Quick HOWTO : Ch20 : The Apache Web Server

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Contents

Introduction

Apache is probably the most popular Linux-based Web server application in use. Once you have DNS correctly setup and your server has access to the Internet, you'll need to configure Apache to accept surfers wanting to access your Web site.

This chapter explains how to configure Apache in a number of commonly encountered scenarios for small web sites.

Download and Install The Apache Package

Most RedHat and Fedora Linux software products are available in the RPM format. When searching for the file, remember that the Apache RPM's filename usually starts with the word httpd followed by a version number, as in httpd-2.0.48-1.2.rpm. It is best to use the latest version of Apache. (For more on RPMs, see Chapter 6, "Installing Linux Software").

When searching for the file, remember that the Redhat / Fedora Apache RPM package's filename usually starts with the word httpd followed by a version number, as in httpd-2.0.48-1.2.rpm. With Ubuntu / Debian the package name will have the apache prefix instead.

Note: Unless otherwise stated, the sample configurations covered in this chapter will be for Redhat / Fedora distributions. If you use Debian / Ubuntu, don’t worry, there will be annotations to make you aware of the differences.

Managing the Apache Server

Managing Apache's httpd daemon is easy to do, but the procedure differs between Linux distributions. Here are some things to keep in mind.

  1. Firstly, different Linux distributions use different daemon management systems. Each system has its own set of commands to do similar operations. The most commonly used daemon management systems are SysV and Systemd.
  2. Secondly, the daemon name needs to be known. In this case the name of the daemon is httpd.

Armed with this information you can know how to:

  1. Start your daemons automatically on booting
  2. Stop, start and restart them later on during troubleshooting or when a configuration file change needs to be applied.

For more details on this, please take a look at the "Managing Daemons" section of Chapter 6 "Installing Linux Software" Note: Remember to configure your daemon to start automatically upon your next reboot.

Configuring DNS For Apache

Remember that you will never receive the correct traffic unless you configure DNS for your domain to make your new Linux box Web server the target of the DNS domain's www entry. To do this, refer to Chapter 18, "Configuring DNS", or Chapter 19, "Dynamic DNS".

DHCP and Apache

As you remember, if your Internet connection uses DHCP to get its IP address, then you need to use dynamic DNS to get the correct Internet DNS entry for your Web server. If your Web server and firewall are different machines, then you probably also need to set up port forwarding for your Web traffic to reach the Web server correctly. (Chapter 19, "Dynamic DNS", explains port forwarding, as well.).

DHCP on your protected home network is different. In the book's sample topology, the web server lives on the 192.168.1.0 home network protected by a firewall. The firewall uses NAT and port forwarding to pass Internet traffic on to the web server. Remember that the IP address of your web server can change if it gets its IP address using DHCP. This could cause your firewall port forwarding, not Dynamic DNS, to break.

In this case I recommend that your web server on the 192.168.1.0 network uses a fixed, or static IP address that is outside of the range of the DHCP server to prevent you from having this problem.

General Configuration Steps

The configuration file used by Apache is /etc/httpd/conf/httpd.conf in Redhat / Fedora distributions and /etc/apache*/httpd.conf in Debian / Ubuntu distributions. As for most Linux applications, you must restart Apache before changes to this configuration file take effect.

Where To Put Your Web Pages

All the statements that define the features of each web site are grouped together inside their own <VirtualHost> section, or container, in the httpd.conf file. The most commonly used statements, or directives, inside a <VirtualHost> container are:

  • servername: Defines the name of the website managed by the <VirtualHost> container. This is needed in named virtual hosting only, as I'll explain soon.
  • DocumentRoot: Defines the directory in which the web pages for the site can be found.

By default, Apache searches the DocumentRoot directory for an index, or home, page named index.html. So for example, if you have a servername of www.my-site.com with a DocumentRoot directory of /home/www/site1/, Apache displays the contents of the file /home/www/site1/index.html when you enter http://www.my-site.com in your browser.

Some editors, such as Microsoft FrontPage, create files with an .htm extension, not .html. This isn't usually a problem if all your HTML files have hyperlinks pointing to files ending in .htm as FrontPage does. The problem occurs with Apache not recognizing the topmost index.htm page. The easiest solution is to create a symbolic link (known as a shortcut to Windows users) called index.html pointing to the file index.htm. This then enables you to edit or copy the file index.htm with index.html being updated automatically. You'll almost never have to worry about index.html and Apache again!

This example creates a symbolic link to index.html in the /home/www/site1 directory.

[root@bigboy tmp]# cd /home/www/site1
[root@bigboy site1]# ln -s index.htm index.html
[root@bigboy site1]# ll index.*
-rw-rw-r--    1 root     root        48590 Jun 18 23:43 index.htm
lrwxrwxrwx    1 root     root            9 Jun 21 18:05 index.html -> index.htm
[root@bigboy site1]#

The l at the very beginning of the index.html entry signifies a link and the -> the link target.

