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| − | = LAB PREPARATION =
| + | #REDIRECT [[OPS235_Lab_7_-_CentOS7_-_SSD2]] |
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| − | ===Purpose / Objectives of Lab 7===
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| − | [[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]]
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| − | 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.
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| − | 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, '''disabling root login''', and '''changing the default ssh communication port''' to mislead potential penetration testers (also known as "pen-testers" or "hackers"). You will also learn how to setup a firewall using the '''iptables''' command in order to control the flow of packets throughout your computer server.
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| − | <u>Main Objectives</u>
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| − | * To set up, configure Secure Shell Services ('''ssh/sshd''')
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| − | * To customize '''sshd''' to create a more private, secure system
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| − | * Use ssh to '''tunnel X applications'''
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| − | * To tunnel network traffic via other communication ports
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| − | * Learn the basics of the '''iptables''' command used to configure and maintain a firewall for protection and troubleshooting a Linux server
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| − | {|cellpadding="15" width="100%" cellspacing="0"
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| − | |- valign="top"
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| − | |colspan="3" style="font-size:16px;font-weight:bold;border-bottom: thin solid black;border-spacing:0px;"|Minimum Required Materials<br>
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| − | |colspan="3" style="font-size:16px;font-weight:bold;border-bottom: thin solid black;border-spacing:0px;padding-left:20px;"|Linux Command Reference<br>
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| − | |- valign="top"
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| − | | |[[Image:harddrive.png|thumb|left|75px|<b>Removable SATA Hard Disk</b>]]
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| − | |style="padding-left:20px;"|[[Image:ubs-key.png|thumb|left|85px|<b>USB key</b><br>(for backups)]]
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| − | |style="padding-left:20px;"|[[Image:log-book.png|thumb|left|70px|<b>Lab7 Log Book</b>]]
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| − | |style="padding-left:20px;" |Networking Utilities<br>
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| − | [http://man7.org/linux/man-pages/man1/ssh.1.html ssh]<br>
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| − | [http://man7.org/linux/man-pages/man1/ssh-keygen.1.html ssh-keygen]<br>
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| − | [http://linux.die.net/man/1/ssh-copy-id ssh-copy-id]<br>
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| − | [http://man7.org/linux/man-pages/man1/scp.1.html scp]<br>
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| − | [http://man7.org/linux/man-pages/man1/sftp.1.html sftp]<br>
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| − | [http://man7.org/linux/man-pages/man8/netstat.8.html netstat]<br>
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| − | [http://man7.org/linux/man-pages/man8/ifconfig.8.html ifconfig]<br>
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| − | [http://man7.org/linux/man-pages/man8/ping.8.html ping]<br>
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| − | [http://man7.org/linux/man-pages/man8/arp.8.html arp]<br>
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| − | [http://zenit.senecac.on.ca/wiki/index.php/IPTables iptables]<br>
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| − | |style="padding-left:20px;"|Additional Utilities<br>
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| − | [http://man7.org/linux/man-pages/man7/hostname.7.html hostname]<br>
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| − | [http://linux.die.net/man/8/restorecon restorecon]<br>
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| − | [http://linux.die.net/man/8/chkconfig chkconfig]<br><br>
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| − | Configuration Files<br>
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| − | [http://linux.about.com/library/cmd/blcmdl5_ssh_config.htm ssh_config]<br>
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| − | [http://linux.about.com/od/commands/l/blcmdl5_sshdcon.htm sshd_config]<br>
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| − | |style="padding-left:20px;"|SSH Reference<br>
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| − | [http://support.suso.com/supki/SSH_Tutorial_for_Linux A good ssh tutorial]<br>
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| − | [http://it.toolbox.com/blogs/locutus/shh-securing-ssh-howto-10640 A good HOW-TO to make ssh more secure]
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| − | |}
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| − | =INVESTIGATION 1: INSTALLING AND MAINTAINING AN SSH SERVER=
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| − | 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. For these common operations, 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) they need to be running the SSH service (i.e. the ssh daemon).
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| − | In this section, you will learn how to install and configure an SSH server on an existing VM. You will also learn how to generate private and public keys in order to allow secure connections between your Linux machines (host as well as VMs).
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| − | === Part 1: Enabling the sshd service. ===
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| − | :'''Perform the following steps:'''
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| − | # Launch your c7host machine and ALL of your VMs.
