Difference between revisions of "OPS235 Lab 7"

From CDOT Wiki
Jump to: navigation, search
(Part 1: Confirming sshd service is Running on VMs.)
 
(114 intermediate revisions by 7 users not shown)
Line 4: Line 4:
 
[[Image:firewall.png|thumb|right|180px|Protecting a computer network from unauthorized access is one of the many day-to-day operations for a Linux system administrator and/or security specialist]]
 
[[Image:firewall.png|thumb|right|180px|Protecting a computer network from unauthorized access is one of the many day-to-day operations for a Linux system administrator and/or security specialist]]
  
 +
Setting up a computer network is very important, but the Linux system administrator must also perform networking maintenance which includes '''trouble-shooting''', '''repairing network connection issues''' and '''maintaining network security'''. System administrators need to '''protect or "harden" their computer networks from "penetration" from unauthorized computer users'''. Hardening a computer system can range from running an '''IDS''' (Intrusion Detection System) to monitoring and flagging suspicious activity to implementing security policies which could range from running firewalls to setting locked screen savers on workstations.
  
Setting up a computer network is very important, but there are many other operations that occur on a daily basis that can include '''trouble-shooting''', '''repairing network connection issues''' as well as '''maintaining network security'''. System administrators need to '''protect or "harden" their computer networks from "penetration" from unauthorized computer users'''. Hardening a computer system can range from running an '''IDS''' (Intrusion Detection System) to monitoring and flagging suspicious activity to implementing screen savers on workstations.
+
In this lab, you will learn how to install and configure the SSH service on a VM to allow users to securely access and share data between authorized personnel. In addition, you will learn various methods of running and configuring an ssh server which include: using '''Public Key Authentication''', setting up an '''SSH tunnel''' in order to securely run graphical applications safely among computers in the network, and '''disabling root login''' into a Linux machine. You will also learn how to set up a firewall using the '''iptables''' command in order to control the flow of packets throughout your computer server.
 
 
In this lab, you will learn how to use '''ssh''', '''scp''', '''sftp''' commands to securely access and share data between authorized personnel. In addition, you will learn various methods of running and configuring an ssh server which include: using '''Public Key Authentication''', setting up an '''SSH tunnel''' in order to securely run graphical applications safely among computers in the network, '''disabling root login''', and '''changing the default ssh communication port''' to mislead potential penetration testers (also known as "pen-testers" or "hackers").
 
  
  
 
<u>Main Objectives</u>
 
<u>Main Objectives</u>
  
* To set up, configure Secure Shell Services ('''ssh/sshd''')
+
# To use the '''ssh''' and '''scp''' to access and copy data among Linux servers in a secure manner
* To use the '''ssh''', '''scp''', and '''sftp''' clients to access, copy, or transfer data among Linux servers in a secure manner
+
# Set up, configure, and start the Secure Shell Service ('''sshd''')
* Use ssh to '''tunnel X applications'''
+
#* To refuse root login from remote Linux servers or limit users that are permitted to ssh into Linux servers
* To tunnel network traffic via other communication ports
+
# Generate Public and Private keys to ensure secure connections between Linux servers
* To customize '''sshd''' to create a more private, secure system
+
# Use ssh to '''tunnel Xwindow applications'''
 +
# Learn about the Linux firewall (via iptables):
 +
#* Use '''iptables''' command used to configure and maintain a firewall for protection and troubleshooting
 +
#* Configure '''iptables''' to set a default policy and add exceptions to the default policy
  
  
Line 45: Line 47:
 
[http://man7.org/linux/man-pages/man8/netstat.8.html netstat]<br>
 
[http://man7.org/linux/man-pages/man8/netstat.8.html netstat]<br>
 
[http://man7.org/linux/man-pages/man8/ifconfig.8.html ifconfig]<br>
 
[http://man7.org/linux/man-pages/man8/ifconfig.8.html ifconfig]<br>
 +
[http://man7.org/linux/man-pages/man8/ip.8.html ip]<br>
 
[http://man7.org/linux/man-pages/man8/ping.8.html ping]<br>
 
[http://man7.org/linux/man-pages/man8/ping.8.html ping]<br>
 
[http://man7.org/linux/man-pages/man8/arp.8.html arp]<br>
 
[http://man7.org/linux/man-pages/man8/arp.8.html arp]<br>
Line 51: Line 54:
 
|style="padding-left:20px;"|Additional Utilities<br>
 
|style="padding-left:20px;"|Additional Utilities<br>
 
[http://man7.org/linux/man-pages/man7/hostname.7.html hostname]<br>
 
[http://man7.org/linux/man-pages/man7/hostname.7.html hostname]<br>
[http://linux.die.net/man/8/restorecon restorecon]<br>
+
[http://linux.die.net/man/8/restorecon restorecon]<br><br>
[http://linux.die.net/man/8/chkconfig chkconfig]<br><br>
+
Managing Services<br>
 +
[http://www.dsm.fordham.edu/cgi-bin/man-cgi.pl?topic=systemctl systemctl]<br><br>
 
Configuration Files<br>
 
Configuration Files<br>
[http://linux.about.com/library/cmd/blcmdl5_ssh_config.htm ssh_config]<br>
+
[https://www.freebsd.org/cgi/man.cgi?query=ssh_config&sektion=5 ssh_config]<br>
[http://linux.about.com/od/commands/l/blcmdl5_sshdcon.htm sshd_config]<br>
+
[https://www.freebsd.org/cgi/man.cgi?sshd_config(5) sshd_config]<br>
 
|style="padding-left:20px;"|SSH Reference<br>
 
|style="padding-left:20px;"|SSH Reference<br>
 
[http://support.suso.com/supki/SSH_Tutorial_for_Linux A good ssh tutorial]<br>
 
[http://support.suso.com/supki/SSH_Tutorial_for_Linux A good ssh tutorial]<br>
Line 61: Line 65:
 
|}
 
|}
  
=INVESTIGATION 1: CONFIGURING AND ESTABLISHING AN SSH CONNECTION=
+
=INVESTIGATION 1: INSTALLING AND MAINTAINING AN SSH SERVER=
 +
 
 +
So far, you have learned to use the '''ssh''' utility to establish a secure connection to a remote server in order to perform Linux administration tasks. You have issued the ''ssh'' command, which is actually the '''client''' application for ssh. In order to connect to a remote server (like your VMs, Matrix, etc) it needs to run the '''SSH service''' (i.e. the '''ssh daemon''').
  
So far, you have learned to use the '''ssh''' utility to establish a secure connection to a remote server to perform tasks, administer the server, etc. For these common operations, you have issued the ''ssh'' command, which is the client application for ssh. In order to connect to a remote server (like your VMs, Matrix, etc) they need to be running the SSH service. In this lab, you will learn how to run an SSH server in a VM, then confirm that you can connect into the server by using the ssh client application.
+
In this section, you will learn how to configure an SSH server and restart the ssh service for an existing VM. You will also learn how to configure, restart, and use SSH in order to create secure connections between your Linux machines (host as well as VMs).
  
  
=== Part 1: Enabling the sshd service. ===
+
=== Part 1: Confirming sshd service is Running on VMs. ===
  
 
:'''Perform the following steps:'''
 
:'''Perform the following steps:'''
  
# Launch ALL of your VMs.
+
# Launch your '''c7host machine''' and your '''centos1''' and '''centos3''' VMs.
 
# Switch to your '''c7host''' VM.
 
# Switch to your '''c7host''' VM.
 +
# Create a file in your current directory of your c7host machine with some text in it called: '''myfile.txt'''
 +
# Ensure you've successfully connected to the VPN required for Matrix (https://inside.senecacollege.ca/its/services/vpn/studentvpn.html). Then issue the following command (using your Matrix login id):<br><b><code><span style="color:#3366CC;font-size:1.2em;">scp &nbsp; myfile.txt &nbsp; yourmatrixid@matrix.senecac.on.ca:/home/yourmatrixid</span></code></b><br>(followed by your Matrix password)<br>What did this command do?
 +
# Issue the following single command (arguments are separated by a space - use your Matrix login id):<br><b><code><span style="color:#3366CC;font-size:1.2em;">ssh &nbsp; yourmatrixid@matrix.senecac.on.ca &nbsp; ls /home/yourmatrixid/myfile.txt</span></code></b><br>(followed by your Matrix password)<br>What did this command do?<br>Issue the following Linux command:<br><b><code><span style="color:#3366CC;font-size:1.2em;">ssh &nbsp; yourmatrixid@matrix.senecac.on.ca &nbsp; cat /home/yourmatrixid/myfile.txt</span></code></b><br>How do these commands differ from using issuing the ssh command without the ls or cat command? How is this useful?<br><br>The client ssh application contains the utlities: '''ssh''', '''scp''' and '''sftp''' (learned in ULI101) to connect to remote Linux servers in order to issue commands or transfer files between Linux servers. You can install the SSH service on your Linux server, although this has already been performed upon installation. We will now confirm that the ssh service is running on all of your VMs.<br><br>
 
# OpenSSH should have been installed by default. Let's confirm this by issuing the command:<br /> <b><code><span style="color:#3366CC;font-size:1.2em;">rpm -qa | grep ssh</span></code></b>
 
# OpenSSH should have been installed by default. Let's confirm this by issuing the command:<br /> <b><code><span style="color:#3366CC;font-size:1.2em;">rpm -qa | grep ssh</span></code></b>
 
# You should see a number of packages installed including <b>openssh-clients</b> and <b>openssh-server</b>  
 
# You should see a number of packages installed including <b>openssh-clients</b> and <b>openssh-server</b>  
# <b><code><span style="color:#3366CC;font-size:1.2em;">openssh-server</span></code></b> installs a service called '''sshd'''. Confirm that this service is running by issuing the command:<br /><b><code><span style="color:#3366CC;font-size:1.2em;">systemctl status sshd</span></code></b>
+
# The '''openssh-server''' package installs a service called '''sshd'''.
<ol><li value="6">Now that you know the service is running, investigate what '''port number''' and '''protocol''' sshd uses by issuing the command:<br /><b><code><span style="color:#3366CC;font-size:1.2em;">netstat -atunp | grep sshd</span></code></b></li>
+
# Login as '''root''' and confirm that this service is running by issuing the command:<br /><b><code><span style="color:#3366CC;font-size:1.2em;">systemctl status sshd</span></code></b>
<li>What protocol and port is the sshd process using? What is the state of the port? Why would you think that UDP ports don't have a state?</li>
+
<ol><li value="10">Now that you know the service is running, investigate what '''port number''' and '''protocol''' sshd uses by issuing the command:<br /><b><code><span style="color:#3366CC;font-size:1.2em;">netstat -atunp | more</span></code></b><br>What protocol and port is the sshd process using? What is the state of the port? Why would you think that UDP ports don't have a state?</li>
 
