LINUX GAZETTE
...making Linux just a little more fun!
Security Administration with Debian GNU/Linux
By Jose Salvador Gonzalez Rivera

  1. Introduction
  2. Installing Debian
  3. Vulnerabilities Analysis
  4. Security Administration
    1. Permissions and Attributes
    2. Sticky Bit
    3. Umask
    4. Quotas and Limit
    5. User Activities
    6. Logs and Services

Introduction

Debian has a package manager (DPKG) that resolves dependency problems automatically. It help us to automatically keep up to date programs looking for new versions on the internet, resolving and completing the files and libraries dependencies which a package requires, making system administration easy and keeping us up to date with the new security changes. It also shows some important and substantial security features: it doesn't have commercial goals, also doesn't obey mercantile urgencies, It has a good pursuit of errors, problems are fixed in less than 48 hours and it's priority is to develop a complete and reliable operating system.

Before Installing

From a security and reliability standpoint, it's better to have separate hard disk partitions for directories that are large, and especially to separate those which are frequently-changing (/tmp and /var) from those that can be mounted read-only except when installing software (/usr). Some people also make separate partitions for /home and /usr/local. Separate partitions mean that if one gets corrupted, the others won't be affected. It also means you can mount some partitions (especially /usr and /boot) read-only except when doing system administration: this decreases the likelihood of corruption or mistakes dramatically. Don't do the distribution default, which is usually to put everything in one partition. Of course, you can go overboard if you use too many partitions, and if you don't anticipate your sizes correctly you may end up with wasted space in some partitions and not enough space in others. In that case you'll either have to back up the files and repartition, or use symbolic links to steal space from another partition. Both strategies are undesirable, so think beforehand about how many partitions are appropriate for this machine, which directories contain irreplaceable data, and leave some extra space for unexpected additions later.

Installing Debian

The Debian installation, text mode, consists of two phases. The first one consists of installing the base system and the second one allows us to configure several details and the installation of additional packages. It is also necessary to identify those services that the system will offer. It doesn't make sense to install packages that could open ports and offer unnecessary services, so we will begin installing just the base system and after that the services our system will offer.

Vulnerability Analysis

There are some software tools to perform vulnerability verification or security auditing in our servers; these tools are intended to detect well-known security problems and also to offer detailed information in how to solve almost any problem you find. This kind of analysis is also called "ethical hacking" because we can check the way our servers can be penetrated as an intruder would do it. Nessus audits insecurity. Its main advantage is that it is totally modernized with the latest attacks, with the possibility to include them in plug-ins form. It is available for any UNIX flavor from its Web site: www.nessus.org It is composed of two programs:

Nessusd

The server performs the exploration. It should be started with root privileges and uses the ports 1241 and 3001 to listen to nessus client's requests. To install it is necessary to type the following command:

# apt-get install nessusd

It only runs in UNIX and the client should be authenticated by means of a login and a password that has to be activated in the system with the different options offered by nessus-adduser command.

Nessus Client

It is the client who communicates with nessusd. This program has its own graphical front end for administrative purposes. It's not just for UNIX but for Windows too. Also one of its tasks is report generation at the end of the exploration, showing the vulnerabilities found and their possible solutions. To install it we have to type:

# apt-get install nessus

Nessus uses a couple of keys stored in the .nessus.keys directory located in user's HOME. They are used to communicate with nessusd.

Security Administration

I do not want to repeat the HOWTO and manuals information so I will focus on specific points and situations not considered frequently, the use of limits and files attributes.

Permissions and Attributes

The Linux permissions and attributes system allows us to restrict file access to non authorized users. The basic permissions are read (r), writ (w) and execute (x).

To visualize a directory permission structure we type ls -l

total 44
drwxr-xr-x††† 2 root†††† root†††††††† 4096 May 272000 backups
drwxr-xr-x††† 4 root†††† root†††††††† 4096 Jul 17 14:36 cache
drwxr-xr-x††† 7 root†††† root†††††††† 4096 Jul 17 09:30 lib
drwxrwsr-x††† 2 root†††† staff††††††† 4096 May 272000 local
drwxrwxrwt††† 2 root†††† root†††††††† 4096 May 272000 lock
drwxr-xr-x††† 5 root ††††root†††††††† 4096 Jul 17 14:35 log
drwxrwsr-x††† 2 root†††† mail†††††††† 4096 Jun 132001 mail
drwxr-xr-x††† 3 root†††† root†††††††† 4096 Jul 17 14:36 run
drwxr-xr-x††† 3 root†††† root†††††††† 4096 Jul 17 14:34 spool
drwxr-xr-x††† 5 root†††† root†††† ††††4096 Jul 17 14:35 state
drwxrwxrwt††† 2 root†††† root†††††††† 4096 May 272000 tmp

The permission column has 10 characters divided in 4 groups:

- rw- rw- r--
 

The first part indicates the file type:

-††††††† common file.
d††††††† directory.
l†††††† symbolic link.
s††††††† socket.