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| − | # Switch to your '''c7host''' VM.
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| − | # 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>
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| − | # You should see a number of packages installed including <b>openssh-clients</b> and <b>openssh-server</b>
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| − | # <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>
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| − | <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>
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| − | <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>
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| − | <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>
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| − | <li>Make sure the '''sshd''' service is running on '''all 3 of your VM's'''</li>
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| − | <li>Remember that you can view the '''/etc/services''' file in case you need to reference a service with a corresponding port number.</li>
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| − | </ol>
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| − | ===Part 2: SSH Server Security Configuration===
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| − | Any time 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.
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| − | 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.
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| − | :'''Perform the following steps:'''
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| − | # For this section, you will still be using your '''c7host''' and '''centos1''' VMs.
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| − | # The next change you can make is to prevent the root account from logging in to sshd altogether.
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| − | # Change to your '''centos1''' VM and open a terminal.
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| − | # 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>'''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. If you need to ssh with root access, ssh as a regular user and use '''su -''' to become root.<br><br>
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| − | # 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 account</span></code></b> using '''your''' login account for account<br>
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| − | # 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>
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| − | # Try SSHing from your '''c7host''' VM to your '''centos1''' VM as '''root'''. Where you successful? Now try SSHing from your c7host VM to your centos1 VM as a regular user that was permitted to connect via ssh. Did it work? What would happen for other user accounts that were not permitted?
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| − | # Next, let us "fool" potential penetration testers by changing the default port number that sshd uses from port 22 to port 2200.
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| − | # Edit the '''/etc/ssh/sshd_config''' file again, un-comment the port option and change the '''port number''' it uses from ''22'' to <b><code><span style="color:#3366CC;font-size:1.2em;">2200</span></code></b>.
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| − | # <u>Restart</u> the '''ssh''' service.
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| − | # Confirm the new port is being used with the '''netstat''' command.
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| − | # Switch to your '''centos1''' VM.
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| − | # Issue the commmand: <b><code><span style="color:#3366CC;font-size:1.2em;">ssh username@centos1</span></code></b>. What happens? What port do you think that command is using by default?
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| − | # 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?
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| − | <ol><li value="17">Finally, as a system administrator, you should periodically monitor your system logs for unauthorized login attempts.</li>
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| − | <li>On CentOS systems the log file that is used is '''/var/log/secure''' </li>
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| − | <li>It also logs all uses of the '''su''' and '''sudo''' commands.</li>
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| − | <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>
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| − | <li>Inspect the log to see what kind of information is logged.</li>
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| − | </ol>
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| − | '''Answer INVESTIGATION 1 observations / questions in your lab log book.'''
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| − | =INVESTIGATION 2: ADDITIONAL METHODS TO SECURE YOUR SSH SERVER =
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| − | {|width="40%" align="right" cellpadding="10"
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| − | |- valign="top"
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| − | |{{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}}
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| − | |}
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| − | === Part 1: Generating Private and Public Keys (Public Key Infrastructure) ===
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| − | 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.
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| − | 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.
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| − | :'''Perform the following steps:'''
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| − | # Switch to your '''centos3''' VM.<br><br>
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| − | | |
| − | :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>
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| − | <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>
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| − | <li>While in your '''centos3''' VM, issue the following command to connect to the same VM via ssh: <b><code><span style="color:#3366CC;font-size:1.2em;">ssh ops235@centos3</span></code></b></li>
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| − | <li>Enter yes at the prompt, and enter your OPS235 password.<br>The output should appear similar as what is shown below:<br><br>
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| − | :<span style="font-family:courier">The authenticity of host 'centos3 (192.168.235.13)' can't be established.</span><br>
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| − | :<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>
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| − | :<span style="font-family:courier">Are you sure you want to continue connecting (yes/no)? yes</span><br>
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| − | :<span style="font-family:courier">Warning: Permanently added 'centos3' (RSA) to the list of known hosts.</span><br><br></li>
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| − | <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>
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| − | [[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>)]] | |
| − | <ol><li value="7">Re-run that same '''netstat pipeline command'''. Any change to the connection status?</li>
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| − | <li>Log-out of your ssh connection by typing <b><code><span style="color:#3366CC;font-size:1.2em;">exit</span></code></b>.
