What is audit daemon or service in Linux

Linux Audit Architecture

auditd is a userspace component to the Linux Auditing System. It’s responsible for writing audit records to the disk. Viewing the logs is done with the ausearch or aureport utilities. Configuring the audit system or loading rules is done with the auditctl utility. During startup, the rules in /etc/audit/audit.rules are read by auditctl and loaded into the kernel. Alternately, there is also an augenrules program that reads rules located in /etc/audit/rules.d/ and compiles them into an audit.rules file. The audit daemon itself has some configuration options that the admin may wish to customize. They are found in the /etc/audit/auditd.conf file.

Topic

  • What is audit daemon or service in Linux?
  • How auditd tracks events on Linux?
  • Linux auditd and its features
  • Information about Linux audit daemon

apt

  • Centos
  • RHEL
  • Scientific Linux
  • Fedora
  • Debian
  • Ubuntu

Solution


understanding /etc/audit/auditd.conf

local_events = yes
write_logs = yes
log_file = /var/log/audit/audit.log
log_group = root
log_format = RAW
flush = INCREMENTAL_ASYNC
freq = 50
max_log_file = 8
num_logs = 5
priority_boost = 4
disp_qos = lossy
dispatcher = /sbin/audispd
name_format = NONE
##name = mydomain
max_log_file_action = ROTATE
space_left = 75
space_left_action = SYSLOG
verify_email = yes
action_mail_acct = root
admin_space_left = 50
admin_space_left_action = SUSPEND
disk_full_action = SUSPEND
disk_error_action = SUSPEND
use_libwrap = yes
##tcp_listen_port = 60
tcp_listen_queue = 5
tcp_max_per_addr = 1
##tcp_client_ports = 1024-65535
tcp_client_max_idle = 0
enable_krb5 = no
krb5_principal = auditd
##krb5_key_file = /etc/audit/audit.key
distribute_network = no

local_events
This yes/no keyword specifies whether or not to include local events. Normally you want local events so the default value is yes. Cases where you would set this to no is when you want to aggregate events only from the network.

write_logs
This yes/no keyword determines whether or not to write logs to the disk.

log_file
This keyword specifies the full path name to the log file where audit records will be stored. It must be a regular file.

log_group
This keyword specifies the group that is applied to the log file’s permissions.

log_format
The log format describes how the information should be stored on disk. There are 2 options: raw and enriched. If set to RAW, the audit records will be stored in a format exactly as the kernel sends it. The ENRICHED option will resolve all uid, gid, syscall, architecture, and socket address information before writing the event to disk.

flush
Valid values are none, incremental, incremental_async, data, and sync. If set to none, no special effort is made to flush the audit records to disk. If set to incremental, Then the freq parameter is used to determine how often an explicit flush to disk is issued. The incremental_async parameter is very much like incremental except the flushing is done asynchronously for higher performance. The data parameter tells the audit daemon to keep the data portion of the disk file sync’d at all times. The sync option tells the audit daemon to keep both the data and meta-data fully sync’d with every write to disk. The default value is incremental_async.

freq
This is a non-negative number that tells the audit daemon how many records to write before issuing an explicit flush to disk command. This value is only valid when the flush keyword is set to incremental or incremental_async.

max_log_file
This keyword specifies the maximum file size in megabytes. When this limit is reached, it will trigger a configurable action. The value given must be numeric.

num_logs
This keyword specifies the number of log files to keep if rotate is given as the max_log_file_action. If the number is < 2, logs are not rotated. This number must be 999 or less. The default is 0 – which means no rotation. As you increase the number of log files being rotated, you may need to adjust the kernel backlog setting upwards since it takes more time to rotate the files. This is typically done in /etc/audit/audit.rules. If log rotation is configured to occur, the daemon will check for excess logs and remove them in effort to keep disk space available. The excess log check is only done on startup and when a reconfigure results in a space check.

priority_boost
This is a non-negative number that tells the audit daemon how much of a priority boost it should take. The default is 4. No change is 0

disp_qos
This option controls whether you want blocking/lossless or non-blocking/lossy communication between the audit daemon and the dispatcher. There is a 128k buffer between the audit daemon and dispatcher. This is good enough for most uses. If lossy is chosen, incoming events going to the dispatcher are discarded when this queue is full

dispatcher
The dispatcher is a program that is started by the audit daemon when it starts up. It will pass a copy of all audit events to that application’s stdin. Make sure you trust the application that you add to this line since it runs with root privileges.