 

The other characters indicate if the owner, the owner group and all others have permission to read, write or execute the file. The chmod command is used to change permission with - + = operators to remove, add or to assign permissions. For example:

$ chmod +x foo

Assigns to foo execution attributes. To remove execution permission to the group members we type:

$ chmod g-r foo

Another way to change the permission schema is by the octal system where each number represents a place-dependant permission for owner, group or all others.

0††††††† no permission
1††††††† execution
2††††††† writing
3††††††† writing and execution
4††††††† reading
5††††††† reading and execution
6††††††† reading and writing
7††††††† reading, writing and execution

For example, if we type:

$ chmod 751 foo

We assign read, write and execute permission to the file owner (7), the group can read it and to execute it (5) and can be executed by everybody else (1).

We can also modify file attributes with chattr and list them with lsattr, this allows us to increase file and directory security. Attributes can be assigned in this way:

A††††††† Do not update the atime file attribute allowing to limit the input and output to disk.
a††††††† Open the file only in update mode.
c††††††† File compressed automatically.
d††††††† Marks file so dump program will not touch it
i††††††† File can not be erased, renamed, modified or linked.
s††††††† Fills the erased file blocks with zeroes.
S††††††† Changes in file will be immediately recorded.
u††††††† File content will be saved when erasing the file.

An example to assign "immutability", so the file can not be modified, erased, linked or renamed would be:

lsattr foo.txt
-------- foo
 
 
chattr +i foo.txt
 
 
lsattr foo.txt
 
 
----i--- foo.txt

Sticky bit

If any user has writing permission on a certain directory, he will be able to erase any file contained in that directory although he is neither the owner nor has privileges. To assign permissions to a directory so that no user can erase another user's files we assign the sticky bit with chmod:

ls -ld temp
 
chmod +t temp
 
ls -ld temp

Umask

When we create files or directories they have predetermined permissions, commonly 664 for files and 775 for directory This is done by the umask value. To assign more restrictive permissions as 666 for files and 777 for directory, it is advisable to establish the umask value at 077 inside each user's profile in ~/.bash_profile

# /etc/profile: system-wide .profile file for the Bourne shell (sh(1))
# and Bourne compatible shells (bash(1), ksh(1), ash(1), ...).
 
PATH="/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/games"
 
if [ "$BASH" ]; then
PS1='\u@\h:\w\$ '
else
if [ "`id -u`" -eq 0 ]; then
††† PS1='# '
else
††† PS1='$ '
fi
fi
 
export PATH PS1
umask 022

Quotas and Limits

Since Linux is a multi-user operating system, it is possible that several users could be filling the hard disk or wasting the disk's resources, so a quota disk can be a good choice. To make this, it is enough to modify the /etc/fstab file adding usrquota, then create two files for the partition: quota.user and quota.grup:

touch /home/quota.user
touch /home/quota.group
chmod 660 /home/quota.user
chmod 660 /home/quota.group

Then restart the system and the assigned quota can be modified with edquota. It is also possible to limit users, i.e. to limit CPU's time usage, the number of open files, data segment size, etc. For this we use the ulimit command, the commands must be placed in /etc/profile and every time a user obtains a shell those commands are executed. The options are:

-a††††††† Show current limits
-c††††††† Maximum core file size
-d††††††† Maximum process data segment size
-f††††††† Maximum files created by shell size
-m††††††† Maximum locked memory size
-s††††††† Maximum stack size
-t††††††† Maximum CPU time in seconds
-p††††††† Pipe size
-n††††††† Maximum opened files number
-u††††††† Maximum process number
-v††††††† Maximum virtual memory size
 
core file size (blocks)†††††††††††† 0
data seg size (kbytes)††††††††††††† unlimited
file size (blocks)††††††††††††††††† unlimited
max locked memory (kbytes)††††††††† unlimited
max memory size (kbytes)††††††††††† unlimited
open files†††††††††††††††† †††††††††1024
pipe size (512 bytes)†††††††††††††† 8
stack size (kbytes)†††††††††††††††† 8192
cpu time (seconds)††††††††††††††††† unlimited
max user processes††††††††††††††††† 256
virtual memory (kbytes)†††††††††††† unlimited

User Activities

The user's command record is stored in the ~/.bash_history file. The user could consult it with the history command, using the direction keys (up and down). However there are several ways to avoid this, for example history-c command erases the current record. Replacing the contents of the environment variable HISTFILE to null is another way. Yet another way is to kill the session with kill -9 or kill -9 0.

