W3CLogger in ASP.NET Core

W3CLogger is a middleware that writes log files in the W3C standard format. The logs contain information about HTTP requests and HTTP responses. W3CLogger provides logs of:

• HTTP request information
• Common properties
• Headers
• HTTP response information
• Metadata about the request/response pair (date/time started, time taken)

W3CLogger is valuable in several scenarios to:

• Record information about incoming requests and responses.
• Filter which parts of the request and response are logged.
• Filter which headers to log.

W3CLogger can reduce the performance of an app. Consider the performance impact when selecting fields to log - the performance reduction will increase as you log more properties. Test the performance impact of the selected logging properties.

Warning

W3CLogger can potentially log personally identifiable information (PII). Consider the risk and avoid logging sensitive information. By default, fields that could contain PII aren't logged.

Enable W3CLogger

Call AddW3CLogging to add the W3CLogger services to the dependency injection container:

builder.Services.AddW3CLogging(logging =>
{
   // Set logging properties here
});

Next, call UseW3CLogging to add the W3CLogger middleware to the request processing pipeline:

app.UseW3CLogging();

By default, log files are written to a logs directory in the app's content root directory (./logs/) with the file name prefix w3clog- (for example, ./logs/w3clog-20210929.0000.txt).

By default, W3CLogger logs common properties such as path, status-code, date, time, and protocol. All information about a single request/response pair is written to the same line.

#Version: 1.0
#Start-Date: 2021-09-29 22:18:28
#Fields: date time c-ip s-computername s-ip s-port cs-method cs-uri-stem cs-uri-query sc-status time-taken cs-version cs-host cs(User-Agent) cs(Referer)
2021-09-29 22:18:28 ::1 DESKTOP-LH3TLTA ::1 5000 GET / - 200 59.9171 HTTP/1.1 localhost:5000 Mozilla/5.0+(Windows+NT+10.0;+WOW64)+AppleWebKit/537.36+(KHTML,+like+Gecko)+Chrome/93.0.4577.82+Safari/537.36 -
2021-09-29 22:18:28 ::1 DESKTOP-LH3TLTA ::1 5000 GET / - 200 0.1802 HTTP/1.1 localhost:5000 Mozilla/5.0+(Windows+NT+10.0;+WOW64)+AppleWebKit/537.36+(KHTML,+like+Gecko)+Chrome/93.0.4577.82+Safari/537.36 -
2021-09-29 22:18:30 ::1 DESKTOP-LH3TLTA ::1 5000 GET / - 200 0.0966 HTTP/1.1 localhost:5000 Mozilla/5.0+(Windows+NT+10.0;+WOW64)+AppleWebKit/537.36+(KHTML,+like+Gecko)+Chrome/93.0.4577.82+Safari/537.36 -

W3CLogger options

To configure the W3CLogger middleware, call AddW3CLogging and configure W3CLoggerOptions in Program.cs:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddW3CLogging(logging =>
{
    // Log all W3C fields
    logging.LoggingFields = W3CLoggingFields.All;

    logging.AdditionalRequestHeaders.Add("x-forwarded-for");
    logging.AdditionalRequestHeaders.Add("x-client-ssl-protocol");
    logging.FileSizeLimit = 5 * 1024 * 1024;
    logging.RetainedFileCountLimit = 2;
    logging.FileName = "MyLogFile";
    logging.LogDirectory = @"C:\logs";
    logging.FlushInterval = TimeSpan.FromSeconds(2);
});

W3CLoggerOptions supports the following properties:

• AdditionalRequestHeaders: Additional request headers to include in the log file.
• FileName: The log file name prefix. Defaults to w3clog-.
• FileSizeLimit: Gets or sets a strictly positive value representing the maximum log size in bytes or null for no limit. Once the log is full, no more messages will be appended. Defaults to 10MB (10 * 1024 * 1024).
• FlushInterval: The period after which logs are flushed to disk. Defaults to 1 second.
• LogDirectory: The directory where log files are written. Defaults to ./logs/ relative to the app's content root directory.
• RetainedFileCountLimit: The maximum number of retained log files. Defaults to 4.
• LoggingFields: A bit flag enumeration that configures specific parts of the request and response to log and other information about the connection. Defaults to include all possible fields except UserName and Cookie. For a complete list of available fields, see W3CLoggingFields.