Apache HTTP Server Version 2.4

Apache > HTTP Server > Documentation > Version 2.4 > Modules

# Apache Module mod_logio

Description: | Logging of input and output bytes per request
Status: | Extension
Module Identifier: | logio_module
Source File: | mod_logio.c

### Summary

This module provides the logging of input and output number of bytes received/sent per request. The numbers reflect the actual bytes as received on the network, which then takes into account the headers and bodies of requests and responses. The counting is done before SSL/TLS on input and after SSL/TLS on output, so the numbers will correctly reflect any changes made by encryption.

This module requires `mod_log_config`.

When KeepAlive connections are used with SSL, the overhead of the SSL handshake is reflected in the byte count of the first request on the connection. When per-directory SSL renegotiation occurs, the bytes are associated with the request that triggered the renegotiation.

### Topics

- Custom Log Formats

### Directives

### Bugfix checklist

- httpd changelog
- Known issues
- Report a bug

### See also

- `mod_log_config`
- Apache Log Files

## Custom Log Formats

This module adds three new logging directives. The characteristics of the request itself are logged by placing "`%`" directives in the format string, which are replaced in the log file by the values as follows:

Format String | Description
--- | ---
`%I` | Bytes received, including request and headers, cannot be zero.
`%O` | Bytes sent, including headers. May be zero in rare cases such as when a request is aborted before a response is sent.
`%S` | Bytes transferred (received and sent), including request and headers, cannot be zero. This is the combination of %I and %O. Available in Apache 2.4.7 and later
`%^FB` | Delay in microseconds between when the request arrived and the first byte of the response headers are written. Only available if `LogIOTrackTTFB` is set to ON. Available in Apache 2.4.13 and later

Usually, the functionality is used like this:

Combined I/O log format:
  `"%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-agent}i\" %I %O"`

## LogIOTrackTTFB Directive

Description: | Enable tracking of time to first byte (TTFB)
Syntax: | `LogIOTrackTTFB ON|OFF`
Default: | `LogIOTrackTTFB OFF`
Context: | server config, virtual host, directory, .htaccess
Override: | All
Status: | Extension
Module: | mod_logio
Compatibility: | Apache HTTP Server 2.4.13 and later

This directive configures whether this module tracks the delay between the request being read and the first byte of the response headers being written. The resulting value may be logged with the `%^FB` format.