I want to get the station information using Netlink by libnl. I issue the command NL80211_CMD_GET_STATION. Before sending the message, I add attributes NL80211_ATTR_MAC and NL80211_ATTR_IFINDEX with their respective values per instructions in nl80211.h file. I have written a simple code with error checking:
int main(int argc, char **argv)
{
struct nl_sock *sk; /* Socket */
struct nl_msg *msg; /* Netlink message struct */
int err; /* Message flags */
unsigned char mac_adr[ETH_ALEN]; /* MAC address */
char path[50]="", if_name[] = "wlp4s0"; /* Interface name */
strcat(path, "/sys/class/net/");
strcat(path, if_name);
strcat(path, "/address");
/* allocate socket */
if (!(sk = nl_socket_alloc()))
{
printf("Error in creating socket!\n");
return -1;
}
/* Connect to generic netlink */
if (genl_connect(sk) < 0)
{
printf("Error in connecting socket to Generic Netlink!\n");
return -1;
}
/* Find the nl80211 driver ID */
NL_family = genl_ctrl_resolve(sk, "nl80211");
if (NL_family < 0)
{
printf("Error in getting nl80211 Netlink message ID!\n");
return -1;
}
/* Attach a callback */
err = nl_socket_modify_cb(sk, NL_CB_VALID, NL_CB_CUSTOM,
nlCallback, NULL);
if (err < 0)
{
printf("Error in a Callback to Netlink socket!\n");
return -1;
}
/* Create a new message */
if (!(msg = nlmsg_alloc()))
{
printf("Error in allocating Netlink message!\n");
return -1;
}
/* Setup the message */
if (!genlmsg_put(msg, 0, NL_AUTO_SEQ, NL_family, 0, 0,
NL80211_CMD_GET_STATION, 0))
{
printf("Error in creating Netlink message!\n");
return -1;
}
/* Add message attributes */
/* Putting MAC address */
if (get_mac(mac_adr, path) < 0)
{
printf ("Error in getting MAC address!\n");
return -1;
}
NLA_PUT(msg, NL80211_ATTR_MAC, sizeof(mac_adr), mac_adr);
/* Interface attribute */
NLA_PUT_U32(msg, NL80211_ATTR_IFINDEX, if_nametoindex(if_name));
/* Send the messge (this frees it) */
err = nl_send_auto(sk, msg);
if (err < 0)
{
printf ("Error in sending the messages!\n");
return -1;
}
printf("Sent %d Bytes\n", err);
/* Block execution to receive the answer from the kernel */
err = nl_recvmsgs_default(sk);
if (err < 0)
{
printf("Error in receiving Netlink message! Error: %s\n", nl_geterror(err));
return -1;
}
return 0;
nla_put_failure:
nlmsg_free(msg);
printf("Error in Attribute Put\n");
return -1;
}
When I run the program, it generates the error object not found while it tries to receive the reply from kernel using function nl_recvmsgs_default(sk) at the end of the code.
I have used the same code to get the type of the same interface with command NL80211_CMD_GET_INTERFACE. I send the message for this command with attribute NL80211_ATTR_IFINDEX. The program works for NL80211_CMD_GET_INTERFACE and it prints the interface type! Therefore, I suspect there is something wrong with the way I get the MAC address.
Here is the fuction I write for getting MAC address from a file:
int get_mac(unsigned char *mac_adr, char *path)
{
int c, i = 0, j = 0, d1, d2;
char mac_str[20];
FILE *file;
file = fopen(path, "r");
if (file) {
while ((c = getc(file)) != EOF)
mac_str[i++] = c;
mac_str[i] = 0;
fclose(file);
}
else return -1;
for (i=0; i<=(ETH_ALEN-1)*3; i+=3)
{
if (mac_str[i]>='0' && mac_str[i]<='9')
d1 = mac_str[i] - '0';
else if (mac_str[i]>='a' && mac_str[i]<='f')
d1 = mac_str[i] - 'a' + 10;
if (mac_str[i+1]>='0' && mac_str[i+1]<='9')
d2 = mac_str[i+1] - '0';
else if (mac_str[i+1]>='a' && mac_str[i+1]<='f')
d2 = mac_str[i+1] - 'a' + 10;
mac_adr[j++] = d1*16+d2;
}
return 1;
}
If the name of interface is wlp4s0 the file containing the MAC address would be in path /sys/class/net/wlp4s0/address. What can cause the problem with performing NL80211_CMD_GET_STATION?
Is it possible that this error shows up because the command NL80211_CMD_GET_STATION needs root user and NL80211_CMD_GET_INTERFACE does not?
But I of course use NL80211_CMD_GET_STATION with root user.
