I am trying to implement a custom network device driver on linux kernel version 4.0 and 3.16.and it is working well on link.however, I found out the device has some problem about the forwarding recently.  (device 1)                 (gateway)            (device 2)[ custom netdevice ] - [ custom netdevice' + eth0' ] - [ eth0 ]I performed a ping test. the followings are results.1st ping test(from device 2 to device 1)The ping request message arrives at the device 1.The ping response message arrives at the gateway.but the response message does not arrive at the device 2.2nd ping test(from device 1 to device 2)The ping request message arrives at the gateway.but the ping request message does not arrive at the device 1.the device driver is implemented without arp tables.eth0' netdevice seemed to receive the packets. eth0' netdevice's RX counter increases, while TX counter does not increase.I saw the packets were delivered to IP layer well at the gateways.I used the command sysctl -w net.ipv6.conf.all.forwarding=1 for the gateway.and utilzed radvd daemon to do stateless auto-configuration. the firewall at the gateway was disabled as using the command ufw disable. this problem occured only in case of using my custom network driver, this symptom seemed to be not the problem regarding eth1 netdevice.Please Help me 
Forwarding problem inside gateway device using custom network driver
linux;networking;linux kernel;network interface