Linux Networking - Routing packets from one network interface to another

Question

I have been recently struggling to grasp what I am doing wrong here. So I have a micro computer which has two separate network interfaces, usb0 and eth0. usb0 is a radio which I can communicate with from an external device (essentially a modem). eth0 is a camera (which is another computer) which I have plugged into this computer.

Is it possible for an external device connected to usb0 to communicate with a device on eth0? I believe it is, but I am starting to second guess it.

Here are the network interfaces on my linux device:

voxl2:/$ sudo ifconfig
bond0: flags=5123<UP,BROADCAST,MASTER,MULTICAST>  mtu 1500
        ether da:49:41:51:a8:bf  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

dummy0: flags=195<UP,BROADCAST,RUNNING,NOARP>  mtu 1500
        inet6 fe80::66b9:adf2:23d1:2eb8  prefixlen 64  scopeid 0x20<link>
        ether 5e:da:25:e6:6b:f6  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 32  bytes 9856 (9.8 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.144.21  netmask 255.255.255.0  broadcast 192.168.144.255
        ether 5c:85:7e:3e:94:33  txqueuelen 1000  (Ethernet)
        RX packets 1476  bytes 88560 (88.5 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 37  bytes 10030 (10.0 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 4  bytes 352 (352.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 4  bytes 352 (352.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

usb0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.168.100  netmask 255.255.255.0  broadcast 192.168.168.255
        inet6 fe80::f27e:51e4:efeb:5592  prefixlen 64  scopeid 0x20<link>
        ether 92:46:8f:a0:15:f2  txqueuelen 1000  (Ethernet)
        RX packets 81226  bytes 4071362 (4.0 MB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 179792  bytes 21740183 (21.7 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

Note that eth0 and usb0 are on different subnets. If I ping the device from this micro computer (ping 192.168.144.52), I get a response.

However, from an external device connected over usb0, pinging the device gives me no response. I am, however, able to ping the ip of the actual interface itself.

ping 192.168.168.100 and ping 192.168.144.21 works.

I first ensured that net.ipv4.ip_forward=1 on /etc/sysctl.conf

Next, I started messing around with the routing table on this linux device.

Here are some variations of routes I tried. I think most were not correct

Variation 1:

voxl2:/$ sudo route add -net 192.168.144.52 netmask 255.255.255.255 gw 192.168.144.1 
voxl2:/$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         UserDevice      0.0.0.0         UG    209    0        0 usb0
link-local      0.0.0.0         255.255.0.0     U     208    0        0 eth0
192.168.144.0   0.0.0.0         255.255.255.0   U     0      0        0 eth0
192.168.144.52  192.168.144.1   255.255.255.255 UGH   0      0        0 eth0
192.168.168.0   0.0.0.0         255.255.255.0   U     209    0        0 usb0

Variation 2:

voxl2:/$ sudo ip route add 192.168.144.0/24 via 192.168.168.100 dev usb0
voxl2:/$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         UserDevice      0.0.0.0         UG    209    0        0 usb0
link-local      0.0.0.0         255.255.0.0     U     208    0        0 eth0
192.168.144.0   192.168.168.100 255.255.255.0   UG    0      0        0 usb0
192.168.168.0   0.0.0.0         255.255.255.0   U     209    0        0 usb0

Variation 3:

voxl2:/$ sudo ip route add 192.168.144.52/32 via 192.168.168.100 dev usb0
voxl2:/$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         UserDevice      0.0.0.0         UG    209    0        0 usb0
link-local      0.0.0.0         255.255.0.0     U     208    0        0 eth0
192.168.144.0   0.0.0.0         255.255.255.0   U     0      0        0 eth0
192.168.144.52  192.168.168.100 255.255.255.255 UGH   0      0        0 usb0
192.168.168.0   0.0.0.0         255.255.255.0   U     209    0        0 usb0

Variation 4:

voxl2:/$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         UserDevice      0.0.0.0         UG    209    0        0 usb0
link-local      0.0.0.0         255.255.0.0     U     208    0        0 eth0
192.168.144.0   0.0.0.0         255.255.255.0   U     0      0        0 eth0
192.168.144.52  0.0.0.0         255.255.255.255 UH    0      0        0 eth0
192.168.168.0   0.0.0.0         255.255.255.0   U     209    0        0 usb0

I think I am misunderstanding what the gateway field is doing. Perhaps if I set gateway to 192.168.144.1, destination to 192.168.168.100, and iface to usb0, it might work. I will try that shortly.