Linux: lxc guest does not get an DHCP address using bridge
Title could also be; lxc networking does not work, using a bridge.
I use a bridge for all my lxc guests (also for docker). This is so that I do not need to setup iptables forwarding to my guests (easy).
But setting up a new Ubuntu 20.04 LTE host gave me some headacke. I could not get my gust to get an ipaddress, and using tcpdump I could see that it did not get an dhcp reply - and looking in the dhcpserver log I could see that nohing arrived there either.
Lots of digging aournd for a good two hours (compaing to another Ubuntu 20.04 LTE), and finally I found the differnce.
On the box where it was workning I had /etc/sysctl.d/99-sysctl-bridge.conf containg: