###########CODE EXECUTED
r = (%x[#{IPTABLES} -t mangle -vnx -L '_AUTH_IN_#{cp_interface}' | grep '#{client_address}'])
puts "IPTABLES_RET = #{r}"

###########ERROR:
IPTABLES_RET =
[Fri Jul 15 18:43:06 +0200 2016]:ERROR get_iptables_user_bytes_counters: Exception! (undefined method `[]' for nil:NilClass)
[Fri Jul 15 18:43:06 +0200 2016]:ERROR ["/home/cloud-user/OpenWISP-Captive-Portals-Manager/lib/custom_workers/captive_portals_upkeep.rb:371:in `get_iptables_user_bytes_counters'", "/home/cloud-user/OpenWISP-Captive-Portals-Manager/lib/custom_workers/captive_portals_upkeep.rb:133:in `online_users_update_activity'", "/home/cloud-user/OpenWISP-Captive-Portals-Manager/lib/custom_workers/captive_portals_upkeep.rb:131:in `each'", "/home/cloud-user/OpenWISP-Captive-Portals-Manager/lib/custom_workers/captive_portals_upkeep.rb:131:in `online_users_update_activity'", "/home/cloud-user/OpenWISP-Captive-Portals-Manager/lib/custom_workers/captive_portals_upkeep.rb:430:in `run_worker'", "/home/cloud-user/OpenWISP-Captive-Portals-Manager/lib/custom_workers/captive_portals_upkeep.rb:455"]