The Default File Location

By default, Apache expects to find all its web page files in the /var/www/html/ directory with a generic DocumentRoot statement at the beginning of httpd.conf. The examples in this chapter use the /home/www directory to illustrate how you can place them in other locations successfully.

File Permissions And Apache

Apache will display Web page files as long as they are world readable. You have to make sure you make all the files and subdirectories in your DocumentRoot have the correct permissions.

It is a good idea to have the files owned by a nonprivileged user so that Web developers can update the files using FTP or SCP without requiring the root password.

To do this:

  1. Create a user with a home directory of /home/www.
  2. Recursively change the file ownership permissions of the /home/www directory and all its subdirectories.
  3. Change the permissions on the /home/www directory to 755, which allows all users, including the Apache's httpd daemon, to read the files inside.
[root@bigboy tmp]# useradd -g users www
[root@bigboy tmp]# chown -R www:users /home/www
[root@bigboy tmp]# chmod 755 /home/www

Now we test for the new ownership with the ll command.

[root@bigboy tmp]# ll /home/www/site1/index.*
-rw-rw-r--    1 www     users       48590 Jun 25 23:43 index.htm
lrwxrwxrwx    1 www     users           9 Jun 25 18:05 index.html -> index.htm
[root@bigboy tmp]#

Note: Be sure to FTP or SCP new files to your web server as this new user. This will make all the transferred files automatically have the correct ownership.

If you browse your Web site after configuring Apache and get a "403 Forbidden" permissions-related error on your screen, then your files or directories under your DocumentRoot most likely have incorrect permissions. Appendix II, "Codes, Scripts, and Configurations," has a short script that you can use to recursively set the file permissions in a directory to match those expected by Apache. You may also have to use the Directory directive to make Apache serve the pages once the file permissions have been correctly set. If you have your files in the default /home/www directory then this second step becomes unnecessary.

Security Contexts For Web Pages

Fedora Core 3 introduced the concept of security contexts as part of the Security Enhanced Linux (SELinux) definition. (See Appendix I, "Miscellaneous Linux Topics," for details.) A Web page may have the right permissions, but the Apache httpd daemon won't be able to read it unless you assign it the correct security context or daemon access permissions. Context-related configuration errors will give "403 Forbidden" browser messages, and in some cases, you will get the default Fedora Apache page where your expected Web page should be.

When a file is created, it inherits the security context of its parent directory. If you decide to place your Web pages in the default /var/www/ directory, then they will inherit the context of that directory and you should have very few problems.

The context of a file depends on the SELinux label it is given. The most important types of security label are listed in Table 20-1.

Table 20-1 SELinux Security Context File Labels

Context Code Description
httpd_sys_content_t The type used by regular static web pages with .html and .htm extensions.
httpd_sys_script_ro_t Required for CGI scripts to read files and directories.
httpd_sys_script_ra_t Same as the httpd_sys_script_ro_t type but also allows appending data to files by the CGI script.
httpd_sys_script_rw_t Files with this type may be changed by a CGI script in any way, including deletion.
httpd_sys_script_exec_t The type required for the execution of CGI scripts

As expected, security contexts become important when Web pages need to be placed in directories that are not the Apache defaults. In this example, user root creates a directory /home/www/site1 in which the pages for a new Web site will be placed. Using the ls -Z command, you can see that the user_home_t security label has been assigned to the directory and the index.html page created in it. This label is not accessible by Apache.

[root@bigboy tmp]# mkdir /home/www/site1
[root@bigboy tmp]# ls -Z /home/www/
drwxr-xr-x  root     root     root:object_r:user_home_t    site1
[root@bigboy tmp]# touch /home/www/site1/index.html
[root@bigboy tmp]# ls -Z /home/www/site1/index.html
-rw-r--r--  root     root     root:object_r:user_home_t        /home/www/site1/index.html
[root@bigboy tmp]#

Accessing the index.html file via a Web browser gets a "Forbidden 403" error on your screen, even though the permissions are correct. Viewing the /var/log/httpd/error_log gives a "Permission Denied" message and the /var/log/messages file shows kernel audit errors.

[root@bigboy tmp]# tail /var/log/httpd/error_log 
[Fri Dec 24 17:59:24 2004] [error] [client 216.10.119.250] (13)Permission denied: access to / denied
[root@bigboy tmp]# tail /var/log/messages
Dec 24 17:59:24 bigboy kernel: audit(1103939964.444:0): avc:   denied  { getattr } for  pid=2188 exe=/usr/sbin/httpd path=/home/www/site1 dev=hda5 ino=73659 scontext=system_u:system_r:httpd_t tcontext=root:object_r:user_home_t tclass=dir
[root@bigboy tmp]#

SELinux security context labels can be modified using the chcon command. Recognizing the error, user root uses chcon with the -R (recursive) and -h (modify symbolic links) qualifiers to modify the label of the directory to httpd_sys_content_t with the -t qualifier.

