For a non-loopback bind, configure_nano_firewall opened the nano port
in firewalld to every source, while the summary told the operator to
restrict it to the proxy. hasJoined takes no token, so on a public
host that port is an auth relay anyone can point a proxy at, spending
this host's Mojang egress until Mojang rate-limits it and the
operator's own players stop getting in.
A new FELIS_NANO_PROXY_CIDR names the proxy. With it, firewalld gets
one rich rule that admits the port from that source only, ipv4 or
ipv6 by the address given. Without it, no port is opened and the
summary prints the rule to add. A re-run closes the port an earlier
installer opened to every source. A rule for a previous
FELIS_NANO_PROXY_CIDR is not tracked and stays until removed by hand.
Hosts without firewalld are handled as before.
The value goes into the rule text, so it is checked up front for an
address with one prefix length and nothing else. firewalld's own
parser accepts both rule forms and refuses an ipv6 address under the
ipv4 family. The harness covers the rule for each family, the
closed-by-default case, the re-run cleanup, the loopback case and the
CIDR check.
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