Essential Apache

• Welcome to linuxzoo
  • System Administration
    • Apache
      • Essential Apache

Configuring Apache

Apache

• Very well known and respected http server.
• Used commercially.
• Freely available from http://www.apache.org
• Plenty of plugins.
• Relatively easy and flexible to configure.
• Fast and Reliable.

Server Architectures

• In most designs of server, you either use
  • Threaded model
  • Forking model
  • Asynchronous Architecture
• A threaded model needs special OS support to provide lightweight threads. Not used in Apache for security and reliability reasons.
• Forking means that each new request which arrives is handled by a whole process. This is the Apache way.
• Asynchronous. Some web servers exist with this model, where one process handles everything with complex IO code. Good for fast processing of simple web pages.

Apache Forking Model

Initial Settings

• These options are important, but often the least likely to be changed from the defaults!

Important Files

• /etc/init.d/httpd - the server control script
• /etc/httpd/conf/http.confg - the main conf file.
• Remember when changing the configurations it is only reread on a server reload or restart.
• Errors and other details are logged by default in /var/log/httpd/ as access_log, error_log, as suexec.log.

Reload or Restart

• Reload is the best option to use.
• With a reload, apache checks your configuration file, and switches to it only if it contains no errors.
• If it has errors, it keeps using the old configuration.
• This allows you to reconfigure a server with no downtime.
• Restart shuts down then starts the server...
• Due to a bug in apache restart can cause problems in linuxzoo.net.
• Look in the error log for help (e.g. /var/log/httpd/error_log), or syslog (e.g. /var/log/messages).

Mimic a Browser

• To understand how a sever is running is it sometimes useful to make requests at the keyboard of a server and see the results as text.
• Telnet can do this, so long as you have learned some basic HTTP commands.
• The two important ones are:
  • HEAD - Give information on a page.
  • GET - Give me the whole page.
• In HTTP 1.1 we can use virtual hosts.
• This allows multiple hosts to share a single server.
• Each host has a different name.
• The name of the host you want to answer a query is given as part of a page request.
• This is only supported in HTTP 1.1 and beyond.

$ telnet linuxzoo.net 80

HEAD / HTTP/1.1
Host: linuxzoo.net

HTTP/1.1 200 OK
Date: Mon, 01 Nov 2008 15:06:44 GMT
Server: Apache/2.0.46 (Red Hat)
Last-Modified: Fri, 29 Oct 2008 14:47:22 GMT
ETag: "4981dd-920-22ea7280"
Accept-Ranges: bytes
Content-Length: 2336
Content-Type: text/html; charset=UTF-8

$ telnet linuxzoo.net 80

HEAD / HTTP/1.1
Host: db.grussell.org

HTTP/1.1 200 OK
Date: Mon, 01 Nov 2008 15:08:52 GMT
Server: Apache/2.0.46 (Red Hat)
Last-Modified: Thu, 21 Oct 2008 09:12:33 GMT
ETag: "3c8066-a37-86c9a240"
Accept-Ranges: bytes
Content-Length: 2615
Content-Type: text/html; charset=UTF-8

VirtualHosts

• The sharing of a single IP to provide multiple hostnames is well supported in Apache.
• The part of the conf file which handles this is called <VirtualHost>
• Each part holds a list of hostnames it can handle
• The first host found in the file is always considered the default, so if no VirtualHost section matches the first block is done instead.

  <VirtualHost>
    ServerAdmin me@grussell.org
    DocumentRoot /home/gordon/public_html
    ServerName grussell.org
    ServerAlias www.grussell.org grussell.org.uk
    ErrorLog logs/gr-error_log
    CustomLog logs/gr-access_log combined
  </VirtualHost>
  

public_html

• Where apache runs on a server used by many different servers, it would be useful for each user to be able to build their own web pages which the server could serve.
• But the virtualhost configuration takes only a single document root, and each user has their own directories in /home.
• You could make the root /home
  • All of the files in /home would be accessible, not just web pages.
  • It's a bit disgusting...
• Instead, apache supports web pages appearing in a users home directory, under the subdirectory public_html.

public_html access

• Urls of the form
  • http://linuxzoo.net/~gordon/file.html
• Refer to
  • /home/gordon/public_html/file.html
• This feature must first be switched on in httpd.conf.
• To activate it, find the line
  • UserDir disable
• Then either delete the line, or put "#" (the comment character) in front of it.
• Remember to reload the server.

Linuxzoo tutorials

• Each time you book a linuxzoo machine, you will likely get a different IP and hostname.
• Each time you come in, check your hostname with "hostname".

  $ hostname
  host-5-5.linuxzoo.net
  

• In this example, virtual hosts vm-5-5.linuxzoo.net, as well as host-5-5 and web-5-5 will be proxied to your machine.
• Warning: If the server on which your virtual machine fails, you will be moved to a different machine and a different IP. You need to check your hostname when you boot!

Web access from the prompt

• The prompt is fast and convenient for admin purposes, but when you are debugging http sometimes "telnet" is not sufficient.
• There are a few other tools you can use at the prompt.
  • elinks
  • lwp-request
  • wget
• However, there is no simple replacement for actually using a real browser to check your pages. $ elinks http://linuxzoo.net

Copy http to your directory

• lwp-request http://linuxzoo.net > file.html
  • The data is obtained and then printed to the screen.
  • In this case that is redirected to file.html
• wget http://linuxzoo.net

  $ wget http://linuxzoo.net
  --19:20:11--  http://linuxzoo.net/
  Resolving linuxzoo.net... 146.176.166.1
  Connecting to linuxzoo.net|146.176.166.1|:80... connected.
  HTTP request sent, awaiting response... 200 OK
  Length: 4785 (4.7K) [text/html]
  Saving to: `index.html'
  100%[=======================================>] 4,785       --.-K/s   in 0s
  19:20:11 (304 MB/s) - `index.html' saved [4785/4785]
  
  

mod_rewrite

URL Rewriting

• A useful module in apache is mod_rewrite.
• This allows us to change URLs dynamically.
• This can be useful to, for example,
  • Change the URL of aliases in a domain so that they always give the name you want.
  • Support directories and files being moved without breaking bookmarked URLs.
  • Provide a variety of proxying methods.

