Difference between loopback and dummy interfaces for use in Linux routing

[Wilhelm Schuster]

Hi,

I’m learning about IP-networking and am in the process of setting up a Linux router using bird. During my research I’ve come across the usage of loopback interfaces. I could gather that this is done, because Ethernet interfaces for example (in contrast to loopbacks) can go down making the addresses configured on them unavailable. In addition, the kernel accepts packets destined to loopbacks addresses on other interfaces making these addresses available on multiple interfaces and not just the loopback.

On Linux I found both the lo(opback), and dummy interfaces recommended to achieve the behavior outlined above. What I’m struggling with is understanding the differences between both interface types (besides the obvious difference in packet processing) and when to use which. Searching this mailing list I’ve seen people use lo, on other sites dummy interfaces are recommended. From a quick test (assigning address to lo/dummy; pinging/receiving pings on the host) I wasn’t able to find a difference.

When should I use which interface?

Cheers, Wilhelm.