<li>Reissue the <b><code><span style="color:#3366CC;font-size:1.2em;">netstat</span></code></b> command without the '''-n''' option. What is the difference?</li>
 
<li>Reissue the <b><code><span style="color:#3366CC;font-size:1.2em;">netstat</span></code></b> command without the '''-n''' option. What is the difference?</li>
 +
<li>You can refer to the '''/etc/services''' file in order to determine a port number for a service. Issue the following command to confirm that port 22 is associated with ssh:<br><b><code><span style="color:#3366CC;font-size:1.2em;">grep ssh /etc/services</span></code></b>
 
<li>Make sure the '''sshd''' service is running on '''all 3 of your VM's'''</li>
 
<li>Make sure the '''sshd''' service is running on '''all 3 of your VM's'''</li>
<li>Remember that you can view the '''/etc/services''' file in case you need to reference a service with a corresponding port number.</li>
 
 
</ol>
 
</ol>
  
 +
===Part 2: SSH Server Security Configuration===
 +
 +
Any time that you configure your computer to allow logins from the network you are leaving yourself '''vulnerable to potential unauthorized access''' by penetration testers or even hackers. Running the sshd service is a fairly common practice but care must be taken to make things more difficult for those individuals that attempt to use '''brute force attacks''' to gain access to your system. Hackers use their knowledge of your system and can use '''password guessing programs''' help to gain access. They know which port is likely open to attack (TCP:22), the administrative account name (root).
 +
 +
The Linux system administrator can '''configure the SSH server''' in order to make the SSH server less vulnerable to attacks. Examples include not permitting root login, and changing the default port number for the ssh service.
 +
 +
:'''Perform the following steps:'''
 +
 +
# For this section, you will still be using your '''c7host''' and '''centos1''' VMs.<br><br>The next change you can make is to prevent the root account from logging in to sshd altogether.<br><br>
 +
# Change to your '''centos1''' VM and open a terminal.
 +
# Edit the file '''/etc/ssh/sshd_config''' and look for the option <b><code><span style="color:#3366CC;font-size:1.2em;">PermitRootLogin</span></code></b>. <u>'''<br>Un-comment the option'''</u> (or add the option if it does not appear) and change the option value to <b><code><span style="color:#3366CC;font-size:1.2em;">no</span></code></b>.<br><br>'''NOTE:''' Now any hacking attempt also has to guess an account name as well as the password.<br>If you need to ssh with root access, ssh as a regular user and use '''su -''' to become root.<br><br>
 +
# Even better, it is possible to restrict access to just specific users that require it:<br>Edit the file '''/etc/ssh/sshd_config''' and '''add''' a new option of <b><code><span style="color:#3366CC;font-size:1.2em;">AllowUsers yourAccountName</span></code></b> (where "yourAccountName" is your regular user accountname for your centos1 VM)<br>
 +
# In order for these changes to take affect, you need to restart the sshd daemon. Issue the following command to restart the '''sshd''' service:<br /><b><code><span style="color:#3366CC;font-size:1.2em;">systemctl restart sshd</span></code></b>
 +
# Try SSHing from your '''c7host''' VM to your '''centos1''' VM as '''root'''. Where you successful?
 +
# Try SSHing from your c7host VM to your centos1 VM as your regular user accountname. Did it work?
 +
# Create another regular user called: '''other'''
 +
# Set the password for the newly-created called '''other'''
 +
# Try SSHing from your c7host VM to your centos1 VM for the account called '''other'''. Why didn't it work?
 +
# Edit the file '''/etc/ssh/sshd_config''' to add the account '''other''' for the '''AllowUsers''' option (use a space to separate usernames instead of a comma).
 +
# Restart the ssh service.
 +
# Try SSHing from your c7host VM to your centos1 VM for the account called '''other'''. Did it work this time?
 +
# Issue the following command to make a backup copy of your sshd_config file to your original regular user's home directory:<br><b><code><span style="color:#3366CC;font-size:1.2em;">cp /etc/ssh/sshd_config /home/regularuserid/sshd_config.bk</span></code></b>
 +
# Issue the following command to allow same group and other group members to view the file contents:<br><b><code><span style="color:#3366CC;font-size:1.2em;">chmod og+r /home/regularuserid/sshd_config.bk</span></code></b>
 +
<ol><li value="16">Finally, as a system administrator, you should periodically monitor your system logs for unauthorized login attempts.</li>
 +
<li>On CentOS systems the log file that is used is '''/var/log/secure''' </li>
 +
<li>It also logs all uses of the '''su''' and '''sudo''' commands.</li>
 +
<li>Attempt to connect to all of your VM's as root and other users using both public key and password authentication. Use some '''su''' and '''sudo''' commands also. </li>
 +
<li>Inspect the log to see what kind of information is logged.</li>
 +
</ol>
 +
 +
 +
 +
'''Answer INVESTIGATION 1 observations / questions in your lab log book.'''
 +
 +
=INVESTIGATION 2: ADDITIONAL METHODS TO SECURE YOUR SSH SERVER =
  
 
{|width="40%" align="right" cellpadding="10"
 
{|width="40%" align="right" cellpadding="10"
Line 87: Line 131:
 
|- valign="top"
 
|- valign="top"
  
|{{Admon/note |Storing Fingerprints | When a user connects to a host using ssh, the host sends a fingerprint or digital signature to the client to establish its identity. The first time a connection is established the identity must be stored for subsequent connections. The fingerprints are stored separately for each user in a file called <code>~/.ssh/known_hosts</code> . <br /><br />From now on when you connect to that host the client will compare the received fingerprint against the list of known hosts before connecting. If the fingerprint does not match it could indicate somebody had setup a system to impersonate the computer you wish to connect to and you would receive a message like this}}
+
|{{Admon/note |Storing Fingerprints | When a user connects to a host using ssh, the host sends a fingerprint or digital signature to the client to establish its identity. The first time a connection is established the identity must be stored for subsequent connections. The fingerprints are stored separately for each user in a file called <code>~/.ssh/known_hosts</code> . <br /><br />From now on when you connect to that host the client will compare the received fingerprint against the list of known hosts before connecting. If the fingerprint does not match it could indicate somebody had setup a system to impersonate the computer you wish to connect.}}
  
 
|}
 
|}
=== Part 2: Establishing a Safe SSH Connection: Public Key Authentication ===
+
=== Part 1: Generating Private and Public Keys (Public Key Infrastructure) ===
  
As a system administrator, you have the ability to generate or create '''public''' and '''private''' keys to ensure safe and secure ssh connections. The system administer can generate these keys for the first time, of if they suspect that a hacker has compromised the server, can remove the existing keys and generate new keys. A common type of attack, Arp Poisoning (Man in the Middle Attack), can be used to redirect packets to a third party while maintaining the illusion that the connection is secure. Therefore, understanding about the generation and management of public/private keys are important to the security of servers.
+
As a system administrator, you have the ability to generate or create '''public''' and '''private''' keys to ensure safe and secure ssh connections. This will require a user to prove who they say they are in order to access a Linux server via SSH (i.e. '''authentication'''). The system administer can generate these keys for the first time, or if the system administrator suspects that a hacker has compromised or trying to penetrate the server, they can remove the existing keys and generate new keys.
  
 +
A common type of attack, '''Arp Poisoning (Man in the Middle Attack)''', can be used to <u>redirect</u> packets to a third party while maintaining the illusion that the connection is secure. Therefore, understanding about the generation and management of public/private keys are important to the security of servers.
  
 
:'''Perform the following steps:'''
 
:'''Perform the following steps:'''
Line 99: Line 144:
 
# Switch to your '''centos3''' VM.<br><br>
 
# Switch to your '''centos3''' VM.<br><br>
  
:We can use the '''netstat''' utility as a trouble-shooting / confirmation tool to view the SSH service and determine which STATE the SSH service is performing: '''LISTENING''', '''ESTABLISHED''', '''CLOSED''' , or '''WAITING''' <br><br>
+
:We can use the '''netstat''' utility as a trouble-shooting tool to view the SSH service and determine which STATE the SSH service is performing:<br> '''LISTENING''', '''ESTABLISHED''', '''CLOSED''' , or '''WAITING''' <br><br>
 
<ol><li value="2">Run the '''netstat -atunp''' command (pipe to "grep sshd") to check the state of a possible ''ssh connection''. What is the state (i.e. LISTENING or ESTABLISHED)?</li>
 
<ol><li value="2">Run the '''netstat -atunp''' command (pipe to "grep sshd") to check the state of a possible ''ssh connection''. What is the state (i.e. LISTENING or ESTABLISHED)?</li>
<li>Open another terminal and establish an ssh connection to your '''centos3''' VM using the command: <b><code><span style="color:#3366CC;font-size:1.2em;">ssh ops235@centos3</span></code></b></li>
+
<li>While in your '''centos3''' VM, issue the following command to connect to '''your same VM''' via ssh: <b><code><span style="color:#3366CC;font-size:1.2em;">ssh ops235@centos3</span></code></b></li>
<li>You should receive a message similar to the following:<br><br>
+
<li>Enter yes at the prompt, and enter your OPS235 password.<br>The output should appear similar as what is shown below:<br><br>
 
:<span style="font-family:courier">The authenticity of host 'centos3 (192.168.235.13)' can't be established.</span><br>
 
:<span style="font-family:courier">The authenticity of host 'centos3 (192.168.235.13)' can't be established.</span><br>
 
:<span style="font-family:courier">RSA key fingerprint is 53:b4:ad:c8:51:17:99:4b:c9:08:ac:c1:b6:05:71:9b.</span><br>
 
:<span style="font-family:courier">RSA key fingerprint is 53:b4:ad:c8:51:17:99:4b:c9:08:ac:c1:b6:05:71:9b.</span><br>
 
:<span style="font-family:courier">Are you sure you want to continue connecting (yes/no)? yes</span><br>
 
:<span style="font-family:courier">Are you sure you want to continue connecting (yes/no)? yes</span><br>
 
:<span style="font-family:courier">Warning: Permanently added 'centos3' (RSA) to the list of known hosts.</span><br><br></li>
 
:<span style="font-family:courier">Warning: Permanently added 'centos3' (RSA) to the list of known hosts.</span><br><br></li>
<li>Answer '''yes''' to add to the list of known hosts.</li>
+
 
