ipsec: Add block rules to avoid conntrack entries

ipsec: Add block rules to avoid conntrack entries

If an IPsec VPN connections is not established, there are
rare cases when packets are supposed to be sent through
that said tunnel and incorrectly handled.

Those packets are sent to the default gateway an entry
for this connection is created in the connection tracking
table (usually only happens to UDP). All following packets
are sent the same route even after the tunnel has been
brought up. That leads to SIP phones not being able to
register among other things.

This patch adds firewall rules that these packets are
rejected. That will sent a notification to the client
that the tunnel is not up and avoid the connection to
be added to the connection tracking table.

Apart from a small performance penalty there should
be no other side-effects.

Fixes: #10908
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>
Cc: tomvend@rymes.com
Cc: daniel.weismueller@ipfire.org
Cc: morlix@morlix.de
Reviewed-by: Timo Eissler <timo.eissler@ipfire.org>