Remote Network Commands

Jens Hartman shows how to use rlogin, rcp, and rsh to transfer and manipulate data on different computers from across the network.

Normally, being connected to some kind of network does not mean that you are able to directly access all resources provided by that network. Some devices, like tape drives and printers, are connected to special computers and are only accessible on these machines. Others, such as disk drives, can be accessed easily, when the system administrators allow it.

Gaining direct access to resources becomes more complicated the larger the network gets. This is partly due to security reasons, and partly due to the simple fact that the more people that have to be convinced you need to mount a disk, the less your chance of success. From this follows the rule that the more resources available, the harder it will usually be to connect to them.

A nice and easy way around this dilemma is using the remote commands rlogin, rcp and rsh. These commands allow access to any account that is owned by you on any computer in the network without the use of a password. rcp and rlogin can be compared with ftp and telnet, whereas rsh offers the possibility to combine commands on different machines in one shell pipeline.

Configuration is extremely easy—in fact, your host network is probably configured correctly already—and you have instant access to these capabilities. In this article, I present these commands, explain the local setup, give some examples to give you a start, show some options, and demonstrate the complexity that can be reached. Most of my examples can easily be replicated on your networked machine.

Setup:

Under normal conditions (your network is up, you can telnet into other machines, and you can be reached by other machines), the only thing you need to do is create a file called .rhosts in your home directory which is readable and writable only by you (mode 600). This file should contain the full hostnames of each of the machines you want to log in from, and the user name on that machine, like this:

apple.groucho.edu          fred
orange.groucho.edu              sam

The .rhosts file specifies the machines and users that are allowed to login to the user on the machine where the .rhosts file resides. If I am logged in as sam on the machine banana.groucho.edu and I have the above .rhosts file in my home directory, then the user sam from orange.groucho.edu and the user fred from the machine apple.groucho.edu have remote access to my account.

Now, I log into apple.groucho.edu (username fred) from my account on banana.groucho.edu. From apple.groucho.edu I run the following command: rlogin banana.groucho.edu -l sam. Once you are logged in, shell commands will work as normal.

If you are asked to enter your password, do not enter a password, but instead quickly switch back to your original login on banana.groucho.edu and type ps -a. In the process list your rlogin request should appear with the name of the machine it came from as an argument. When this is different from the name you entered in the .rhosts file, you will need to enter the new name. Sometimes a machine uses different lines or a common server for such communication, although its name doesn't change. If there is still no connection, you should ask the system administrator. Some machines simply don't allow any rlogin commands.

In order to respond to any rlogin request, your Linux machine's inetd.conf should have the following two lines:

shell   stream  tcp     nowait  root /usr/sbin/tcpd  /usr/sbin/in.rshd
login   stream  tcp     nowait  root /usr/sbin/tcpd  /usr/sbin/in.rlogind

When you are a member of a domain and share usernames, you might want to include the hosts you frequently connect to in /etc/hosts.equiv. In this case your .rhosts file may contain only the nickname (which is commonly just the machine name without the domain information) together with the username. The above example .rhosts file on the machine named banana.groucho.edu would then look like:

apple.groucho.edu               fred
orange                          sam
RCP:

The rcp command copies files or directories from one machine to another. It is used like the cp command. For instance, I can copy a file named test.dat from the remote machine banana to my local machine orange. (For this example to work the two machines must share usernames.)

rcp banana:test.dat .

or

rcp banana.groucho.edu:test.dat .

The file test.dat is situated in my home directory on banana and is copied to my current directory on orange.

If I want to copy my Mail directory and its contents to orange into the directory Mail.banana (again, from orange):

rcp -r banana:Mail Mail.banana

To preserve the time stamp I would type:

rcp -r -p banana:Mail Mail.banana

Making a remote copy from the machine apple where I have a different account, apple:

rcp fred@apple:test.dat test

Of course, things also work the other way around. Here is a remote copy to apple:

rcp test.dat fred@apple:test.dat

The last interesting thing would be a copy from apple to banana, while you're logged into banana. Unfortunately, this works on every other machine except my Linux machines:

rcp fred@apple:test.dat banana:test.dat

You see that rcp is a bit handier than ftp.

rlogin:

With rlogin you perform a remote login to another machine. It can be used instead of telnet:

rlogin orange

or

rlogin -l fred apple

or

rlogin  apple -l fred

(for some versions of Unix)

I integrate every machine in my window-menu with an rlogin. This makes login very efficient. As an example, here are two descriptions—one for windows manager fvwm and one for olvwm--to add a menu and a shortcut key for rlogin to orange. The xhost can be omitted, but it is useful for other things. For fvwm:

Popup "Rlogin"
Title "Rlogin"
Exec "banana F1" xhost +banana;\
exec xterm -fn fixed -T banana -sb -e rlogin banana & EndPopup
Key F1  A N  Exec "banana" xhost +banana;\
exec xterm -fn fixed -T banana -sb -e rlogin banana &

For olvwm:

"Login" MENU
"Rlogin" TITLE PIN
"banana"  xhost +banana; exec xterm -T banana\
-sb -e rlogin banana
"Login" END
______________________

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