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I wrote this for testing purpose.

route_write.c

/* $Id$ */
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/param.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/sysctl.h>
#include <sys/ioctl.h>
#include <netinet/in.h>
#include <net/route.h>
#include <net/if.h>
#include <net/if_dl.h>
#include <strings.h>
#include <arpa/inet.h>
 
int main(void) {
       char *c_dstip = "10.2.3.0";
       char *c_maskip = "255.255.255.0";
       char *c_gwip = "192.168.56.1";
 
       u_long dstip, gwip, maskip;
 
       u_short flags;
       int     s, rc;
       struct {
               struct  rt_msghdr hdr;
               struct  sockaddr_in dst;
               struct  sockaddr_in gateway;
               struct  sockaddr_in mask;
       } rtmsg;
 
       s = socket(PF_ROUTE, SOCK_RAW, 0);
 
       if (s < 0) {
            printf("error: socket\n");
            return -1;
       }
 
       shutdown(s, SHUT_RD);
 
       flags |= (RTF_UP | RTF_GATEWAY);
 
       bzero(&rtmsg, sizeof(rtmsg));
 
       rtmsg.hdr.rtm_type = RTM_ADD;
       rtmsg.hdr.rtm_version = RTM_VERSION;
       rtmsg.hdr.rtm_addrs = 0;
       rtmsg.hdr.rtm_addrs |= RTA_DST;
       rtmsg.hdr.rtm_addrs |= RTA_GATEWAY;
       rtmsg.hdr.rtm_addrs |= RTA_NETMASK ;
 
       rtmsg.hdr.rtm_flags = 0;
       rtmsg.hdr.rtm_flags |= RTF_GATEWAY;
//       rtmsg.hdr.rtm_flags |= RTF_HOST;
       rtmsg.hdr.rtm_flags |= RTF_GATEWAY;
 
       rtmsg.hdr.rtm_addrs |= RTA_GATEWAY;
 
 
// ----- dstip  -----
 
       int r = inet_pton(AF_INET, c_dstip, &dstip);
 
       rtmsg.dst.sin_len = sizeof(rtmsg.dst);
       rtmsg.dst.sin_family = AF_INET;
       rtmsg.dst.sin_addr.s_addr = dstip;
 
 
// ------ mask -----
 
       r = inet_pton(AF_INET, c_maskip, &maskip);
 
       rtmsg.mask.sin_len = sizeof(rtmsg.mask);
       rtmsg.mask.sin_family = AF_INET;
       rtmsg.mask.sin_addr.s_addr = maskip;
 
// ------ gwip -----
 
       r = inet_pton(AF_INET, c_gwip, &gwip);
 
       rtmsg.gateway.sin_len = sizeof(rtmsg.gateway);
       rtmsg.gateway.sin_family = AF_INET;
       rtmsg.gateway.sin_addr.s_addr = gwip;
 
 
       rc = sizeof(rtmsg);
       rtmsg.hdr.rtm_msglen = rc + 4;
 
       if ((rc = write(s, &rtmsg, rc)) < 0) {
               printf("error: writing to routing socket\n");
               return -1;
       }
       return (rc);
 
}
/* EOF */