4.3.4 Lab – Linux Servers Answers

Lab – Linux Servers (Answers Version)

Answers Note: Red font color or gray highlights indicate text that appears in the instructor copy only.

In this lab, you will use the Linux command line to identify servers running on a given computer.

Part 1: Servers

Part 2: Using Telnet to Test TCP Services

  • CyberOps Workstation virtual machine

Answers Note: This lab can be done using the virtual machine created in a previous lab.

Part 1:  Servers

Servers are essentially programs written to provide specific information upon request. Clients, which are also programs, reach out to the server, place the request, and wait for the server response. Many different client-server communication technologies can be used, with the most common being IP networks. This lab focuses on IP network-based servers and clients.

Step 1:  Access the command line.

  1. Log on to the CyberOps Workstation VM as the analyst, using the password cyberops. The account analyst is used as the example user account throughout this lab.
  2. To access the command line, click the terminal icon located in the Dock, at the bottom of VM screen. The terminal emulator opens.

Screenshot of the Dock images with the Terminal circled.

Step 2:  Display the services currently running.

Many different programs can be running on a given computer, especially a computer running a Linux operating system. Many programs run in the background so users may not immediately detect what programs are running on a given computer. In Linux, running programs are also called processes.

Note: The output of your ps command will differ because it will be based on the state of your CyberOps Workstation VM.

  1. Use the ps command to display all the programs running in the background:

[[email protected] ~]$ sudo ps –elf

[sudo] password for analyst:


4 S root         1     0  0  80   0 –  2250 SyS_ep Feb27 ?        00:00:00 /sbin/init

1 S root         2     0  0  80   0 –     0 kthrea Feb27 ?        00:00:00 [kthreadd]

1 S root         3     2  0  80   0 –     0 smpboo Feb27 ?        00:00:00 [ksoftirqd/0]

1 S root         5     2  0  60 -20 –     0 worker Feb27 ?        00:00:00 [kworker/0:0H]

1 S root         7     2  0  80   0 –     0 rcu_gp Feb27 ?        00:00:00 [rcu_preempt]

1 S root         8     2  0  80   0 –     0 rcu_gp Feb27 ?        00:00:00 [rcu_sched]

1 S root         9     2  0  80   0 –     0 rcu_gp Feb27 ?        00:00:00 [rcu_bh]

1 S root        10     2  0 -40   – –     0 smpboo Feb27 ?        00:00:00 [migration/0]

1 S root        11     2  0  60 -20 –     0 rescue Feb27 ?        00:00:00 [lru-add-drain]

5 S root        12     2  0 -40   – –     0 smpboo Feb27 ?        00:00:00 [watchdog/0]

1 S root        13     2  0  80   0 –     0 smpboo Feb27 ?        00:00:00 [cpuhp/0]

5 S root        14     2  0  80   0 –     0 devtmp Feb27 ?        00:00:00 [kdevtmpfs]

1 S root        15     2  0  60 -20 –     0 rescue Feb27 ?        00:00:00 [netns]

1 S root        16     2  0  80   0 –     0 watchd Feb27 ?        00:00:00 [khungtaskd]

1 S root        17     2  0  80   0 –     0 oom_re Feb27 ?        00:00:00 [oom_reaper]

<some output omitted>


Why was it necessary to run ps as root (prefacing the command with sudo)?

Type your answers here.

Some processes do not belong to the analyst user and may not be displayed if ps was executed as analyst, which is a regular user account.

  1. In Linux, programs can also call other programs. The ps command can also be used to display such process hierarchy. Use –ejH options to display the currently running process tree after starting the nginx webserver with elevated privileges.

Note: The process information for the nginx service is highlighted. Your PID values will be different.

[[email protected] ~]$ sudo /usr/sbin/nginx

[[email protected] ~]$ sudo ps –ejH

[sudo] password for analyst:


    1     1     1 ?        00:00:00 systemd

  167   167   167 ?        00:00:01   systemd-journal

  193   193   193 ?        00:00:00   systemd-udevd

  209   209   209 ?        00:00:00   rsyslogd

  210   210   210 ?        00:01:41   java

  212   212   212 ?        00:00:01   ovsdb-server

  213   213   213 ?        00:00:00   start_pox.sh

  224   213   213 ?        00:01:18     python2.7

  214   214   214 ?        00:00:00   systemd-logind

  216   216   216 ?        00:00:01   dbus-daemon

  221   221   221 ?        00:00:05   filebeat

  239   239   239 ?        00:00:05   VBoxService

  287   287   287 ?        00:00:00   ovs-vswitchd

  382   382   382 ?        00:00:00   dhcpcd

  387   387   387 ?        00:00:00   lightdm

  410   410   410 tty7     00:00:10     Xorg

  460   387   387 ?        00:00:00     lightdm

  492   492   492 ?        00:00:00       sh

  503   492   492 ?        00:00:00         xfce4-session

  513   492   492 ?        00:00:00           xfwm4

  517   492   492 ?        00:00:00           Thunar

 1592   492   492 ?        00:00:00             thunar-volman

  519   492   492 ?        00:00:00           xfce4-panel

  554   492   492 ?        00:00:00             panel-6-systray

  559   492   492 ?        00:00:00             panel-2-actions

  523   492   492 ?        00:00:01           xfdesktop

  530   492   492 ?        00:00:00           polkit-gnome-au

  395   395   395 ?        00:00:00   nginx

  396   395   395 ?        00:00:00     nginx

  408   384   384 ?        00:01:58   java

  414   414   414 ?        00:00:00   accounts-daemon

  418   418   418 ?        00:00:00   polkitd

<some output omitted>


How is the process hierarchy represented by ps?

Type your answers here.

Through indentation.

  1. As mentioned before, servers are essentially programs, often started by the system itself at boot time. The task performed by a server is called a service. In such fashion, a web server provides web services.

The netstat command is a great tool to help identify the network servers running on a computer. The power of netstat lies on its ability to display network connections.

Note: Your output maybe different depending on the number of open network connections on your VM.

In the terminal window, type netstat.

[[email protected] ~]$ netstat

Active Internet connections (w/o servers)

Proto Recv-Q Send-Q Local Address           Foreign Address         State     

tcp        0      0 localhost.localdo:48746 localhost.local:wap-wsp ESTABLISHED

tcp        0      0 localhost.localdo:48748 localhost.local:wap-wsp ESTABLISHED

tcp6       0      0 localhost.local:wap-wsp localhost.localdo:48748 ESTABLISHED

tcp6       0      0 localhost.local:wap-wsp localhost.localdo:48746 ESTABLISHED

tcp6       0      0 localhost.local:wap-wsp localhost.localdo:48744 ESTABLISHED

tcp6       0      0 localhost.localdo:48744 localhost.local:wap-wsp ESTABLISHED

Active UNIX domain sockets (w/o servers)

Proto RefCnt Flags       Type       State         I-Node   Path

unix  3      [ ]         DGRAM                    8472     /run/systemd/notify

unix  2      [ ]         DGRAM                    8474     /run/systemd/cgroups-agent<some output omitted>

As seen above, netstat returns lots of information when used without options. Many options can be used to filter and format the output of netstat, making it more useful.

  1. Use netstat with the –tunap options to adjust the output of netstat. Notice that netstat allows multiple options to be grouped together under the same ““ sign.

The information for the nginx server is highlighted.

[[email protected] ~]$ sudo netstat -tunap

[sudo] password for analyst:

Active Internet connections (servers and established)

Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name 

tcp        0      0  *               LISTEN      257/python2.7        

tcp        0      0    *               LISTEN      395/nginx: master

tcp        0      0    *               LISTEN      279/vsftpd         

tcp        0      0    *               LISTEN      277/sshd: /usr/bin           

tcp6       0      0 :::22                   :::*                    LISTEN      277/sshd: /usr/bin             

udp        0      0*                           237/systemd-network


What is the meaning of the –t, -u, –n, –a and –p options in netstat? (use man netstat to answer)

Type your answers here.

-a: shows both listen and non-listening sockets. -n: use numeric output (no DNS, service port or username resolution), -p: show the PID of the connection owner process. -t: shows TCP connections. –u: shows UDP connections

Is the order of the options important to netstat?

Type your answer here.

No, the option order is irrelevant.

Clients will connect to a port and, using the correct protocol, request information from a server. The netstat output above displays a number of services that are currently listening on specific ports. Interesting columns are:

o         The first column shows the Layer 4 protocol in use (UDP or TCP, in this case).

o         The third column uses the <ADDRESS:PORT> format to display the local IP address and port on which a specific server is reachable. The IP address signifies that the server is currently listening on all IP addresses configured in the computer.

o         The fourth column uses the same socket format <ADDRESS:PORT> to display the address and port of the device on the remote end of the connection.* means that no remote device is currently utilizing the connection.

o         The fifth column displays the state of the connection.

o         The sixth column displays the process ID (PID) of the process responsible for the connection. It also displays a short name associated to the process.


Based on the netstat output shown in item (d), what is the Layer 4 protocol, connection status, and PID of the process running on port 80?

Type your answers here.

TCP, LISTEN and 395.

While port numbers are just a convention, can you guess what kind of service is running on port 80 TCP?

Type your answers here.

This is probably a web server.

