OPS235 Lab 6 - Fedora17

Configuring a Network Using Virtual Machines

Objectives

• Configure a virtual network for Virtual Machines
• Use the Fedora GUI program to configure network interfaces with static IP configuration and host name resolution
• Use the find command to locate the configuration files modified by the GUI network configuration program

• To examine some of the Linux's TCP/IP configuration files in the /etc/ directory
• To configure a Fedora host with static network configuration without a GUI tool
• To use and interpret the netstat command to troubleshoot and monitor network services

• To configure the linux firewall iptables to allow/disallow/forward different types of network traffic using simple rules

Reference

• man pages for find, ifconfig, ping, netstat, NetworkManager, nslookup, iptables, arp
• Online reading material for week 8.

Required materials

• Fedora 12 Live CD or a classmate on the same pod
• USB flash drive, 64 MB or more in size (Warning: the contents of this drive will be erased)
• One SATA hard disk in a removable drive tray with Fedora host and 3 Fedora Virtual Machines installed

Current Configuration

Currently you should have the following network configuration:

• Fedora host has 1 active network interface (probably eth0)that receives IP configuration from the School's DHCP server.
• Fedora host has 1 active network interface (virbr0) that has a static default configuration of 192.168.122.1/255.255.255.0
• Fedora1 VM has 1 active interface (eth0) that receives a dynamic configuration from your Fedora Host
• Fedora2 VM has 1 active interface (eth0) that receives a dynamic configuration from your Fedora Host
• Fedora3 VM has 1 active interface (eth0) that receives a dynamic configuration from your Fedora Host

Lab Preparation

Important
It is advisable to perform a yum update on your Fedora host and all 3 VM's.

Important
The fedora1 VM was installed from Live CD. It is missing the GUI Network Configuration tool we will be using. Use the command yum install system-config-network to install it.

Important We will be working with your 3 VM's
Backup your VM disk images and then stop all 3 VM's for until told to start them.

Lab Investigations

Investigation 1: How do you create a new virtual network.

Note!
Complete this investigation on your fedora host.

Before configuring our network we want to turn off dynamic network configuration for our Virtual Machines by turning off the "default" virtual network.

1. On the fedora host start Virtual Machine Manager
2. Under Edit->Host Details select the Virtual Networks tab
3. Disable the default configuration from starting at boot by deselecting the "Autostart On Boot" checkbox.
4. Stop the default network configuration by clicking on the stop button at the bottom of the window.
5. Click on the add button to add a new network configuration.
6. Give your new network a name (network1)
7. Enter in the new network IP address space:
   • 192.168.235.0/24
8. Disable DHCP by deselecting the check box.
9. Enable Network Forwarding by Selecting "Forwarding to physical network"

1. The destination should be "Any physical device" and the mode should be "NAT"
2. Now we need to add our new virtual network "network1" to the 3 VM's
   1. Select the fedora1 VM and edit the VM details
   2. Under View select Details
   3. In the left pane select the NIC and note that this NIC is on the "default" virtual network
   4. Click on the Remove button
   5. Click on "Add Hardware" and add a new network
   6. For the host device select "Virtual Network network1" : NAT
3. Repeat these steps for fedora2 and fedora3 VM's
4. Answer the Investigation 1 question in your lab log book.

Investigation 2: How do you configure a static network using system-config-network.

Note!
Complete this investigation on your fedora2 VM.

1. Start fedora2 VM and login
2. On your fedora host run ifconfig and make note of the IP address assigned to the virbr1 interface. This will be your default gateway for your Vm's.
3. To configure a new interface on fedora2 go to System->Administration->Network and click on the "New" button
4. Select Ethernet Device and choose the network card named eth1

1. Statically set the IP configuration:
   • IP Address 192.168.235.12
   • Subnet Mask 255.255.255.0
   • Default Gateway 192.168.235.1 (The IP address of virbr1 on your fedora host.)
2. Click on the DNS tab and add 192.168.235.1 as the primary DNS server.
3. Save then Quit
4. Restart your network on fedora2. service NetworkManager restart
5. Verify your new interface by examining the output of ifconfig
6. To verify that fedora2 has the correct default gateway configured, enter the command route -n
7. Verify the network by using ping 192.168.235.1
8. Record the answer to Investigation 2 in your logbook.

