This document covers the SSH client on the Linux Operating System and other OSes that use OpenSSH. If you use Windows, please read the document SSH Tutorial for Windows
If you use Mac OS X or other Unix based system, you should already have
OpenSSH installed and can use this document as a reference.
This article is one of the top tutorials
covering SSH on the Internet. It was originally written back in 1999 and
was completely revised in 2006 to include new and more accurate
information. As of October, 2008, it has been read by over 473,600
people and consistently appears at the top of Google's search results
for SSH Tutorial and Linux SSH.
What Is SSH?
There are a couple of ways that you can access a shell
(command line) remotely on most Linux/Unix systems. One of the older
ways is to use the telnet program, which is available on most network
capable operating systems. Accessing a shell account through the telnet
method though poses a danger in that everything that you send or receive
over that telnet session is visible in plain text on your local
network, and the local network of the machine you are connecting to. So
anyone who can "sniff" the connection in-between can see your username,
password, email that you read, and commands that you run. For these
reasons you need a more sophisticated program than telnet to connect to a
remote host.
An unencrypted telnet session
SSH, which is an acronym for Secure SHell, was designed and created
to provide the best security when accessing another computer remotely.
Not only does it encrypt the session, it also provides better
authentication facilities, as well as features like secure file
transfer, X session forwarding, port forwarding and more so that you can
increase the security of other protocols. It can use different forms of
encryption ranging anywhere from 512 bit on up to as high as 32768 bits
and includes ciphers like AES (Advanced Encryption Scheme), Triple DES,
Blowfish, CAST128 or Arcfour. Of course, the higher the bits, the
longer it will take to generate and use keys as well as the longer it
will take to pass data over the connection.
These two diagrams on the left show how a telnet session can be
viewed by anyone on the network by using a sniffing program like
Ethereal (now called Wireshark) or tcpdump. It is really rather trivial
to do this and so anyone on the network can steal your passwords and
other information. The first diagram shows user jsmith logging in to a
remote server through a telnet connection. He types his username jsmith
and password C0lts06!, which are viewable by anyone who is using the
same networks that he is using.
The second diagram shows how the data in an encrypted connection
like SSH is encrypted on the network and so cannot be read by anyone who
doesn't have the session-negotiated keys, which is just a fancy way of
saying the data is scrambled. The server still can read the information,
but only after negotiating the encrypted session with the client.
When I say scrambled, I don't mean like the
old cable pay channels where you can still kinda see things and hear the
sound, I mean really scrambled. Usually encryption means that the data
has been changed to such a degree that unless you have the key, its
really hard to crack the code with a computer. It will take on the order
of years for commonly available computer hardware to crack the
encrypted data. The premise being that by the time you could crack it,
the data is worthless.
Getting Started
This tutorial isn't going to cover how to install SSH, but will cover
how to use it for a variety of tasks. Consult your Linux distribution's
document for information on how to setup OpenSSH.
Chances are that if you are using a version of Linux that was
released after 2002, that you already have OpenSSH installed. The
version of SSH that you will want to use on Linux is called OpenSSH. As
of this writing (October 2009), the latest version available is 5.3, but
you may encounter versions from 3.6 on up. If you are using anything
lower than version 3.9, I'd strongly advise you to upgrade it.
OpenSSH can be obtained from http://www.openssh.org/
To really make ssh useful, you need a shell account on a remote machine, such as on a Suso account.
The first thing we'll do is simply connect to a remote machine.
This is accomplished by running 'ssh hostname' on your local machine.
The hostname that you supply as an argument is the hostname of the
remote machine that you want to connect to. By default ssh will assume
that you want to authenticate as the same user you use on your local
machine. To override this and use a different user, simply use
remoteusername@hostname as the argument. Such as in this example:
ssh username@username.suso.org
The first time around it will ask you if you wish to add the remote host to a list of known_hosts, go ahead and say yes.
The authenticity of host 'arvo.suso.org (216.9.132.134)' can't be established.
RSA key fingerprint is 53:b4:ad:c8:51:17:99:4b:c9:08:ac:c1:b6:05:71:9b.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'arvo.suso.org' (RSA) to the list of known hosts.
