Linux audit files to see who made changes to a file

by on March 19, 2007 · 53 comments· LAST UPDATED September 7, 2007

in , ,

This is one of the key questions many new sys admin ask:

How do I audit file events such as read / write etc? How can I use audit to see who changed a file in Linux?

The answer is to use 2.6 kernel’s audit system. Modern Linux kernel (2.6.x) comes with auditd daemon. It’s responsible for writing audit records to the disk. During startup, the rules in /etc/audit.rules are read by this daemon. You can open /etc/audit.rules file and make changes such as setup audit file log location and other option. The default file is good enough to get started with auditd.

In order to use audit facility you need to use following utilities
=> auditctl - a command to assist controlling the kernel’s audit system. You can get status, and add or delete rules into kernel audit system. Setting a watch on a file is accomplished using this command:

=> ausearch - a command that can query the audit daemon logs based for events based on different search criteria.

=> aureport - a tool that produces summary reports of the audit system logs.

Note that following all instructions are tested on CentOS 4.x and Fedora Core and RHEL 4/5 Linux.

Task: install audit package

The audit package contains the user space utilities for storing and searching the audit records generate by the audit subsystem in the Linux 2.6 kernel. CentOS/Red Hat and Fedora core includes audit rpm package. Use yum or up2date command to install package
# yum install audit
or
# up2date install audit

Auto start auditd service on boot
# ntsysv
OR
# chkconfig auditd on
Now start service:
# /etc/init.d/auditd start

How do I set a watch on a file for auditing?

Let us say you would like to audit a /etc/passwd file. You need to type command as follows:
# auditctl -w /etc/passwd -p war -k password-file

Where,

  • -w /etc/passwd : Insert a watch for the file system object at given path i.e. watch file called /etc/passwd
  • -p war : Set permissions filter for a file system watch. It can be r for read, w for write, x for execute, a for append.
  • -k password-file : Set a filter key on a /etc/passwd file (watch). The password-file is a filterkey (string of text that can be up to 31 bytes long). It can uniquely identify the audit records produced by the watch. You need to use password-file string or phrase while searching audit logs.

In short you are monitoring (read as watching) a /etc/passwd file for anyone (including syscall) that may perform a write, append or read operation on a file.

Wait for some time or as a normal user run command as follows:
$ grep 'something' /etc/passwd
$ vi /etc/passwd

Following are more examples:

File System audit rules

Add a watch on "/etc/shadow" with the arbitrary filterkey "shadow-file" that generates records for "reads, writes, executes, and appends" on "shadow"
# auditctl -w /etc/shadow -k shadow-file -p rwxa

syscall audit rule

The next rule suppresses auditing for mount syscall exits
# auditctl -a exit,never -S mount

File system audit rule

Add a watch "tmp" with a NULL filterkey that generates records "executes" on "/tmp" (good for a webserver)
# auditctl -w /tmp -p e -k webserver-watch-tmp

syscall audit rule using pid

To see all syscalls made by a program called sshd (pid - 1005):
# auditctl -a entry,always -S all -F pid=1005

How do I find out who changed or accessed a file /etc/passwd?

Use ausearch command as follows:
# ausearch -f /etc/passwd
OR
# ausearch -f /etc/passwd | less
OR
# ausearch -f /etc/passwd -i | less
Where,

  • -f /etc/passwd : Only search for this file
  • -i : Interpret numeric entities into text. For example, uid is converted to account name.

Output:

----
type=PATH msg=audit(03/16/2007 14:52:59.985:55) : name=/etc/passwd flags=follow,open inode=23087346 dev=08:02 mode=file,644 ouid=root ogid=root rdev=00:00
type=CWD msg=audit(03/16/2007 14:52:59.985:55) :  cwd=/webroot/home/lighttpd
type=FS_INODE msg=audit(03/16/2007 14:52:59.985:55) : inode=23087346 inode_uid=root inode_gid=root inode_dev=08:02 inode_rdev=00:00
type=FS_WATCH msg=audit(03/16/2007 14:52:59.985:55) : watch_inode=23087346 watch=passwd filterkey=password-file perm=read,write,append perm_mask=read
type=SYSCALL msg=audit(03/16/2007 14:52:59.985:55) : arch=x86_64 syscall=open success=yes exit=3 a0=7fbffffcb4 a1=0 a2=2 a3=6171d0 items=1 pid=12551 auid=unknown(4294967295) uid=lighttpd gid=lighttpd euid=lighttpd suid=lighttpd fsuid=lighttpd egid=lighttpd sgid=lighttpd fsgid=lighttpd comm=grep exe=/bin/grep

Let us try to understand output

  • audit(03/16/2007 14:52:59.985:55) : Audit log time
  • uid=lighttpd gid=lighttpd : User ids in numerical format. By passing -i option to command you can convert most of numeric data to human readable format. In our example user is lighttpd used grep command to open a file
  • exe="/bin/grep" : Command grep used to access /etc/passwd file
  • perm_mask=read : File was open for read operation

So from log files you can clearly see who read file using grep or made changes to a file using vi/vim text editor. Log provides tons of other information. You need to read man pages and documentation to understand raw log format.

