Getting started with SSH
The following sections hope to provide enough information to setup a user new to ssh with the
appropriate files necessary for accessing remote hosts in a secure manner. Improvements to and
comments about this document are welcome.
Updated: The examples now show ssh version 2 commands, using OpenSSH. There are several
security vulnerabilities in protocol version 1. Everyone should have already migrated to version
2 by now, so it was about time for me to also reflect best practises in this document. I also
updated the link section at the end of the document to better reflect today’s software selection.
Kimmo Suominen
kimmoX@Xsuominen.com
(21.7.2004)
Contents










About public key cryptography
Creating your authentication key
o Changing your pass-phrase
Authorizing access
o Directory and file permissions
Logging into remote systems
Keeping authentication keys in memory
o Running X on a local display
o Running X with an xdm session
Managing keys in memory
Running commands on remote systems
Copying files between systems
Changing default settings
Sources of more information
About public key cryptography
Public key cryptography uses a public key to encrypt data and a private key to decrypt it. The
name public key comes from the fact that you can make the encryption key public without
compromising the secrecy of the data or the decryption key.
What this means is that it is safe to send your public key (i.e. the contents of the
~/.ssh/id_rsa.pub file) in electronic mail or by other means e.g. to have a system
administrator of a remote site install that key into your ~/.ssh/authorized_keys file. For
anyone to actually gain access they need the corresponding private key (i.e. the decrypted
contents of ~/.ssh/id_rsa) to identify themselves.
To further protect your private key you should enter a passphrase to encrypt the key when it is
stored in the filesystem. This will prevent people from using it even if they gain access to your
files.
Creating your authentication key
The very first step is to use ssh-keygen to create an authentication key for yourself. In most
cases the defaults for this command are what you want.
Always, always, type in a good pass-phrase when prompted for one. It can be multiple words
(i.e. spaces are just fine within the phrase), so you could choose a sentence that you can
remember. Changing some of the words by misspelling them or by changing some of the letters
into digits is highly recommended to increase the strength of your pass phrase.
Here is a sample session, your input is in bold. Note that the pass-phrase is not echoed back as
you type it.
beowulf% ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/u/kim/.ssh/id_rsa): [RETURN]
Enter passphrase (empty for no passphrase): 1amp jumb3d
Enter same passphrase again: 1amp jumb3d
Your identification has been saved in /u/kim/.ssh/id_rsa.
Your public key has been saved in /u/kim/.ssh/id_rsa.pub.
The key fingerprint is:
b7:18:ad:3b:0b:50:5c:e1:da:2d:6f:5b:65:82:94:c5 kim@beowulf.gw.com
If you have multiple accounts you might want to create a separate key on each of them. I have
separate keys for




my office environment
my private systems
my Internet service provider (ISP) systems
my university account
This allows me to limit access between these organizations, e.g. not allowing the university
account to access my ISP account or the machines in the office. This enhances the overall
security in case any of the authentication keys are compromised for some reason.
Changing your pass-phrase
You can change the pass-phrase at any time by using the -p option of ssh-keygen.
beowulf% ssh-keygen -p
Enter file in which the key is (/u/kim/.ssh/id_rsa): [RETURN]
Enter old passphrase: 1amp jumb3d
Key has comment '/u/kim/.ssh/id_rsa'
Enter new passphrase (empty for no passphrase): cow 3ats grass
Enter same passphrase again: cow 3ats grass
Your identification has been saved with the new passphrase.
The pass-phrases are not echoed as you type them.
Authorizing access
To allow access to a system for a given identity place the public key in your
~/.ssh/authorized_keys file on that system. All keys listed in that file are allowed access.
Usually you will want to authorize access to the local system using the local key (especially in an
environment where multiple systems share the same home directory e.g. using NFS). Thus a good
start is to copy the public key for your default identity into the ~/.ssh/authorized_keys file.
beowulf% cd ~/.ssh
beowulf% cp id_rsa.pub authorized_keys
You could now copy the ~/.ssh/authorized_keys file to other systems to allow access from
the local system. One way to copy the file is to use the scp command, like this:
beowulf% scp -p ~/.ssh/authorized_keys hrothgar:.ssh/
kim@hrothgar's password: YourPasswordHere
authorized_keys
100% 1839
1.2MB/s
00:00
As no identities have been authorized yet on the remote system, you will be asked for your
regular login password as seen above. It will not be echoed as you type it. The Copying files
between systems section has more information about scp.
