AUSEARCH-EXPRESSION
Section: Linux Audit (5)
Updated: Feb 2008
Page Index
NAME
ausearch-expression - audit search expression format
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