NAME
twpolicy - Tripwire policy file reference
DESCRIPTION
The policy file describes system objects to be monitored by Tripwire,
and specifies what properties for each object should be collected and
stored in the database file. Each object in the policy file is
associated with a property mask, which describes what changes to the
file or directory Tripwire should monitor, and which ones can safely be
ignored. By customizing the various aspects of the policy file, the
system administrator can very closely control how Tripwire checks the
integrity of any system.
During installation, an encoded and signed policy file (tw.pol) will be
created in the /etc/tripwire directory, and a plain text copy of this
policy file (twpol.txt) will be generated in the same directory. An
additional text file (policyguide.txt) illustrates all of the features
of the policy language. Both of the text files are heavily commented
and can be used as a reference during policy file creation.
A new policy file is first created using the twadmin --create-polfile
command. With this command, the user can designate an existing plain
text file as the current policy file. Using the current site key and
passphrase, the new configuration file is encoded, signed and saved.
Once the initial policy file has been generated, any changes should be
made with the tripwire --update-policy command, rather than by simply
overwriting the policy file with the twadmin --create-polfile command.
This is an important distinction‐‐when a new policy file is created,
the Tripwire database must be re-initialized. If an intruder has
modified files since the last integrity check, these changes will not
be detected, and will be included as part of the new "baseline"
database.
COMPONENTS OF THE POLICY FILE
The basic components of policy files are comments, rules, directives,
and variables. Each of these components is described in greater detail
below.
Comments
In a policy file, any text following a ’#’, up to the next line break,
is considered a comment. Example:
# This is a comment.
/bin -> $(ReadOnly); # A comment can go here, too.
Rules
Policy rules determine whether and to what extent Tripwire will check
particular files and directories. There are two kinds of policy rules
recognized by Tripwire:
1) Normal rules define which properties of a particular file or
directory tree Tripwire scans.
2) Stop points tell Tripwire not to scan a particular file or
directory.
Normal Rules
The format for a normal rule is:
object_name -> property_mask;
where the object_name is the fully qualified pathname for a directory
or file, and property_mask specifies what properties of an object to
examine or ignore. The ’->’ token separates the pathname and the
property mask. Whitespace must separate the object name and ’->’
token. A semicolon must terminate the rule. If the pathname specified
is a directory, the directory and all of its descendants will be
scanned with the indicated property mask. If the pathname refers to an
individual file, only that file will be scanned with the specified
property mask. Examples:
# Defines Tripwire behavior for entire /bin directory tree.
/bin -> $(ReadOnly);
# Defines Tripwire behavior for a single file. In this case,
# Tripwire watches for all properties of hostname.hme0.
/etc/hostname.hme0 -> $(IgnoreNone) -ar;
# Scan the entire /etc directory tree using mask1, except the
# file /etc/passwd, which should be scanned using mask2.
/etc -> $(mask1);
/etc/passwd -> $(mask2);
Only one rule may be associated with any given object. If any object
has more than one rule in a policy file, Tripwire will print an error
message and exit without scanning any files. For example:
# This is an example of an illegal construct.
/usr/bin -> $(mask3);
/usr/bin -> $(mask4);
Object Names
In this document, policy file objects are fully qualified pathnames of
files and directories. Environment variables are not allowed for
security reasons. Examples:
/etc # valid object name.
/etc/passwd # valid object name.
$HOME # not valid.
Property Masks
Property masks designate which Tripwire properties of a given object
should be examined. A property mask consists of a series of single-
character symbols, each of which may be preceded by an optional plus or
minus sign. Each character symbol stands for a particular Tripwire
property to be examined during integrity checking. If the character is
preceded by a plus, checking is done for that property; if preceded by
a minus, checking is not done for that property. For example:
+p # compare permissions.
-p # ignore permissions.
Each rule in the policy file must have a property mask. Examples:
/etc -> $(IgnoreAll); #valid property mask.
/etc -> +p-p; #valid property mask.
/etc -> ; #invalid property mask.
