Linux Audit system
The Linux Audit system provides fine-grained logging of security-related events, known as Linux audit logs. It is enabled by creating Linux auditing rules that specify which events to log. For example, you can add rules to audit:
-
Access to a specific file or directory
-
Specific system calls
-
Commands executed by a user
-
Authentication events
-
Network access
The Audit system architecture comprises:
-
A kernel component that generates events
-
The
auditddaemon collects events from the kernel component and writes them to a log file -
The
audispdispatcher daemon relays events to other applications for additional processing -
The
auditctlcontrol utility to configure the kernel component
It also provides a set of tools for reading the Audit log files:
-
aulastlists the last logged-in users -
aulastlogoutputs the last login for all machine users -
aureportproduces summary reports of the Audit logs -
ausearchsearches Audit logs for events matching given criteria -
auvirtlists virtual machine sessions found in the Audit logs
For more information about the audit system, see System Auditing in the Red Hat Enterprise Linux Security Guide, your operating system manual, and the Linux Audit Documentation Project repository on GitHub.
Linux Audit rules
The Linux Audit system generates events according to Audit rules.
These rules can be set dynamically with the auditctl utility or stored persistently in the /etc/audit/rules.d folder.
Persistent rule files are automatically compiled to /etc/audit/audit.rules when auditd is initialized.
There are three types of rules:
-
A control rule modifies the auditing behavior
-
A file system rule watches a file or directory
-
A system call rule logs an event for a particular system call
See Defining Audit Rules in the Red Hat Enterprise Linux Security Guide for further details on Linux Audit rules.
Some common control rules are:
-
-b backlogto set the maximum number of audit buffers. Specify a bigger value for busier systems with a high log volume. -
-Dto delete all rules and watches. This rule is typically the first one. -
-e [0..2]to control auditing. The accepted values are0to disable auditing temporarily,1to enable it, or2to lock the configuration until the next reboot (typically the last rule).
The following is a set of basic rules commonly found in rulesets.
# Delete all rules (typically the first rule) -D # Increase buffers from the default 64 to 320 -b 320 # Lock Audit rules until the next reboot (the last rule) -e 2
To create a file system rule, use -w path -p permissions -k key_name as follows:
-
The
pathargument defines the file or directory you want to audit. -
The
permissionsargument sets the type of access to audit. It accepts a combination ofr(read access),w(write access),x(execute access), anda(attribute change). -
The
key_nameargument is an optional tag for identifying the rule.
This rule monitors /etc/passwd for modifications and tags access events with passwd.
-w /etc/passwd -p wa -k passwd
To create a system call rule, use -a action,filter -S system_call -F field=value -k key_name as follows:
-
The
actionargument can be eitheralways(to generate a log entry) ornever(to suppress a log entry). Generally, you addalwaysrules first because rules are matched from top to bottom. -
The
filterargument is one oftask(when a task is created),exit(when a system call exits),user(when a call originates from user space), orexclude(to filter events). -
The
system_callargument specifies the system call name. You can specify the-Sflag more than once to monitor multiple system calls. -
The
field=valuepair can be used for additional filtering options and can also be specified more than once. -
The
key_nameargument is an optional tag for identifying the rule.
This rule audits system time changes.
-a always,exit -F arch=b64 -S adjtimex -S settimeofday -k system_time
System call rules can audit activities related to files, such as:
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Creation
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Modification
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Deletion
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Changes in access permissions and ownership
This rule audits file deletions with the unlink or rename command.
-a always,exit -F arch=b64 -S unlink,unlinkat,rename,renameat -F success=1 -F auid>=1000 -F auid!=unset -F key=successful-delete
You can also audit network connections with system call rules.
This rule audits successful incoming or outgoing external network connections.
-a always,exit -F arch=b64 -S accept,connect -F key=external-access
Combine the different rules to create a ruleset.
Below is a simple Linux Audit ruleset based on the above examples.
# Delete all rules -D # Increase buffers from the default 64 to 320 -b 320 # Watch /etc/passwd for modifications and tag with 'passwd' -w /etc/passwd -p wa -k passwd # Audit system time changes -a always,exit -F arch=b64 -S adjtimex -S settimeofday -k system_time # Lock Audit rules until the next reboot -e 2
See The Linux Audit Project and auditd-attack repositories on GitHub for more examples.