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| − | <li>Run that same '''netstat''' command again. Wait a few minutes and then check again. Record your observations.</li>
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| − | </ol>
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| − | 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'''.
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| − | '''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.
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| − | While it doesn't mean the message is <u>secure</u> as anybody could decrypt it with the public key, it does establish my <u>identity</u>, if the host can successfully decrypt the message then it must have come from the one person in possession of the private key.
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| − | <ol>
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| − | <li value="10">Switch to your '''centos2''' VM.</li>
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| − | <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>
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| − | <li>Make <u>certain</u> that you are in your centos you are NOT logged in as root!''' (you have been warned!)</li>
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| − | <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>
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| − | <li>Press ENTER to accept the default, then enter a pass-phrase used to establish your identity, and re-enter the pass-phrase to verify.<br>The output should appear similar as what is shown below:</li>
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| − | </ol>
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| − | | |
| − | <pre style="font-family:monospace;background-color:white;border-style:none;padding-left:50px;">
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| − | Generating public/private rsa key pair.
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| − | Enter file in which to save the key (/home/ops235/.ssh/id_rsa):
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| − | Enter passphrase (empty for no passphrase):
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| − | Enter passphrase again:
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| − | Your public key has been saved in /home/ops235/.ssh/id_rsa.pub.
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| − | The key fingerprint is:
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| − | ef:de:31:67:f7:15:a4:43:39:15:5d:78:1b:e8:97:74 ops235@centos3
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| − | The key's randomart image is:
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| − | +--[ RSA 2048]----+
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| − | | .+=|
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| − | | .+oE|
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| − | | .+.o=|
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| − | | ..++ |
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| − | | S o.. |
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| − | | . . .|
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| − | | . o o o|
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| − | | . . = .o|
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| − | | .o . .|
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| − | +-----------------+
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| − | </pre>
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| − | | |
| − | <ol><li value="15"> 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>
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| − | <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>
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| − | <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>
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| − | <li>When prompted for password, enter OPS235's root password</li>
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| − | <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>
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| − | <li> Issue the '''hostname''' command to verify that you are successfully logged into your '''centos3''' VM.</li>
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| − | <li>Make certain to logout of your '''centos3''' system. Use the '''hostname''' command to verify you are back in your centos2 server.</li>
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| − | </ol>
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| − | {|width="40%" align="right" cellpadding="10"
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| − | |- valign="top"
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| − | [[Image:tunel-gedit.png|thumb|right|550px|You can use an SSH tunnel with options to allow running of applications on remote Linux servers.]]
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| − | |}
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| − | | |
| − | === Part 2: Securely Running Graphical Applications Between Linux Servers===
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| − | 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.
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| − | :'''Perform the following steps:'''
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| − | | |
| − | # For this section, you will be using your '''c7host''' and '''centos1''' VMs.
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| − | # Switch to your c7host VM, open a terminal and remain logged in as a regular user.
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| − | # 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> (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>
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| − | # Once the connection is properly established, run the command <b><code><span style="color:#3366CC;font-size:1.2em;">gedit</span></code></b>
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| − | # The ''gedit'' window will display on your '''c7host''' VM, but in reality, this application is running on your '''centos1''' VM!
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| − | # Enter some text and save your editing session.
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| − | # Exit the '''gedit''' application.
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| − | # In which VM was the file saved? What does that tell you about the use of tunneling for this section?
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| − | # Experiment with running other GUI applications through '''ssh'''.
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| − | '''Answer INVESTIGATION 2 observations / questions in your lab log book.'''
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| − | | |
| − | =INVESTIGATION 3: MANAGING FIREWALLS FOR PROTECTION & TROUBLESHOOTING =
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| − | {| width="40%" align="right" cellpadding="10"
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| − | |- valign="top"
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| − | |
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| − | [[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. ]]
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| − | |}
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| − | ==== Linux Firewall (iptables) Concepts====
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| − | Since Linux servers can be connected to the Internet, it is very important to run a firewall to control what comes into the computer system, what goes out of the computer system, and what may be forwarded to another computer. A utility called '''iptables''' can be used to set the firewall rules on a Linux server.
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| − | 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'').
