Servlets:%20HTTP%20Request%20Header%20Contents%20and%20Responses

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ServletsHTTP Request Header Contentsand
Responses
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Road Map

• Recap and Overview
• Reading HTTP Request Headers
• Generating the Server Response
• Case Study 1 Search Engines
• Case Study 2 Basic Web Security
• Restricting by User Name/Password

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Recap and Overview
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Overview

• Interaction between browser and web server.

Request
Web Browser
Web Server
Response
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Client Request Data

• When a user submits a browser request to a web
  server, it sends two categories of data
• Form Data Data that the user explicitly typed
  into an HTML form.
• For example registration information.
• HTTP Request Header Data Data that is
  automatically appended to the HTTP Request from
  the client.
• For example cookies, browser type, etc,

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Reading HTTP Request Headers
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Sample HTTP Request

• A sample HTTP Request to Yahoo.com
• GET / HTTP/1.1
• Accept /
• Accept-Language en-us
• Accept-Encoding gzip, deflate
• User-Agent Mozilla/4.0 (compatible MSIE 5.0
  Windows NT DigExt)
• Host www.yahoo.com
• Connection Keep-Alive
• Cookie B2td79o0sjlf5rb2

Tip Check out http//www.web-sniffer.net
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Accessing HTTP Headers

• As in the SnoopServlet Example
• To access any of these Headers, use the
  HTTPServletRequest getHeader() method.
• For example
• String connection req.getHeader(Connection)
• To retrieve a list of all the Header Names, use
  the getHeaderNames() method.
• getHeaderNames() returns an Enumeration object.
• For example
• Enumeration enum req.getHeaderNames()

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Additional HTTP Information

• getMethod()
• Indicates the request method, e.g. GET or POST.
• getRequestURI()
• Returns the part of the URL that comes after the
  host and port. For example, for the URL
  http//randomhost.com/servlet/search, the request
  URI would be /servlet/search.
• getProtocol()
• Returns the protocol version, e.g. HTTP/1.0 or
  HTTP/1.1

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Reading Browser Types

• The User-Agent HTTP header indicates the browser
  and operating system.
• For example
• user-agent Mozilla/4.0 (compatible MSIE 6.0
  Windows NT 5.1)
• You can use this header to differentiate browser
  types or simply log browser requests.

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Example User-Agents

• Internet Explorer
• user-agent Mozilla/4.0 (compatible MSIE 6.0
  Windows NT 5.1)
• Mozilla
• Mozilla/5.0 (Windows U Windows NT 5.1 en-US
  rv1.4) Gecko/20030624
• For strange historical reasons, IE identifies
  itself as Mozilla

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Generating the Server Response
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Sample HTTP Response

• As a refresher, heres a sample HTTP response
• HTTP/1.1 200 OK
• Date Mon, 06 Dec 2004 205426 GMT
• Server Apache/1.3.6 (Unix)
• Last-Modified Fri, 04 Oct 2002 140611 GMT
• Content-length 327
• Connection close
• Content-type text/html
• lttitlegtSample Homepagelt/titlegt
• ltimg src"/images/oreilly_mast.gif"gt
• lth1gtWelcomelt/h2gtHi there, this is a simple web
  page. Granted, it may

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Generating Responses

• Servlets can return any HTTP response they want.
• Useful for lots of scenarios
• Redirecting to another web site.
• Restricting access to approved users.
• Specifying content-type other than text/html.
• Return images instead of HTML.

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Setting the HTTP Status Code

• Normally, your Servlet will return an HTTP Status
  code of 200 OK to indicate that everything went
  fine.
• To return a different status code, use the
  setStatus() method of the HttpServletResponse
  object.
• Be sure to set the status code before sending any
  document content to the client.

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Using setStatus()

• setStatus takes an integer value. But, its best
  to use the predefined integers in the
  HttpServletResponse. Here are a few
• SC_BAD_REQUEST
• Status code (400) indicating the request sent by
  the client was syntactically incorrect.
• SC_FORBIDDEN
• Status code (403) indicating the server
  understood the request but refused to fulfill it.
• SC_INTERNAL_SERVER_ERROR
• Status code (500) indicating an error inside the
  HTTP server which prevented it from fulfilling
  the request.
• SC_NOT_FOUND
• Status code (404) indicating that the requested
  resource is not available.

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Sending Redirects

• You can redirect the browser to a different URL
  by issuing a Moved Temporarily Status Code
• SC_MOVED_TEMPORARILY Status code (302)
  indicating that the resource has temporarily
  moved to another location.
• Because this is so common, the HttpServletResponse
  interface also has a sendRedirect() method.
• Example
• res.sendRedirect( http//www.yahoo.com)

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Example Search Engines
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Multiple Search Engines

• SearchEngines Servlet
• Enables users to submit a search query to one of
  four search engines.
• Google
• AllTheWeb
• Yahoo
• AltaVista, etc.
• The code exploits the HTTP Response Header to
  redirect the user to the correct search engine.

