Oracle backups fail with an error "ORA-09925: Unable to create audit trail file"

When backing up an Oracle instance, the backup job fails almost immediately with the following information shown in the RMAN section of the job log:
 

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of startup command at 07/10/2009 12:22:15
RMAN-04014: startup failed: ORA-09925: Unable to create audit trail file
 

Verify the current configuration of the Oracle auditing by entering SQL*Plus:

SQL> show parameter audit;

Auditing is disabled by default, but can be enabled by setting the AUDIT_TRAIL variable
Accepted values: { none | os | db | db,extended | xml | xml,extended }

When OS or XML extended option is used, the AUDIT_FILE_DEST variable specifies the OS directory used for the audit trail file.

1) Set AUDIT_TRAIL = OS, so all the audit records are written into the OS audit trail.
2) Set AUDIT_FILE_DEST pointing to an existing folder in the Linux host.

To set AUDIT_TRAIL and AUDIT_FILE_DEST edit the initSID.ora file (where SID is the database name in the $ORACLE_HOME/dbs directory) and add the following lines:

audit_trail OS
audit_file_dest /u01/app/oracle/admin/orcl/adu

Also, it can be done dynamically using SQL*Plus commands. Contact your Oracle DBA for further information.

3) Verify that the account used to access the Linux host has these two variables in its environment.
4) Verify that the account used to access the Linux host has enough permissions to write to the specified folder.

Note:
Windows Servers do not use the AUDIT_FILE_DEST parameter. Operating System auditing changes are recorded in the Event Viewer.