audit.rules

AUDIT.RULES:(7)         System Administration Utilities        AUDIT.RULES:(7)



NAME
       audit.rules - a set of rules loaded in the kernel audit system

DESCRIPTION
       audit.rules is a file containing audit rules that will be loaded by the
       audit daemon's init script whenever the daemon is started. The auditctl
       program is used by the initscripts to perform this operation. The
       syntax for the rules is essentially the same as when typing in an
       auditctl command at a shell prompt except you do not need to type the
       auditctl command name since that is implied. The audit rules come in 3
       varieties: control, file, and syscall.


   Control
       Control commands generally involve configuring the audit system rather
       than telling it what to watch for. These commands typically include
       deleting all rules, setting the size of the kernel's backlog queue,
       setting the failure mode, setting the event rate limit, or to tell
       auditctl to ignore syntax errors in the rules and continue loading.
       Generally, these rules are at the top of the rules file.


   File System
       File System rules are sometimes called watches. These rules are used to
       audit access to particular files or directories that you may be
       interested in. If the path given in a watch rule is a directory, then
       the rule used is recursive to the bottom of the directory tree
       excluding any directories that may be mount points. The syntax of these
       watch rules generally follow this format:

       -w path-to-file -p permissions -k keyname

       where the permission are any one of the following:


              r - read of the file

              w - write to the file

              x - execute the file

              a - change in the file's attribute

       Watches can also be created using the syscall format described below
       which allow for greater flexibility and options. Using syscall rules
       you can choose between path and dir which is against a specific inode
       or directory tree respectively. It should also be noted that the
       recursive directory watch will stop if there is a mount point below the
       parent directory. There is an option to make the mounted subdirectory
       equivalent by using a -q rule.


   System Call
       The system call rules are loaded into a matching engine that intercepts
       each syscall that all programs on the system makes. Therefore it is
       very important to only use syscall rules when you have to since these
       affect performance. The more rules, the bigger the performance hit. You
       can help the performance, though, by combining syscalls into one rule
       whenever possible.

       The Linux kernel has 4 rule matching lists or filters as they are
       sometimes called. They are: task, exit, user, and exclude. The task
       list is checked only during the fork or clone syscalls. It is rarely
       used in practice.

       The exit filter is the place where all syscall and file system audit
       requests are evaluated.

       The user filter is used to filter (remove) some events that originate
       in user space.  By default, any event originating in user space is
       allowed. So, if there are some events that you do not want to see, then
       this is a place where some can be removed. See auditctl(8) for fields
       that are valid.

       The exclude filter is used to exclude certain events from being
       emitted. The msgtype and a number of subject attribute fields can be
       used to tell the kernel which message types you do not want to record.
       This filter can remove the event as a whole and is not selective about
       any other attribute. The user and exit filters are better suited to
       selectively auditing events.  The action is ignored for this filter,
       defaulting to "never".

       Syscall rules take the general form of:

       -a action,list -S syscall -F field=value -k keyname

       The -a option tells the kernel's rule matching engine that we want to
       append a rule at the end of the rule list. But we need to specify which
       rule list it goes on and what action to take when it triggers. Valid
       actions are:


              always - always create an event

              never  - never create an event

       The action and list are separated by a comma but no space in between.
       Valid lists are: task, exit, user, and exclude. Their meaning was
       explained earlier.

       Next in the rule would normally be the -S option. This field can either
       be the syscall name or number. For readability, the name is almost
       always used. You may give more than one syscall in a rule by specifying
       another -S option. When sent into the kernel, all syscall fields are
       put into a mask so that one compare can determine if the syscall is of
       interest. So, adding multiple syscalls in one rule is very efficient.
       When you specify a syscall name, auditctl will look up the name and get
       its syscall number. This leads to some problems on bi-arch machines.
       The 32 and 64 bit syscall numbers sometimes, but not always, line up.
       So, to solve this problem, you would generally need to break the rule
       into 2 with one specifying -F arch=b32 and the other specifying -F
       arch=b64. This needs to go in front of the -S option so that auditctl
       looks at the right lookup table when returning the number.

       After the syscall is specified, you would normally have one or more -F
       options that fine tune what to match against. Rather than list all the
       valid field types here, the reader should look at the auditctl man page
       which has a full listing of each field and what it means. But it's
       worth mentioning a couple things.

       The audit system considers uids to be unsigned numbers. The audit
       system uses the number -1 to indicate that a loginuid is not set. This
       means that when it's printed out, it looks like 4294967295. If you
       write a rule that you wanted try to get the valid users of the system,
       you need to look in /etc/login.defs to see where user accounts start.
       For example, if UID_MIN is  500, then you would also need to take into
       account that the unsigned representation of -1 is higher than 500. So
       you would address this with the following piece of a rule:

       -F auid>=500 -F auid!=4294967295

       These individual checks are "anded" and both have to be true.