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Architecture
SearchEngines Servlet
I want to search for Bill Gates on Google
Web Browser
Go to Google
I want to search for Bill Gates on Google
Google
Your results
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SearchSpec.java

• The SearchSpec object contains information about
  connecting to a specific search engine
• public String makeURL (String searchString,
  String numResults)
• You provide this method with a search string and
  the number of results, and it returns the URL and
  search query specific to Google, Yahoo, HotBot,
  etc.
• Class is contained in SearchEngines.java on acad

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SearchUtilities.java

• The SearchUtilities.java code has an array of
  SearchSpec objects one for Google, one for
  Yahoo, etc.
• It also provides a makeUrl method

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SearchEngines.java

• The main servlet code.
• This code
• Extracts the searchEngine parameter.
• If no such parameter exists, it sends an HTTP
  Error.
• Otherwise, it calls SearchUtilities to construct
  the correct URL.
• Finally, it redirects the user to this new URL.

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Example Basic Web Security
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HTTP Authentication

• The HTTP Protocol Includes a built-in
  authentication mechanism.
• Useful for protecting web pages or servlets that
  require user name / password access.
• First, lets examine the basic mechanism and the
  HTTP Headers involved.
• Then, lets figure out how to build a servlet
  that exploits this mechanism.

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Basic Authentication

• If a web page is protected, the Web Server will
  issue an authentication challenge
• HTTP/1.1 401 Authorization Required
• Date Sun, 27 Aug 2000 175125 GMT
• Server Apache/1.3.12 (Unix) ApacheJServ/1.1
  PHP/4.0.0 mod_ssl/2.6.6 OpenSSL/0.9.5a
• WWW-Authenticate BASIC realm"privileged-few"
• Keep-Alive timeout90, max150
• Connection Keep-Alive
• Transfer-Encoding chunked
• Content-Type text/html

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WWW-Authenticate

• WWW-Authenticate BASIC realmrealm"
• When you issue a return status code of 401,
  Authorization Required, you need to tell the
  browser what type of authentication is required.
• You do this via the WWW-Authenticate Header.
  This header has two parameters
• BASIC Basic authorization requiring user name
  and password.
• Realm you can create multiple realms of
  authentication for different users, e.g. Admin,
  User, Super_User, etc.

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Basic Authentication Cont.

• Upon receiving an authentication challenge, the
  browser will prompt the user with a pop-up box
  requesting the user name and password.
• Browser takes the usernamepassword from the
  user and encrypts it using the Base 64 Encoding
  Algorithm.
• For example if the string is martymartypd,
  the Base 64 string is bWFydHk6bWFydHlwdw
• We will not cover the details of Base 64, but
  remember that Base 64 is easy to decode.
  Therefore, even if your page is protected,
  someone can easily intercept your Base 64 string
  and decode it.

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Basic Authentication Cont.

• The browser reissues the request for the page.
  In the HTTP request, the browser indicates the
  Authorization string
• GET /servlet/coreservlets.ProtectedPage HTTP/1.1
• Accept image/gif, /
• Accept-Language en-us
• Accept-Encoding gzip, deflate
• User-Agent Mozilla/4.0 (compatible MSIE 5.0
  Windows NT DigExt)
• Host www.ecerami.com
• Connection Keep-Alive
• Authorization Basic bWFydHk6bWFydHlwdw

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Basic Authentication Cont.

• Web Server checks the user name and password.
• If User Name/Password is correct, web server
  displays the protected page.
• If the User Name/Password is incorrect, web
  server issues a second authentication challenge.

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Almost there

• Before we examine the actual servlet code, there
  are two pieces of Java coding we need to examine
• sun.misc.BASE64Decoder.
• java.util.Properties

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Base 64 Encoding

• Sun provides a class called sun.misc.BASE64Decod
  er.
• You can use the decodeBuffer() method to decode
  the Base 64 String sent from the user
• String userInfo bWFydHk6bWFydHlwdw
• BASE64Decoder decoder new BASE64Decoder()
• String nameAndPassword
• new String(decoder.decodeBuffer(userInfo))
  
• After this code, nameAndPassword will be set to
  martymartypd

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java.util.Properties

• A utility class for reading in property files.
• For example, suppose you have the following
  password.properties file
• Passwords
• Sat Aug 26 111542 EDT 2000
• nathannathanpw
• martymartypw
• lindsaylindsaypw
• bjbjpw

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java.util.Properties

• You can easily and automatically load the
  password file and parse its contents
• passwordFile "passwords.properties"
• passwords new Properties()
• passwords.load(new FileInputStream(passwordFile))
  
• Then, you can extract the password for a specific
  user name
• String password properties.getProperty
  ("marty)

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ProtectedPage.java

• Heres how the Servlet Works
• Initialization Read in a Password file of valid
  user names and passwords.
• Check for the HTTP Authorization Header.
• Decode the Authorization Header using Base 64 to
  obtain user name and password.
• Check the User Name and Password against the
  valid names list.
• If valid, show protected page.
• Else, issue another authentication challenge.

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Form Authentication System

• BASE64 not secure
• Need secure solution!
• Use HTML form
• Example FormAuthenticate
• Access of servlet attempts to access protected
  data
• User redirected to login form web page
• Example takes any combination
• Once authenticated, redirected to desired page
• Session object used to store desired destination
  during login diversion

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Summary

• Lots of hidden HTTP data, including headers and
  cookies are sent from browser to the server.
• HTTP Header data can also be sent from server to
  the browser, e.g. error codes, redirection codes,
  etc.