[root@bigboy tmp]# chcon -R -h -t httpd_sys_content_t /home/www/site1
[root@bigboy tmp]# ls -Z /home/www/site1/
-rw-r--r--  root     root     root:object_r:httpd_sys_content_t index.html
[root@bigboy tmp]#

Browsing now works without errors. User root won't have to run the chcon command again for the directory, because new files created in the directory will inherit the SELinux security label of the parent directory. You can see this when the file /home/www/site1/test.txt is created.

[root@bigboy tmp]# touch /home/www/site1/test.txt
[root@bigboy tmp]# ls -Z /home/www/site1/ 
-rw-r--r--  root     root     root:object_r:httpd_sys_content_t index.html
-rw-r--r--  root     root     root:object_r:httpd_sys_content_t test.txt
[root@bigboy tmp]#

Security Contexts For CGI Scripts

You can use Apache to trigger the execution of programs called Common Gateway Interface (CGI) scripts. CGI scripts can be written in a variety of languages, including PERL and PHP, and can be used to do such things as generate new Web page output or update data files. A Web page's Submit button usually has a CGI script lurking somewhere beneath. By default, CGI scripts are placed in the /var/www/cgi-bin/ directory as defined by the ScriptAlias directive you'll find in the httpd.conf file, which I'll discuss in more detail later.

ScriptAlias /cgi-bin/ "/var/www/cgi-bin/"

In the default case, any URL with the string /cgi-bin/ will trigger Apache to search for an equivalent executable file in this directory. So, for example, the URL, http://192.168.1.100/cgi-bin/test/test.cgi actually executes the script file /var/www/cgi-bin/test/test.cgi.

SELinux contexts have to be modified according to the values in Table 20.1 for a CGI script to be run in another directory or to access data files. In the example case, the PERL script test.cgi was created to display the word "Success" on the screen of your Web browser.

#!/usr/bin/perl

# CGI Script "test.cgi"

print qq(
<html>
<head>
<meta http-equiv="Content-Language" content="en-us">
<meta http-equiv="Content-Type" content="text/html">
<title>Linux Home Networking</title>
</head>
<body>
Success!
</body>
</html>
);

The ScriptAlias directive has been set to point to /home/www/cgi-bin/ instead of /var/www/cgi-bin/.

ScriptAlias /cgi-bin/ "/home/www/cgi-bin/"

User root creates the /home/www/cgi-bin/ directory, changes the directory's security context label to httpd_sys_script_exec_t, and then creates the script /home/www/cgi-bin/test/test.cgi mentioned previously with the correct executable file permissions.

[root@bigboy tmp]# mkdir -p /home/www/cgi-bin/test
[root@bigboy tmp]# chcon -h -t httpd_sys_script_exec_t /home/www/cgi-bin/
[root@bigboy tmp]# mkdir /home/www/cgi-bin/test
[root@bigboy tmp]# ls -Z /home/www/cgi-bin
drwxr-xr-x  root     root     root:object_r:httpd_sys_script_exec_t test
[root@bigboy tmp]# vi /home/www/cgi-bin/test/test.cgi
[root@bigboy tmp]# chmod o+x /home/www/cgi-bin/test/test.cgi
[root@bigboy tmp]#

Accessing the URL http://192.168.1.100/cgi-bin/test/test.cgi is successful. Problems occur when the same test.cgi file needs to be used by a second Web site housed on the same Web server. The file is copied to a directory /web/cgi-bin/site2/ governed by the ScriptAlias in the second Web site's <VirtualHost> container (explained later), but the security context label isn't copied along with it.

ScriptAlias /cgi-bin/ "/web/cgi-bin/site2/"

The file inherits the context of its new parent.

[root@bigboy tmp]# cp /home/www/cgi-bin/test/test.cgi /web/cgi-bin/site2/test.cgi
[root@bigboy tmp]# ls -Z /web/cgi-bin/site2/test.cgi
-rw-r--r-x  root     root     root:object_r:tmp_t              /web/cgi-bin/site2/test.cgi
[root@bigboy tmp]#

Permission denied and kernel audit errors occur once more; you can fix them only by changing the security context of the test.cgi file.

[root@bigboy tmp]# tail /var/log/httpd/error_log
[Fri Dec 24 18:36:08 2004] [error] [client 216.10.119.250] (13)Permission denied: access to /cgi-bin/texcelon/test.cgi denied
[root@bigboy tmp]# tail /var/log/messages
Dec 24 18:36:08 bigboy kernel: audit(1103942168.549:0): avc:   denied  { getattr } for  pid=2191 exe=/usr/sbin/httpd path=/web/cgi-bin/site2/test.cgi dev=hda5 ino=77491 scontext=system_u:system_r:httpd_t tcontext=root:object_r:tmp_t tclass=file
[root@bigboy tmp]#

Note: If you find security contexts too restrictive, you can turn them off system wide by editing your /etc/selinux/config file, modifying the SELINUX parameter to disabled. SELinux will be disabled after your next reboot.

