Dubious communication between iPad and iPhone on incoming call

Question

I'm owner of an iPad and an iPhone, both running iOS 8.1.3 although this phenomenon did already appear with earlier iOS versions.
In the following scenario something weird is happening:
My iPad is at home in a private network (192.168.x.0) and has the address 192.168.x.y.
My iPhone is at office in a private network (10.0.1.0) and has the address 10.0.1.z)
Someone is calling me on my iPhone
Now the iPad tries to communicate with my iPhone by sending an ip packet from ip/port 192.168.x.y:57864 to ip/port 10.0.1.z:58590. Normally it is not possible to communicate between private networks directly, i.e. this packet makes no sense and would be discarded by my home router.
In this special case however, there is a VPN connection between my home and my office networks. Thus the packet is not discarded but forwarded by VPN from my home to my office network. The office firewall however is treating this packet as an attack and thus triggering an alarm mail to the admin.
The iPad appears to know the iPhone's address (probably through iCloud) although I'm in my office (together with my iPhone ).
It is trying to communicate with the iPhone using private network addresses which in general is not possible. This is why I suspect that there is a bug somewhere in the FaceTime / telephony protocol.
I could change some of our firewall rules to avoid the alarm mail but that is not the point here.
Can someone explain if the behaviour described above makes sense in any situation or if it's really a bug (as I'm suspecting)?

Answer 1

You can disagree all you want, It will not change the truth. a VPN Brdiges two networks so that the remote one can use the local one as if it where physically there. From Wikipedia's page on VPNs:
A virtual private network (VPN) extends a private network across a public network, such as the Internet. It enables a computer or network-enabled device to send and receive data across shared or public networks as if it were directly connected to the private network, while benefiting from the functionality, security and management policies of the private network.[1] A VPN is created by establishing a virtual point-to-point connection through the use of dedicated connections, virtual tunneling protocols, or traffic encryptions.
VPN Definition
If you are facetime-ing to a friend not on the same VPN or network, Facetime uses a connection Via Apple servers to establish a connection to the other user. Since there is no way for your iPad to know the internal network IP address of the other user. And which would also be pointless, since the only thing available to the outside world is the routers public IP. What IP they have in their network is irrelevant to Facetime. Facetime connects through Apple Servers, not directly by IP.
However, an iPhone and a iPad using Continuity are connected through iCloud. Since Apple has not published any specs on how continuity works, I can only surmise from what I see on my devices that when they are connected to the same network both devices are aware of each other. How I cannot say. but they are, otherwise Continuity would not work.
Hagen-1 wrote:
I continue to claim that there is a bug in the FaceTime / Continuity protocol. Two iOS devices must not try to communicate with each other using private ip addresses when they are not sure that they are in the same network.
That is where your theory falls. The VPN means they are for traffic purposes on the same network. i.e they can communicate with each other. Expecting them not to when the VPN is active is wrong.
Again the fact that it is being flagged as an attack is the issue. Everything else is working as designed. What you expect should happen simply does not match what actually should happen and is happening.
Just like your computer can use office resources from home through the VPN, so can the iPad. Since the resource its looking for is your iPhone it sends packets to it.
This is not a bug.
You can send Apple feedback if you wish. But there's nothing at all wrong with what is happening.,
If you do not want this traffic, then disconnect the iPad from your Wifi network before leaving, or turn off the VPN on your home network so there is no access to the office network.