1. Dynamic content 1WUCM1

2. 2 Basic architecture of the web

3. Dynamic web pages Stages: – Collect data from user in browser Usually using an HTML form – Send data in HTTP request to server – Server processes request (dynamically) Different models of how to do this – Server sends HTTP response to browser – Browser displays response 3WUCM1

4. HTML forms Which lecture are you missing? (Give the lecture number not title) When will you come to collect them? Next lecture Next tutorial At 5:00pm on Friday this week Your CAM number? 4WUCM1

5. Passing data to the server HTTP methods – GET for data retrieval – POST for data update – (and others we can ignore) WUCM15

6. Passing data to the server Using GET GET /cgi-bin/mycgi.bat?WUCMI_unit=78&collect_type=NextTutorial&dis_num=67 HTTP/1.0 Using POST POST /cgi-bin/mycgi.bat HTTP/1.0 WUCMI_unit=78&collect_type=NextTutorial&dis_num=67 WUCM16

7. 7 HTTP encoding Queries – ? separates path from query Parameters – & separates name/value pairs – = separates name and value Encoding – + for space – %xx for special characters (e.g. %7E for ~)

8. WUCM18 Dynamic web pages Four models: – Server-side includes (SSI) – CGI – Server modules – Auxiliary servers

9. WUCM19 CGI architecture

10. WUCM110 CGI characteristics Web server creates a new process for each request that maps onto a program Data passed according to CGI Server reads output of program from program CGI spec: http://hoohoo.ncsa.uiuc.edu/cgi/http://hoohoo.ncsa.uiuc.edu/cgi/ Can use pretty much any programming language – best known Perl, Python, C/C++

11. Pros and cons of CGI Pros: – Independent of server - if program crashes it cannot affect the server – The web server takes up less memory if it does not load any server modules – Any memory (or other resources) used by the CGI program is released when the CGI program terminates Cons: – The time to create a new process to handle the CGI request is relatively long – For programs that access databases, each new process must establish a new database connection 11WUCM1

12. 12 Server module

13. Server module characteristics Web server invokes interpreter via API for each request that maps onto a program Data passed via API Server gets output via API Popular for: – PHP – ASP.NET – Perl (as an alternative to CGI) 13WUCM1

14. Pros and cons of server modules Pros: – No need to create a separate process, therefore faster – For programs that access databases, the server can maintain a persistent connection to a database, saving reconnection time Cons: – Server and program inextricably linked - a crash within the server module may crash the server – The web server will occupy more memory because of the size of the server module(s) it loads – If any server module needs a lot of memory, that memory will not be released (at least not until the server dies) 14WUCM1

15. 15 Auxiliary server

16. Auxiliary server characteristics Auxiliary server runs on a different TCP/IP port (and potentially on a different machine) Relevant requests forwarded by web server to auxiliary server Server passes response back Common for: – Java – PL/SQL (Oracle) 16WUCM1

17. Pros and cons of auxiliary servers Pros: – No need to create a new process for each request – Can maintain state (if desired) including database connections – Separate from the main web server Cons: – Overhead of resending HTTP requests and responses 17WUCM1

18. Big benefits of auxiliary servers Enterprise scalability – add new web servers – add new auxiliary servers – cross-connect between them – fits in with database scalability Resilience and reliability 18WUCM1

19. Web programming languages Programmatic – Output HTML in print statements – Use normal programming language constructs Examples: – Perl – Java (servlets) – C/C++ Better when the complexity is in the data capture and processing Output-based – HTML page with programming statements embedded – Can require contrived programming language constructs Examples: – PHP – ASP – Java (Java Server Pages) Better when the complexity is in the output format 19WUCM1

20. Examples (both Java) protected void processRequest (HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType ("text/html"); PrintWriter out = response.getWriter (); out.println(" "); out.println(" Hello world servlet "); out.println(" "); String name = request.getParameter ("name"); out.println(" Hello " + name); out.println(" "); out.close (); } Hello JSP Page Hello 20WUCM1

21. CGI 21WUCM1

22. Script interaction – CGI A CGI script is a program run on the server: – How can it get its input? – What should it do with its output? CGI scripts are loaded and executed at the request of the web server Server passes details about the request through environment variables WUCM122

23. CGI input Environment variables pass details on: – The URL the script was called from – Request parameters passed from web browser – The HTTP method used – General information about the request In addition, input can come via the standard input (STDIN) if the method was POST WUCM123