<li>Issue the following command to confirm that you connected to your centos3 VM: <b><code><span style="color:#3366CC;font-size:1.2em;">hostname</span></code></b></li></ol>
+
<li>Issue the following command to confirm that you connected to your centos3 VM: <b><code><span style="color:#3366CC;font-size:1.2em;">hostname</span></code></b></li><li>Re-run that same '''netstat pipeline command'''. Any change to the connection status?</li>
[[Image:spoof.png|thumb|right|485px|If you receive a message like the one displayed above, you should investigate why it is happening as it could indicate a '''serious security issue''', or it could just mean that something on '''the host has changed'''(i.e. the OS was <u>reinstalled</u>)]]
+
<li>Log-out of your ssh connection by typing <b><code><span style="color:#3366CC;font-size:1.2em;">exit</span></code></b>.
<ol><li value="7">Switch back to your '''centos3''' VM and re-run that same '''netstat pipeline command'''. Any change to the connection status?</li><li>Return to the second terminal, and log-out of your ssh connection by typing <b><code><span style="color:#3366CC;font-size:1.2em;">exit</span></code></b>.
+
<li>Run that same '''netstat''' command again. Wait a few minutes and then check again. Record your observations.</li>
<li>Run that same '''netstat''' command in the original terminal and check the state of the connection after logging out. Wait a few minutes and then check again. Record your observations.</li>
 
<li>Make certain to exit all connections, and remain in your '''centos2''' VM. When using ssh to connect to other servers, it is very easy to forget which server you are currently using. Verify that you are in your '''centos2''' VM by entering the command: <b><code><span style="color:#3366CC;font-size:1.2em;">hostname</span></code></b>
 
<li>Use the Internet to search for '''TCP 3 way handshake''' to see how TCP connections are established and closed.<br><br></li>
 
 
</ol>
 
</ol>
 +
 +
 +
[[Image:spoof.png|thumb|right|485px|If you ever receive a message like the one displayed above, you should investigate why it is happening as it could indicate a '''serious security issue''', or it could just mean that something on '''the host has changed'''(i.e. the OS was <u>reinstalled</u>)]]
 
So far, we have learned to establish an ssh connection to another host using a password to establish your identity. But '''passwords are not the only or even the best way of authenticating your identit'''y. We can also use '''Public/Private key encryption'''.  
 
So far, we have learned to establish an ssh connection to another host using a password to establish your identity. But '''passwords are not the only or even the best way of authenticating your identit'''y. We can also use '''Public/Private key encryption'''.  
  
Line 123: Line 168:
  
 
<ol>
 
<ol>
<li value="12"> Make <u>certain</u> that you are NOT logged in as root!''' (you have been warned!)</li>
+
<li value="10">Switch to your '''centos2''' VM.</li>
 +
<li>Confirm you are in your centos2 VM by entering the command: <b><code><span style="color:#3366CC;font-size:1.2em;">hostname</span></code></b></li>
 +
<li>Make <u>certain</u> that you are in your centos2 VM and that you are logged in as a '''regular user''' (i.e. NOT root!) (you have been warned!)</li>
 
<li>To generate a keypair (public/private keys), issue the following command: <b><code><span style="color:#3366CC;font-size:1.2em;">ssh-keygen</span></code></b></li>
 
<li>To generate a keypair (public/private keys), issue the following command: <b><code><span style="color:#3366CC;font-size:1.2em;">ssh-keygen</span></code></b></li>
<li>That should generate output similar to the following:<br><br></li>
+
<li value="14"> After generating the keys it prompts you for the location to save the keys. The default is '''~/.ssh''' Your private key will be saved as <b>id_rsa</b> and your public key will be saved as '''id_rsa.pub'''. Press ENTER to accept the default.</li>
</ol>
+
<li>You will then be prompted for a '''pass-phrase'''. The pass-phrase must be entered in order to use your private key. Pass-phrases are more secure than passwords and should be lengthy, hard to guess and easy to remember. For example one pass-phrase that meets this criteria might be ''"seneca students like to dance at 4:00am"''. Avoid famous phrases such as ''"to be or not to be"'' as they are easy to guess. It is possible to leave the pass-phrase blank but this is dangerous. It means that if a hacker were able to get into your account they could then use your private key to access other systems you use.<br><br>The output should appear similar as what is shown below:</li></ol>
 +
 
 +
<pre style="font-family:monospace;background-color:white;border-style:none;padding-left:50px;">
 +
Generating public/private rsa key pair.
 +
Enter file in which to save the key (/home/ops235/.ssh/id_rsa):
 +
Enter passphrase (empty for no passphrase):
 +
Enter passphrase again:
 +
Your public key has been saved in /home/ops235/.ssh/id_rsa.pub.
 +
The key fingerprint is:
 +
ef:de:31:67:f7:15:a4:43:39:15:5d:78:1b:e8:97:74 ops235@centos3
 +
The key's randomart image is:
 +
+--[ RSA 2048]----+
 +
|              .+=|
 +
|            .+oE|
 +
|            .+.o=|
 +
|            ..++ |
 +
|        S    o.. |
 +
|        .    . .|
 +
|          . o o o|
 +
|        . . = .o|
 +
|        .o .  .|
 +
+-----------------+
 +
</pre>
  
::<span style="font-family:courier">Generating public/private rsa key pair.</span><br>
+
<ol><li value=16>Now issue the command <b><code><span style="color:#3366CC;font-size:1.2em;">ssh-copy-id -i ~/.ssh/id_rsa.pub ops235@centos3</span></code></b></li>
::<span style="font-family:courier">Enter file in which to save the key (/home/user1/.ssh/id_rsa):</span> <br>
+
<li>When prompted for password, enter OPS235's root password</li>
::<span style="font-family:courier">Enter passphrase (empty for no passphrase): </span><br>
+
  <li>Try using ssh to now log into your '''centos3''' VM <u>from</u> your '''centos2''' VM. What happens? Were you required to use your pass-phrase?</li>
::<span style="font-family:courier">Enter same passphrase again: </span><br>
 
::<span style="font-family:courier">Your identification has been saved in /home/user1/.ssh/id_rsa.</span><br>
 
::<span style="font-family:courier">Your public key has been saved in /home/user1/.ssh/id_rsa.pub.</span><br>
 
::<span style="font-family:courier">The key fingerprint is:</span><br>
 
::<span style="font-family:courier">93:58:20:56:72:d7:bd:14:86:9f:42:aa:82:3d:f8:e5 user1@centos2</span><br><br>
 
<ol><li value="16"> After generating the keys it prompts you for the location to save the keys. The default is '''~/.ssh''' Your private key will be saved as <b>id_rsa</b> and your public key will be saved as '''id_rsa.pub'''</li>
 
<li>You will then be prompted for a pass-phrase. The pass-phrase must be entered in order to use your private key. Pass-phrases are more secure than passwords and should be lengthy, hard to guess and easy to remember. For example one pass-phrase that meets this criteria might be "seneca students like fish at 4:00am". Avoid famous phrases such as "to be or not to be" as they are easy to guess. It is possible to leave the pass-phrase blank but this is dangerous. It means that if a hacker were able to get into your account they could then use your private key to access other systems you use.<br><br></li>
 
<li>Now issue the command <b><code><span style="color:#3366CC;font-size:1.2em;">ssh-copy-id -i ~/.ssh/id_rsa.pub ops235@centos3</span></code></b></li>
 
  <li>Try using ssh to now log into your '''centos3''' VM <u>from</u> your '''centos2''' VM. What happens? Were you required to use your pass-phrase?<li>
 
 
<li> Issue the '''hostname''' command to verify that you are successfully logged into your '''centos3''' VM.</li>
 
<li> Issue the '''hostname''' command to verify that you are successfully logged into your '''centos3''' VM.</li>
 
<li>Make certain to logout of your '''centos3''' system. Use the '''hostname''' command to verify you are back in your centos2 server.</li>
 
<li>Make certain to logout of your '''centos3''' system. Use the '''hostname''' command to verify you are back in your centos2 server.</li>
 
</ol>
 
</ol>
  
 
'''Answer INVESTIGATION 1 observations / questions in your lab log book.'''
 
 
 
=INVESTIGATION 2: USING SSH AND OTHER SECURE SHELL UTILITIES=
 
 
=== Part 1: How do you use scp and sftp. ===
 
 
When you have created an SSH server, then users can take advantage of secure shell tools (including the '''scp''' and '''sftp''' utilities)
 
 
The ssh client utility also contains many options to provide useful features or options when establishing secure connections between servers. One of these features is referred to as '''tunnelling''' - this term refers to running programs on remote servers (i.e. running the program on a remote server, yet interacting and viewing program on your local server. Since '''X-windows''' in Linux is a support <u>layer</u> to transmit graphical information efficiently between servers, ssh tunnelling becomes more useful and important to allow organizations to work efficiently and securely in a user-friendly environment.
 
 
:'''Perform the following steps:'''
 
 
# Switch to your '''centos1''' VM.
 
# To connect to a remote host type the command:<br /><b><code><span style="color:#3366CC;font-size:1.2em;">sftp ops235@centos3</span></code></b>
 
# This will establish an interactive session after authentication.
 
# Type <b><code><span style="color:#3366CC;font-size:1.2em;">help</span></code></b> to see the list of sftp commands at any time.
 
# The 2 main commands are <b><code><span style="color:#3366CC;font-size:1.2em;">put</span></code></b> to copy a file from the local host to the remote host (upload) and <b><code><span style="color:#3366CC;font-size:1.2em;">get</span></code></b> to copy a file from the remote host to the local host (download).
 
# Try using '''sftp''' to transfer files back and forth between hosts.
 
# Confirm that the files were transfer and then enter the command '''exit''' to quit your sftp session.
 
<ol>
 
<li value="8">You can also use the '''scp''' command to copy files to and from remote hosts and even from one remote host to another.</li>
 
<li>Use '''scp''' to copy your services file to the centos3 host into the /tmp directory. (The path on a remote host follows the ''':''') using the command: <br /><b><code><span style="color:#3366CC;font-size:1.2em;">scp /etc/services  ops235@centos3:/tmp</span></code></b></li>
 
<li>Here is a neat trick: You can run commands remotely using ssh by typing the command as an argument after the ssh command. Issue the following command in your '''centos2''' VM:<br><b><code><span style="color:#3366CC;font-size:1.2em;">ssh ops235@centos3 ls /tmp</span></code></b></li>
 
<li>What happened when you issued that command? Where you able to successfully using scp to copy the '''/etc/services''' file to '''centos3's /tmp''' directory?</li>
 
<li>Experiment with '''scp''' to copy a file from '''centos3''' directly to '''centos1'''.</li>
 
</ol>
 
  
  
Line 180: Line 212:
 
|}
 
|}
  
=== Part 2: How do you use ssh to tunnel X. ===
+
=== Part 2: Securely Running Graphical Applications Between Linux Servers===
  
 
You can also use ssh to '''tunnel window and bitmap information''', allowing us to login to a remote desktop host and '''run a Xwindows application''' such as <b>gedit</b> or <b>firefox</b> and the application will run on the remote host but be displayed on the local host.
 
