How do I monitor a tcp port?

Icinga 2
2

you probably want something like this:

object Host "host with webproxy" {
  ...
  address = "x.x.x.x"
  vars.tcp_ports = [ 3128 ]
}

apply Service "TCP Webproxy " for (port in host.vars.tcp_ports) {
  ...
  check_command = "tcp"
   display_name = "Webproxy ports 3128"
   vars.tcp_port = port
}