First we select the protocols for Captive Portal availability in a separate class, as it is described in Assignment of priorities. These protocols typically are http/https/dns.

bittorrent 0
http 8
https 8
dns 8
default 16

Next we create the configuration file captive_portal.cfg. It specifies the bandwidth limits in Captive Portal mode for each of 8 classes1)More details on bandwidth control.

htb_root=rate 1mbit
htb_class0=rate 8bit ceil 8bit
htb_class1=rate 1mbit ceil 1mbit
htb_class2=rate 8bit ceil 8bit
htb_class3=rate 8bit ceil 8bit
htb_class4=rate 8bit ceil 8bit
htb_class5=rate 8bit ceil 8bit
htb_class6=rate 8bit ceil 8bit
htb_class7=rate 8bit ceil 8bit

htb_root - is the root class. It specifies the total bandwidth. The bandwidth is redistributed within this class
rate - is the lower bandwidth limit
ceil - is the upper bandwidth limit, that can be borrowed from the root class if available

We place a subscriber into Cpative Portal when his account runs out of money:

fdpi_ctrl load --policing captive_portal.cfg --ip 192.168.0.1
fdpi_ctrl load --service 5 --ip 192.168.0.1

Here we unblock the subscriber upon a deposit to his account:

fdpi_ctrl load --policing rate_plan1.cfg --ip 192.168.0.1
fdpi_ctrl del --service 5 --ip 192.168.0.1

Future developments: a simplified (one step) method to put a subscriber into Captive Portal; support of various Captive Portal modes.