14.1-R rc.conf/ifconfig netmask issue was really hard to figure out

I had a machine with this line in /etc/rc.conf:

	ifconfig_bla0="192.168.87.11"

I found out the hard way, that this defaults to /8 now.

The main symptom was that DNS was /really/ busted, which makes sense
when none of the DNS servers in the 192/8 "swamp" can be reached.

Since we all know that it is always DNS(SEC), I spent a lot of time
having fun with that, before I noticed the /8 netmask on the interface.

I agree that the class A/B/C netmask assumptions should have died long ago.

But from a foot-shooting point of view, it makes no sense to default
192.168/16 to a /8 netmask.

If we're going to default to /8, at the very least ifconfig should
spitting out a very noisy warning and wait 5 seconds before proceeding,
when the netmask is not explicitly specified.

But I also think we can do better than /8.

One option is to go for "limit the damage in RFC1918" and default
them according to their size: reach:

	10/8
	172.16/12
	192.168/16

That will prevent the DNS weirdness I had to figure out, and probably
still DWIM in most cases.

Another option is to default all three to /24, which in my experience
is how people deploy RFC1918.

A third option is to default any missing netmask to /24 instead of /8,
which would be what I would personally have done in the first place.

Poul-Henning

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.