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

[Christian Ehrhardt ]

Hi,
I come by trying to clear old bugs that were dormant for too long either resolving or reviving them.

First of all I can confirm what you have found (insmod vs modprobe
changes), but I also found why.


I've found that this makes modprobe work like before:
  modprobe dummy numdummies=1

So I wondered if only the default config changed.

modprobe uses config files and it picks up defaults from
/lib/modprobe.d/ and /etc/modprobe.d/.

And in there I found:
root at b:~# cat /lib/modprobe.d/systemd.conf
...
# When bonding module is loaded, it creates bond0 by default due to max_bonds
# option default value 1. This interferes with the network configuration
# management / networkd, as it is not possible to detect whether this bond0 was
# intentionally configured by the user, or should be managed by
# networkd/NM/etc. Therefore disable bond0 creation.
options bonding max_bonds=0
# Do the same for dummy0.
options dummy numdummies=0


That is the reason the new default number of dummies is zero when using modprobe.
You can change via a config file or pass numdummies=1 to modprobe to resolve that.

The default value was discussed upstream (denied) and the same problem but from a "where to configure" POV in Ubuntu (see bug 1937953).
Following that bug 1937953 and marking this one invalid as I think it is explained and not something that will be fixed/changed in the package.
That bug also has some hints on how to overwrite that default config as there are some intricacies in regard to "which overwrites which" for these conffiles.

** Changed in: net-tools (Ubuntu)
       Status: New => Invalid

** Changed in: ifupdown (Ubuntu)
       Status: New => Invalid

** Also affects: kmod (Ubuntu)
   Importance: Undecided
       Status: New

** Also affects: systemd (Ubuntu)
   Importance: Undecided
       Status: New

** Changed in: kmod (Ubuntu)
       Status: New => Invalid

** Changed in: systemd (Ubuntu)
       Status: New => Invalid

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

Title:
  ifconfig dummy0 : Device not found

Status in ifupdown package in Ubuntu:
  Invalid
Status in kmod package in Ubuntu:
  Invalid
Status in net-tools package in Ubuntu:
  Invalid
Status in systemd package in Ubuntu:
  Invalid

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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