Investigation 3: What files does the system-config-network GUI tool change?.

Note!
Complete this investigation on your fedora1 VM.

1. Start fedora1 VM and login
2. Before we configure fedora1 we should create a timestamp file that can be used to see which files have changed as a result of using the GUI tool.
   • date > /tmp/timestamp
3. Run the network configuration tool and enter the following static configuration in the same way that you configured fedora2.
   • IP Address: 192.168.235.11
   • Subnetmask: 255.255.255.0
   • Default Gateway: 192.168.235.1
   • DNS Server: 192.168.235.1
4. Save and quit and restart NetworkManager
5. Verify the configuration by pinging fedora host (192.168.235.1) and fedora2 (192.168.235.12)
6. To verify that fedora1 has the correct default gateway configured, enter the command route -n
7. Use the find command to locate the configuration files modified by the GUI network configuration program
   • Previously, you created a file called /tmp/timestamp. Any files that were modified by the GUI network configuration program should have a timestamp later (or newer) than the "timestamp" file you created. All the Linux TCP/IP configuration files are stored under the "/etc" directory or its sub-directories. The following command when run as root will give you a list of all the files under the /etc directory with a file modification date newer than the date of the "timestamp" file:
     • find /etc -newer /tmp/timestamp
8. Capture the output to a file called netcfg.lst under user root's home directory.

1. Create a new directory called /tmp/lab6 and copy all the files from the find command to it.
   • There are a number of ways to accomplish this:
     • Edit netcfg.lst and turn it into a bash script.
     • Copy the files manually using the cp command.
     • Investigate the -exec option of the find command to see how to find and copy the files with a single command.
2. Record the answer to the investigation in your lab log.

Investigation 4: How do I configure the network without a GUI tool?.

Note!
Complete this investigation on your fedora3 VM.

1. Start fedora3 VM and login as root
2. Use the command ifconfig to list active interfaces, you should see one with a name of eth1 or a similar name.
3. To configure your card with a static address use the command:
   • ifconfig eth1 192.168.235.13 netmask 255.255.255.0
4. To configure a default gateway for that interface enter the command:
   • route add default gw 192.168.235.1
5. To configure your DNS server edit the file /etc/resolv.conf. Change the nameserver line to be:
   • nameserver 192.168.235.1
6. Confirm your settings work by doing the following:
   • ifconfig
   • route -n
   • ping your other VM's and fedora host.
   • ssh to your matrix account to test DNS
7. Restart the fedora3 VM
8. Login and test your configuration again. What happened?
9. 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:
   • Change to the /etc/sysconfig/network-scripts directory on fedora3
   • List the contents of the directory and you should see 2 different types of files, network config scripts and network configuration files.
   • look for the config file for your original interface, it should be named ifcfg-eth0
   • Copy that file to ifcfg-eth1 or whatever name matches your current eth interface.
   • Edit the new file for you interface and give it the following settings:
     • GATEWAY=192.168.235.1
     • DNS1=192.168.235.1
     • DEVICE=eth1 <-- or the interface name you have
     • BOOTPROTO=none
     • NETMASK=255.255.255.0
     • TYPE=Ethernet
     • HWADDR=52:54:00:3f:5c:fa <--use the HWADDR for your interface
     • IPADDR=192.168.235.13
     • IPV6INIT=no
     • ONBOOT=yes
     • USERCTL=no
10. Save the file and restart the NetworkManager service
11. Verify your configuration as you did before.
12. Finally the kickstart file used to install this VM did not set the hostname. Edit the file /etc/sysconfig/network and set the hostname to fedora3
13. Restart the fedora3 VM.
14. Login and attempt to ssh to your matrix account to verify the settings.
15. Answer the Investigation question in your logbook.

Investigation 5: How do I setup local hostname resolution?

Note!
Complete this investigation on all of your VM's and the Fedora host.

