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Here is a sample event when using to_syslog_snare() in the nxlog.conf:

<14>Jan 27 10:03:39 event_computer MSWinEventLog        1        Security        32630749        Wed Jan 27 10:03:39 2016        4624        Microsoft-Windows-Security-Auditing        N/A        N/A        Success Audit        event_computer        Logon                An account was successfully logged on.    Subject:   Security ID:  S-1-0-0   Account Name:  -   Account Domain:  -   Logon ID:  0x0    Logon Type:   3    Impersonation Level:  Impersonation    New Logon:   Security ID:  S-1-5-21-2705889813-1605608894-1661845433-43745   Account Name:  account_name   Account Domain:  account_domain   Logon ID:  0x23820B882   Logon GUID:  {00000000-0000-0000-0000-000000000000}    Process Information:   Process ID:  0x0   Process Name:  -    Network Information:   Workstation Name: workstation_name   Source Network Address: source_address   Source Port:  54241    Detailed Authentication Information:   Logon Process:  NtLmSsp    Authentication Package: NTLM   Transited Services: -   Package Name (NTLM only): NTLM V2   Key Length:  0    This event is generated when a logon session is created. It is generated on the computer that was accessed.    The subject fields indicate the account on the local system which requested the logon. This is most commonly a service such as the Server service, or a local process such as Winlogon.exe or Services.exe.    The logon type field indicates the kind of logon that occurred. The most common types are 2 (interactive) and 3 (network).    The New Logon fields indicate the account for whom the new logon was created, i.e. the account that was logged on.    The network fields indicate where a remote logon request originated. Workstation name is not always available and may be left blank in some cases.    The impersonation level field indicates the extent to which a process in the logon session can impersonate.    The authentication information fields provide detailed information about this specific logon request.   - Logon GUID is a unique identifier that can be used to correlate this event with a KDC event.   - Transited services indicate which intermediate services have participated in this logon request.   - Package name indicates which sub-protocol was used among the NTLM protocols.   - Key length indicates the length of the generated session key. This will be 0 if no session key was requested.        35284558

My issue is that I would NOT want to collect the "informational text" representing the event - in this case everything starting from the string "This event is generated---" all the way up until "--was requested."

Before I go any deeper into this, let me state that in the logs of this format I call the "<14>Jan 27 10:03:39 event_computer MSWinEventLog        1        Security        32630749        Wed Jan 27 10:03:39 2016        4624        Microsoft-Windows-Security-Auditing        N/A        N/A        Success Audit        event_computer        Logon" portion of the whole log message the HEADER, and the rest is called MESSAGE.

Putting it another way, I would like to forward the message using syslog in a format constructed according to the pseudocode below:

parse fields from windows event /* e.g. SubjectUserName, LogonType, IpAddress, etc. */
/* print the header "as is" already in the to_syslog_snare() format, i.e. from "<14>---" until and including "---Logon"
print HEADER /* e.g. event_time,event_computer,event_type,event_id,... */
for all fields parsed
    print "'field_name=field_value'" /* e.g. SubjectUserName=value,LogonType=value,IpAddress=value,... /*
    
The reason I would like to do this is that the informational text, which gets appended to some Windows events (not all, it seems), takes a lot of space, and we do not really need this information text for anything.

Another way to do this would be to statically list all the fields POSSIBLY found in an Windows event and construct the message that way, but this would often leave me with a lot of empty key-value pairs. THUS I would only like to print out those fields that were found in that specific log message while leaving out the informational message.

I do acknowledge, though, that especially Application and System events might not contain most or any of the fields that are present in a Security log event. Take for example the following System log event:

<14>Jan 27 11:09:21 event_computer MSWinEventLog        1        System        32633951        Wed Jan 27 11:09:21 2016        7036        Service Control Manager        N/A        N/A        Information        event_computer        N/A                The Remote Registry service entered the stopped state.        319889

In the example above, the "header" portion of the whole message only contains the string "The Remote Registry service entered the stopped state." I do hope, though, that the variable where this string is stored is actually the same that hosted the string "An account was successfully logged on.", which would mean that my approach in the pseudocode would still work (i.e. the array or list of fields that is iterated and printed would only contain one field. The HEADER portion of the field is exactly the same in all messages.

The description of to_syslog_snare() in the nxlog documentation states:

"Create a SNARE Syslog formatted log message in $raw_event. Uses the following fields to construct $raw_event: $EventTime, $Hostname, $SeverityValue, $FileName, $EventID, $SourceName, $AccountName, $AccountType, $EventType, $Category, $Message."

Thus when reflecting back to what I said, it seems that what I call the HEADER includes all the fields from $EventTime to (and including) $Category - this I would like to keep as it is. But according to the documentation, the $Message variable actually then holds all the other information in the log, or what I call the MESSAGE portion. So I guess the question is that can the contents of the $Message variable be further filtered, as it obviously is constructed from e.g. EventData's Data fields listed below. I would like to only change the $Message contents so that it would never contain the informational text if there exists such a message in a given log message, and that preferably the Data fields inside $Message would be formatted using key-value pairs instead of the to_syslog_snare format seen in the first example (one or more whitespace as delimiter).

AskedJanuary 27, 2016 - 10:25am

Answer (1)

You can rewrite and strip out unnecessary data from the $Message field as needed with the regexp replacement operator for example:

<Exec>
 if ($EventId == $4242) {
    $Message =~ s/This event is generated .+//;
 }
 to_syslog_snare();
</Exec>

Snare calls this "Truncation of Verbose Event Text".