Извините за задержку.
Сделал с таким конфигом:
INET_IP="ххх"
GSERV_IP="ххх"
LAN_1="ххх/25"
NCARD1="enp3s0"
NCARD2="enp4s5"
iptables -F
iptables -F -t nat
iptables -F -t mangle
iptables -X
iptables -X -t nat
iptables -X -t mangle
iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT ACCEPT
iptables -A INPUT -p ICMP -m limit --limit 3/second --limit-burst 6 -j ACCEPT
iptables -A INPUT -p TCP --dport 22 -j ACCEPT
iptables -A INPUT -i $NCARD2 -j ACCEPT
iptables -A INPUT -i $NCARD2 -s $LAN_1 -j ACCEPT
iptables -A INPUT -i lo -j ACCEPT
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -i $NCARD2 -o $NCARD1 -j ACCEPT
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -t nat -A PREROUTING -i $NCARD2 ! -d $LAN_1 -p tcp -m multiport --dport 80,81,82,83,88,8000,8001,8002,8080,8081 -j REDIRECT --to-ports 3128
netfilter-persistent save