1) Yes you should still use them as you should never be seeing these addresses coming from the Internet as legitimate traffic.
2) A packet with a source ip address of 10.x.x.x will still reach your firewall. It is only when it tries to route back to that address that it fails. But often people sending these packets are not interested in the return traffic eg when they are issuing a snmp set commmand to a router.
If they have spoofed the source no it is not really possible to trace the real address.
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We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...