IPv6 with firewalld

If you are using firewalld and want IPv6 to work correctly with --uid option, do the following:

$ sudoedit /etc/firewalld/firewalld.conf

# IPv6_rpfilter
# Performs a reverse path filter test on a packet for IPv6. If a reply to the
# packet would be sent via the same interface that the packet arrived on, the 
# packet will match and be accepted, otherwise dropped.
# The rp_filter for IPv4 is controlled using sysctl.
# Note: This feature has a performance impact. See man page FIREWALLD.CONF(5)
# for details.
# Default: yes
IPv6_rpfilter=yes

Set IPv6_rpfilter=no.