Novell LINUX ENTERPRISE 11 SP1 - LINUX AUDIT Manuale utente

Linux audit allows you to comprehensively log and track any access to les, directories, or resources of your

system and trace system calls. It enables you to monitor your system for application misbehavior or code mal-

functions. By creating a sophisticated set of rules including le watches and system call auditing, you can make

sure that any violation of your security policies is noticed and properly addressed.

To set up Linux audit on your system, proceed as follows:

1. Stop the audit daemon that is running by default with

the rcauditd stop command.

2. Adjust the system conguration for audit and enable

audit.

3. Congure the audit daemon.

4. Determine which system components to audit and set

up audit rules.

5. Start the audit daemon after you have completed the

conguration of the audit system using the rcauditd

start command.

6. Determine which reports to run and congure these

reports.

7. Analyze the audit logs and reports.

8. (Optional) Analyze individual system calls with autrace.

IMPORTANT: Users Entitled to Work

with Audit

The audit tools, conguration les, and logs are

only available to root. This protects audit from

ordinary users of the system. To manipulate any

aspect of audit, you must be logged in as root.

Enabling Audit

Your rst task enabling audit is to activate system call audit-

ing, since system call auditing capabilities are needed even

when you are only conguring plain le or directory

watches:

Enabling System Call Auditing for One Session Only

Enable with auditctl -e 1 and disable with au-

ditctl -e 0. These settings are not persistent and do

not survive a reboot.

Enabling System Call Auditing Permanently

Permanently enable audit contexts for system calls by

changing AUDITD_DISABLE_CONTEXTS in /etc/

sysconfig/auditd from yes to no. To permanently

disable audit contexts for system calls, revert this setting

to yes. THis conguration will be applied with the next

start of the audit daemon.

Conguring Audit

The conguration of the audit daemon is contained in the

/etc/audit/auditd.conf conguration le. The default

settings as shipped with SUSE Linux Enterprise should be

sufcient for most setups.

log_file = /var/log/audit/audit.log

log_format = RAW

log_group = root

priority_boost = 4

Linux Audit Quick Start

SUSE Linux Enterprise 11 SP1

NOVELL® QUICK START CARD

flush = INCREMENTAL

freq = 20

num_logs = 4

disp_qos = lossy

dispatcher = /usr/sbin/audispd

name_format = NONE

#name = mydomain

max_log_file = 5

max_log_file_action = ROTATE

space_left = 75

space_left_action = SYSLOG

action_mail_acct = root

admin_space_left = 50

admin_space_left_action = SUSPEND

disk_full_action = SUSPEND

disk_error_action = SUSPEND

#tcp_listen_port =

tcp_listen_queue = 5

#tcp_client_ports = 1024-65535

tcp_client_max_idle = 0

Most of the settings in this le concern the audit log les

and how the logging is done. The most important settings

all concern the actions the daemon should take when en-

countering certain critical conditions or errors (system low

on disk space, system out of disk space, or disk error) and

when to warn the administrator about these conditions.

These actions are customizable and range from a mere

warning in syslog to a complete halt of the system. For more

information about /etc/audit/auditd.conf, refer to

The Linux Audit Framework manual and the manual page

of auditd.conf (auditd.conf(8)).

Setting Up Audit Rules

Audit rules are used to specify which components of your

system are audited. There are three basic types of audit

rules:

• Basic audit system parameters

• File and directory watches

• System call audits

Before creating an audit rule set and before rolling it out

to your system, carefully determine which components to

audit. Extensive auditing causes a substantial logging load.

Make sure that your system provides enough disk space to

store large audit logs and test your audit rule set extensively

before rolling it out to a production system.

Audit rules can either be passed to the audit system by the

command line using auditctl or bundled into a rules le

located under /etc/audit/audit.rules that is read

during the start of the audit daemon:

# basic audit system parameters

-D

-b 8192

-f 1

-e 1

# some file and directory watches

-w /var/log/audit/

-w /etc/audit/auditd.conf -p rxwa

-w /etc/audit/audit.rules -p rxwa

-w /etc/passwd -p rwxa

-w /etc/sysconfig/

# an example system call rule

-a entry,always -S umask

The basic audit system parameters include a rule to delete

any preexisting rules (-D) to avoid clashes with the new

rules, a rule that sets the number of outstanding audit

buffers (-b), the failure ag (-f), and the enable ag (-e):

-b

Depending on the audit load of your system, increase

or decrease the number of outstanding audit buffers. If

there are no more buffers left, the kernel checks the

failure ag for action.

-f

The failure ag controls the kernel's reaction to critical

errors. Possible values are 0(silent), 1(printk, print a

failure message), and 2(panic, bring the system

down—no clean shutdown and risk of data loss or cor-

ruption).

