Hi, I'm a novice in Kernel development. This is the first time I'm writing here. I need to develop something similar to FTP ALG on Active mode. That is, I will inspect some packets while passing through the NAT then based on some conditions I will set permission for an IP of another device/NAT on the internet so that it can send packets directly to an internal device. I'm reading some books on Linux kernel device driver development. I know I'll have to do that using the Netfilter subsystem of the linux kernel. To learn how ftp alg is implemented I've been searching for source in kernel source, although I found code that mangles the packet of PORT command of FTP connection but I can't find where the mapping thing is done for Active mode of FTP. Which file should I read to know about how FTP alg for active mode makes a server connect to a client behind NAT? I'd appreciate any replies.