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Linux Administrator's Security Guide
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Linux Limiting and Monitoring Users

By Kurt Seifried [email protected]



Many security problems arise from the simple fact that you must allow user access to your systems. In some cases users can be malicious (i.e. internal attacker, university students, etc.) or they may accidentally expose their password to an attacker or simply use a weak password (like their username). In any event it is all to possible for users to attack a system, so it is advisable to monitor users, and limit the amount of damage they can do.

Limiting users

Users require resources like CPU time, memory and drive space to do their work. On many systems it is possible for users to hog resources, reducing the usefulness of the system to other users or in some cases actually bringing the server to a slow halt or crashing it. Users can also inadvertently use up more resources then they mean to, limiting their resources can prevent several classes of problems. Programs can crash. generating huge core dumps, or go crazy and user up all the available memory. Something to remember: global limits apply to root, so be careful! If you do not allow root to run enough processes for example cron jobs may fail or you may not even be able to log in to fix any problems.


Almost all Linux distributions ship with PAM support making it universally available. PAM limits provide a single standardized interface to setting user limits, instead of having to write complex shell configuration files (such as /etc/profile) you simply edit the "limits.conf" file. As well applying limits selectively through the command shell is very difficult, whereas with PAM applying limits globally, on groups or on individual users is quite simple. Documentation is available on PAM usually in the "/usr/share/doc/" tree. To enable PAM limits you need to add a line such as:

session		required	/lib/security/

to the appropriate Pam configuration file (i.e. /etc/pam.d/sshd). You can then define limits, typically these are in "/etc/security/limits.conf" or a similar location. Because most of these limits are enforced by the shell the system cannot log all violations of limits (i.e. you will be notified in syslog when a user exceeds the number of times they are allowed to login, however you will not receive a warning if the user tries to use more disk space then they are allowed to).

The available limits are:

core -- Limits the core file size (KB); usually set to 0 for most users to prevent core dumps.
data -- Maximum data size (KB).
fsize -- Maximum file size (KB).
memlock -- Maximum locked-in-memory address space (KB).
nofile -- Maximum number of open files.
rss -- Maximum resident set size (KB).
stack -- Maximum stack size (KB).
cpu -- Maximum CPU time (MIN).
nproc -- Maximum number of processes.
as -- Address space limit.
maxlogins -- Maximum number of logins for this user or group.
priority -- The priority to run user process with.

For example you can limit the amount of memory that user "bob" is allowed to use:

user		hard	memlock		4096

This would place a hard (absolute) limit of 4 megabytes on memory usage for "bob". Limits can be placed on users by listing the user name, groups by using the syntax "@group" or globally by using "*".

core files can be created when a program crashes. They have been used in security exploits, overwriting system files, or by containing sensitive information (such as passwords). You can easily disable core dumps using PAM, and generally speaking most users will not notice, however if you have software developers they may complain.

*		hard	core		0

fsize is generally a good idea to set, many users will have a large filesystem quota (i.e. tens of megabytes to several hundred or several gigabytes), however if they are allowed to create a single file that is abnormally large they can easily hog disk I/O resources (i.e. create a large file and copy/delete the copy repeatedly). Setting this limit globally can also prevent an attacker from trying to fill up the partitions your log files are stored on, for example if you only have a single / partition an attacker can easily fill it up by generating a lot of log events.

@notroot	hard	data		102400

Of course limiting CPU time is one of the classic administrative tasks, this is very useful for preventing run-away processes from eating up all the cpu time, and it ensures that if a user leaves something running in background (such as a packet sniffer) it will eventually be killed. Limiting CPU time will have several side effects however, once of which will be limiting the amount of time a user can spend logged in (eventually they will run out of CPU time and the session will be killed), this can lead to problems if users spend long periods logged in. As well depending on the CPU(s) present in your machine the limits can vary greatly (one minute on a 386 is quite a bit different then one minute on a 1.3 GHz Athlon).

