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Security in MySQL  /  ...  /  Configuring Audit Logging Characteristics

6.5.5 Configuring Audit Logging Characteristics

This section describes how to configure audit logging characteristics, such as the file to which the audit log plugin writes events, the format of written events, and whether to enable log file compression and encryption.

For additional information about the user-defined functions and system variables that affect audit logging, see Audit Log Functions, and Audit Log Options and Variables.

The audit log plugin can also control which audited events are written to the audit log file, based on event content or the account from which events originate. See Section 6.5.7, “Audit Log Filtering”.

Naming Conventions for Audit Log Files

To configure the audit log file name, set the audit_log_file system variable at server startup. The default name is audit.log in the server data directory. For best security, write the audit log to a directory accessible only to the MySQL server and to users with a legitimate reason to view the log.

As of MySQL 5.7.21, the plugin interprets the audit_log_file value as composed of an optional leading directory name, a base name, and an optional suffix. If compression or encryption are enabled, the effective file name (the name actually used to create the log file) differs from the configured file name because it has additional suffixes:

  • If compression is enabled, the plugin adds a suffix of .gz.

  • If encryption is enabled, the plugin adds a suffix of .enc. The audit log plugin stores the encryption password in the keyring (see Encrypting Audit Log Files.

The effective audit log file name is the name resulting from the addition of applicable compression and encryption suffixes to the configured file name. For example, if the configured audit_log_file value is audit.log, the effective file name is one of the values shown in the following table.

Enabled Features Effective File Name
No compression or encryption audit.log
Compression audit.log.gz
Encryption audit.log.enc
Compression, encryption audit.log.gz.enc

Prior to MySQL 5.7.21, the configured and effective log file names are the same. For example, if the configured audit_log_file value is audit.log, the audit log plugin writes to audit.log.

The audit log plugin performs certain actions during initialization and termination based on the effective audit log file name:

As of MySQL 5.7.21:

  • During initialization, the plugin checks whether a file with the audit log file name already exists and renames it if so. (In this case, the plugin assumes that the previous server invocation exited unexpectedly with the audit log plugin running.) The plugin then writes to a new empty audit log file.

  • During termination, the plugin renames the audit log file.

  • File renaming (whether during plugin initialization or termination) occurs according to the usual rules for automatic log file rotation; see Automatic Audit Log File Rotation.

Prior to MySQL 5.7.21, only the XML log formats are available and the plugin performs rudimentary integrity checking:

  • During initialization, the plugin checks whether the file ends with an </AUDIT> tag and truncates the tag before writing any <AUDIT_RECORD> elements. If the log file exists but does not end with </AUDIT> or the </AUDIT> tag cannot be truncated, the plugin considers the file malformed and renames it. (Such renaming can occur if the server exits unexpectedly with the audit log plugin running.) The plugin then writes to a new empty audit log file.

  • At termination, no file renaming occurs.

  • When renaming occurs at plugin initialization, the renamed file has .corrupted, a timestamp, and .xml added to the end. For example, if the file name is audit.log, the plugin renames it to a value such as audit.log.corrupted.15081807937726520.xml. The timestamp value is similar to a Unix timestamp, with the last 7 digits representing the fractional second part. For information about interpreting the timestamp, see Space Management and Name Rotation of Audit Log Files.

Selecting Audit Log File Format

To configure the audit log file format, set the audit_log_format system variable at server startup. By default, the format is NEW (new-style XML format). For details about each format, see Section 6.5.4, “Audit Log File Formats”.

If you change audit_log_format, it is recommended that you also change audit_log_file. Otherwise, there will be two sets of log files with the same base name but different formats.

Note

Prior to MySQL 5.7.21, changing the value of audit_log_format can result in writing log entries in one format to an existing log file that contains entries in a different format. To avoid this issue, use the following procedure:

  1. Stop the server.

  2. Either change the value of the audit_log_file system variable so the plugin writes to a different file, or rename the current audit log file manually.

  3. Restart the server with the new value of audit_log_format. The audit log plugin creates a new log file and writes entries to it in the selected format.

Compressing Audit Log Files

Audit log file compression is available as of MySQL 5.7.21. Compression can be enabled for any log format.

To configure audit log file compression, set the audit_log_compression system variable at server startup. Permitted values are NONE (no compression; the default) and GZIP (GNU Zip compression).