You can also use ssh to '''tunnel window and bitmap information''', allowing us to login to a remote desktop host and '''run a Xwindows application''' such as <b>gedit</b> or <b>firefox</b> and the application will run on the remote host but be displayed on the local host.
Line 187: Line 219:
  
 
# For this section, you will be using your '''c7host''' and '''centos1''' VMs.
 
# For this section, you will be using your '''c7host''' and '''centos1''' VMs.
# From your '''c7host''' VM issue the ssh command to connect to your '''centos1''' VM   using the following command:<br /><b><code><span style="color:#3366CC;font-size:1.2em;">ssh -X -C username@centos1</span></code></b> &nbsp; (where 'username' is your learn account on centos1)<br>(The '''-X''' option enables the forwarding of X window information, and the '''-C''' option enables compression for better performance).<br><br>
+
# Switch to your c7host VM, open a terminal and remain logged in as a regular user.
# Once the connection is properly established, run the '''gedit''' application. (Gnome Text Editor)
+
# Issue the following command to connect to your '''centos1''' VM:<br /><b><code><span style="color:#3366CC;font-size:1.2em;">ssh -X -C yourUserID@centos1</span></code></b> &nbsp; (where 'yourUserID' is your user account name on centos1)<br>(The '''-X''' option enables the forwarding of X window information, and the '''-C''' option enables compression for better performance).<br><br>
 +
# Once the connection is properly established, run the command  <b><code><span style="color:#3366CC;font-size:1.2em;">gedit</span></code></b>
 
# The ''gedit'' window will display on your '''c7host''' VM, but in reality, this application is running on your '''centos1''' VM!
 
# The ''gedit'' window will display on your '''c7host''' VM, but in reality, this application is running on your '''centos1''' VM!
# Enter some text and save a file with '''gedit'''.  
+
# Enter some text and save your editing session.  
 
# Exit the '''gedit''' application.
 
# Exit the '''gedit''' application.
# In which VM was the file saved?
+
# In which VM was the file saved? What does that tell you about the use of tunneling for this section?
# Experiment with running other GUI applications through '''ssh'''.
+
# Run the graphical program remotely by issuing only one Linux command:<br> <b><code><span style="color:#3366CC;font-size:1.2em;">ssh -X -C yourUserID@centos1 &nbsp; gedit</span></code></b>  (Note: ignore warning messages).
 +
# Exit the gedit application.
 +
# Experiment with running other GUI applications (in the /bin directory with applications starting with the letter "x" via '''ssh''' (for example: xev or xchat).
 +
 
 +
 
 +
'''Answer INVESTIGATION 2 observations / questions in your lab log book.'''
  
 +
=INVESTIGATION 3: MANAGING FIREWALLS FOR PROTECTION &amp; TROUBLESHOOTING =
  
{{Admon/important | centos1 VM iptables and ssh service | You may need to adjust the firewall on your '''centos1''' host to complete this section, and verify that the '''sshd''' service is running on that VM.}}
 
  
'''Answer INVESTIGATION 2 observations / questions in your lab log book.'''
+
{| width="40%" align="right" cellpadding="10"
 +
|- valign="top"
 +
|
 +
[[Image:chains.png|thumb|600px|right|When using iptables packets must pass-through "a chain of policy rules" in order to handle packets. If a packet matches a rule, then an action is taken (some examples include: '''ACCEPT''', '''DROP''', '''REJECT''', or '''LOG'''); otherwise, the packet will be directed to the default policy chain.  ]]
 +
|}
 +
==== Linux Firewall (iptables) Concepts====
 +
 
 +
Since Linux servers can be connected to the Internet, it is very important to run a '''firewall''' to control what packets might come into the computer system, what packets might go out of the computer system, and what packets might be forwarded to another computer. We are currently using the utility called '''iptables''' can be used to set the firewall rules on a Linux server.
  
  
=INVESTIGATION 3: SECURING THE SSH CONNECTION=
+
Basically, there is a list ('''chain''') of policy rules that <u>'''packets'''</u> must pass-through in order to handle packets. If a packet matches a rule, then an action is taken (some examples include: '''ACCEPT''', '''DROP''', '''REJECT''', or '''LOG'''). If the packet passes through the chain of rules without a match, then the packet is directed to the default policy chain (for example: ''ACCEPT'', ''REJECT'', or ''DROP'').
  
  
[[Image:tunnel-trick.png|thumb|right|550px|You can also use an ssh connection to '''tunnel other types of traffic'''. There could be different reasons for doing this. For example tunneling traffic for an unencrypted application/protocol through ssh can '''increase the security of that application''' (i.e. deceive potential hackers).<br><br>Alternatively you could use it to '''circumvent a firewall that is blocking traffic''' you wish to use but allows ssh traffic to pass through.]]
+
You can create your own '''customized chains''' (which you will learn in the OPS335 course) but to keep thing simple, we only deal with 3 '''common predefined chains''':
=== Part 1: How do you use ssh to tunnel other traffic. ===
 
  
To help harden (protect a server from attack or "penetration"), system or security administrators have the ability to "trick" or "mislead" a potential hacker in order to prevent system penetration.
+
:*'''INPUT''': Packets coming into current Linux server
 +
:*'''OUTPUT''': Packets leaving current Linux server
 +
:*'''FORWARD''': Packets being routed between Linux servers
  
In this part, you will learn to use a combination of SSH server configuration and iptables rules to redirect the SSH port to allow secure data traffic via another port (as opposed to the default port: 22), and use iptables to reject (better log) incoming tcp traffic via the default port.
 
  
What is the result of this?
+
=== Part 1: Listing &amp; Clearing Existing iptables Rules ===
Simple. Permit the SSH service for the organization, and yet trick and confound the potential hacker into thinking that ssh traffic is used on a port that is no longer available (but they may not know this!)
 
  
Sneaky! >;)
+
Let's get some practice using the iptables command such as listing CHAIN rules, and clearing the CHAIN rules:
  
 
:'''Perform the following steps:'''
 
:'''Perform the following steps:'''
  
# For this section, you will still be using your '''c7host''' and '''centos1''' VMs.
+
# For the remainder of this section, use your '''c7host''' machine.
# We will be bypassing a firewall that blocks http traffic.
+
# Issue the following command to list the existing iptables policy rules: <b><code><span style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.2em;">iptables -L</span></code></b>
# In this investigation, '''c7host''' will be your '''http server''' and '''centos1''' will be your <u>client</u>.
+
# Were there already iptables policy rules that already existed by default?
# Use the '''hostname''' command to verify that you are in your centos1 VM (as opposed to another VM by mistake via ssh!)
+
# Before we proceed, we need to understand various methods to list iptables rules:<br><br>'''Listing iptables Rules:'''<br><table width="100%" cellpadding="10" cellspacing="0" border="1"><tr><td width="30%">'''iptables -L'''</td><td>List all iptables rules (eg. INPUT, OUTPUT. FORWARD, and any customized chains (if any)</td></tr><tr><td>'''iptables -L -v'''</td><td>Verbosely List all iptables rules including information such as total size of packets affected by rules</td></tr><tr><td>'''iptables -L CHAIN-NAME'''</td><td>List all iptables rules for that particular chain-name for less clutter (eg. INPUT or OUTPUT, etc)</td></tr></table><br>
# On the HTTP server, make sure that the Apache web server is installed by typing the command:<br /><b><code><span style="color:#3366CC;font-size:1.2em;">rpm -q httpd</span></code></b>
+
# Issue the following Linux command: <b><code><span style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.2em;">iptables -L INPUT</span></code></b><br>What do you notice is different with this command compared to the previous iptables command?
# If this is not installed, make sure to install '''httpd'''.
+
# Issue the iptables command separately to display the rules for the '''OUTPUT''' chain and for the '''FORWARD''' chain.
# When ''httpd'' is properly installed, check the configuration of the service to see if it is automatically started at any run-levels by using the command:<br><b><code><span style="color:#3366CC;font-size:1.2em;">systemctl list-unit-files | grep httpd</span></code></b>
+
# Issue the following command: <b><code><span style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.2em;">iptables -L -v </span></code></b><br>What do you notice about this command as opposed to the first iptables command you issued?<br>What sort of additional information does this command provide regarding affected packets?<br><br>
# If it has not been started automatically start the service using the '''service''' command to start the ''httpd'' service.
+
# Sometimes it may be useful to completely clear the rules for all or a particular chain. Note the options that can be used to clear (or flush) the iptables rules,<br><br>'''Clearing (Flushing) iptables Rules:'''<br><table width="100%" cellpadding="10" cellspacing="0" border="1"><tr><td width="30%">'''iptables -F'''</td><td>Clears the rules for ALL of the chains</td></tr><tr><td>'''iptables -F CHAIN-NAME'''</td><td>Clears the rules for only the specified CHAIN-NAME (eg. INPUT or OUTPUT)</td></tr></table><br>
# It is also a good idea to enable the httpd service so it starts automatically from boot-up. To do this, issue the command:<br><b><code><span style="color:#3366CC;font-size:1.2em;">chkconfig httpd on</span></code></b>
+
# Issue the following command to reset the iptables rules for the INPUT chain: <b><code><span style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.2em;">iptables -F INPUT</span></code></b>
# Issue a previous command to verify that the httpd service is enabled.
+
# Issue the '''iptables -L INPUT''' command to verify that the iptables rules for the INPUT chain have been cleared.
# Confirm that httpd is listening to TCP/80 using the '''netstat''' command.
+
# Now, issue the command: <b><code><span style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.2em;">iptables -F</span></code></b><br>and then issue the command: <b><code><span style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.2em;">iptables -L</span></code></b><br>What do you notice?
# Create a small html document called '''/var/www/html/index.html''' that displays a short message. If you do not know how to use HTML markup language, just type a simple text message...
+
 
# <u>Restart</u> your '''c7host''' VM.
+
 
# On your '''c7host''' VM (i.e. the http server), confirm everything is working locally by using a browser to connect to '''http://localhost'''
+
 
# Set the default firewall configuration on centos1 to '''REJECT''' incoming requests to http (TCP/80)
+
=== Part 2: Setting a Default Policy / Setting Policy Exceptions (iptables) ===
# NOTE: '''c7host''': if '''http://localhost''' stops working locally, add the following iptables rule to centos1, as root <br /><b><code><span style="color:#3366CC;font-size:1.2em;">iptables -I INPUT -i lo -j ACCEPT</span></code></b>  
+
 
# On '''centos1''' confirm that the httpd service is stopped so it cannot interfere with your observations.
+
Usually when setting policy rules with iptables, a general "overall" policy is set (default policy chain). A good way to think about setting policies is to have a '''"safety-net"''' to take some sort of action to prevent un-handled packets from passing through the firewall by mistake. After the default policy is set-up, then specific exceptions to the default policy can be added to control specific network traffic.
# On '''centos1''' confirm that you can't connect by using firefox to centos1 '''http://centos1/'''
+
 
# The next step is to establish a <u>tunnel</u>. When you establish a tunnel you make an ssh connection to a remote host and open a new port on the local host. That local host port is then connected to a port on the remote host through the established tunnel. When you send requests to the local port it is forwarded through the tunnel to the remote port.
+
An example would be to set a default policy for incoming network traffic (INPUT chain) to DROP everything, and then set an exception certain exceptions (like ssh connections). Note the following table below for policy setting examples.<br><br>
# In a terminal in your '''centos2''' VM, '''make certain you are NOT logged in as root!'''
 