24. CGI Output The script passes information back via standard output (STDOUT) Server will ensure that data presented to STDOUT is routed to the browser Server will output the relevant data from the web browser so can be read from STDIN But HOW? - via forms, e.g. WUCM124

25. Forms for dynamic programming The significant part of the html is: Notice that: – the FORM method is GET – the FORM action is to run the mycgi.bat program WUCM125

26. GET as the form method Any data is passed as a “query string” Separator is ‘?’ Parameters concatenated with ‘&’ to form the “query string” GET /cgi-bin/mycgi.bat?WUCMI_unit=78&collect_type=NextTutorial&dis_num=67 HTTP/1.0 Complications: – If the user enters any characters not permitted in URLs, like spaces or &, =, ? – The data is encoded using the ISO8859 rules, e.g. for a space, this would give %20, its ASCII code Decoding these is best left to library code – CGI.pm in the case of Perl WUCM126

27. POST as the form method The data is sent in the request body The web server will present this data to the CGI script as input on STDIN Browser indicates how much data is being transferred, so that the CGI script knows how much to expect on STDIN WUCM127

28. When to use which method? Use GET for actions – that are safe – that are idempotent – where the total length of the URI is less than 256 chars – where it's OK for the parameters to be visible to the user – where it's OK for the parameters to be preserved as part of a bookmark Data retrieval Use POST for – everything else Data update WUCM128

29. Common CGI environment variables REQUEST_METHOD – how the script was called, i.e. GET or POST PATH_INFO – the relative path of the requested resource PATH_TRANSLATED – the absolute path of the requested resource QUERY_STRING – additional supplied parameters, if any SCRIPT_NAME – the actual name of the script WUCM129

30. Simple Apache CGI configuration CGI scripts are handled by mod_cgi Apache needs to be told: – Which directory contains scripts – How to recognise them as executable programs rather than files to be delivered The choice is basically: – Use ScriptAlias in the config file, setting up a safe directory – outside tree – Use AddHandler or SetHandler to set a handler type of cgi-script for script files in a directory in the document tree WUCM130

31. Example - ScriptAlias Assume: – A cgi-bin directory parallel to the htdocs directory in your web server's space – e.g. C:\Apache\Roger\cgi-bin Then the httpd.conf file could be WUCM131

32. Example httpd.conf file # permit access to cgi-bin directory # default deny from Prac04 Options –Indexes +ExecCGI AllowOverride None Order allow,deny Allow from all TransferLog "logs/access.log" ErrorLog "logs/error.log" LogLevel warn # tell Apache where cgi scripta are. ScriptAlias /cgi-bin/ "C:/Apache/Roger/cgi-bin/" # tell Apache where to put the script errors. ScriptLog "logs/script.log" WUCM132

33. Marking scripts as executable This is OS specific Under Windows: – Usually determined by the extension,.cmd,.bat,.pl or.exe Under Unix: – chmod +x filename – #!/bin/perl first line WUCM133

34. Debugging scripts Since CGI scripts run through Apache they are more difficult to debug For a simple test, e.g. filling in the form and then clicking “submit” where do the error & debug messages go? Usual answer is the log files: – both ErrorLog – and ScriptLog – depending on what has gone wrong WUCM134 If Apache config problem.

35. Debugging scripts Some CGI libraries, e.g. Perl CGI::Carp, direct error messages to the browser for debugging purposes – not useful for production systems For debugging messages, send output to STDERR – they will be added to ScriptLog Problems with testing CGI scripts from the command line – no web server to generate input WUCM135

36. A few security points Apache's privileges (and hence those of any CGI scripts it runs) – Apache must start as root to bind to port 80 In a well configured server, once this initial binding is done, Apache will drop back to a very low privilege user, (nobody, webuser etc) If due to a poor configuration file it does not, then any later executed CGI script will be running as root – potentially very dangerous WUCM136

37. A few security points Editors used to edit CGI scripts – Often produce backup files with standard extensions, e.g. PFE uses.$$$ – To protect deny from suspect extensions: WUCM137 Order allow,deny Deny from all

38. suEXEC and CGIWrap To increase the security when running CGI scripts, they are often “wrapped” The CGI wrappers can: – control the ownership of CGI scripts – subject the script to stringent security tests The two main alternatives are: – suEXEC –bundled with Apache – CGIWrap –produced by an independent group WUCM138