@students	hard	cpu		2

Limiting the number of times a user can login is strongly advised, for most situations users should not need to log in to a server more then once, and allowing them to do so let's them use more resources then you might intend. As well it can be used to detect suspicious activity, if users know they can only login once then attempts to log in multiple times can be viewed as suspicious activity (i.e. an attacker with a stolen password trying to access the account).

@users		hard	maxlogins	1

Additionally when someone violated this limit it will be logged in syslog:

Apr 15 15:09:26 stench PAM_unix[9993]: (sshd) session opened for user test by (uid=0)
Apr 15 15:09:32 stench pam_limits[10015]: Too many logins (max 1) for test

Soft limit violations will not be logged (i.e. a soft limit of 1 and a hard limit of 2).


Bash has built in limits, accessed via “ulimit”. Any hard limits cannot be set higher, so if you have limits defined in /etc/profile, or in the users .bash_profile (assuming they cannot edit/delete .bash_profile) you can enforce limits on users with Bash shells. This is useful for older Linux distributions that lack PAM support (however this is increasingly rare and PAM should be used if possible). You must also ensure that the user cannot change their login shell, if they use "chsh" to change their shell to ksh for example the next time they login they will have no limits (assuming you cave not put limits on ksh). Documentation is available on ulimit, log in using bash and issue:

[[email protected] /root]# <strong>help ulimit</strong>
ulimit: ulimit [-SHacdflmnpstuv] [limit]
    Ulimit provides control over the resources available to processes
    started by the shell, on systems that allow such control.  If an
    option is given, it is interpreted as follows:
        -S      use the `soft' resource limit
        -H      use the `hard' resource limit
        -a      all current limits are reported
        -c      the maximum size of core files created
        -d      the maximum size of a process's data segment
        -f      the maximum size of files created by the shell
        -l      the maximum size a process may lock into memory
        -m      the maximum resident set size
        -n      the maximum number of open file descriptors
        -p      the pipe buffer size
        -s      the maximum stack size
        -t      the maximum amount of cpu time in seconds
        -u      the maximum number of user processes
        -v      the size of virtual memory
    If LIMIT is given, it is the new value of the specified resource.
    Otherwise, the current value of the specified resource is printed.
    If no option is given, then -f is assumed.  Values are in 1024-byte
    increments, except for -t, which is in seconds, -p, which is in
    increments of 512 bytes, and -u, which is an unscaled number of

To disallow core files (by setting the maximum size to 0) for example you would add:

ulimit -Hc 0

To set limits globally you would need to edit "/etc/profile", of course this will also affect root, so be careful! To set limits on groups you would need to add scripting to "/etc/profile" that would check for the user's membership in a group and then apply the statements, however doing this with PAM is recommended as it is a lot simpler.


Quota allows you to enforce user and group limits on disk usage. Quotas are most often used to prevents users from hogging disk space that is shared with other users (i.e. "/home"). There are several benefits to using quota instead of PAM/shell limitations; the first being that you can apply it selectively to disk systems, i.e. you can place a limit on /home/ but not on /tmp/. Additionally you can place limits on the number of inodes used as well as space, running out of inodes is as bad as running out of space since you won't be able to create new files. To see how much space a disk has (in kilobytes):

[[email protected] /]# df -k
Filesystem           1k-blocks      Used Available Use% Mounted on
/dev/hda2              2925900    948936   1828336  35% /
/dev/hda1                49743      2485     44690   6% /boot

And to show inode usage:

[[email protected] /]# df -i
Filesystem            Inodes   IUsed   IFree IUse% Mounted on
/dev/hda2             371680   58737  312943   16% /
/dev/hda1              12880      27   12853    1% /boot

Quota support must be compiled into the kernel (this is the default with most vendors), and the tools must be installed, usually in a package called "quota". Once you have done this you need to edit your "/etc/fstab" file and add either the keyword "grpquota" or "usrquota" in the options:

LABEL=/		/		ext2    defaults		1 1
LABEL=/home	/home		ext2    defaults,grpquota	1 2

Then for each filesystem that you have enabled quotas on you must create two files in the root of the filesystem:

[[email protected] /]# <strong> touch /home/quota.user </strong>
[[email protected] /]# <strong> touch /home/ </strong> 

These files should only be accessible to root. You will then need to reboot the system.