Named Virtual Hosting

You can make your Web server host more than one site per IP address by using Apache's named virtual hosting feature. You use the NameVirtualHost directive in the /etc/httpd/conf/httpd.conf file to tell Apache which IP addresses will participate in this feature.

The <VirtualHost> containers in the file then tell Apache where it should look for the Web pages used on each Web site. You must specify the IP address for which each <VirtualHost> container applies.

Named Virtual Hosting Example

Consider an example in which the server is configured to provide content on 97.158.253.26. In the code that follows, notice that within each <VirtualHost> container you specify the primary Web site domain name for that IP address with the ServerName directive. The DocumentRoot directive defines the directory that contains the index page for that site.

You can also list secondary domain names that will serve the same content as the primary ServerName using the ServerAlias directive.

Apache searches for a perfect match of NameVirtualHost, <VirtualHost>, and ServerName when making a decision as to which content to send to the remote user's Web browser. If there is no match, then Apache uses the first <VirtualHost> in the list that matches the target IP address of the request.

This is why the first <VirtualHost> statement contains an asterisk: to indicate it should be used for all other Web queries.

NameVirtualHost 97.158.253.26
 
<VirtualHost *>
   Default Directives. (In other words, not site #1 or site #2)
</VirtualHost>

<VirtualHost 97.158.253.26>
   servername www.my-site.com
   Directives for site #1
</VirtualHost>

<VirtualHost 97.158.253.26>
   servername www.another-site.com
   Directives for site #2
</VirtualHost>

Be careful with using the asterisk in other containers. A <VirtualHost> with a specific IP address always gets higher priority than a <VirtualHost> statement with an * intended to cover the same IP address, even if the ServerName directive doesn't match. To get consistent results, try to limit the use of your <VirtualHost *> statements to the beginning of the list to cover any other IP addresses your server may have.

You can also have multiple NameVirtualHost directives, each with a single IP address, in cases where your Web server has more than one IP address.

IP-Based Virtual Hosting

The other virtual hosting option is to have one IP address per Web site, which is also known as IP-based virtual hosting. In this case, you will not have a NameVirtualHost directive for the IP address, and you must only have a single <VirtualHost> container per IP address.

Also, because there is only one Web site per IP address, the ServerName directive isn't needed in each <VirtualHost> container, unlike in named virtual hosting.

IP Virtual Hosting Example: Single Wild Card

In this example, Apache listens on all interfaces, but gives the same content. Apache displays the content in the first <VirtualHost *> directive even if you add another right after it. Apache also seems to enforce the single <VirtualHost> container per IP address requirement by ignoring any ServerName directives you may use inside it.

<VirtualHost *>
   DocumentRoot /home/www/site1
</VirtualHost>

IP Virtual Hosting Example: Wild Card and IP addresses

In this example, Apache listens on all interfaces, but gives different content for addresses 97.158.253.26 and 97.158.253.27. Web surfers get the site1 content if they try to access the web server on any of its other IP addresses.

<VirtualHost *>
   DocumentRoot /home/www/site1
</VirtualHost>

<VirtualHost 97.158.253.26>
   DocumentRoot /home/www/site2
</VirtualHost>

<VirtualHost 97.158.253.27>
   DocumentRoot /home/www/site3
</VirtualHost>

A Note On Virtual Hosting And SSL

Because it makes configuration easier, system administrators commonly replace the IP address in the <VirtualHost> and NameVirtualHost directives with the * wildcard character to indicate all IP addresses.

If you installed Apache with support for secure HTTPS/SSL, which is used frequently in credit card and shopping cart Web pages, then wild cards won't work. The Apache SSL module demands at least one explicit <VirtualHost> directive for IP-based virtual hosting. When you use wild cards, Apache interprets it as an overlap of name-based and IP-based <VirtualHost> directives and gives error messages because it can't make up its mind about which method to use:

Starting httpd: [Sat Oct 12 21:21:49 2002] [error] VirtualHost _default_:443 -- mixing * ports and non-* ports with a NameVirtualHost address is not supported, proceeding with undefined results

If you try to load any Web page on your web server, you'll see the error:

Bad request!

Your browser (or proxy) sent a request that this server could not understand.
If you think this is a server error, please contact the webmaster

The best solution to this problem is to use wild cards more sparingly. Don't use virtual hosting statements with wild cards except for the very first <VirtualHost> directive that defines the web pages to be displayed when matches to the other <VirtualHost> directives cannot be found. Here is an example.

