Fourth day in Addis

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Unix file permissions:

    drwxr-xr-x   2 root root    38 2006-07-14
    |
    +- Is a directory

    drwxr-xr-x   2 root root    38 2006-07-14
     ---
      |
      +- User permissions (u)

    drwxr-xr-x   2 root root    38 2006-07-14
        ---
         |
         +- Group permissions (g)

    drwxr-xr-x   2 root root    38 2006-07-14
           ---
            |
            +- Permissions for others (o)

    drwxr-xr-x   2 root root    38 2006-07-14
                   ----
                    |
                    +- Owner user

    drwxr-xr-x   2 root root    38 2006-07-14
                        ----
                         |
            Owner group -+

Other bits:

4000 Set user ID:
For executable files: run as the user who owns the file, instead of the user who runs the file
For directories: I think it's not used
2000 Set group ID:
For executable files: run as the group who owns the file, instead of the group of the user who runs the file
For directories: when a file is created inside the directory, it belongs to the group of the directory instead of the default group of the user who created the file
1000 Sticky bit:
For files: I think it's not used anymore
For directories: only the owner of a file can delete or rename the file

The executable bit for directories means "can access the files in the directory".

If a directory is readable but not executable, then I can see the list of files (with ls) but I cannot access the files.

To access a file, all the directories of its path up to / need to be executable.

Commands to manipulate permissions:

chown - change file owner and group
chgrp - change group ownership
chmod - change file access permissions
sudo adduser enrico www-data adds the user enrico to the group www-data.

Example setup for a website for students:

    # Create the group 'students'
    mkdir /var/www/students
    chgrp students /var/www/students
    chmod 2775 /var/www/students

    # If you don't want other users to read the files of the students:

    chmod 2770 /var/www/students
    adduser www-data students
     (this way the web server can read the
      pages)

    # when you add a user to a group, it does not affect running processes:

     - users need to log out and in again
     - servers need to be restarted

Apache:

To install apache2 without a graphical interface:

apt-cache search apache2 | less
sudo apt-get install apache2

By default, /var/www is where is the static website.
By default, ~/public_html is the personal webspace for every user, accessible as: http://localhost/~user
By default, /usr/lib/cgi-bin contains scripts that are executed when someone browses http://website/cgi-bin/script
By default, apache reads the server name from the DNS. If we don't have a name in the DNS and we want to use the IP, we need to set:
```
ServerName 10.4.15.158
```

in /etc/apache/apache2.conf (set it to your IP address)

To access the Apache manual: http://localhost/doc/apache2-doc/manual/
http://localhost/doc/apache2-doc/manual/mod/mod_access.html The access control module
http://localhost/doc/apache2-doc/manual/mod/mod_auth.html The user authentication module

To edit a user password file, use:

htpasswd - Manage user files for basic authentication

Example .htaccess file to password protect a directory:

AuthUserFile /etc/apache2/students
AuthType Basic
AuthName "Students"
Require valid-user

Information about .htaccess is in http://localhost/doc/apache2-doc/manual/howto/htaccess.html
If you need to tell apache to listen on different ports, add a Listen directive to /etc/apache2/ports.conf. Then you can use:
```
<VirtualHost www.training.aau.edu.et:9000>
[...]
</VirtualHost>
```
To setup an HTTPS website:
- Documentation is in http://localhost/doc/apache2-doc/manual/ssl/
- How to create a certificate: http://www.tc.umn.edu/~brams006/selfsign.html
- Create a certificate:
  
  /usr/sbin/apache2-ssl-certificate -days 365
- Create a virtual host on port 443:
  
  [...]
- Enable SSL in the VirtualHost:
  
  SSLEngine On SSLCertificateFile /etc/apache2/ssl/apache.pem
- Enable listening on the HTTPS port (/etc/apache2/ports.conf):
  
  Listen 443

Apache troubleshooting:

check that there are no errors in the configuration file:
```
apache2ctl configtest
```

This it is always a good thing to do before restarting or reloading apache.

read logs in /var/log/apache2/
if you made a change but you don't see it on the web, it can be that you have the old page in the cache of the browser: try reloading a few times.

To install PHP

apt-get install libapache2-module-php5
then by default, every file .php is executed as php code
Small but useful test php file:
```
<? phpinfo() ?>
```

To install MySQL

apt-get install mysql-client mysql-server
for administration run mysql as root:
- Create a database with:
  
  create database students

Give a user access to the database:

# Without password
grant all on students.* to enrico;

# With password
grant all on students.* to enrico identified by "SECRET";

More information can be found at http://www-css.fnal.gov/dsg/external/freeware/mysqlAdmin.html

To use MySQL from PHP:

    apt-get install php5-mysqli php5-mysql

Problems found today:

the apache2 manual in /usr/share/doc/manual can only be viewed using apache because it uses MultiView. So you need to have a working apache to read how to have a working apache.
chmod does not have examples in the manpage.