Other useful examples

Search for events with date and time stamps. if the date is omitted, today is assumed. If the time is omitted, now is assumed. Use 24 hour clock time rather than AM or PM to specify time. An example date is 10/24/05. An example of time is 18:00:00.
# ausearch -ts today -k password-file
# ausearch -ts 3/12/07 -k password-file

Search for an event matching the given executable name using -x option. For example find out who has accessed /etc/passwd using rm command:
# ausearch -ts today -k password-file -x rm
# ausearch -ts 3/12/07 -k password-file -x rm

Search for an event with the given user name (UID). For example find out if user vivek (uid 506) try to open /etc/passwd:
# ausearch -ts today -k password-file -x rm -ui 506
# ausearch -k password-file -ui 506

Other auditing related posts

Further readings

  • Read man pages - auditd, ausearch, auditctl

Updated for accuracy.

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{ 53 comments… read them below or add one }

1 James Musil March 21, 2007 at 3:42 pm

In the line “auditctl -w /etc/passwd -k shadow-file -p rwxa” you mean /etc/shadow not /etc/passwd.

Reply

2 nixCraft March 21, 2007 at 4:41 pm

James,

Thanks for heads up, post has been updated.

Reply

3 GH Snijders March 22, 2007 at 8:12 am

Very interesting article, thanks alot.

I did spot one small detail, though:

“So from log files you can clearly see who made changes to a file using grep commands.”

Grep is a tool to *read* files, not change them… ;)

Reply

4 Jawed Abbasi February 3, 2014 at 5:21 pm

I know its old post but I don’t see anything wrong with this line quoted below
“So from log files you can clearly see who made changes to a file using grep commands.”
As I read it as you can look at auditd log files and clearly see who made any changes by grepping either usernames or files names or any other string from auditd log files.
that does not say that someone used “grep” tool/utility to make changes.

Reply

5 nixCraft March 22, 2007 at 8:29 am

GH,

Heh… I was suppose to use vim as an example but somehow I did pickup grep. Anyway post has been updated

Appreciate your post.

Reply

6 Rodrigo March 27, 2007 at 8:32 pm

Question, i need a file monitor to tell me which files are being used on a few folders, can i use auditd? is it compatible with Redhat 7.3? is there a GUI to use with this?

If this is not what i need.. can you point me to what i need or something close?

Reply

7 nixCraft March 28, 2007 at 5:54 am

Rodrigo,

RH 7.3 does not support auditd; also a big security risk for such old disro.

Get Cent OS 4.x or FC 6/7

Reply

8 Rodrigo March 28, 2007 at 11:29 am

Sadly the box running RH 7.3 is a live production box for a multinational company, I cant just get a new OS installed on that server, we will be at least another 6 months before migrating to a new system.

Do you perhaps have an idea of what tool I could use to monitor files in a folder that have been accessed during a period of time?

BTW… great site.

Reply

9 motumboe March 30, 2007 at 7:22 am

Found this article following this link: http://beranger.org/index.php?article=2722

Two great blogs, my comps
:-)

Reply

10 nixCraft March 30, 2007 at 5:26 pm

@motumboe, thanks for feedback :D

@Rodrigo you can write your own perl scripts

Reply

11 Stome January 24, 2014 at 2:20 am

Dear nixCraft,
how can i change log file from /var/log/message? when i look to default configuration file in less /etc/audisp/audispd.conf
q_depth = 120
overflow_action = SYSLOG

that is correct?

Reply

12 Ken September 6, 2007 at 10:40 pm

When I try to set up a file watch, it fails. When I do an auditctl -l, i get this at the bottom:

File system watches not supported

Any ideas on whats wrong?

(btw, I’m guessing that I can get around this by tracing syscalls based on the files’ inode numbers, but thats messy, and hard to maintain…)

Reply

13 tiger74 January 25, 2008 at 2:23 am

@nixcraft,
Thank you for such a great article.
But, I’m confused, it seems that there is no man page for the audit.rules?