NameVirtualHost *

<VirtualHost *>
   Directives for other sites
</VirtualHost>

<VirtualHost 97.158.253.28>
   Directives for site that also run on SSL
</VirtualHost>

Configuration - Multiple Sites And IP Addresses

To help you better understand the edits needed to configure the /etc/httpd/conf/httpd.conf file, I'll walk you through an example scenario. The parameters are:

  • The web site's systems administrator previously created DNS entries for www.my-site.com, my-site.com, www.my-cool-site.com and www.default-site.com to map the IP address 97.158.253.26 on this web server. The domain www.another-site.com is also configured to point to alias IP address 97.158.253.27. The administrator wants to be able to get to www.test-site.com on all the IP addresses.
  • Traffic to www.my-site.com, my-site.com, and www.my-cool-site.com must get content from subdirectory site2. Hitting these URLs causes Apache to display the contents of file index.html in this directory.
  • Traffic to www.test-site.com must get content from subdirectory site3.
  • Named virtual hosting will be required for 97.158.253.26 as in this case we have a single IP address serving different content for a variety of domains. A NameVirtualHost directive for 97.158.253.26 is therefore required.
  • Traffic going to www.another-site.com will get content from directory site4.
  • All other domains pointing to this server that don't have a matching ServerName directive will get Web pages from the directory defined in the very first <VirtualHost> container: directory site1. Site www.default-site.com falls in this category.

Table 20-2 summarizes these requirements.

Table 20-2 Web Hosting Scenario Summary

Domain IP Address Directory Type of Virtual Hosting
www.my-site.com

my-site.com

www.my-cool-site.com

97.158.253.26 Site2 Name Based
www.test-site.com 97.158.253.27 Site3 Name Based (Wild card)
www.another-site.com 97.158.253.27 Site4 Name Based
www.default-site.com

All other domains

97.158.253.26 Site1 Name Based


How do these requirements translate into code? Here is a sample snippet of a working httpd.conf file:


ServerName localhost
NameVirtualHost 97.158.253.26
NameVirtualHost 97.158.253.27

#
# Match a webpage directory with each website
#
<VirtualHost *>
    DocumentRoot /home/www/site1
 </VirtualHost>

<VirtualHost 97.158.253.26>
    DocumentRoot /home/www/site2
   ServerName www.my-site.com
    ServerAlias my-site.com, www.my-cool-site.com
</VirtualHost>
 
<VirtualHost 97.158.253.27>
    DocumentRoot /home/www/site3
   ServerName www.test-site.com
</VirtualHost>
 
<VirtualHost 97.158.253.27>
    DocumentRoot /home/www/site4
   ServerName www.another-site.com
</VirtualHost>
 
 
#
# Make sure the directories specified above
# have restricted access to read-only.
#
<Directory "/home/www/*">
    Order allow,deny
   Allow from all
 
    AllowOverride FileInfo AuthConfig Limit
   Options MultiViews Indexes SymLinksIfOwnerMatch IncludesNoExec
   <Limit GET POST OPTIONS>
     Order allow,deny
     Allow from all
    </Limit>
    <LimitExcept GET POST OPTIONS>
     Order deny,allow
     Deny from all
   </LimitExcept>
 
</Directory>

These statements would normally be found at the very bottom of the file where the virtual hosting statements reside. The last section of this configuration snippet has some additional statements to ensure read-only access to your Web pages with the exception of Web-based forms using POSTs (pages with "submit" buttons). Remember to restart Apache every time you update the httpd.conf file for the changes to take effect on the running process.

Note: You will have to configure your DNS server to point to the correct IP address used for each of the Web sites you host. Chapter 18, "Configuring DNS", shows you how to configure multiple domains, such as my-site.com and another-site.com, on your DNS server.

Testing Your Website Before DNS Is Fixed

You may not be able to wait for DNS to be configured correctly before starting your project. The easiest way to temporarily bypass this is to modify the hosts file on the Web developer's client PC or workstation (not the Apache server). By default, PCs and Linux workstations query the hosts file first before checking DNS, so if a value for www.my-site.com is listed in the file, that's what the client will use.

The Windows equivalent of the Linux /etc/hosts file is named C:\WINDOWS\system32\drivers\etc\hosts. You need to open and edit it with a text editor, such as Notepad. Here you could add an entry similar to:

97.158.253.26          www.my-site.com

Do not remove the localhost entry in this file

Disabling Directory Listings

Be careful to include an index.html pages in each subdirectories under your DocumentRoot directory, as if one isn't found, Apache will default to giving a listing of all the files in that subdirectory.

Say, for example, you create a subdirectory named /home/www/site1/example under www.my-site.com's DocumentRoot of /home/www/site1/. Now you'll be able to view the contents of the file my-example.html in this subdirectory if you point your browser to:

http://www.my-site.com/example/my-example.html

If curious surfers decide to see what the index page is for www.my-site.com/example, they would type the link:

http://www.my-site.com/example

Apache lists all the contents of the files in the example directory if it can't find the index.html file. You can disable the directory listing by using a -Indexes option in the <Directory> directive for the DocumentRoot like this:

<Directory "/home/www/*">
 ...
 ...
 ...
 Options MultiViews -Indexes SymLinksIfOwnerMatch IncludesNoExec

Remember to restart Apache after the changes. Users attempting to access the nonexistent index page will now get a "403 Access denied" message.

Note: When setting up a yum server it's best to enable directory listings for the RPM subdirectories. This allows web surfers to double check the locations of files through their browsers.

