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       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  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 the 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  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

   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  field  is 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.

       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 its 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 its 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.


       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 -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

       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

       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

       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 open -k access

       would be rewritten as

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


       auditctl(8), auditd(8).


       Steve Grubb

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