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| − | 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''':
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| − | | |
| − | :*'''INPUT''': Packets coming into current Linux server
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| − | :*'''OUTPUT''': Packets leaving current Linux server
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| − | :*'''FORWARD''': Packets being routed between Linux servers
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| − | | |
| − | | |
| − | === Part 1: Listing & Clearing Existing iptables Rules ===
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| − | Let's get some practice using the iptables command as we introduce concepts such as listing CHAIN rules, and clearing the CHAIN rules:
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| − | :'''Perform the following steps:'''
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| − | # For the remainder of this section, use your '''c7host''' machine.
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| − | # 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>.
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| − | # Were there already iptables policy rules that already existed by default?
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| − | # 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>
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| − | # 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?
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| − | # Issue the iptables command separately to display the rules for the '''OUTPUT''' chain and for the '''FORWARD''' chain.
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| − | # 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? What sort of additional information does this command provide regarding affected packets?<br><br>
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| − | # 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>
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| − | # 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>
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| − | # Issue the '''iptables -L INPUT''' command to verify that the iptables rules for the INPUT chain have been cleared.
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| − | # 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> . What do you notice?
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| − | | |
| − | | |
| − | | |
| − | === Part 2: Setting a Default Policy / Setting Policy Exceptions (iptables) ===
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| − | | |
| − | 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.
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| − | | |
| − | 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>
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| − | | |
| − | :'''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>
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| − | | |
| − | :'''Perform the following steps:'''
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| − | | |
| − | # 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>
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| − | # Issue the iptables command verify the INPUT policy has been set (look for the Policy section, NOT INPUT).<br><br>After the overall default policy is set, then you can create policy rules that are "exceptions" to the default policy rules. These iptables commands 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>
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| − | | |
| − | :'''iptables Command Structure (for setting exceptions):'''<br><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 to Take<br>(i.e. "jump to target")</td><td>Target</td></tr><tr><td>'''-A''' (add / Append to bottom of chain)<br>'''-I''' (insert at top of chain)<br>'''-i CHAIN-NAME 5''' (insert before line #5 in CHAIN-NAME)</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>(refer to '''/etc/protocols''' for available network protocols)</td><td>'''-s IPADDR''' (originating IPADDR)<br>'''-d IPADDR''' (destination IPADDR)</td><td>'''--sport 22''' (originating port 22 - SSH)<br>'''--sport 80''' (originating port 80 - http)<br>'''--dport 22''' (originating port 22 - SSH)<br>'''--dport 80''' (destination port 80 - http)<br><br>(refer to '''/etc/services''' for available port numbers)</td><td>'''-j''' </td><td>'''ACCEPT'''<br>'''REJECT'''<br>'''DROP'''<br>'''LOG'''</td>
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| − | </tr></table><br><br>
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| − | | |
| − | The <b><code><span style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.5em;">-j</span></code></b> option is used to redirect (jump) packets to actions (ACCEPT, REJECT, DROP, LOG) if the packet match that policy rule. The option <b><code><span style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.5em;">-p</span></code></b> will indicate the protocol used (eg. tcp, upd, icmp). The options <b><code><span style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.5em;">--dport</span></code></b> or <b><code><span style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.5em;">--sport</span></code></b> indicate the "destination communication port" or "source communication port" respectively. You can view the file '''/etc/services''' to determine the communication port number for the appropriate network service. The option <b><code><span style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.5em;">-A</span></code></b> is used to append the policy rule to the <u>bottom</u> of the chain. The option <b><code><span style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.5em;">-I</span></code></b> is used to insert a policy rule before an existing policy line number (if used with no number, will insert at the <u>top</u> of the chain)
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| − | | |
| − | '''Examples:'''
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| − | | |
| − | <table width="100%" cellpadding="10" cellspacing="0" border="1">
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| − | <tr>
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| − | <td width="40%">'''iptables -A INPUT -p tcp --sport 80 -j ACCEPT'''</td><td>Append policy to <u>'''bottom'''</u> of INPUT chain to accept all tcp packets from port 80</td>
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| − | </tr><tr>
