kern/51388: IPv4 with 31 bit netmasks and RFC 3021

[zafer%netbsd.org@localhost]

>Number:         51388
>Category:       kern
>Synopsis:       IPv4 with 31 bit netmasks and RFC 3021
>Confidential:   no
>Severity:       non-critical
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          change-request
>Submitter-Id:   net
>Arrival-Date:   Fri Aug 05 10:30:00 +0000 2016
>Originator:     Zafer Aydogan
>Release:        7.0
>Organization:
TNF
>Environment:
amd64
>Description:
While deploying a server at a cloud provider, I noticed that the provider only serves /31 networks via dhcp. This left me disconnected with the internet, because the gateway was on the subnet ip.

NetBSD's behaviour is correct, but there is an RFC 3021 that is widely adopted that allows to work with 31 bit netmasks which mitigates the lack of IPv4 addresses.




>How-To-Repeat:
Create a network where the router is on the subnet ip of a 31 bit netmask.
Then try to reach the router.

>Fix:
Please adopt RFC 3021:
https://tools.ietf.org/html/rfc3021

The changes are minimal:

https://secure.freshbsd.org/commit/freebsd/r226402?diff=head%2Fsys%2Fnetinet%2Fin.c