Methods

• mod_rewrite has many functions...
• The key functions are:
  • RewriteCondition - an IF statement
  • RewriteRule - an action (doit) statement.
• These can be placed almost anywhere in the apache configuration files.
• We will concentrate on their use in VirtualHost areas of httpd.conf.
• To work, the area must also have: RewriteEngine on

rewriteRule

• Basic for of this rule is: RewriteRule URL-reg-exp New-URL
• For instance, you have moved /old.txt to /new.txt RewriteRule /old.txt /new.txt

Regular Expressions

• The match comparison is a regular expression.
• Useful aspects of regular expressions include:
• Text matching: . Any single Character [chars] One of the characters in chars [^chars] None of the characters in chars Text1|Text2 Either "Text1" or "Text2"

Quantifiers and Grouping

• Quantifiers: ? 0 or 1 of the preceding text * 0 or N of the preceding text + 1 or N of the preceding text
• Grouping (text) A text group - Can mark the border of an alternative or for RHS reference as $N

Anchors and Escaping

• Anchors: ^ Start of the URL $ End of the URL
• Escaping \char Allows you to use a character as the "char". For instance, \^ is the ^ character and not the start of the URL.

Back References

• $N corresponds to a group from the URL match.
• For example, rewrite any URL ending in .txt to .html one could write: RewriteRule (.*)\.txt $1.html

More complex example.

• Rewrite a URL ending with directory /demo/ to use /hia/ instead... RewriteRule ^(.*)/demo/(.*)$ $1/hia/$2

Additional Flags

• At the end of the RewriteRule can be a number of flags.
• The Flags are listed in [brackets], eg [F,G] for flags F and G.
• These change or enhance the behaviour of the match.

Options:

• R or R=code - This sends the browser the new URL as an external REDIRECTION. The code can be the type or redirection, such as 302 for MOVED TEMPORARILY (the default).
• F - Send back FORBIDDEN.
• G - Send back GONE
• P - Proxy - Forward the request
• L - Last - do not look at any more rules.
• C - chain - If the pattern matches do the next rule, otherwise ignore the remaining rules.
• NC - case insensitive.
• There are many more options, but these are the important ones.

Complex example

• If the URL has /work/ in it, rewrite /work/ to /home/.
• In addition, if the URL did have /work/ in it, replace "hello.txt" with "bye.txt".

  RewriteRule ^(.*)/work/(.*)$	$1/home/$2 	[C]
  RewriteRule ^(.*)hello.txt$	  	$1/bye.txt 	[L]
  

RewriteCond

• This command performs tests or RULES.
• If the test matches, then the next test is checked.
• If all tests match, then the RewriteRule which follows the tests is performed.
• If any Cond does not match, processing skips on till after the Rule(s) in this block.
• Basic Form of RewriteCond RewriteCond TestString ConditionString
• The value of the TestString is compared to the conditionstring.
• Condition String can be any type of regular expression.
• TestString can be one of a huge variety of things, including variables and file tests.

Variables:

• Here are some of the important variables:
  • REMOTE_ADDR
  • REMOTE_HOST
  • HTTP_HOST
  • REQUEST_URI (e.g. /index.html)
  • REQUEST_FILENAME (e.g. /home/gordon/...)
• You use these as %{REMOTE_ADDR} etc.
• There are over 20 variables available.

Flags

• RewriteCond can take flags in the same way as RewriteRule.
• There are only 2 flags:
  • NC - case insensitive
  • OR - or the Conds together.
• Normally all rules have to be true before the Rule is done, with OR the rule is done if ANY Cond is true.

Example 1:

• If 10.20.0.5 tries to view /gordon/index.html, redirect the page reference to /gordon/bye.html.

  RewriteCond %{REMOTE_ADDR} ^10\.20\.0\.5$
  RewriteRule ^/gordon/index.html$ /gordon/bye.html [L]
  

Example 2:

• My VirtualHost has grussell.org, www.grussell.org, and www.grussell.org.uk.
• Rewrite all requests to grussell.org.

  RewriteEngine on
  RewriteCond %{HTTP_HOST} !^grussell\.org$
  RewriteRule ^(.*)$ http://grussell.org$1 [L,R]
  

Example 3:

• Rewrite *.grussell.org to grussell.org, and *.grussell.org.uk to grussell.org.uk.

  RewriteEngine on
  RewriteCond %{HTTP_HOST} ^.+grussell\.org$
  RewriteRule ^(.*)$ http://grussell.org$1 [L,R]
  
  RewriteCond %{HTTP_HOST} ^.+grussell\.org\.uk$
  RewriteRule ^(.*)$ http://grussell.org.uk$1 [L,R]
  

Discussions

Discussion

• Apache runs as a user, usually "apache" or "httpd". For apache to serve a file from a user's public_html directory, what permissions would be required?

Discussion

• Here are some mock exam questions you should now be able to answer:

Question 1

• To test a web server which is hosting the virtual host "grussell.org", using only telnet, what would you type at the telnet prompt?

Question 2

Question 3