At which layer of TCP/IP does nf_hook - kernel works?

Question

At which layer of TCP/IP does packet filtering works. I am writing a kernel module that has a nf_hook that will simply drop the packet based on the port - for example TCP or UDP. I want to know at what layer does the nf_hook works in TCP/IP stack?

Answer 1

Layer 3: Network layer

More information on networking kernel can be found here.