systemctl stop networking hang / timeout

it does reproduce after that with simply:
% systemctl start networking
% systemctl stop networking

does not seem to be lxc specific. Ii reproduced on a cloud-image also.

Maybe that is actually related to the dhclient problem. I wonder, if you kill that manually before the ifdown (kill -11) does get rid of the long delay?

Stefan, yeah. kill -11 "fix"es
root@test-x1:~# systemctl start networking
root@test-x1:~# pidof dhclient
1332
root@test-x1:~# kill -11 $(pidof dhclient)
root@test-x1:~# time systemctl stop networking

real 0m0.212s
user 0m0.008s
sys 0m0.000s

Have no idea if related to here but just recently on plain old Ubuntu Desktop installs of 16.04 restarts have slowed done considerably.
For me about 4-5 times slower, actual numbers are 2-3 sec. normally, now 10-15 secs. Others seemed affected with different numbers.
If I disconnect from Network, ( ethernet) prior to a restart then expected 2-3 sec. returns

That sounds like it is related. While doing more debug I found that network-manager starts dhclient in forground mode (-d) and that is not affected by bug 1551351. But regardless of how it was started, dhclient seem not to react on SIGHUP. Maybe disconnecting from the network manually uses a more aggressive method of stopping the dhclient than just doing a shutdown or reboot. The system probably goes into the mode of first trying to send SIGHUP to running processes, then wait a bit and finally doing a SIGTERM. That would explain the increased time.

how do I debug this?

if i manually run 'ifup -a --read-environment' or 'systemctl start networking' it brings all stuff up cleanly.
by 'debug' i mean more than just looking at journalctl -u networking

below contents of network/interfaces, journalctl -u networking and contents of netwroking unit

---------

[Unit]
Description=Raise network interfaces
Documentation=man:interfaces(5)
DefaultDependencies=no
Wants=network.target
After=local-fs.target network-pre.target apparmor.service systemd-sysctl.service systemd-modules-load.service
Before=network.target shutdown.target network-online.target
Conflicts=shutdown.target

[Install]
WantedBy=multi-user.target
WantedBy=network-online.target

[Service]
Type=oneshot
EnvironmentFile=-/etc/default/networking
ExecStartPre=-/bin/sh -c '[ "$CONFIGURE_INTERFACES" != "no" ] && [ -n "$(ifquery --read-environment --list --exclude=lo)" ] && udevadm settle'
ExecStart=/sbin/ifup -a --read-environment
ExecStop=/sbin/ifdown -a --read-environment --exclude=lo
RemainAfterExit=true
TimeoutStartSec=3sec #modified from default which just made the machine hang on boot until timeout

------

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
#allow-hotplug eno1
#allow-hotplug ipsec0
auto eno1
auto ipsec0

iface eno1 inet manual
        up ip link set eno1 up

iface ipsec0 inet static
        bridge_ports eno1
        address 192.168.15.113
        netmask 255.255.255.0
        gateway 192.168.15.254
        # dns-* options are implemented by the resolvconf package, if installed
        dns-nameservers 8.8.8.8
        #link-speed 1000
        #link-duplex full
        #ethernet-autoneg off
        #dns-search project.local
        #up dnsmasq -s lan -S //192.168.15.113 -u root --strict-order --pid-file=/run/dnsmasq-lan.pid --dhcp-no-override --except-interface=lo --interface=ipsec0 --dhcp-leasefile=/var/lib/dnsmasq.lan.leases --dhcp-authoritative --listen-address 192.168.15.113 --dhcp-range 192.168.15.101,192.168.15.199 --dhcp-lease-max=252 --bind-interfaces --dhcp-option-force=option:router,192.168.15.254 --dhcp-option-force=option:ntp-server,192.168.15.113
        up dnsmasq -s project -S //192.168.15.113 -u root --strict-order --pid-file=/run/dnsmasq-project.pid --dhcp-no-override --except-interface=lo --interface=ipsec0 --dhcp-leasefile=/var/lib/dnsmasq.project.leases --dhcp-authoritative --listen-address 192.168.15.113 --dhcp-range 192.168.15.101,192.168.15.199 --dhcp-lease-max=252 --bind-interfaces --dhcp-option-force=option:router,192.168.15.254 --dhcp-option-force=option:ntp-server,192.168.15.113
        down kill $(cat /run/dnsmasq-project.pid)

#allow-hotplug eno2
auto eno2
iface eno2 inet static
        address 192.168.100.100
        netmask 255.255.255.0
        #link-speed 100
        #link-duplex full
        #ethernet-autoneg off
        #gateway
        # dns-* options are implemented by the resolvconf package, if installed
        #dns-nameservers 10.10.200.10
        #dns-search proj...

this is workaround for me that runs correctly:

in rc.local:

(systemctl stop networking.service; systemctl start networking.service; systemctl restart networking.service; true) &

if i put the commands one by one, rc.local unit will fail because systemctl start exits non-zero. notice the 'restart' action for the second command.