# Establish a tunnel using a local port on centos2 of 20808, that connects to the remote port on '''c7host''' of 80, using the following command on '''centos1''':<br /><b><code><span style="color:#3366CC;font-size:1.2em;">ssh -L 20808:centos1:80  username@centos1</span></code></b><br><br> '''Note:'''<br>The '''-L''' option (which means Local port) takes one argument:<br><span style="courier">&lt;local-port&gt;:&lt;connect-to-host&gt;:&lt;connect-to-port&gt;</span><br><br> The command basically connects your local port of 20808 to the remote port of 80 on '''c7host'''.<br>This means all requests to 20808 on the <u>localhost</u> ('''centos1''') are actually tunneled through your ssh connection<br>to port 22 on '''c7host''' and then delivered to port 80 on '''c7host''', bypassing the firewall.<br><br>
 
# Once the tunnel is established use '''netstat''' to verify the port 20808 is listening on '''centos1'''
 
# Now using the browser on '''centos1''' connect to '''http://localhost:20808'''
 
#You should see the '''index.html''' page on '''c7host'''.
 
# Close the ssh connection and verify that the port 20808 is no longer listening.
 
  
 +
:'''Policy Setting Examples:'''<br><table width="100%" cellpadding="10" cellspacing="0" border="1"><tr><td width="30%">'''iptables -P INPUT DROP'''</td><td>Drops all incoming packets regardless of protocol (eg. tcp, udp, icmp), port numbers (eg. 22, 80) or source or destination IP Addresses. Setting a default rule to DROP all incoming traffic would make it easier to specify a few exceptions.</td></tr><tr><td>'''iptables -P INPUT ACCEPT'''</td><td>Accepts all incoming packets regardless of protocol (eg. tcp, udp, icmp), port numbers (eg. 22, 80) or source or destination IP Addresses. It would seem that setting a default rule to ACCEPT all incoming traffic would require A LOT of exceptions to help "lock-down" the server for protection! It really depends on the server set-up and what the Linux system administrator wants to accomplish.</td></tr></table><br><br>
  
{|width="40%" align="right" cellpadding="10"
+
:'''Perform the following steps:'''
|- valign="top"
 
  
|{{Admon/tip |Troubleshooting Tips for SSH|Cannot connect via SSH? To fix issues with the ability to ssh, on both machines:<ul><li>Ensure ssh is '''running'''.</li><li>Disable '''SELinux'''</li><li>'''Flush iptables''' (iptables -F)</li><li>For '''scp''', use the access the option (eg.  '''scp -P 2200''' )</li></ul> }}
+
# Make certain you are in your '''c7host''' machine.
 +
# Issue the following Linux command: <b><code><span  style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.2em;">iptables -P INPUT DROP</span></code></b>
 +
# Issue the '''iptables -L''' command. Can you see the policy to DROP all incoming connections?
 +
# Although you have set a default policy to DROP all incoming connections, there is a problem: now, you cannot browse the Internet. You can confirm that by opening a SEPARATE web-browser and perform a Net-search.<br><br>In order to fix that problem, you can make an exception to allow incoming web-based traffic (via port 80). Those iptables commands to create exceptions are more complex since you need to determine: <ul><li>'''Where each rules appears in the chain'''? (order can be important)</li><li>'''Which protocol(s)''' are affected (eg. tcp, udp, icmp)</li><li>'''What source or destination IP Addresses''' are affected?</li><li>'''What port numbers''' are affected?</li><li>'''What action to take''' if all of the above conditions are met? (eg. ACCEPT, REJECT, DROP, or LOG)</li></ul><br>'''iptables Command Structure (for setting exceptions):<br>(NOTE: If element in column is not specified in the iptables command, then rule relates to ALL elements)'''<table width="100%" cellpadding="10" cellspacing="0" border="1"><tr valign="top><td>Place Rule in Chain</td><td>Chain Name</td><td>Specify Protocol</td><td>Source/Destination IPADDR</td><td>Port Number</td><td>Action<br> -&gt;</td><td>Target</td></tr><tr valign="top"><td>'''-A''' (add / Append to bottom of chain)<br>'''-I''' (insert at top of chain)<br>'''-I CHAIN-NAME 5''' (insert before line 5) </td><td>'''INPUT'''<br>'''OUTPUT'''<br>'''FORWARD'''<br>'''CHAIN-NAME'''</td><td>'''-p tcp''' (tcp packets)<br>'''-p udp''' (datagram packets)<br>'''-p tcp,udp,icmp''' (combined)<br><br>(refer to '''/etc/protocols''' )</td><td>'''-s IPADDR''' (originating IPADDR)<br>'''-d IPADDR''' (destination IPADDR)</td><td>'''<span style="font-family:courier">--</span>sport 22''' (originating port 22 - SSH)<br>'''<span style="font-family:courier">--</span>dport 80''' (destined port 80 - http)<br><br>(refer to '''/etc/services''')</td><td>'''-j'''  </td><td>'''ACCEPT'''<br>'''REJECT'''<br>'''DROP'''<br>'''LOG'''</td></tr></table><br>
 +
# Issue the following Linux commands to ensure the loopback interface is not affected by these rules. The computer should be able to communicate with itself with any state and protocol:<br><b><code><span  style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.2em;">iptables -A INPUT -i lo -p all -j ACCEPT</span></code><br><code><span  style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.2em;">iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT</span></code></b>
 +
# Issue the following Linux command to ADD an exception to the INPUT chain to allow web-based incoming traffic (ie. port 80):<br><b><code><span style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.2em;">iptables -A INPUT -p tcp --dport 80 -j ACCEPT</span></code></b>
 +
# Issue an iptables command to confirm that their is an exception rule to handle incoming tcp packets over port 80.
 +
# Use your other web-browser to confirm that you can now browse the Internet. If you cannot, contact your lab assistant or professor for help.
 +
# Determine the '''external facing address''' of your c7host machine.<br>('''Tip:''' in a web-browser, enter the term: '''"ip address"'''. The external facing IP Address should start with '''"10."''').
 +
# Provide your external facing address, and provide another lab-mate to ping that external facing address. Were they successful?
 +
# Have your lab-mate determine THEIR external facing address and obtain that IP Address.
 +
# Issue the following iptables command to allow an exception for pings from your lab-mate:<br><b><code><span  style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.2em;">iptables -A INPUT -p icmp -s {neighbour's external facing address} -j ACCEPT</span></code></b>
 +
# Have your neighbour repeat pinging your external facing IP Address. What happened? Why?
 +
# Have your neighbour try to SSH into YOUR c7host. Were they Successful?
 +
# Issue an iptables rule (in a similar way as with the previous iptables command) to allow an exception for incoming ssh traffic (eg. port #22) from your neighbour's external facing IP address.
 +
# Have your neighbour try to SSH into YOUR c7host (at least to get a password prompt). Were they Successful? If so, why?
 +
# Shutdown all VMs and restart your c7host Linux machine.
 +
# List the iptables rules for the INPUT chain. What happened to your iptables rules for the INPUT chain?
 +
# Proceed to the next part to learn how to learn how to make your iptables rules persistent.
  
|}
+
=== Part 3: Making iptables Policies Persistent ===
===Part 2: Making sshd More Secure ===
 
  
Anytime you configure your computer to allow logins from the network you are leaving yourself '''vulnerable to potential unauthorized access''' by so called "hackers". Running the sshd service is a fairly common practice but '''care must be taken to make things more difficult for those hackers that attempt to use "brute force" attacks to gain access to your system. Hackers use their knowledge of your system and many password guesses to gain access'''. They know which port is likely open to attack (TCP:22), the administrative account name (root), all they need to do is to "guess" the password.<br><br> Making your root password (and all other accounts!) both quite complex but easy to remember is not hard.
+
Any changes to your iptables policy rules will be lost when you restart your Linux server, unless you make your iptables rules persistent. Failure to perform the following steps after setting up your firewall rules can cause confusion and wasted time.
  
The Linux system administrator can also '''configure the SSH server to make the SSH server more secure'''. Examples include not permitting root login, and change the default port number for ssh.
 
