Small blog about system administration.

HOWTO: Configure Logging and Log Rotation in Nginx

One of the easiest ways to save yourself trouble with your web server is to configure appropriate logging today. Logging information on your server gives you access to the data that will help you troubleshoot and assess situations as they arise.


The Error_log Directive

Nginx uses a few different directives to control system logging. The one included in the core module is called "error_log".

Error_log Syntax

The "error_log" directive is used to handle logging general error messages. If you are coming from Apache, this is very similar to Apache's "ErrorLog" directive.
The error_log directive takes the following syntax:
error_log log_file [ log_level ]
The "log_file" in the example specifies the file where the logs will be written. The "log_level" specifies the lowest level of logging that you would like to record.

The Access_log Directive

The access_log directive uses some similar syntax to the error_log directive, but is more flexible. It is used to configure custom logging.
The access_log directive uses the following syntax:
access_log /path/to/log/location [ format_of_log buffer_size ];
The default value for access_log is the "combined" format we saw in the log_format section. You can use any format defined by a log_format definition.
The buffer size is the maximum size of data that Nginx will hold before writing it all to the log. You can also specify compression of the log file by adding "gzip" into the definition:
access_log location format gzip;
Unlike the error_log directive, if you do not want logging, you can turn it off by specifying:
access_log off;
It is not necessary to write to "/dev/null" in this case.

Log Rotation

As log files grow, it becomes necessary to manage the logging mechanisms to avoid filling up disk space. Log rotation is the process of switching out log files and possibly archiving old files for a set amount of time.
Nginx does not provide tools to manage log files, but it does include mechanisms that make log rotation simple.
if ($time_iso8601 ~ "^(\d{4})-(\d{2})-(\d{2})") {
    set $year $1;
    set $month $2;
    set $day $3;
}
access_log /var/log/nginx/$year-$month-$day-access.log;

Conclusion

Proper log configuration and management can save you time and energy in the event of a problem with your server. Having easy access to the information that will help you diagnose a problem can be the difference between a trivial fix and a persistent headache.
It is important to keep an eye on server logs in order to maintain a functional site and ensure that you are not exposing sensitive information. This guide should serve only as an introduction to your experience with logging.

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