If you have any corrections, additions or constructive comments, please send them to me.
This document was written by Rami Lehti <rammer@cs.tut.fi>AIDE (Advanced intrusion detection environment) is an intrusion detection program. More specifically a file integrity checker.
Aide constructs a database of the files specified in Aide.conf, Aide's configuration file. The Aide database stores various file attributes including: permissions, inode number, user, group, file size, mtime and ctime, atime, growing size and number of links. Aide also creates a cryptographic checksum or hash of each file using one or a combination of the following message digest algorithms: sha1, md5, rmd160, tiger (crc32, haval and gost can be compiled in if mhash support is available).
Typically, a system administrator will create an AIDE database on a new system before it is brought onto the network. This first AIDE database is a snapshot of the system in it's normal state and the yardstick by which all subsequent updates and changes will be measured. The database should contain information about key system binaries, libraries, header files, all files that are expected to remain the same over time. The database probably should not contain information about files which change frequently like log files, mail spools, proc filesystems, user's home directories, or temporary directories.
After a break-in, an administrator may begin by examinining the system using system tools like ls, ps, netstat, and who --- the very tools most likely to be trojaned. Imagine that ls has been doctored to not show any file named "sniffedpackets.log" and that ps and netstat have been rewritten to not show any information for a process named "sniffdaemond". Even an administrator who had previously printed out on paper the dates and sizes of these key system files can not be certain by comparison that they have not been modified in some way. File dates and sizes can be manipulated, some better root-kits make this trivial.
While it is possible to manipulate file dates and sizes, it is much more difficult to manipulate a single cryptographic checksum like md5, and exponentially more difficult to manipulate each of the entire array of checksums that Aide supports. By rerunning Aide after a break-in, a system administrator can quickly identify changes to key files and have a fairly high degree of confidence as to the accuracy of these findings.
Unfortunately, Aide can not provide absolute sureness about change in files. Like any other system files, Aide's binary and/or database can also be altered.
After you have installed all the necessary sofware do
./configure;make;make install in the main Aide
directory of the unpacked source tree. You should carefully think
about the configuration and what a possible hacker can do if
he/her/they/it has root access.
Before you can compile Aide you must have certain things.
Once you have the source code of Aide you should unpack it. If you
have GNU tar then the command is tar zxvf
aide-version.tar.gz .
Next you must use the configure script found in Aide's source code package to configure the compilation process.
There are several options you can give to configure. You can find out
what options are available with ./configure --help
command. Most of the time you do not need to give any options.
You can just use configure without any parameters.
However, if you want more digest functions you should consider
using --with-mhash option. This enables mhash support. You
have to have mhash library installed to use this option. The mhash
support enables RMD160, HAVAL, GOST and CRC32 digest
functions.
If you want to use the bundled gnu regular expression package
you can give the --with-gnu-regexp option.
If you want to change the directory where Aide is installed you can
use --prefix option. For example ./configure --prefix=/usr
The compilation is done by simply typing make. You can
now type make install to install the binary and the
manual pages. The binary however should be installed on read-only
media or in some other tamperproof place. Also the databases should
be kept somewhere where a possible intruder cannot change them.
Next you have to create a configuration file. You can find documentation for this in aide.conf(5) manual page. Here are a few pointers for what to look for.
There are three types of lines in aide.conf:
Here is an example configuration.
#AIDE conf # Here are all the things we can check - these are the default rules # #p: permissions #i: inode #n: number of links #u: user #g: group #s: size #b: block count #m: mtime #a: atime #c: ctime #S: check for growing size #md5: md5 checksum #sha1: sha1 checksum #rmd160: rmd160 checksum #tiger: tiger checksum #R: p+i+n+u+g+s+m+c+md5 #L: p+i+n+u+g #E: Empty group #>: Growing logfile p+u+g+i+n+S # You can alse create custom rules - my home made rule definition goes like this # MyRule = p+i+n+u+g+s+b+m+c+md5+sha1 # Next decide what directories/files you want in the database /etc p+i+u+g #check only permissions, inode, user and group for etc /bin MyRule # apply the custom rule to the files in bin /sbin MyRule # apply the same custom rule to the files in sbin /var MyRule !/var/log/.* # ignore the log dir it changes too often !/var/spool/.* # ignore spool dirs as they change too often !/var/adm/utmp$ # ignore the file /var/adm/utmp
Here we include files in /etc, /bin and /sbin. We also include /var but ignore /var/log, /var/spool and a single file /var/adm/utmp.
It is generally a good idea to ignore directories that frequently change, unless you want to read long reports. It is good practice to exclude tmp directories, mail spools, log directories, proc filesystems, user's home directories, web content directories, anything that changes regularly. It is good practice to include all system binaries, libraries, include files, system source files. It will also be a good idea to include directories you don't often look in like /dev /usr/man/.*usr/. Of course you'll want to include as many files as practical, but think about what you include.
#############For example if you have a block device whose owner keeps changing you can just record the attributes that do not normally change (inode,number of links,ctime).
Note that if you are referring to a single file you should add $ to the end of the regexp. This matches to the name of the file exactly and does not include any other files that might have the same beginning. In the example if there were no dollar sign at the end of the last line it would mean that all filenames beginning with /var/adm/utmp would be ignored. An intruder could then create a directory called /var/adm/utmp_root_kit and place all the files he/she/they wanted there and they would be ignored by AIDE.