  
 
:'''Perform the following steps:'''
 
:'''Perform the following steps:'''
  
# For this section, you will still be using your '''c7host''' and '''centos1''' VMs.
+
# Flush all of your iptables rules by issuing the following command: <b><code><span style="color:#3366CC;font-size:1.2em;">iptables -F</span></code></b>
# Think of a good quality password and change your root passwords on all 3 VM's to be more secure. (It would be a good idea to do this for non-root accounts also)
+
# Set the default INPUT policy to ACCEPT by issuing the following command: <b><code><span style="color:#3366CC;font-size:1.2em;">iptables -P INPUT ACCEPT</span></code></b>
# The next change you can make is to prevent the root account from logging in to sshd altogether.
+
# Verify there are no iptables rules by issuing the command: <b><code><span style="color:#3366CC;font-size:1.2em;">iptables -L</span></code></b>
# Change to your '''centos1''' VM and open a terminal.
+
# Make a backup of the file '''/etc/sysconfig/iptables''' by issuing the command:<br><b><code><span style="color:#3366CC;font-size:1.2em;">cp /etc/sysconfig/iptables /etc/sysconfig/iptables.bk</span></code></b>
# Edit the file '''/etc/ssh/sshd_config''' and look for the option '''PermitRootLogin'''. Un-comment the option (or add the option if it does not appear) and change the option value to '''"no"'''.<br><br>'''NOTE:''' Now any hacking attempt also has to guess an account name as well as the password. If you need to ssh with root access, ssh as a regular user and use '''su -''' to become root.<br><br>
+
#To make the iptables rules '''persistent''' (i.e. keeps rules when system restarts), you issue the command: <br><b><code><span style="color:#3366CC;font-size:1.2em;">iptables-save > /etc/sysconfig/iptables</span></code></b>
# Even better, it is possible to restrict access to just specific users that require it.
+
# Verify that the file '''/etc/sysconfig/iptables''' exists.
# Edit the file '''/etc/ssh/sshd_config''' and add a new option of '''"AllowUsers account"''' using your login account for account
+
# Restart your iptables service and test your configuration.
# In order for these changes to be effective, issue the following command to restart the sshd service:<br /><b><code><span style="color:#3366CC;font-size:1.2em;">service sshd restart</span></code></b>
+
 
# Try sshing from your '''c7host''' VM to your '''centos1''' VM. Where you successful? Would it work if you let "AllowUsers account" without a username, or a non-existent username? Do not do this for your machine!
 
# Next change the default port number that sshd uses (TCP:22).
 
# Edit the '''/etc/ssh/sshd_config''' file again, un-comment the port option and change the port number it uses from ''22'' to '''2200'''.
 
# <u>Restart</u> the service.
 
# Confirm the new port is being used with the '''netstat''' command.
 
# Before we can use this new port we must change our firewall to allow traffic through the new port number and block access to port 22 by issuing the command:<br /><b><code><span style="color:#3366CC;font-size:1.2em;">iptables -I INPUT -p tcp -s0/0 --dport 2200 -j ACCEPT</span></code></b>
 
# Next, we will drop any incoming traffic to port 22 by issuing the command:<br><b><code><span style="color:#3366CC;font-size:1.2em;">iptables -I INPUT  -p tcp -s0/0 --dport 22 -j DROP</span></code></b>
 
# We have now possibly mislead a potential "hacker" to the true port for our ssh server's communication channel (port).
 
# Switch to your '''centos1''' VM.
 
# Issue the commmand: <b><code><span style="color:#3366CC;font-size:1.2em;">ssh username@centos2</span></code></b>. What happens? What port do you think that command is using by default?
 
# Now issue the following command to ssh via port "2200": <b><code><span style="color:#3366CC;font-size:1.2em;">ssh -p 2200 username@centos1</span></code></b>. Where you able to connect?
 
<ol><li value="17">Finally, as a system administrator, you should periodically monitor your system logs for unauthorized login attempts.</li>
 
<li>On CentOS systems the log file that is used is '''/var/log/secure''' </li>
 
<li>It also logs all uses of the '''su''' and '''sudo''' commands.</li>
 
<li>Attempt to connect to all of your VM's as root and other users using both public key and password authentication. Use some '''su''' and '''sudo''' commands also. </li>
 
<li>Inspect the log to see what kind of information is logged.</li>
 
</ol>
 
  
 
'''Answer INVESTIGATION 3 observations / questions in your lab log book.'''
 
'''Answer INVESTIGATION 3 observations / questions in your lab log book.'''
  
 
= LAB 7 SIGN-OFF (SHOW INSTRUCTOR) =
 
= LAB 7 SIGN-OFF (SHOW INSTRUCTOR) =
 +
===Exclusively for Summer 2020 term, submissions are accepted only online!===
 +
Follow the submission instructions for lab 7 on Blackboard.
 
{{Admon/important|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.}}
 
{{Admon/important|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.}}
  
Line 294: Line 333:
 
# Switch to your '''c7host''' VM and '''su -''' into root.
 
# Switch to your '''c7host''' VM and '''su -''' into root.
 
# Change to the '''/root/bin''' directory.
 
# Change to the '''/root/bin''' directory.
# Issue the Linux command: <b><code><span style="color:#3366CC;font-size:1.2em;">wget http://matrix.senecac.on.ca/~murray.saul/ops235/lab7-check.bash</span></code></b>
+
# Issue the Linux command: <b><code><span style="color:#3366CC;font-size:1.2em;">wget https://ict.senecacollege.ca/~ops235/labs/lab7-check.bash</span></code></b>
 
# Give the '''lab7-check.bash''' file execute permissions (for the file owner).
 
# Give the '''lab7-check.bash''' file execute permissions (for the file owner).
 
# Run the shell script and if any warnings, make fixes and re-run shell script until you receive "congratulations" message.
 
# Run the shell script and if any warnings, make fixes and re-run shell script until you receive "congratulations" message.
#Arrange proof of the following on the screen:<br><blockquote><span style="color:green;font-size:1.5em;">&#x2713;</span> '''centos2''' VM:<blockquote><ul><li>have logged into centos3 VM using '''public key authentication''' (with a pass-phrase)</li></ul></blockquote><span style="color:green;font-size:1.5em;">&#x2713;</span> '''c7host''' Machine:<blockquote><ul><li>have tunneled Xwindows application from '''centos1''' via ssh</li><li>have tunneled http through firewall using ssh (on web-browser</li><li>Run the '''lab7-check.bash''' script in front of your instructor (must have all <b><code><span style="color:#66cc00;border:thin solid black;font-size:1.2em;">&nbsp;OK&nbsp;</span></code></b> messages)</li></ul></blockquote><span style="color:green;font-size:1.5em;">&#x2713;</span> '''Lab7''' log-book filled out.
+
#Arrange proof of the following on the screen:<br><blockquote><span style="color:green;font-size:1.5em;">&#x2713;</span> '''centos2''' VM:<blockquote><ul><li>have logged into centos3 VM using '''public key authentication''' (with a pass-phrase)</li></ul></blockquote><span style="color:green;font-size:1.5em;">&#x2713;</span> '''c7host''' Machine:<blockquote><ul><li>have tunneled Xwindows application from '''centos1''' via ssh</li><li>Run the '''lab7-check.bash''' script in front of your instructor (must have all <b><code><span style="color:#66cc00;border:thin solid black;font-size:1.2em;">&nbsp;OK&nbsp;</span></code></b> messages)</li></ul></blockquote><span style="color:green;font-size:1.5em;">&#x2713;</span> '''Lab7''' log-book filled out.
 
 
  
 
= Practice For Quizzes, Tests, Midterm &amp; Final Exam =
 
= Practice For Quizzes, Tests, Midterm &amp; Final Exam =
Line 304: Line 342:
 
# What port does sshd use by defaults?  
 
# What port does sshd use by defaults?  
 
# What file is used to configure sshd?
 
# What file is used to configure sshd?
# What sftp commands are used to upload/download files?
 
 
# What kind of files are stored in the "~/.ssh/" directory?
 
# What kind of files are stored in the "~/.ssh/" directory?
 
# How do you determine whether the sshd service is running on your system or not?
 
# How do you determine whether the sshd service is running on your system or not?
Line 312: Line 349:
 
# How do you stop the sshd service?
 
# How do you stop the sshd service?
 
# How do you tunnel XWindows applications?
 
# How do you tunnel XWindows applications?
# What port is the default scp port?
+
# What port is the default ssh port?
 
# What port(s) is/are used by httpd service?
 
# What port(s) is/are used by httpd service?
  
  
 
[[Category:OPS235]]
 
[[Category:OPS235]]
 +
[[Category:OPS235 Labs]]
 +
[[Category:CentOSS 7]]
 +
[[Category:SSD2]]
 +
[[Category:Digital Classroom]]

Latest revision as of 17:52, 25 November 2020

LAB PREPARATION

Purpose / Objectives of Lab 7

Protecting a computer network from unauthorized access is one of the many day-to-day operations for a Linux system administrator and/or security specialist

Setting up a computer network is very important, but the Linux system administrator must also perform networking maintenance which includes trouble-shooting, repairing network connection issues and maintaining network security. System administrators need to protect or "harden" their computer networks from "penetration" from unauthorized computer users. Hardening a computer system can range from running an IDS (Intrusion Detection System) to monitoring and flagging suspicious activity to implementing security policies which could range from running firewalls to setting locked screen savers on workstations.

In this lab, you will learn how to install and configure the SSH service on a VM to allow users to securely access and share data between authorized personnel. In addition, you will learn various methods of running and configuring an ssh server which include: using Public Key Authentication, setting up an SSH tunnel in order to securely run graphical applications safely among computers in the network, and disabling root login into a Linux machine. You will also learn how to set up a firewall using the iptables command in order to control the flow of packets throughout your computer server.


Main Objectives

  1. To use the ssh and scp to access and copy data among Linux servers in a secure manner
  2. Set up, configure, and start the Secure Shell Service (sshd)
    • To refuse root login from remote Linux servers or limit users that are permitted to ssh into Linux servers
  3. Generate Public and Private keys to ensure secure connections between Linux servers
  4. Use ssh to tunnel Xwindow applications
  5. Learn about the Linux firewall (via iptables):
    • Use iptables command used to configure and maintain a firewall for protection and troubleshooting
    • Configure iptables to set a default policy and add exceptions to the default policy


Minimum Required Materials
Linux Command Reference
Solid State Drive
USB key
(for backups)
Lab7 Log Book


Networking Utilities

ssh
ssh-keygen
ssh-copy-id
scp
sftp
netstat
ifconfig
ip
ping
arp
iptables

Additional Utilities

hostname
restorecon

Managing Services
systemctl

Configuration Files
ssh_config
sshd_config

SSH Reference

A good ssh tutorial
A good HOW-TO to make ssh more secure

INVESTIGATION 1: INSTALLING AND MAINTAINING AN SSH SERVER

So far, you have learned to use the ssh utility to establish a secure connection to a remote server in order to perform Linux administration tasks. You have issued the ssh command, which is actually the client application for ssh. In order to connect to a remote server (like your VMs, Matrix, etc) it needs to run the SSH service (i.e. the ssh daemon).

In this section, you will learn how to configure an SSH server and restart the ssh service for an existing VM. You will also learn how to configure, restart, and use SSH in order to create secure connections between your Linux machines (host as well as VMs).


Part 1: Confirming sshd service is Running on VMs.

