OPS235 Lab 6 - CentOS7 - HD2

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Setting up networks is an essential operation for a system administrator. Maintaining network connectivity and securing the network are also essential operations. In this lab, we will configure a Virtual Private Network using static IP addresses (eg. wired workstation connections). We will learn how to setup a DHCP network (eg. for notebook, table and smartphones) in lab 8.

Purpose / Objectives of Lab 6

In this lab, you will learn the basics of networking by using your Virtual Machines. You will first set up a virtual private network (VPN) among those machines. In addition, you will learn to set up network names (to associate with server's IP Addresses), associate network services with port numbers for troubleshooting purposes, and use shell scripts with arrays to store network configuration data.

Main Objectives

  1. Configure a private (virtual) network for your VMs and your c7host machine
  2. Configure network interfaces for your Virtual Machines using both graphical and command-line utilities.
  3. Use local hostname resolution to resolve simple server names with their corresponding IP Addresses
  4. Use common networking utilities to associate network services with port numbers for troubleshooting purposes
  5. Use shell scripts with arrays to store network configuration data

Minimum Required Materials
Linux Command Reference
Removable SATA Hard Disk
USB key
(for backups)
Lab6 Log Book

Networking Utilities


Additional Utilities



For the remainder of this course, we will focus on networking involving our VMs. This lab will focus on setting up a virtual private network (VPN), connecting our VMs and c7host machine to the VPN, and configuring our VPN to make more convenient to use, troubleshoot and protect. Lab 7 will focus on configuring SSH and making access to the VPN more secure. Finally, lab 8 will focus on configuring mobile (as well as wired devices) via DHCP to automatically assign an IP address.

There are several reasons for creating VPNs. The main reason is to safely connect servers together (i.e. to safely limit but allow the sharing of information among computer network users). This allows for a secure connection of computers yet controlling access to and monitoring (protecting) access to permitted users (discussed later in lab7).

This diagram shows the current network configuration of your c7host machine in relation to your Virtual Machines. In this section, you will be learning to change the default network settings for both your c7host machine and VMs to belong to a virtual network using fixed IP Addresses.

Part 1: Configuring a Private Network (Via Virtual Machine Manager)

If we are going to setup a private network, there are a number of steps to perform: First, define a new private network in the Virtual Manager application; and second, configure each of our VMs to connect to this new private network. In Part 1, we will be perform the first operation. In part 2, we will be performing the second operation for all VMS (graphical and command-line).

Before configuring our network, we want to turn off dynamic network configuration for our Virtual Machines by turning off the "default" virtual network. We will then define our virtual private network.

Perform the following steps:
  1. Launch your c7host VM and start the Virtual Machine Manager.
  2. Make certain that the centos1, centos2, and centos3 virtual machines are powered off.
  3. In the Virtual Machine Manager dialog box, select Edit-> Connection Details.
  4. In the Connection Details dialog box, select the Virtual Networks tab
  5. Click to de-select the Autostart (on boot) check-box options and click the Apply button.
  6. Stop the default network configuration by clicking on the stop button at the bottom left-side of the dialog box.
  7. Click the add button (the button resembles a "plus sign") to add a new network configuration.
  8. Type the network name called: network1, and then click the Forward button.
  9. In the next screen, enter the new network IP address space called:
  10. Disable the DHCP4 check box and click the Forward button.
  11. Click the Forward button again to accept the default in the next screen.
  12. Enable Network Forwarding by Selecting Forwarding to physical network, the destination should be Any physical device and the mode should be NAT
  13. Proceed with changes, and click Finish.
  1. We will now reconfigure each of our VMs to use our new virtual network network1
    1. Let's start with our centos1 VM. Double-click on your centos1 VM, but instead of running the VM, click on the view menu, and select: Details
      (Note: the Virtual Machine window will appear - do not start virtual machine)
    2. In the left pane of the Virtual Machine window, select NIC: and note that this NIC is on the "default" virtual network
    3. Change it to Virtual Network network1: NAT (i.e. the VPN that you just created) and click the Apply button.

Although the private network has been setup via the Virtual Machine Manager, each virtual machine requires to change its own network setting individually (either graphically or by command line).

Part 2: Configuring Network For centos1 VM

In this section, we will be using a graphical tool to connect our centos1 VM to our private network.

