The Linux Operating System

1
The Linux Operating System
Tonga Institute of Higher Education
Lecture 5 Apache
2
Apache

• One of the most important things your Linux
  computer might do is run a webserver.
• More and more companies, as well as individuals
  are having their own web servers, and it's a good
  idea to know how to set it up and make sure it's
  running
• The most popular web server in the world is
  called Apache. It runs on Linux as well as almost
  every other operating system. It is also free to
  download.
• Many tests have shown it to be one of the fastest
  and most secure web servers, so it's a good
  choice to use. It also comes installed with
  RedHat, but whenever you install a new system you
  should always upgrade apache. (There might be
  security holes in older versions)

3
Understanding communication

• The Internet uses what's called the client-server
  relationship, where a client will ask a server
  for data. Then they will communicate and send the
  data. This is what Apache does as well
• For the server and client to talk, the server
  opens what is called a 'socket.' This is like
  opening a door for two computers to send data.
• A socket needs to know the port, the service and
  the protocol to send the data correctly
• Apache, and most web servers will use port 80
  (but you can change that), the 'www' service and
  the 'HTTP' protocol for the sockets that they open

4
Web communication

• When you open your browser and type in a website,
  what happens is that the operating system will
  take the address, put together a request for that
  webpage in a TCP/IP data packet and wrap it all
  up in the HTTP protocol.
• When that packet arrives at the server, the
  operating system on the server will figure out
  that it wants the www service and it sends it the
  packet to the webserver to deal with the data
• The webserver will use the HTTP protocol to
  figure out what the client computer wants and
  then it opens up the socket for the data to be
  sent

5
Apache as a webserver

• To make apache run, we can use the chkconfig
  commands to turn on, or the service commands,
  like
• root_at_comp root chkconfig httpd on
• root_at_comp root service httpd restart
• Notice the program name of the webserver is httpd
  (stands for http daemon), but everyone will call
  it apache.
• You can also use the program apachectl to start
  and stop the server
• root_at_comp root apachectl restart
• Our goal is to be able to get the Apache server
  to do a few different things, so we need to
  change the configuration file. This is how Apache
  figures out what to do

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Apache configuration

• The apache configuration file is usually in
  /etc/httpd/httpd.conf if you're using RedHat.
• If it's not in that location, then use the
  'locate' program to find where it is
• Once you open the file using 'pico' or 'vi'
  you'll be able to change values that affect how
  the server runs and what it does. After you save
  the changes you made, you'll also have to restart
  the server (using the 'service' program).
• The parts that we will look at are the global
  settings, Directories, Virtual Hosts and Handlers

7
Global Settings

• In the configuration file you will see many lines
  starting with ''
• This just means that it's a comment and you can
  skip through it.
• The first real setting you will come to will say
  something like
• ServerRoot /etc/httpd/
• The comments from above this line say
• ServerRoot The top of the directory tree under
  which the server's configuration, error, and log
  files are kept.
• The comments tell us that it is the place where
  the program will expect to find all the
  configuration files, logs and other important info

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Global Settings

• Other important global settings to change
• The Listen setting tells Apache to listen on
  what port and what IP address. Notice it has the
  in front of '80'. This means it will listen on
  for any IP address and on port 80
• Listen 80
• user and group settings tell the webserver what
  user they should run as. Normally there is a
  separate user that will run the webserver, called
  apache
• User apache
• Group apache
• ServerAdmin tells what email address should be
  notified if there are problems and ServerName
  says what the hostname will be of the main
  webserver
• ServerAdmin root_at_tonga-server.to
• ServerName tonga-server.to

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Global Settings and Directories

• The last global setting you'll need to know is
  DocumentRoot. This tells Apache where the website
  files are. It will be a folder on your Linux
  computer
• DocumentRoot /var/www/html
• This leads into the Directory part of the
  configuration file.
• Directories are a way to control how different
  parts of the website are accessed. For example,
  you can say that a folder on your computer at
  /home/user1 can be accessed at the website if you
  type in http//www.website.com/user1 and you can
  also say who can access this folder and who is
  allowed to see what is inside

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Directories

• The first directory listing looks like this
• ltDirectory /gt
• Options FollowSymLinks
• AllowOverride None
• lt/Directorygt
• It says that the root directory on the website
  has an option of FollowSymLinks and no Overiding.
• There are many options like the above. The format
  is that they go between to tags that say
  directory. (ltDirectorygt)
• You might also see an entry for the directory of
  the main folder, like the following
• ltDirectory "/home/www/html"gt
• Options Indexes FollowSymLinks
• Order allow,deny
• Allow from all
• lt/Directorygt