Perform the following steps:
  1. Launch your c7host machine and your centos1 and centos3 VMs.
  2. Switch to your c7host VM.
  3. Create a file in your current directory of your c7host machine with some text in it called: myfile.txt
  4. Ensure you've successfully connected to the VPN required for Matrix (https://inside.senecacollege.ca/its/services/vpn/studentvpn.html). Then issue the following command (using your Matrix login id):
    scp   myfile.txt   yourmatrixid@matrix.senecac.on.ca:/home/yourmatrixid
    (followed by your Matrix password)
    What did this command do?
  5. Issue the following single command (arguments are separated by a space - use your Matrix login id):
    ssh   yourmatrixid@matrix.senecac.on.ca   ls /home/yourmatrixid/myfile.txt
    (followed by your Matrix password)
    What did this command do?
    Issue the following Linux command:
    ssh   yourmatrixid@matrix.senecac.on.ca   cat /home/yourmatrixid/myfile.txt
    How do these commands differ from using issuing the ssh command without the ls or cat command? How is this useful?

    The client ssh application contains the utlities: ssh, scp and sftp (learned in ULI101) to connect to remote Linux servers in order to issue commands or transfer files between Linux servers. You can install the SSH service on your Linux server, although this has already been performed upon installation. We will now confirm that the ssh service is running on all of your VMs.

  6. OpenSSH should have been installed by default. Let's confirm this by issuing the command:
    rpm -qa | grep ssh
  7. You should see a number of packages installed including openssh-clients and openssh-server
  8. The openssh-server package installs a service called sshd.
  9. Login as root and confirm that this service is running by issuing the command:
    systemctl status sshd
  1. Now that you know the service is running, investigate what port number and protocol sshd uses by issuing the command:
    netstat -atunp | more
    What protocol and port is the sshd process using? What is the state of the port? Why would you think that UDP ports don't have a state?
  2. Reissue the netstat command without the -n option. What is the difference?
  3. You can refer to the /etc/services file in order to determine a port number for a service. Issue the following command to confirm that port 22 is associated with ssh:
    grep ssh /etc/services
  4. Make sure the sshd service is running on all 3 of your VM's

Part 2: SSH Server Security Configuration

Any time that you configure your computer to allow logins from the network you are leaving yourself vulnerable to potential unauthorized access by penetration testers or even hackers. Running the sshd service is a fairly common practice but care must be taken to make things more difficult for those individuals that attempt to use brute force attacks to gain access to your system. Hackers use their knowledge of your system and can use password guessing programs help to gain access. They know which port is likely open to attack (TCP:22), the administrative account name (root).

The Linux system administrator can configure the SSH server in order to make the SSH server less vulnerable to attacks. Examples include not permitting root login, and changing the default port number for the ssh service.

Perform the following steps:
  1. For this section, you will still be using your c7host and centos1 VMs.

    The next change you can make is to prevent the root account from logging in to sshd altogether.

  2. Change to your centos1 VM and open a terminal.
  3. Edit the file /etc/ssh/sshd_config and look for the option PermitRootLogin.
    Un-comment the option
    (or add the option if it does not appear) and change the option value to no.

    NOTE: Now any hacking attempt also has to guess an account name as well as the password.
    If you need to ssh with root access, ssh as a regular user and use su - to become root.

  4. Even better, it is possible to restrict access to just specific users that require it:
    Edit the file /etc/ssh/sshd_config and add a new option of AllowUsers yourAccountName (where "yourAccountName" is your regular user accountname for your centos1 VM)
  5. In order for these changes to take affect, you need to restart the sshd daemon. Issue the following command to restart the sshd service:
    systemctl restart sshd
  6. Try SSHing from your c7host VM to your centos1 VM as root. Where you successful?
  7. Try SSHing from your c7host VM to your centos1 VM as your regular user accountname. Did it work?
  8. Create another regular user called: other
  9. Set the password for the newly-created called other
  10. Try SSHing from your c7host VM to your centos1 VM for the account called other. Why didn't it work?
  11. Edit the file /etc/ssh/sshd_config to add the account other for the AllowUsers option (use a space to separate usernames instead of a comma).
  12. Restart the ssh service.
  13. Try SSHing from your c7host VM to your centos1 VM for the account called other. Did it work this time?
  14. Issue the following command to make a backup copy of your sshd_config file to your original regular user's home directory:
    cp /etc/ssh/sshd_config /home/regularuserid/sshd_config.bk
  15. Issue the following command to allow same group and other group members to view the file contents:
    chmod og+r /home/regularuserid/sshd_config.bk
  1. Finally, as a system administrator, you should periodically monitor your system logs for unauthorized login attempts.
  2. On CentOS systems the log file that is used is /var/log/secure
  3. It also logs all uses of the su and sudo commands.
  4. Attempt to connect to all of your VM's as root and other users using both public key and password authentication. Use some su and sudo commands also.
  5. Inspect the log to see what kind of information is logged.


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

INVESTIGATION 2: ADDITIONAL METHODS TO SECURE YOUR SSH SERVER

Note.png
Storing Fingerprints
When a user connects to a host using ssh, the host sends a fingerprint or digital signature to the client to establish its identity. The first time a connection is established the identity must be stored for subsequent connections. The fingerprints are stored separately for each user in a file called ~/.ssh/known_hosts .

From now on when you connect to that host the client will compare the received fingerprint against the list of known hosts before connecting. If the fingerprint does not match it could indicate somebody had setup a system to impersonate the computer you wish to connect.

Part 1: Generating Private and Public Keys (Public Key Infrastructure)

As a system administrator, you have the ability to generate or create public and private keys to ensure safe and secure ssh connections. This will require a user to prove who they say they are in order to access a Linux server via SSH (i.e. authentication). The system administer can generate these keys for the first time, or if the system administrator suspects that a hacker has compromised or trying to penetrate the server, they can remove the existing keys and generate new keys.

A common type of attack, Arp Poisoning (Man in the Middle Attack), can be used to redirect packets to a third party while maintaining the illusion that the connection is secure. Therefore, understanding about the generation and management of public/private keys are important to the security of servers.

Perform the following steps:
  1. Switch to your centos3 VM.

We can use the netstat utility as a trouble-shooting tool to view the SSH service and determine which STATE the SSH service is performing:
LISTENING, ESTABLISHED, CLOSED , or WAITING

  1. Run the netstat -atunp command (pipe to "grep sshd") to check the state of a possible ssh connection. What is the state (i.e. LISTENING or ESTABLISHED)?
  2. While in your centos3 VM, issue the following command to connect to your same VM via ssh: ssh ops235@centos3
  3. Enter yes at the prompt, and enter your OPS235 password.
    The output should appear similar as what is shown below:

    The authenticity of host 'centos3 (192.168.235.13)' can't be established.
    RSA key fingerprint is 53:b4:ad:c8:51:17:99:4b:c9:08:ac:c1:b6:05:71:9b.
    Are you sure you want to continue connecting (yes/no)? yes
    Warning: Permanently added 'centos3' (RSA) to the list of known hosts.

  4. Issue the following command to confirm that you connected to your centos3 VM: hostname
  5. Re-run that same netstat pipeline command. Any change to the connection status?
  6. Log-out of your ssh connection by typing exit.
  7. Run that same netstat command again. Wait a few minutes and then check again. Record your observations.


If you ever receive a message like the one displayed above, you should investigate why it is happening as it could indicate a serious security issue, or it could just mean that something on the host has changed(i.e. the OS was reinstalled)

So far, we have learned to establish an ssh connection to another host using a password to establish your identity. But passwords are not the only or even the best way of authenticating your identity. We can also use Public/Private key encryption.

Public Key authentication is a method of establishing identity using a pair of encryption keys that are designed to work together. One key is known as your private key (which as the name suggests should remain private and protected) and the other is known as the public key (which as the name suggests can be freely distributed) The keys are designed to work together to encrypt data asymmetrically, that is to say that when we encrypt data with one of the keys it can only be decrypted with the other key from the pair.

While it doesn't mean the message is secure as anybody could decrypt it with the public key, it does establish my identity, if the host can successfully decrypt the message then it must have come from the one person in possession of the private key.


  1. Switch to your centos2 VM.
  2. Confirm you are in your centos2 VM by entering the command: hostname
  3. Make certain that you are in your centos2 VM and that you are logged in as a regular user (i.e. NOT root!) (you have been warned!)
  4. To generate a keypair (public/private keys), issue the following command: ssh-keygen
  5. After generating the keys it prompts you for the location to save the keys. The default is ~/.ssh Your private key will be saved as id_rsa and your public key will be saved as id_rsa.pub. Press ENTER to accept the default.
  6. You will then be prompted for a pass-phrase. The pass-phrase must be entered in order to use your private key. Pass-phrases are more secure than passwords and should be lengthy, hard to guess and easy to remember. For example one pass-phrase that meets this criteria might be "seneca students like to dance at 4:00am". Avoid famous phrases such as "to be or not to be" as they are easy to guess. It is possible to leave the pass-phrase blank but this is dangerous. It means that if a hacker were able to get into your account they could then use your private key to access other systems you use.

    The output should appear similar as what is shown below:
Generating public/private rsa key pair.
Enter file in which to save the key (/home/ops235/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase):
Enter passphrase again:
Your public key has been saved in /home/ops235/.ssh/id_rsa.pub.
The key fingerprint is:
ef:de:31:67:f7:15:a4:43:39:15:5d:78:1b:e8:97:74 ops235@centos3
The key's randomart image is:
+--[ RSA 2048]----+
|              .+=|
|             .+oE|
|            .+.o=|
|            ..++ |
|        S    o.. |
|         .    . .|
|          . o o o|
|         . . = .o|
|         .o .   .|
+-----------------+
  1. Now issue the command ssh-copy-id -i ~/.ssh/id_rsa.pub ops235@centos3
  2. When prompted for password, enter OPS235's root password
  3. Try using ssh to now log into your centos3 VM from your centos2 VM. What happens? Were you required to use your pass-phrase?
  4. Issue the hostname command to verify that you are successfully logged into your centos3 VM.
  5. Make certain to logout of your centos3 system. Use the hostname command to verify you are back in your centos2 server.


You can use an SSH tunnel with options to allow running of applications on remote Linux servers.

Part 2: Securely Running Graphical Applications Between Linux Servers

You can also use ssh to tunnel window and bitmap information, allowing us to login to a remote desktop host and run a Xwindows application such as gedit or firefox and the application will run on the remote host but be displayed on the local host.

Perform the following steps:
  1. For this section, you will be using your c7host and centos1 VMs.
  2. Switch to your c7host VM, open a terminal and remain logged in as a regular user.
  3. Issue the following command to connect to your centos1 VM:
    ssh -X -C yourUserID@centos1   (where 'yourUserID' is your user account name on centos1)
    (The -X option enables the forwarding of X window information, and the -C option enables compression for better performance).

