Guideline on Linux NAT feature

[Abdul Matin]

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.
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