Technical Tip: 'Deny: DNS error' and 'Deny: IP connection error'
Description
This article describes that starting FortiOS v5.4, log 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.
Scope
FortiGate.
Solution
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 and returns any status other 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, oversized packets, or 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.
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 and returns any status other 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, oversized packets, or 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.
Related documents:
Technical Tip: Application Control with Explicit Proxy policy error
Technical Tip: How FortiGate can block Duolingo in different ways. Blocks web application.
Technical Tip: DNS error logs showing in FortiAnalyzer