name_format
This option controls how computer node names are inserted into the audit event stream. It has the following choices: none, hostname, fqd, numeric, and user. None means that no computer name is inserted into the audit event. hostname is the name returned by the gethostname syscall. The fqd means that it takes the hostname and resolves it with dns for a fully qualified domain name of that machine. Numeric is similar to fqd except it resolves the IP address of the machine. In order to use this option, you might want to test that ‘hostname -i’ or ‘domainname -i’ returns a numeric address. Also, this option is not recommended if dhcp is used because you could have different addresses over time for the same machine. User is an admin defined string from the name option. The default value is none.

name
This is the admin defined string that identifies the machine if user is given as the name_format option.

max_log_file_action
This parameter tells the system what action to take when the system has detected that the max file size limit has been reached. Valid values are ignore, syslog, suspend, rotate and keep_logs. If set to ignore, the audit daemon does nothing. syslog means that it will issue a warning to syslog. suspend will cause the audit daemon to stop writing records to the disk. The daemon will still be alive. The rotate option will cause the audit daemon to rotate the logs. It should be noted that logs with higher numbers are older than logs with lower numbers. This is the same convention used by the logrotate utility. The keep_logs option is similar to rotate except it does not use the num_logs setting. This prevents audit logs from being overwritten. The effect is that logs accumulate and are not deleted – which will trigger the space_left_action if the volume fills up. This is best used in combination with an external script used to archive logs on a periodic basis.

space_left
This is a numeric value in megabytes that tells the audit daemon when to perform a configurable action because the system is starting to run low on disk space. You may also append a percent sign (e.g. 5%) to the number to have the audit daemon calculate the number based on the disk partition size.

space_left_action
This parameter tells the system what action to take when the system has detected that it is starting to get low on disk space. Valid values are ignore, syslog, rotate, email, exec, suspend, single, and halt. If set to ignore, the audit daemon does nothing. syslog means that it will issue a warning to syslog. rotate will rotate logs, losing the oldest to free up space. Email means that it will send a warning to the email account specified in action_mail_acct as well as sending the message to syslog. exec /path-to-script will execute the script. You cannot pass parameters to the script. The script is also responsible for telling the auditd daemon to resume logging once its completed its action. This can be done by adding service auditd resume to the script. suspend will cause the audit daemon to stop writing records to the disk. The daemon will still be alive. The single option will cause the audit daemon to put the computer system in single user mode. The halt option will cause the audit daemon to shutdown the computer system.

verify_email
This option determines if the email address given in action_mail_acct is checked to see if the domain name can be resolved. This option must be given before action_mail_acct or the default value of yes will be used.

action_mail_acct
This option should contain a valid email address or alias. The default address is root. If the email address is not local to the machine, you must make sure you have email properly configured on your machine and network. Also, this option requires that /usr/lib/sendmail exists on the machine.

admin_space_left
This is a numeric value in megabytes that tells the audit daemon when to perform a configurable action because the system is running low on disk space. This should be considered the last chance to do something before running out of disk space. The numeric value for this parameter should be lower than the number for space_left. You may also append a percent sign (e.g. 1%) to the number to have the audit daemon calculate the number based on the disk partition size.

admin_space_left_action
This parameter tells the system what action to take when the system has detected that it is low on disk space. Valid values are ignore, syslog, rotate, email, exec, suspend, single, and halt. If set to ignore, the audit daemon does nothing. Syslog means that it will issue a warning to syslog. rotate will rotate logs, losing the oldest to free up space. Email means that it will send a warning to the email account specified in action_mail_acct as well as sending the message to syslog. exec /path-to-script will execute the script. You cannot pass parameters to the script. The script is also responsible for telling the auditd daemon to resume logging once its completed its action. This can be done by adding service auditd resume to the script. Suspend will cause the audit daemon to stop writing records to the disk. The daemon will still be alive. The single option will cause the audit daemon to put the computer system in single user mode. The halt option will cause the audit daemon to shutdown the computer system.

disk_full_action
This parameter tells the system what action to take when the system has detected that the partition to which log files are written has become full. Valid values are ignore, syslog, rotate, exec, suspend, single, and halt. If set to ignore, the audit daemon will issue a syslog message but no other action is taken. Syslog means that it will issue a warning to syslog. rotate will rotate logs, losing the oldest to free up space. exec /path-to-script will execute the script. You cannot pass parameters to the script. The script is also responsible for telling the auditd daemon to resume logging g once its completed its action. This can be done by adding service auditd resume to the script. Suspend will cause the audit daemon to stop writing records to the disk. The daemon will still be alive. The single option will cause the audit daemon to put the computer system in single user mode. halt option will cause the audit daemon to shutdown the computer system.

