debug systemd restart networking problem

[Jeremy Ardley <jeremy@ardley.org>]

I have been doing various changes to my network but have now got to the 
stage where I have errors running

systemd restart networking

systemctl status networking
● networking.service - Raise network interfaces
     Loaded: loaded (/lib/systemd/system/networking.service; enabled; 
vendor preset: enabled)
    Drop-In: /etc/systemd/system/networking.service.d
             └─override.conf
     Active: failed (Result: exit-code) since Sat 2022-03-26 20:50:03 
AWST; 13min ago
       Docs: man:interfaces(5)
   Main PID: 1935 (code=exited, status=1/FAILURE)
        CPU: 326m

cat /etc/network/interfaces
source /etc/network/interfaces.d/*

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp

iface eth0 inet6 dhcp
    request_prefix 1
    accept_ra 2

auto lan0

iface lan0 inet static
        address 10.31.40.1/24

iface lan0 inet static
        address 10.31.40.5/24

iface lan0 inet6 static
        address <redacted>::1/64

iface lan0 inet6 static
        address <redacted>::5/64


I check the interfaces using ip a I see all the ipv4 interfaces as 
configured in /etc/network/interfaces but none of the ipv6 ones. I do 
have ipv6 though as lan0 has has an fe80 address

I have tried a lot of things to debug this but no luck. e.g.

systemctl edit networking.service command

[Service]
# remove existing ExecStart rule
ExecStart=
# start ifup with verbose option
ExecStart=/sbin/ifup -av

Any assistance in resolving this problem appreciated.


--
Jeremy

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