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| − | <td>'''iptables -A OUTPUT -p tcp --dport 80 -j ACCEPT'''</td><td>Append policy to <u>'''bottom'''</u> of OUTPUT chain to accept all tcp packets to port 80</td>
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| − | </tr><tr>
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| − | <td>'''iptables -I INPUT -p tcp --sport 22 -j LOG'''</td><td>Insert policy at '''<u>top</u>''' of INPUT chain to log all tcp packets from port 22</td>
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| − | </tr><tr>
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| − | <td>'''iptables -I INPUT 3 -p tcp --dport 22 -j LOG'''</td><td>Insert policy <u>'''before line 3'''</u> of INPUT chain to log all tcp packets from port 22</td>
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| − | </tr>
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| − | </table>
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| − | | |
| − | | |
| − | You can also set exceptions to the default policy for specific IP Addresses by using the options <b><code><span style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.5em;">-d IPADDR</span></code></b> or <b><code><span style="pointer-events: none;cursor: default;color:#3366CC;font-size:1.5em;">-s IPADDR</span></code></b>
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| − | | |
| − | | |
| − | <table width="100%" cellpadding="10" cellspacing="0" border="1">
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| − | <tr>
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| − | <td width="45%">'''iptables -A INPUT -p tcp -s 192.168.0.0/24 -sport 22 -j ACCEPT'''</td><td>Append policy to bottom of INPUT chain to ACCEPT tcp packets from IP Address 192.168.0.0 from communication port 22</td>
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| − | </tr><tr>
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| − | <td>'''iptables -A OUTPUT -p tcp -d 192.168.0.138/24 -dport 80 -j REJECT'''</td><td>Append policy to bottom of OUTPUT chain to REJECT tcp packets to IP Address 192.168.0.138 via communication port 80</td>
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| − | </tr>
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| − | </table>
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| − | | |
| − | | |
| − | :'''Perform the following steps:'''
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| − | | |
| − | # Remain in your '''c7host''' VM for this section.
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| − | # Set the default policy for the INPUT chain to DROP by issuing the command:<br><b><code><span style="color:#3366CC;font-size:1.2em;">iptables -P INPUT DROP</span></code></b>
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| − | # Now try on your own to change the default policies for the OUTPUT chain to DROP
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| − | # Issue the commmand <b><code><span style="color:#3366CC;font-size:1.2em;">iptables -L</span></code></b> to verify that the policies on your INPUT and OUTPUT chain are set to DROP
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| − | # Open a browser and attempt to access the Internet. Were you successful?
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| − | # Using the commands you have learned so far, change the policies on the INPUT and OUTPUT chains to ACCEPT
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| − | # Open your browser and attempt to access the Internet again. Were you successful?
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| − | # Change the policies on all of the chains to DROP
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| − | #In the OUTPUT chain, add the following rule:<br><b><code><span style="color:#3366CC;font-size:1.2em;">iptables -A OUTPUT -j LOG</span></code></b>. The above rule tells '''iptables''' to log packets and relevant information to '''/var/log/messages'''.
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| − | #Try to access the Internet again. Because the policies have been set to DROP, you should be unsuccessful. However, every packet of data that your PC attempted to send out was logged.
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| − | # Let's have a look at the log file and analyze the data: <b><code><span style="font-family:courier;color:#3366CC;font-size:1.2em;">tail /var/log/messages</span></code></b><br><br>
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| − | ::This command shows us the last 10 lines of the file. While there are many things being logged to this file, the last thing we did was try to access the Internet so we should be able to see the data we need. Look for a line that looks similar to the following:<br><code style="font-family:courier;font-size:1.2em;">Jun 24 12:41:26 c7host kernel: IN= OUT=lo SRC=127.0.0.1 DST=127.0.0.1 LEN=52 TOS=0x00 PREC=0x00 TTL=64 ID=16442 DF PROTO=TCP SPT=57151 DPT=5902 WINDOW=1024 RES=0x00 ACK URGP=0</code><br><br>Your IP, host names and date will be different, but the one thing that should be the same is the DPT=80 value. When your computer tried to send OUT a request to connect to the Internet using the WWW, the computer used a destination port of 80. This is the standard port for the WWW. Because we have set the default policy to DROP it drops these packets. The problem is we are dropping all packets. What if we just want to drop the WWW packets?<br><br>
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| − | <ol>
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| − | <li value="11">Using the commands we already know, change the default policies on all of your chains to ACCEPT.</li>
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| − | <li>Open a browser and confirm that you can access the world wide web.</li>
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| − | <li>Enter the command:<br><b><code><span style="color:#3366CC;font-size:1.2em;">iptables -I OUTPUT -p tcp -s0/0 -d 0/0 --dport 80 -j DROP</span></code></b></li>
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| − | <li>Try to access the Web. If you have done everything right, you should not have been successful.</li>
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| − | <li>After you have completed the test execute the following command:<br><b><code><span style="color:#3366CC;font-size:1.2em;">iptables -F</span></code></b><br></li>
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| − | <li>Using the information you have learned, try on your own to achieve the same goal as above (block www access to your computer) by using the INPUT chain instead of the OUTPUT chain.</li>
