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13.3. Configuring an OpenSSH Client

To connect to an OpenSSH server from a client machine, you must have the openssh-clients and openssh packages installed on the client machine.

13.3.1. Using the ssh Command

The ssh command is a secure replacement for the rlogin, rsh, and telnet commands. It allows you to log in to a remote machine as well as 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 (~/.ssh/known_hosts/) as seen in the following message:

Warning: Permanently added 'penguin.example.net' (RSA) to the list of known hosts.

Next, you will 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 do not specify a username 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 username@penguin.example.net

You can also use the syntax ssh -l username penguin.example.net.

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 the remote directory /usr/share/doc will be displayed, and you will return to your local shell prompt.

13.3.2. Using the scp Command

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 as follows:

scp <localfile> username@tohostname:<remotefile>

The <localfile> specifies the source including path to the file, such as /var/log/maillog. The <remotefile> specifies the destination, which can be a new filename such as /tmp/hostname-maillog. For the remote system, if you do not have a preceding /, the path will be relative to the home directory of username, typically /home/username/.

To transfer the local file shadowman to the home directory of your account on penguin.example.net, type the following at a shell prompt (replace username with your username):

scp shadowman username@penguin.example.net:shadowman

This will transfer the local file shadowman to /home/username/shadowman on penguin.example.net. Alternately, you can leave off the final shadowman in the scp command.

The general syntax to transfer a remote file to the local system is as follows:

scp username@tohostname:<remotefile> <newlocalfile>

The <remotefile> specifies the source including path, and <newlocalfile> specifies the destination including path.

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/* username@penguin.example.net:uploads/

13.3.3. Using the sftp Command

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 username@hostname.com. Once authenticated, you can use a set of commands similar to those used by FTP. Refer to the sftp man page for a list of these commands. To read the man page, execute the command man sftp at a shell prompt. The sftp utility is only available in OpenSSH version 2.5.0p1 and higher.

13.3.4. Generating Key Pairs

If you do not want to enter your password every time you use ssh, scp, or sftp to connect to a remote machine, you can generate an authorization key pair.

Keys must be generated for each user. To generate keys for a user, use the following steps as the user who wants to connect to remote machines. If you complete the steps as root, only root will be able to use the keys.

Starting with OpenSSH version 3.0, ~/.ssh/authorized_keys2, ~/.ssh/known_hosts2, and /etc/ssh_known_hosts2 are obsolete. SSH Protocol 1 and 2 share the ~/.ssh/authorized_keys, ~/.ssh/known_hosts, and /etc/ssh/ssh_known_hosts files.

Red Hat Enterprise Linux 4 uses SSH Protocol 2 and RSA keys by default.

TipTip
 

If you reinstall and want to save your generated key pair, backup the .ssh directory in your home directory. After reinstalling, copy this directory back to your home directory. This process can be done for all users on your system, including root.

13.3.4.1. Generating an RSA Key Pair for Version 2

Use the following steps to generate an RSA key pair for version 2 of the SSH protocol. This is the default starting with OpenSSH 2.9.

  1. To generate an RSA key pair to work with version 2 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.

    The public key is written to ~/.ssh/id_rsa.pub. The private key is written to ~/.ssh/id_rsa. Never distribute your private key to anyone.

  2. Change the permissions of the .ssh directory using the following command:

    chmod 755 ~/.ssh
  3. Copy the contents of ~/.ssh/id_rsa.pub into the file ~/.ssh/authorized_keys on the machine to which you want to connect. If the file ~/.ssh/authorized_keys exist, append the contents of the file ~/.ssh/id_rsa.pub to the file ~/.ssh/authorized_keys on the other machine.

  4. Change the permissions of the authorized_keys file using the following command:

    chmod 644 ~/.ssh/authorized_keys
  5. If you are running GNOME, skip to Section 13.3.4.4 Configuring ssh-agent with GNOME. If you are not running the X Window System, skip to Section 13.3.4.5 Configuring ssh-agent.

13.3.4.2. Generating a DSA Key Pair for Version 2

Use the following steps to generate a DSA key pair for version 2 of the SSH Protocol.

  1. To generate a DSA key pair to work with version 2 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.

    TipTip
     

    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 a single word.

    The public key is written to ~/.ssh/id_dsa.pub. The private key is written to ~/.ssh/id_dsa. It is important never to give anyone the private key.

