link-local fallback

["Michel, Matthias" <matthias michel siemens com>]

Hi
I'm using the dhcp=systemd setting in the networkmanager v1.30.2.
I have a requirement to switch to a fallback link-local when the dhcp
is not available and to return from link-local to dhcp when it is back.

What I was able to configure was the fallback to link-local. But
switching to dhcp automatically was not possible.

my connections with networkmanager:

cat br-lan.nmconnection 
[connection]
id=br-lan
uuid=0d924e20-a7ca-4eec-9ad1-28b6d2ddb0c2
autoconnect-priority=100
autoconnect-retries=1
interface-name=br-lan
llmnr=2
permissions=
zone=lan

[bridge]
stp=false

[match]
kernel-command-line=!test;

[ipv4]
dhcp-timeout=3
dns-search=
may-fail=false
method=auto
route1=224.0.0.0/4,0.0.0.0,0

[ipv6]
addr-gen-mode=stable-privacy
dns-search=
method=auto
[proxy]


cat br-lan-ll.nmconnection 
[connection]
id=br-lan-ll
uuid=1d924e20-a7ca-4eec-9ad1-28b6d2ddb0c2
type=bridge
autoconnect=true
autoconnect-priority=50
interface-name=br-lan
llmnr=2
permissions=
timestamp=1617958884
zone=lan

[bridge]
stp=false

[match]
kernel-command-line=!test;

[ipv4]
dns-search=
method=link-local
route1=224.0.0.0/4,0.0.0.0,0

[ipv6]
addr-gen-mode=stable-privacy
dns-search=
method=disabled

[proxy]


my connection with networkd:

cat /etc/systemd/network/lan.network 
[Match]
Name=enp44s0u2u1

[Network]
Bridge=br0

cat /etc/systemd/network/br0.netdev 
[NetDev]
Name=br0
Kind=bridge

cat /etc/systemd/network/bridge.network 
[Match]
Name=br0

[Network]
DHCP=ipv4
LinkLocalAddressing=fallback

[DHCPv4]
MaxAttempts=1


I studied the code of networkmanager and networkd.

networkd:
https://github.com/systemd/systemd/blob/3a1e9d8083b83f611a23cc84ba69a8175e659629/src/network/networkd-dhcp4.c#L1128


networkmanager:
https://github.com/NetworkManager/NetworkManager/blob/af360238be198257934b89d42f24431401550721/src/core/dhcp/nm-dhcp-systemd.c#L495

Is there a design reason why this behavior is implemented differently?

Is it mainly a configuration error on my part?

Would the community accept changes to get the same behavior for
networkmanager as is the case in networkd?

Best regards
Matthias Michel