Use a text editor to add more keys to the file. If you use cut and paste to copy the key make sure
each key entry is a single line in the file. The keys to add are always the public keys (from files
with the .pub extension).
NOTE: To gain access to restricted systems you might need to send your public key in
electronic mail to the administrator of the system. Just include the contents of the
~/.ssh/id_rsa.pub file in the message.
Directory and file permissions
If access to the remote system is still denied you should check the permissions of the following
files on it:



the home directory itself
the ~/.ssh directory
the ~/.ssh/authorized_keys file
The permissions should allow writing only by you (the owner). This example shows the most
relaxed permissions you could use.
hrothgar% cd
hrothgar% ls -ld . .ssh .ssh/authorized_keys
drwxr-xr-x 36 kim kim 4096 Jul 25 02:24 .
drwxr-xr-x 2 kim kim 512 Apr 10 02:30 .ssh
-rw-r--r-- 1 kim kim 1674 Apr 10 02:29 .ssh/authorized_keys
To make the remote system allow access you must change the permissions to disallow writing by
others than the owner.
hrothgar% cd
hrothgar% chmod go-w . .ssh .ssh/authorized_keys
Remember to do this on all the systems you want to have access to.
Logging into remote systems
To establish an interactive connection to a remote system you would use either the slogin or the
ssh command. The only parameter is the name of the remote system. The pass-phrase is not
echoed back to you when you type it.
beowulf% slogin hrothgar
Enter passphrase for key '/u/kim/.ssh/id_rsa': 1amp jumb3d
Last login: Wed Jul 21 00:00:49 2004 from beowulf.gw.com
[more output from the remote machine]
hrothgar%
You can avoid the pass-phrase prompts by keeping the authentication keys in memory. You only
need to type the pass-phrase when you add a key into memory.
If your account name on the remote system differs from the one on the local system (the system
you are connecting from) you can use the -l switch to specify the remote account name.
beowulf% slogin -l suominen panix.com
Last login: Sun Jun 13 14:55:17 2004 from beowulf.gw.com
[more output from the remote machine]
panix%
You can change the default remote account name by creating a configuration file entry for the
host.
Keeping authentication keys in memory
If you frequently open connections to remote systems you can run your session under the sshagent. The agent will provide decrypted authentication keys to all the commands when new
connections are created.
When you start ssh-agent you need to provide it a command to spawn. This is usually either
your shell or a command to start a windowing environment. When you exit the command all
keys will be removed from memory.
beowulf% ssh-agent $SHELL
beowulf%
You will now need to add keys into memory to have them available for other commands.
Running X on a local display
If you have workstation where you start the X window system after logging in you can have the
whole windowing environment benefit from the keys in memory. The X window system is
normally started with startx and the initial clients are in your ~/.xinitrc file.
beowulf% ssh-agent startx &
If your workstation has virtual consoles it is good to put the X window system in the background
so the current virtual console can still be used for more commands if necessary. It won’t hurt to
background the command even without virtual consoles.
NOTE: Your system might have a non-standard way of starting the X window system. Replace
startx with the appropriate command if necessary. Please ask your system administrator for the
exact command to use.
Running X with an xdm session
If you use an X-terminal or your workstation is running xdm you need to arrange for the clients
to run under ssh-agent. The easiest way (which is conveniently compatible with the method
used without xdm) is to put all initial clients into the ~/.xinitrc file which in turn is called from
the ~/.xsession file.
An example ~/.xsession file is below. It runs ssh-agent only if you have a ~/.ssh directory.
#!/bin/sh
if [ -d $HOME/.ssh ]
then EXEC="exec ssh-agent"
else EXEC="exec"
fi
if [ -x $HOME/.xinitrc ]
then $EXEC $HOME/.xinitrc
else $EXEC xterm -geometry 80x24+0-60 -ls
fi
Make sure the files are executable. The following command will change the permissions
suitably.
beowulf% chmod a+x ~/.xinitrc ~/.xsession
NOTE: If you are using an X-terminal keep in mind that your session is most likely not secure.
Usually anything you type can be captured on the local area network you are connected to.