Making a config file is a lot of hard work and must be done on a case by case bases. Don't give up simply because you don't get it right the first time around. This section gives you a few hints howto debug your config.
You can use aide --verbose=255 to generate a lot of debug
output to help you see which files get added and which are discarded.
To really understand Aide's rule matching you should probably read a
good book about regexps and the following section.
Before reading this you should have basic understanding of how regular expressions work. There are several good books about this. Several Perl-books have also decent explanations about this subject. Just remember that Perl has some extensions to the standard regexps. There are also some differences in how different platforms handle regexps if you are using your platforms own regexp implementation. For example GNU regexps have their own extensions. Try reading the manual page of your system in this case. It might be a pain to read but it is worth it.
In the initialisation process Aide creates a tree of the regexp
rules. Each type of rule is placed in a separate list for each node in
the tree. So we have an equals rule list,a select rule list and a
negative selection rule list for all nodes. These lists may be empty.
The node in which a rule is placed is determined by the first special
regexp character in the rule. For example !/proc would be
placed in the root node. While !/proc/.* would be placed
in /proc node. Also in front of each rule Aide adds an implicit ^.
When Aide does rule matching it uses the following algorithm.
The following is a pseudocode adaptation from src/gen_list.c.
check_node_for_match(node,filename)
if(no deeper match found)
check(equals list for this node)
if(no deeper match found)
check(select list for this node)
check_node_for_match(nodes parent,filename)
if(this file is about to be added)
check(negative select list for this node)
return (info about whether this file should be added or not and how)
There are some side-effects from this algorithm that might seem
strange at first. For example if you have the following rules:
The result would be that /etc and all files in it and in /etc/ppp
except /etc/ppp/logs would be added to the database. This is perfectly
normal. This happens because the =/etc matches not only /etc but all
the files under it. Remember that regexps match always just the part
they are referring to. The rest of the line is included by default.
So
/ R
=/etc R+a
!/etc/ppp/logs
=/etc$ R+a would be the correct form. If you don't
have the !/etc/ppp/logs you would get the results that
you are looking for because. There is no node /etc in the regexp
tree and there for it is not checked when Aide constructs the list of
files to add to the database. But when you have the negative rules the
nodes /etc and /etc/ppp get created and they get checked when the file
list is generated. So the =/etc is used to find a match in those nodes
and it succeeds.
Consider the following rules:
This is what you might write if you want to check /var/log/messages
but not /var/log/messages.0 and /var/log/messages.1 etc. However since
the negative selection rules are checked last and .* can match to an
empty string /var/log/messages is not added to the database. The
following is a more correct way of doing it.
/ R
=/var/log/messages$ R+a
!/var/log/messages.*
Now only messages files ending in number 0-9 and not included in the
database. Note an intruder could disguise a rootkit by creating a
directory called messages.9. If messages.9 does not already exist that
is.
/ R
=/var/log/messages$ R+a
!/var/log/messages\.[0-9]$
First you must create a database against which future checks are
performed. This should be done immediately after the operating system
and applications have been installed, before the machine is plugged
into a network. You can do this by giving the command
aide --init.
This creates a database that contains all of the files that selected
by your config file. The newly created database should now be
moved to a secure location such as read-only media. You should also
place the configuration file and the Aide binary and preferably the
manual pages and this manual on that media also. Please remember to
edit the configuration file so that the input database is read from
that read-only media. The config file should not be kept on the
target machine. The attacker could read the config file and alter it
or even if he does alter it he could place his rootkit to place that
Aide does not check. So the read-only media should be accessible only
during the check.
Now you are all set to go. You can now check the integrity of the
files. This can be done by giving the command
aide --check.
Aide now reads the database and compares it to the files found on
disk. Aide may find changes in places that might not expect. For
instance tty devices often change owners and permissions. You may want
to read long reports and that is up to you to decide. But most of us
do not have the time or the inclination read through tons of garbage
every day. So you should trim the config file to include only the
files and attributes of certain files that should not change. But keep
in mind that you should not ignore too much as that leaves you open
for an attack. An intruder might place his/her/its/their root kit in a
directory that you have ignored completely. One good example is
/var/spool/lp or something similar. This is the place that lp daemon
stores its temporary files. You should not ignore it completely
however. You should only ignore the format of files that you lp daemon
keeps creating. And remember to use the $-sign at the end of your
regexps. This stops someone from creating a directory that is ignored
along with its contents.
Now that you have trimmed your config file you should update the
database this can be done
aide --update
command. The update command also does the same thing as check but it
also creates a new database. This database should now be placed on
that read-only media along with the new config file. The check, trim,
update cycle should be repeated as long as necessary. I recommend that
the config file should be reviewed once in a while. The definition of
"a while" depends on your paranoia. Some might want do it daily after
each check. Some might want to do it weekly.
There is usually some drift in the databases. What I mean by drift is that new files are created, config files of applications are edited, tons of small changes pile up until the report becomes unreadable. This can be avoided by updating the database once in a while. I myself run the update every night. But, I don't replace the input database nearly as often. The replacement of the input datbase should always be a manual operation. This should not be automated.
The Aide database can be used to find the real names and places of files that have been moved to lost+found directory by fsck.