Please take note!
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 fedoraproject.org to the computer friendly 32bit numeric addresses used by the IP protocol. On smaller ad-hoc networks we can use the /etc/hosts on each system to resolve names to addresses.

1. Use the hostname and ifconfig commands on your fedora host and all 3 VM's to gather the information needed to configure the /etc/hosts file on the fedora host and the 3 VM's.
2. Edit the /etc/hosts file on each of them so that they can all ping each other by name.
3. Answer the investigation in your logbook.

Sample /etc/hosts file
# hostname fedora1 added to /etc/hosts by anaconda 127.0.0.1 localhost.localdomain localhost fedora1 ::1 localhost6.localdomain6 localhost6 fedora1 192.168.235.1 f12host 192.168.235.11 fedora1 192.168.235.12 fedora2 192.168.235.13 fedora3

Investigation 6: How do I collect the MAC (Hardware) addresses of computers on my network?

Note!
Complete this investigation on your Fedora host.

While we use 32bit IP addresses to communicate over an internetwork, on the local ethernet network packets are delivered to a 48bit hardware address (sometimes called a MAC address). The ARP protocol resolves 32bit IP addresses to 48bit MAC addresses by using a broadcast and caching the results. We can examine the ARP cache to get the MAC addresses of other computers on our local network.

1. On the fedora host ping each of your VM's
2. Examine the contents of the ARP cache by using the command arp
3. Check the contents of the cache again by using the command arp -n
4. What was the difference in output?
5. Answer the investigation question in your logbook

Investigation 7: How can I see what network services or ports are active on my Fedora system?

Note!
Complete this investigation on all of your VM's and the Fedora host.

Please take note!
When our Fedora system provides any services on a network, those services are accessible through a port number. All network services are configured to be accessed on a particular port number. By examining which ports are active on our system we can know what services (and points of attack) are available on our system. The ability to examine this information is important for troubleshooting network services and securing our systems. One great tool for this is the netstat command.

• On your fedora host execute the 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.
• From one of your VM's login to your host using ssh
• On the fedora host rerun the 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.
• Exit your ssh connection from the VM and rerun the command on the fedora host. Instead of ESTABLISHED it should now show a state of CLOSE_WAIT. Indicating that the TCP connection is being closed.
• On your fedora host 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)
• Examine the /etc/services file and find which ports are used for the services ssh ftp http
• Now execute the command netstat -au What is the difference between -at and -au?
• When examining UDP ports why is there no state?

• Using the netstat man page and experimentation make sure you understand how the following options work.
  • -at
  • -au
  • -atp
  • -aup
  • -atn
  • -aun
  • -autnp
• Answer the investigation your logbook

Investigation 8: How do I view and configure the IPTABLES firewall? -- Basic Function/Configuration

Iptables is the built-in firewall for LINUX. While this program can be controlled by different GUI's, we are going to investigate the powerful command line interface for this program to choose what data is allowed into, out of and through our computer.

Essentially, Iptables is a list of rules. Each rule is placed into a particular chain and when data is sent into, out of or through a PC the data is checked against these rules. If the data matches a particular rule, it then must “jump” to a condition. Simple conditions include ACCEPT, DROP and LOG but there are also more complex conditions that can be applied and there is even the option to create your own conditions.

Iptables consists of three chains of rules – INPUT, OUTPUT and FORWARD. Here as brief explanation of these chains.

OUTPUT – When you want to do some research on the Web for something, you open a browser on your PC and navigate to http://www.google.ca. When you do you are attempting to establish an HTTP or HTTPS session with the web server at http://www.google.ca. A data packet is built with appropriate IP and TCP information and sent out of your computer but before it goes out to the Internet it will be compared to all of the rules in the OUTPUT chain to see if this data is allowed to go “out” of the PC. If it is not allowed then the packet is dropped.

INPUT – If your data was allowed out and a request was sent to http://www.google.ca, this web server will send data back to your PC with an acknowledgement. Before this data can be processed by your browser, it must first be checked against the INPUT chain to see if it is allowed into the PC. If it is, your browser will process the data and move to it's next task. If it is not, the packet will be dropped.

