Access to MAC addresses of local network interfaces in macOS 27

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// clang++ -std=c++23 -o ifprobe ifprobe.cpp && ./ifprobe
#include <ifaddrs.h>
#include <net/bpf.h>
#include <net/if.h>
#include <net/if_dl.h>
#include <net/route.h>
#include <sys/ioctl.h>
#include <sys/sockio.h>
#include <sys/sysctl.h>
#include <fcntl.h>
#include <unistd.h>
#include <cerrno>
#include <cstdio>
#include <cstring>
#include <string>
#include <vector>

static int open_bpf()
{
    for ( int i = 0; i < 256; ++i )
    {
        std::string p = "/dev/bpf" + std::to_string( i );
        int fd = open( p.c_str(), O_RDWR );
        if ( fd != -1 ) return fd;
        if ( errno != EBUSY ) break;
    }
    return -1;
}

static const char* functional_type_name( unsigned t )
{
    switch ( t )
    {
        case IFRTYPE_FUNCTIONAL_LOOPBACK:   return "LOOPBACK";
        case IFRTYPE_FUNCTIONAL_WIRED:      return "WIRED";
        case IFRTYPE_FUNCTIONAL_WIFI_INFRA: return "WIFI_INFRA";
        case IFRTYPE_FUNCTIONAL_WIFI_AWDL:  return "WIFI_AWDL";
        case IFRTYPE_FUNCTIONAL_CELLULAR:   return "CELLULAR";
        default:                            return "UNKNOWN";
    }
}

static size_t sockaddr_aligned_size(const sockaddr* address)
{
    if (address->sa_len == 0)
        return sizeof(long);

    return (address->sa_len + sizeof(long) - 1) &
           ~(sizeof(long) - 1);
}

static void print_sysctl_mac(const char* name)
{
    const unsigned int index = if_nametoindex(name);
    if (index == 0)
    {
        printf(" sysctl_mac=<unknown interface>");
        return;
    }

    int mib[] = {
        CTL_NET, PF_ROUTE, 0, 0, NET_RT_IFLIST,
        static_cast<int>(index)
    };

    size_t length = 0;
    if (sysctl(mib, 6, nullptr, &length, nullptr, 0) != 0)
    {
        printf(" sysctl_mac=<size query failed errno=%d>", errno);
        return;
    }

    std::vector<unsigned char> buffer(length);
    if (sysctl(mib, 6, buffer.data(), &length, nullptr, 0) != 0)
    {
        printf(" sysctl_mac=<read failed errno=%d>", errno);
        return;
    }

    auto* message = reinterpret_cast<const if_msghdr*>(buffer.data());
    const auto* address = reinterpret_cast<const sockaddr*>(
        message + 1);

    for (int address_index = 0;
         address_index < RTAX_MAX;
         ++address_index)
    {
        if ((message->ifm_addrs & (1 << address_index)) == 0)
            continue;

        if (address_index == RTAX_IFP &&
            address->sa_family == AF_LINK)
        {
            auto* link_address =
                reinterpret_cast<const sockaddr_dl*>(address);

            if (link_address->sdl_alen == 6)
            {
                const auto* mac =
                    reinterpret_cast<const unsigned char*>(
                        LLADDR(link_address));

                printf(" sysctl_mac=%02x:%02x:%02x:%02x:%02x:%02x",
                       mac[0], mac[1], mac[2],
                       mac[3], mac[4], mac[5]);
                return;
            }
        }

        address = reinterpret_cast<const sockaddr*>(
            reinterpret_cast<const unsigned char*>(address) +
            sockaddr_aligned_size(address));
    }

    printf(" sysctl_mac=<unavailable>");
}

int main()
{
    ifaddrs* ifap = nullptr;
    if ( getifaddrs( &ifap ) != 0 ) return 1;

    int probe = socket( AF_INET, SOCK_DGRAM, 0 );

    for ( ifaddrs* p = ifap; p; p = p->ifa_next )
    {
        if ( !p->ifa_addr || p->ifa_addr->sa_family != AF_LINK ) continue;

        auto* dl = reinterpret_cast< sockaddr_dl* >( p->ifa_addr );

        unsigned ftype = 0;
        ifreq fr{};
        strncpy( fr.ifr_name, p->ifa_name, IFNAMSIZ - 1 );
        if ( ioctl( probe, SIOCGIFFUNCTIONALTYPE, &fr ) == 0 )
            ftype = fr.ifr_ifru.ifru_functional_type;

        // BIOCGDLT is the decisive check: DLT_EN10MB means our 14-byte Ethernet header is correct framing.
        int dlt = -1;
        int fd = open_bpf();
        if ( fd >= 0 )
        {
            ifreq br{};
            strncpy( br.ifr_name, p->ifa_name, IFNAMSIZ - 1 );
            if ( ioctl( fd, BIOCSETIF, &br ) == 0 ) ioctl( fd, BIOCGDLT, &dlt );
            close( fd );
        }

        printf( "%-10s maclen=%-2u ftype=%-11s dlt=%-3d up=%d running=%d ",
                p->ifa_name, dl->sdl_alen, functional_type_name( ftype ), dlt,
                ( p->ifa_flags & IFF_UP ) != 0, ( p->ifa_flags & IFF_RUNNING ) != 0 );
        print_sysctl_mac(p->ifa_name);
        printf("\n");
    }

    close( probe );
    freeifaddrs( ifap );
}