What's the socket protocol with value 17?

Question

Recall the declaration of a socket in C has the following signature:

int socket(int domain, int type, int protocol);

I met some reverse-engineered code where protocol = 17. Does anyone know what protocol this refers to? The net seems to be lacking of such int values; they have only the names, at best.

Answer 1

Protocol 17 would be UDP per IANA specifications, which is referred to at least in the Linux socket documentation. The name of the protocol should also be available via getprotoent if it’s supported by the platform.

Microsoft also uses same numbers for the protocols in socket.