  4. Once the connection is properly established, run the command gedit
  5. The gedit window will display on your c7host VM, but in reality, this application is running on your centos1 VM!
  6. Enter some text and save your editing session.
  7. Exit the gedit application.
  8. In which VM was the file saved? What does that tell you about the use of tunneling for this section?
  9. Run the graphical program remotely by issuing only one Linux command:
    ssh -X -C yourUserID@centos1   gedit (Note: ignore warning messages).
  10. Exit the gedit application.
  11. Experiment with running other GUI applications (in the /bin directory with applications starting with the letter "x" via ssh (for example: xev or xchat).


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

INVESTIGATION 3: MANAGING FIREWALLS FOR PROTECTION & TROUBLESHOOTING

When using iptables packets must pass-through "a chain of policy rules" in order to handle packets. If a packet matches a rule, then an action is taken (some examples include: ACCEPT, DROP, REJECT, or LOG); otherwise, the packet will be directed to the default policy chain.

Linux Firewall (iptables) Concepts

Since Linux servers can be connected to the Internet, it is very important to run a firewall to control what packets might come into the computer system, what packets might go out of the computer system, and what packets might be forwarded to another computer. We are currently using the utility called iptables can be used to set the firewall rules on a Linux server.


Basically, there is a list (chain) of policy rules that packets must pass-through in order to handle packets. If a packet matches a rule, then an action is taken (some examples include: ACCEPT, DROP, REJECT, or LOG). If the packet passes through the chain of rules without a match, then the packet is directed to the default policy chain (for example: ACCEPT, REJECT, or DROP).


You can create your own customized chains (which you will learn in the OPS335 course) but to keep thing simple, we only deal with 3 common predefined chains:

  • INPUT: Packets coming into current Linux server
  • OUTPUT: Packets leaving current Linux server
  • FORWARD: Packets being routed between Linux servers


Part 1: Listing & Clearing Existing iptables Rules

Let's get some practice using the iptables command such as listing CHAIN rules, and clearing the CHAIN rules:

Perform the following steps:
  1. For the remainder of this section, use your c7host machine.
  2. Issue the following command to list the existing iptables policy rules: iptables -L
  3. Were there already iptables policy rules that already existed by default?
  4. Before we proceed, we need to understand various methods to list iptables rules:

    Listing iptables Rules:
    iptables -LList all iptables rules (eg. INPUT, OUTPUT. FORWARD, and any customized chains (if any)
    iptables -L -vVerbosely List all iptables rules including information such as total size of packets affected by rules
    iptables -L CHAIN-NAMEList all iptables rules for that particular chain-name for less clutter (eg. INPUT or OUTPUT, etc)

  5. Issue the following Linux command: iptables -L INPUT
    What do you notice is different with this command compared to the previous iptables command?
  6. Issue the iptables command separately to display the rules for the OUTPUT chain and for the FORWARD chain.
  7. Issue the following command: iptables -L -v
    What do you notice about this command as opposed to the first iptables command you issued?
    What sort of additional information does this command provide regarding affected packets?

  8. Sometimes it may be useful to completely clear the rules for all or a particular chain. Note the options that can be used to clear (or flush) the iptables rules,

    Clearing (Flushing) iptables Rules:
    iptables -FClears the rules for ALL of the chains
    iptables -F CHAIN-NAMEClears the rules for only the specified CHAIN-NAME (eg. INPUT or OUTPUT)

  9. Issue the following command to reset the iptables rules for the INPUT chain: iptables -F INPUT
  10. Issue the iptables -L INPUT command to verify that the iptables rules for the INPUT chain have been cleared.
  11. Now, issue the command: iptables -F
    and then issue the command: iptables -L
    What do you notice?


Part 2: Setting a Default Policy / Setting Policy Exceptions (iptables)

Usually when setting policy rules with iptables, a general "overall" policy is set (default policy chain). A good way to think about setting policies is to have a "safety-net" to take some sort of action to prevent un-handled packets from passing through the firewall by mistake. After the default policy is set-up, then specific exceptions to the default policy can be added to control specific network traffic.

An example would be to set a default policy for incoming network traffic (INPUT chain) to DROP everything, and then set an exception certain exceptions (like ssh connections). Note the following table below for policy setting examples.

Policy Setting Examples:
iptables -P INPUT DROPDrops all incoming packets regardless of protocol (eg. tcp, udp, icmp), port numbers (eg. 22, 80) or source or destination IP Addresses. Setting a default rule to DROP all incoming traffic would make it easier to specify a few exceptions.
iptables -P INPUT ACCEPTAccepts all incoming packets regardless of protocol (eg. tcp, udp, icmp), port numbers (eg. 22, 80) or source or destination IP Addresses. It would seem that setting a default rule to ACCEPT all incoming traffic would require A LOT of exceptions to help "lock-down" the server for protection! It really depends on the server set-up and what the Linux system administrator wants to accomplish.


Perform the following steps:
  1. Make certain you are in your c7host machine.
  2. Issue the following Linux command: iptables -P INPUT DROP
  3. Issue the iptables -L command. Can you see the policy to DROP all incoming connections?
  4. Although you have set a default policy to DROP all incoming connections, there is a problem: now, you cannot browse the Internet. You can confirm that by opening a SEPARATE web-browser and perform a Net-search.

    In order to fix that problem, you can make an exception to allow incoming web-based traffic (via port 80). Those iptables commands to create exceptions are more complex since you need to determine:
    • Where each rules appears in the chain? (order can be important)
    • Which protocol(s) are affected (eg. tcp, udp, icmp)
    • What source or destination IP Addresses are affected?
    • What port numbers are affected?
    • What action to take if all of the above conditions are met? (eg. ACCEPT, REJECT, DROP, or LOG)

    iptables Command Structure (for setting exceptions):
    (NOTE: If element in column is not specified in the iptables command, then rule relates to ALL elements)
    Place Rule in ChainChain NameSpecify ProtocolSource/Destination IPADDRPort NumberAction
    ->
    Target
    -A (add / Append to bottom of chain)
    -I (insert at top of chain)
    -I CHAIN-NAME 5 (insert before line 5)
    INPUT
    OUTPUT
    FORWARD
    CHAIN-NAME
    -p tcp (tcp packets)
    -p udp (datagram packets)
    -p tcp,udp,icmp (combined)

    (refer to /etc/protocols )
    -s IPADDR (originating IPADDR)
    -d IPADDR (destination IPADDR)
    --sport 22 (originating port 22 - SSH)
    --dport 80 (destined port 80 - http)

    (refer to /etc/services)
    -j ACCEPT
    REJECT
    DROP
    LOG

  5. Issue the following Linux commands to ensure the loopback interface is not affected by these rules. The computer should be able to communicate with itself with any state and protocol:
    iptables -A INPUT -i lo -p all -j ACCEPT
    iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
  6. Issue the following Linux command to ADD an exception to the INPUT chain to allow web-based incoming traffic (ie. port 80):
    iptables -A INPUT -p tcp --dport 80 -j ACCEPT
  7. Issue an iptables command to confirm that their is an exception rule to handle incoming tcp packets over port 80.
  8. Use your other web-browser to confirm that you can now browse the Internet. If you cannot, contact your lab assistant or professor for help.
  9. Determine the external facing address of your c7host machine.
    (Tip: in a web-browser, enter the term: "ip address". The external facing IP Address should start with "10.").
  10. Provide your external facing address, and provide another lab-mate to ping that external facing address. Were they successful?
  11. Have your lab-mate determine THEIR external facing address and obtain that IP Address.
  12. Issue the following iptables command to allow an exception for pings from your lab-mate:
    iptables -A INPUT -p icmp -s {neighbour's external facing address} -j ACCEPT
  13. Have your neighbour repeat pinging your external facing IP Address. What happened? Why?
  14. Have your neighbour try to SSH into YOUR c7host. Were they Successful?
  15. Issue an iptables rule (in a similar way as with the previous iptables command) to allow an exception for incoming ssh traffic (eg. port #22) from your neighbour's external facing IP address.
  16. Have your neighbour try to SSH into YOUR c7host (at least to get a password prompt). Were they Successful? If so, why?
  17. Shutdown all VMs and restart your c7host Linux machine.
  18. List the iptables rules for the INPUT chain. What happened to your iptables rules for the INPUT chain?
  19. Proceed to the next part to learn how to learn how to make your iptables rules persistent.

Part 3: Making iptables Policies Persistent

Any changes to your iptables policy rules will be lost when you restart your Linux server, unless you make your iptables rules persistent. Failure to perform the following steps after setting up your firewall rules can cause confusion and wasted time.


Perform the following steps:
  1. Flush all of your iptables rules by issuing the following command: iptables -F
  2. Set the default INPUT policy to ACCEPT by issuing the following command: iptables -P INPUT ACCEPT
  3. Verify there are no iptables rules by issuing the command: iptables -L
  4. Make a backup of the file /etc/sysconfig/iptables by issuing the command:
    cp /etc/sysconfig/iptables /etc/sysconfig/iptables.bk
  5. To make the iptables rules persistent (i.e. keeps rules when system restarts), you issue the command:
    iptables-save > /etc/sysconfig/iptables
  6. Verify that the file /etc/sysconfig/iptables exists.
  7. Restart your iptables service and test your configuration.


Answer INVESTIGATION 3 observations / questions in your lab log book.

LAB 7 SIGN-OFF (SHOW INSTRUCTOR)

Exclusively for Summer 2020 term, submissions are accepted only online!

Follow the submission instructions for lab 7 on Blackboard.

Important.png
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 ALL of your VMs are running.
  2. Switch to your c7host VM and su - into root.
  3. Change to the /root/bin directory.
  4. Issue the Linux command: wget https://ict.senecacollege.ca/~ops235/labs/lab7-check.bash
  5. Give the lab7-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:
    • have logged into centos3 VM using public key authentication (with a pass-phrase)
    c7host Machine:
    • have tunneled Xwindows application from centos1 via ssh
    • Run the lab7-check.bash script in front of your instructor (must have all  OK  messages)
    Lab7 log-book filled out.

Practice For Quizzes, Tests, Midterm & Final Exam

  1. What port does sshd use by defaults?
  2. What file is used to configure sshd?
  3. What kind of files are stored in the "~/.ssh/" directory?
  4. How do you determine whether the sshd service is running on your system or not?
  5. What is the purpose of the ~/.ssh/known_hosts file?
  6. What is the purpose of the ~/.ssh/authorized_keys file?
  7. Which system log file records each use of the sudo command?
  8. How do you stop the sshd service?
  9. How do you tunnel XWindows applications?
  10. What port is the default ssh port?
  11. What port(s) is/are used by httpd service?