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Directories

• For example, if we have a folder at /home/user
  and we want to have it part of the website, but
  the DocumentRoot is at /var/www how can we
  include it as part of the website?
• First we add an Alias setting and then we add the
  Directory setting
• Alias /userfolder/ /home/user
• ltDirectory /home/usergt
• Options Indexes MultiViews
• AllowOverride None
• Order allow,deny
• Allow from all
• lt/Directorygt

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Directory Options

• Some common directory options that you might want
  to use inside the ltDirectorygt
• You should always include the following for
  security reasons. It means that previous security
  settings do not affect the current folder
• AllowOverride None
• If you have a directory and no web pages, just
  files, and you want people to see all the files,
  use
• Options Indexes
• If you want to control who accesses this folder
  you can use Order and Allow settings. The
  following will deny access to anyone trying to
  get to this folder that is not from usp.to
• Order deny,allow
• Allow from computer-tonga.to

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Handlers

• Handlers extend the ability of apache to do
  different thing. A handler is like an extra
  program that Apache uses to handle certain types
  of requests for webpages.
• For example, you might want a program that can be
  run on your webpage with Perl. Since it is a
  program that needs to be run and not just a
  regular file that needs to be sent, Apache must
  run a special program that will itself run the
  program on your webpage.
• This is what the idea of a Handler does. It
  handles certain types of data.

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Handlers

• One common type of handler is the following
• AddHandler cgi-script .cgi
• This says that if a file ends in .cgi that it
  is a cgi-script, which means it's a special
  program.
• Another common programming done with websites is
  called Server Side Includes. It's little
  programming codes that are put in webpages,
  except they end in .shtml.
• Apache needs to do something special with these
  files. To let your webserver use the SSI, you can
  add in the Handler below
• AddType text/html .shtml
• AddHandler server-parsed .shtml

15
Virtual Hosts

• One of the biggest computing issues that has come
  up is how to have more than one website on one
  computer?
• Apache solved this problem by creating was it
  called a 'Virtual Host'
• This means that even though your server might be
  called usp.to and you might have a webpage that
  goes to this, you can ALSO have other domain
  names point to your server, and your server will
  figure out which domain name is the one the
  person actually wants.
• There are two types of virtual hosts Name-based
  virtual hosts and IP-based hosts

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Virtual Hosts

• Named virtual hosting this is when you
  determine which website will be displayed based
  on the domain name that the client asks for. It
  is easier to set up, because you just need to
  point all the domain names to the one IP address
  of the server
• IP virtual hosting This differs from the named
  virtual hosting by using the IP address that user
  wants to connect to. This means that your server
  will be using more than one IP address. Apache
  will then figure out which website to display
  based on the IP address that the user went to.
• This is harder because of having the multiple IP
  addresses on one computer
• Generally, it's easiest and best to use the Name
  virtual hosts

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Virtual Hosts

• The virtual host section is at the bottom of the
  Apache configuration file.
• To enable it, first uncomment the following line
• NameVirtualHost
• Then you need to add in the Virtual Host that
  will be a new website. Example
• ltVirtualHost gt
• ServerName new-website.to
• DocumentRoot /home/new-website
• DirectoryIndex index.php
• CustomLog /var/log/httpd/new-access_log common
• lt/VirtualHostgt

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Virtual Hosts

• So the options that are important for the virtual
  hosts, are
• ServerName this decides what virtual host we
  are talking about
• DocumentRoot this decides where the files for
  the webpage are located
• DirectoryIndex this decides the first page that
  will come up when someone types in the webpage
• CustomLog this lets you keep track of who came
  to your website. Each virtual host can have their
  own separate log that keeps track of visitors.

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Apache Configuration

• As you look through the configuration file,
  you'll see a lot more options. Most of them are
  well-commented, so you can figure out what they
  are for.
• If you're still unsure, try looking at the manual
  that is installed on your own website. It will be
  
• http//www.your-website.com/manual/
• This is the Apache manual that is included with
  all of their servers.
• When you restart the apache server, you'll know
  if there is an error right away because you'll
  get a message that says it could not start. It
  will even tell you the line that the problem is
  on so you can fix it.
• If you start Apache and it says, OK, then your
  configuration file has no problems.

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Webpages and HTML

• We should have an Apache webserver set up. If you
  type in http//localhost/ you should see a
  webpage come up.
• The localhost is another name for saying, look at
  this computer that I am using. If you are using
  another computer, you can type in the address for
  your server, and you will also see a webpage come
  up
• So now you'll need to set up some webpages for
  your website. Your webpages are just files on
  your Linux server located in a folder, so you can
  make them using 'pico' or 'vi'

21
HTML

• Go to the folder where your website is located
  such as /var/www/html and you'll want to open
  up a file called index.html
• This is the default file that is sent. This can
  be changed in your Apache configuration file
• When you have opened up that file, we can add in
  HTML and make a webpage. As soon as you save the
  file, it will be available on the web and you can
  see the changes you have made.
• The stuff that you type inside the index.html
  file is just text. There is nothing special about
  it. You could even use only text for a webpage,
  no HTML and it would work just fine.

