This message is logged when the PIX Firewall discards a packet with an invalid source address. Invalid source addresses are those addresses belonging to the following:
Loopback network (127.0.0.0)
Broadcast (limited, net-directed, subnet-directed, and all-subnets-directed)
The destination host
Furthermore, if the sysopt connection enforcesubnet command is enabled, PIX Firewall discards packets with a source address belonging to the destination subnet from traversing the PIX Firewall and logs this message.
To further enhance spoof packet detection, use the conduit command to configure the PIX Firewall to discard packets with source addresses belonging to the internal network.
To resolve this Determine if an external user is trying to compromise the protected network. Check for misconfigured clients