * (8 bytes). Thus, only 7 bytes + null byte operator is copied into the buffer, resulting in “HTTP/1.”.

After changing that:

$ nc localhost 12345
send me the html please # from the client
HTTP/1.0 200 OK
Content-Type: text/html
Content-Length: 37

<html><body>hello world</body></html>

Nice! Testing in a browser reveals the same thing:

testing munin hello world

next steps

  • parse the request
  • send 200 OK for properly formatted
  • otherwise, respond 400 or 5xx