Perform the following steps:
  1. On your c7host machine, run ifconfig and make note of the IP address assigned to the virbr1 (i.e. "Virtual Bridge) interface. This will be the default gateway and DNS server for your VMs.
  2. Select the Console view (instead of Details), start your centos1 VM and login.
  3. Within your centos1 VM, click Applications menu, then select System Tools, and then Settings.
  4. In the Settings Dialog Box, click on the Network icon.
  5. For the Wired connection, click the settings button (The icon appears as a gear located at the bottom right-hand corner of the dialog box).
  6. Select the IPv4 tab. Change Address from Automatic (DHCP) to Manual.
  7. In the Addresses section, enter the following information:
    IP Address:
    Subnet Mask:
    Default Gateway: The IP address of virbr1 on your centos host.
  8. Click on the DNS' field and add The IP address (virbr1 on your centos host) as the primary DNS server.
  9. When finished, check your settings, and then click the Apply button.
  10. Open a terminal and issue the ifconfig command to confirm the IP ADDRESS settings change.
  11. Verify that centos1VM is now connected to the VPN by issuing the following command from your c7host machine:

Although you can use the ifconfig command to temporarily create a static IP address connection to a network, you need to add the network settings in the /etc/sysconfig/network-scripts directory to automatically connect to the network upon Linux system boot-up.

Part 3: Configuring VM Network Setup via Command Line (centos3 and centos2)

The centos3 and centos2 VMs are text-based only systems, thus we cannot use a graphical tool to configure centos3 to connect to our private network. Therefore we will learn how to perform this task by using command-line tools.

Perform the following steps:
  1. Configure your centos3 VM (in the View -> Details menu of Virtual Machine Manager) to configure the NIC interface to network1, click Apply, and switch your centos3 VM view from details to console.
  2. Start your centos3 VM, login, and su to root.
  3. Use the command called: ifconfig to list active interfaces, you should see one with a name of eth0 or a similar name.
  4. To configure your card with a static address use the following command:
    ifconfig eth0 netmask
  5. To configure a default gateway for that interface enter the command:
    route add default gw
  6. To configure a DNS server for this VM, edit the file /etc/resolv.conf. Change the nameserver line to read:
  7. Confirm your settings work by doing the following (you might need to do the steps 3 and 4 a few times before it works; keep checking with the commands below and wait a bit before each attempt): ifconfig, route -n, ping (other VM's IP address), and ssh ( to your Seneca's Matrix account to test your DNS)
  8. Restart the centos3 VM, or just wait a few minutes.
  9. Login and test your configuration again. What happened?
  10. While we can configure network settings from the command line those settings are not persistent. To configure persistent network configurations we need to edit the configuration files.
  11. Change to the /etc/sysconfig/network-scripts directory
    This diagram should show the newer network configuration of your c7host machine in relation to your Virtual Machines.
  12. List the contents of this directory. You should see 2 different types of files, network config scripts and network configuration files.
  13. Look for the configuration file for your original interface, it should be named ifcfg-eth0
  14. Edit the new file for your interface and give it the following settings (or create a brand new file, might be easier than editing the old one):
    HWADDR=xx:xx:xx:xx:xx:xx <-- # Use YOUR centos3 VM's MAC ADDRESS!!!
Keep consistent with Quotation or no Quotation
You have the option to either place quotation marks around the values, or no to use quotation at all. Regardless of the method you chose, KEEP CONSISTENT. For example, if beginning value with a double-quote, end value with a double-quote. If beginning value without a double-quote, do not end with any quotation.
  1. Save the file and then restart the network connection by issuing the commands:
    ifdown eth0 and ifup eth0
  2. Verify your configuration as you did before.
  3. Restart the centos3 VM.
  4. Use the ifconfig and ping commands to verify your network connection to other VMs.
  5. Login and attempt to ssh to your matrix account to verify the settings.

    We need to also configure your centos2 VM for a persistent network connection as well:

  6. Configure the centos2 VM (in the View -> Details menu of Virtual Machine Manager) to configure the NIC interface to network1, click Apply, and switch your centos2 VM view from details to console.
  7. Start your centos2 VM, login, and su to root.
  8. Issue the command: ip address and write down the MAC address for your eth0 network interface.
  9. Edit the /etc/sysconfig/network-scripts/ifcfg-eth0 file using the IPADDR: , and the same NETMASK, GATEWAY, DNS1 and MAC address information for you centos2 VM (i.e. recorded previously).
  10. Save changes, re-issue the ifdown and ifup commands, and then issue the ip address and ping commands to verify that you can connect to other VMs on your network.

