LPI Linux Certification/Secure Shell (SSH)
Candidates should be able to configure an SSH daemon. This objective includes managing keys and configuring SSH for users. Candidates should also be able to forward an application protocol over SSH and manage the SSH login.
- Key knowledge area(s):
- SSH (OpenSSH) configuration files, tools and utilities
- Differences between SSH versions 1 and 2
- Login restrictions for the superuser and the normal users
- Managing and using server and client keys to login with and without password
- Usage of XWindow and other application protocols through SSH tunnels
- Configuration of ssh-agent
- Usage of multiple connections from multiple hosts to guard against loss of connection to remote host following configuration changes
- The following is a partial list of the used files, terms and utilities:
- ~/.ssh/identity.pub and identity
Secure Shell (OpenSSH)
Description: The candidate should be able to configure sshd to allow or deny root logins, enable or disable X forwarding. This objective includes generating server keys, generating a user's public/private key pair, adding a public key to a user's authorized_keys file, and configuring ssh-agent for all users. Candidates should also be able to configure port forwarding to tunnel an application protocol over ssh, configure ssh to support the ssh protocol versions 1 and 2, disable non-root logins during system maintenance, configure trusted clients for ssh logins without a password, and make multiple connections from multiple hosts to guard against loss of connection to remote host following configuration changes.
Key files, terms, and utilities include:
ssh, sshd /etc/ssh/sshd_config ~/.ssh/identity.pub, ~/.ssh/identity ~/.ssh/authorized_keys .shosts, .rhosts
OpenSSH is a free, open source implementation of the SSH (Secure SHell) protocols. It replaces telnet, ftp, rlogin, rsh, and rcp with secure, encrypted network connectivity tools. OpenSSH supports versions 1.3, 1.5, and 2.0 of the SSH protocol.
If you use OpenSSH tools, you are enhancing the security of your machine. All communications using OpenSSH tools, including passwords, are encrypted. Telnet and ftp use plaintext passwords and send all information unencrypted. The information can be intercepted, the passwords can be retrieved, and then your system can be compromised by an unauthorized person logging in to your system using one of the intercepted passwords. The OpenSSH set of utilities should be used whenever possible to avoid these security problems. Another reason to use OpenSSH is that it automatically forwards the DISPLAY variable to the client machine. In other words, if you are running the X Window System on your local machine, and you log in to a remote machine using the ssh command, when you execute a program on the remote machine that requires X, it will be displayed on your local machine. This is convenient if you prefer graphical system administration tools but do not always have physical access to your server.
The ssh command is a secure replacement for the rlogin, rsh, and telnet commands. It allows you to log in to and execute commands on a remote machine.
Logging in to a remote machine with ssh is similar to using telnet. To log in to a remote machine named penguin.example.net, type the following command at a shell prompt: ssh penguin.example.net
The first time you ssh to a remote machine, you will see a message similar to the following: The authenticity of host 'penguin.example.net' can't be established.
DSA key fingerprint is 94:68:3a:3a:bc:f3:9a:9b:01:5d:b3:07:38:e2:11:0c. Are you sure you want to continue connecting (yes/no)? Type yes to continue. This will add the server to your list of known hosts as seen in the following message: Warning: Permanently added 'penguin.example.net' (DSA) to the list of known hosts.
Next, you'll see a prompt asking for your password for the remote machine. After entering your password, you will be at a shell prompt for the remote machine. If you use ssh without any command line options, the username that you are logged in as on the local client machine is passed to the remote machine. If you want to specify a different username, use the following command:
ssh -l username penguin.example.net
You can also use the syntax ssh email@example.com. The ssh command can be used to execute a command on the remote machine without logging in to a shell prompt. The syntax is ssh hostname command. For example, if you want to execute the command ls /usr/share/doc on the remote machine penguin.example.net, type the following command at a shell prompt:
ssh penguin.example.net ls /usr/share/doc
After you enter the correct password, the contents of /usr/share/doc will be displayed, and you will return to your shell prompt.
The scp command can be used to transfer files between machines over a secure, encrypted connection. It is similar to rcp.
