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OK, how does this work?
http://www.x404.co.uk/forum/viewtopic.php?f=4&t=1465		 Page 1 of 1
Author:  jonbwfc [ Thu Jun 25, 2009 7:53 pm ]
Post subject:  OK, how does this work?
I have an Apple TV. And an iPhone.

There is an application for the iPhone which allows you to use the phone as a remote control for the AppleTV. Today Apple issued an update for both, to allow you control the AppleTV by 'swiping' on the iPhone. It's not actually any better than clicking up and down buttons but I suppose it looks cooler.

Anyway, I update the app, and I update the AppleTV. And it all works and I'm swiping away and flipping through the menus. Then I notice half of the options aren't available. I go and check and the update has caused the AppleTV to forget it's IP address. I don't have a DHCP server on my home network and it can't pick one up automatically, so it's using a '169.<blah>' address. For those of you who don't know networking, a 169 address basically means 'I have no idea what my address is, I can't get it to work so I'll use this one to fill the boxes in'. 169 addresses effectively mean 'no networking'. Even if the network was working nothing else on my network uses a 169 address so it's effectively on the wrong phone line to talk to anything else.

Yet the iPhone remote app was still working. There was no way to communicate with the AppleTV via the network, because the Apple TV's networking was 'broken'. The standard Apple remote is infra-red and there's no infra-red sender on the iPhone. So how exactly were they talking to each other?

I can't figure this one out. I was thinking maybe bonjour but again the AppleTV isn't on the proper network. Can't be bluetooth, the AppleTV doesn't have that. Can't be IR, the iPhone doesn't have that. So how is this working, machine telepathy? Anyone hazard a guess?

Jon
Author:  saspro [ Thu Jun 25, 2009 8:15 pm ]
Post subject:  Re: OK, how does this work?
Without a dhcp server everything on the network will issue itself a 192. address, these are always in the same range so everything is actually networked but just can't get out on to the internet.
Author:  jonbwfc [ Thu Jun 25, 2009 10:13 pm ]
Post subject:  Re: OK, how does this work?
saspro wrote:
Without a dhcp server everything on the network will issue itself a 192. address, these are always in the same range so everything is actually networked but just can't get out on to the internet.

Not in this case, for several reasons

1) The box wasn't using a 192 address, it was using a 169 address. It said so quite clearly.

2) 169 addresses really do have a specific function. To quote wikipedia

"RFC 3330 defines an address block, 169.254.0.0/16, for the special use in link-local addressing for IPv4 networks ... These addresses are only valid on the link, such as a local network segment or point-to-point connection, that a host is connected to. These addresses are not routable and like private addresses cannot be the source or destination of packets traversing the Internet."

The use of the 192 address block is slightly different and is explained on the same page, under 'IPv4 private addresses' and is far too long to reprint here :).

3) The rest of my stuff isn't actually set up to use 192. something addresses either, because I want to make it as hard as possible for someone getting onto my wifi network to do pass any traffic or do any harm. Thus no DHCP and everything's configured using a non-standard set of IP addresses which are then NATed to the outside world. It's not foolproof but it's considerably easier to crack my two neighbour's wifi networks than mine :). In any case, a 192.<something> device on my network won't be able to see anything else, so my phone still shouldn't have been able to see the AppleTV.

Anyway, I did a bit of digging around and I think i know what it's doing. It's to do with bonjour and the subnet mask it sets up by default and, as this is already more than a little dry, I won't go into the full details. Sufficed to say Apple are actually doing something somewhat naughty but in doing so they've got the AppleTV to work in such a way that you can plug it into a home network and it will work without any configuration at all under any circumstance, at a cost of putting a lot of 'noise' on the network. Cheeky.

Jon
Author:  forquare1 [ Thu Jun 25, 2009 10:31 pm ]
Post subject:  Re: OK, how does this work?
My guess would have been it had something to do with IPv6...

Out of curiosity, why don't you run a DHCP server?
Author:  jonbwfc [ Fri Jun 26, 2009 7:00 am ]
Post subject:  Re: OK, how does this work?
forquare1 wrote:
My guess would have been it had something to do with IPv6... Out of curiosity, why don't you run a DHCP server?

Firstly, it's kind of a security thing. If someone 'cracked' the wireless security on my network and I had a DHCP server, they would then have free run of the place. This way, just cracking the security won't get you anywhere, because you won't get a usable IP address and you'll be a bit stuck. Now this is not an insurmountable hurdle to a determined hacker - once past the encryption you could packet sniff the network to figure out what subnet I was using - but it would be enough to make them go find another network to use. Like for example the one at the school opposite that still uses WEP encryption :LOL:

It's also a functionality thing. Because I specifically allocate things IP addresses I always know what device is on what IP so it makes it easier to do port forwarding from the router to a particular device. I know you can 'hard allocate' DHCP addresses to particular MAC addresses but it doesn't look like my router does that, or if it does it's far more faff than using static IPs.

Fundamentally, given I don't share my WLAN with anyone and rarely put new devices on it, there's no great need for me to run a DHCP server.

Jon
Author:  saspro [ Fri Jun 26, 2009 8:48 am ]
Post subject:  Re: OK, how does this work?
Sorry my mistake, I meand 169. but for some reason typed 192. :oops:
Author:  didgeman [ Fri Jun 26, 2009 11:12 am ]
Post subject:  Re: OK, how does this work?
a UDP broadcast packet can jump subnets as long as the devices are on the same physical network.

often used for device discovery .. plug in the device .. run the setup app on your local machine which broadcasts across the entire physical network looking for devices that are then configured.

hth

didge
Author:  tombolt [ Fri Jun 26, 2009 12:59 pm ]
Post subject:  Re: OK, how does this work?
Could it be communicating via appletalk?

Actually, I was having a problem with my network that I didn't notice for ages. I'd typed the wrong subnet into one of my Macs and didn't notice until I tried to connect to one of the PCs. It managed to find all the other Macs fine but couldn't see any of the PCs. Took me a while to figure it out.
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