Characters in a property mask without a preceding plus or minus sign
are assumed to be plus. If a property is not specified in the property
mask, it is ignored, which is equivalent to turning it off with the
minus sign. Examples:
# Examine permissions and link count.
# All three of the following are equivalent.
+p+n
pn
pn-g
Characters used in property masks, with descriptions:
- Ignore the following properties
+ Record and check the following properties
a Access timestamp
b Number of blocks allocated
c Inode timestamp (create/modify)
d ID of device on which inode resides
g File owner’s group ID
i Inode number
l File is increasing in size (a "growing file")
m Modification timestamp
n Number of links (inode reference count)
p Permissions and file mode bits
r ID of device pointed to by inode
(valid only for device objects)
s File size
t File type
u File owner’s user ID
C CRC-32 hash value
H Haval hash value
M MD5 hash value
S SHA hash value
Stop Points
Stop points are used to specify specific files or directories that
Tripwire should not scan. The syntax for stop points is:
! object_name ;
For example:
!/etc/init.d;
# The directory /etc/init.d will not be scanned.
/etc -> $(ReadOnly);
!/etc/rc.d;
!/etc/mnttab;
# Scan all of /etc, but do not scan two particular
# files in the /etc hierarchy.
Rule Attributes
Rule attributes work with normal rules to modify their behavior or
provide additional information. Multiple attributes can be assigned to
each rule. Rule attributes are not case-sensitive. Rule attributes
may be applied to a single rule using the following syntax:
object_name -> property_mask (rule attribute = value);
For example:
/usr/lib -> $(ReadOnly) (emailto = admin@foo.com, severity = 80);
#This rule will notify the admin if any violations of the
#rule occur and designate the severity as 80.
Rule attributes can also be specified for a group of rules, using the
format:
(attribute list)
{
rule list;
}
For example:
(emailto = admin@foo.com, severity = 80)
{
/usr/lib -> $(ReadOnly);
}
is equivalent to the attribute example above.
The following four rule attributes are supported by Tripwire:
rulename The rulename attribute is used to associate a rule or
set of rules with a specific name. In a report file,
this name will be associated with violations to the
specified rule. This feature is useful if you want to
track certain objects within a large Tripwire database.
For instance, if you associate the rule name "watchme"
with important files, you can sort through the Tripwire
report using "watchme" as a sorting key.
Example:
/etc -> +ug (rulename=watchme);
emailto The emailto attribute associates one or more email
addresses with a rule or group of rules. When an
integrity check is run with the --email-report option
and a rule is violated, a report of that violation will
be sent to the specified email address(es), using the
report format specified by the EMAILREPORTLEVEL variable
in the configuration file.
Example:
/etc -> +ug (emailto=admin@domain.com);
To specify multiple email addresses, include them as a
quoted, semicolon-delimited list.
/etc -> +ug (emailto="admin@foo.com;admin2@foo.com");
severity The severity attribute associates a numeric severity
level with a rule. When Tripwire is run in Integrity
Checking mode, it is possible to specify that only rules
exceeding a certain severity level are used. The
default severity level is 0, and values can range from 0
to 1,000,000.
Example:
/etc -> +ug (severity=50);
recurse The recurse attribute specifies how a rule will scan
directories. Valid values for recurse are true, false,
or a number from -1 to 1,000,000. If recurse is set to
true (or -1), tripwire will recursively scan the entire
contents of the directory (both files and
subdirectories). When recurse is set to false (or 0),
and the rule refers to a directory, Tripwire will scan
the inode of the directory but none of the files or
subdirectories contained therein. For positive recurse
value n, the rule will monitor all objects up to n
levels below the start point. Stop points within the
recursed directory still apply, and will prevent the
specified file or directory from being scanned.
When a rule refers to a file, specifying the recurse
attribute has no effect‐‐files will be scanned no matter
what value is given for recurse. The default value for
recurse is true.
Example:
/etc -> +ug (recurse=2);
Directives
Tripwire supports a small number of directives that allow conditional
interpretation of the policy file and certain diagnostic and debugging
operations. The primary purpose of directives is to support sharing of
a policy file among multiple machines. Directives use the following
syntax:
@@ directive_name [arguments]
Where the directive name is one of the directives listed below:
@@section # Designates a section of the policy file.
