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Candidates should know how to set up a basic level of host security.

Key Knowledge Areas

  • Awareness of shadow passwords and how they work.
  • Turn off network services not in use.
  • Understand the role of TCP wrappers.

Terms and Utilities

  • /etc/nologin
  • /etc/passwd
  • /etc/shadow
  • /etc/xinetd.d/
  • /etc/xinetd.conf
  • systemd.socket
  • /etc/inittab
  • /etc/init.d/
  • /etc/hosts.allow
  • /etc/hosts.deny

shadow passwords

The /dev/passwd is already discussed. It contains the passwords of the users but there is a logical problem: if a user should be able to change her own password, he should have access to this file and if this is the case, he can see other people's passwords. This is not interesting even when the passwords are hashed (shown as a more complex form using a one way function).

$ ls -ltrh /etc/passwd
-rw-r--r-- 1 root root 2.5K Jun  5 19:14 /etc/passwd

To prevent this the /etc/shadow file is introduced. In modern systems, we only show a * at the location of the password in /etc/passwd and store the real password in /etc/shadow which has a very limited file access:

jadi@funlife ~$ grep jadi /etc/passwd
jadi@funlife ~$ grep jadi /etc/shadow
grep: /etc/shadow: Permission denied
jadi@funlife ~$ sudo grep jadi /etc/shadow
[sudo] password for jadi:
jadi@funlife ~$ sudo ls -ltrh /etc/shadow
-rw-r----- 1 root shadow 1.5K Jun 11 17:36 /etc/shadow


The /etc/nologin is a cool file! If you create and write something in it, the content will be shown to any person who tries to login into the system and with that file the login attempt will fail. It is useful for maintenance time. Delete this file and the users will be able to login again.

the root user will be able to login even in the presence of /etc/nologin

Please also note that there is a dummy shell called nologin and you can set it as shell for any user you want to prevent from being able to login into the system via a shell. Note that such a user still has an active account and will be able to use other services (say email or ftp) but wont be able to enter the shell.

sudo usermod -s /sbin/nologin baduser


A super-server or sometimes called a service dispatcher is a type of daemon running mostly on Unix-like systems for security and resource management reasons. It listens for requests its configured for and starts the target services when needed to answer the requests. This adds a layer of security to your communications and also lets some of the services to be inactive when we do not need them. You can see a super server or superdaemon as a TCP (and UDP or even ICMP) wrapper around other services.

Super Server

Few GNU/Linuxes use TCP wrappers like xinetd these days but you might see it in some installations or traces of it in your /etc/xinet.d. If needed it is also possible to configure the systemd.socket as a TCP wrapper for other services.

Here is a sample xinetd configuration file:

service telnet
        disable         = no
        flags           = REUSE
        socket_type     = stream
        wait            = no
        user            = root
        server          = /usr/sbin/in.telnetd
        log_on_failure  += USERID
        no_access       =
        log_on_success  += PID HOST EXIT
        access_times    = 09:45-16:15

If we change the disable to yes and restart the xinetd, the telnet daemon will start running. There are a few files to control xinetd related files.

As mentioned, xinetd is replaced by the systemd.socket units. Some services like ssh and cups might have a socket unit alongside the service unit in your distribution. In this case its enough to stop & disable the ssh.service and start the ssh.docekt instead. Now the systemd.socekt is acting as a wrapper around the port 22 and IF someones needs the service, starts the ssh server to answer.

/etc/hosts.allow & /etc/hosts.deny

These two files will allow or deny access from specific hosts. Its logic is like cron.deny and cron.allow. If something is allowed, everything else is denied but if you add something to the /etc/hosts.deny, only that specific thing is denied (and every other thing is allowed).

jadi@funlife ~$ cat /etc/hosts.allow
# /etc/hosts.allow: list of hosts that are allowed to access the system.
#                   See the manual pages hosts_access(5) and hosts_options(5).
# Example:    ALL: LOCAL @some_netgroup
#             ALL: .foobar.edu EXCEPT terminalserver.foobar.edu
# If you're going to protect the portmapper use the name "rpcbind" for the
# daemon name. See rpcbind(8) and rpc.mountd(8) for further information.

vsftpd: 10.10.100.

How the vsftpd knows about this file? Because it uses the libwrap library in its source and the libwrap understands the wrapper tools. You can check this claim by searching for libwrap in the list of libraries vsftpd uses:

โžœ  ~ ldd /usr/sbin/vsftpd | grep libwrap
    libwrap.so.0 => /lib/x86_64-linux-gnu/libwrap.so.0 (0x00007fb293921000)

Here the vsftpd service is only allowed from 10.10.100.* . It is possible to use ALL as the service name to allow or deny ALL services.

after changing this file, xinetd should be restarted

As mentioned, super servers are not being used anymore and most distributions use standalone services.

Removing unused services

Based on your distribution, you can check the running services using the service or systemctl command. If you are using the SysV init system (older distros mainly) or you have the compatibility tools installed, do as follow:

~ sudo service --status-all
 [ - ]  alsa-utils
 [ + ]  ufw
 [ + ]  unattended-upgrades
 [ + ]  uuidd
 [ + ]  vpn-unlimited-daemon
 [ + ]  vsftpd
 [ - ]  whoopsie
 [ - ]  x11-common

On a RedHat based machine, you can stop a service via:

sudo chkconfig vsfptd off

and On debian machines:

sudo update-rc.d vsftpd remove

If you are using the systemd, do as follow to check and stop / disable a service:

systemctl list-units --state active --type service
systemctl status
systemctl disable vsftpd.service --now

Please also remember that on older systems, we used to have all the Init scripts in /etc/init.d and /etc/rcX.d folders. There were also a /etc/inittab file which was a configuration file for initializing a Linux system using SysV . It contains lines in this format:

This would tell the init system to do actions on the process on a specific runlevel. For example:


It tells the init to start (and respawn if killed), the mingetty tty1 command on runlevels 2, 3, 4, & 5.

1:2345:respawn:/sbin/mingetty tty1

As the final note, this was the most important line in the inittab file because it tells the init to start in run level 3.


For more information about runlevels , please refer to Chapter 101.3.

โ† 110.1 Perform security administration tasks
Chapter List
110.3 Securing data with encryption โ†’