iptables LOG implementation

I'm trying to forward traffic from one virtual interface (eth3) to a tun (tun0) interface within a container. I'm able to forward ping and iperf traffic with adding the following iptable rules

iptables -t nat -A POSTROUTING -o connectify0 -j MASQUERADE

iptables -A FORWARD -i connectify0 -o eth3 -m state --state RELATED,ESTABLISHED -j ACCEPT

iptables -A FORWARD -i eth3 -o connectify0 -j ACCEPT

I'm unable to forward TRex generated traffic. I see it in eth3 using tcpdump but it isn't getting forwarded to tun0.
Looking at ways to debug it using iptables LOG option. How would I LOG the cause for eth3 not forwarding traffic to tun0 ?