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 hardware interfaces, it may be important for the protocol argument to set up de-multiplexing. For example, raw sockets in the Internet domain can be used to implement a new protocol on IP, and the socket receives packets only for the protocol specified. To obtain a particular protocol, determine the protocol number as defined in the protocol domain. For the Internet domain, use one of the library routines 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_INET, SOCK_STREAM, pp->p_proto) ;

This results in a socket s using a stream-based connection, but with protocol type of newtcp instead of the default tcp.