[Bug 256681] [route] Incorrect loopback route for aliases IP addresses

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=256681

--- Comment #4 from Zhenlei Huang <zlei.huang@gmail.com> ---
(In reply to Rodney W. Grimes from comment #3)
Comparing to stable/12 and stable/11, I call the route is "expected".

Spent hours to repeat this on previous releases.

Prior to FreeBSD 8.0, upon adding IP address to interface, only the on-link
prefix route will be created. As for /32 aliases, the prefix route is /32. For
output routine, ip_output() will consult rtalloc() to "generate" the loopback
route for local IP address and use it.

From FreeBSD 8.0 to FreeBSD 12.2, the on-link prefix route will be created, as
well as the loopback route for local IP address. See [1]. 

From FreeBSD 13.0 and onward, due to significant rework of routing subsystem,
route for the /32 aliases is treated specially. See [2].

I reviewed the design of loopback route. If I understand correctly, the
loopback route short cut the out path of packets those are destined for local,
it directly forward it to local and bypass the physical interface. See [3].

So in some cases, if the loopback route is disabled and the hardware/logical
interface can not forward those packets destined for local, 'No route to host'
should be generated.

PS, there is a 'net.link.ether.inet.useloopback' sysctl tunable, it is default
on, and  controls whether installing a loopback route or not. This tunable
existed at least since 4.11 and was removed from stable/11. See [4].


[1]:
https://cgit.freebsd.org/src/commit/sys/netinet/in.c?h=stable/8&id=ebc90701ac6c1f814c5bd6f3e19f0113ebe06156
[2]: https://reviews.freebsd.org/D28668
[3]: https://cgit.freebsd.org/src/tree/sys/netinet/if_ether.c?h=stable/4#n109
[4]:
https://cgit.freebsd.org/src/commit/sys/netinet/if_ether.c?h=stable/11&id=b1b9dcae46803ae79255a9994584cb03d2a77048

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