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OPS235 Lab 7 - Fedora17

Revision as of 20:43, 19 March 2011 by Cheping (talk | contribs) (Investigation 1: How do you enable the sshd service.)

Setup and Configure Secure Shell Services (ssh) Using Virtual Machines


  • To set up, configure Secure Shell Services (ssh/sshd)
  • To use the ssh, scp, and sftp clients to access another host securely
  • Use ssh to tunnel X applications
  • Use ssh to tunnel other traffic
  • To customize sshd to create a more private, secure system


Required materials

  • Fedora 12 Live CD or a classmate on the same pod
  • One SATA hard disk in a removable drive tray with Fedora host and 3 Fedora Virtual Machines installed
  • Completion of Lab 6

Lab Preparation

Update your systems
It is advisable to perform a yum update on your Fedora host and all 3 VM's.
Backup your VMs before proceeding
If you did not do it at the end of Lab 6, stop all of your VMs and backup your VM disk images.

Lab Investigations

Investigation 1: How do you enable the sshd service.

Complete the following steps on your fedora host.
  1. OpenSSH should have been installed by default. Lets confirm this by issuing the command:
    rpm -qa | grep ssh
  2. You should see a number of packages installed including openssh-clients and openssh-server
  3. openssh-server<code> installs a service called <code>sshd<code>, confirm this service is running by issuing the command:
    <code>service sshd status
  4. Now check that the sshd service is configured to start for runlevels 2, 3, 4, & 5, by issuing the command:
    chkconfig --list sshd
  5. If the service is not configured correctly fix it by issuing the command:
    chkconfig --level 2345 sshd on
  6. Now that you know the service is running investigate what port number and protocol sshd uses by issuing the command:
    netstat -atunp
  7. What protocol and port is the sshd process using?
  8. What is the state of the port?
  9. Why do UDP ports not have a state?
  10. Reissue the netstat command without the n option.
  11. What is the difference?
  12. The n option tells netstat to list everything with numerical values, without it netstat resolves IP addresses and port numbers to host names and protocol names using the files /etc/hosts and /etc/services
  13. netstat is a very useful command for anything to do with networking. Read its man page and make sure you understand its output.
  14. Make sure your sshd service is running on all 3 of your VM's
  15. Answer the Investigation 1 question in your lab log book.

Investigation 2: How do you establish an ssh connection.

Complete this investigation on your fedora2 VM.
  1. Establish an ssh connection to your fedora3 VM using the command:
    ssh ops235@fedora3
  2. Where 'ops235' is the account on fedora3 and 'fedora3' is the hostname of the fedora3 VM.
  3. You should receive a message similar to the following:
The authenticity of host 'fedora3 (' 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 'fedora3' (RSA) to the list of known hosts.

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 .
  1. Answer yes to add to the list of 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 to and you would receive a message like this:
The RSA host key for fedora3 has changed,
and the key for the according IP address
is unchanged. This could either mean that
DNS SPOOFING is happening or the IP address for the host
and its host key have changed at the same time.
Offending key for IP in /home/user1/.ssh/known_hosts:10
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
Please contact your system administrator.
Add correct host key in /home/user1/.ssh/known_hosts to get rid of this message.
Offending key in /home/user1/.ssh/known_hosts:53
RSA host key for fedora3 has changed and you have requested strict
Host key verification failed.

If you receive a message like this 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.
  1. When prompted enter your password for your ops235 account on fedora3.
  2. Establish an ssh connection using your learn account from fedora3 to fedora2.

When you have both ssh connections established between fedora2 and fedora3 check your network connections using the netstat command. You should now see at least 2 TCP connections with a state of ESTABLISHED. One connection represents the connection from fedora2 to fedora3 and the other represents the connection from fedora3 to fedora2. You should also see that you still have ssh listening to TCP port 22. Notice that the client side of each connection uses a random port number in the upper ranges. This common behaviour for client side applications.
  1. Logout of your ssh connection by typing exit.
  2. Check the state of the connection after logging out. Wait a few minutes and then check again. Record your observations.
  3. Make certain to exit all connections, and that your shell is located in your Fedora2 server. You can verify this by entering the command: hostname
  4. Use the Internet to search for "TCP 3 way handshake" to see how TCP connections are established and closed.
In this part of the lab you established 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.
  1. Answer the Investigation 2 question in your lab log book.

Investigation 3: How do you establish an ssh connection using Public Key Authentication.

Complete this investigation on your fedora2 VM.