Typically you will only need to place quota on filesystems directly writeable to the user, usually this is:


To edit user and group quotas you use the program "edquota", for example:

[[email protected] /]# <strong>edquota -u </strong><em><strong>username</strong></em>

This will then let you edit the appropriate quota.user or file (these files should not be edited directly!). The editor uses vi by default and the data looks like:

Quotas for user username: 
/dev/hda1: blocks in use: 255, limits (soft = 5120, hard = 10240) 
         inodes in use: 123, limits (soft = 1000, hard = 1500)

Soft limits will generate warnings and hard limits will stop the user from using more disk space or inodes. Documentation on this can be found in "man quota" or in the Quota mini-howto available at:

Monitoring users

Monitoring users may be a requirement of your security policy, or you may have discovered an account that has been compromised and you wish to monitor the attacker when they use it. Unfortunately while this used to be relatively easy there are now a number of new issues that make monitoring users problematic. The increased usage of encrypted protocols like SSH and SSL mean you can no longer simply run a packet sniffer and monitor the attacker. The change in Linux's tty's to pseudo terminals has "broken" many older software packages as well. Typically the best way to monitor users is transparently, packet sniffers are ideal for this as you need not make any changes on the server at all. The second best is to use some form of monitoring on the server, this used to be possible with ttysnoop however ttysnoop does not appear to be actively maintained anymore. While you can use tricks like shell logging this will be obvious to a skilled attacker as the first thing they usually do is disable shell logging. If you are monitoring legitimate users you should inform them (this is a legal requirement in some countries and states). If you are monitoring an attacker and intend to use the log files as evidence you should probably consult a lawyer to make sure it is done correctly.


This is actually a really useful tool I stumbled across. It allows you to monitor all traffic going in and coming out of a process, such as an interactive shell. The bad news is that it does not appear to be completely reliable. While testing it sshsniff reliably captured keystrokes sent to the shell and the replies, however when files were cat'ed, i.e. "cat /etc/passwd" the file was not always displayed on the monitoring shell:

[[email protected] test]$ ???\033???[???Acat /etc/passwd???
--- SIGCHLD (Child exited) ---
[[email protected] test]$ ???\033???[???Acat /etc/passwd???
--- SIGCHLD (Child exited) ---
[[email protected] test]$ ???\033???[???Acat /etc/passwd???

[pid 1417]
ftp:x:14:50:FTP User:/home/ftp:/bin/true
rpc:x:32:32:Portmapper RPC user:/:/bin/false
+++ exited (status 0) +++

[pid 1380]
--- SIGCHLD (Child exited) ---

sshsniff is not perfect but it is able to bind to any PID and not just terminals, making it very flexible. You can download it from: One note there is a file called col.c that contains an entry for /etc/passwd, it does not appear to be used but it does raise some questions. If you plan to use this code I strongly suggest you audit it.


With protocols like SSH and SSL being widely deployed simple packet sniffers are no longer as useful as they once were. So people started writing more complex packet sniffers, like dsniff. dsniff is capable of doing man in the middle attacks against SSL and SSH. If you control the server then you can easily proxy connections (as you have access to the secret keys on the server) and thus decrypt the attackers SSH session and monitor what is going on. dsniff is not capable of dealing with all version of the SSH protocol however, so forcing only protocol 1 support on the server can solve this, in your sshd_config file:

Protocol 1

is specified and not:

Protocol 2,1

dsniff is available from:


One new capability in Linux 2.4 that should prove quite useful is the ability to filter some types of outgoing packet based on who owns the process that created them. Of course this is not 100% effective as some types of packets are not owned ny anyone specifically (i.e. ICMP responses). Using the various flags:


which are discussed in more detail in the firewall section, documentation is available via "man iptables".

Linux Administrator's Security Guide
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  Copyright Kurt Seifried 2001 [email protected]. Published under the terms of the Open Content License Design by Interspire