It is important to pay attention to this question however because this
is one of SSH's major features. Host validation. To put it simply, ssh
will check to make sure that you are connecting to the host that you
think you are connecting to. That way if someone tries to trick you into
logging into their machine instead so that they can sniff your SSH
session, you will have some warning, like this:
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: POSSIBLE DNS SPOOFING DETECTED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
The RSA host key for arvo.suso.org has changed,
and the key for the according IP address 216.9.137.122
is unchanged. This could either mean that
DNS SPOOFING is happening or the IP address for the host
and its host key have changed at the same time.
Offending key for IP in /home/suso/.ssh/known_hosts:10
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
96:92:62:15:90:ec:40:12:47:08:00:b8:f8:4b:df:5b.
Please contact your system administrator.
Add correct host key in /home/suso/.ssh/known_hosts to get rid of this message.
Offending key in /home/suso/.ssh/known_hosts:53
RSA host key for arvo.suso.org has changed and you have requested strict
checking.
Host key verification failed.
If you ever get a warning like this, you should stop and determine if
there is a reason for the remote server's host key to change (such as
if SSH was upgraded or the server itself was upgraded). If there is no
good reason for the host key to change, then you should not try to
connect to that machine until you have contacted its administrator about
the situation. If this is your own machine that you are trying to
connect to, you should do some computer forensics to determine if the
machine was hacked (yes, Linux can be hacked). Or maybe your home
computer's IP address has changed such as if you have a dynamic IP
address for DSL. One time I received this message when trying to connect
to my home machine's DSL line. I thought it was odd since I hadn't
upgraded SSH or anything on my home machine and so I choose not to try
to override the cached key. It was a good thing that I didn't try
because I found out that my dynamic IP address had changed and that out
of chance, another Linux machine running OpenSSH took my old IP.
After saying yes, it will prompt you for your password on the
remote system. If the username that you specified exists and you type in
the remote password for it correctly then the system should let you in.
If it doesn't, try again and if it still fails, you might check with
the administrator that you have an account on that machine and that your
username and password is correct.
Generating a key
Now that you have spent all that time reading and are now connected,
go ahead and logout. ;-) Once you're back to your local computer's
command prompt enter the command 'ssh-keygen -b 4096' to generate a
strong key.
ssh-keygen -b 4096
It should begin spitting out the following:
Generating public/private rsa key pair.
Enter file in which to save the key (/home/localuser/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/localuser/.ssh/id_rsa.
Your public key has been saved in /home/localuser/.ssh/id_rsa.pub.
The key fingerprint is:
e7:06:7f:2c:32:bf:84:a8:5b:8d:63:98:f3:ee:a2:8b localuser@mybox.home.com
The key's randomart image is:
+---[RSA 4096]----+
| |
| |
| |
| |
| S . |
| o + * . |
| + * = * o |
| . .* . * o |
|E ooo=+ o. |
+-----------------+
It will prompt you for the location of the keyfile. Unless you have
already created a keyfile in the default location, you can accept the
default by pressing 'enter'.
Next it will ask you for a passphrase and ask you to confirm it.
The idea behind what you should use for a passphrase is different from
that of a password. Ideally, you should choose something unique and
unguessable, just like your password, but it should probably be
something much longer, like a whole sentence. Here are some examples of
passphrases I've used in the past:
The right thing changes from state to state
the purpose of life is to give it purpose
They're not going to guess this passphrase!
The RIAA can just suck my big ass
It is never a good day at Teletron
Some passphrases that I've used have had as many as 60 characters
along with punctuation and numbers. This makes the passphrase harder to
guess. To give you an idea of how much more secure a passphrase is than a
password. Consider this. Even if you narrowed down the number of words
someone could use in a passphrase to 2000 potential words, if that
person used 5 words in a sentence from that 2000 word set, it would mean
there are 32,000,000,000,000,000 different combinations. Compare this
with 6,095,689,385,410,816, which is the total possible combinations in
an 8 character password using upper and lower case characters, numbers
and punctuation (about 94 potential characters). So an 8 character
password has 5.25 times less combinations than a 5 word
passphrase. In actuality, most people choose words from a set of 10,000
or more words, bringing the complexity of a 5 word passphrase to 16,405
or more times greater than that of a 8 character password. So on
average, the difficulty of cracking a passphrase is much greater than
any password that could be used. Interestingly, the potential number of
combinations of 8 word passphrase of someone with an adult vocabulary
(8000 words or more) is almost equal to the number of 8 character
password combinations multiplied by itself or about
16,777,216,000,000,000,000,000,000,000,000 combinations.