Handling Missing Pages

You can tell Apache to display a predefined HTML file whenever a surfer attempts to access a non-index page that doesn't exist. You can place this statement in the httpd.conf file, which will make Apache display the contents of missing.htm instead of a generic "404 file Not Found" message:

ErrorDocument 404 /missing.htm

Remember to put a file with this name in each DocumentRoot directory. You can see the missing.htm file I use by trying the nonexistent link.

http://www.linuxhomenetworking.com/bogus-file.htm

Notice that this gives the same output as

http://www.linuxhomenetworking.com/missing.htm.

Using Data Compression On Web Pages

Apache also has the ability to dynamically compress static Web pages into gzip format and then send the result to the remote Web surfers' Web browser. Most current Web browsers support this format, transparently uncompressing the data and presenting it on the screen. This can significantly reduce bandwidth charges if you are paying for Internet access by the megabyte.

First you need to load Apache version 2's deflate module in your httpd.conf file and then use Location directives to specify which type of files to compress. After making these modifications and restarting Apache, you will be able to verify from your /var/log/httpd/access_log file that the sizes of the transmitted HTML pages have shrunk.

Compare the file sizes in this Apache log.

[root@ bigboy tmp]# grep dns-static /var/log/httpd/access_log
...
...
67.119.25.115 - - [15/Feb/2003:23:06:51 -0800] "GET /dns-static.htm HTTP/1.1" 200 15190 "http://www.linuxhomenetworking.com/sendmail.htm" "Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 4.0; AT&T CSM6.0; YComp 5.0.2.6)"
...
...
[root@ bigboy tmp]#

and the corresponding directory listing

[root@ bigboy tmp]# ll /web-dir/dns-static.htm
-rw-r--r--    1 user      group       78350 Feb 15 00:53 /home/www/ccie/dns-static.htm
[root@bigboy tmp]#

As you can see, 78,350 bytes shrunk to 15,190 bytes, that's almost 80% compression.

Compression Configuration Example

You can insert these statements just before your virtual hosting section of your httpd.conf file to activate the compression of static pages. Remember to restart Apache when you do.

Note: Fedora's version of httpd.conf loads the compression module mod_deflate by default. This means that the LoadModule line (the first line of the example snippet) is not required for Fedora. The location statements are required, however.

LoadModule deflate_module modules/mod_deflate.so
 
<Location />
 
     # Insert filter
     SetOutputFilter DEFLATE
 
     # Netscape 4.x has some problems...
     BrowserMatch ^Mozilla/4 gzip-only-text/html
 
     # Netscape 4.06-4.08 have some more problems
     BrowserMatch ^Mozilla/4\.0[678] no-gzip
 
     # MSIE masquerades as Netscape, but it is fine
     BrowserMatch \bMSIE !no-gzip !gzip-only-text/html
 
     # Don't compress images
     SetEnvIfNoCase Request_URI \
       \.(?:gif|jpe?g|png)$ no-gzip dont-vary
 
 
     # Make sure proxies don't deliver the wrong content
     Header append Vary User-Agent env=!dont-vary
 
</Location>

Apache Running On A Server Behind A NAT Firewall

If your Web server is behind a NAT firewall and you are logged on a machine behind the firewall as well, then you may encounter problems when trying to access www.mysite.com of www.another-site.com. Because of NAT (network address translation), firewalls frequently don't allow access from their protected network to IP addresses that they masquerade on the outside.

For example, Linux Web server bigboy has an internal IP address of 192.168.1.100, but the firewall presents it to the world with an external IP address of 97.158.253.26 via NAT/masquerading. If you are on the inside, 192.168.1.X network, you may find it impossible to hit URLs that resolve in DNS to 97.158.253.26.

There is a two part solution to this problem:

Step 1: Configure Virtual Hosting on Multiple IPs

You can configure Apache to serve the correct content when accessing www.mysite.com or www.another-site.com from the outside, and also when accessing the specific IP address 192.168.1.100 from the inside. Fortunately Apache allows you to specify multiple IP addresses in the <VirtualHost> statements to help you overcome this problem.

Here is an example:

NameVirtualHost 192.168.1.100
NameVirtualHost 97.158.253.26

<VirtualHost 192.168.1.100 97.158.253.26>
   DocumentRoot /www/server1
   ServerName www.my-site.com
   ServerAlias bigboy, www.my-site-192-168-1-100.com
</VirtualHost>

Step 2: Configure DNS "Views"

You now need to fix the DNS problem that NAT creates. Users on the Internet need to access IP address 97.158.253.26 when visiting www.my-site.com and users on your home network need to access IP address 192.168.1.100 when visiting the same site.

You can configure your DNS server to use views which makes your DNS server give different results depending on the source IP address of the Web surfer's PC doing the query. Chapter 18, "Configuring DNS", explains how to do this in detail.

Note: If you have to rely on someone else to do the DNS change, then you can edit your PC's hosts file as a quick and dirty temporary solution to the problem. Remember that this will fix the problem on your PC alone.

