[FS#283] NAT Loopback ("reflections") not working correctly.

[LEDE Bugs]

The following task has a new comment added:

FS#283 - NAT Loopback ("reflections") not working correctly.
User who did this - Jo-Philipp Wich (jow-)

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It should be fixable in mbim.sh as long as the actual WAN IPv4 address is known somewhere in its context. 

The handler should do something like

    . /lib/netifd/netifd-proto.sh
    proto_init_update wwan0
    proto_add_ipv4_address 1.2.3.4 32
    proto_add_ipv4_route 0.0.0.0 0 5.6.7.8
    proto_send_update wan

Where...

 - `wan` is the logical name of the config interface uci section
 - `wwan0` is the physical device
 - `1.2.3.4` is the WAN IPv4 address
 - `32` is the bit length of the netmask (30 for 255.255.255.252)
 - `5.6.7.8` is the default gateway IP address
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=283#comment979