C/C++ Linux MAC Address of all interfaces

Question

I am using the following code to retrieve all MAC addresses for current computer:

ifreq ifr;
ifconf ifc;
char buf[1024];

int sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_IP);
if (sock == -1) { ... };

ifc.ifc_len = sizeof(buf);
ifc.ifc_buf = buf;
if (ioctl(sock, SIOCGIFCONF, &ifc) == -1) { ... }

ifreq *it = ifc.ifc_req;
const ifreq* const end = it + (ifc.ifc_len / sizeof(ifreq));

for (; it != end; ++it) {
    strcpy(ifr.ifr_name, it->ifr_name);
    if (ioctl(sock, SIOCGIFFLAGS, &ifr) == 0) {
        if (!(ifr.ifr_flags & IFF_LOOPBACK)) {
            if (ioctl(sock, SIOCGIFHWADDR, &ifr) == 0) {
               unsigned char mac_address[6];
               memcpy(mac_address, ifr.ifr_hwaddr.sa_data, 6);
               ...
            }
        }
    }
    else { ... }
}

By running simple shell command ifconfig i can see lo, eth0 and wlan0. I would like to retrieve MAC addresses for eth0 and wlan0 by my C/C++ code. But only wlan0 is returned - eth0 is missing (I got ifr_names lo, lo, wlan0). Probably because eth0 is not active (no ethernet cable connected, with cable it is returned). Can I somehow alter that ioctl(SIOCGIFCONF) command to retrieve eth0 too even if it is "turned off"?

I can get its HW address by using directly

  struct ifreq s;
  int fd = socket(PF_INET, SOCK_DGRAM, IPPROTO_IP);

  strcpy(s.ifr_name, "eth0");
  if (0 == ioctl(fd, SIOCGIFHWADDR, &s)) { ... }

but what if the name would be not eth0 but something else (eth1, em0,...)? I would like to get all of them. Thanks for help.

Answer 1

You should stop using net-tools and the archaic ioctl interface, and start on using the modern Netlink/sysfs interfaces. You have no less than 5 possibilities:

• write your own Netlink-interfacing code
• your own NL code, in combination utilizing libmnl (-> see rtnl-link-dump in Examples
• or utilize autonomous libs like libnl3
• parse text output of ip -o link (-o is to get output meant for text parsing, unlike ifconfig)
• or use sysfs and just look at /sys/class/net/eth0/address