Can't trace a host even when it is in ARP table

Asked by Oscar Ramos

I have an ubuntu machine with 3 eth interfaces and 1 wlan, I have create next routes:

    post-up /sbin/route add -host dev eth1
    pre-down /sbin/route del -host dev eth1
    post-up /sbin/route add -host dev eth2
    pre-down /sbin/route del -host dev eth2

my route table is:

Destination Gateway Genmask Flags Metric Ref Use Iface UH 0 0 0 eth2 UH 0 0 0 eth1 U 0 0 0 eth0 U 0 0 0 wlan0 UG 0 0 0 wlan0

my arp table is

? ( at 00:00:0c:07:ac:cb [ether] on wlan0
? ( at 00:d0:dc:07:3e:4d [ether] on eth1
? ( at 00:20:98:01:d7:63 [ether] on eth2
? ( at 00:0c:29:04:88:49 [ether] on wlan0
? ( at 00:0c:29:a0:71:1c [ether] on wlan0
? ( at dc:9f:db:22:92:e2 [ether] on wlan0

When I do a tcpdump -i eth2 I can see traffic from going to, but I can't see any traffic from to and also I can't ping the I have also noticed when eth1 is not connected I can see traffic from but as soon as I coonect eth1 I just can see the traffic from eth1 to eth2 on eth2 port

Any help will be appreciated


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Oscar Ramos (ramses-vii) said :

one correction, the route I created on eth1 was not 166

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Oscar Ramos (ramses-vii) said :

one more correction: I can't ping, but I can ping

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Thomas Krüger (thkrueger) said :

Your network setup is kinda ubnusual. What are you actually trying to do and how does your physical network topology look like?

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Oscar Ramos (ramses-vii) said :

well on the Eth2 I have a device that send out information over Ethernet (IP:, there is no way to change the IP; on the Eth1 there is a windows device with a software on it that collect and process all the info from the device but it only has 1 Ethernet port and it is necessary to send all the info through the network, that is why I am using the Linux - Ubuntu Machine and trying to route all the traffic between both devices and sent it over ethernet

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Thomas Krüger (thkrueger) said :

So, it looks like this?

------- ----------------- -------
| D1 | --- (2)| Ubuntu PC |(1) --- | D2 |
------- ----------------- -------

And you want D1 to communicate with D2?
I that case you have to setup two different subnets (and not only hosts routes) on both eth1 and eth2 and set the default route (or a specific route to the other network) on both devices (D1, D2) to the IP of the router in the subnet they are connected to.

D1: default gw
D2: default gw
  eth1: default gw (your router)

Or you have to bridge the adapter.

But frankly it would be far more easy to use one network with a switch and assign a secondary IP to the devices that need to communicate with device that has a fixed IP.

Can you help with this problem?

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