Don't use any famous quotes or phrases
for your passphrase, they may be easily guessed by another person or by a
brute force cracking program.
The reason why you would generate a key file is so that you can
increase the security of your SSH session by not using your system
password. When you generate a key, you are actually generating two key
files. One private key and one public key, which is different from the
private key. The private key should always stay on your local computer
and you should take care not to lose it or let it fall into the wrong
hands. Your public key can be put on the machines you want to connect to
in a file called .ssh/authorized_keys. The public key is safe to be
viewed by anybody and mathematically cannot be used to derive the
private key. Its just like if I gave you a number 38,147,918,357 and
asked you to find the numbers and operations I used to generate that
number. There are nearly infinite possibilities.
Whenever you connect via ssh to a host that has your public key
loaded in the authorized_keys file, it will use a challenge response
type of authentication which uses your private key and public key to
determine if you should be granted access to that computer It will ask
you for your key passphrase though. But this is your local ssh process
that is asking for your passphrase, not the ssh server on the remote
side. It is asking to authenticate you according to data in your private
key. Using key based authentication instead of system password
authentication may not seem like much of a gain at first, but there are
other benefits that will be explained later, such as logging in
automatically from X windows.
Recent versions of OpenSSH will print out a randomart ASCII art
image representing your key's fingerprint. This is meant to make it
easier to identify a matching key when compared with another key's
randomart.
Installing your public key manually
If you do not have the ssh-copy-id program available, then you must
use this manual method for installing your ssh key on the remote host.
Even if you do have the ssh-copy-id program, its good to do the manual
installation at least once so that you have a good understanding of what
is going on, because this is where a lot of people end up having
problems.
Go ahead and copy your public key which is in ~/.ssh/id_rsa.pub to the remote machine.
scp ~/.ssh/id_rsa.pub username@arvo.suso.org:.ssh/authorized_keys
It will ask you for your system password on the remote machine and
after authenticating it will transfer the file. You may have to create
the .ssh directory in your home directory on the remote machine first.
By the way, scp is a file transfer program that uses ssh. We'll talk
more about it later.
Now when ssh to the remote machine, it should ask you for your
key passphrase instead of your password. If it doesn't, it could be that
the permissions and mode of the authorized_keys file and .ssh directory
on the remote server need to be set more restrictively. You can do that
with these commands on the remote server:
chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys
You can also put the public key in the remote authorized_keys file by
simply copying it into your paste buffer, logging into the remote
machine and pasting it directly into the file from an editor like vi,
emacs or nano. I would recommend using the 'cat' program to view the
contents of the public key file though because using less will end up
breaking the single line into multiple lines.
cat ~/.ssh/id_rsa.pub
Installing your public key automatically
A newer way that you can quite easily install your public ssh key on a remote host is with the ssh-copy-id program like this:
ssh-copy-id yourusername@your.website.com
It will prompt you for your password on the remote host and take care
of the rest. That was easy. So why didn't I just tell you how to use
this program in the first place? Well, in my experience, many of the
problems people have with ssh revolve around trying to get their ssh
public key installed correctly. Its a good thing that they've made a
program to do the dirty work for you, but in the interest of building
your skills, you should at least do the manual install once so that you
know what is involved.
Using the ssh-agent program
The true usefulness of using key based authentication comes in the
use of the ssh-agent program. Usually, the ssh-agent program is a
program that starts up before starting X windows and in turn starts X
windows for you. All X windows programs inherit a connection back to the
ssh-agent, including your terminal windows like Gnome Terminal,
Konsole, xfce4-terminal, aterm, xterm and so on. What this means is that
after you've started up X windows through ssh-agent, you can use the
ssh-add program to add your passphrase one time to the agent and the
agent will in turn pass this authentication information automatically
every time you need to use your passphrase. So the next time you run:
ssh username@arvo.suso.org
you will be logged in automatically without having to enter a
passphrase or password. Most recent distributions will automatically
start ssh-agent when you login to X windows through a session manager
like gdm (graphical login). I found that as of this writing the
following distributions started ssh-agent by default.