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. This means that when we connect, ssh can use the private key that only exists on my system in my account, to encrypt a message. That message can be decrypted by anybody with the corresponding public key. 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. This basic method of authentication is used extensively in many network communications protocols that require the ability to authenticate identity.
  1. Start by generating a keypair as your learn account on fedora2 using the command:
    ssh-keygen -t dsa
  2. That should generate output similar to the following:
Generating public/private dsa key pair.
Enter file in which to save the key (/home/user1/.ssh/id_dsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/user1/.ssh/id_dsa.
Your public key has been saved in /home/user1/.ssh/
The key fingerprint is:
93:58:20:56:72:d7:bd:14:86:9f:42:aa:82:3d:f8:e5 user1@fedora2
  1. 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_dsa and your public key will be saved as
You will then be prompted for a passphrase. The passphrase must be entered in order to use your private key. Passphrases are more secure than passwords and should be lengthy, hard to guess and easy to remember. For example one passphrase 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 passphrase 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.
  1. Once your keys have been saved you should check to make sure the permissions of the .ssh directory and your key files are secure.
  2. Use the following commands to secure them:
    chmod 700 ~/.ssh
    chmod 600 ~/.ssh/id_dsa*
  3. The next step is to copy your public key to fedora3 (the remote host). You can use the scp command to do it.
  4. scp (secure copy) is used to copy files between hosts over the ssh protocol. The files are sent over an encrypted channel as is all ssh traffic.
  5. Issue the command:
    scp ~/.ssh/ ops235@fedora3:
  6. This will copy your public key to your ops235 home directory on fedora3.
  7. The : is important as it separates the hostname from the path where it is copied to.
  8. The default location is the users home directory on the remote host but you can also use an absolute or relative path after the :
  9. Enter your password to complete the copy.
  10. Now ssh to fedora3 using a password to authenticate.
  11. Once logged in to fedora3 we need to add the public key to your list of authorized_keys
  12. he safest way to do this is to append the contents of to the .ssh/authorized_keys file. To do this use the command:
    cat ~/ >> ~/.ssh/authorized_keys
You don't want to overwrite the file as it may contain multiple public keys. Make sure you are using the append redirection >> and not >.
  1. Once again you should secure the .ssh directory and the authorized_keys file using the following commands:
    chmod 700 ~/.ssh
    chmod 600 ~/.ssh/authorized_keys
  2. Logout of fedora3 and log back in again.
  3. You should be prompted to enter your passphrase to unlock your private key.
If you are connecting from terminal started from in your GUI environment you can also choose it to remember your passphrase every time you login to the GUI.
  1. Now we can ssh into fedora3 from fedora2 using 2 different authentication methods.
  2. MaKe certain to logout of your Fedora3 system. Use the hostname command to verify you are back in your Fedora2 server.
  3. Answer the Investigation 3 question in your lab log book.

Investigation 4: How do you use scp and sftp.

Complete this investigation on your fedora2 VM.

There are 2 common command line tools for transferring files between hosts over an encrypted ssh connection, scp and sftp. sftp is an interactive file transfer program that functions much like an ftp client.
  1. To connect to a remote host type the command:
    sftp ops235@fedora3
  2. This will establish an interactive session after authentication.
  3. Type help to see the list of sftp commands at any time.
  4. The 2 main commands are put to copy a file from the local host to the remote host (upload) and get to copy a file from the remote host to the local host (download).
  5. Try using sftp to transfer files back and forth between hosts.
  6. As you did previously you can also use the scp command to copy files to and from remote hosts and even from one remote host to another.
  7. Use scp to copy your services file to the fedora3 host into the /tmp directory. (The path on a remote host follows the :) using the command:
    scp /etc/services ops235@fedora3:/tmp
  8. Experiment with scp to copy a file from fedora3 directly to fedora1.
SELinux may prevent ssh from accessing your home directories on Fedora 1 because you created a new filesystem there. You can reset the security context of the /home directory with this command: restorecon -Rv /home
  1. Answer the Investigation 4 question in your lab log book.

Investigation 5: How do you use ssh to tunnel X.

Complete this investigation on your fedora2 and fedora1 VM's.
fedora1 VM iptables and ssh service
You may need to adjust the firewall on your fedora1 host to complete this section, and verify that the sshd service is running on that VM.

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.

  1. From fedora2 ssh to fedora1 using the following command:
    ssh -X -C user@fedora1 (Where 'user' is your learn account on fedora1). The -X enables the forwarding of X window information, and the -C enables compression for better performance.
  2. Once connected run the gedit application. (Gnome Text Editor)
  3. The gedit window will display on fedora2 but it is running fedora1.
  4. Enter some text and save a file with gedit.
  5. Exit gedit.
  6. Where was the file saved?
  7. Experiment with running other GUI applications through ssh.
  8. Answer the Investigation 5 question in your lab log book.

Investigation 6: How do you use ssh to tunnel other traffic.

Complete this investigation on your fedora2 and fedora1 VM's.

