IOCTL returns No such device - sockets

Question

I have written a code to create and set the properties of VLAN interface. I am able to set the IP address, however the I am facing problem in setting the gateway address and reading the device flags. Please have a look at code and suggest what could be wrong.
I am getting error in the function generic_ioctrlcall.
/**
* Create socket function
*/
int create_socket()
{
int sockfd = 0;
sockfd = socket(AF_INET, SOCK_DGRAM, 0);
if(sockfd == -1){
fprintf(stderr, "Could not get socket.\n");
return -1;
}
return sockfd;
}
/**
* Generic ioctrlcall to reduce code size
*/
int generic_ioctrlcall(int sockfd, u_long *flags, struct ifreq *ifr) {
if (ioctl(sockfd, (long unsigned int)flags, &ifr) < 0) {
fprintf(stderr, "ioctl: %s\n", (char *)flags);
return -1;
}
return 1;
}
/**
* Set route with metric 100
*/
int set_route(int sockfd, char *gateway_addr, struct sockaddr_in *addr) {
struct rtentry route;
int err = 0;
memset(&route, 0, sizeof(route));
addr = (struct sockaddr_in*) &route.rt_gateway;
addr->sin_family = AF_INET;
addr->sin_addr.s_addr = inet_addr(gateway_addr);
addr = (struct sockaddr_in*) &route.rt_dst;
addr->sin_family = AF_INET;
addr->sin_addr.s_addr = inet_addr("0.0.0.0");
addr = (struct sockaddr_in*) &route.rt_genmask;
addr->sin_family = AF_INET;
addr->sin_addr.s_addr = inet_addr("0.0.0.0");
route.rt_flags = RTF_UP | RTF_GATEWAY;
route.rt_metric = 100;
if ((err = ioctl(sockfd, SIOCADDRT, &route)) < 0) {
fprintf(stderr, "ioctl: %s\n", (char *)flags);
return -1;
}
return 1;
}
/**
* Set ip function
*/
int set_ip(char *iface_name, char *ip_addr, char *gateway_addr)
{
if(!iface_name)
return -1;
struct ifreq ifr;
struct sockaddr_in sin;
int sockfd = create_socket();
sin.sin_family = AF_INET;
// Convert IP from numbers and dots to binary notation
inet_aton(ip_addr,&sin.sin_addr.s_addr);
/* get interface name */
strncpy(ifr.ifr_name, iface_name, IFNAMSIZ);
/* Read interface flags */
generic_ioctrlcall(sockfd, (u_long *)"SIOCGIFFLAGS", &ifr);
/*
* Expected in <net/if.h> according to
* "UNIX Network Programming".
*/
#ifdef ifr_flags
# define IRFFLAGS ifr_flags
#else /* Present on kFreeBSD */
# define IRFFLAGS ifr_flagshigh
#endif
// If interface is down, bring it up
if (ifr.IRFFLAGS | ~(IFF_UP)) {
ifr.IRFFLAGS |= IFF_UP;
generic_ioctrlcall(sockfd, (u_long *)"SIOCSIFFLAGS", &ifr);
}
// Set route
set_route(sockfd, gateway_addr, &sin);
memcpy(&ifr.ifr_addr, &sin, sizeof(struct sockaddr));
// Set interface address
if (ioctl(sockfd, SIOCSIFADDR, &ifr) < 0) {
fprintf(stderr, "Cannot set IP address. ");
perror(ifr.ifr_name);
return -1;
}
#undef IRFFLAGS
return 0;
}

Answer 1

Many problems here. The proximate cause of your difficulty is that the second argument to ioctl is a long integer, not a pointer to a character string.
generic_ioctrlcall(sockfd, (u_long *)"SIOCGIFFLAGS", &ifr);
does not create a pointer to the integer value of SIOCGIFFLAGS which is presumably what you intended. Other issues:
(1) Since ioctl takes a long integer as the second argument, why did you want to pass a pointer to an integer? Just pass the integer as argument.
(2) If you do wish to pass a pointer to an integer, you would need to do something like this:
u_long ioctl_arg = SIOCGIFFLAGS;
generic_ioctrlcall(sockfd, &ioctl_arg, &ifr);
(3) You cannot convert a pointed-to integer to an integer simply by casting it. You need to dereference the pointer. That is:
Change:
if (ioctl(sockfd, (long unsigned int)flags, &ifr) < 0) {
to:
if (ioctl(sockfd, *flags, &ifr) < 0) {
(4) on an ioctl failure, you should be printing out errno (or, better yet, its text translation) after the failure in order to figure out what went wrong:
if (ioctl(sockfd, *flags, &ifr) < 0) {
fprintf(stderr, "ioctl: flags %ld errno %d/%s\n", *flags, errno, strerror(errno));
And by the way, it seems unlikely that the errno from any of your generic_ioctl calls would be ENODEV (No such device) -- as your title implies. It seems much more likely that the errno would be EINVAL since the chances of your pointer-to-character-string forming a valid ioctl integer argument are very slim.