How To Protect Web Page Directories With Passwords

You can password protect content in both the main and subdirectories of your DocumentRoot fairly easily. I know people who allow normal access to their regular Web pages, but require passwords for directories or pages that show MRTG or Webalizer data. This example shows how to password protect the /home/www directory.

1) Use Apache's htpasswd password utility to create username/password combinations independent of your system login password for Web page access. You have to specify the location of the password file, and if it doesn't yet exist, you have to include a -c, or create, switch on the command line. I recommend placing the file in your /etc/httpd/conf directory, away from the DocumentRoot tree where Web users could possibly view it. Here is an example for a first user named peter and a second named paul:

[root@bigboy tmp]# htpasswd -c /etc/httpd/conf/.htpasswd peter
New password:
Re-type new password:
Adding password for user peter
[root@bigboy tmp]#

[root@bigboy tmp]# htpasswd /etc/httpd/conf/.htpasswd paul
New password:
Re-type new password:
Adding password for user paul
[root@bigboy tmp]#

2) Make the .htpasswd file readable by all users.

[root@bigboy tmp]# chmod 644 /etc/httpd/conf/.htpasswd

3) Create a .htaccess file in the directory to which you want password control with these entries.

AuthUserFile /etc/httpd/conf/.htpasswd
AuthGroupFile /dev/null
AuthName EnterPassword
AuthType Basic
require user peter

Remember this password protects the directory and all its subdirectories. The AuthUserFile tells Apache to use the .htpasswd file. The require user statement tells Apache that only user peter in the .htpasswd file should have access. If you want all .htpasswd users to have access, replace this line with require valid-user. AuthType Basic instructs Apache to accept basic unencrypted passwords from the remote users' Web browser.

4) Set the correct file protections on your new .htaccess file in the directory /home/www.

[root@bigboy tmp]# chmod 644 /home/www/.htaccess

5) Make sure your /etc/httpd/conf/http.conf file has an AllowOverride statement in a <Directory> directive for any directory in the tree above /home/www. In this example below, all directories below /var/www/ require password authorization.

<Directory /home/www/*>
   AllowOverride AuthConfig
</Directory>

6) Make sure that you have a <VirtualHost> directive that defines access to /home/www or another directory higher up in the tree.

<VirtualHost *>
   ServerName 97.158.253.26
   DocumentRoot /home/www
</VirtualHost>

7) Restart Apache.

Try accessing the web site and you'll be prompted for a password.

The conf.d Directory

Files in the /etc/httpd/conf.d (Redhat / Fedora) or the /etc/apache*/conf.d (Debian / Ubuntu) directory are read and automatically appended to the configuration in the httpd.conf file every time Apache is restarted. In complicated configurations, in which a Web server has to host many Web sites, you can create one configuration file per Web site each with its own set of <VirtualHost> and <Directory> containers. This can make Web site management much simpler. To do this correctly:

  1. Backup your httpd.conf file, in case you make a mistake.
  2. Create the files located in this directory that contain the Apache required <VirtualHost> and <Directory> containers and directives.
  3. If each site has a dedicated IP address, then place the NameVirtualHost statements in the corresponding conf.d directory file. If it is shared, it'll need to remain in the main httpd.conf file.
  4. Remove the corresponding directives from the httpd.conf file.
  5. Restart Apache, and test.

The files located in the conf.d directory don't have to have any special names, and you don't have to refer to them in the httpd.conf file.

Troubleshooting Apache

Troubleshooting a basic Apache configuration is fairly straightforward; you'll find errors in the /var/log/httpd/error_log file during normal operation or displayed on the screen when Apache starts up. Most of the errors you'll encounter will probably be related to incompatible syntax in the <VirtualHosts> statement caused by typing errors.

Testing Basic HTTP Connectivity

The very first step is to determine whether your web server is accessible on TCP port 80 (HTTP).

Lack of connectivity could be caused by a firewall with incorrect permit, NAT, or port forwarding rules to your Web server. Other sources of failure include Apache not being started at all, the server being down, or network-related failures.

If you can connect on port 80 but no pages are being served, then the problem is usually due to a bad Web application, not the Web server software itself.

It is best to test this from both inside your network and from the Internet. Troubleshooting with TELNET is covered in Chapter 4, "Simple Network Troubleshooting".

Browser 403 Forbidden Messages

Browser 403 Forbidden messages are usually caused by file permissions and security context issues. Please refer to the "General Configuration Steps" section for further details.

A sure sign of problems related to security context are "avc: denied" messages in your /var/log/messages log file.

Nov 21 20:41:23 bigboy kernel: audit(1101098483.897:0): avc:  denied  { getattr } for  pid=1377 exe=/usr/sbin/httpd path=/home/www/index.html dev=hda5 ino=12 scontext=root:system_r:httpd_t tcontext=root:object_r:home_root_t tclass=file

Only The Default Apache Page Appears

When only the default Apache page appears, there are two main causes. The first is the lack of an index.html file in your Web site's DocumentRoot directory. The second cause is usually related to an incorrect security context for the Web page's file. Please refer to the "General Configuration Steps" section for further details.