@rodrigo,
You can use tripwire with similar function. It detects file changes.

Reply

14 ike April 27, 2008 at 7:49 pm

:-) Wow. This is great article.

Reply

15 Ken May 22, 2008 at 11:11 am

I got the same error:

File system watches not supported

Did you ever resolve this?

Thanks John

Reply

16 Nguyen Dang December 14, 2008 at 12:50 am

Hi, thanks for the article.

How do I redirect auditd to not generate log message but call a user-defined program (for an selected event)? Is it possible?

Thank you very much.

Reply

17 Relay February 11, 2009 at 7:03 pm

In the description for the ‘-p’ option, ‘a’ is for “attribute”, not “append” the man page has a full explaination.


-p war : Set permissions filter for a file system watch. It can be r for read, w for write, x for execute, a for append.

Reply

18 John Doe April 3, 2012 at 12:51 am

Newer versions seem to use ‘a’ for attribute changes, my manpage reads like this:

-p [r|w|x|a]
Set permissions filter for a file system watch. r=read, w=write, x=execute, a=attribute change.

Nice article though, exactly what i needed. :)

Reply

19 john May 9, 2009 at 12:09 pm

Great article. I’ve checked the man pages and am still left with two questions:

1. It doesn’t appear that the options to the “p” switch allow for logging file deletions? How do we log when a file is deleted?

2. The kernel does not allow us set a watch on the / directory. If I wanted to log all file deletions, would I be best served by setting watches on all my top level directories (bin,boot,dev,etc…)?

Thanks again for the great resource!
- John

Reply

20 J.C. Denton July 3, 2009 at 3:44 pm

After a system restart or a manual one (sudo /etc/init.d/auditd restart) all my file monitoring is gone. sudo auditctl -l says “no rules” then. do I have to save the rules to a textfile or something? Please help (using (X)ubuntu 8.04 LTS)! ;-)

Reply

21 Frans July 20, 2009 at 6:40 am

Is this also working on Vmware ESX server 3.5? Because this is a modified RedHat distrobution.

Reply

22 Stef November 12, 2009 at 9:28 am

Hi,

thanks for this article. Helps me a lot!

regards

Reply

23 sushil December 18, 2009 at 5:15 pm

hello,
good article…………..

Reply

24 asdasdsd December 22, 2009 at 1:23 pm

# /etc/init.d/audit start
# auditctl -w /etc/passwd -p war -k _etc_passwd
# auditctl -w /etc/shadow -k etc_shadow -p rwxa
# vipw (make a change)
# ausearch -f etc_passwd

Not a lot of use this idea… :(

Reply

25 asdasdsd December 22, 2009 at 1:25 pm

/edit:
# ausearch -f etc_passwd
\

Had to escape the greater and less than sign because this comments section thought that it was some HTML!

Reply

26 david January 6, 2010 at 9:38 pm

@ Rodrigo

compile a newer kernel, it takes about 2 hours or use a propietary software such as powerbroker from beyondtrust

Reply

27 Anonymous July 5, 2010 at 9:04 pm

is it possible to use it from NIS.. we use ypcat

Reply

28 Jagadeesh July 9, 2010 at 5:07 am

Hi,

This is very nice article. In my company we have NFS mounted home directories. Anyone can access files from anybody’s home. This will help me monitoring who comes to my home :-)

Thanks for this article

Reply

29 Hello1971 July 14, 2010 at 2:03 am

Hi, Did this work on exported directory. I mean, if any one read/write a file through NFS, The audit system will log them??

Reply

30 nima0102 September 21, 2010 at 1:51 pm

Good Article :):)

Reply

31 Dave Marcus October 7, 2010 at 9:07 pm

Is there anyway to place an audit on a directory? And yes it’s a very good article, I have it bookmarked.

Reply

32 Yzhar November 11, 2010 at 10:27 am

I’m a Varins inc eng that had research this stuff for a while.

Unix (any), lacks such abilities and the best it can do is audit pre define objects.
scale is poor and some file operations are missing.

We have successfully build such framework (for about any unix platforms).
it is running on hundreds production sites for 3 years now. and I can tell you it wasn’t easy.

I don’t want to sound like a sales man (I’m not), but hope I can save you some time if you are looking for such solution.

btw,
very nice article.

Reply

33 Aldian November 22, 2010 at 10:34 am

You forgot to explain how to stop monitoring once not needed anymore

Reply

34 Sandy December 12, 2010 at 7:42 pm

Does auditd work over NFS ? . I mean, if any one read/write a file through NFS, The audit system will log them?? I have not been able to configure this. auditd captures read/write access from FTP and even CIFS – but not from NFS ? Anyone has any Clue ?

