How android knows it's on an expensive connection
=================================================

Keywords: android, hotspot, tethering, `ANDROID_METERED`, expensive

When an android phone is using another android phone's hotspot, it knows it's
on a "metered connection" and therefore disables the expensive sync options (
eg: photo sync).  How does it know this?  The android hotspot sends DHCP Option
43 (Vendor specific options) with the value `ANDROID_METERED`. The client, if
it sees `ANDROID_METERED` anywhere in the option 43 values, turns on the
"expensive data connection" option.

The problem with this is that the DHCP server isn't supposed to send DHCP
options unless the DHCP client asked for it, and the android DHCP client
doesn't ask for option 43 (at least under ≤4.3).  But the android DHCP server,
just returns option 43 anyway.  It really shouldn't do that, and the client
listens for options it's not requested, which it shouldn't do either.  The
practical upshot is that if you're trying to set this option, remember to force
it rather than wait for the client to ask for it.

From [TetherController](https://android.googlesource.com/platform/system/netd/+/7b90f090d9e48e76d5fb3f674eb7bfd5fbdfaf7d/TetherController.cpp):

    args[0] = (char *)"/system/bin/dnsmasq";
    args[1] = (char *)"--keep-in-foreground";
    args[2] = (char *)"--no-resolv";
    args[3] = (char *)"--no-poll";
    // TODO: pipe through metered status from ConnService
    args[4] = (char *)"--dhcp-option-force=43,ANDROID_METERED";
    args[5] = (char *)"--pid-file";
    args[6] = (char *)"";

The comment here shows that all tethering is assumed to be "metered", and there
is a todo to fix it.

And from [core.java.android.net.DhcpInfoInternal](https://bitbucket.org/seandroid/frameworks-base/src/33034b13cae1/core/java/android/net/DhcpInfoInternal.java)

    public boolean hasMeteredHint() {
        if (vendorInfo != null) {
            return vendorInfo.contains("ANDROID_METERED");
        } else {
            return false;
        }
    }

Notes
-----
I'm unaware of any laptops that listen to this attribute, but it wouldn't be
hard to add.  I'm also unaware of any support in iOS for this attribute, or
another similar attribute.

There's Settings > Data usage > ⋮ > Mobile Hotspots, which appears to be a
similar thing.  "Apps can be restricted from using these networks when in the
background and can also warn you about using these networks for large
downloads."

Ideas
-----
There are some obvious ideas that might be helpful.  If you're on an expensive
internet connection, then using this option to tell android to back off is of
course fairly obvious.  If you're a laptop doing sync, or auto updates, 
listening for this option would be very welcome as well.

### Using this to get android phones to back off on congested wifi
One idea would be to look at the amount of spare air time there is on the 
wifi interface, and if it starts looking busy, then start handing this option
out.  If the airtime is mostly free then don't hand it out.

Airtime is fairly hard to measure, and bit of a layer violation, so a good
proxy for airtime might be combined in/out bandwidth on the interface.  
Bandwidth is fairly hard for a dhcp server to measure too, so you might want
to use the number of associated stations as a proxy for bandwidth.  And since
number of associated stations is annoying to measure from a DHCP server, you
might just want to count the number of DHCP leases as a proxy for number of
associations.

### Using this to stop your laptop from trying to update while tethered
If you're tethering, you might not want your laptop to automatically try and
update everything.  Even if you have enough of a data cap to deal with this,
you might not want to have it congest all your bandwidth while you're trying to
get some emergency work done.

(UNTESTED)<br>
You can trigger this by dropping an executable script in 
`/etc/dhcp/dhclient-enter-hooks.d/`, this should look for an environmental
variable called `new_vendor_encapsulated_options` and see if it contains the
string `ANDROID_METERED`.

Some things to do:

     gconftool --set --type bool /apps/apps-notifier/auto_launch false
     dconf write /com/ubuntu/update-notifier/auto_launch false

You might also want to look at `gconftool -R / | egrep 'auto|sync'` and 
`dtool dump / | egrep 'auto|sync'` for other things you might want to flip
off.

If I ever get around to testing it, I'll put an example script here.