Incompatible httpd.conf Files When Upgrading

Your old configuration files will be incompatible when upgrading from Apache version 1.3 to Apache 2.X. In Redhat / Fedora, the new version 2.X default configuration file is stored in /etc/httpd/conf/httpd.conf.rpmnew. For the simple virtual hosting example above, it would be easiest to:

  1. Save the old httpd.conf file with another name, httpd.conf-version-1.x for example. Copy the ServerName, NameVirtualHost, and VirtualHost containers from the old file and place them in the and place them in the new httpd.conf.rpmnew file.
  2. Copy the httpd.conf.rpmnew file an name it httpd.conf
  3. Restart Apache

With other distributions, the procedure is similar; just place your containers in the new default configuration file and restart Apache.

Server Name Errors

All ServerName directives must list a domain that is resolvable in DNS, or else you'll get an error similar to these when starting httpd.

Starting httpd: httpd: Could not determine the server's fully qualified domain name, using 127.0.0.1 for ServerName

Starting httpd: [Wed Feb 04 21:18:16 2004] [error] (EAI 2)Name or service not known: Failed to resolve server name for 192.16.1.100 (check DNS) -- or specify an explicit ServerName

You can avoid this by adding a default generic ServerName directive at the top of the httpd.conf file that references localhost instead of the default new.host.name:80.

#ServerName new.host.name:80
ServerName localhost

The Apache Status Log Files

The /var/log/httpd/access_log file is updated after every HTTP query and is a good source of general purpose information about your website. There is a fixed formatting style with each entry being separated by spaces or quotation marks. Table 20-3 lists the layout.

Table 20-3 Apache Log File Format

Field Number Description Separator
1 IP Address of the remote web surfer Spaces
2 Time Stamp Square Brackets []
3 HTTP query including the web page served Quotes ""
4 HTTP result code Spaces
5 The amount of data in bytes sent to the remote web browser Spaces
6 The web page that contained the link to the page served. Quotes ""
7 The version of the web browser used to get the page Quotes ""

Upon examining the entry, you can determine that someone at IP address 67.119.25.115 on February 15, looked at the web page /dns-static.htm returning a successful 200 status code. The amount of data sent was 15190 bytes and the surfer got to the site by clicking on the link http://www.linuxhomenetworking.com/sendmail.htm using Microsoft Internet Explorer version 5.5.

67.119.25.115 - - [15/Feb/2003:23:06:51 -0800] "GET /dns-static.htm HTTP/1.1" 200 15190
"http://www.linuxhomenetworking.com/sendmail.htm" 
"Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 4.0; AT&T CSM6.0; YComp 5.0.2.6)"

The HTTP status code can provide some insight into the types of operations surfers are trying to attempt and may help to isolate problems with your pages, not the operation of the Apache. For example 404 errors are generated when someone tries to access a web page that doesn't exist anymore. This could be caused by incorrect URL links in other pages on you site. Table 20-4 has some of the more common examples.

Table 20-4 HTTP Status Codes

HTTP Code Description
200 Successful request
304 Successful request, but the web page requested hasn't been modified since the current version in the remote web browser's cache. This means the web page will not be sent to the remote browser, it will just use its cached version instead. Frequently occurs when a surfer is browsing back and forth on a site.
401 Unauthorized access. Someone entered an incorrect username / password on a password protected page.
403 Forbidden. File permissions or contexts prevents Apache from reading the file. Often occurs when the web page file is owned by user "root" even though it has universal read access.
404 Not found. Page requested doesn't exist.
500 Internal server error. Frequently generated by CGI scripts that fail due to bad syntax. Check your error_log file for further details on the script's error message.

The Apache Error Log Files

The /var/log/httpd/error_log file is a good source for error information. Unlike the /var/log/httpd/access_log file, there is no standardized formatting.

Typical errors that you'll find here are HTTP queries for files that don't exist or forbidden requests for directory listings. The file will also include Apache startup errors which can be very useful.

The /var/log/httpd/error_log file also is the location where CGI script errors are written. Many times CGI scripts fail with a blank screen on your browser; the /var/log/httpd/error_log file most likely lists the cause of the problem.

Conclusion

Web sites both personal and commercial can be very rewarding exercises as they share your interests with the world and allow you to meet new people with whom to develop friendships or transact business.

Unfortunately, even the best Web sites can be impersonal as they frequently only provide information that the designer expects the visitor to need. E-mail, although ancient in comparison to newer personalized interactive Internet technologies, such as IP telephony and instant messaging, has the advantage of being able to relay documents and other information without interrupting the addressee. This allows them to schedule a response when they are better prepared to answer, a valuable quality when replies need to be complex.

Chapter 21, "Configuring Linux Mail Servers", explains how to configure a Linux e-smail server to reduce spam and provide personalized addresses across multiple domains. No Web site should be without one.