[Bug 1828749] Re: ifconfig dummy0 : Device not found

[John Denker]

Msg 1 of 2:  Here is a use-case for ifup dummy0 and ifdown dummy0:

I have multiple laptops.  Sally has multiple laptops.  Sometimes they
are wired (for speed) and sometimes wireless (for portability).
Sometimes we are at the same workplace, or at my abode, or her abode, or
the local library, or the local cantina.  Sometimes I am at some
unfamiliar place, tasked with figuring out what's wrong with the local
computer setup.

That means many different DHCP servers, most of which I do not control.
They do not all perform dynamic DNS in any reasonable way.  It would of
course be elegant to set up dynamic DNS, but that would be a lot of work
in the best case, and would not be reliable in corner cases.

I find it advantageous to bring up a dummy interface with a static
address, and match that with a static entry in /etc/hosts.  That
simplifies the task of contacting this-or-that machine.  No routing is
required, because the interface will ARP the dummy address as well as
the DHCP-assigned address.

See next message for test and usage details.

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https://bugs.launchpad.net/bugs/1828749

Title:
  ifconfig dummy0 : Device not found

Status in ifupdown package in Ubuntu:
  Incomplete
Status in net-tools package in Ubuntu:
  New

Bug description:
  Desired behavior:
    The ifconfig command should be able to deal with the
    dummy device.  This worked fine until recently.

  Observed behavior:
    :; ifconfig dummy0
    dummy0: error fetching interface information: Device not found

    This problem appeared when I upgraded to bionic.

  Highly informative workaround:
    :; ip link add dummy0 type dummy
    That command works, and makes the problem go away permanently.
    The ifconfig command works fine after that.
    The ifup and ifdown commands also work fine after that.

    For convenient debugging, you can use the command:
    :; ip link del dummy0 type dummy
    which makes the problem come back.
    You can also experiment with dummy1 et cetera.

  Package ownership issues:
    Compare: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=909204
    That report was filed against ippusbxd, which is almost certainly
    not the relevant package.

    For that matter, I have no idea whether the root cause is in the
    net-tools package or the kernel networking stack.  All I know is
    the ip command plays nicely with the kernel while the ifconfig
    command does not.

  
  Notes:
    The kernel module for the dummy interface is preloaded in
    all situations described here.  That's not the issue.

    An apport file is attached, to describe the environment.
    Also, since you asked:

    :; apt-cache policy net-tools
  net-tools:
    Installed: 1.60-26ubuntu1
    Candidate: 1.60-26ubuntu1
    Version table:
   *** 1.60-26ubuntu1 500
          500 http://ubuntu.cs.utah.edu/ubuntu xenial/main amd64 Packages
          100 /var/lib/dpkg/status

    :; lsb_release -rd
    Description:    Ubuntu 16.04.6 LTS
    Release:        16.04

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