If both compression and encryption are enabled, compression occurs before encryption. To recover the original file manually, first decrypt it, then uncompress it. See Manually Uncompressing and Decrypting Audit Log Files.

Encrypting Audit Log Files

Audit log file encryption is available as of MySQL 5.7.21. Encryption can be enabled for any log format. Encryption is based on a user-defined password (with the exception of the initial password, which the audit log plugin generates). To use this feature, the MySQL keyring must be enabled because audit logging uses it for password storage. Any keyring plugin can be used; for instructions, see Section 6.4, “The MySQL Keyring”.

To configure audit log file encryption, set the audit_log_encryption system variable at server startup. Permitted values are NONE (no encryption; the default) and AES (AES-256-CBC cipher encryption).

To set or get an encryption password at runtime, use these user-defined functions (UDFs):

  • To set the current encryption password, invoke audit_log_encryption_password_set(). This function stores the new password in the keyring. If encryption is enabled, it also performs a log file rotation operation that renames the current log file, and begins a new log file encrypted with the password. File renaming occurs according to the usual rules for automatic log file rotation; see Automatic Audit Log File Rotation.

    Previously written audit log files are not re-encrypted with the new password. Keep a record of the previous password should you need to decrypt those files manually.

  • To get the current encryption password, invoke audit_log_encryption_password_get(), which retrieves the password from the keyring.

For additional information about audit log encryption UDFs, see Audit Log Functions.

When the audit log plugin initializes, if it finds that log file encryption is enabled, it checks whether the keyring contains an audit log encryption password. If not, the plugin automatically generates a random initial encryption password and stores it in the keyring. To discover this password, invoke audit_log_encryption_password_get().

If both compression and encryption are enabled, compression occurs before encryption. To recover the original file manually, first decrypt it, then uncompress it. See Manually Uncompressing and Decrypting Audit Log Files.

Manually Uncompressing and Decrypting Audit Log Files

Audit log files can be uncompressed and decrypted using standard tools. This should be done only for log files that have been closed (archived) and are no longer in use, not for the log file that the audit log plugin is currently writing. You can recognize archived log files because they have been renamed by the audit log plugin to include a timestamp in the file name just after the base name.

For this discussion, assume that audit_log_file is set to audit.log. In that case, an archived audit log file has one of the names shown in the following table.

Enabled Features Archived File Name
No compression or encryption audit.timestamp.log
Compression audit.timestamp.log.gz
Encryption audit.timestamp.log.enc
Compression, encryption audit.timestamp.log.gz.enc

To uncompress a compressed log file manually, use gunzip, gzip -d, or equivalent command. For example:

gunzip -c audit.timestamp.log.gz > audit.timestamp.log

To decrypt an encrypted log file manually, use the openssl command. For example:

openssl enc -d -aes-256-cbc -pass pass:password -md sha256
    -in audit.timestamp.log.enc
    -out audit.timestamp.log

If both compression and encryption are enabled for audit logging, compression occurs before encryption. In this case, the file name has .gz and .enc suffixes added, corresponding to the order in which those operations occur. To recover the original file manually, perform the operations in reverse. That is, first decrypt the file, then uncompress it:

openssl enc -d -aes-256-cbc -pass pass:password -md sha256
    -in audit.timestamp.log.gz.enc
    -out audit.timestamp.log.gz
gunzip -c audit.timestamp.log.gz > audit.timestamp.log

Space Management and Name Rotation of Audit Log Files

The audit log file has the potential to grow very large and consume a lot of disk space. To enable management of the space used by its log files, the audit log plugin provides for log file rotation, either manually or automatically. Rotation capabilities use the audit_log_flush and audit_log_rotate_on_size system variables:

  • By default, audit_log_rotate_on_size=0 and no log rotation occurs unless performed manually. In this case, use audit_log_flush to close and reopen the current log file after manually renaming it.

  • If audit_log_rotate_on_size is greater than 0, automatic rotation occurs when a write to the current log file causes its size to exceed this value. The audit log plugin closes the file, renames it, and opens a new log file. With automatic rotation enabled, audit_log_flush has no effect.

  • Automatic rotation also occurs under several other conditions, described later.

Note

Renamed log files are not removed automatically. For example, with size-based log file rotation, renamed log files do not rotate off the end of the name sequence. Instead, they have unique names and accumulate indefinitely. To avoid excessive space use, remove old files periodically, backing them up first as necessary.