Reply

35 Prashant October 17, 2011 at 5:48 am

Hi Sandy,

Were you about to get the answer for your query..
As even I want to get statistics on NFS / CIFS / FTP etc..
please let me know if you got any tips !

thnx
Prashant

Reply

36 Roumen Semov December 16, 2010 at 12:39 am

Hmmm, appending text to a watched file does not show up in the audit logs:
echo ‘hello world’ >> /etc/passwd
Any idea why?

Reply

37 RG May 27, 2011 at 5:00 am

You might need to turn auditing on by changing the default “yes” to “no”
at the bottom of the file /etc/sysconfig/auditd:
—————————————–
# This option disables syscall auditing by default. This can also be
# accomplished by auditctl -e.
AUDITD_DISABLE_CONTEXTS=”no”
—————————————–
Set to no for full audit functionality including file and directory watches and system call auditing.

Reply

38 DarenTay February 25, 2011 at 8:04 am

If a user su to root, how do we manage that? Can we identify who’s the original user?

Reply

39 joe March 21, 2011 at 5:43 pm

Daren Tay
For SU install sudo and which uses su log.

Reply

40 Cristian Rusu April 27, 2011 at 7:52 am

Hello

Is there any way to figure out what php script modified a file on the system?
I got a bug where all the images in some folders are converted to an black empty png and I can’t figure out what does this for months.

Thank you for any hint

Cris

Reply

41 David May 23, 2011 at 9:35 pm

I’d change the permissions on the PNG files to read-only – possibly by changing the extended attributes if necessary – and see what breaks. Might have to change the directory permissions if the mysterious program is actually creating a new file and moving deleting the old one – as these steps don’t require file permissions, just directory permissions.

Reply

42 Tha_Duck May 26, 2011 at 11:38 am

# auditctl -w /tmp -p e -k webserver-watch-tmp

Shouldn’t that be:
# auditctl -w /tmp -p x -k webserver-watch-tmp

?

Reply

43 dreamingkat July 9, 2011 at 8:10 am

according to the man page, a isn’t for append, it’s for attribute changes.

Reply

44 Funutation October 13, 2011 at 5:45 pm

anyone know whether SELinux includes these features? I assume that it does, and does even more but I cannot find details (easily :-)

thanx

Reply

45 ceooph November 21, 2011 at 9:15 am

Hi,
Thanks for this article and your whole site. I have a problem with auditd.
Can you audit a directory (yes) and all subdirectory ??
I want to audit a complete map point with folder, sub-folder, sub-sub-folder, …

Thanks a lot for your help

Reply

46 John Gonzalez November 29, 2011 at 11:55 pm

Thank You…!!!

Reply

47 ritesh June 4, 2012 at 11:46 am

HI,
I configured samba as a file server. Server is running successfully. I wanted to see logs which user currently accessing a file and which file got deleted from user.
Is this possible in samba ?
Thanks in advacne.
Ritesh

Reply

48 thomas October 19, 2012 at 3:26 am

i have set up my audit.rules file.

when i start auditd, i get an output stating:

the audit system is in immutable mode. no audit rules loaded.

when i type auditctl -l it says no rules loaded.

i thought when i set up my audit.rules file and started auditd, that’s all i had to do. after reading this article, i believe there is more i need to do.

can someone elaborate on how the audit.rules file comes into play?

Reply

49 ibeam7 November 28, 2012 at 11:20 pm

thomas
Check to see if you have
-e 2
at the end of your audit.rules file. If so, once you reload or restart your auditd service you will not be able to modify your rules file without bouncing the server. If doing testing, it’s best to use
-e 1
which just enables the rules but doesn’t lock them.

Reply

50 Kirk March 26, 2013 at 11:14 pm

Is it possible to write rules to detect modifications to any file in /var/www with the name settings.php? This doesn’t work, but it captures what I’d like to be able to do:

auditctl -w /var/www/vhosts/*/settings.php -k config-watch -prwa

Thanks.

Reply

51 Sarfraz September 2, 2013 at 12:27 pm

Do we enable file auditing for files accessed from SFTP like winscp tool?

Reply

52 AW November 14, 2013 at 10:37 pm

How does one ensure that the logs send up in syslog?
I regularly send the auth.* @IP address, what would be the equivalent in this case?

Reply

53 satan May 13, 2014 at 6:21 pm

Option “a” doesn’t mean append. From official manual: a – change in the file’s or directory’s attribute.

Reply

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