HTTP is appliation layer protocol often based on TCP/IP layer (reliable) and follows classical client server model.

Requests are sent by ony entity, the user-agent. Usually it is Web browser but an be a robot that crawls the web to populate and maintain a search engine index. Server is usually not a single machine, but a collection of servers sharing the load (load balancing), cache, database server… Between server and web browser there could be other proxies (logging, authentication, filtering).

Http is stateless, there is no link between two requests, but using Http Headers we can create Http Cookies allowing session creation to share some context.

Http/1.0 open a TCP connection for each request/response exchange, so it is not so efficient. Http/2 use multiplexing messages over a single connection, keep the connection warm and more efficient.

GET / HTTP/1.1

body line

Request consists of start line, headers, empty line and a body. Start line defines:

  • Path usually without protocol, domain, port, only absolute path. But it can vary depending on method (for example CONNECT asd.asd:80 HTTP/1.1 OPTIONS * HTTP/1.1)
  • Protocol Version (HTTP/1.1)
HTTP/1.1 200 OK
Date: 1 Jan 2017
Server: Apache

Response consists of status line, headers, empty line and a body. Status line defines:

  • Protocol Version (HTTP/1.1)
  • Status code (200)
  • Status message (non authoritative short description of status code, purely informational)

Using Http headers (not available in Http/0.9) we can set varius properties of request and response.

Header syntax is: case insensitive string followed by colon and value on single line. They can be:

  • general headers: like via, connection, upgrade-insecure-requests
  • request headers: like host, user-agent, accept, accept-type, accept-language
  • response headers: like accept-ranges, vary, access-control-allow-origin, etag, server, set-cookie
  • entity headers: like content-type content-length, content-encoding, last-modified
    • content-type is it used to define body (usually POST requests) which can be single resource or multiple resource body (mime is multipart/form-data). Single resource usually has content-lenght but can have unknown length in case of transfer-encoding

Identifying resources on the web

link target of the Http request is “resourse” which is identified with Uniform Resource Identifier URI. URL Uniform Resource Locator (is kind of URI) is most common form of URI and is known as web address (location) It consists scheme or protocol (http, data, file, ftp, mailto, ssh, tel, urn, view-source, ws/wss). Than follows domain name or ip address, port, path (handled by web server), query (key value pairs separated with &) and fragment (hash, anchor is never sent to the server). URN is URI that identifies resource by name in namespace: urn:ietf:rfc:7230 Data URI can be used to embed small files inline in documents: data:'image/jpeg',<data> or data:text/plain,<script>alert('hi');</script>. To encode data into base64 format you can use uuencode command line tool. Do not forget to add , before data segment.

Mime types

mime type has format type/subtype (lowercase without space). File extension is not used in browser, only mime type. list of all mime types. Most common type(subtype) is one of the discrete:

  • text (plain, html, css, javascript)
    • for css files we have to use mime type text/css not (text/plain)
    • text/csv is used for csv (chrome does not open, but chromium opens libre office). Usually do not need to specify since browser can detect that based on request (you can simple render text: 'col1,col2')
  • image (gif, png, jpeg, bmp, webp)
  • audio (midi, mpeg, webm, ogg, wav)
  • video (webm, ogg)
  • application (xml, pdf, octet-stream)
    • application/octet-stream is unknown binary file, same as if Content-Disposition: attachment header is set, so browser will “Save as”
    • content-type: application/x-www-form-urlencoded and body name=Joe%20User&request=Send%20me%20one%20of%20your%20catalogue is another example of request.

or multipart:

  • multipart/form-data is used for html forms and consists multiple parts separated with boundary string that starts with -- and each part has its own mime type (inside Content-Type header) and Content-Disposition, for example

    Content-Type: multipart/form-data; boundary=aBoundaryString
    (other headers associated with the multipart document as a whole)
    Content-Disposition: form-data; name="myFile"; filename="img.jpg"
    Content-Type: image/jpeg
    Content-Disposition: form-data; name="myField"
  • multipart/byteranges is used to send partial content of the files

Mime sniffing is used when client believes that mime type is incorrect, so they try to guess the correct value by looking at the resource. Server can block sniffing by sending X-Content-Type-Options.

