Selecting Specific Protocols

If the third argument of the socket call is 0, socket selects a default protocol to use with the returned socket of the type requested. The default protocol is usually correct, and alternate choices are not usually available. When using raw sockets to communicate directly with lower-level protocols or lower-level hardware interfaces, set up de-multiplexing with the protocol argument. For more information, see the socket(3C) man page.

Using raw sockets in the Internet family to implement a new protocol on IP ensures that the socket only receives packets for the specified protocol. To obtain a particular protocol, determine the protocol number as defined in the protocol family. For the Internet family, use one of the library routines that are discussed in Standard Routines, such as getprotobyname.

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
...
pp = getprotobyname("newtcp");
s = socket(AF_INET6, SOCK_STREAM, pp->p_proto);

Using getprotobyname() results in a socket s by using a stream-based connection, but with a protocol type of newtcp instead of the default tcp.