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

Answer 2

Maybe not as elegant, but you could capture & parse the results from ifconfig, since it sounds like it has just what you are looking for.

Answer 3

You can find a solution here: Get mac address given a specific interface

You can just skip the specific interface part.

Answer 4

Inspired by jørgensen's answer, the C++17 code below uses Netlink to get the MAC address and name of each interface. It is a modified version of this gist, which helped me overcome my shyness about using Netlink. It really isn't as complex as it seems at first.

#include <linux/rtnetlink.h>
#include <net/if_arp.h>
#include <unistd.h>

#include <cstddef>
#include <cstring>
#include <iostream>
#include <string>
#include <tuple>
#include <vector>

using std::byte;
using std::string;
using std::vector;

struct RtReq {
    nlmsghdr header;
    ifinfomsg msg;
};

class fd_wrapper
{
  public:
    fd_wrapper( int const fd_arg ) : fd_( fd_arg ) {}
    ~fd_wrapper() { close( fd_ ); }
    int fd() const noexcept { return fd_; }

  private:
    int fd_;
};

void print_all_macs()
{
    fd_wrapper fd = ( []() -> fd_wrapper {
        RtReq req = ( []() {
            RtReq req;
            memset( &req, 0, sizeof( req ) );
            req.header.nlmsg_len = NLMSG_LENGTH( sizeof( struct ifinfomsg ) );
            req.header.nlmsg_flags = NLM_F_REQUEST | NLM_F_ROOT;
            req.header.nlmsg_type = RTM_GETLINK;
            req.header.nlmsg_seq = 1; // We're only sending one message
            req.msg.ifi_family = AF_UNSPEC;
            req.msg.ifi_change = 0xffffffff;
            return std::move( req );
        } )();

        sockaddr_nl sa;
        memset( &sa, 0, sizeof( sa ) );
        sa.nl_family = AF_NETLINK;
        iovec iov = {&req, req.header.nlmsg_len};
        msghdr msg = {&sa, sizeof( sa ), &iov, 1, nullptr, 0, 0};

        int fd = socket( AF_NETLINK, SOCK_RAW, NETLINK_ROUTE );

        sendmsg( fd, &msg, 0 );

        return fd_wrapper( fd );
    } )();

    enum class Result { not_done, done };

    auto read_message = []( fd_wrapper const& fd ) -> Result {
        char buf[16 * 1024];
        iovec iov = {buf, sizeof( buf )};
        sockaddr_nl sa;
        msghdr msg = {&sa, sizeof( sa ), &iov, 1, nullptr, 0, 0};
        int len = recvmsg( fd.fd(), &msg, 0 );
        for( nlmsghdr* nh = (nlmsghdr*)buf; NLMSG_OK( nh, len ); nh = NLMSG_NEXT( nh, len ) ) {
            if( nh->nlmsg_type == NLMSG_DONE ) {
                return Result::done;
            }

            if( nh->nlmsg_type != RTM_BASE )
                continue;

            ifinfomsg* msg = (ifinfomsg*)NLMSG_DATA( nh );
            if( msg->ifi_type != ARPHRD_ETHER )
                continue;

            rtattr* rta = IFLA_RTA( msg );
            int alen = nh->nlmsg_len - NLMSG_LENGTH( sizeof( *msg ) );

            string name;
            vector<byte> addr;

            for( ; RTA_OK( rta, alen ); rta = RTA_NEXT( rta, alen ) ) {
                if( rta->rta_type == IFLA_ADDRESS ) {
                    auto const p = reinterpret_cast<byte const*>( RTA_DATA( rta ) );
                    addr.insert( addr.begin(), p, p + 6 );
                }

                if( rta->rta_type == IFLA_IFNAME ) {
                    name = string{(char*)RTA_DATA( rta )};
                }
            }

            auto const c = reinterpret_cast<uint8_t const*>( addr.data() );
            printf( "%02x:%02x:%02x:%02x:%02x:%02x %s\n", c[0], c[1], c[2], c[3], c[4], c[5],
                    name.c_str() );
        }
    };

    Result result;
    while( ( result = read_message( fd ) ) == Result::not_done )
        ;
}

int main( int /*argc*/, char** /*argv*/ )
{
    print_all_macs();
    return 0;
}