man ip, and search for IP_HDRINCL:

   RAW IP SOCKETS
     Raw IP sockets are connectionless, and are normally used with the
     sendto(2) and recvfrom(2) calls, though the connect(2) call may also be
     used to fix the destination for future packets (in which case the read(2)
     or recv(2) and write(2) or send(2) system calls may be used).

     If proto is 0, the default protocol IPPROTO_RAW is used for outgoing
     packets, and only incoming packets destined for that protocol are
     received.  If proto is non-zero, that protocol number will be used on
     outgoing packets and to filter incoming packets.

     Outgoing packets automatically have an IP header prepended to them (based
     on the destination address and the protocol number the socket is created
     with), unless the IP_HDRINCL option has been set.  Incoming packets are
     received with IP header and options intact.

     IP_HDRINCL indicates the complete IP header is included with the data and
     may be used only with the SOCK_RAW type.

     #include <netinet/ip.h>

     int hincl = 1;                  /* 1 = on, 0 = off */
     setsockopt(s, IPPROTO_IP, IP_HDRINCL, &hincl, sizeof(hincl));

     Unlike previous BSD releases, the program must set all the fields of the
     IP header, including the following:

     ip->ip_v = IPVERSION;
     ip->ip_hl = hlen >> 2;
     ip->ip_id = 0;  /* 0 means kernel set appropriate value */
     ip->ip_off = offset;

     If the header source address is set to INADDR_ANY, the kernel will choose
     an appropriate address.



-- 
        Greg Troxel <gdt@ir.bbn.com>