ausearch-expression

Overview

This man page describes the format of "ausearch expressions". Parsing and evaluation of these expressions is provided by libauparse and is common to applications that use this library.

Lexical Structure

White space (ASCII space, tab and new-line characters) between tokens is ignored. The following tokens are recognized:

Punctuation

( ) \

Logical operators

! && ||

Comparison operators

< <= == > >= !== i= i!= r= r!=

Unquoted strings

Any non-empty sequence of ASCII letters, digits, and the _ symbol.

Quoted strings

A sequence of characters surrounded by the " quotes. The \ character starts an escape sequence. The only defined escape sequences are \\ and \". The semantics of other escape sequences is undefined.

Regexps

A sequence of characters surrounded by the / characters. The \ character starts an escape sequence. The only defined escape sequences are \\ and \/. The semantics of other escape sequences is undefined.

Anywhere an unquoted string is valid, a quoted string is valid as well, and vice versa. In particular, field names may be specified using quoted strings, and field values may be specified using unquoted strings.

Expression Syntax

The primary expression has one of the following forms:

field comparison-operator value

\regexp string-or-regexp

field is either a string, which specifies the first field with that name within the current audit record, or the \ escape character followed by a string, which specifies a virtual field with the specified name (virtual fields are defined in a later section).

field is a string. operator specifies the comparison to perform

r= r!=

Get the "raw" string of field, and compare it to value. For fields in audit records, the "raw" string is the exact string stored in the audit record (with all escaping and unprintable character encoding left alone); applications can read the "raw" string using auparse_get_field_str(3). Each virtual field may define a "raw" string. If field is not present or does not define a "raw" string, the result of the comparison is false (regardless of the operator).

i= i!=

Get the "interpreted" string of field, and compare it to value. For fields in audit records, the "interpreted" string is an "user-readable" interpretation of the field value; applications can read the "interpreted" string using auparse_interpret_field(3). Each virtual field may define an "interpreted" string. If field is not present or does not define an "interpreted" string, the result of the comparison is false (regardless of the operator).

< <= == > >= !==

Evaluate the "value" of field, and compare it to value. A "value" may be defined for any field or virtual field, but no "value" is currently defined for any audit record field. The rules of parsing value for comparing it with the "value" of field are specific for each field. If field is not present, the result of the comparison is false (regardless of the operator). If field does not define a "value", an error is reported when parsing the expression.

In the special case of \regexp regexp-or-string, the current audit record is taken as a string (without interpreting field values), and matched against regexp-or-string. regexp-or-string is an extended regular expression, using a string or regexp token (in other words, delimited by " or /).

If E1 and E2 are valid expressions, then ! E1, E1 && E2, and E1 || E2 are valid expressions as well, with the usual C semantics and evaluation priorities. Note that ! field op value is interpreted as !(field op value), not as (!field) op value.

Virtual Fields

The following virtual fields are defined:

\timestamp

The value is the timestamp of the current event. value must be formatted as:

ts:seconds.milli

where seconds and milli are decimal numbers specifying the seconds and milliseconds part of the timestamp, respectively.

\timestamp_ex

This is similar to \timestamp but also includes the event's serial number. value must be formatted as:

ts:seconds.milli:serial

where serial is a decimal number specifying the event's serial number.

\record_type

The value is the type of the current record. value is either the record type name, or a decimal number specifying the type.

Semantics

The expression as a whole applies to a single record. The expression is true for a specified event if it is true for any record associated with the event.

Examples

As a demonstration of the semantics of handling missing fields, the following expression is true if field is present:

(field r= "") || (field r!= "")

and the same expression surrounded by !( and ) is true if field is not present.

Future Directions

New escape sequences for quoted strings may be defined.

For currently defined virtual fields that do not define a "raw" or "interpreted" string, the definition may be added. Therefore, don't rely on the fact that comparing the "raw" or "interpreted" string of the field with any value is false.

New formats of value constants for the \timestamp virtual field may be added.

Author

Miloslav Trmac

Referenced By

audit-viewer(8), ausearch_add_expression(3), ausearch_add_interpreted_item(3), ausearch_add_item(3), ausearch_add_timestamp_item(3), ausearch_add_timestamp_item_ex(3).