Subject: Re: default route and private networks
To: Bill Studenmund <>
From: Jason Thorpe <>
List: tech-net
Date: 04/25/2005 16:47:12
On Apr 25, 2005, at 12:36 PM, Bill Studenmund wrote:

>> Tho' I dont know if it works anymore on NetBD, with hashed lookup of
>> local-IP-addrs it certainly does on other BSD derivatves. If it truly
>> isn't predictable on NetBSD, then that strikes me as a darn good
>> reason to distinguish first-class local addresses from
>> explicitly-marked secondary or "alias" addresses.
> And what if we want multiple "first-class" addresses?

And to address this point... nothing in an ifaddr explicitly marks  
the address as an "alias".  It is an alias only by virtue of not  
being the first on the list.  If you delete the first address on the  
list, then I am pretty sure that the next one is suddenly no longer  
just an "alias".

I.e. it is only by convention that these things are called  
"aliases".  It's all due to undocumented magic that the semantics are  
the way they are.

To me, an address that is truly an "alias" would never be used as a  
source address unless it is on the passive side of a TCP handshake.   
But there are legitimate reasons for having multiple "non-alias" IPv4  
addresses on an interface.  If we want to support both, then  
something needs to mark those alias addresses as such (I would call  
them "passive" addresses, myself).  The "passive" semantics I  
envision would map equally well to both IPv4 and IPv6, or any other  
address family, for that matter.

-- thorpej