-e

If set to 1, this enables audit and audit contexts for sys-

tem calls. Setting it to 2does the same, but also locks

down the conguration. Set to 0, audit is disabled. This

ag is used to enable or disable audit temporarily.

File system watches can be added whenever you want to

track les or directories for unauthorized access. Typical

examples would include watching the audit conguration

and logs and user and security databases. Use permission

ltering to focus on those system calls requesting the per-

missions in which you are interested:

-w /etc/audit/audit.rules -p rxwa

The -p ag enables permission ltering. This example has

permission ltering turned on for read, write, execute, and

attribute change permissions.

Note the following limitations to le system watches:

• Directory watches produce less verbose logs than exact

le watches. When in need of detailed le-related

records, enable separate le watches for all les of inter-

est.

•Pathname globbing of any kind is not supported by audit.

Always use the exact pathnames.

•Auditing can only be performed on existing les. Any les

added while the audit daemon is already running are ig-

nored until the audit rule set is updated to watch the

new les.

Assigning keys to your audit rules helps you to identify any

records related to this rule in the logs. An example rule plus

key:

-w /etc/var/log/audit/ -k LOG_audit

The -k option attaches a text string to any event that is

recorded in the logs due to this rule. Using the ausearch

log analyzer, you can easily lter for any events related to

this particular rule.

A sample system call audit rule could look like the following:

-a entry,always -S umask

This adds the rule to the system call entry list (-a) and logs

an event whenever this system call is used (entry,al-

ways). The -S option precedes the actual system call,

umask in this example. Using -F, you could add optional

ltering to this rule. For more information about audit rules,

refer to The Linux Audit Framework and the manual page

of auditctl (auditctl(8)).

Generating Reports

Every audit event is recorded in the audit log, /var/log/

audit/audit.log. To avoid having to read the raw audit

log, congure custom audit reports with aureport and run

them regularly. Use the aureport tool to create various types

of reports ltering for different elds of the audit records

in the log. The output of any aureport command is

printed in column format and can easily be piped to other

commands for further processing. Because the aureport

commands are scriptable, you can easily create custom re-

port scripts to run at certain intervals to gather the audit

information for you.

aureport --summary

Run this report to get a rough overview of the current

audit statistics (events, logins, processes, etc.). To get

detailed information about any of the event categories

listed, run individual reports for the event type.

aureport --success

Run this report to get statistics of successful events on

your system. This report includes the same event cate-

gories as the summary report. To get detailed informa-

tion for a particular event type, run the individual report

adding the --success option to lter for successful

events of this type, for example, aureport -f --

success to display all successful le-related events.

aureport --failed

Run this report to get statistics of failed events on your

system. This report includes the same event categories

as the summary report. To get detailed information for

a particular event type, run the individual report adding

the --failed option to lter for failed events of this

type, such as aureport -f --failed to display all

failed le-related events.

aureport -l

Run this command to generate a numbered list of all

ID, host and terminal used, name of the executable,

success or failure of the attempt, and an event ID.

aureport -p

Run this report to generate a numbered list of all pro-

cess-related events. This command generates a numbered

list of all process events including date, time, process ID,

name of the executable, system call, audit ID, and event

number.

aureport -f

Run this report to generate a numbered list of all le-

related events. This command generates a numbered list

of all process events including date, time, process ID,

name of the executable, system call, audit ID and event

number.

aureport -u

Run this report to nd out which users are running what

executables on your system. This command generates a

numbered list of all user-related events including date,

time, audit ID, terminal used, host, name of the exe-

cutable, and an event ID.

Use the -ts and -te (for start time and end time) options

with any of the above commands to limit your reports to

a certain time frame. Use the -i option with any of these

commands to transform numeric entities to human-readable

text. The following command creates a le report for the

time between 8 am and 5:30 pm on the current day and

converts numeric entries to text.

aureport -ts 8:00 -te 17:30 -f -i

Analyzing Audit Log Files and Reports

While aureport helps you generate custom reports focusing

on a certain area, ausearch helps you to nd the detailed

log entry of individual events:

ausearch -a audit_event_id

Run this search to view all records carrying a particular

audit event ID. Each audit event message is logged along

with a message ID consisting of a UNIX epoch time stamp

plus a unique event ID separated by a colon. All events

that are logged from one application's system call have

the same event ID. For example, use ausearch -a

1234 to display all audit events carrying this audit event

ID. As one application's system call may trigger several

events to be logged, you are likely to retrieve more than

one record from the log.