In order to record users behavior there is a tool called snoopy which logs this activity, however it could be considered a privacy issue, so if you implement it would be wise to create policies and let users know that all their activities are registered. It can be installed with apt-get install snoopy At this moment the last version is 1.3-3.

A way to identify the processes using user's files is by the fuser command; this is very useful in order to know what users have open files that disallow umounting a certain file system. Another useful command to know the open files and sockets list is lsof. To identify what process is using a certain socket we can type for example:

lsoft -i -n -P | grep 80| grep LISTEN

Logs and Services

The faillog and lastlog files are inside /var/log which register the last successful and failed connections, they will be analyzed in the intruders' detection section, but they are accessible to everybody and it is convenient to limit their access with:

chmod 660 /var/log/faillog

And

chmod 660 /var/log/lastlog

The lilo.conf file is also accessible to all. It has the Linux loader configuration and by this is why it is advisable to limit its access with:

chmod 600 /etc/lilo.conf

The setuid is when a program makes a system call to assign itself a UID to identify a process. Programs recorded with setuid can be executed by the owner or by a process that reaches the appropriate privileges, being able to adopt the programís owner UID. To determine what files are setuid and setgid we can carry out a search with:

$ find / -perm -4000 -print

When installed, every UNIX opens many services but many of them are not necessary, depending on the kind of server built. For example in my linux box I have the following services:

$ netstat -pn -l -A inet
 
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address†††††††††† Foreign Address†††††††† State†††††† PID/Program name
tcp††††††† 0††††† 0 0.0.0.0:22††††††††††††† 0.0.0.0:*†††††††††††††† LISTEN††††† 200/sshd
tcp††††††† 0††††† 0 0.0.0.0:515†††††††††††† 0.0.0.0:*†††††††††††††† LISTEN††††† 193/lpd
tcp††††††† 0††††† 0 0.0.0.0:113††††††††† †††0.0.0.0:*†††††††††††††† LISTEN††††† 189/inetd
tcp††††††† 0††††† 0 0.0.0.0:25††††††††††††† 0.0.0.0:*†††††††††††††† LISTEN††††† 189/inetd
tcp††††††† 0††††† 0 0.0.0.0:37††††††††††††† 0.0.0.0:*†††††††††††††† LISTEN††††† 189/inetd
tcp††††††† 0††††† 0 0.0.0.0:13††††††††††††† 0.0.0.0:*†††††††††††††† LISTEN††††† 189/inetd
tcp††††††† 0††††† 0 0.0.0.0:9†††††††††††††† 0.0.0.0:*†††††††††††††† LISTEN††††† 189/inetd
tcp††††††† 0††††† 0 0.0.0.0:1024††††††††††† 0.0.0.0:*†††††††††††††† LISTEN††††† 180/rpc.statd
tcp††††† ††0††††† 0 0.0.0.0:111†††††††††††† 0.0.0.0:*†††††††††††††† LISTEN††††† 116/portmap
udp††††††† 0††††† 0 0.0.0.0:9†††††††††††††† 0.0.0.0:*†††††††††††††††††††††††††† 189/inetd
udp††††††† 0††††† 0 0.0.0.0:1024††††††††††† 0.0.0.0:*†††††††††††††††††††††††††† 180/rpc.statd
udp††††††† 0††††† 0 0.0.0.0:780†††††††††††† 0.0.0.0:*†††††††††††††††††††††††††† 180/rpc.statd
udp††††††† 0††††† 0 0.0.0.0:111†††††††††††† 0.0.0.0:*†††††††††††††††††††††††††† 116/portmap
udp††††††† 0††††† 0 0.0.0.0:68††††††††††††† 0.0.0.0:*††††† †††††††††††††††††††††112/dhclient-2.2.x
raw††††††† 0††††† 0 0.0.0.0:1†††††††††††††† 0.0.0.0:*†††††††††††††† 7†††††††††† -
raw††††††† 0††††† 0 0.0.0.0:6†††††††††††††† 0.0.0.0:*†††††††††††††† 7†††††††††† -

This shows information such as the protocol type, address and port as well as the state it is in. With lsof we can obtain more precise and summarized information

$ lsof -i | grep LISTEN
 
portmap†† 116 root††† 4uIPv4†††† 73†††††† TCP *:sunrpc (LISTEN)
rpc.statd 180 root††† 5uIPv4††† 118†††††† TCP *:1024 (LISTEN)
inetd†††† 189 root††† 4uIPv4††† 126†††††† TCP *:discard (LISTEN)
inetd†††† 189 root††† 6uIPv4††† 128†††††† TCP *:daytime (LISTEN)
inetd†††† 189 root††† 7uIPv4††† 129†††††† TCP *:time (LISTEN)
inetd†††† 189 root††† 8uIPv4††† 130†††††† TCP *:smtp (LISTEN)
inetd†††† 189 root††† 9uIPv4††† 131†††††† TCP *:auth (LISTEN)
lpd†††††† 193 root††† 6uIPv4††† 140†††††† TCP *:printer (LISTEN)
sshd††††† 200 root††† 3uIPv4††† 142†††††† TCP *:ssh (LISTEN)

