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...making Linux just a little more fun!
Pass on Passwords with scp
By Dave Sirof

Pass on Passwords with scp Home | VizagInfo Mirror of LG

Pass on Passwords with scp

In this article I'll show you how to use scp without passwords. Then I'll show you how to use this in two cool scripts. One script lets you copy a file to multiple linux boxes on your network and the other allows you to easily back up all your linux boxes.

If you're a linux sysadmin, you frequently need to copy files from one linux box to another. Or you need to distribute a file to multiple boxes. You could use ftp, but there are many advantages to using scp instead. Scp is much more secure than ftp, as scp travels across the LAN /WAN encrypted, while ftp uses clear text (even for passwords.

But what I like best about scp is that it's easily scriptable. Suppose you have a file that you need to distribute to 100 linux boxes. I'd rather write a script to do it than type 100 sets of copy commands. If you use ftp in your script it can get pretty messy, because each linux box you log into is going to ask for a password. But if you use scp in your script, you can set things up so the remote linux boxes don't ask for a password. Believe it or not, this is actually much more secure than using ftp!

Here's an example demonstrating the most basic syntax for scp. To copy a file named 'abc.tgz' from your local pc, to the /tmp dir of a remote pc called 'bozo' use:

scp abc.tgz root@bozo:/tmp

You will now be asked for bozo's root password. So we're not quite there yet. It's still asking for a password so it's not easily scriptable. To fix that, follow this one time procedure (then you can do endless "passwordless" scp copies):

1. Decide which user on the local machine will be using scp later on. Of course root gives you the most power, and that's how I personally have done it. I won't give you a lecture on the dangers of root here, so if you don't understand them, use a different user. Whatever you choose, log in as that user now for the rest of the procedure, and log in as that user when you use scp later on.

2. Generate a public / private key pair on the local machine. Say What? If you're not familiar with Public Key Cryptography, here's the 15 second explanation. In Public Key Cryptography, you generate a pair of mathematically related keys, one public and one private. Then you give your public key to anyone and everyone in the world, but you never ever give out your private key. The magic is in the mathematical makeup of the keys - anyone with your public key can encrypt a message with it, but only you can decrypt it with your private key. Anyway, the syntax to create the key pair is:

ssh-keygen -t rsa

3. In response you'll see:
"Generating public/private rsa key pair"
"Enter file in which to save the key ... "
Just hit enter to accept this.

4. In response you'll see:
"Enter passphrase (empty for no passphrase):"
You don't need a passphrase, so just hit enter twice.

5. In response you'll see:
"Your identification has been saved in ... "
"Your public key has been saved in ... "
Note the name and location of the public key just generated (it will always end in .pub).

6. Copy the public key just generated all your remote linux boxes. You can use scp or ftp or whatever to do the copy. Assuming your're using root (again see my warning in step 1. above), the key must be contained in the file /root/.ssh/authorized_keys (watch spelling!). Or if you are logging in as a user, e.g. clyde, it would be in /home/clyde/authorized_keys. Note that the authorized_keys file can contain keys from other PC's. So if the file already exists and contains text in it, you need to append the contents of your public key file to it.

That's it. Now with a little luck you should be able to scp a file to the remote box without using a password. So let's test it by trying our first example again. Copy a file named 'xyz.tgz' from your local pc, to the /tmp dir of a remote pc called 'bozo'

scp xyz.tgz root@bozo:/tmp

Wow !!! It copied with no password!!

A word about security before we go on. This local PC just became pretty powerful, since it now has access to all the remote PC's with only the one local password. So that one password better be very strong and well guarded.

Now for the fun part. Let's write a short script to copy a file called 'houdini' from the local PC to the /tmp dir of ten remote PC's, in ten different cities (with only 5 minutes work). Of course it would work just the same with 100 or 1000 PC's. Suppose the 10 PC's are called: brooklyn, oshkosh, paris, bejing, winslow, rio, gnome, miami, minsk and tokyo. Here's the script:

for CITY in brooklyn oshkosh paris bejing winslow rio gnome miami minsk tokyo
scp houdini root@$CITY:/tmp
echo $CITY " is copied"

Works liek magic. With the echo line in the script you should be able to watch as each city is completed one after the next.

By the way, if you're new to shell scripting, here's a pretty good tutorial:

As you may know, scp is just one part of the much broader ssh. Here's the cool part. When you followed my 6 stop procedure above, you also gained the ability sit at your local PC and execute any command you like on any of the remote PC's (without password of course!). Here's a simple example, to view the date & time on the remote PC brooklyn:

ssh brooklyn "date"

Now let's put these 2 concepts together for one final and seriously cool script. It's a down and dirty way to backup all your remote linux boxes. The example backs up the /home dir on each box. It's primitive compared to the abilities of commercial backup software, but you can't beat the price. Consider the fact that most commercial backup software charges licence fees for each machine you back. If you use such a package, instead of paying licence fees to back remote 100 PC's, you could use the script back the 100 PC's to one local PC. Then back the local PC to your commercial package and save the license fee for 99 PC's ! Anyway the script demostates the concepts so you can write you own to suit your situation. Just put this script in a cron job on your local PC (no script is required on the remote PC's). Please read the comments carefully, as they explain everything you need to know:


# Variables are upper case for clarity

# before using the script you need to create a dir called '/tmp/backups' on each
# remote box & a dir called '/usr/backups' on the local box

# on this local PC
# Set the variable "DATE" & format the date cmd output to look pretty
DATE=$(date +%b%d)

# this 'for loop' has 3 separate functions

for CITY in brooklyn oshkosh paris bejing winslow rio gnome miami minsk tokyo

# remove tarball on remote box from the previous time the script ran # to avoid filling up your HD
# then echo it for troubleshooting
ssh -1 $CITY "rm -f /tmp/backups/*.tgz"
echo $CITY " old tarball removed"

# create a tarball of the /home dir on each remote box & put it in /tmp/backups
# name the tarball uniquely with the date & city name
ssh $CITY "tar -zcvpf /tmp/backups/$CITY.$DATE.tgz /home/"
echo $CITY " is tarred"

# copy the tarball just create from the remote box to the /usr/backups dir on
# the local box
scp root@$CITY:/tmp/backups/$CITY.$DATE.tgz /usr/backups
echo $CITY " is copied"


# the rest of the script is for error checking only, so it's optional:

# on this local PC
# create error file w todays date.
# If any box doesn't get backed, it gets written to this file
touch /u01/backup/scp_error_$DATE

for CITY in brooklyn oshkosh paris bejing winslow rio gnome miami minsk tokyo


# Check if tarball was copied to local box. If not write to error file
# note the use of '||' which says do what's after it if what's before it is not # true
ls /u01/backup/$CITY.$DATE.tgz || echo " $CITY did not copy" >> scp_error_$DATE

# Check if tarball can be opened w/o errors. If errors write to error file.
tar ztvf /u01/backup/$CITY.$DATE.tgz || echo "tarball of $CITY is No Good" >> scp_error_$DATE


That's about it. In this article I've tried to give examples that demonstate the concepts, not necessarily to be use "as is". Some of the syntax may not work in all distros, but in the interest of brevity I could not include all the possibilities. For example, if you are using Red Hat 6.2 or before, the syntax will require some changes (I'd be happy to give it to you if you email me). So be creative and hopefully you can use some of this in your own environment.
Unless otherwise mentioned, this work copyright © 2003-2004 by SSC, Inc. All rights reserved.


[BIO] None provided.

Copyright © 2004, Dave Sirof. Copying license
Published in Issue 98 of Linux Gazette, January 2004

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