If it is for executing some script on successful connection of any device to wifi (and thus an IP is assigned), with dhcpd, it can be achieved with the following config:

  subnet 192.168.1.0 netmask 255.255.255.0 {
    on commit {
      set ip = binary-to-ascii (10, 8, ".", leased-address);
     execute ("/usr/local/bin/on-device-connect", "commit", ip);
    }
  }

I have setup exactly this to automate stuff unlocking the door in my office (in the morning, whenever some one arrives and phone connects to wifi).

My original source: http://jpmens.net/2011/07/06/execute-a-script-when-isc-dhcp-...

Edit: Fix code formatting.