This shows us the service, port, proprietor and protocol used. To list the demons that have inet.d we can revise their configuration file in /etc/inetd.conf:

$ grep -v "^#" /etc/inetd.conf | sort -u
 
daytime†††††††††††† stream††††††† tcp††††††† nowait††††† root††††††† internal
discard†††††††††††† dgram†††††††† udp††††††† wait††††††† root††††††† internal
discard†††††††††††† stream††††††† tcp††††††† nowait††††† root††††††† internal
ident†††††††††††††† stream††††††† tcp††††††† wait††††††† identd††††† /usr/sbin/identd††††††† identd
smtp††††††††††††††† stream††††††† tcp††††††† nowait††††† mail††††††† /usr/sbin/exim exim -bs
time††††††††††††††† stream††††††† tcp††††††† nowait††††† root††††††† internal

 

And to stop and disable a service, in this case we will disable the time, we have the command:

 

$ update-inetd -disable time
 

and the file inetd.conf is modified like this:

 

daytime††††††††††† stream††††††† tcp††††††† nowait††††† root††††††† internal
discard††††††††††† dgram†††††††† udp††††††† wait††††††† root††††††† internal
discard††††††††††† stream†††† †††tcp††††††† nowait††††† root††††††† internal
ident††††††††††††† stream††††††† tcp††††††† wait††††††† identd††††† /usr/sbin/identd††††††† identd
smtp†††††††††††††† stream††††††† tcp††††††† nowait††††† mail††††††† /usr/sbin/exim exim -bs

To restart the daemon inetd we can use the command:

$ /etc/init.d/inetd restart

To disable unnecessary services, I made the following shell script, remembering that you can adapt it for your purposes.

#!/bin/bash
# ----------------------------------------------------------------------
# Securing configuration files and deactivating unnecessary services
# Jose Salvador Gonzalez Rivera jsgr@linuxpuebla.org
# ----------------------------------------------------------------------
clear
raiz=0
if [ "$UID" -eq "$raiz" ]
 then
echo -e "Ok, Inits Shell Script...\n"
 else
echo -e "You need to be ROOT to run this este script...\a\n"
exit
fi
 
echo "Securing Logs..."
chmod 700 /bin/dmesg††††††††††††††††† # Limits the kernel messages
chmod 600 /var/log/messages†††††††††† # Messages to the console
chmod 600 /var/log/lastlog††††††††††† # Register connections
chmod 600 /var/log/faillog††††††††††† # Register failed connections
chmod 600 /var/log/wtmp†††††††††††††††††††††† # Data Input and Output (last)
chmod 600 /var/run/utmp†††††††††††††††††††††† # Logged user data
††††††††††††††††††††††††††††††††††††† ††††††††††† # commands who,w,users,finger
echo "Securing configurations..."
chmod 600 /etc/lilo.conf††††††† # Configuration and password for LiLo
chmod 600 /etc/syslog.conf††††† # Syslog configuration
chmod -R 700 /etc/init.d†††††† ††† # Init files directory
 
echo "Removing the guilty bit..."
find / -perm -4000 -exec chmod a-s {} \;
find / -perm -2000 -exec chmod a-s {} \;
 
echo "Removing the unnecessary services..."
/etc/init.d/lpd stop
update-rc.d -f lpd remove
/etc/init.d/nfs-common stop
update-rc.d -f nfs-common remove
/etc/init.d/portmap stop
update-rc.d -f portmap remove
update-inetd --disable time
update-inetd --disable daytime
update-inetd --disable discard
update-inetd --disable echo
update-inetd --disable chargen
update-inetd --disable ident
 
echo "Restarting super daemon...\n"
/etc/init.d/inetd restart
cd && echo -e "Ok, Finishing the Shell Script...\n"
 

Well, for all this I use the man pages of the programs, I hope this can help people get interested a little bit more in Linux security, and specifically with Debian.

 

[BIO] Currently I'm an active member of the Puebla Linux User Group (GULP) in México. I frequently participate in events to promove the use of Free Software and Linux mainly. I accept any questions, comments or suggestions by email.


Copyright © 2003, Jose Salvador Gonzalez Rivera. Copying license http://www.linuxgazette.com/copying.html
Published in Issue 89 of Linux Gazette, April 2003