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From: jw@sics.UUCP
Newsgroups: comp.bugs.4bsd
Subject: On the importance of a correctly specified netmask
Message-ID: <1300@sics.UUCP>
Date: Thu, 28-May-87 11:15:07 EDT
Article-I.D.: sics.1300
Posted: Thu May 28 11:15:07 1987
Date-Received: Sat, 30-May-87 11:04:08 EDT
Reply-To: jw@sics.UUCP (Johan Widen)
Distribution: world
Organization: Swedish Institute of Computer Science, Kista
Lines: 40

Index: /etc/ifconfig BSD4.3 and some BSD4.2

When I converted from BSD4.2 to BSD4.3 I made the following mistake.
I did not know a lot about subnets, and we did not feel a need for them,
so I decided to skip all that for the time being. I hoped (foolish me)
that /etc/rc.local from BSD4.2 would work without much change. It almost
did except for the ifconfig lines.

I had read the manuals so I knew that the format of the ifconfig command
had changed. The problem was that ifconfig had a lot of new options which
I did not know how to set appropriately. My rule: if you do not understand
an option, do not specify that option. Programs should provide good
default values.

This brings us to the netmask option. The netmask option specifies
how much of the address to reserve for subdividing networks into
sub-networks. The netmask is used by the network interface to recognise
broadcast packets. It turns out that the vax ifconfig does not set
a default netmask. Ifconfig assumes that this is done in the driver.

The DEUNA driver does not set a reasonable default netmask. If you fail
to specify a mask then the interface will become promiscuous and receive
a lot af packets not intended for the machine. The OS will process these
packets and answer several of them!

The vax became severely loaded from time to time. More seriously, our Sun
clients had trouble starting up NFS.

It was fairly difficult to find the cause of the problem. The Sun clients
did not have the netmask specified either, but the Sun ifconfig appears
to set a reasonable default value.

To sum up: set the netmask explicitly when you use ifconfig.


-- 
Johan Widen
Swedish Institute of Computer Science
PO Box 1263, S-163 13 SPANGA, SWEDEN
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