OPS235 Lab 7 - CentOS6

From CDOT Wiki
Jump to: navigation, search

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

Overview

  • In this lab, you will learn how to use the ssh, scp, sftp commands to work between computer systems via a network.
  • In addition, you will learn various methods of how to communicate between computer networks using SSH tunneling, and simplify secure network communication by using Public Key Authentication.

Objectives

  • 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 (Bring to All Labs)

  • CentOS 6.5 x86_64 Live DVD
  • CentOS 6.5 x86_64 Installation DVD1
  • SATA Hard Disk (in removable disk tray)
  • USB Memory Stick
  • Lab Logbook

Prerequisites

Linux Command Online Reference

Each Link below displays online manpages for each command (via http://linuxmanpages.com):

Networking Utilities Additional Utilities: Configuration Files:


Resources on the web

Additional links to tutorials and HOWTOs:


Configuring & Establishing an SSH Connection

Investigation 1: How do you enable the sshd service.

Note.png
Use your c6host computer system
Complete the following steps on your CentOS 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 installs a service called sshd, confirm this service is running by issuing the command:
    service sshd status
  4. Now check that the sshd service is configured to start automatically: chkconfig --list sshd
  5. Now that you know the service is running investigate what port number and protocol sshd uses by issuing the command:
    netstat -atunp
  6. What protocol and port is the sshd process using?
  7. What is the state of the port?
  8. Why do UDP ports not have a state?
  9. Reissue the netstat command without the n option.
  10. What is the difference? How is the file /etc/services related to the difference?
  11. netstat is a very useful command for anything to do with networking. Read its man page and make sure you understand its output.
  12. Make sure your sshd service is running on all 3 of your VM's

Answer the Investigation 1 observations / questions in your lab log book.

Investigation 2: How do you establish an ssh connection.

Note.png
Use your centos2 VM
Complete this investigation on your centos2 VM.
  1. Establish an ssh connection to your centos3 VM using the command:
    ssh ops235@centos3
    (Where 'ops235' is the account on centos3 and 'centos3' is the hostname of the centos3 VM.)
  2. You should receive a message similar to the following:
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.
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 to and you would receive a message like this
  1. Answer yes to add to the list of known hosts.
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: POSSIBLE DNS SPOOFING DETECTED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
The RSA host key for centos3 has changed,
and the key for the according IP address 192.168.235.13
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
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
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
96:92:62:15:90:ec:40:12:47:08:00:b8:f8:4b:df:5b.
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 centos3 has changed and you have requested strict
checking.
Host key verification failed.
Note.png
POSSIBLE DNS SPOOFING DETECTED
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 reinstalled.
  1. When prompted enter your password for your ops235 account on centos3.
  2. Establish an ssh connection using your learn account from centos3 to centos2.
Note.png
Proof of Established SSH Connection
When you have both ssh connections established between centos2 and centos3 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 centos2 to centos3 and the other represents the connection from centos3 to centos2. 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 centos3 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.
Idea.png
Methods of Authenticating User Identity
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. We will be looking at Public Key Authentication in the next investigation

Answer the Investigation 2 observations / questions in your lab log book.

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

Note.png
Use your centos2 VM
Complete this investigation on your centos2 VM.
Note.png

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 centos2 using the command:
    ssh-keygen
  2. That should generate output similar to the following:
Generating public/private rsa key pair.
Enter file in which to save the key (/home/user1/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/user1/.ssh/id_rsa.
Your public key has been saved in /home/user1/.ssh/id_rsa.pub.
The key fingerprint is:
93:58:20:56:72:d7:bd:14:86:9f:42:aa:82:3d:f8:e5 user1@centos2
  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_rsa and your public key will be saved as id_rsa.pub
Idea.png
Lengthy Passphrases
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. Now issue the command ssh-copy-id -i ~/.ssh/id_rsa.pub centos3
  2. Now we can ssh into centos3 from centos2 using two different authentication methods.
  3. Make certain to logout of your centos3 system. Use the hostname command to verify you are back in your centos2 server.

Answer the Investigation 3 observations / questions in your lab log book.

Using SSH & Other Secure Shell Utilities

Investigation 4: How do you use scp and sftp.

Note.png
Use your centos2 VM
Complete this investigation on your centos2 VM.
Note.png

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@centos3
  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 centos3 host into the /tmp directory. (The path on a remote host follows the :) using the command:
    scp /etc/services ops235@centos3:/tmp
  8. Experiment with scp to copy a file from centos3 directly to centos1.
Idea.png
SELinux
SELinux may prevent ssh from accessing your home directories on centos1 because you created a new filesystem there. You can reset the security context of the /home directory with this command: restorecon -Rv /home


Answer the Investigation 4 observations / questions in your lab log book.


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

Note.png
Use your centos2 and centos1 VMs
Complete this investigation on your centos2 and centos1 VMs.
Idea.png
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.

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 centos2 ssh to centos1 using the following command:
    ssh -X -C user@centos1 (Where 'user' is your learn account on centos1). 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 centos2 but it is running centos1.
  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.

Answer the Investigation 5 observations / questions in your lab log book.

Further Securing your Secure Shell Connection

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

Note.png
Use your centos2 and centos1 VMs
Complete this investigation on your centos2 and centos1 VMs.
Note.png

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 centos1 will be your http server and centos2 will be your client.
  3. On the HTTP server 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 using the chkconfig h command.
  5. If it has not been started automatically start the service using the service command.
  6. Confirm that httpd is listening to TCP/80 using the netstat command.
  7. Create a small html document called /var/www/html/index.html that displays a short message.*
  8. On the centos1 (the http server) confirm everything is working locally by using a browser to connect to http://localhost
  9. Set the default firewall configuration on centos1 to REJECT incoming requests to http (TCP/80)
  10. Confirm that you can't connect by using firefox on centos to connect to centos1 http://centos1/
  11. On centos2 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.
Tunnel.png
  1. Establish a tunnel using a local port on centos2 of 20808, that connects to the remote port on centos1 of 80, using the following command on centos2:
    ssh -L 20808:centos1:80 user@centos1
Note.png
Note!
The -L (which means Local port) takes one argument of
<local-port>:<connect-to-host>:<connect-to-port>
The command basically connects your local port of 20808 to the remote port of 80 on centos1. This means all requests to 20808 on the localhost (centos2) are actually tunnelled through your ssh connection to port 22 on centos1 and then delivered to port 80 on centos1, bypassing the firewall.
  1. Once the tunnel is established use netstat to verify the port 20808 is listening on centos2
  2. Now using the browser on centos2 connect to http://localhost:20808
  3. You should see the index.html page on centos1.
  4. Close the ssh connection and verify that the port 20808 is no longer listening.

Answer the Investigation 6 observations / questions in your lab log book.

Investigation 7: How do you make sshd more secure

Note.png
Note!
Complete this investigation on your centos2 and centos1 VM's.
Note.png

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.
Idea.png
Tip!
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 centos1 from centos2 using the following command:
    ssh -p 2200 user@centos1
Important.png
Cannot connect via SSH?
To fix issues with the ability to ssh, on both machines:
  • Ensure ssh is running. Systemctl status sshd.service.
  • Disable selinux by going into /etc/selinux/config and change "enforcing" to "disabled"; "targeted" to "minimum".
  • If your are still encountering problems flush iptables.
Idea.png
Tip!
For scp access the option to be used is: scp -P 2200
Idea.png
Tip!
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 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 the Investigation 7 observations / questions in your lab log book.

Completing the lab

Important.png
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 Quizzes

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