OPS235 Lab 7 - Fedora17

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

Important/warning!
This lab is heavily under construction, It will undergo major changes until this warning is removed.

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

Reference

• man pages for ssh, ssh-keygen, sshd_config, ssh_config, scp, netstat, sftp, ifconfig, ping, arp, netstat, service
• A good ssh tutorial
• A good HOW-TO to make ssh more secure

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.

Note!
Complete the following steps on your fedora host.

• 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-clients and openssh-server
• openssh-server<code> installs a service called <code>sshd, confirm this service is running by issuing the command:
  • service sshd status
• Now check that the sshd service is configured to start for runlevels 2, 3, 4, & 5, by issuing the command:
  • chkconfig --list sshd
• If the service is not configured correctly fix it by issuing the command:
  • chkconfig --level 2345 sshd on
• Now that you know the service is running investigate what port number and protocol sshd uses by issuing the command:
  • netstat -atunp
• 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 netstat command without the n option.
• What is the difference?
• 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
• netstat is a very useful command for anything to do with networking. Read its man page and make sure you understand its output.
• Make sure your sshd service is running on all 3 of your VM's
• Answer the Investigation 1 question in your lab log book.

Investigation 2: How do you establish an ssh connection.

Note!
Complete this investigation on your fedora2 VM.

• As your learn account establish an ssh connection to your fedora3 VM using the command:
  • ssh ops235@fedora3
• Where 'ops235' is the account on fedora3 and 'fedora3' is the hostname of the fedora3 VM.
• You should receive a message similar to the following:

The authenticity of host 'fedora3 (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 '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 .

• 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:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: POSSIBLE DNS SPOOFING DETECTED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
The RSA host key for fedora3 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 fedora3 has changed and you have requested strict
checking.
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.

• When prompted enter your password for your ops235 account on fedora3.
• 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.

• Logout of your ssh connection by typing exit.
• Check the state of the connection after logging out. Wait a few minutes and then check again. Record your observations.
• Use the Internet to search for "TCP 3 way handshake" to see how TCP connections are established and closed.

Tip:
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.

• Answer the Investigation 2 question in your lab log book.

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

Note!
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.

• Start by generating a keypair as your learn account on fedora2 using the command:
  • ssh-keygen -t dsa
• 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/id_dsa.pub.
The key fingerprint is:
93:58:20:56:72:d7:bd:14:86:9f:42:aa:82:3d:f8:e5 user1@fedora2

• 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 id_dsa.pub

Tip:
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.

• Once your keys have been saved you should check to make sure the permissions of the .ssh directory and your key files are secure.
• Use the following commands to secure them:
  • chmod 700 ~/.ssh
  • chmod 600 ~/.ssh/id_dsa*
• The next step is to copy your public key to fedora3 (the remote host). You can use the scp command to do it.
• 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.
• Issue the command:
  • scp ~/.ssh/id_dsa.pub ops235@fedora3:
• This will copy your public key to your ops235 home directory on fedora3.
• The : is important as it separates the hostname from the path where it is copied to.
• The default location is the users home directory on the remote host but you can also use an absolute or relative path after the :
• Enter your password to complete the copy.
• Now ssh to fedora3 using a password to authenticate.
• Once logged in to fedora3 we need to add the public key to your list of authorized_keys
• The safest way to do this is to append the contents of id_dsa.pub to the .ssh/authorized_keys file. To do this use the command:
  • cat ~/id_dsa.pub >> ~/.ssh/authorized_keys

Note!
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 >.