[Bug 1375821] [NEW] ifupdown doesn't bring loopback to state up

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You have been subscribed to a public bug by Robie Basak (racb):

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?

** Affects: ifupdown (Ubuntu)
     Importance: Undecided
         Status: Fix Released

** Affects: mailman (Ubuntu)
     Importance: Undecided
         Status: Invalid

** Affects: ifupdown (Ubuntu Trusty)
     Importance: Undecided
         Status: Triaged

** Affects: mailman (Ubuntu Trusty)
     Importance: Undecided
         Status: Invalid


** Tags: sts trusty
-- 
ifupdown doesn't bring loopback to state up
https://bugs.launchpad.net/bugs/1375821
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