Commenting on some of the comments in Bug #123773.

In Bug #123773
Jamie Strandboge wrote:
> ...though ifup (and ifdown) will give that error, the interface
> comes up fine, and is pingable

In this case the interface doesn't come up.


In comment
https://bugs.launchpad.net/ubuntu/+source/ifupdown/+bug/123773/comments/11
behemot wrote:
> ...that removing package wireless-tools...helps.

Removing, moving, or disabling the execute bits on /etc/network/if-pre-up.d/wireless-tools seems to have no impact on this problem.


In comment
https://bugs.launchpad.net/ubuntu/+source/ifupdown/+bug/123773/comments/12
Alessandro Lo Forte wrote:
> I have located the problem in the file /etc/if-pre-up.d/wireless-tools...
> In fact the error code is generated when /sbin/ifconfig is invoked to
> bring up an alias (like eth0:0) interface.

This suggests both problems likely have a common underlying cause. My research shows that /sbin/ifconfig, being invoked by dhclient, is also the culprit here. (See comments below.)

This may also mean that this bug should be reclassified as being against the net-tools package instead of dhcp3.


In comment
https://bugs.launchpad.net/ubuntu/+source/ifupdown/+bug/123773/comments/22
Juuso Tähkäpää wrote:
> sudo ifconfig eth0:1 1.2.3.4
> dhcpcd -I someid eth0:1
> ...but still ifconfig shows no eth0:1. 
> Manually assigned aliases (ifconfig eth0:2 $ip) do show up
> in ifconfig listings just as they should.

Although I don't have dhcpcd installed, I've observed similar results with dhclient, where it fails to configure the interface, but invoking ifconfig directly with a static address works.

This gives some hope for a possible workaround hack. I'm thinking in /etc/network/interfaces one could invoke a script using the 'up' argument for the main (real) interface, which then would invoke dhclient in some fashion or maybe using a simpler DHCP client like udhcpc that returns an IP address to a shell variable, and then invokes ifconfig, passing the address. It would have limitations, like requiring perpetual leases (I think, as there would be no process left running to handle an expired lease).


In comment
https://bugs.launchpad.net/ubuntu/+source/ifupdown/+bug/123773/comments/17
gumptravels wrote:
> ...makes me think that you may not be able use dhcp on virtual interfaces
> since the mac address would be the same as all the other virtual interfaces
> on that same parent, and the same as the parent itself.

While a MAC address is often used as the default way to identify a DHCP client, the DHCP client can optionally supply other identifying information. For example, this theoretically correct (untested, due to this bug) stanza from /etc/network/interfaces specifies that when the DHCP client requests an address for the virtual interface that it identifies itself with the alternate host name, "indianpoint."

auto eth0:1
iface eth0:1 inet dhcp
  hostname indianpoint

Being able to use DHCP on virtual interfaces is highly desirable in order to keep maintenance to a minimum, especially if you are using an integrated DHCP/DNS service, like Dnsmasq. In that case the act of requesting an address with the alternate host name will automatically generate a corresponding DNS A-record for that hast name. Thus DNS requires far less central administration.

 -Tom