@@ifhost # Allow conditional interpretation
@@else # of the policy file.
@@endif
@@print # Print a message to standard output.
@@error # Print a message to standard output and then exit.
@@end # Marks the logical end-of-file.
The @@section directive is used to designate sections of the policy
file that are OS-specific. With Tripwire 2.3.1, valid arguments for
the @@section directive are FS and GLOBAL. If no @@section directive is
specified, FS will be assumed. If the argument to the @@section
directive is either NTFS or NTREG (which have meaning only on Windows
NT systems), Tripwire will skip all policy file text down to the next
valid @@section directive. Any other argument will cause an error.
The @@ifhost, @@else, and @@endif directives are used to allow
conditional interpretation of the policy file. With the @@ifhost
directive, multiple hostnames can be expressed as arguments, but they
must be separated by an ’||’, interpreted as the logical ’OR’. This
example illustrates how one might employ directives to use one policy
file with multiple hosts.
@@ifhost spock || kirk
/bin -> $(ReadOnly);
@@endif
@@ifhost chekov || uhura
/usr/bin -> +pinug;
@@else
/usr/bin -> +pinugsmC;
@@endif
The @@print and @@error directives are intended for debugging and
remote diagnostics. The syntax for these commands is:
@@print "string"
@@error "string"
The @@print directive prints string to stdout, while the @@error
directive prints string to stdout and causes the calling program to
exit with a non-zero status.
The @@end directive marks the end of the policy file. Any text
appearing after this directive will be ignored by Tripwire.
Variables
For user convenience, Tripwire’s policy file supports variables for
string substitution. Variables can be defined anywhere between rules.
The syntax for variable definition is:
variable = value;
Variable substitution is legal anywhere that a string could appear.
The syntax for variable substitution is:
$( variable )
Examples of variable definition and variable substitution on the left,
right, and both sides of rules. Note that variable names are case
sensitive.
param1 = +SMCH; # Set variable param1.
dir1 = /etc/inet; # Set variable dir1.
DIR1 = /etc/init.d; # Variables are case sensitive.
$(dir1) -> +tbamc; # Left hand substitution.
/etc/inet -> $(param1); # Right hand substitution.
$(DIR1) -> $(param1); # Double substitution.
A number of variables are predefined by Tripwire and may not be
changed. These variables represent different ways that files can
change, and can be used on the right side of rules to design a policy
file quickly.
ReadOnly ReadOnly is good for files that are widely available but
are intended to be read-only.
Value: +pinugtsdbmCM-rlacSH
Dynamic Dynamic is good for monitoring user directories and
files that tend to be dynamic in behavior.
Value: +pinugtd-srlbamcCMSH
Growing The Growing variable is intended for files that should
only get larger.
Value: +pinugtdl-srbamcCMSH
Device Device is good for devices or other files that Tripwire
should not attempt to open.
Value: +pugsdr-intlbamcCMSH
IgnoreAll IgnoreAll tracks a file’s presence or absence, but
doesn’t check any other properties.
Value: -pinugtsdrlbamcCMSH
IgnoreNone IgnoreNone turns on all properties and provides a
convenient starting point for defining your own property
masks. (For example, mymask = $(IgnoreNone) -ar;)
Value: +pinugtsdrbamcCMSH-l
VERSION INFORMATION
This man page describes Tripwire 2.3.1.
AUTHORS
Tripwire, Inc.
COPYING PERMISSIONS
Permission is granted to make and distribute verbatim copies of this
man page provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of this
man page under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.
Permission is granted to copy and distribute translations of this man
page into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by Tripwire, Inc.
Copyright 2000 Tripwire, Inc. Tripwire is a registered trademark of
Tripwire, Inc. in the United States and other countries. All rights
reserved.
SEE ALSO
twintro(8), tripwire(8), twadmin(8), twprint(8), siggen(8),
twconfig(4), twfiles(5)
1 July 2000