To connect to an OpenSSH server from a client machine, you must have the
openssh-clients and openssh
packages installed on the client machine.
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:
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
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
/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
username@tohostname:/newfilename. The
localfile specifies the source, and the
group of username@tohostname:/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 username with your username):
scp shadowman username@penguin.example.net:/home/username |
This will transfer the local file shadowman to
/home/username/shadowman
on penguin.example.net.
The general syntax to transfer a remote file to the local system is
scp
username@tohostname:/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/* username@penguin.example.net:/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
username@hostname.com. 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.
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.
 | Separate Authorization Key Pairs |
|---|
| | You must have separate authorization key pairs for SSH Protocol 1
(RSA) and SSH Protocol 2 (DSA).
|
 | 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 Linux 7.1.
To generate a DSA key pair to work with version 2.0 of the
protocol, type the following command at a shell prompt:
Accept the default file location of
~/.ssh/id_dsa. Enter a passphrase
different from your account password and confirm it by
entering it again.
[1]
 | What is a Passphrase? |
|---|
| | 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.
|
Change the permissions of your .ssh
directory using the command chmod 755
~/.ssh.
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.[1]
If you are running GNOME, skip to the section called Configuring ssh-agent with GNOME. If you are not running the X
Window System, skip to the section called Configuring ssh-agent.
Use the following steps to generate a RSA key pair for version 2.0
of the SSH protocol.
To generate a RSA key pair to work with version 2.0 of the
protocol, type the following command at a shell prompt:
Accept the default file location of
~/.ssh/id_rsa. Enter a passphrase
different from your account password and confirm it by entering
it again.
[1]
Change the permissions of your .ssh directory
using the command chmod 755 ~/.ssh.
Copy 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.[1]
If you are running GNOME, skip to the section called Configuring ssh-agent with GNOME. If you are not running the X
Window System, skip to the section called Configuring ssh-agent.
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 Red Hat Linux 7.1 systems, you do not need an RSA key pair.
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.
Change the permissions of your .ssh
directory and your keys with the commands chmod 755
~/.ssh and chmod 644
~/.ssh/identity.pub.
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.
[1]
If you are running GNOME, skip to the section called Configuring ssh-agent with GNOME. If you are not running GNOME,
skip to the section called Configuring ssh-agent.
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 utility can be 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 the section called Configuring ssh-agent.
To save your passphrase during your GNOME session, follow the
following steps:
You'll 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 CD-ROM set, from a Red Hat FTP mirror site, or using
Red Hat Network.
If you do not have an ~/.Xclients file,
you can run switchdesk to create it. In
your ~/.Xclients file, edit the following
line:
exec $HOME/.Xclients-default |
Change the line so that it instead reads:
exec /usr/bin/ssh-agent $HOME/.Xclients-default |
Open the GNOME Control Center ( => =>
=> ) and go to
Session => Startup
Programs. 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 OK to save your settings, and
exit the GNOME Control Center.
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.
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 (see the section called 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, such as an
xterm. However, your passphrase will only be remembered for that
terminal window; it is not a global setting.
At a shell prompt, type the following command:
exec /usr/bin/ssh-agent $SHELL |
Then type the command
and enter your passphrase(s). If you have both DSA and RSA
key pairs configured, you will be prompted for both.
When you log out, your passphrase will be forgotten. You must
execute these two commands each time you log in to a virtual
console or open a terminal window.
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