DDoS

To display how many http connections are open at the moment, enter:

netstat | grep http | wc -l

To test your server for vulnerability, you can use Backbox Linux, it has a various testing tools preinstalled.
For example, to test your domain for resistance to DDoS attacks you can use slowhttptest which is in Backbox installation. Example command:

slowhttptest -c 1000 -B -g -o output-file-name -i 100 -r 300 -s 10240 -u http://www.example.com/url/page.html -x 20

Now here are detailed options:

-a start start value of ranges-specifier for range header test
-b bytes limit of range-specifier for range header test
-c number of connections limited to 1024
-H, B, or R specify to slow down in headers section or in message body.
           -R enables range test
-g generate statistics in CSV and HTML formats, pattern is slow_xxx.csv/html,
  where xxx is the time and date
-i seconds interval between follow up data in seconds, per connection
-l seconds test duration in seconds
-o file custom output file path and/or name, effective if -g is specified
-r connections per second connection rate
-s bytes value of Content-Length header, if -B specified
-t verb custom verb to use
-u URL target URL, the same format you type in browser, e.g https://host:port/
-v level verbosity level of log 0-4
-x bytes max length of follow up data