Ubuntu – Route only specific intranet traffic via eth0; everything else via wlan0

16.04ethernetnetwork-managernetworking

I am relatively new to networking so bear with me if you please. I am using Ubuntu 16.04 on an NVIDIA Jetson TX2. I have a Velodyne Lidar VLP-16 Lite connected directly to the Ethernet port and am connected to the internet via wlan0. When I connect to eth0 (AKA the Lidar) while I was still connected to wlan0, I could not ping google. If I disconnected from eth0, internet access came back as I was always connected to wlan0.

First solution
I went into the network manager and modified the eth0 settings to have an ip of 192.168.2.227 and net mask 255.255.255.0. I also edited the IPv4 routes to include Address: 192.168.2.1 and net mask 255.255.255.0 and clicked the radio button "use this connection only for resources on its network".

This allowed me to connect both to the internet via wlan0 and to communicate with the LIDAR peripheral directly via its host IP 192.168.2.201 (which I set).

So What's the Problem? I believe I am losing packets, perhaps randomly.

So what's the question? How do I route everything that is coming from or going to a specific address to a specific interface and make everything else go to the other interface?

That is: How do I make all traffic that is sent to IP address 192.168.2.201 (Me => the LIDAR) to go through eth0, and all traffic to IP address 192.168.2.202 (Lidar => me) go to eth0? All the while making everything else go through wlan0?

Currently my system looks like this:

ubuntu@VPro-test2:~$ netstat -rn
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG        0 0          0 wlan0
169.254.0.0     0.0.0.0         255.255.0.0     U         0 0          0 eth0
192.168.1.0     0.0.0.0         255.255.255.0   U         0 0          0 wlan0
192.168.2.0     0.0.0.0         255.255.255.0   U         0 0          0 eth0

ubuntu@VPro-test2:~$ ip route
default via 192.168.1.1 dev wlan0  proto static  metric 600 
169.254.0.0/16 dev eth0  scope link  metric 1000 
192.168.1.0/24 dev wlan0  proto kernel  scope link  src 192.168.1.45  metric 600 
192.168.2.0/24 dev eth0  proto kernel  scope link  src 192.168.2.202  metric 100

ubuntu@VPro-test2:~$ ifconfig
eth0      Link encap:Ethernet  HWaddr 00:04:4b:8c:c1:0c  
          inet addr:192.168.2.202  Bcast:192.168.2.255  Mask:255.255.255.0
          inet6 addr: fe80::1a8f:c89c:e32c:8de0/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:131110 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1494 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:72618144 (72.6 MB)  TX bytes:824212 (824.2 KB)
          Interrupt:42 

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:43 errors:0 dropped:0 overruns:0 frame:0
          TX packets:43 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1 
          RX bytes:2467 (2.4 KB)  TX bytes:2467 (2.4 KB)

wlan0     Link encap:Ethernet  HWaddr 00:04:4b:8c:c1:0a  
          inet addr:192.168.1.45  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::9679:8dae:defd:aafa/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:6277 errors:0 dropped:0 overruns:0 frame:0
          TX packets:382 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:783723 (783.7 KB)  TX bytes:49404 (49.4 KB)

I am looking explicitly for the configuration to enable this, be that command line or graphical interface. Thanks!

Best Answer

I am not an expert, just autodidact. Hopefully this is going to help you somehow, since nobody gave you any answer at all actually.

You believe you are losing packets, perhaps randomly - how to you come to this conclusion?

How do I route everything that is coming from or going to a specific address to a specific interface and make everything else go to the other interface?

By setting up routes as you already did using the gui.

It should work like this from command line:

Ethernetdevice connected to the LIDAR

sudo ip route add ip-of-a-specific-server-or-device-or-of-whole-subnet via standardgateway1

  • Assuming your LIDAR has ip 192.168.2.202
  • and the router or standardgateway on that net has the IP 192.168.2.1

the command could look like this

sudo ip route add 192.168.2.202 via 192.168.2.1

or if you want to route all traffic to any device inside that subnet: 

sudo ip route add 192.168.2.0/24 via 192.168.2.1

/24 is a subnetmask of 255.255.255.0 that is 8 bit + 8 bit + 8 bit + 0 bit = 24 bit

Ethernetdevice connected to the internet

sudo ip route add all-the-rest-to-the-internet via standardgateway2

  • The rest/internet has the default ip of 0.0.0.0
  • Your router or modem might have the ip 192.168.1.1

The command should look like this: 

sudo ip route add 0.0.0.0 via 192.168.1.1

Problems that sometimes can occur when routing:

You are sending ip packets with a bad source ip. For example it can happen that you send a source ip of 192.168.1.x to a device at 192.168.2.x. To check this you could use tcpdump -i interface -vvv and check what is shown as your outgoing ip. To repair this you need to use iptables and MASQUERADE your traffic for example with sudo iptables -A POSTROUTING -t nat -o specific-interface-for-example-eth0 -j MASQUERADE

To Monitor your traffic I would suggest to use bmon as it will show you also collisions / errors per device.

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