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:

When you rewrite URL using the Nginx's rewrite directive, you might be confused that what are the exact difference between last and break?

From the official site: http://wiki.nginx.org/HttpRewriteModule#rewrite

last - completes processing of rewrite directives, after which searches for corresponding URI and location
break - completes processing of rewrite directives and breakes location lookup cycle by not doing any location lookup and internal jump at all

So: both directives will end the rewrite cycle, but break will not do any location lookup and internal jump afterward.

Answer:

When you want to confirm you have configured Nginx correctly, e.g. make sure you have the right rewrite rules. The easiest way is to use the echo module to debug.

E.g.

..
location / {
    rewrite ^/foo /bar break;
    echo $uri; # try with other variables you want to trace
}

When you use curl to request the page, you should receive something like:

# curl http://example.com/foo
bar

Answer:

Sometimes, you need to enable the debug log feature on Nginx for troubleshooting some nasty issues.

Firstly, you need to make sure you have configured nginx to build with the debugging feature enabled

# ./configure --with-debug ...

If you are using the installing using apt-get on Ubuntu, you are ready to go by setting the following in the Nginx config:

...
error_log  /path/to/log  debug;
...

Don't forget to restart the Nginx to take effect.

Answer:

If you use Nginx as a web server, the max. number of client can be served at the same time by Nginx should be calculated by the following formula:

max_clients = worker_processes * worker_connections

But this does not equal to the number of users can be served at the same time by Nginx, since a lot of browsers open 2 connections to the web server at the same time.

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:

Enabling Expire header for static contents is useful to reduce the loading of your web server.

If you are using nginx, it is easy by adding the following lines in your nginx.conf

location ~* \.(ico|css|js|gif|jpe?g|png)$ {
    expires max;
}

So all files with the above extensions will be delivered with the maximum expire header.

If you want user to update the cached contents, make sure you have change the path (i.e. rename) of these resources.

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