NetworkManager creating a new connection ‘eth0’ that does not work, Part 4

Further to my previous post, this is to report the result of another experiment. By doing all the following I can stop NetworkManager creating an invalid second eth0 connection:

  • Enable IPv6 system-wide in /etc/modprobe.d/aliases.conf by commenting-out ‘alias net-pf-10 off‘.
  • Disable use of IPv6 by the Avahi daemon in /etc/avahi/avahi-daemon.conf (see the four additional lines given in my previous post).
  • Use plasma-nm to edit the connection profile for ‘eth0’ that I had already created. Click on the IPv6 tab and ensure ‘Method: Ignored‘ is selected. Click on the IPv4 tab and ensure ‘Method: Automatic‘ is selected and ‘IPv4 is required for this connection‘ is ticked. Ticking ‘IPv4 is required for this connection‘ adds the line ‘may-fail=false‘ in the [ipv4] section in the file /etc/NetworkManager/system-connections/eth0 (the default value for may-fail is ‘true‘ if the box has not been ticked and may-fail has not been assigned in the file).

The various experiments I have conducted are summarised in the following table:

Laptop WiFi switch off off off off off on
IPv6 enabled in aliases.conf yes no yes yes yes yes
IPv6 enabled in avahi-daemon.conf yes yes no no yes yes
[ipv6] method= ignore ignore ignore ignore ignore ignore
[ipv4] method= auto auto auto auto auto auto
[ipv4] may-fail= true true true false false false
Invalid second eth0 created usually no usually no yes yes

As disabling IPv6 system-wide makes it impossible for NetworkManager to use IPv6, the above table can actually be written as follows:

Laptop WiFi switch off off off off off on
IPv6 enabled in aliases.conf yes no yes yes yes yes
IPv6 enabled in avahi-daemon.conf yes yes||no no no yes yes
[ipv6] method= ignore ignore ignore ignore ignore ignore
[ipv4] method= auto auto auto auto auto auto
[ipv4] may-fail= true true||false true false false false
Invalid second eth0 created usually no usually no yes yes

I still think there is a bug in NetworkManager. I would not have expected NetworkManager to create a second eth0 connection and make it an IPv6 Link-Local connection when all the following are true:

  • /etc/NetworkManager.conf has ‘no-auto-default=eth0‘ in the [main] section.
  • IPv4 is required for this connection‘ is not ticked in plasma-nm (i.e. the [ipv4] section in /etc/NetworkManager/system-connections/eth0 contains either the line ‘may-fail=true‘ or the line ‘may-fail=‘).
  • Method: Automatic‘ is selected for IPv4 (‘method=auto‘ under [ipv4] in /etc/NetworkManager/system-connections/eth0).
  • Method: Ignored‘ is selected for IPv6 (‘method=ignore‘ under [ipv6] in /etc/NetworkManager/system-connections/eth0) and the other fields on the IPv6 tab have been rendered unselectable as a result.

Anyway, I will keep IPv6 disabled in /etc/avahi/avahi-daemon.conf and IPv6 enabled system-wide. This seems to be the first thing to try if you’re experiencing the creation of an invalid additional eth0 connection with an IPv6 Link-Local address and you’re sure that none of the net.* services are running.

About Fitzcarraldo
A Linux user with an interest in all things technical.

One Response to NetworkManager creating a new connection ‘eth0’ that does not work, Part 4

  1. Fitzcarraldo says:

    Actually, since making this blog post I have found that, very, very occasionally, an invalid second eth0 connection is created even in the following situation:

    IPv6 enabled in aliases.conf: yes
    IPv6 enabled in avahi-daemon.conf: no
    [ipv6] method=ignore
    [ipv4] method=auto
    [ipv4] may-fail=false

    So it seems the only sure-fire way of stopping NetworkManager from creating an invalid second eth0 connection is to disable IPv6 system-wide.

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