FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
In FortiOS v5.4, logs entries with a result of 'Deny: DNS error' and 'Deny: IP connection error' are frequently seen. This article provides information on these logs and why they occur.
Deny: DNS error
This log entry is an expected behavior in v5.4.
By design FortiGate looks for invalid/failed DNS traffic and will mark it as action=dns or in the GUI as "Action Deny: DNS error".
This happens if the DNS query is not successful returns any other status than NOERROR.
The firewall action itself is allow/pass so the bad reply from the server is still forwarded back to the requesting client. The session is marked with the "Deny: DNS Error" message.
Some examples of Invalid DNS traffic could be UDP packets on port 53 that are not DNS traffic, packets which are over sized, packets with a bad checksum. This error message should also be seen for DNS traffic that causes an error, for example, the request to or reply from a DNS server that is invalid.
Deny: IP connection error
FortiGate did not receive any reply packet the result is "Deny: IP connection error" and the session is closed.