OpenSSH
4:50 AM
By
Rajashekar Reddy BusiReddy
OpenSSH
OpenSSH is a free, open source
implementation of the SSH (Secure
SH ell) protocols. It replaces telnet,
ftp, rlogin, rsh, and rcp with secure, encrypted
network connectivity tools.
All communications using OpenSSH tools,
including passwords, are encrypted. Telnet and ftp use plain text
passwords andsend all information unencrypted. T he information can be
intercepted, the passwords can be retrieved,and your system could be
compromised by an unauthorized person logging in to your system using oneof the
intercepted passwords. T he OpenSSH set of utilities should be used whenever
possible to avoidthese security problems.
Service Profile
Service:
sshd
Package:
openssh
Configuration file:
/etc/ssh/ssh_config
/etc/ssh/sshd_config
/etc/ssh/ssh_host_dsa_key
/etc/ssh/ssh_host_dsa_key.pub
/etc/ssh/ssh_host_key
/etc/ssh/ssh_host_key.pub
/etc/ssh/ssh_host_rsa_key
/etc/ssh/ssh_host_rsa_key.pub
Daemon:sshd
Objective : To provide secure
communications between two systems using a client/server architecture and
allows users to log into server host systems remotely.
Install the package
[root@raju~]#yum install openssh* -y
Run the service
[root@raju~]#chkconfig sshd on
To
connect to an OpenSSH server from a client machine, you must have the
openssh-clients and openssh packages installed on the client machine.
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 desktop1.example.com
type the following command at a shell
prompt:
ssh
desktop1.example.com
The
first time when you ssh use to a remote
machine, you will see a message similar to the following:
The
authenticity of host ' desktop1.example.com’
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 ‘desktop1.example.com '
(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@desktop1.example.com
You
can also use the syntax ssh -l username desktop1.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 desktop1.example.com , type the following command at a
shell
prompt:
ssh
desktop1.example.com 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.
Using
the scp Command
T
he 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/m aillog. T he
<remotefile>
specifies the destination
For
the remote system, if you do not have a preceding /, the path will be relative
to the home directory of username, typically /hom e/username/.
The
general syntax to transfer a remote file to the local system is as follows:
scp
username@tohostname:<remotefile><newlocalfile>
T
he <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 desktop1.example.com type
the following at a shell prompt:
scp
downloads/* username@ desktop1.example.com:upl o ads/
Using
the sftp Command
The
sftp utility can be used to open a secure, interactive FT P 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 FT P.
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 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 userwho wants to connect to remote
machines. If you complete the steps as root, only root will be able to use the
keys.
To generate an RSA key pair for version 2 of the SSH
protocol, follow these steps:
2.
ssh-keygen -t rsa
3. Generating
public/private rsa key pair.
4. Enter
file in which to save the key (/home/john/.ssh/id_rsa):
5.
Press Enter to confirm the default location
(that is, ~/.ssh/id_rsa) for the newly created key.
6.
Enter a passphrase, and confirm it by entering it again when
prompted to do so. For security reasons, avoid using the same
password as you use to log in to your account.
After this, you will be presented with a message similar to
this:
Your identification has been saved in
/home/john/.ssh/id_rsa.
Your public key has been saved in
/home/john/.ssh/id_rsa.pub.
The key fingerprint is:
e7:97:c7:e2:0e:f9:0e:fc:c4:d7:cb:e5:31:11:92:14
john@penguin.example.com
The key's randomart image is:
+--[ RSA 2048]----+
| E. |
| . . |
| o . |
| . .|
| S . . |
| + o o ..|
| * * +oo|
| O +..=|
| o* o.|
+-----------------+
7.
Change the permissions of the ~/.ssh/ directory:
8.
chmod 755 ~/.ssh
9.
Copy the content of ~/.ssh/id_rsa.pub into
the ~/.ssh/authorized_keys on the machine to which you want to
connect, appending it to its end if the file already exists.
10.
Change the permissions of the ~/.ssh/authorized_keys file
using the following command:
chmod 644 ~/.ssh/authorized_keys
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