You can open the "full text log files" created by Simple DNS Plus with notepad, or watch the most recent log entries using the Active Log View.
Log lines starting with "->" are details for a previous line.
Writing log files to disk can be activated in the Options dialog / Logging / Log files section.
In addition to the logs, some events can be recorded to the Windows Event Log, which in turn can be used to send notification via network messages, e-mail, etc. See Options dialog / Logging / Windows Event Log section.
*** Error/Warning:... messages
For details and explanations see Event IDs / Error Messages.
-> Header:... messages
-> Header: Format Error
Means that the DNS request was not formatted correctly.
This could be caused by network problems, a malfunctioning DNS server, or another network program incorrectly using the DNS port (53).
-> Header: Server Failure
Usually means that a DNS server did not respond or that no NS-record (or associated A-record) exists for a domain name.
Often follows the "*** Warning: Lame delegation..." message (see Event IDs / Error Messages).
This could also be caused by network connectivity problems.
-> Header: Non-Existent Domain
Means that the domain name specified in the request does not exist.
If this happens for all outside domain names that you try to resolve, make sure you don't have a <root> zone in the DNS Records window.
-> Header: Not implemented
Means that the DNS server does not support this type of query.
-> Header: Refused
The queried DNS server refuses to respond to this query - usually due to local security settings.
This most often happens in connection with zone transfers - make sure the primary DNS server allows the secondary servers to zone transfer the zone (see Zone Properties dialog).
-> Header: Name exists when it should not
This header is returned in a response to a dynamic update request.
The update could not be completed because the prerequisites specified by the update request were not met.
-> Header: RR Set exists when it should not
This header is returned in a response to a dynamic update request.
The update could not be completed because the prerequisites specified by the update request were not met.
-> Header: RR Set that should exist does not
This header is returned in a response to a dynamic update request.
The update could not be completed because the prerequisites specified by the update request were not met.
-> Header: Server not authoritative for zone
This header is returned in a response to a dynamic update request.
The update could not be completed because the server responding is not configured with the zone specified in the update request.
-> Header: Name not contained in zone
This header is returned in a response to a dynamic update request.
The update could not be completed because the update name is not contained within the zone specified in the update request.
-> Header: TSIG Signature Failure
This header is returned in a response to a TSIG signed dynamic update or zone transfer request.
The operation was not allowed because the TSIG signature in the update request was invalid.
-> Header: Key not recognized
This header is returned in a response to a TSIG signed dynamic update or zone transfer request.
The operation was not allowed because the server responding is not configured with the TSIG key or signature algorithm used in the update request for the update name.
-> Header: Signature out of time window
This header is returned in a response to a TSIG signed dynamic update or zone transfer request.
The operation was not allowed because the time stamp in the TSIG signature did not match the server's time (not within the requested "fudge" interval).