- Debian
- Fedora
- Gentoo
- SuSE
- Ubuntu
Most distributions prior to about 2002 did not start it.
Don't worry if you don't see your distro listed in here. You can check if it is already running by running this command.
ps auxw
If there is an ssh-agent process listed there, then you can just
start using it, otherwise, you should consult your distribution's
documentation on OpenSSH and running the ssh-agent.
Once you've verified that ssh-agent is running, you can add your ssh key to it by running the ssh-add command:
ssh-add
If the program finds the RSA key that you created above, it will
prompt you for the passphrase. Once you have done so it should tell you
that it has added your identity to the ssh-agent:
Identity added: /home/username/.ssh/id_rsa (/home/username/.ssh/id_rsa)
Now you can try logging into that remote machine again and this time
you will notice that it just logs you right in without prompting you for
any password or passphrase.
To make adding your passphrase easier, you can add the ssh-add
program to your desktop session startup programs and it will bring up a
prompt in X windows to ask for your passphrase every time you login to
your desktop. You should also have the gtk2-askpass program installed.
Or x11-askpass. They are the real programs that actually prompt you for
your password. ssh-add just runs them if its not being run in a
terminal. Below is a screenshot of the Gnome Sessions Configuration
dialog with ssh-add added to the startup programs.
Gnome Session with ssh-add program set to run and prompt for your key's passphrase
X11 Session Forwarding
One lesser known feature of X windows is its network transparency. It
was designed to be able to transmit window and bitmap information over a
network connection. So essentially you can login to a remote desktop
machine and run some X windows program like Gnumeric, Gimp or even
Firefox and the program will run on the remote computer, but will
display its graphical output on your local computer.
To try this out, you will need an account on a remote computer
that has X windows installed with some X windows applications. suso.org
servers do not have any such programs so you will need to either login
to one of your other workstations or another server that does have them.
The key to making it work is using the -X option, which means "forward
the X connection through the SSH connection". This is a form of
tunneling.
ssh -X username@desktopmachine.domain.com
If this doesn't work, you may have to setup the SSH daemon on the
remote computer to allow X11Forwarding, check that the following lines
are set in /etc/ssh/sshd_config on that computer:
X11Forwarding yes
X11DisplayOffset 10
X11UseLocalhost yes
For some newer programs and newer versions of X windows, you may need
to use the -Y option instead for trusted X11 forwarding. Try using this
option if your X11 windows program fails to start running with a
message like this one that was for Gimp:
The program 'gimp-2.2' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadWindow (invalid Window parameter)'.
(Details: serial 154 error_code 3 request_code 38 minor_code 0)
(Note to programmers: normally, X errors are reported asynchronously;
that is, you will receive the error a while after causing it.
To debug your program, run it with the --sync command line
option to change this behavior. You can then get a meaningful
backtrace from your debugger if you break on the gdk_x_error()
function.)
TCP Port Forwarding
Like X11 session forwarding, SSH can also forward other TCP
application level ports both forward and backwards across the SSH
session that you establish.
For example, you can setup a port forward for your connection
from your home machine to arvo.suso.org so that it will take connections
to localhost port 3306 and forward them to the remote side
mysql.suso.org port 3306. Port 3306 is the port that the MySQL server
listens on, so this would allow you to bypass the normal host checks
that the MySQL server would make and allow you to run GUI MySQL programs
on your local computer while using the database on your suso account.
Here is the command to accomplish this:
ssh -L 3306:mysql.suso.org:3306 username@arvo.suso.org
The -L (which means Local port) takes one argument of
<local-port>:<connect-to-host>:<connect-to-port>
so you specify what host and port the connection will go to on
the other side of the SSH connection. When you make a connection to the
<local-port> port, it sends the data through the SSH connection
and then connects to <connect-to-host>:<connect-to-port> on
the other side. From the point of view of <connect-to-host>, its
as if the connection came from the SSH server that you login to. In the
case above, arvo.suso.org.
This is much like a VPN connection allows you to act like you are making connections from the remote network that you VPN into.
Take a moment to think of other useful connections you can make with this type of network tunnel.
