accessing low level network interface

I would like be able to access packets coming off the network with the hopes of coding a primitive packet sniffer for learning purposes. I would also like to be able to piece together my own packets and send them out on my network. Eventually, I would also like to write a simple firewall so I want the capability to drop packets as well as let them pass. I assume I will have to access the kernel with some system calls to do this, any one have any information or resources that could help me on this project?

you could take a look at the tcpdump source code to see how it does the packet sniffing