PF_PACKET Protocol Family

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When you open a socket with the standard call sock = socket(domain, type, protocol) you have to specify which domain (or protocol family) you are going to use with that socket. Commonly used families are PF_UNIX, for communications bounded on the local machine, and PF_INET, for communications based on IPv4 protocols. Furthermore, you have to specify a type for your socket and possible values depend on the family you specified. Common values for type, when dealing with the PF_INET family, include SOCK_STREAM (typically associated with TCP) and SOCK_DGRAM (associated with UDP). Socket types influence how packets are handled by the kernel before being passed up to the application. Finally, you specify the protocol that will handle the packets flowing through the socket (more details on this can be found on the socket(3) man page).

In recent versions [original article was written in 2001] of the Linux kernel (post-2.0 releases) a new protocol family has been introduced, named PF_PACKET. This family allows an application to send and receive packets dealing directly with the network card driver, thus avoiding the usual protocol stack-handling (e.g., IP/TCP or IP/UDP processing). That is, any packet sent through the socket will be directly passed to the Ethernet interface, and any packet received through the interface will be directly passed to the application.

The PF_PACKET family supports two slightly different socket types, SOCK_DGRAM and SOCK_RAW. The former leaves to the kernel the burden of adding and removing Ethernet level headers. The latter gives the application complete control over the Ethernet header. The protocol field in the socket() call must match one of the Ethernet IDs defined in /usr/include/linux/if_ether.h [IEEE 802.3 Ethernet magic constants], which represents the registered protocols that can be shipped in an Ethernet frame. Unless dealing with very specific protocols, you typically use ETH_P_IP, which encompasses all of the IP-suite protocols (e.g., TCP, UDP, ICMP, raw IP and so on).

Since they have pretty serious security implications (for example, you may forge a frame with a spoofed MAC address), PF_PACKET-family sockets may only be used by root.