Running Ubuntu core.
Both Wifi and Ethernet connections work initially but much later ssh or ping from other hosts on the LAN don't work.

I can connect to the web for surfing, updates etc.

I can ping hosts on the LAN from the M3 on the Ethernet but the M3 is unreachable from other hosts. M3 --> other hosts on the LAN via wifi unreachable.

Although Ethernet pings look OK, tcpdump on the other hosts don't show any ping requests and arp only shows the firewall box.

root@NanoPi-M3:# arp
Address HWtype HWaddress Flags Mask Iface
ipfirewall.localdomain ether 90:2b:34:57:c5:5b C eth0
ipfirewall.localdomain ether 90:2b:34:57:c5:5b C eth0

After a reboot all connections are restored only much later does it exhibit this problem.

The link to ipfirewall is always OK and I can ssh into ipfirewall all the time.

ipfirewall is running IPFire x86, other x86 hosts running openSUSE Tumbleweed and Kubuntu 17.04 and ARM boards running Debian and Ubuntu 16.04 aarch64.