       The last thing to know about syscall rules is that you can add a key
       field which is a free form text string that you want inserted into the
       event to help identify its meaning. This is discussed in more detail in
       the NOTES section.


NOTES
       The purpose of auditing is to be able to do an investigation
       periodically or whenever an incident occurs. A few simple steps in
       planning up front will make this job easier. The best advice is to use
       keys in both the watches and system call rules to give the rule a
       meaning. If rules are related or together meet a specific requirement,
       then give them a common key name. You can use this during your
       investigation to select only results with a specific meaning.

       When doing an investigation, you would normally start off with the main
       aureport output to just get an idea about what is happening on the
       system. This report mostly tells you about events that are hard coded
       by the audit system such as login/out, uses of authentication, system
       anomalies, how many users have been on the machine, and if SE Linux has
       detected any AVCs.

       aureport --start this-week

       After looking at the report, you probably want to get a second view
       about what rules you loaded that have been triggering. This is where
       keys become important. You would generally run the key summary report
       like this:

       aureport --start this-week --key --summary

       This will give an ordered listing of the keys associated with rules
       that have been triggering. If, for example, you had a syscall audit
       rule that triggered on the failure to open files with EPERM that had a
       key field of access like this:

       -a always,exit -F arch=b64 -S open -S openat -F exit=-EPERM -k access

       Then you can isolate these failures with ausearch and pipe the results
       to aureport for display. Suppose your investigation noticed a lot of
       the access denied events. If you wanted to see the files that
       unauthorized access has been attempted, you could run the following
       command:

       ausearch --start this-week -k access --raw | aureport --file --summary

       This will give an ordered list showing which files are being accessed
       with the EPERM failure. Suppose you wanted to see which users might be
       having failed access, you would run the following command:

       ausearch --start this-week -k access --raw | aureport --user --summary

       If your investigation showed a lot of failed accesses to a particular
       file, you could run the following report to see who is doing it:

       ausearch --start this-week -k access -f /path-to/file --raw | aureport
       --user -i

       This report will give you the individual access attempts by person. If
       you needed to see the actual audit event that is being reported, you
       would look at the date, time, and event columns. Assuming the event was
       822 and it occurred at 2:30 on 09/01/2009 and you use the en_US.utf8
       locale, the command would look something like this:

       ausearch --start 09/01/2009 02:30 -a 822 -i --just-one

       This will select the first event from that day and time with the
       matching event id and interpret the numeric values into human readable
       values.

       The most important step in being able to do this kind of analysis is
       setting up key fields when the rules were originally written. It should
       also be pointed out that you can have more than one key field
       associated with any given rule.


TROUBLESHOOTING
       If you are not getting events on syscall rules that you think you
       should, try running a test program under strace so that you can see the
       syscalls. There is a chance that you might have identified the wrong
       syscall.

       If you get a warning from auditctl saying, "32/64 bit syscall mismatch
       in line XX, you should specify an arch". This means that you specified
       a syscall rule on a bi-arch system where the syscall has a different
       syscall number for the 32 and 64 bit interfaces. This means that on one
       of those interfaces you are likely auditing the wrong syscall. To solve
       the problem, re-write the rule as two rules specifying the intended
       arch for each rule. For example,

       -always,exit -S openat -k access

       would be rewritten as

       -always,exit -F arch=b32 -S openat -k access
       -always,exit -F arch=b64 -S openat -k access

       If you get a warning that says, "entry rules deprecated, changing to
       exit rule". This means that you have a rule intended for the entry
       filter, but that filter is no longer available. Auditctl moved your
       rule to the exit filter so that it's not lost. But to solve this so
       that you do not get the warning any more, you need to change the
       offending rule from entry to exit.


EXAMPLES
       The following rule shows how to audit failed access to files due to
       permission problems. Note that it takes two rules for each arch ABI to
       audit this since file access can fail with two different failure codes
       indicating permission problems.

       -a always,exit -F arch=b32 -S open -S openat -F exit=-EACCES -k access
       -a always,exit -F arch=b32 -S open -S openat -F exit=-EPERM -k access
       -a always,exit -F arch=b64 -S open -S openat -F exit=-EACCES -k access
       -a always,exit -F arch=b64 -S open -S openat -F exit=-EPERM -k access


HARD WIRED EVENTS
       If auditing is enabled, then you can get any event that is not caused
       by syscall or file watch rules (because you don't have any rules
       loaded). So, that means, any event from 1100-1299, 1326, 1328, 1331 and
       higher can be emitted. The reason that there are a number of events
       that are hardwired is because they are required by regulatory
       compliance and are sent automatically as a convenience. (For example,
       logon/logoff is a mandatory event in all security guidance.) If you
       don't want this, you can use the exclude filter to drop events that you
       do not want.

       -a always,exclude -F msgtype=CRED_REFR


SEE ALSO
       auditctl(8), auditd(8).


AUTHOR
       Steve Grubb



Red Hat                            Jan 2019                    AUDIT.RULES:(7)