The Linux Operating System

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

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)

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

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

Apache as a webserver • To make apache run, we can use the chkconfig commands to turn on, or the service commands, like • [root@comp root]# chkconfig httpd on • [root@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@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

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

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

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@tonga-server.to • ServerName tonga-server.to

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

Directories • The first directory listing looks like this <Directory /> Options FollowSymLinks AllowOverride None </Directory> • 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. (<Directory>) • You might also see an entry for the directory of the main folder, like the following • <Directory "/home/www/html"> • Options Indexes FollowSymLinks • Order allow,deny • Allow from all • </Directory>

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” <Directory “/home/user”> Options Indexes MultiViews AllowOverride None Order allow,deny Allow from all </Directory>

Directory Options • Some common directory options that you might want to use inside the <Directory>: • 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

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.

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

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

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

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: <VirtualHost *> ServerName new-website.to DocumentRoot /home/new-website DirectoryIndex index.php CustomLog /var/log/httpd/new-access_log common </VirtualHost>

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.

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.

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'

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.

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: Text is bold • To make text italic: Text is italic • To make text underline: Text is underline

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 <HTML> tag and end at the bottom with a </HTML> 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

HTML Parts Example <html> <head> <title>My page</title> </head> <body> Hello World This is my page Thank you for coming here </body> </html>

More HTML • HTML is a fairly simple language. It also uses the same format, of having some command between a < > • Sometimes there are also attributes within the tag, like: • - changes font size • <hr width='400'> - makes a line on the webpage • <a href='http://www.yahoo.com'> - creates a link to yahoo.com • <body bgcolor='blue'> - changes the background color of the webpage to blue

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

Using SSI • SSI directives have the following syntax: •  • 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 attribute=value part will provide some sort of argument to the command that element gives. • Example which outputs the current date: •

SSI Examples • Add a line that tells the last modification date of a file •  • Include part of another file (good if you want all your webpages to look the same) •  • Execute a command on your computer and then display the results •  •  •

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

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.

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.

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.

The Linux Operating System