Another useful one is for when you are away from home and can't
send mail through your home ISP's mail server because it only allows
local connections to block spam. You can create an SSH tunnel to an SSH
server that is local to your ISP and then have your GUI mail client like
Thunderbird make a connection to localhost port 8025 to send the mail.
Here is the command to create the tunnel:
ssh -L 8025:smtp.homeisp.net:25 username@shell.homeisp.net
One thing to note is that non-root users do not normally have the
ability to listen on network ports lower than 1024, so listening on port
25 would not work, thus we use 8025. It really doesn't matter, you can
use any port as long as your email client can connect to it.
You can also reverse the direction and create a reverse port
forward. This can be useful if you want to connect to a machine remotely
to allow connections back in. For instance, I use this sometimes so
that I can create a reverse port 22 (SSH) tunnel so that I can reconnect
through SSH to a machine that is behind a firewall once I have gone
away from that network.
ssh -R 8022:localhost:22 username@my.home.ip.address
This will connect to my home machine and start listening on port 8022
there. Once I get home, I can then connect back to the machine I
created the connection from using the following command:
ssh -p 8022 username@localhost
Remember to use the right username for the machine that you started
the tunnel from. It can get confusing. You also have to keep in mind
that since you are connecting to the host called localhost, but its
really a port going to a different SSH server, you may wind up with a
different host key for localhost the next time you connect to localhost.
In that case you would need to edit your .ssh/known_hosts file to
remove the localhost line. You really should know more about SSH before
doing this blindly.
As a final exercise, you can keep your reverse port forward open all the time by starting the connection with this loop:
while true ; do ssh -R 8022:localhost:22 suso@my.home.ip.address ; sleep 60 ; done
This way, if you happen to reboot your home machine, the reverse
tunnel will try to reconnect after 60 seconds. Provided you've setup
keys and your ssh-agent on the remote machine. ;-)
SOCKS5 proxying
So thats great and all, but eventually you are going to want to know
how you can do tunneling without having to specify the address that you
want to forward to.
This is accomplished through the -D SOCKS5 option.
ssh -D 9999 username@remotehost.net
Any application that supports the SOCKS5 protocol (and most of the
big network programs do) can forward its network connection over SSH and
dynamically forward to any hostname that you specify. So for a web
browser, any URL that you type in the URL field, would be sent through
the SSH tunnel. Firefox, Xchat, Gaim and many others all support using
SOCKS5. The setting is usually under preferences in the connection
settings.
Remember, in the words of Benjamin "Uncle
Ben" Parker, with great power comes great responsibility. Just because
you can get around firewalls and use other hosts for sending network
traffic, doesn't mean that some system administrator isn't going to
notice you.
Running Commands Over SSH
Sometimes you don't really want to run a shell like Bash on the host
you are connecting to. Maybe you just want to run a command and exit.
This is very simply accomplished by putting the command you wish to run
at the end of your ssh connection command.
ssh username@remotehost.net ls -l /
This will probably generate output similar to the following.
total 220
drwxr-xr-x 2 root root 4096 Nov 9 04:08 bin
drwxr-xr-x 3 root root 4096 Nov 11 09:29 boot
drwxr-xr-x 23 root root 122880 Nov 14 02:36 dev
drwxr-xr-x 68 root root 12288 Jan 10 04:03 etc
drwxr-xr-x 189 root root 4096 Jan 9 00:40 home
drwxr-xr-x 2 root root 4096 Mar 12 2004 initrd
drwxr-xr-x 9 root root 4096 Nov 9 04:07 lib
drwx------ 2 root root 16384 Sep 26 2004 lost+found
drwxr-xr-x 2 root root 4096 Apr 14 2004 misc
drwxr-xr-x 6 root root 4096 Nov 12 02:11 mnt
drwxr-xr-x 3 root root 4096 Oct 15 22:17 opt
dr-xr-xr-x 307 root root 0 Nov 14 02:36 proc
drwx------ 44 root root 8192 Jan 9 16:23 root
drwxr-xr-x 2 root root 8192 Nov 9 04:08 sbin
drwxr-xr-x 2 root root 4096 Mar 12 2004 selinux
drwxr-xr-x 9 root root 0 Nov 14 02:36 sys
drwxrwxrwt 20 root root 4096 Jan 10 06:46 tmp
drwxr-xr-x 17 root root 4096 Dec 7 2004 usr
drwxr-xr-x 26 root root 4096 Jan 10 2005 var
Then you can process the output however you want using the normal shell conventions.
