Download check_squid.pl.rar

  tar -zxvf check_squid.pl.rar

copy into the folder /usr/lib/nagios/plugins/

change the definition command for squid in /etc/nagios-plugins/config/http.cfg

  
  # 'check_squid' - tries to access an URL through proxy
  define command {
      command_name    check_squid
      command_line    $USER1$/check_squid.pl --proxy=$HOSTADDRESS$ --proxyport=$ARG1$ --proxyuser=$ARG2$ --proxypass=$ARG3$ --url=$ARG4$ --status=$ARG5$
      }

Define the service into the icinga or nagios server config file

   define service {
      service_description             squid
      use                             generic-service
      host_name                       proxy
      check_command                   check_squid!3128!squid!squid!http://www.google.com!200
     }