FORWARD – LINUX PC's are often used as routers or gateways for other PC's. This means that data may have to be passed through this LINUX box, but the data is not intended for the LINUX PC nor is it being sent by the LINUX PC. Even though the data will go into this PC and it will exit this PC, the INPUT and OUTPUT chains do not apply here. Because the PC is acting as a router it does not actually send or receive data, it FORWARDS data from one machine to another. When this process happens, the data is checked against the FORWARD chain to see if it is allowed through. If it is the router will forward the data to it's destination. If not, the packet is dropped.

Note!
We will complete this lab on the host machine.

It should be noted that all of the commands that we do here with iptables will not be persistent.

That means if you re-boot, the default iptables configuration will be loaded.

• As root on the fedora host enter the following commands at the prompt:
  • iptables -F (This flushes out or clears all of your rules from the chains)
  • iptables -L
• You should see something similar to this:

    Chain INPUT (policy ACCEPT)

    target prot opt source destination

    Chain FORWARD (policy ACCEPT)

    target prot opt source destination

    Chain OUTPUT (policy ACCEPT)

    target prot opt source destination

Each chain has a default policy. In my example here the default policy is ACCEPT. This means that if data packets are checked and there is no rule that matches that packet in the chain the data will be allowed to pass to it's destination. Conversely, if the policy is set to DROP then the packet will be dropped if there is no match. See the MAN page for iptables for detailed information on switches and options.

• Set the default policy for the INPUT chain to DROP:
  • iptables -P INPUT DROP

The -P switch stands for POLICY. This tells iptables that we are changing the policy for the INPUT chain. The policy is to be changed to DROP. Note that iptables commands (as well as all other Linux commands) are case sensitive. Check your syntax carefully.

• Now try on your own to change the default policies for the OUPUT and FORWARD chains to DROP
• Write the commands you executed in your lab book.
• Can we mix these policies? Try to set the FORWARD chain policy to ACCEPT. Did it work?

Testing policies

• Execute the command iptables -L and check that the policies on your INPUT and OUTPUT chain are set to DROP
• Open a browser and attempt to access the Internet. Were you successful?
• Using the commands you have learned so far, change the policies on the INPUT and OUTPUT chains to ACCEPT
• Open your browser and attempt to access the Internet again. Were you successful?

• Change the policies on all of the chains to DROP
• In the OUTPUT chain, add the following rule:
  • iptables -A OUTPUT -j LOG
• The above rule tells iptables to log packets and relevant information to /var/log/messages.
• This entry in the OUTPUT policy will therefore log all packets being sent out of the machine.
• Try to access the Internet again. Because the policies have been set to DROP, you should be unsuccessful. However, every packet of data that your PC attempted to send out was logged. Let's have a look at the log file and analyze the data.
  • tail /var/log/messages
• This command shows us the last 10 lines of the file. While there are many things being logged to this file, the last thing we did was try to access the Internet so we should be able to see the data we need. Look for a line that looks similar to the following:

Mar 3 09:21:03 koala-laptop kernel: [90775.407304] IN= OUT=wlan0 SRC=192.168.1.107 DST=66.249.90.104 LEN=1470 TOS=0x00 PREC=0x00 TTL=64 ID=19752 DF PROTO=TCP SPT=45431 DPT=80 WINDOW=108 RES=0x00 ACK PSH URGP=0

• Your IP, host names and date will be different, but the one thing that should be the same is the DPT=80 value.
• When your computer tried to send OUT a request to connect to the Internet using the WWW, the computer used a destination port of 80. This is the standard port for the WWW. Because we have set the default policy to DROP it drops these packets. The problem is we are dropping all packets. What if we just want to drop the WWW packets?
• Using the commands we already know, change the default policies on all of your chains to ACCEPT.
• Open a browser and confirm that you can access the world wide web.
• Enter the command:
  • iptables -I OUTPUT -p tcp -s0/0 -d 0/0 –dport 80 -j DROP

This command can be read like this, Insert a line into the iptables OUTPUT chain that will look at tcp information and DROP any packet with a source address from anywhere, to a destination address to anywhere if the destination port is 80. Let's break down the command to see how it works.

