Bug in PF_ROUTE on macOS? - macos

Question

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).
A couple of other side notes: If I try to do if_indextoname(), 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).
Also, if I call getifaddrs() in a loop (with a little sleeping between calls) after receiving the IPv6 RTM_NEWADDR event to try to find the missing IPv4 address, it can take a long time for it to show up in the returned data. I have seen it take up to 8 seconds on my system. Note that the IP address is up and usable long before this as a continuous ping to an external address readily confirmed.
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;
}