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I have a strange behaviour on my network as my unix-like hosts seem to have problems to reach the gateway.

Digging into the network traffic via tcpdump, I've seen that two different hosts answer to the arp request for the 192.168.0.1 ip, which is my gateway address.

The first one is the proper gateway, the second one is a Xen Server machine which has the same ip but on another network interface.

Network topology:

enter image description here

In the image you can see the focal points of my network:
ARP requests for 192.168.0.1 from 192.168.0.116 receive the following answers:

tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
15:03:28.848836 ARP, Ethernet (len 6), IPv4 (len 4), Reply 192.168.0.1 is-at 00:15:17:70:e5:00 (oui Unknown), length 46
15:03:28.848879 ARP, Ethernet (len 6), IPv4 (len 4), Reply 192.168.0.1 is-at 00:1a:8c:13:bf:70 (oui Unknown), length 46
15:04:15.851796 ARP, Ethernet (len 6), IPv4 (len 4), Reply 192.168.0.1 is-at 00:15:17:70:e5:00 (oui Unknown), length 46
15:04:59.851789 ARP, Ethernet (len 6), IPv4 (len 4), Reply 192.168.0.1 is-at 00:15:17:70:e5:00 (oui Unknown), length 46

The Citrix server answer even if it has the 192.168.0.1 interface connected to a different network.

How can I prevent it?

  • How can I prevent it? - Don't assign the same ip address to multiple hosts or interfaces. – joeqwerty May 4 '16 at 14:18
  • I agree but these interfaces are on different networks... – luca3003 May 4 '16 at 14:20
  • You can't have "different" networks with the same IP. That makes them the "same" network. – Ron Trunk May 4 '16 at 15:56
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I'm fairly certain that XenServer uses the weak host model, which explains the behavior you're seeing. You probably have two options:

  1. Configure the XenServer to use the strong host model, if possible.

  2. Assign a different ip address to the interface in question.

For an explanation of the strong and weak host models, have a read here:

http://tools.ietf.org/html/rfc1122

and here:

https://technet.microsoft.com/en-us/magazine/2007.09.cableguy.aspx

  • Yes, it is! FInally I solved the issue connecting the xen server management interface on a different switch, so it doesn't receive arp requests from my 192.168.0.0/28 network. Thank you Joe! – luca3003 May 5 '16 at 8:43

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