Answer INVESTIGATION 1 observations / questions in your lab log book.


Creating private networks are an important task, but a system administrator also needs to manage the network to make it convenient to use, and troubleshoot network connectivity problems.

This investigation will expose you to useful "tweaks" and utilities to help accomplish this task. Lab 7 requires that you understand these concepts and have a good general understanding how to use troubleshooting utilities (like netstat).

Hosts files vs. the Domain Name System
On large public networks like the Internet or even large private networks we use a network service called Domain Name System (DNS) to resolve the human friendly hostnames like centos.org to the numeric addresses used by the IP protocol. On smaller networks we can use the /etc/hosts on each system to resolve names to addresses.

Part 1: Using /etc/host File for Local Hostname Resolution

After setting up a private network, it can be hard to try to remember IP addresses. In this section, we will setup your network to associate easy-to-remember server names with IP ADDRESSES.

Perform the following steps:
  1. Complete this investigation on all of your VMs and the c7host machine.
  2. Use the hostname and ifconfig commands on your c7host machine and all of your 3 VM's to gather the information needed to configure the /etc/hosts file on all of your Linux systems.
  3. Edit the /etc/hosts file for the c7host, centos1, centos2 and centos3 VMs. Add the following contents to the bottom of the /etc/hosts file: c7host centos1 centos2 centos3

  4. Verify that you can now ping any VM by their hostname instead of the IPADDR.

Common Network Troubleshooting Tools
Purpose Command(s)
Network Connectivity ping
Network Service Status netstat

Part 2: Network Connectivity & Network Service Troubleshooting Utilities

Troubleshooting network problems is an extremely important and frequent task for a Linux/Unix system administrator. Since network services (such as file-server, print-servers, web-servers, and email-servers) depend on network connectivity, as Linux/Unix sysadmin must be able to quickly and effectively pin-point sources of network problems in order to resolve them.

Network service problems may not be entirely related to a "broken" network connection, but a service that is not running or not running correctly. The following table lists the most common listing of utilities to assist with detection of network connectivity or network service problems to help correct the problem.

Perform the following steps:
  1. Switch to your c7host machine.
  2. Issue the ping command to test connectivity to your centos1, centos2, and centos3 VMs.
  3. Examine the contents of the ARP cache by using the command: arp What is the purpose of ARP?
  4. Check the contents of the cache again by using the command: arp -n What was the difference in output?
  5. Issue the following command: netstat -at This command will list all active TCP ports. Note the state of your ports.

TCP is a connection oriented protocol that uses a handshaking mechanism to establish a connection. Those ports that show a state of LISTEN are waiting for connection requests to a particular service. For example you should see the ssh service in a LISTEN state as it is waiting for connections.

  1. From one of your VM's login to your host using the ssh command.
  2. On your c7host VM rerun the netstat -at command and in addition to the LISTEN port it should list a 2nd entry with a state of ESTABLISHED. This shows that there is a current connection to your ssh server.
  3. Exit your ssh connection from the VM and rerun the command on the CentOS host. Instead of ESTABLISHED it should now show a state of CLOSE_WAIT. Indicating that the TCP connection is being closed.
  4. On your c7host VM, try the command: netstat -atn How is this output different? Without the -n option netstat attempts to resolve IP addresses to host names (using /etc/hosts) and port numbers to service names (using /etc/services)
  5. Examine the /etc/services file and find which ports are used for the services: ssh, sftp, http
  6. Now execute the command netstat -au What is the difference between the options: -at and -au? When examining UDP ports why is there no state?

Part 3: Using Arrays to Collect VM Network Information

Bash Shell Scripting Tips:


  • Incredibly useful tool that works like a "storage container" of variables. Arrays have names like variables, but are numbered for quick storage and access of related elements. Indices are I.D. numbers for array access. The first index starts at zero instead of 1.

  • Instead of using index numbers, you can use I.D. names: this is referred to as an Associative Array.

