Hi!

I have just been tearing out my hair about this one, on ubuntu though.
I've checked the source and it's the same error.

Without network-manager loopback isn't brought up on boot by ifupdown scripts.

Copying bug report from ubuntu: https://bugs.launchpad.net/ubuntu/+source/ifupdown/+bug/1375821

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Upon booting a machine with the following setup on a relatively slimmed down netboot machine the loopback interface isn't brought up properly:

cat /etc/network/interfaces
auto lo
iface lo inet loopback
iface lo inet6 loopback

auto eth0
iface eth0 inet6 auto
dhcp 1
#iface eth0 inet ipv4ll

auto eth1
iface eth1 inet6 auto
dhcp 1
#iface eth1 inet ipv4ll

Upon boot Loopback is unconfigured:
root@<hostname>:~# ip addr show dev lo
1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN group default
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00

Try to bring the interface up:
root@<hostname>:~# ifup -v lo
ifup: interface lo already configured

Bring it down and up:
root@<hostname>:~# ifdown -v lo
Configuring interface lo=lo (inet)
run-parts --verbose /etc/network/if-down.d
run-parts: executing /etc/network/if-down.d/avahi-autoipd
run-parts: executing /etc/network/if-down.d/upstart
run-parts --verbose /etc/network/if-post-down.d
Configuring interface lo=lo (inet)
run-parts --verbose /etc/network/if-down.d
run-parts: executing /etc/network/if-down.d/avahi-autoipd
run-parts: executing /etc/network/if-down.d/upstart
run-parts --verbose /etc/network/if-post-down.d
Configuring interface lo=lo (inet6)
run-parts --verbose /etc/network/if-down.d
run-parts: executing /etc/network/if-down.d/avahi-autoipd
run-parts: executing /etc/network/if-down.d/upstart
run-parts --verbose /etc/network/if-post-down.d

root@<hostname>:~# ifup -v lo
Configuring interface lo=lo (inet)
run-parts --verbose /etc/network/if-pre-up.d
run-parts --verbose /etc/network/if-up.d
run-parts: executing /etc/network/if-up.d/avahi-autoipd
run-parts: executing /etc/network/if-up.d/openssh-server
run-parts: executing /etc/network/if-up.d/upstart
Configuring interface lo=lo (inet)
run-parts --verbose /etc/network/if-pre-up.d
run-parts --verbose /etc/network/if-up.d
run-parts: executing /etc/network/if-up.d/avahi-autoipd
run-parts: executing /etc/network/if-up.d/openssh-server
run-parts: executing /etc/network/if-up.d/upstart
Configuring interface lo=lo (inet6)
run-parts --verbose /etc/network/if-pre-up.d
run-parts --verbose /etc/network/if-up.d
run-parts: executing /etc/network/if-up.d/avahi-autoipd
run-parts: executing /etc/network/if-up.d/openssh-server
run-parts: executing /etc/network/if-up.d/upstart

no change:
root@<hostname>:~# ip addr show dev lo
1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN group default
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00

bring it down again:
root@<hostname>:~# ifdown lo

And up, but this time with --no-loopback:
root@<hostname>:~# ifup -v --no-loopback lo
Configuring interface lo=lo (inet)
run-parts --verbose /etc/network/if-pre-up.d
ip link set dev lo up
run-parts --verbose /etc/network/if-up.d
run-parts: executing /etc/network/if-up.d/avahi-autoipd
run-parts: executing /etc/network/if-up.d/openssh-server
run-parts: executing /etc/network/if-up.d/upstart
Configuring interface lo=lo (inet6)
run-parts --verbose /etc/network/if-pre-up.d
ip link set dev lo up 2>/dev/null
ip addr add dev lo ::1 2>/dev/null
run-parts --verbose /etc/network/if-up.d
run-parts: executing /etc/network/if-up.d/avahi-autoipd
run-parts: executing /etc/network/if-up.d/openssh-server
run-parts: executing /etc/network/if-up.d/upstart

now it seems like something actually happened!

check:
root@<hostname>:~# ip addr show dev lo
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever

All seems fine, everything is working from here on.

My expectation is that ifup should configure the interface and set state to up, and ifdown should set state to down, even though it's a loopback that may or may not be magically autoconfigured somewhere else.

And a question:
If the case is that ifup shouldn't configure the loopback, what should configure it?

----

other interfaces (eth*) are configured as expected upon boot:

root@<hostname>:~# ip addr show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 00:9c:02:ab:cb:d8 brd ff:ff:ff:ff:ff:ff
    inet6 my:ipv6:address:1/64 scope global temporary dynamic
       valid_lft 603348sec preferred_lft 84348sec
    inet6 my:ipv6:address:2/64 scope global dynamic
       valid_lft 2591995sec preferred_lft 604795sec
    inet6 fe80::29c:2ff:feab:cbd8/64 scope link
       valid_lft forever preferred_lft forever
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 00:9c:02:ab:cb:d9 brd ff:ff:ff:ff:ff:ff
    inet6 my:ipv6:address:3/64 scope global temporary dynamic
       valid_lft 603348sec preferred_lft 84348sec
    inet6 my:ipv6:address:4/64 scope global dynamic
       valid_lft 2591995sec preferred_lft 604795sec
    inet6 fe80::29c:2ff:feab:cbd9/64 scope link
       valid_lft forever preferred_lft forever


It also seems like ifup is actively not configuring loopback twice, but running all scripts for it..

loopback should be up properly on both ipv4 and ipv6.

I've subscribed and i'll gladly help if anything is missing to get this resolved.


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