Security is one of the main factor that we need to consider. We must maintain it because we don't want someone steal our data. Security includes many things. Audit, is one of it.
On Linux system, we know that we have a tool named auditd. This tool is by default exist in most of Linux operating system. What is auditd tool and how to use it? We will cover it below.
What is auditd?
Auditd or audit daemon, is a userspace component to the Linux Auditing System. It’s responsible for writing audit records to the disk.
On Ubuntu based system , we can use wajig tool or apt-get tool to install auditd.
Just follow the instruction to get it done. Once it finish it will install some tools related to auditd tool. Here are the tools :
- auditctl ; is a tool to control the behaviour of the daemon on the fly, adding rules, etc
- /etc/audit/audit.rules ; is the file that contains audit rules
- aureport ; is tool to generate and view the audit report
- ausearch ; is a tool to search various events
- auditspd ; is a tool which can be used to relay event notifications to other applications instead of writing them to disk in the audit log
- autrace ; is a command that can be used to trace a process
- /etc/audit/auditd.conf ; is the configuration file of auditd tool
- When the first time we install auditd, there will be no rules available yet.
We can check it using this command :
$ sudo auditctl -l
To add rules on auditd, let’s continue to the section below.
How to use it
Audit files and directories access
One of the basic need for us to use an audit tool are, how can we know if someone change a file(s) or directories? Using auditd tool, we can do with those commands (please remember, we will need root privileges to configure auditd tool):
$ sudo auditctl -w /etc/passwd -p rwxa
- -w path ; this parameter will insert a watch for the file system object at path. On the example above, auditd will wacth /etc/passwd file
- -p ; this parameter describes the permission access type that a file system watch will trigger on
- rwxa ; are the attributes which bind to -p parameter above. r is read, w is write, x is execute and a is attribute
To audit directories, we will use a similar command. Let’s take a look at the command below :
$ sudo auditctl -w /production/
The above command will watch any access to the /production folder.
Now, if we run auditctl -l command again, we will see that new rules are added.
Now let’s see the audit log says.
Viewing the audit log
After rules are added, now we can see how auditd in action. To view audit log, we can use ausearch tool.
We already add rule to watch /etc/passwd file. Now we will try to use ausearch tool to view the audit log.
$ sudo ausearch -f /etc/passwd
- -f parameter told ausearch to investigate /etc/passwd file
- The result is shown below :
time->Mon Dec 22 09:39:16 2014
type=PATH msg=audit(1419215956.471:194): item=0 name="/etc/passwd" inode=142512 dev=08:01 mode=0100644 ouid=0 ogid=0 rdev=00:00 nametype=NORMAL
type=CWD msg=audit(1419215956.471:194): cwd="/home/pungki"
type=SYSCALL msg=audit(1419215956.471:194): arch=40000003 syscall=5 success=yes exit=3 a0=b779694b a1=80000 a2=1b6 a3=b8776aa8 items=1 ppid=2090 pid=2231 auid=4294967295 uid=1000 gid=1000 euid=0 suid=0 fsuid=0 egid=1000 sgid=1000 fsgid=1000 tty=pts0 ses=4294967295 comm="sudo" exe="/usr/bin/sudo" key=(null)
Now let’s we understand the result.
- time ; is when the audit is done
- name ; is the object name to be audited
- cwd ; is the current directory
- syscall ; is related syscall
- auid ; is the audit user ID
- uid and gid ; are User ID and Group ID of the user who access the file
- comm ; is the command that the user is used to access the file
- exe ; is the location of the command of comm parameter above
- The above audit log is the original file.
Next, we are going to add a new user, to see how the auditd record the activity to /etc/passwd file.
time->Mon Dec 22 11:25:23 2014
type=PATH msg=audit(1419222323.628:510): item=1 name="/etc/passwd.lock" inode=143992 dev=08:01 mode=0100600 ouid=0 ogid=0 rdev=00:00 nametype=DELETE
type=PATH msg=audit(1419222323.628:510): item=0 name="/etc/" inode=131073 dev=08:01 mode=040755 ouid=0 ogid=0 rdev=00:00 nametype=PARENT
type=CWD msg=audit(1419222323.628:510): cwd="/root"
type=SYSCALL msg=audit(1419222323.628:510): arch=40000003 syscall=10 success=yes exit=0 a0=bfc0ceec a1=0 a2=bfc0ceec a3=897764c items=2 ppid=2978 pid=2994 auid=4294967295 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=4294967295 comm="chfn" exe="/usr/bin/chfn" key=(null)
As we can see above, that on that particular time, /etc/passwd was accessed by user root (uid = 0 and gid = 0) from directory /root (cwd = /root). The /etc/passwd file was accessed using chfn command which located in /usr/bin/chfn
If we type man chfn on the console, we will see more detail about what is chfn.
Now we take a look at another example.
We already told auditd to watch directory /production/ . That is a new directory. So when we try to use ausearch tool at the first time, it found nothing.
Next, root account try to list the /production directory using ls command. The second time we use ausearch tool, it will show us some information.
time->Mon Dec 22 14:18:28 2014
type=PATH msg=audit(1419232708.344:527): item=0 name="/production/" inode=797104 dev=08:01 mode=040755 ouid=0 ogid=0 rdev=00:00 nametype=NORMAL
type=CWD msg=audit(1419232708.344:527): cwd="/root"
type=SYSCALL msg=audit(1419232708.344:527): arch=40000003 syscall=295 success=yes exit=3 a0=ffffff9c a1=95761e8 a2=98800 a3=0 items=1 ppid=3033 pid=3444 auid=4294967295 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=4294967295 comm="ls" exe="/bin/ls" key=(null)
Similar with the previous one, we can determine that /production folder was looked by root account (uid=0 gid=0) using ls command (comm = ls) and the ls command is located in /bin/ls folder.
Viewing the audit reports
Once we put the audit rules, it will run automatically. And after a period of time, we want to see how auditd can help us to track them.
Auditd comes with another tool called aureport. As we can guess from its name, aureport is a tool that produces summary reports of the audit system log.
We already told auditd to track /etc/passwd before. And a moment after the auditd parameter is developed, the audit.log file is created.
To generate the report of audit, we can use aureport tool. Without any parameters, aureport will generate a summary report of audit activity.
$ sudo aureport
As we can see, there are some information available which cover most important area.
On the picture above we see there are 3 times failed authentication. Using aureport, we can drill down to that information.
We can use this command to look deeper on failed authentication :
$ sudo aureport -au
As we can see on the picture above, there are two users which at the particular time are failed to authenticated
If we want to see all events related to account modification, we can use -m parameter.
$ sudo aureport -m
Auditd configuration file
Previously we already added :
- $ sudo auditctl -w /etc/passwd -p rwxa
- $ sudo auditctl -w /production/
- Now, if we sure the rules are OK, we can add it into
Then don’t forget to restart auditd daemon.
# /etc/init.d/auditd restart
# service auditd restart
Auditd is one of the audit tool that available on Linux system. You can explore more detail about auditd and its related tools by reading its manual page. For example, just type man auditd to see more detail about auditd. Or type man ausearch to see more detail about ausearch tool.
Please be careful before creating rules. It will increase your log file size significantly if too much information to record.