  2. Change the permissions of the .ssh directory with the following command:

    chmod 755 ~/.ssh
  3. Copy the contents of ~/.ssh/id_dsa.pub into the file ~/.ssh/authorized_keys on the machine to which you want to connect. If the file ~/.ssh/authorized_keys exist, append the contents of the file ~/.ssh/id_dsa.pub to the file ~/.ssh/authorized_keys on the other machine.

  4. Change the permissions of the authorized_keys file using the following command:

    chmod 644 ~/.ssh/authorized_keys
  5. If you are running GNOME, skip to Section 13.3.4.4 Configuring ssh-agent with GNOME. If you are not running the X Window System, skip to Section 13.3.4.5 Configuring ssh-agent.

13.3.4.3. Generating an RSA Key Pair for Version 1.3 and 1.5

Use the following steps to generate an RSA key pair, which is used by version 1 of the SSH Protocol. If you are only connecting between systems that use DSA, you do not need an RSA version 1.3 or RSA version 1.5 key pair.

  1. To generate an RSA (for version 1.3 and 1.5 protocol) key pair, type the following command at a shell prompt:

    ssh-keygen -t rsa1

    Accept the default file location (~/.ssh/identity). Enter a passphrase different from your account password. Confirm the passphrase by entering it again.

    The public key is written to ~/.ssh/identity.pub. The private key is written to ~/.ssh/identity. Do not give anyone the private key.

  2. Change the permissions of your .ssh directory and your key with the commands chmod 755 ~/.ssh and chmod 644 ~/.ssh/identity.pub.

  3. Copy the contents of ~/.ssh/identity.pub into the file ~/.ssh/authorized_keys on the machine to which you wish to connect. If the file ~/.ssh/authorized_keys does not exist, you can copy the file ~/.ssh/identity.pub to the file ~/.ssh/authorized_keys on the remote machine.

  4. If you are running GNOME, skip to Section 13.3.4.4 Configuring ssh-agent with GNOME. If you are not running GNOME, skip to Section 13.3.4.5 Configuring ssh-agent.

13.3.4.4. Configuring ssh-agent with GNOME

The ssh-agent utility can be used to save your passphrase so that you do not have to enter it each time you initiate an ssh or scp connection. If you are using GNOME, the openssh-askpass-gnome package contains the application used to prompt you for your passphrase when you log in to GNOME and save it until you log out of GNOME. You will not have to enter your password or passphrase for any ssh or scp connection made during that GNOME session. If you are not using GNOME, refer to Section 13.3.4.5 Configuring ssh-agent.

To save your passphrase during your GNOME session, follow the following steps:

  1. You will need to have the package openssh-askpass-gnome installed; you can use the command rpm -q openssh-askpass-gnome to determine if it is installed or not. If it is not installed, install it from your Red Hat Enterprise Linux CD-ROM set, from a Red Hat FTP mirror site, or using Red Hat Network.

  2. Select Main Menu Button (on the Panel) => Preferences => More Preferences => Sessions, and click on the Startup Programs tab. Click Add and enter /usr/bin/ssh-add in the Startup Command text area. Set it a priority to a number higher than any existing commands to ensure that it is executed last. A good priority number for ssh-add is 70 or higher. The higher the priority number, the lower the priority. If you have other programs listed, this one should have the lowest priority. Click Close to exit the program.

  3. Log out and then log back into GNOME; in other words, restart X. After GNOME is started, a dialog box will appear prompting you for your passphrase(s). Enter the passphrase requested. If you have both DSA and RSA key pairs configured, you will be prompted for both. From this point on, you should not be prompted for a password by ssh, scp, or sftp.

13.3.4.5. Configuring ssh-agent

The ssh-agent can be used to store your passphrase so that you do not have to enter it each time you make a ssh or scp connection. If you are not running the X Window System, follow these steps from a shell prompt. If you are running GNOME but you do not want to configure it to prompt you for your passphrase when you log in (refer to Section 13.3.4.4 Configuring ssh-agent with GNOME), this procedure will work in a terminal window, such as an XTerm. If you are running X but not GNOME, this procedure will work in a terminal window. However, your passphrase will only be remembered for that terminal window; it is not a global setting.

  1. At a shell prompt, type the following command:

    exec /usr/bin/ssh-agent $SHELL
  2. Then type the command:

    ssh-add 

    and enter your passphrase(s). If you have more than one key pair configured, you will be prompted for each one.

  3. When you log out, your passphrase(s) will be forgotten. You must execute these two commands each time you log in to a virtual console or open a terminal window.