  1. Sometimes it is useful to cross the information provided by netstat with ps. Based on the output of item (d), it is known that a process with PID 395 is bound to TCP port 80. Port 395 is used in this example. Use ps and grep to list all lines of the ps output that contain PID 395. Replace 395 with the PID number for your particular running instance of nginx:

[[email protected] ~]$ sudo ps -elf | grep 395

[sudo] password for analyst:

1 S root       395     1  0  80   0 –  1829 19:33 ?        00:00:00 nginx: master process /usr/bin/nginx

5 S http       396   395  0  80   0 –  1866 19:33 ?        00:00:00 nginx: worker process

0 S analyst   3789  1872  0  80   0 –  1190 19:53 pts/0    00:00:00 grep 395

In the output above, the ps command is piped through the grep command to filter for only the lines containing the number 395. The result is three lines with text wrapping.

The first line shows a process owned by the root user (third column), started by another process with PID 1 (fifth column), at 19:33 (twelfth column)

The second line shows a process with PID 396, owned by the http user, started by process 395, at 19:33.

The third line shows a process owned by the analyst user, with PID 3789, started by a process with PID 1872, as the grep 395 command.


The process PID 395 is nginx. How could that be concluded from the output above?

Type your answers here.

Based on the last column of line 1, the output shows nginx command line.

What is nginx? What is its function? (Use google to learn about nginx)

Type your answers here.

nginx is a lightweight webserver. A quick google search is extremely helpful on finding information about unidentified processes.

The second line shows that process 396 is owned by a user named http and has process number 395 as its parent process. What does that mean? Is this common behavior?

Type your answers here.

It means that nginx started process 396 under the http username. This is normal as nginx runs itself for every client that connects to port 80 TCP.

Why is the last line showing grep 395?

Type your answers here.

Because the grep 395 was used to filter the ps output, when the output was compiled, grep 395 was still running and therefore, it appeared in the list.

Part 2:  Using Telnet to Test TCP Services

Telnet is a simple remote shell application. Telnet is considered insecure because it does not provide encryption. Administrators who choose to use Telnet to remotely manage network devices and servers will expose login credentials to that server, as Telnet will transmit session data in clear text. While Telnet is not recommended as a remote shell application, it can be very useful for quickly testing or gathering information about TCP services.

The Telnet protocol operates on port 23 using TCP by default. The telnet client however, allows for a different port to be specified. By changing the port and connecting to a server, the telnet client allows for a network analyst to quickly assess the nature of a specific server by communicating directly to it.

Note: It is strongly recommended that ssh be used as remote shell application instead of telnet.

  1. In Part 1, nginx was found to be running and assigned to port 80 TCP. Although a quick internet search revealed that nginx is a lightweight web server, how would an analyst be sure of that? What if an attacker changed the name of a malware program to nginx, just to make it look like the popular webserver? Use telnet to connect to the local host on port 80 TCP:

[[email protected] ~]$ telnet 80


Connected to

Escape character is ‘^]’.

  1. Press a few letters on the keyboard. Any key will work. After a few keys are pressed, press ENTER. Below is the full output, including the Telnet connection establishment and the random keys pressed (fdsafsdaf, this case):


HTTP/1.1 400 Bad Request

Server: nginx/1.16.1

Date: Tue, 28 Apr 2020 20:09:37 GMT

Content-Type: text/html

Content-Length: 173

Connection: close



<head><title>400 Bad Request</title></head>

<body bgcolor=”white”>

<center><h1>400 Bad Request</h1></center>




Connection closed by foreign host.

Thanks to the Telnet protocol, a clear text TCP connection was established, by the Telnet client, directly to the nginx server, listening on port 80 TCP. This connection allows us to send data directly to the server. Because nginx is a web server, it does not understand the sequence of random letters sent to it and returns an error in the format of a web page.

Why was the error sent as a web page?

Type your answers here.

Nginx is a web server and as such, only speaks the HTTP protocol.

While the server reported an error and terminated the connection, we were able to learn a lot. We learned that:

1)      The nginx with PID 395 is in fact a web server.

2)      The version of nginx is 1.16.1.

3)      The network stack of our CyberOps Workstation VM is fully functional all the way to Layer 7.

Not all services are equal. Some services are designed to accept unformatted data and will not terminate if garbage is entered via keyboard. Below is an example of such a service:

  1. Looking at the netstat output presented earlier, it is possible to see a process attached to port 22. Use Telnet to connect to it.

Port 22 TCP is assigned to SSH service. SSH allows an administrator to connect to a remote computer securely.

Below is the output:

[[email protected] ~]$ telnet 22


Connected to

Escape character is ‘^]’.



Invalid SSH identification string.

Connection closed by foreign host.

Use Telnet to connect to port 68. What happens? Explain.

Type your answers here.

Unable to connect because the connection is refused. Telnet is a TCP-based protocol and will not be able to connect to UDP ports.

  1. What are the advantages of using netstat?

Type your answers here.

Netstat allows for an analyst to display all the connections currently present on a computer. Source and destination addresses, ports, and process IDs can also be displayed, providing a quick overview of all connections present on a computer.

  1. What are the advantages of using Telnet? Is it safe?

Type your answers here.

Yes, as long it is not used as a remote shell. It is perfectly safe to quickly test or gather information about a given network service.

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