linux guide

Remote Administration Telnet, SSH and telnet:
Telnet
Telnet is basically a virtual terminal program that helps you to configure a system remotely. In production environment, telnet is rarely used as it transfer keystroke in clear text.To enable telnet all you have to do is, run the following two commands
chkconfig telnet on
service xinetd restart
By default, telnet to remote system using root user is disabled. You have to log into the telnet server as a non-privileged user then you can change your identity as root using the following command –
su – root

SSH

Unlike telnet, ssh is considered a Secure Shell Service. Ssh runs on public/private key infrastructure using rsa or dsa technology.
It is very easy to start ssh. Execute service sshd restart command.
By default any user can establish ssh session. However, this can be controlled by modifying the /etc/ssh/sshd_config file. See the following screen shot. Here I have denied atanu and allowed all other users to establish an ssh session.
We can also allow or deny one or multiple group using AllowGroups or DenyGroups directive, like AllowGroups acct mrkt.
The modification can only be effected when you restart the ssh server.
Check the last two lines in the screen shot in the page –


In my network, I always use ssh to configure remote servers. Generally I configure my linux laptop as an ssh client. I have a RHEL 4 system and I use this particular computer to administer my proxy server that is running on 172.16.1.12. See, what I usually do –In my laptop I execute the following command to create the key –
ssh-keygen –t dsa
You can also use rsa here. See the following screen shot –


Check the output, in the screen shot, carefully. In the 3rd Line, it is saying the key will be saved in /root/.ssh directory. If the .ssh directory is not exist in /root, the sshkeygen command will create it, check the 4th line. After creating /root/.ssh directory, the system will ask you to supply the passphrase. I enter empty passphrase, means just press enter twice. My identification is now saved in /root/.ssh/id_dsa file and the public key is id_dsa.pub.
It is time to copy the id_dsa.pub in the /root/.ssh/ directory in my proxy server and the name of the file will be authorized_keys. I will be using the following command assuming in my proxy server, running on 172.16.1.12, the /root/.ssh directory exists.
scp –rp /root/.ssh/id_dsa.pub root@172.16.1.12:/root/.ssh/authorized_keys
See the output in the following screenshot. After executing the above command the system will ask your confirmation, type yes and press enter, supply root’s password for 172.16.1.12 and the file will be copied securely.

The scp will be discussed shortly.
Now, from your client computer if you execute ssh 172.16.1.12 the system will never ask you to supply the password!!


SCP

Using scp, you can copy files from or to a remote linux system. In ssh section above, see how we use scp to copy id_dsa.pub. To refer a remote system we use user@machin_name_or_ip_address:/any/directory and a source can also be a remote system.
While working with Windows system, I use winscp to communicate with linux system. The winscp382setup.exe is freely available for download in sourceforge.net.
Please be noted, when you establish an ssh or scp session with a computer for the first time, the remote system related information is added in /root/.ssh/known_hosts text file. And if the remote system is re-installed you will be failed to establish ssh or scp session until you remove the system’s information from the /root/.ssh/known_hosts file.

Labels: allow, command, conf, configuration, edit, guide to linux, Linux, modify, network, password, proxy, redhat server, Remote Administration Telnet, rhel, scp, services, squid, SSH, telnet, user