I have a server with 2+ network interfaces (pick a number). Each of the interfaces is defined as such:

auto ethx
iface ethx inet dhcp

Each of the interfaces has been assigned an IP address based on its MAC address; that is, the network admin has created a manual DHCP lease for that MAC address and only that MAC address. We are also using sticky-ARPs, meaning that no other MAC address can obtain the IP of another without have the ARP entry cleared.

The problem begins occuring 5 minutes after each NIC obtains its IP address, and continues to appear in the logs in five minute increments. Errors begin showing up in the DHCP server logs indicating that the NICs are attempting to use the IP addresses of other interfaces.

For example, eth0 obtains ip address 1.2.3.4, eth1 obtains address 1.2.3.5, and eth2 obtains 1.2.3.6. The DHCP server soon begins reporting that the MAC address of eth2 is attempting to send traffic through the address of eth0, eth1 is attempting to send traffic through the address of eth2, and so on. Ifconfig shows that the ip and MAC addresses match up properly, as does to the leases in the DHCP server.

If we use only two network interfaces, this problem does not manifest. It only shows up when using more than two interfaces.

Is this a problem with multiple adapters? Can this problem be fixed by using static addresses? Is this problem only evident in Debian Linux?

Chris Rees
Enterprise Systems Administrator
Missouri State University

Are the IP addresses on different subnets? The examples given don't indicate this. If you are using 3 cards then the addresses should be like;

eth0 range 192.168.0.1 - 254
eth1 range 192.168.1.1 - 254
eth2 range 192.168.2.1 - 254

They cannot operate together in the same subnet.

Two of the three NICs are on the same subnet. The third is not. However, we are using VLANS, and all three are on the same VLAN.

For example:

eth0 1.2.3.4 - 254
eth1 1.2.2.4 - 254
eth2 1.2.2.5 - 254

Where the VLAN is .2, which contains address subnets .2 and .3.

I am not a DNS, VLAN, etc. guru, so pardon if there is any misunderstanding or confusion on my part.

You cannot have network cards in the same box on the the same subnet and achieve any form of routing because the network gets confused as you have found out. They must be on different subnets as described above in order to achieve routing. The boxes attached to each card however, must be on the same subnet as the card they are attached to.