Comment # 2 on bug 959707 from Frerk Meyer

I got the same problem (using IPv6 local network).

I have a standard install of Leap 42.1 and since my internet connection is IPv6
based (cable modem) the internal network is IPv6 based too.

If I connect my network cable, dmesg says:
[ 9874.060239] e1000e: eth0 NIC Link is Down
[ 9885.981017] e1000e: eth0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control:
Rx/Tx
[ 9888.124224] SFW2-INext-DROP-DEFLT IN=eth0 OUT= MAC=
SRC=fe80:0000:0000:0000:0222:68ff:fe11:03f7
DST=ff02:0000:0000:0000:0000:0000:0000:00fb LEN=406 TC=0 HOPLIMIT=255 FLOWLBL=0
PROTO=UDP SPT=5353 DPT=5353 LEN=366 
[ 9888.131894] SFW2-INext-DROP-DEFLT IN=eth0 OUT= MAC=
SRC=fe80:0000:0000:0000:0222:68ff:fe11:03f7
DST=ff02:0000:0000:0000:0000:0000:0000:00fb LEN=240 TC=0 HOPLIMIT=255 FLOWLBL=0
PROTO=UDP SPT=5353 DPT=5353 LEN=200 
[ 9888.375013] SFW2-INext-DROP-DEFLT IN=eth0 OUT= MAC=
SRC=fe80:0000:0000:0000:0222:68ff:fe11:03f7
DST=ff02:0000:0000:0000:0000:0000:0000:00fb LEN=406 TC=0 HOPLIMIT=255 FLOWLBL=0
PROTO=UDP SPT=5353 DPT=5353 LEN=366 
[ 9888.625598] SFW2-INext-DROP-DEFLT IN=eth0 OUT= MAC=
SRC=fe80:0000:0000:0000:0222:68ff:fe11:03f7
DST=ff02:0000:0000:0000:0000:0000:0000:00fb LEN=406 TC=0 HOPLIMIT=255 FLOWLBL=0
PROTO=UDP SPT=5353 DPT=5353 LEN=366 
[ 9888.826124] SFW2-INext-DROP-DEFLT IN=eth0 OUT= MAC=
SRC=fe80:0000:0000:0000:0222:68ff:fe11:03f7
DST=ff02:0000:0000:0000:0000:0000:0000:00fb LEN=376 TC=0 HOPLIMIT=255 FLOWLBL=0
PROTO=UDP SPT=5353 DPT=5353 LEN=336 

The journal (kernel log) says:
Feb 04 08:33:40 moby.fritz.box nm-dispatcher[22019]: Dispatching action 'up'
for eth0
Feb 04 08:33:40 moby.fritz.box automount[22152]: lookup_init:136: lookup(yp):
map auto.master: Local domain name not set
Feb 04 08:33:42 moby.fritz.box avahi-daemon[865]: Joining mDNS multicast group
on interface eth0.IPv6 with address fe80::222:68ff:fe11:3f7.
Feb 04 08:33:42 moby.fritz.box avahi-daemon[865]: New relevant interface
eth0.IPv6 for mDNS.
Feb 04 08:33:42 moby.fritz.box avahi-daemon[865]: Registering new address
record for fe80::222:68ff:fe11:3f7 on eth0.*.
Feb 04 08:33:42 moby.fritz.box NetworkManager[874]: <info>  Activation (eth0)
Beginning DHCPv6 transaction (timeout in 45 seconds)
Feb 04 08:33:42 moby.fritz.box NetworkManager[874]: <info>  dhclient started
with pid 22163
Feb 04 08:33:42 moby.fritz.box kernel: SFW2-INext-DROP-DEFLT IN=eth0 OUT= MAC=
SRC=fe80:0000:0000:0000:0222:68ff:fe11:03f7
DST=ff02:0000:0000:0000:0000:0000:0000:00fb LEN=406 TC=0 HOPLIMIT=255 FLOWLBL=0
Feb 04 08:33:42 moby.fritz.box kernel: SFW2-INext-DROP-DEFLT IN=eth0 OUT= MAC=
SRC=fe80:0000:0000:0000:0222:68ff:fe11:03f7
DST=ff02:0000:0000:0000:0000:0000:0000:00fb LEN=240 TC=0 HOPLIMIT=255 FLOWLBL=0
Feb 04 08:33:42 moby.fritz.box kernel: SFW2-INext-DROP-DEFLT IN=eth0 OUT= MAC=
SRC=fe80:0000:0000:0000:0222:68ff:fe11:03f7
DST=ff02:0000:0000:0000:0000:0000:0000:00fb LEN=406 TC=0 HOPLIMIT=255 FLOWLBL=0
Feb 04 08:33:42 moby.fritz.box NetworkManager[874]: <info>    nameserver
'fd00::ca0e:14ff:fe83:8d05'
Feb 04 08:33:42 moby.fritz.box NetworkManager[874]: <info>  (eth0): DHCPv6
state changed unknown -> bound
Feb 04 08:33:42 moby.fritz.box NetworkManager[874]: <info>  Policy set
'Zuhause' (eth0) as default for IPv6 routing and DNS.
Feb 04 08:33:42 moby.fritz.box kernel: SFW2-INext-DROP-DEFLT IN=eth0 OUT= MAC=
SRC=fe80:0000:0000:0000:0222:68ff:fe11:03f7
DST=ff02:0000:0000:0000:0000:0000:0000:00fb LEN=406 TC=0 HOPLIMIT=255 FLOWLBL=0
Feb 04 08:33:42 moby.fritz.box NetworkManager[874]: <info>  (eth0): DHCPv6
client pid 22163 exited with status 0
Feb 04 08:33:42 moby.fritz.box NetworkManager[874]: <info>  (eth0): DHCPv6
state changed bound -> done
Feb 04 08:33:42 moby.fritz.box nm-dispatcher[22019]: Dispatching action
'dhcp6-change' for eth0
Feb 04 08:33:42 moby.fritz.box kernel: SFW2-INext-DROP-DEFLT IN=eth0 OUT= MAC=
SRC=fe80:0000:0000:0000:0222:68ff:fe11:03f7
DST=ff02:0000:0000:0000:0000:0000:0000:00fb LEN=376 TC=0 HOPLIMIT=255 FLOWLBL=0
Feb 04 08:33:43 moby.fritz.box avahi-daemon[865]: Leaving mDNS multicast group
on interface eth0.IPv6 with address fe80::222:68ff:fe11:3f7.
Feb 04 08:33:43 moby.fritz.box avahi-daemon[865]: Joining mDNS multicast group
on interface eth0.IPv6 with address 2a02:8108:41c0:1fbc:222:68ff:fe11:3f7.
Feb 04 08:33:43 moby.fritz.box avahi-daemon[865]: Registering new address
record for 2a02:8108:41c0:1fbc:222:68ff:fe11:3f7 on eth0.*.
Feb 04 08:33:43 moby.fritz.box avahi-daemon[865]: Withdrawing address record
for fe80::222:68ff:fe11:3f7 on eth0.
Feb 04 08:35:10 moby.fritz.box kernel: nf_conntrack: automatic helper
assignment is deprecated and it will be removed soon. Use the iptables CT
target to attach helpers instead.
lines

So Avahi has a problems with the firewall rules.