Forwarding Linux Audit logs to syslog
You can configure the Linux Audit system to forward log messages to a local syslog daemon.
Enable the Linux Audit syslog plugin to forward logs to the /dev/log socket.
The example below shows a modified /etc/audisp/plugins.d/syslog.conf file to enable log forwarding to syslog.
active = yes direction = out path = builtin_syslog type = builtin args = LOG_INFO format = string
To verify your configuration, you can add a test rule in /etc/audit/rules.d/audit.rules to monitor modifications to a file, such as /tmp/audit_syslog.
-w /tmp/audit_syslog -p wa
You can then configure NXLog Agent to collect syslog messages from the /dev/log socket.
The configuration below uses the im_uds input module to collect logs from the dev/log socket.
The Exec block filters for messages containing the string audit_syslog and parses matching messages with the parse_syslog_bsd() procedure of the xm_syslog module.
Finally, it converts records to JSON format using the to_json() procedure of the xm_json module.
<Extension syslog>
Module xm_syslog
</Extension>
<Extension json>
Module xm_json
</Extension>
<Input linux_audit>
Module im_uds
UDS /dev/log
<Exec>
if not ($raw_event =~/.+audit_syslog.+/) {
drop();
}
parse_syslog_bsd();
to_json();
</Exec>
</Input>Restart auditd and NXLog Agent to apply the configuration changes:
# systemctl restart auditd # systemctl restart nxlog
{
"EventReceivedTime": "2022-09-28T21:09:13.959876+00:00",
"SourceModuleName": "linux_audit",
"SourceModuleType": "im_uds",
"SyslogFacilityValue": 1,
"SyslogFacility": "USER",
"SyslogSeverityValue": 6,
"SyslogSeverity": "INFO",
"SeverityValue": 2,
"Severity": "INFO",
"Hostname": "administrator",
"EventTime": "2022-09-28T21:09:13.000000+00:00",
"SourceName": "audispd",
"Message": "node=administrator type=SYSCALL msg=audit(1588108153.953:1246): arch=c000003e syscall=257 success=yes exit=3 a0=ffffff9c a1=55a4e3921110 a2=41 a3=1a4 items=2 ppid=2417 pid=3374 auid=1000 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=3 comm=\"vim\" exe=\"/usr/bin/vim.basic\" key=\"audit_syslog\""
}Collecting Linux Audit logs with im_linuxaudit
NXLog Agent includes the im_linuxaudit input module, which directly accesses the kernel component of the Linux Audit system.
With this module, you can configure audit rules and collect logs all within NXLog Agent without requiring tools such as auditd or other userspace software.
| If a Linux Audit System (im_linuxaudit) module instance is suspended and the Linux Audit backlog limit is exceeded, all system processes that generate audit messages will be blocked. For this reason, we recommend disabling FlowControl for im_linuxaudit module instances. When flow control is disabled, NXLog Agent will discard messages instead of suspending log processing. In addition, to reduce the risk of data loss, you can increase the LogQueueSize of subsequent module instances or introduce a pm_buffer instance. |
This example uses im_linuxaudit to configure and collect Linux Audit logs. It specifies the ruleset in the configuration with the Rules block directive.
<Input linux_audit>
Module im_linuxaudit
FlowControl FALSE
<Rules>
# Delete all rules (This rule has no affect; it is performed
# automatically by im_linuxaudit)
-D
# Increase buffers from the default 64 to 320
-b 320
# Watch /etc/passwd for modifications and tag with 'passwd'
-w /etc/passwd -p wa -k passwd
# Audit system time changes
-a always,exit -F arch=b64 -S adjtimex -S settimeofday -k system_time
# Lock audit rules until the next reboot
-e 2
</Rules>
</Input>Alternative to the configuration example above, you can use the LoadRule directive to load the rules from a file.
You must place the file in a location accessible by NXLog Agent and different than the Linux Audit rules folder /etc/audit/rules.d.
<Input linux_audit>
Module im_linuxaudit
FlowControl FALSE
LoadRule '/opt/nxlog/etc/audit.rules'
</Input>Collecting Linux Audit logs with userspace tools
There are several ways to collect logs via the regular Linux Audit userspace tools, including from auditd logs and by network via audispd.
