This file documents the support in OpenVPN for Android using the
VPNService API (https://developer.android.com/reference/android/net/VpnService)
that has been introduced in Android 4.0 (API 14).

This support is primarily used in the "OpenVPN for Android" app
(https://github.com/schwabe/ics-openvpn). For building see the developer
README: https://github.com/schwabe/ics-openvpn/blob/master/doc/README.txt

Android provides the VPNService API
(https://developer.android.com/reference/android/net/VpnService)
which allows establishing VPN connections without rooting the device.

Unlike on other platforms, the tun device is openend by UI instead of
OpenVPN itself.  The VpnService API needs the following parameters:

- IP and netmask of tun interface
- Networks that should be routed to the tun interface
- DNS Servers and DNS Domain
- MTU

All IPs/Routes are in CIDR style.  Non-CIDR routes are not supported.
Notable is the lack of support for setting routes to other interfaces
usually used to avoid the server connection going over the tun
interface.  However, Android 13 adds support for exclusion routes that
serve the same purpose.  The Android VPNService API has the concept
of protecting a socket from being routed over an interface. Calling
protect (fd) will internally bind the socket to the interface used for the
external connection (usually WiFi or mobile data).

To use OpenVPN with the VPNService API OpenVPN must be built with
the TARGET_ANDROID compile option.  Also the UI must use a UNIX
domain socket to connect to OpenVPN.  When compiled as TARGET_ANDROID
OpenVPN will use management callbacks instead of executing traditional
ifconfig/route commands use the need-ok callback mechanism which
will ask

> NEED-OK command

where command can be:

IFCONFIG6 IPv6/netmask
IFCONFIG local remoteOrNetmask MTU topology

To tell the UI which IPs addresses OpenVPN expects on the interface.
Topology is one of "net30","p2p","subnet" or "undef".

ROUTE6 network/netmask
ROUTE network netmask

To tell the UI which routes should be set on the tun interface.

DNSSERVER IP server address
DNS6SERVER IPv6 server address
DNSDOMAIN searchdomain

To set the DNS server and search domain.

The GUI will then respond with a "needok 'command' ok' or "needok
'command' cancel', e.g. "needok 'IFCONFIG' ok".

PERSIST_TUN_ACTION

When OpenVPN wants to open an fd it will do this query via management.
The UI should compare the last configuration of the tun device with the current
tun configuration and reply with either NOACTION (or always respond with
OPEN_BEFORE_CLOSE).

- NOACTION: Keep using the old fd
- OPEN_BEFORE_CLOSE: the normal behaviour when the VPN configuration changed

For example the UI could respond with
needok 'PERSIST_TUN_ACTION' OPEN_BEFORE_CLOSE

To protect a socket the OpenVPN will send a PROTECTFD to the UI.
When sending the PROTECTFD command command to the UI it will send
the fd of the socket as ancillary message over the UNIX socket.
The UI will then call protect(fd) on the received socket protecting
it from being routed over the VPN.

When opening a tun device the OpenVPN process will first send all
route, ifconfig and DNS related configuration to the UI and after
that calls the OPENTUN command to receive a tun fd with the requested
configuration.  The UI will then use the collected information to
call the VPNService's establish() method to receive a fd which in
turn is send to the OpenVPN process as ancillary message to the
"needok 'OPENTUN' ok' response.

The OpenVPN for Android UI extensively uses other features that
are not specific to Android but are rarely used on other platform.
For example using SIGUSR1 and management-hold to restart, pause,
continue the VPN on network changes or the external key management
--management-external-key option and inline files.

To better support handover between networks, a the management command

network-change [samenetwork]

is used on the Android platform.  It tells OpenVPN to do the necessary
action when the network changes.  Currently this is just calling
the protect callback when using peer-id regardless of the samenetwork.
Without peer-id OpenVPN will generate USR1 when samenetwork is not set.
