ifconfig alias: same subnet netmask question

[Oleg Cherevko]

Darn, I have to correct myself once again.

Oleg Cherevko wrote:
> 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).

The above two prefixes should read "(192.168.1.0/24 and 192.168.1.2/32)", of course.

> 
> 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).

This one should read "(192.168.1.0/24)" as well.

> 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 Or manpage text is simply outdated?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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>>> http://lists.freebsd.org/mailman/listinfo/freebsd-net
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> 
> 


-- 
Olwi