Desired behaviour of "ifconfig -alias"

Oliver Fromme olli at
Mon Feb 12 14:26:26 UTC 2007

JoaoBR <joao at> wrote:
 > Brooks Davis wrote:
 > > Jeremy Chadwick wrote:
 > > > Kevin Way wrote:
 > > > > I recently ran into a bug in the jail startup scripts that caused this
 > > > > command to be executed:
 > > > > 
 > > > >     ifconfig bce0 -alias
 > > > > 
 > > > > It turns out that this command eliminated the primary IP for the
 > > > > device.
 > > > > 
 > ....
 > > 
 > > It's way to late to make this change.  This is known behavior and has
 > > been for ages.  If there's a bug it's in the documentation.
 > wellwell, we also were apes for ages but does not mean that we stay behaving
 > like them  and if some still does so it is also never to late to change
 > that  ;)

Changing the behaviour of tools always involves a certain
danegr of breaking existing script.  That's especially true
for symstem administration commands such as ifconfig that
are running in automated scripts, and people depend on them
for booting their machines remotely.

I'm not saying that people are intentionally using that
syntax ...  Maybe they are, maybe not.  But you also should
take into accounts that there might be scripts that use the
syntax inadvertantly and happen to work correctly because
of the current behaviour.

I'm also _not_ saying that the behaviour must not be changed
at all.  But it should be done carefully, i.e. first to
-current, with proper "heads up" warnings.  Don't change
it in RELENG_6 without warning and expect evrybody to be

 > "ifconfig nic -alias" is obviously a wired and confusing behaviour

It might be confusing to you.  Personally I think that the
current behaviour isn't that far off.

First you need to be aware that there is no distinction
between a primary IP and alias IPs (such a distinction
existed historically, but it's gone).  All of the IPs on
an interface are equal, just like hardlinks on a file.
Maybe the manual page should be clearer about that.

The "-alias" parameter simply removes an address from an
interface.  The term "alias" should really be avoided
because it is misleading.  You can use "delete" or "remove"
which do the same thing.  I think "-alias" should really
be regarded to exist for backwards compatibility only.
Personally I always use "delete".

If no IP address is specified, then it's not completely
nonsensical to remove the first address.  In fact I've
used that short-cut to quickly remove the only address
from an interface.  I've used "ifconfig xyz0 delete"
quite a lot.

 > then already beeing here there is more, ifconfig nic alias does not return 
 > anything at all

That's the same as "ifconfig nic add".  If you don't tell
it anything to add, then it won't add anything, of course.
Remember that UNIX always tries to do exactly what you
tell it to dao.  ;-)

 > and ifconfig nic -alias on a nic w/o ip returns "can't assign 
 > requested address" ...

That error corresponds to EADDRNOTAVAIL, which is the
correct errno to return, because there's no address left
on the interface.  However, I agree that the message is
a bit confusing to the unfamiliar.

Best regards

Oliver Fromme, secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing b. M.
Handelsregister: Registergericht Muenchen, HRA 74606, USt-Id: DE204219783
Any opinions expressed in this message are personal to the author and may
not necessarily reflect the opinions of secnetix GmbH & Co KG in any way.
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