You can use mime url data inside inline html and css, for example:

<a href="data:text/plain;base64,SGVsbG8sIFdvcmxkIQ%3D%3D" download>Click here to
download file</a>

<img alt="Embedded Image" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAADIA..." />

div.image {

If you chose that apex domain is your canonical location, than you need to add http 301 redirection for to Or you can use special link element in head part of the page: <link href="http://example.cin/whaddup" rel="canonical">

Http/1.1 enable warm connections so it does not close after first request is completed Connection: keep-alive header. Drawback is server need to consume resources to keep open connections (DOS attack). Pipeling ie second request is send before answer for the first is received. Only idempotent methods (GET, HEAD, PUT, DELETE) can be pipelined. It is difficult to implement and browser do not use it (anyway Http/2 multiplexing solve this problem). Browser simply opens 6 different connections with a server and use them in parallel. Cache control mechanism is introduced, HOST header enable multiple domains at the same ip address.

SSL (and later TLS) protocol is used to secure communication between server and web browser (https):

  • connection is private (secure) by using symmetic cryptography
  • identity (typically server) can be authenticated using publi-key crypthography
  • connection ensures integrity

REST representational state transfer) pattern was introduced in 2000 for complex applications (API defined using specific URI with basic Http/1.1 methods).

In 2005 extension like server-sent events and websockets. In 2010 Google implements protocol SPDY which increase in responsiveness and solving a problem of data transmitted duplication (inspiration for Http/2) In 2015 Http/2 standardized with a several differences from Http/1.1 link

  • it is binary protocol rather than text, so it can no longer be read and created manually
  • it is multiplexed so parallel requests can be send on one connection
  • compress headers
  • allows server to populate data in client cache

http/2 is transparent to web developers, it is just additional step between http/1.1 and transport protocol. When it is available on server and browser it is automatically switched on and used.

In 2016 some important extensions are:

  • Alt-Svc header for location of the resource
  • Client-Hints information about client or web browser



Using Http cookies you can link requests to some state of the server (even http is state-less protocol).

CORS Web browsers enforce strict separation between web sites. Only resources from Same origin can be accessed. But there are exceptions. Cross origin resource sharing CORS

Allow cors on server so options request are resolved

echo "gem 'rack-cors', require: 'rack/cors'" >> Gemfile

sed -i config/application.rb -e '/^  end/i \
    config.middleware.insert_before 0, "Rack::Cors" do\
      allow do\
        origins "*"\
          headers: :any,\
          methods: :any,\
          expose: [\
            "Content-Range", # clean_pagination\
            "Accept-Ranges", # clean_pagination\

You can enable CORS only for specific actions. For example if you are loading form on other sites to create and update, you can enable cors for that actions. When you are loading javascript from other domains like

<script type="text/javascript" src="//localhost:3002/widget/1"></script>

than on your server you will get error

ActionController::InvalidCrossOriginRequest (Security warning: an embedded <script> tag on another site requested protected JavaScript. If you know what you're doing, go ahead and disable forgery protection on this action to permit cross-origin JavaScript embedding.):

To solve that add

  skip_before_action :verify_authenticity_token, only: [:widget]

When you want to submit form from another domain you get this error

ActionController::InvalidAuthenticityToken (ActionController::InvalidAuthenticityToken):

So I solve that by adding

  skip_before_action :verify_authenticity_token, only: [:widget, :create, :update]

This is when your controller is protected from CSRF attack (it contains protect_from_forgery) .

To see response from submitted form request, you see in javascript console this warning and $.ajax success callback is never triggered

Failed to load http://localhost:3002/days/1: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost' is therefore not allowed access.

To solve that add

  after_action :set_access_control_headers, :only => [:create, :update]

  def set_access_control_headers
    headers['Access-Control-Allow-Origin'] = '*'



Content security policy CSP


You can use WWW-Authenticate header or setting sessiong key using Http cookie.