Setting up auditd
First, install and configure the Linux Audit userspace components:
-
Install the
auditpackage. Include theaudispd-pluginspackage if you will be usingaudispd. See Forwarding audit logs over the network with audispd below.-
On Red Hat/CentOS:
# yum install audit -
On Debian/Ubuntu:
# apt-get install auditd
-
-
Configure Auditd by editing the
/etc/audit/auditd.conffile. See Configuring the Audit Service in the Red Hat Enterprise Linux Security Guide and the auditd.conf(5) man page. -
Enable and restart the Auditd service to reload the configuration. Make sure that the rules are not locked, or changes will not be applied until the next system reboot.
-
On Red Hat/CentOS:
# service auditd start # systemctl enable auditd -
On Debian/Ubuntu:
# systemctl restart auditd
-
Collecting Auditd logs
By default, Auditd writes events to /var/log/audit/audit.log.
NXLog Agent can read logs from this file.
The Auditd log file requires root privileges to access it.
NXLog Agent will not be able to read the file when running as the restricted nxlog user.
You can either omit the User and Group global directives from nxlog.conf to run NXLog Agent as root or adjust the permissions as follows.
See Reading Rsyslog log files for more information.
-
Use the
log_groupoption in/etc/audit/audit.confto set the group ownership for Auditd log files. -
Change the current ownership of the log directory and files with
chgrp -R adm /var/log/audit. -
Add the
nxloguser to theadmgroup withusermod -a -G adm nxlog. -
Configure NXLog Agent and restart the system to apply the changes.
This configuration uses the im_file input module to read the Auditd log file.
Next, it parses key-value pairs with the xm_kvp module and enriches log records with the $Hostname, $FQDN, $Tag, and $SourceName fields.
Finally, it converts log records to JSON format with the to_json() procedure of the xm_json module.
<Extension json>
Module xm_json
</Extension>
<Extension audit_parser>
Module xm_kvp
KVPDelimiter ' '
KVDelimiter =
EscapeChar '\'
</Extension>
<Input linux_audit>
Module im_file
File '/var/log/audit/audit.log'
<Exec>
audit_parser->parse_kvp();
$Hostname = hostname();
$FQDN = hostname_fqdn();
$Tag = "audit";
$SourceName = "selinux";
to_json();
</Exec>
</Input>Forwarding audit logs over the network with audispd
The Audit dispatcher audispd can forward events to a remote server with the audisp-remote plugin included in the audispd-plugins package.
-
Configure the
audisp-remoteplugin. Specify the appropriateremote_serverandformatfor your environment./etc/audisp/audisp-remote.confremote_server = 127.0.0.1 port = 60 transport = tcp queue_file = /var/spool/audit/remote.log mode = immediate queue_depth = 2048 format = ascii network_retry_time = 1 max_tries_per_record = 3 max_time_per_record = 5 heartbeat_timeout = 0 network_failure_action = stop disk_low_action = ignore disk_full_action = ignore disk_error_action = syslog remote_ending_action = reconnect generic_error_action = syslog generic_warning_action = syslog overflow_action = syslog
-
Activate the plugin from
/etc/audisp/plugins.d/au-remote.confby settingactive = yes. -
Optionally, you can disable logging to file by Auditd. Edit
/etc/audit/auditd.confand setwrite_logs = no(this option is the same aslog_format = NOLOG). -
Configure and restart NXLog Agent.
-
Restart the Auditd service.
This configuration uses the im_tcp input module to listen for logs on TCP port 60.
It parses key-value pairs with the xm_kvp module and enriches log records with the $Hostname, $FQDN, $Tag, and $SourceName fields.
<Extension audit_parser>
Module xm_kvp
KVPDelimiter ' '
KVDelimiter =
EscapeChar '\'
</Extension>
<Input linux_audit>
Module im_tcp
ListenAddr 0.0.0.0:60
<Exec>
audit_parser->parse_kvp();
$Hostname = hostname();
$FQDN = hostname_fqdn();
$Tag = "audit";
$SourceName = "auditd";
</Exec>
</Input>