You can also do something called forced-command where you force
any login attempt to run a specific command regardless of what is
specified on the command line by the client.
To do this, you put this variable and the command you want to force in the authorized_keys file on the remote host:
command="/usr/bin/backup" ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAIEAvna.....
Put the variable before the start of the line for the key. There are
other variables you can use here like from="" to allow only from a
specific host. These variables can be put together separated by commas.
(This space is intentionally left blank)
Using SCP
SCP is basically a program that uses the SSH protocol to send files
between hosts over and encrypted connection. You can transfer files from
your local computer to a remote host or vice versa or even from a
remote host to another remote host.
Here is a basic command that copies a file called report.doc from
the local computer to a file by the same name on the remote computer.
scp report.doc username@remote.host.net:
Note how the lack of a destination filename just preserves the
original name of the file. This is also the case if the remote
destination includes the path to a directory on the remote host.
To copy the file back from the server, you just reverse the from and to.
scp username@remote.host.net:report.doc report.doc
If you want to specify a new name for the file on the remote computer, simply give the name after the colon on the to side.
scp report.doc username@remote.host.net:monday.doc
Or if you want to copy it to a directory relative to the home directory for the remote user specified.
scp report.doc username@remote.host.net:reports/monday.doc
You can also use fullpaths which are preceded with a /.
To copy a whole directory recursively to a remote location, use
the -r option. The following command copies a directory named mail to
the home directory of the user on the remote computer.
scp -r mail username@remote.host.net:
Sometimes you will want to preserve the timestamps of the files and
directories and if possible, the users, groups and permissions. To do
this, use the -p option.
scp -rp mail username@remote.host.net:
Keeping Your SSH Session Alive
Sometimes you may have trouble keeping your SSH session up and idle.
For whatever reason, the connection just dies after X minutes of
inactivity. Usually this happens because there is a firewall between you
and the internet that is configured to only keep stateful connections
in its memory for 15 or so minutes.
Fortunately, in recent versions of OpenSSH, there is a fix for this problem. Simply put the following:
Host *
Protocol 2
TCPKeepAlive yes
ServerAliveInterval 60
in the file
~/.ssh/config
The file above can be used for any client side SSH configuration. See
the ssh_config man page for more details. The 'TCPKeepAlive yes'
directive tells the ssh client that it should send a little bit of data
over the connection periodically to let the server know that it is still
there. 'ServerAliveInterval 60' sets this time period for these
messages to 60 seconds. This tricks many firewalls that would otherwise
drop the connection, to keep your connection going.
Ending your SSH session
All good things come to an end. And there are many common ways to end your SSH session.
exit
logout
(Ctrl-d)
The last one is actually the user pressing the 'Ctrl' key and the
letter 'd' at the same time. These all are ways of terminating the SSH
session from the server side. They usually exit the shell which in turn
logs you off the machine.
What you may not know, is that there is another way to close an
SSH session. This is useful if you lose connectivity with the machine
and you have no way of ending your shell session. For example, this
happens momentarily if you stay logged into a machine while it is
shutdown. SSH has its own command line escape sequences. These can be
used to end connections, create new port forwards or list current ones
and a few other functions. To end a connection even when you don't have a
command prompt, type return twice (for good measure) and then the
sequence '~.'. That's a tilde followed by a period.
(RETURN) (RETURN) ~.
This will terminate the SSH connection from the client end instead of the server end.
Happy SSH'ing!
External References
Here are some links where you can find more information about SSH
Credits
- Original document, graphics and examples by Mark Krenz (mark@suso.org)
- Thank you to the following people for sending corrections:
- Zake Stahl (Several corrections)
- Christopher Mylonas (noticing that MySQL should be 3306, not 3066)
- Tehiri Tehiri (Suggesting a clarification in the username and password prompt section)
- Behrang Saeedzadeh (Noticing contact typo)
- Pratik Mallya (Suggesting I mention ssh-copy-id)
- dysonlu . (Suggesting the switch to RSA 4096 bit keys, this document was way overdue for that change, thanks)
- Other people listed on the history page of this document.