disk_error_action
This parameter tells the system what action to take whenever there is an error detected when writing audit events to disk or rotating logs. Valid values are ignore, syslog, exec, suspend, single, and halt. If set to ignore, the audit daemon will not take any action. Syslog means that it will issue no more than 5 consecutive warnings to syslog. exec /path-to-script will execute the script. You cannot pass parameters to the script. Suspend will cause the audit daemon to stop writing records to the disk. The daemon will still be alive. The single option will cause the audit daemon to put the computer system in single user mode. halt option will cause the audit daemon to shutdown the computer system.

use_libwrap
This setting determines whether or not to use tcp_wrappers to discern connection attempts that are from allowed machines. Legal values are either yes, or no The default value is yes.

tcp_listen_port
This is a numeric value in the range 1..65535 which, if specified, causes auditd to listen on the corresponding TCP port for audit records from remote systems. The audit daemon may be linked with tcp_wrappers. You may want to control access with an entry in the hosts.allow and deny files. If this is deployed on a systemd based OS, then you may need to adjust the ‘After’ directive. See the note in the auditd.service file.

tcp_listen_queue
This is a numeric value which indicates how many pending (requested but unaccepted) connections are allowed. The default is 5. Setting this too small may cause connections to be rejected if too many hosts start up at exactly the same time, such as after a power failure.

tcp_max_per_addr
This is a numeric value which indicates how many concurrent connections from one IP address is allowed. The default is 1 and the maximum is 1024. Setting this too large may allow for a Denial of Service attack on the logging server. Also note that the kernel has an internal maximum that will eventually prevent this even if auditd allows it by config. The default should be adequate in most cases unless a custom written recovery script runs to forward unsent events. In this case you would increase the number only large enough to let it in too.

tcp_client_ports
This parameter may be a single numeric value or two values separated by a dash (no spaces allowed). It indicates which client ports are allowed for incoming connections. If not specified, any port is allowed. Allowed values are 1..65535. For example, to require the client use a priviledged port, specify 1-1023 for this parameter. You will also need to set the local_port option in the audisp-remote.conf file. Making sure that clients send from a privileged port is a security feature to prevent log injection attacks by untrusted users.

tcp_client_max_idle
This parameter indicates the number of seconds that a client may be idle (i.e. no data from them at all) before auditd complains. This is used to close inactive connections if the client machine has a problem where it cannot shutdown the connection cleanly. Note that this is a global setting, and must be higher than any individual client heartbeat_timeout setting, preferably by a factor of two. The default is zero, which disables this check.

enable_krb5
If set to "yes", Kerberos 5 will be used for authentication and encryption. The default is "no".

krb5_principal
This is the principal for this server. The default is "auditd". Given this default, the server will look for a key named like auditd/hostname@EXAMPLE.COM stored in /etc/audit/audit.key to authenticate itself, where hostname is the canonical name for the server’s host, as returned by a DNS lookup of its IP address.

krb5_key_file
Location of the key for this client’s principal. Note that the key file must be owned by root and mode 0400. The default is /etc/audit/audit.key

distribute_network
If set to "yes", network originating events will be distributed to the audit dispatcher for processing. The default is "no".

Adding audit rules

  • Add all your rules in /etc/audit/rules.d/audit.rules file.

  • Validate the rules with the following command.

    # augenrules --check
    /usr/sbin/augenrules: Rules have changed and should be updated

    Note: Above command will also check if there are any syntax errors.

  • Execute the following command to update the rules in /etc/audit/audit.rules file.

    # augenrules --load
    No rules
    enabled 1
    failure 1
    pid 699
    rate_limit 0
    backlog_limit 8192
    lost 0
    backlog 1
    enabled 1
    failure 1
    pid 699
    rate_limit 0
    backlog_limit 8192
    lost 0
    backlog 1

    Validate that rules have been updated in /etc/audit/audit.rules file.

  • Restart auditd daemon
    service restart auditd

Note It is also possible that we can add rules directly in /etc/audit/rules.d/audit.rules. Upon restarting auditd daemon, rules from /etc/audit/rules.d/audit.rules will be appended in /etc/audit/audit.rules file.

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