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NAME | DESCRIPTION | NOTES | TROUBLESHOOTING | EXAMPLES | IO_URING RULES | HARD WIRED EVENTS | SEE ALSO | AUTHOR | COLOPHON |
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AUDIT.RULES(7) System Administration Utilities AUDIT.RULES(7)
audit.rules - a set of rules loaded in the kernel audit system
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:
-a always,exit -F arch=b64 -F path=path-to-file -F perm=permissions -F key=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
For best performance, you should supply an arch field in the rule.
The individual permissions will cause the selection of specific
system calls that use that kind of access. Not supplying the arch
will cause the selection of all system calls which will affect
performance as all system calls will be evaluated.
Watches can also be created using the deprecated -w format which
allows for backwards compatibility at the expense of system
performance as explained. Using syscall rules as shown above, 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 6 rule matching lists or filters as they are
sometimes called. They are: task, exit, user, exclude, filesystem,
and io_uring. 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".
The io_uring filter is used to watch underlying syscalls performed
by io_uring operations.
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, exclude, filesystem,
and io_uring. 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.
But when you write rules, you can use either "unset" which is easy
to remember, or -1, or 4294967295. They are all equivalent. 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 1000, 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>=1000 -F auid!=unset
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.
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 -S openat2 -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.
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,
-a always,exit -S openat -k access
would be rewritten as
-a always,exit -F arch=b32 -S openat -k access
-a 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.
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,openat,openat2 -F exit=-EACCES -k access
-a always,exit -F arch=b32 -S open,openat,openat2 -F exit=-EPERM -k access
-a always,exit -F arch=b64 -S open,openat,openat2 -F exit=-EACCES -k access
-a always,exit -F arch=b64 -S open,openat,openat2 -F exit=-EPERM -k access
Io_uring rules do not take an arch field. It is implicit in the
specification of the filter. The following example rule watches
for file opens through the io_uring subsystem:
-a always,io_uring -S openat,openat2 -F key=access
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
auditctl(8), auditd(8).
Steve Grubb
This page is part of the audit (Linux Audit) project. Information
about the project can be found at
⟨http://people.redhat.com/sgrubb/audit/⟩. If you have a bug report
for this manual page, send it to linux-audit@redhat.com. This
page was obtained from the project's upstream Git repository
⟨https://github.com/linux-audit/audit-userspace.git⟩ on
2025-02-02. (At that time, the date of the most recent commit
that was found in the repository was 2025-01-31.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
Red Hat Sep 2023 AUDIT.RULES(7)
Pages that refer to this page: auditctl(8), auditd(8), augenrules(8)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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