OPS235 Lab 7 - CentOS6
- 1 Setup and Configure Secure Shell Services (ssh) Using Virtual Machines
- 1.1 Overview
- 1.2 Objectives
- 1.3 Required Materials (Bring to All Labs)
- 1.4 Prerequisites
- 1.5 Linux Command Online Reference
- 1.6 Resources on the web
- 1.7 Configuring & Establishing an SSH Connection
- 1.8 Using SSH & Other Secure Shell Utilities
- 1.9 Further Securing your Secure Shell Connection
- 1.10 Completing the lab
- 1.11 Preparing for Quizzes
Setup and Configure Secure Shell Services (ssh) Using Virtual Machines
- 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.
- 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
- Completion and Instructor "Sign-off" of Lab 6: OPS235 Lab 6 - CentOS6
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.
- OpenSSH should have been installed by default. Lets confirm this by issuing the command:
rpm -qa | grep ssh
- You should see a number of packages installed including
openssh-serverinstalls a service called
sshd, confirm this service is running by issuing the command:
service sshd status
- Now check that the sshd service is configured to start automatically:
chkconfig --list sshd
- Now that you know the service is running investigate what port number and protocol sshd uses by issuing the command:
- What protocol and port is the sshd process using?
- What is the state of the port?
- Why do UDP ports not have a state?
- Reissue the
netstatcommand without the
- What is the difference? How is the file /etc/services related to the difference?
netstatis a very useful command for anything to do with networking. Read its man page and make sure you understand its output.
- Make sure your
sshdservice 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.
- Establish an ssh connection to your centos3 VM using the command:
(Where 'ops235' is the account on centos3 and 'centos3' is the hostname of the centos3 VM.)
- 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.
- 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.
- When prompted enter your password for your ops235 account on centos3.
- Establish an ssh connection using your learn account from centos3 to centos2.
- Logout of your ssh connection by typing
- Check the state of the connection after logging out. Wait a few minutes and then check again. Record your observations.
- Make certain to exit all connections, and that your shell is located in your centos3 server. You can verify this by entering the command:
- Use the Internet to search for "TCP 3 way handshake" to see how TCP connections are established and closed.
Answer the Investigation 2 observations / questions in your lab log book.
Investigation 3: How do you establish an ssh connection using Public Key Authentication.
- Start by generating a keypair as your learn account on centos2 using the command:
- 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
- After generating the keys it prompts you for the location to save the keys. The default is
~/.sshYour private key will be saved as
id_rsaand your public key will be saved as
- Now issue the command
ssh-copy-id -i ~/.ssh/id_rsa.pub centos3
- Now we can ssh into centos3 from centos2 using two different authentication methods.
- Make certain to logout of your centos3 system. Use the
hostnamecommand 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.
- To connect to a remote host type the command:
- This will establish an interactive session after authentication.
helpto see the list of sftp commands at any time.
- The 2 main commands are
putto copy a file from the local host to the remote host (upload) and
getto copy a file from the remote host to the local host (download).
- Try using
sftpto transfer files back and forth between hosts.
- As you did previously you can also use the
scpcommand to copy files to and from remote hosts and even from one remote host to another.
scpto 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
- Experiment with
scpto copy a file from centos3 directly to centos1.
Answer the Investigation 4 observations / questions in your lab log book.
Investigation 5: How do you use ssh to tunnel X.
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
firefox and the application will run on the remote host but be displayed on the local host.
- From centos2
sshto centos1 using the following command:
ssh -X -C user@centos1(Where 'user' is your learn account on centos1). The
-Xenables the forwarding of X window information, and the
-Cenables compression for better performance.
- Once connected run the
geditapplication. (Gnome Text Editor)
- The gedit window will display on centos2 but it is running centos1.
- Enter some text and save a file with
- Where was the file saved?
- Experiment with running other GUI applications through
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.
- You will be working with the 2nd scenario of bypassing a firewall that blocks http traffic.
- In this investigation centos1 will be your http server and centos2 will be your client.
- On the HTTP server make sure the Apache web server is installed by typing the command:
rpm -q httpd
- If it is installed check the configuration of the service to see if it is automatically started at any runlevels by using the
- If it has not been started automatically start the service using the service command.
- Confirm that httpd is listening to TCP/80 using the netstat command.
- Create a small html document called
/var/www/html/index.htmlthat displays a short message.*
- On the centos1 (the http server) confirm everything is working locally by using a browser to connect to http://localhost
- Set the default firewall configuration on centos1 to REJECT incoming requests to http (TCP/80)
- Confirm that you can't connect by using firefox on centos to connect to centos1 http://centos1/
- On centos2 confirm that the httpd service is stopped so it cannot interfere with your observations.
- 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.
- 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
- Once the tunnel is established use netstat to verify the port 20808 is listening on centos2
- Now using the browser on centos2 connect to http://localhost:20808
- You should see the index.html page on centos1.
- 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
- 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)
- The next change you can make is to prevent the root account from logging in to sshd altogether.
- Edit the file
/etc/ssh/sshd_configand look for the option
PermitRootLogin. Uncomment the option (or add the option if it does not appear) and change the option value to
- Even better, it is possible to restrict access to just specific users that require it.
- Edit the file
/etc/ssh/sshd_configand add a new option of
"AllowUsers account"using your login account for account.
- In order for these changes to be effective, issue the following command to restart the sshd service:
service sshd restart
- 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.
- Next change the default port number that sshd uses (TCP:22).
- Edit the sshd configuration file again and change the port number it uses to 2200.
- Restart the service.
- Confirm the new port is being used with a
- 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
- To test the new port connect to centos1 from centos2 using the following command:
ssh -p 2200 user@centos1
- Finally as a system administrator you should periodically monitor your system logs for unauthorized login attempts.
- On CentOS systems the log file that is used is
- It also logs all uses of the
- 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.
- 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
Arrange proof of the following on the screen:
- have configured sshd to allow connections over a non default port.
- have logged in to a VM using public key authentication
- have scp'd and sftp'd files to a VM.
- have tunneled Xwindows applications through ssh
- have tunneled http through firewall using ssh
- have secured ssh against root access
Preparing for 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?