%PIX-2-106001 Denied inbound


One of the most common critical messages appears to be the %PIX-2-106001 one, and it is recorded when an external IP is attempting to connect to a port that is not configured for inbound traffic. A network probe will often be detected this way. For example:

Inbound TCP connection denied from 219.84.2.38/nnnn to 208.76.111.142/25 flags SYN on interface outside

Most probably, this IP 219.84.2.38 (a dynamically assigned IP in Taiwan) probed for an open SMTP relay. This message will be listed in both FireGen “Detailed messages” section and the “Denials” one. The Denials will also have the result of the reverse DNS resolution. There is little one can do these days to block this kind of probes. Unless there’s quite a large number of such probes they can be ignored. If anything, they are a constant reminder that obscurity is not a good security policy as people (or scripts!) will keep looking for you and your vulnerable spots. Keep an eye on the ports used in these probes, may indicate a new vulnerability.

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