INFO: Header and Library Requirement When Set/Get Socket Options at the IPPROTO_IP Level (257460)

MORE INFORMATION

The IPPROTO_IP level socket options are defined differently in Winsock.h than in Ws2tcpip.h. In Winsock.h, the definitions are the following:

/*
 * Options for use with [gs]etsockopt at the IP level.
 */ 
#define IP_OPTIONS          1           /* set/get IP per-packet options    */ 
#define IP_MULTICAST_IF     2           /* set/get IP multicast interface   */ 
#define IP_MULTICAST_TTL    3           /* set/get IP multicast timetolive  */ 
#define IP_MULTICAST_LOOP   4           /* set/get IP multicast loopback    */ 
#define IP_ADD_MEMBERSHIP   5           /* add  an IP group membership      */ 
#define IP_DROP_MEMBERSHIP  6           /* drop an IP group membership      */ 
#define IP_TTL              7           /* set/get IP Time To Live          */ 
#define IP_TOS              8           /* set/get IP Type Of Service       */ 
#define IP_DONTFRAGMENT     9           /* set/get IP Don't Fragment flag   */ 


#define IP_DEFAULT_MULTICAST_TTL   1    /* normally limit m'casts to 1 hop  */ 
#define IP_DEFAULT_MULTICAST_LOOP  1    /* normally hear sends if a member  */ 
#define IP_MAX_MEMBERSHIPS         20   /* per socket; must fit in one mbuf */ 
				

In contrast, the options at this level are defined in Ws2tcpip.h as:

/* Option to use with [gs]etsockopt at the IPPROTO_IP level */ 

#define	IP_OPTIONS		1 /* set/get IP options */ 
#define	IP_HDRINCL		2 /* header is included with data */ 
#define	IP_TOS			3 /* IP type of service and preced*/ 
#define	IP_TTL			4 /* IP time to live */ 
#define	IP_MULTICAST_IF		9 /* set/get IP multicast i/f  */ 
#define	IP_MULTICAST_TTL       10 /* set/get IP multicast ttl */ 
#define	IP_MULTICAST_LOOP      11 /*set/get IP multicast loopback */ 
#define	IP_ADD_MEMBERSHIP      12 /* add an IP group membership */ 
#define	IP_DROP_MEMBERSHIP     13/* drop an IP group membership */ 
#define IP_DONTFRAGMENT     14 /* don't fragment IP datagrams */ 
				

If you do not correctly match the header and library files, setsockopt or getsockopt may fail with runtime error 10042 (WSAENOPROTOOPT) or the option value you set or get may not be as you would have expected.

Case 1. Runtime Error 10042 (WSAENOPROTOOPT)

Options that fall into this category include:

IP_ADD_MEMBERSHIP
IP_DROP_MEMBERSHIP
IP_TTL
IP_TOS

Assume that you intend to join a multicast group by running code similar to the following:

#include <stdio.h>
#include <stdlib.h>
#include <winsock.h>

int main(int argc, char* argv[])
{
   ...
   ...
   ...
   if (setsockopt(sock, 
                  IPPROTO_IP, 
                  IP_ADD_MEMBERSHIP, 
                  (char FAR *)&mreq, 
                  sizeof (mreq)) == SOCKET_ERROR)
   {
      printf ("setsockopt failed: %d"), WSAGetLastError());
      closesocket (sock);
      return FALSE;
   }
   ...
   ...
   ...
}
				

Note that this includes Winsock.h. If the project is linked with Ws2_32.lib, setsockopt will fail with runtime error 10042 (WSAENOPROTOOPT). This is because in Winsock.h, IP_ADD_MEMBERSHIP is defined as "5". The corresponding Winsock runtime can not resolve option 5 at the IPPROTO_IP level, so the failure occurs with error code 10042.

Case 2. Options Set or Get Appear Do Not Take Effect

Options that fall into this category include:

IP_MULTICAST_IF
IP_MULTICAST_TTL
IP_MULTICAST_LOOP
IP_DONTFRAGMENT

Take IP_MULTICAST_TTL as an example. In Winsock.h, IP_MULTICAST_TTL is defined as "3". In ws2tcpip.h, the constant is defined as "10" and IP_TOS is defined as "3". Try to change the default TTL value:

#include <stdio.h>
#include <stdlib.h>
#include <winsock.h>

int main(int argc, char* argv[])
{
   int ttl = 7 ; // Arbitrary TTL value.
   ...
   ...
   ...
   source_sin.sin_family = AF_INET;
   source_sin.sin_port = htons(0);    
   source_sin.sin_addr.s_addr = htonl (INADDR_ANY);

   if (bind(sock, 
            (struct sockaddr FAR *)&source_sin, 
            sizeof(source_sin)) == SOCKET_ERROR) 
   {
      printf ("bind() failed: %d"), WSAGetLastError());
      closesocket (sock);
      return FALSE;
   }

   if (setsockopt(sock,
                  IPPROTO_IP,
                  IP_MULTICAST_TTL,
                  (char *)&ttl,
                  sizeof(ttl))) == SOCKET_ERROR)
   {
      printf ("setsockopt failed: %d"), WSAGetLastError());
      closesocket (sock);
      return FALSE;
   }
   ...
   ...
   ...
}
				

If you link the project with Ws2_32.lib and run the application, setsockopt with IP_MULTICAST_TTL will succeed. However, the multicast TTL setting will not take effect. If you examine the network trace, you see that the TTL value still remains as "1" (the default).