Peer to peer adhoc network setup using network manager. ARP, gateways and routes...

[linux guy]

I need to connect a wifi device to my Linux computer. There is no router in
between the two. Just my computer and the device. The device has a fixed
IP, 192.168.0.10. It has its own Wifi interface built in  It isn't capable
of connecting to a router.  It has a fixed IP address.

My home wifi network uses 192.168.1.x. It has a router. It works fine. My
computer connects to it via its built in wifi interface.

I have added a USB wifi interface (dongle) to my computer to connect to the
device. My computer displays the device properly in a network manager scan.
So the device is available for connections.

Next I edit the connection for the device in network manager. The SSID is
filled in from the scan. I select Ad-hoc mode. Band, Channel, etc. are all
automatic.

Next I edit wifi security. The device is open security wise, so I select
None.

Next I edit the IPV4 tab. Under connection method I select Manual.

The next fields are DNS server and Search Domains. In a point to point
connection, there will be no DNS server. I leave the boxes blank.

Next up is adding the static IP address, netmask and gateway.

I set the address of my interface to be 192.168.0.123, because it was
recommended by another user who connected with an Apple device. I set
netmask to 255.255.255.0.

The next thing to set is the gateway address. Here is where I get
confused.  What is a gateway in the context of a peer to peer connection ?
I try to leave it blank, but it won't let me.  So I put in 0.0.0.0.   I
also tried the address of the interface (192.168.0.123) and the address of
the device (192.168.0.10)

Then there is a route field.  I add a route for the ip address
192.168.0.10, using a netmask of 255.255.255.0 and again it asks for a
gateway. I tried 0.0.0.0, the device address and the interface address.

I don't touch any of the IPV6 settings because the device doesn't support
IPV6 addresses.

I save everything and connect to the device successfully.

I run "arp -a" and the device is found connected to the interface.  Yippee !

Pinging the device fails.  Destination Host Unreachable.

When I ping the device and watch in wireshark I see an ARP broadcast
message "Who has 192.168.0.10 ? Tell 192.168.0.123". 192.168.0.123 is the
static IP for the interface. And that is as far as the process gets.

When I watch the other interface and do a ping to a local device, I see the
same ARP broadcast message, but also a reply message to it. 192.168.1.142
is at <device MAC> And then the process continues successfully.

So what am I missing in the setup of my adhoc connection such that the OS
knows that a message sent to my device (192.168.0.10) gets routed through
the USB wifi interface, ie the ARP broadcast message gets a reply ?

Thanks.
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