Difference between loopback and dummy interfaces for use in Linux routing

[Wilhelm Schuster]

On 2018-04-27 13:08, Daniel Suchy wrote:
> There's quite good article discussing that:
> https://b4ldr.wordpress.com/2014/09/06/dummies-link-local-and-the-loop-back/

Thank you for providing the link. Though I'm not sure if I fully
understand the implications of the post: In the conclusion they wrote:

> Using the loopback interface caused other issues due to the fact that there is no link-local address. with the loopback interface the next hop for the default route becomes unreachable due to the lack of a link-local address so it is probably best to stick with the dummy interface
> 
> root at router1:~# ip -6 route
> 2001:db8:1::/64 dev zzz0  proto kernel  metric 256
> unreachable 2001:db8::/47 dev lo  proto kernel  metric 256  error -101
> fe80::/64 dev zzz0  proto kernel  metric 256
> unreachable fe80::/64 dev lo  proto kernel  metric 256  error -101
> unreachable default dev lo  proto zebra  metric 1024  error -101

Why does the lack of a link-local address result in the default route
being marked unreachable? I'm guessing this is, because the lack of a
link-local address implies that the interface is not on a IPv6 link and
thus won't be able to reach another host (since the v6 spec requires
interfaces to at least have a link-local address).

> On 04/27/2018 10:52 AM, Wilhelm Schuster wrote:
>> 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.
>>