Difference between loopback and dummy interfaces for use in Linux routing

[Grant Taylor]

On 04/27/2018 02:52 AM, Wilhelm Schuster wrote:
> Hi,

Hi,

> 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.

That's one of the reasons that loopbacks (or dummies) are used.  Another 
is the fact that you can have an arbitrary interface that does not 
require associated hardware.

> 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.

My experience (primarily in Linux) suggests that this is the case with 
all interfaces by default, not just loopback (or dummy) interfaces. 
There are some /proc (and likely /sys) tuneables that can alter this 
default behavior.

> 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.

My understanding is that it's not possible to add additional loopback 
interfaces on Linux.  Instead dummy interfaces are added and removed on 
Linux like loopbacks are added and removed on other NOSs.  (I know that 
you can create and remove loopback interfaces at whim on classic Cisco IOS.)

I personally use lo exclusively for localhost and dummy interfaces for 
anything else that needs a loopback like interface.

The only real nuanced difference that I've been able to find is that the 
lo interface automatically adds the 127.0.0.1/8 IP address when you 
bring the interface up.  Where as dummy interfaces do not add any IP 
addresses automatically.  The only other thing that I've found is that 
lo doesn't add the 127.0.0.0/8 prefix to the routing table.  Conversely 
if you down lo and add 127.0.0.1/8 to dummy0, you do get the 127.0.0.0/8 
prefix in the main routing table.

I feel like the lo interface does some tricks and / or takes some 
shortcuts to try to be helpful.



-- 
Grant. . . .
unix || die

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