Bug in PF_ROUTE on macOS?

I have a question about using PF_ROUTE on macOS to detect IP address changes. Basically, it seems to me that it is broken for IPv4. I have put together a sample program that simply creates the PF_ROUTE socket and then prints out when RTM_NEWADDR, RTM_DELADDR and RTM_IFINFO are received.

What I notice is that when I use a single interface (wifi or ethernet cable) and disconnect the network adapter (disable wifi or unplug the cable) I get nothing at all. If I then reconnect (enable wifi or plug in the cable) I get RTM_NEWADDR but no RTM_IFINFO.

If I have both the wifi and the cable connected at the same time, both disconnecting and then reconnecting one of the interfaces (e.g. disable wifi then re-enable wifi) produces no events at all.

IPv6 seems to work. If I test IPv6 in the same manner, I get an RTM_NEWADDR on connection and RTM_DELADDR on disconnection (the address is the IPv6 link local address - my DHCP server does not serve up IPv6 addresses).

Also if I try to do if_indextoname() when I get the RTM_IFINFO event, it doesn't always work. I need to insert a sleep to be able to consistently get the name back (I chose 500 milliseconds, I didn't spend any time trying other values to see if a lower value would work).

I have tested this program on a MacBook Pro running 10.13, an iMac running 10.14 and a VM running 10.12 - all behave the same way.

So, my question is: is this a bug in the OS, or do I have a fundamental misunderstanding of how the PF_ROUTE socket is supposed to work?

Thanks,

Kevin

#include <SystemConfiguration/SystemConfiguration.h>
#include <net/route.h>
#include <errno.h>

struct cmn_msghdr
{
    u_short    msglen;
    u_char    version;
    u_char    type;
};

int main(int argc, const char * argv[])
{
    char buf[1024];
    size_t len;
    int skt, family = AF_UNSPEC;

    if ( argv[1] && argv[1][0] == '4' )
        family = AF_INET;
    else if ( argv[1] && argv[1][0] == '6' )
        family = AF_INET6;

    // Create a PF_ROUTE socket over which we will receive change messages
    skt = socket( PF_ROUTE, SOCK_RAW, family );
    if ( skt == -1 )
    {
        printf( "ERR: Failed to create PF_ROUTE socket. error %d\n", errno );
        return -1;
    }

    printf( "Watching for %s address changes. Press Ctrl-C to exit\n",
           family == AF_UNSPEC ? "IP" : ( family == AF_INET6 ? "IPv6" : "IPv4" ) );

    // Loop forever waiting for messages
    for (;;)
    {
        len = recv( skt, buf, sizeof(buf), 0 );
        if ( len < 0 )
        {
            switch (errno)
            {
                case EINTR:
                case EAGAIN:
                    printf( "ERR: EINTR or EAGAIN on PF_ROUTE socket\n" );
                    continue;
                default:
                    printf( "ERR: Failed to receive on PF_ROUTE socket. error %d\n", errno );
                    continue;
            }
        }
        if ( len < sizeof( cmn_msghdr ) )
        {
            printf( "ERR: Data received on PF_ROUTE socket too small: %ld bytes\n", len );
            continue;
        }

        struct cmn_msghdr *hdr = (struct cmn_msghdr *)buf;
        if ( hdr->version != RTM_VERSION )
        {
            printf( "ERR: RTM version %d is not supported\n", hdr->version );
            continue;
        }

        switch( hdr->type )
        {
            case RTM_NEWADDR:
                printf( "RTM_NEWADDR\n" );
                break;
            case RTM_DELADDR:
                printf( "RTM_DELADDR\n" );
                break;
            case RTM_IFINFO:
                printf( "RTM_IFINFO\n" );
                break;
            default:
                // Don't care
                continue;
        }
    }

    return 0;
}