lsmod | grep iptable
должен быть примерно такой вывод
root@ubuntu:~# lsmod | grep iptable
iptable_nat 4414 1
nf_nat 15735 2 ipt_MASQUERADE,iptable_nat
nf_conntrack_ipv4 10672 3 iptable_nat,nf_nat
nf_conntrack 61583 4 ipt_MASQUERADE,iptable_nat,nf_nat,nf_conntrack_ipv4
iptable_mangle 2771 0
iptable_filter 2271 0
ip_tables 9991 3 iptable_nat,iptable_mangle,iptable_filter
x_tables 14299 4 xt_tcpudp,ipt_MASQUERADE,iptable_nat,ip_tables