Comment # 3 on bug 1003886 from Takashi Iwai

I guess these are entries in /usr/lib/sysctl.d/50-default.conf from aaa_base:

# Disable NAT'ing on Linuxbridges (bnc#845496)
net.bridge.bridge-nf-call-ip6tables = 0
net.bridge.bridge-nf-call-iptables = 0