Managing keys in memory
Before your connections can be authenticated without prompts for a pass-phrase you have to use
ssh-add to add the necessary keys to memory. To add the default key on the current system to
memory no options are needed. The pass-phrase is prompted for to decrypt the key. It is not
echoed back as you type it.
beowulf% ssh-add
Enter passphrase for /u/kim/.ssh/id_rsa: 1amp jumb3d
Identity added: /u/kim/.ssh/id_rsa (/u/kim/.ssh/id_rsa)
You can specify the file that contains the key if you have other identities than the default. You
must use the private key file (the one that does not have the .pub extension).
The -d option will have the key deleted from memory. There is no ssh-delete command.
beowulf% ssh-add -d ~/.ssh/isp
To list all keys currently in memory use the -l option.
beowulf% ssh-add -l
1024 [hex numbers] /u/kim/.ssh/id_rsa (RSA)
1024 [hex numbers] /u/kim/.ssh/isp (RSA)
You can delete all keys from memory at once with the -D option.
beowulf% ssh-add -D
This is useful if you have added keys into memory on remote systems and don’t want to
reconnect just to delete the keys.
Running commands on remote systems
The ssh command can also be used to run commands on remote systems without logging in. The
output of the command is displayed and control returns to the local system. Here is an example
which will display all the users logged in on the remote system.
beowulf% ssh hrothgar who
christos ttyp8
Oct 17 20:42
beowulf%
(milou)
If you are using the X Window System you can use this capability to start a terminal window to
start an interactive session on the remote system.
beowulf% ssh -n hrothgar xterm &
[1] 15866
beowulf%
Use the -n to prevent the remote system from trying to read from the terminal starting the xterm
and put the process in the background. A new window from the remote system should appear
shortly on your display.
Copying files between systems
You can copy files from the local system to a remote system or vice versa, or even between two
remote systems using the scp command. To specify a file on a remote system simply prefix it
with the name of the remote host followed by a colon.
If you leave off the filename of the copy or specify a directory only the name of the source file
will be used. An easy way of retrieving a copy of a remote file into the current directory while
keeping the name of the source file is to use a single dot as the destination.
beowulf% scp -p hrothgar:aliases .
beowulf%
The -p option is not required. It indicates that the modification and access times as well as
modes of the source file should be preserved on the copy. This is usually desirable.
You can copy several files in a single command if the destination is a directory.
beowulf% scp -p hrothgar:.login hrothgar:.logout panix.com:.
beowulf%
Relative filenames resolve differently on the local system than on the remote system. On the
local system the current directory is assumed (as usual with all commands). On the remote
system the command runs in the home directory! Thus relative filenames will be relative to the
home directory of the remote account.
NOTE: When you specify remote machines in both the source and the destination the
connection to copy the files is made directly between those hosts. The files are not copied
through the local system. Sometimes this makes a difference in a firewalled or otherwise
restricted environment.
Changing default settings
The defaults for the ssh-related commands can be altered for each account in a configuration file
~/.ssh/config (there is also a system-wide file, usually /etc/ssh/ssh_config). Each entry
starts with a Host keyword. You can use wildcards to match all the appropriate systems:


?
*
matches any single character
matches any sequence of zero or more characters
Usual keywords include (defaults in parenthesis):
Compression
yes/no (no)
Controls whether compression is used on the connection.
CompressionLevel 1-9 (6)
Level of compression: 1 is fastest, 9 is slowest (achieves best compression). Compression
is good for slow links (saves bandwidth) and fast machines.
FallBackToRsh yes/no (yes)
If a secure connection to the remote system cannot be established the commands can try
unsecure connections (a warning will be displayed if this happens). On highly secure
systems this could be disabled in the system-wide configuration.
KeepAlive yes/no (yes)
Controls whether TCP keepalive messages are used. When enabled it is possible to detect
network outages and automatically close your connections (which is good). However, if
you are connected over a dialup link that automatically dials when there is traffic, you
will want to turn this off to avoid unnecessarily bringing up the line.
User account (local account)
Specify the remote account name. Add this to avoid having to use the -l option when
issuing commands.
Here is an example ~/.ssh/config file.
Host *panix.com
User suominen
Compression no
Host *gw.com
FallBackToRsh no
Host *
Compression yes
CompressionLevel 9
FallBackToRsh yes
KeepAlive no
Options are accumulated over entries, but a more specific entry will override a less specific one.
E.g. in the above compression will not be used for hosts that match *panix.com but will be used
for hosts that match *gw.com (and all other hosts since the * entry matches all hosts).
For a complete list of options please refer to the manual pages of both ssh and sshd.