alias netmask bug?

[Tom Judge]

Jeremy Chadwick wrote:
> On Tue, Jul 24, 2007 at 03:34:54AM +0300, Evren Yurtesen wrote:
>>  I remember from earlier versions of FreeBSD that it had a restriction about 
>>  alias IP netmasks (somewhere in 3.x,4.x days)...
>>
>>  as explained here:
>>  http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/configtuning-virtual-hosts.html
>>
>>  However I tried on the loopback interface to add a 2nd IP with the same 
>>  netmask as the original IP and it accepts it fine. (on 6.2)
>>
>>  lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
>>          inet 127.0.0.1 netmask 0xff000000
>>          inet 127.0.0.2 netmask 0xff000000
>>
>>  Is FreeBSD allowing this a bug or ???
> 
> It's not a bug.  Why did you disregard the instructions, though?
> 
>>  If this is not a bug. What happens if an interface which is connecting to 
>>  the machine has 2 IP addresses with same netmask which is not 
>>  255.255.255.255? Does FreeBSD use each IP randomly?
> 
> Don't know, but my guess is no, it probably does not use "each IP
> randomly", and I cannot even fathom the network breakage that would
> ensue as a result of such.
> 

I have seen systems that have been configured like this (on physical 
interfaces) have some strange problems.  I'm sure one box that got a 
config like this by accident kicked one of the IP's off the interface 
several hours after it was set up.  Never been able to reproduce that 
though.

Tom