cleaning up INET: deprecating network class A/B/C

From: Mike Karels <>
Date: Tue, 19 Oct 2021 14:47:40 UTC
I plan to do some cleanup of the residual code defining and using the
old Internet network classes (A/B/C), which have been obsolete since
CIDR took hold.  This is an outline of what I plan, as it will happen
in a number of steps and reviews, and I would like feedback on some
of it.

I want to reduce the use of the obsolete definitions and interfaces,
and make it less likely for them to be used going forward.  I plan
to hide the Class A/B/C bit definitions unless a feature test macro
is defined; that will be the default for user code for the moment.
A few files in the kernel will need to define the feature test macro
for now (but see the next two paragraphs).

Several of the uses of the historical network class macros have to
do with generating a default network mask when none is provided.
The worst of these is in the code for SIOCAIFADDR (add interface
address).  I want to have ifconfig and/or the kernel warn about this;
the default is most likely wrong.  After some time with a warning,
it should become an error to set an Internet interface address
without a mask (except for loopback and point-to-point interfaces,
where the mask is meaningless).

I am tempted to define a new default mask, e.g. 24 bits, for those
places that must be able to generate one.  An example is NFS BOOTP
code.  I am interested in feedback on this idea.  It would help to
reduce use of the old masks, and 8- or 16-bit prefixes are highly
unlikely to be correct.  Comments on adding a default mask?  This
would eliminate the use of the old class macros in the kernel.

The C library routines inet_netof() and inet_lnaof() should be
deprecated, as they use the historical masks.  inet_makeaddr() is
almost as bad; it works almost by accident as long as a mask is a
multiple of 8 bits.  I'd like to remove their use from the base
system.  Unfortunately, I have no idea how much other software uses
them.  We can at least document them as deprecated and unsafe.