The general syntax to transfer a local file to a remote system is scp localfile user@hostname:/newfilename. The localfile specifies the source, and the group of user@hostname:/newfilename specifies the destination. To transfer the local file shadowman to your account on penguin.example.net, type the following at a shell prompt (replace user with your username):
scp shadowman firstname.lastname@example.org:/home/user
This will transfer the local file shadowman to /home/user/shadowman on penguin.example.net. The general syntax to transfer a remote file to the local system is scp user@hostname:/remotefile /newlocalfile. The remotefile specifies the source, and newlocalfile specifies the destination.
Multiple files can be specified as the source files. For example, to transfer the contents of the directory /downloads to an existing directory called uploads on the remote machine penguin.example.net, type the following at a shell prompt:
scp /downloads/* email@example.com:/uploads/
The sftp utility can be used to open a secure, interactive FTP session. It is similar to ftp except that it uses a secure, encrypted connection. The general syntax is sftp firstname.lastname@example.org. Once authenticated, you can use a set of commands similar to using FTP. Refer to the sftp manual page for a list of these commands. To read the manual page, execute the command man sftp at a shell prompt. The sftp utility is only available in OpenSSH version 2.5.0p1 and higher.
Generating Key Pairs
If you do not want to enter your password every time you ssh, scp, or sftp to a remote machine, you can generate an authorization key pair.
Note: Separate Authorization Key Pairs You must have separate authorization key pairs for SSH Protocol 1 (RSA) and SSH Protocol 2 (DSA).
Warning : Each User Needs Their Own Key Pair !
Keys must be generated for each user. To generate keys for a user, follow the following steps as the user who wants to connect to remote machines. If you complete the following steps as root, only root will be able to use the keys.
Use the following steps to generate a DSA key pair. DSA is used by SSH Protocol 2 and is the default for Red Hat. 1. To generate a DSA key pair to work with version 2.0 of the protocol, type the following command at a shell prompt:
ssh-keygen -t dsa
Accept the default file location of ~/.ssh/id_dsa. Enter a passphrase different from your account password and confirm it by entering it again.
(A passphrase is a string of words and characters used to authenticate a user. Passphrases differ from passwords in that you can use spaces or tabs in the passphrase. Passphrases are generally longer than passwords because they are usually phrases instead of just a word.)
2. Change the permissions of your .ssh directory using the command chmod 755 ~/.ssh.
3. Copy the contents of ~/.ssh/id_dsa.pub to ~/.ssh/authorized_keys2 on the machine to which you want to connect. If the file ~/.ssh/authorized_keys2 doesn't exist, you can copy the file ~/.ssh/id_dsa.pub to the file ~/.ssh/authorized_keys2 on the other machine.
Use the following steps to generate a RSA key pair for version 2.0 of the SSH protocol.
1. To generate a RSA key pair to work with version 2.0 of the protocol, type the following command at a shell prompt:
ssh-keygen -t rsa
Accept the default file location of ~/.ssh/id_rsa. Enter a passphrase different from your account password and confirm it by entering it again. 
2. Change the permissions of your .ssh directory using the command chmod 755 ~/.ssh.
3. Append the contents of ~/.ssh/id_rsa.pub to ~/.ssh/authorized_keys2 on the machine to which you want to connect. If the file ~/.ssh/authorized_keys2 doesn't exist, you can copy the file ~/.ssh/id_rsa.pub to the file ~/.ssh/authorized_keys2 on the other machine.
Use the following steps to generate an RSA key pair, which is used by version 1 of the SSH Protocol.
1. To generate an RSA (for version 1.3 and 1.5 protocol) key pair, type the following command at a shell prompt:
Accept the default file location (~/.ssh/identity). Enter a passphrase different from your account password. Confirm the passphrase by entering it again.
2. Change the permissions of your .ssh directory and your keys with the commands chmod 755 ~/.ssh and chmod 644 ~/.ssh/identity.pub.
3. Copy the contents of ~/.ssh/identity.pub to the file ~/.ssh/authorized_keys on the machine to which you wish to connect. If the file ~/.ssh/authorized_keys doesn't exist, you can copy the file ~/.ssh/identity.pub to the file ~/.ssh/authorized_keys on the remote machine.
You can forward the X11 port through SSH to enable encrypted X11 connections. There's no need to export a DISPLAY variable or to call the xhost utility.
On the server-side you must check the file /etc/ssh/sshd_config to be sure that the "X11Forwarding" option is set to "yes".
On the client-side, use the -X option :
ssh -X user@remotehost
When the remote host prompt appears, start a X11 application:
A xterm window from the remote host will open on your local desktop.