22
HTML

• But we can also use HTML to make our webpages
  look a little better.
• HTML is the programming language for websites.
  It's really called a markup language, which means
  it surrounds regular text with little commands
  that tell it to change the way things look or how
  they are formatted
• Some common examples of HTML
• To make text bold ltbgtText is boldlt/bgt
• To make text italic ltigtText is italiclt/igt
• To make text underline
• ltugtText is underlinelt/ugt

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HTML Parts

• There are two parts to an HTML file and you
  should generally follow the format so your HTML
  is correct.
• An HTML file will always start at the top with an
  ltHTMLgt tag and end at the bottom with a lt/HTMLgt
  tag
• Between, there should be a HEAD part and a BODY
  part. The HEAD is for special information (like a
  title for the page). The BODY is where all the
  things that will display will be put

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HTML Parts Example

• lthtmlgt
• ltheadgt
• lttitlegtMy pagelt/titlegt
• lt/headgt
• ltbodygt
• ltbgtHello Worldlt/bgt This is my pageltBRgt
• ltigtThank you for coming herelt/igt
• lt/bodygt
• lt/htmlgt

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More HTML

• HTML is a fairly simple language. It also uses
  the same format, of having some command between a
  lt gt
• Sometimes there are also attributes within the
  tag, like
• ltfont size'3'gt - changes font size
• lthr width'400'gt - makes a line on the webpage
• lta href'http//www.yahoo.com'gt - creates a link
  to yahoo.com
• ltbody bgcolor'blue'gt - changes the background
  color of the webpage to blue

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Using SSI

• SSI (Server Side Includes) is a little way to
  make your web page files have some programming in
  it.
• They are directives that are placed in HTML
  pages, and evaluated on the server while the
  pages are being sent to the user.
• They let you add things that can change to an
  existing HTML page (which usually does not change
  each time someone goes to it).
• These are called dynamic webpages. Normally, HTML
  pages are thought of as static webpages, because
  they do not change.
• SSI lets webpages change though depending on
  something that happens

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Using SSI

• SSI directives have the following syntax
• lt!--element attributevalue --gt
• It is formatted like an HTML comment, so if you
  don't have SSI correctly enabled, the browser
  will ignore it,
• The element part will tell what kind of command
  will be run
• The attributevalue part will provide some sort
  of argument to the command that element gives.
• Example which outputs the current date
• lt!--echo var"DATE_LOCAL" --gt

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SSI Examples

• Add a line that tells the last modification date
  of a file
• lt!--flastmod file"index.html" --gt
• Include part of another file (good if you want
  all your webpages to look the same)
• lt!--include virtual"/footer.html" --gt
• Execute a command on your computer and then
  display the results
• lt!--exec cmd"/bin/ls" --gt
• lt!--exec cmd/bin/netstat -n --gt
• lt!--exec cmd/usr/bin/last --gt

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Using SSI

• To make sure that Apache realizes you're using
  SSI, you need to save your file with a .shtml
  at the end.
• Normally, webpage end in .html or .htm
• To tell Apache to do something different, put a
  .shtml at the end
• Example
• index.shtml
• page1.shtml

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SSI

• SSI is a good way to add dynamic content to
  your webpage, or things that will change
  depending on different circumstances
• It is not a complete replacement for server-side
  programming, like Perl, PHP or ASP, which is what
  will be covered next.
• While static pages are good for websites that
  just want to display information, for websites to
  interact with the people who come to them, we
  need ways to generate dynamic content.
• SSI is one way to do it, but there are certainly
  better ways.

31
Installing a new Apache

• Every now and then there is a security hole in
  Apache, or a major upgrade. Sometimes it can be
  troublesome to install Apache
• The main reason for the problems is that RedHat
  puts configuration files for Apache in
  /etc/httpd/ and when you install Apache, it wants
  to put it in /usr/local/apache2/
• Then we also must make sure the startup scripts
  are updated so that it will run the new apache
  and not some old apache that is hiding.

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Summary

• Apache is one of the most valuable tools for a
  Linux server to use.
• Almost 70 of all web servers run Apache. They
  are fast, reliable and generally secure
• To understand the workings of Apache, a good
  understanding of the configuration file is
  necessary.
• Once you are able to control Apache, you will be
  able to create a well-maintained and successful
  webserver.