This document describes the design and use of logging through NetLog.

Adding new NetLogging code

Adding information to the NetLog helps debugging. However, logging also requires careful review as it can impact performance, privacy, and security.

Please add a net/log/OWNERS reviewer when adding new NetLog parameters, or adding information to existing ones.

The high level objectives when adding net logging code are:

  • No performance cost when capturing is off.
  • Logs captured using kDefault are safe to upload and share publicly.
  • Capturing using kDefault has a low performance impact.
  • Logs captured using kDefault are small enough to upload to bug reports.
  • Events that may emit sensitive information have accompanying unit-tests.
  • The event and its possible parameters are documented in net_log_event_type_list.h

To avoid doing work when logging is off, logging code should generally be conditional on NetLog::IsCapturing(). Note that when specifying parameters via a lambda, the lambda is already conditional on IsCapturing().

Binary data and strings

NetLog parameters are specified as a JSON serializable base::Value. This has some subtle implications:

  • Do not use base::Value::Type::STRING with non-UTF-8 data.
  • Do not use base::Value::Type::BINARY (the JSON serializer can't handle it)


  • If the string is likely ASCII or UTF-8, use NetLogStringValue().
  • If the string is arbitrary data, use NetLogBinaryValue().
  • If the string is guaranteed to be valid UTF-8, you can use base::Value::Type::STRING

Also consider the maximum size of any string parameters:

  • If the string could be large, truncate or omit it when using the default capture mode. Large strings should be relegated to the kEverything capture mode.

64-bit integers

NetLog parameters are specified as a JSON serializable base::Value which does not support 64-bit integers.

Be careful when using base::Value::SetIntKey() as it will truncate 64-bit values to 32-bits.

Instead use NetLogNumberValue().

Backwards compatibility

There is no backwards compatibility requirement for NetLog events and their parameters, so you are free to change their structure/value as needed.

That said, changing core events may have consequences for external consumers of NetLogs, which rely on the structure and parameters to events for pretty printing and log analysis.

The NetLog viewer for instance pretty prints certain parameters based on their names, and the event name that added them.

Example 1

Add an PROXY_RESOLUTION_SERVICE event without any parameters, at all capture modes.



  • Privacy: Logging the event at all capture modes only reveals timing information.
  • Performance: When not logging, has the overhead of an unconditional function call (BeginEvent), and then a branch (test on IsCapturing()).
  • Size: Minimal data added to NetLog - just one parameterless event per URL request.

Example 2

Add a FTP_COMMAND_SENT event, at all capture modes, along with parameters that describe the FTP command.

if (net_log.IsCapturing()) {
  std::string command = BuildCommandForLog();
                                   "command", command);


  • Privacy: Low risk given FTP traffic is unencrypted. BuildCommandForString() should additionally best-effort strip any identity information, as this is being logged at all capture modes.
  • Performance: Costs one branch when not capturing. The call to BuildCommandForString() is only executed when capturing.
  • Size: Cost is proportional to the average FTP command length and frequency of FTP, both of which are low. BuildCommandForLog() needn't strictly bound the string length. If a huge FTP command makes it to a NetLog, there is a good chance that is the problem being debugged.

Example 3

Add a SSL_CERTIFICATES_RECEIVED event, along with the full certificate chain, at all capture modes.

net_log.AddEvent(NetLogEventType::SSL_CERTIFICATES_RECEIVED, [&] {
  base::Value dict(base::Value::Type::DICTIONARY);
  base::Value certs(base::Value::Type::LIST);
  std::vector<std::string> encoded_chain;
  for (auto& pem : encoded_chain)
  dict.SetKey("certificates", std::move(certs));
  return dict;


  • Privacy: Low risk as server certificates are generally public data.
  • Performance: Costs one branch when logging is off (hidden by template expansion). The code in the lambda which builds the base::Value parameters is only executed when capturing.
  • Size: On average 8K worth of data per request (average of 2K/certificate, chain length of 3, and the overhead of PEM-encoding). This is heavy-weight for inclusion at kDefault capture mode, however justified based on how useful the data is.

Example 4

Add a COOKIE_STORE_COOKIE_ADDED event at all capture modes. Moreover, if the capture mode is kIncludeSensitive or kEverything, also logs the cookie's name and value.

                 [&](NetLogCaptureMode capture_mode) {
                   if (!NetLogCaptureIncludesSensitive(capture_mode))
                     return base::Value();
                   base::Value dict(base::Value::Type::DICTIONARY);
                   dict.SetStringKey("name", cookie->Name());
                   dict.SetStringKey("value", cookie->Value());
                   return dict;


  • Privacy: The cookie name and value are not included at the kDefault capture mode, so only cookie counts and timing information is revealed.
  • Performance: Costs one branch when logging is off (hidden by template expansion). The code in the lambda which builds the base::Value parameters is only executed when capturing.
  • Size: For default captured logs, has a file size cost proportional to the number of cookies added. This is borderline justifiable. It would be better in this case to simply omit the event all together at kDefault than to log a parameterless event, as the parameterless event is not broadly useful.