Answer:

Assume you server is listening on port 80, you can check how many established connection to this port with the following command:

netstat -an | grep :80 | grep ESTABLISHED | wc -l
4919

Answer:

The reason is pretty simple: If you are using a single IP but with many virtual hosts, each virtual hosts has their own certificate and private key. Without looking at the Host header in the incoming HTTP request, Apache will not be able to route the request to the designated virtual hosts. The core problem is, the incoming requests are encrypted so we are not able to tell which key should be used for decryption!

So only IP-based virtual hosts works if you need SSL.

Answer:

Apache httpd web server might listen IPv6 address if your system support IPv6.

To disable IPv6 supprt, add the following lines to the Apache configuration (httpd.conf)

# Listen 80
Listen 0.0.0.0:80 # New value

Restart Apache to take effect

# apachectl -k graceful

Answer:

If you want to block request to your web site, by user agent string, you can change by editing the Apache's configuration.

Add the following lines to the Apache configuration (httpd.conf)

RewriteCond %{HTTP_USER_AGENT} googlebot
RewriteRule . - [F,L]

The above config block all requests from Google's Bot. (Make sure mod_rewrite is installed and enabled)

Also, remember to restart Apache to take effect

# apachectl -k graceful

Answer:

If you web site's directory without a valid default index page, e.g. index.html, Apache might list all files inside the directory. If you want to turn off, do the following.

Add the following lines to the Apache configuration (httpd.conf)

Options All -Indexes  

Restart Apache to take effect

# apachectl -k graceful

Answer:

If you have not set the AllowOverride to None in your Apache's configuration, Apache will read the file .htaccess on every request. If you want to use a file name other than .htaccess, you can set using the AccessFileName directive

AccessFileName .acl

Then Apache will read the file .acl instead, remember to restart your Apache to take effect.

Answer:

The following command display the most top 10 processes used up the most of the memory in your Linux system.

Example:

# ps aux | sort -nk +4 | tail

www-data 27972  0.0  2.8 253896 14916 ?        S    13:10   0:00 /usr/sbin/apache2 -k start
www-data 27725  0.0  2.9 254028 15572 ?        S    12:52   0:00 /usr/sbin/apache2 -k start
www-data 27912  0.0  2.9 253896 15660 ?        S    13:06   0:00 /usr/sbin/apache2 -k start
www-data 27946  0.0  2.9 253764 15492 ?        S    13:08   0:00 /usr/sbin/apache2 -k start
root      2710  0.0  3.1 252996 16444 ?        Ss   Dec08   1:20 /usr/sbin/apache2 -k start
www-data 27885  0.0  3.4 253508 18152 ?        S    13:05   0:00 /usr/sbin/apache2 -k start
www-data 27892  0.1  6.7 261748 35472 ?        S    13:06   0:00 /usr/sbin/apache2 -k start
www-data 27975  0.3  7.0 263036 37128 ?        S    13:10   0:00 /usr/sbin/apache2 -k start
www-data 27964  0.7  8.2 268532 43452 ?        S    13:09   0:00 /usr/sbin/apache2 -k start
mysql     2480  0.4  8.7 223920 46096 ?        Sl   Dec08  21:07 /usr/sbin/mysqld 

Since the apache2 was configured to run in the prefork mode, so you see multiple apache processes there.

Answer:

When your Apache server is overloaded, you might want to check how many active connections are there.

# netstat -anpt|grep apache2 |grep ESTABLISHED

Replace apache2 with httpd if you are using CentOS/Fedora/RHEL.

Answer:

To check which Apache modules are loaded, in RHEL/Fedora/CentOS, type

# httpd -M

In Debian/Ubuntu

# sudo apache2ctl -M

Answer:

To deny access for Subversion (svn) related files under Apache, add the following lines to the Apache configuration (httpd.conf)

<DirectoryMatch .*\.svn/.*>
    Deny From All
</DirectoryMatch>

And restart Apache to take effect.

# apachectl -k graceful