The following discussion describes log file rotation methods in greater detail.

Manual Audit Log File Rotation

If audit_log_rotate_on_size=0 (the default), no log rotation occurs unless performed manually. In this case, the audit log plugin closes and reopens the log file when the audit_log_flush value changes from disabled to enabled. Log file renaming must be done externally to the server. Suppose that the log file name is audit.log and you want to maintain the three most recent log files, cycling through the names audit.log.1 through audit.log.3. On Unix, perform rotation manually like this:

  1. From the command line, rename the current log files:

    mv audit.log.2 audit.log.3
    mv audit.log.1 audit.log.2
    mv audit.log audit.log.1

    This strategy overwrites the current audit.log.3 contents, placing a bound on the number of archived log files and the space they use.

  2. At this point, the plugin is still writing to the current log file, which has been renamed to audit.log.1. Connect to the server and flush the log file so the plugin closes it and reopens a new audit.log file:

    SET GLOBAL audit_log_flush = ON;

    audit_log_flush is special in that its value remains OFF so that you need not disable it explicitly before enabling it again to perform another flush.

Note

For JSON-format logging, renaming audit log files manually makes them unavailable to the log-reading functions because the audit log plugin no longer can determine that they are part of the log file sequence (see Section 6.5.6, “Reading Audit Log Files”). Consider setting audit_log_rotate_on_size greater than 0 to use size-based rotation instead.

Automatic Audit Log File Rotation

If audit_log_rotate_on_size is greater than 0, setting audit_log_flush has no effect. Instead, whenever a write to the current log file causes its size to exceed the audit_log_rotate_on_size value, the audit log plugin closes the file, renames it, and opens a new log file.

Automatic rotation also occurs under these conditions:

The plugin renames the original file as follows:

  • As of MySQL 5.7.21, the renamed file has a timestamp inserted after its base name and before its suffix. For example, if the file name is audit.log, the plugin renames it to a value such as audit.20180115T140633.log. The timestamp is a UTC value in YYYYMMDDThhmmss format. The timestamp indicates rotation time for XML logging, and the timestamp of the last event written to the file for JSON logging.

  • Prior to MySQL 5.7.21, the renamed file has a timestamp and .xml added to the end. For example, if the file name is audit.log, the plugin renames it to a value such as audit.log.15159344437726520.xml. The timestamp value is similar to a Unix timestamp, with the last 7 digits representing the fractional second part. By inserting a decimal point, the value can be interpreted using the FROM_UNIXTIME() function:

    mysql> SELECT FROM_UNIXTIME(1515934443.7726520);
    +-----------------------------------+
    | FROM_UNIXTIME(1515934443.7726520) |
    +-----------------------------------+
    | 2018-01-14 06:54:03.772652        |
    +-----------------------------------+

Write Strategies for Audit Logging

The audit log plugin can use any of several strategies for log writes. Regardless of strategy, logging occurs on a best-effort basis, with no guarantee of consistency.

To specify a write strategy, set the audit_log_strategy system variable at server startup. By default, the strategy value is ASYNCHRONOUS and the plugin logs asynchronously to a buffer, waiting if the buffer is full. It's possible to tell the plugin not to wait (PERFORMANCE) or to log synchronously, either using file system caching (SEMISYNCHRONOUS) or forcing output with a sync() call after each write request (SYNCHRONOUS).

For asynchronous write strategy, the audit_log_buffer_size system variable is the buffer size in bytes. Set this variable at server startup to change the buffer size. The plugin uses a single buffer, which it allocates when it initializes and removes when it terminates. The plugin does not allocate this buffer for nonasynchronous write strategies.

Asynchronous logging strategy has these characteristics:

  • Minimal impact on server performance and scalability.

  • Blocking of threads that generate audit events for the shortest possible time; that is, time to allocate the buffer plus time to copy the event to the buffer.

  • Output goes to the buffer. A separate thread handles writes from the buffer to the log file.

With asynchronous logging, the integrity of the log file may be compromised if a problem occurs during a write to the file or if the plugin does not shut down cleanly (for example, in the event that the server host exits unexpectedly). To reduce this risk, set audit_log_strategy to use synchronous logging.

A disadvantage of PERFORMANCE strategy is that it drops events when the buffer is full. For a heavily loaded server, the audit log may have events missing.