pppd 强制设置默认网关？

int sifdefaultroute (int unit, u_int32_t ouraddr, u_int32_t gateway)
{
    struct rtentry rt;
        
/*  if (defaultroute_exists(&rt) && strcmp(rt.rt_dev, ifname) != 0) {
        u_int32_t old_gateway = SIN_ADDR(rt.rt_gateway);

        if (old_gateway != gateway)
            error("not replacing existing default route to %s [%I]",
                  rt.rt_dev, old_gateway);

        return 0;
    }
*/
    memset (&rt, '\0', sizeof (rt));
    SET_SA_FAMILY (rt.rt_dst,     AF_INET);
    SET_SA_FAMILY (rt.rt_gateway, AF_INET);

    rt.rt_dev = ifname;

    if (kernel_version > KVERSION(2,1,0)) {
        SET_SA_FAMILY (rt.rt_genmask, AF_INET);
        SIN_ADDR(rt.rt_genmask) = 0L;
    }

    SIN_ADDR(rt.rt_gateway) = gateway;

    rt.rt_flags = RTF_UP | RTF_GATEWAY;
    if (ioctl(sock_fd, SIOCADDRT, &rt) < 0) {
        if ( ! ok_error ( errno ))
            error("default route ioctl(SIOCADDRT): %m");
        
        return 0;
    }

    default_route_gateway = gateway;
    return 1;
}