ifconfig alias: same subnet netmask question

[Oleg Cherevko]

Li, Qing wrote:
> First of all, are you encountering any issues ?

Well, for the last 14+ years I used to setup aliases with 0xffffffff netmask and 
everything worked OK. However recently I encountered situation where 
0xffffffff-style alias triggered some unwanted network behavior.

When one sets alias like this:
	ifconfig em0 inet 192.168.1.1 netmask 0xffffff00
	ifconfig em0 inet 192.168.1.2 netmask 0xffffffff alias
and then exports connected networks via OSPF ASE, two prefixes end up being 
exported (192.168.1.1/24 and 192.168.1.2/32).

In case of "identical netmask" setup:
	ifconfig em0 inet 192.168.1.1 netmask 0xffffff00
	ifconfig em0 inet 192.168.1.2 netmask 0xffffff00 alias
only one prefix gets exported (192.168.1.1/24).

In my particular situation two exported prefixes led to wrong behavior of some 
equipment (other than FreeBSD machine in question). When I changed to "identical 
netmask" setup (one exported prefix) everything started to work flawlessly.

So far I encountered no issues with this "identical netmask" setup.
So I'd like to know why ifconfig manpage still recommends old way of setting 
aliases? Perhaps there are some pitfalls that I'm not aware of? Or manpage text 
is simply outdated?

> There is an outstanding issue with the address alias and improper routing
> table update that I am actively working on.
> 
> --Qing
> 
> 
>> -----Original Message-----
>> From: owner-freebsd-net at freebsd.org [mailto:owner-freebsd-
>> net at freebsd.org] On Behalf Of Oleg Cherevko
>> Sent: Wednesday, June 29, 2011 8:11 AM
>> To: freebsd-net at freebsd.org
>> Subject: ifconfig alias: same subnet netmask question
>>
>> Hi All,
>>
>> When describing the "alias" parameter ifconfig manpage claims that "If
>> the address is on the same subnet as the first network address for this
>> interface, a non-conflicting netmask must be given. Usually 0xffffffff
>> is most appropriate."
>>
>> Taking into account that FreeBSD supports aliases from the same subnet
>> with identical netmask for 6+ years now, does this statement still make
>> sense? And what does this "conflicting netmask" stand for (I mean in
>> the
>> context of more or less recent FreeBSD versions, say 8.0+)?
>>
>> Are there any drawbacks in setting aliases like this:
>> 	ifconfig em0 inet 192.168.1.1 netmask 0xffffff00
>> 	ifconfig em0 inet 192.168.1.2 netmask 0xffffff00
>> instead of traditional:
>> 	ifconfig em0 inet 192.168.1.1 netmask 0xffffff00
>> 	ifconfig em0 inet 192.168.1.2 netmask 0xffffffff
>> (again, for more or less recent FreeBSD versions)?
>>
>> --
>> Olwi
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-- 
Olwi