ausearch -ul login_id

Run this search to view records associated with a partic-

ular login user ID. It displays any records related to the

user login ID specied provided that user had been able

to log in successfully. For example, use ausearch -ul

root to list all processes owned by the given login user

ID.

ausearch -k key

Run this search to nd records that contain a certain key

assigned in the audit rule set. For example, use ause-

arch -k CFG_etc to display any records containing

the CFG_etc key.

ausearch -m message_type

Run this search to nd records related to a particular

message type. Examples of valid message types include

PATH,SYSCALL,USER_LOGIN. Invoking ausearch

-m without a message type displays a list of all message

types.

ausearch -f filename

Run this search to nd records containing a certain le-

name. For example, run ausearch -f /foo/bar for

all records related to the /foo/bar le. Using the le-

name alone would work as well, but using relative paths

would not.

ausearch -p process_id

Run this to search for records related to a certain process

ID. For example, use ausearch -p 13368 to search

for all records related to this process ID.

Use the -ts and -te (for start time and end time) options

with any of these commands to limit your reports to a cer-

tain time frame. Use the -i option with any of these to

transform numeric entities to human readable text. The

following command searches for any le event related to

audit.log that took place any time between 8 am and

5:30 pm on the current day and converts numeric entries

to text.

ausearch -ts 8:00 -te 17:30 -f audit.log -i

Analyzing Individual System Calls

Perform dedicated audits of individual processes using the

autrace command. autrace works similarly to the strace

command, but gathers slightly different information. The

output of autrace is written to /var/log/audit/audit

.log and does not look any different from the standard

audit log entries.

When performing an autrace on a process, make sure that

any audit rules are purged from the queue to avoid having

these rules clash with the ones autrace adds itself. Delete

the audit rules with the auditctl -D command.

autrace /usr/bin/less /etc/sysconfig/auditd

Waiting to execute: /usr/bin/less

Cleaning up...

No rules

Trace complete. You can locate the records

with 'ausearch -i -p 7642'

Always use the full path to the executable to autrace. After

the trace is complete, autrace provides you with the event

ID of the trace, so you can analyze the entire data trail with

ausearch. To restore the audit system to use the audit rule

set again, just restart the audit daemon by calling rcauditd

restart.

Audit Tool Set

auditctl

Controls the audit system. Check the audit daemon's

status and rule set, delete rules, or create new ones.

aureport

Create various types of reports from the audit daemon

logs.

ausearch

Create custom queries to search the audit daemon logs.

autrace

Add audit rules to trace a process. Similar to strace.

rcauditd

Controls the audit init script.

Files

/etc/audit/auditd.conf

Contains conguration options specic to the audit

daemon, such as log le location, log rotation, maximum

size of the log le, and various actions to take when the

system starts to run low on disk space.

/etc/sysconfig/auditd

Controls conguration aspects of auditd that are not

covered in /etc/audit/auditd.conf, such as the

locale to use with audit, the use of audit contexts with

system calls, and whether rules and watches should be

deleted on shutdown of the system.

/etc/audit/audit.rules

Controls the rules auditd processes to track system calls

and le and directory access.

/var/log/audit/audit.log

The audit log le.

For More Information

For a more detailed introduction to the Linux audit frame-

work, refer to the The Linux Audit Framework manual that

is available at http://www.novell.com/

documentation/sles11/http://www.novell.com/

documentation/sled11/.

Legal Notice

This manual is protected under Novell intellectual property

rights. By reproducing, duplicating or distributing this

manual you explicitly agree to conform to the terms and

conditions of this license agreement.

This manual may be freely reproduced, duplicated and dis-

tributed either as such or as part of a bundled package in

electronic and/or printed format, provided however that

the following conditions are fullled:

That this copyright notice and the names of authors and

contributors appear clearly and distinctively on all repro-

duced, duplicated and distributed copies. That this manual,

specically for the printed format, is reproduced and/or

distributed for noncommercial use only. The express autho-

rization of Novell, Inc must be obtained prior to any other

use of any manual or part thereof.

For Novell trademarks, see the Novell Trademark and Ser-

vice Mark list http://www.novell.com/company/

legal/trademarks/tmlist.html. Linux* is a regis-

tered trademark of Linus Torvalds. All other third party

trademarks are the property of their respective owners. A

trademark symbol (®, ™ etc.) denotes a Novell trademark;

an asterisk (*) denotes a third party trademark.

All information found in this book has been compiled with

utmost attention to detail. However, this does not guarantee

complete accuracy. Neither Novell, Inc., SUSE LINUX Prod-

ucts GmbH, the authors, nor the translators shall be held

liable for possible errors or the consequences thereof.

Created by SUSE® with XSL-FO

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