  • Examples:

    for((x=0; x<5; x++)) # Example of a regular array
      read -p "Enter item #$((x+1)): " item[xx]
    for((y=0; y<5; y++))
      echo ${item[y]}

    set a b c # Example of an "Associative Array"
    for x
      read -p "Enter item $x: " item["$x"]
      echo item[$x] is: ${item["$x"]}

Working with Temporary Files

  • When creating temporary files, it is important NOT to store on a user's account (to avoid overwriting their existing files). Instead, temporary files can be created in the /tmp directory.

  • The $$ variable can be used as the filename extension which assigns the current PID of the shell script running to make the filename unique, and allow easy removal at the end of the shell script by deleting ALL files in the /tmp directory with the extension: .$$

  • Example:

    ls -lR > /tmp/temp-file.$$
    grep secret /tmp/temp-file.$$
    rm /tmp.$$

We finish shell scripting by using arrays by reading and storing networking information for each VM (centos1, centos2, and centos3) to be stored in a report in your c7host machine. We will use the ssh command in order to grab the network information (one VM at a time), and store the network setup into an Associative array in our c7host machine.

If you do not entirely understand the script that you will enter, just realize that we are using these "special storage variables" in order to make this work.

Perform the following steps:
  1. Perform this section in your c7host machine.
  2. Open a shell terminal and login as root.
  3. Change to the /root/bin directory.
  4. Using a text editor, create a Bash shell script called network-info.bash and enter the following content below:


# Author: *** INSERT YOUR NAME ***
# Date: *** CURRENT DATE ***
# Purpose: Creates system info report
# USAGE: ./network-info.bash [full pathname to ifcfg-eno* file]

if [ $USER != "root" ] # only runs if logged in as root
 echo "You must be logged in as root." >&2
 exit 1

if [ $# -ne 1 ]
 echo "You need to provide the full pathname to ifcfg-eno file (eg eno1 or eno followed by numbers)" >&2
 echo "USAGE: $0 [full pathname to ifcfg-eno* file]" >&2
 exit 0

  1. We will now use a while loop to read every line in the network configuration file and store into an array as a separate array element. Add the following content to your shell script editing session:

while read line
done < $1

  1. Finally, we will generate a web-page document, use a loop to printout the results of the data (stored in an array) and open the web-page document in the firefox web-browser.

cat > /root/network-info.html <<+
<!DOCTYPE html>
   <title>Network Information</title>
   <h3>Network Information for: $1</h3>
   <table cellpadding="5" cellspacing="0" border="1">

for((x=0; x<index; x++))
 echo "<tr><td>${network[$x]}</td></tr>" >> /root/network-info.html

cat >> /root/network-info.html <<+

/bin/firefox /root/network-info.html

  1. Give this shell script execute permissions and run this shell script.
  2. You will need to determine the full pathname of the ifcfg-en0 file in the /etc/sysconfig/network-scripts directory to use as an argument for this shell script.
  3. Run the shell script. What do you notice from the report that the shell script generated?
  4. The the wget command to download another variation of the network-info.bash shell script using the awk command to separate the variables and values into separate columns:

Answer INVESTIGATION 2 observations / questions in your lab log book.


Time for a new backup!
If you have successfully completed this lab, make a new backup of your virtual machines as well as your host machine.
Perform the Following Steps:
  1. Make certain that ALL of your VMs are running.
  2. Switch to your c7host VM.
  3. Change to the /root/bin directory.
  4. Issue the Linux command: wget http://matrix.senecac.on.ca/~murray.saul/ops235/lab6-check.bash
  5. Give the lab6-check.bash file execute permissions (for the file owner).
  6. Run the shell script and if any warnings, make fixes and re-run shell script until you receive "congratulations" message.
  7. Arrange proof of the following on the screen:
    centos2 VM:
    • ssh from centos2 to c7host VM.
    c7host machine
    • Output from running the network-info.bash shell script
    • Run the lab6-check.bash script in front of your instructor (must have all  OK  messages)
    Lab6 log-book filled out.

Practice For Quizzes, Tests, Midterm & Final Exam

  1. What is a port?
  2. What command will set your IP configuration to ?
  3. What is the difference between UDP and TCP?
  4. What port number is used for DHCP servers?
  5. What is the function of the file /etc/hosts ?
  6. What is the purpose of the file /etc/sysconfig/network-scripts/ifcfg-eth0 ?
  7. What tool is used to show you a list of current TCP connections?