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| − | <li>After you have completed this task, flush the iptables again.</li>
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| − | <li>Make sure that your ssh server is running on the host machine and try to access it from a virtual machine of your choice.</li>
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| − | <li>Once you have confirmed that ssh is running on the host machine, insert an iptables rule on the host machine to prevent access to the ssh server from all VM's on the virtual network.</li>
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| − | <li>Confirm that your rule works by testing from your VM's</li>
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| − | <li>Does iptables close the port? Check using '''netstat'''</li>
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| − | <li>Now insert a rule on the CentOS host that would ACCEPT connections from the centos3 VM only.</li>
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| − | <li>Fully test your configuration.</li>
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| − | <li>Flush the iptables rules for INPUT, OUTPUT and FORWARD chains.</li></ol>
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| − | | |
| − | {{Admon/important|Make Certain iptables rules are Flushed Before Saving|In the next section, you will learn to keep your iptables rules persistent, so they remain even if the Linux system is rebooted. If you do NOT flush the iptables rules prior to the next section, your lab6-checking script will not generate all OKs, and you may experience problems in lab7.}}
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| − | === Part 3: Making iptables Policies Persistent ===
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| − | | |
| − | 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.
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| − | | |
| − | | |
| − | :'''Perform the following steps:'''
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| − | | |
| − | # 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>
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| − | # 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>
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| − | # Make a backup of the file '''/etc/sysconfig/iptables''' by issuing the command:<br><b><code><span style="color:#3366CC;font-size:1.2em;">iptables-save > /etc/sysconfig/iptables.bk</span></code></b>
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| − | #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>
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| − | # Verify that the file '''/etc/sysconfig/iptables''' exists.
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| − | # Restart your iptables service and test your configuration.
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| − | | |
| − | | |
| − | # 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>
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| − | # 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>
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| − | # We have now possibly mislead a potential "hacker" to the true port for our ssh server's communication channel (port).
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| − | | |
| − | | |
| − | | |
| − | '''Answer INVESTIGATION 3 observations / questions in your lab log book.'''
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| − | | |
| − | = LAB 7 SIGN-OFF (SHOW INSTRUCTOR) =
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| − | {{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.}}
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| − | | |
| − | :'''Perform the Following Steps:'''
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| − | # Make certain ALL of your VMs are running.
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| − | # Switch to your '''c7host''' VM and '''su -''' into root.
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| − | # Change to the '''/root/bin''' directory.
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| − | # 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>
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| − | # Give the '''lab7-check.bash''' file execute permissions (for the file owner).
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| − | # Run the shell script and if any warnings, make fixes and re-run shell script until you receive "congratulations" message.
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| − | #Arrange proof of the following on the screen:<br><blockquote><span style="color:green;font-size:1.5em;">✓</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;">✓</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;"> OK </span></code></b> messages)</li></ul></blockquote><span style="color:green;font-size:1.5em;">✓</span> '''Lab7''' log-book filled out.
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| − | | |
| − | | |
| − | = Practice For Quizzes, Tests, Midterm & Final Exam =
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| − | | |
| − | # What port does sshd use by defaults?
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| − | # What file is used to configure sshd?
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| − | # What sftp commands are used to upload/download files?
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| − | # What kind of files are stored in the "~/.ssh/" directory?
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| − | # How do you determine whether the sshd service is running on your system or not?
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| − | # What is the purpose of the ~/.ssh/known_hosts file?
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| − | # What is the purpose of the ~/.ssh/authorized_keys file?
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| − | # Which system log file records each use of the sudo command?
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| − | # How do you stop the sshd service?
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| − | # How do you tunnel XWindows applications?
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| − | # What port is the default scp port?
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| − | # What port(s) is/are used by httpd service?
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| − | | |
| − | | |
| − | [[Category:OPS235]]
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