The -I switch tells iptables to INSERT this line into the OUTPUT policy. This means it will be the first line in the policy. If we used a -A switch it would have appended the line and it would be the last line of the policy. If you are writing complex iptables rules where multiple matches can occur, it is important that the lines go in the right order. Most people simply write scripts to ensure this.

The -p tcp switch tells iptables to filter data by looking at TCP information in the packets. For now, we simply want to filter data by source and destination TCP ports so we need this switch.

The -s0/0 switch specifies the source IP address. 0/0 means a source address of “anywhere.” this has been put into the lab because your ip address will change because it is dynamically assigned. You can change this value if you want to the IP address that has been specifically assigned to your PC.

The -d0/0 switch specifies the destination address. It makes sense that this address is set to “anywhere” because if we want to block all requests to the WWW, we will never know the specific IP address of web server that is trying to be accessed.

The switch –dport 80 tells iptables to look at the destination port in the packet and see if it is equal to 80

-j means jump to a particular condition – Basic conditions are ACCEPT, DROP and LOG. There are others and you also have the ability to create your own.

DROP means drop the packet – do not let it leave the computer.

• Try to access the Web. If you have done everything right, you should not have been successful.
• After you have completed the test execute the following command:
  • iptables -F

Additional Exercises

• Using the information you have learned, try on your own to achieve the same goal as above (block www access to your computer) by using the INPUT chain instead of the OUTPUT chain.
• After you have completed this task, flush the iptables again.
• Make sure that your ssh server is running on the host machine and try to access it from a virtual machine of your choice.
• Once you have confirmed that ssh is running on the host machine, append an iptables rule on the host machine to prevent access to the ssh server from all VM's on the virtual network.
• Confirm that your rule works by testing from your VM's
• Does iptables close the port? Check using netstat
• Now append a rule on the fedora host that would ACCEPT connections from the fedora2 VM only.
• Fully test your configuration.

When your iptables service starts or at boot time it has to load the rules from the file /etc/sysconfig/iptables.

• Make a backup of the file /etc/sysconfig/iptables
• Examine the file to see how rules are added.
• Enter in your 2 new rules created above.

Carefull!
Remember mistakes in iptables can have a significant impact on your system. Make sure you are appending the rules to the INPUT chain

• Restart your iptables service and test your configuration.
• Write a short bash script to append a rule allowing the fedora1 and fedora3 VM's to connect to ssh on the fedora host.
• Answer the investigation in your logbook

New Configuration

Now you should have the following network configuration:

• Fedora host has 1 active network interface (probably eth0)that receives IP configuration from the School's DHCP server.
• Fedora host has 1 active network interface (virbr1) that has a static default configuration of 192.168.235.1/255.255.255.0
• Fedora1 VM has 1 active interface (eth1) that has a static configuration of 192.168.235.11/255.255.255.0
• Fedora2 VM has 1 active interface (eth1) that has a static configuration of 192.168.235.12/255.255.255.0
• Fedora3 VM has 1 active interface (eth1) that has a static configuration of 192.168.235.13/255.255.255.0

Completing the lab

Time for a new backup!
If you have successfully completed this lab, make a new backup of your virtual machines.

Arrange proof of the following on the screen:

1. A list of your iptables rules.
2. The contents of your arp cache.
3. ssh from fedora2 to fedora host.
4. ifconfig from all 3 VM's
5. Contents of /tmp/lab6 directory.
6. Fresh backup of the virtual machines.
7. Run your bash script to allow VM's to access ssh on your fedora host.

Preparing for the Quizzes

1. What is a port?
2. What command will set your IP configuration to 192.168.55.22/255.255.255.0 ?
3. What file contains the systems iptables rules?
4. What is the difference between UDP and TCP?
5. What port number is used for DHCP servers?
6. What is the function of the file /etc/services ?
7. What is the function of the file /etc/hosts ?
8. What is the purpose of the file /etc/sysconfig/network-scripts/ifcfg-eth0 ?
9. What tool is used to show you a list of current TCP connections?