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. Alternatively you could use it to circumvent a firewall that is blocking traffic you wish to use but allows ssh traffic to pass through.
  1. You will be working with the 2nd scenario of bypassing a firewall that blocks http traffic.
  2. In this investigation fedora1 will be your http server and fedora2 will be your client.
  3. On the HTTP server (fedora1), make sure the Apache web server is installed by typing the command:
    rpm -q httpd
  4. If it is installed check the configuration of the service to see if it is automatically started at any runlevels by issuing the command:
    chkconfig --list httpd
  5. If it has not been started automatically start the service using the command:
    ** service httpd start
  6. Confirm that httpd is listening to TCP/80 using the command:
    netstat -atnp
  7. Create a small html document called /var/www/html/index.html that displays a short message.*
  8. On the fedora1 (the http server) confirm everything is working locally by using a browser to connect to http://localhost
  9. The default firewall configuration on fedora1 is to REJECT incoming requests to http (TCP/80)
  10. Confirm that you can't connect by using firefox on fedora2 to connect to fedora1 http://fedora1/
  11. On fedora2 confirm that the httpd service is stopped so it cannot interfere with your observations.
  12. The next step is to establish a tunnel. 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.
  1. Establish a tunnel using a local port on fedora2 of 20808, that connects to the remote port on fedora1 of 80, using the following command on fedora2:
    ssh -L 20808:fedora1:80 user@fedora1
The -L (which means Local port) takes one argument of
The command basically connects your local port of 20808 to the remote port of 80 on fedora1. This means all requests to 20808 on the localhost (fedora2) are actually tunnelled through your ssh connection to port 22 on fedora1 and then delivered to port 80 on fedora1, bypassing the firewall.
  1. Once the tunnel is established use netstat to verify the port 20808 is listening on fedora2
  2. Now using the browser on fedora2 connect to http://localhost:20808
  3. You should see the index.html page on fedora1.
  4. Close the ssh connection and verify that the port 20808 is no longer listening.
  5. nswer the Investigation 6 question in your lab log book.

Investigation 7: How do you make sshd more secure.

Complete this investigation on your fedora2 and fedora1 VM's.

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.
Making your root password (and all other accounts!) both quite complex but easy to remember is not hard. Passwords should be a minimum of 8 characters long, preferably longer, contain upper and lower case letters, numbers, and special characters. A good example of a strong password might be "LotR3--RotK." This is not that hard to remember as it corresponds to a book title. "Lord of the Rings 3 Return of the King." The password "P@ssw0rd!" is not as good because it is quite obvious and common.
  1. 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)
  2. The next change you can make is to prevent the root account from logging in to sshd altogether.
  3. Edit the file /etc/ssh/sshd_config and look for the option PermitRootLogin. Uncomment the option (or add the option if it does not appear) and change the option value to "no".
  4. Even better, it is possible to restrict access to just specific users that require it.
  5. Edit the file /etc/ssh/sshd_config and add a new option of "AllowUsers account" using your login account for account.
  6. In order for these changes to be effective, issue the following command to restart the sshd service:
    service sshd restart
  7. 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.
  8. Next change the default port number that sshd uses (TCP:22).
  9. Edit the sshd configuration file again and change the port number it uses to 2200.
  10. Restart the service.
  11. Confirm the new port is being used with a netstat command.
  12. 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:
    iptables -I INPUT -p tcp -s0/0 --dport 22 -j DROP
    iptables -I INPUT -p tcp -s0/0 --dport 2200 -j ACCEPT
  13. To test the new port connect to fedora1 from fedora2 using the following command:
    ssh -p 2200 user@fedora1
For scp access the option to be used is: scp -P 2200
For more ideas on making sshd more secure consult the HOW-TO link above.
  1. Finally as a system administrator you should periodically monitor your system logs for unauthorized login attempts.
  2. On Fedora 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.
  6. Answer the Investigation 7 question in your lab log book.

Completing the lab

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

Arrange proof of the following on the screen:

  1. have configured sshd to allow connections over a non default port.
  2. have logged in to a VM using public key authentication
  3. have scp'd and sftp'd files to a VM.
  4. have tunneled Xwindows applications through ssh
  5. have tunneled http through firewall using ssh
  6. have secured ssh against root access

Preparing for the Quizzes

  • What port does sshd use by defaults?
  • 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?
  • How do you determine whether the sshd service is running on your system or not?
  • What is the purpose of the ~/.ssh/known_hosts file?
  • What is the purpose of the ~/.ssh/authorized_keys file?
  • Which system log file records each use of the sudo command?
  • How do you stop the sshd service?
  • How do you tunnel XWindows applications?
  • What port is the default scp port?
  • What port(s) is/are used by httpd service?