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Table of Contents
Understanding and using ssh correctly
Everything you always wanted to know about ssh, ssh keys, the passphrase and ssh agent, but were afraid to ask
At least everything you need to know in order to work efficiently, without getting bored to death
A quick introduction
ssh
is a program for connecting securely to a remote server and for executing commands on this server- More precisely
ssh
is an SSH client using the SSH protocol
- We assume below that you have a
my_login
account on the remoteremote_server
computer, and you know your password
- Instead of a password, you can also use a set of private and public keys and a passphrase
- This is the only kind of authentication that will work if you need to use the IPSL Mésocentre ESPRI (aka ciclad and climserv)!
- Many programs are said to work over ssh when they implicitly use the ssh protocol to securely transfer data from one server to another:
scp
(copy remote directories and files),rsync
(synchronize remote directories and files), …
- Some history: before you were born, and the world and internet were a safer place, people used less secure programs like
telnet
,rlogin
,rsh
,ftp
, …
Using ssh
Standard usage
- The following will work in a Linux terminal, but can also work in a terminal on a Mac or on a Windows 10 computer
- On Windows 10,
ssh
is directly available in aWindows Powershell
, a Windows Terminal or the oldcmd
, but the most user-friendly way to usessh
is to use PuTTY
ssh [options] [my_login@]remote_server
- If your login is the same on the local and remote computer, you can omit the optional
my_login@
part:
e.g. simply usessh ssh1.lsce.ipsl.fr
instead ofssh my_login@ssh1.lsce.ipsl.fr
- The first time you connect to a new server,
ssh
will ask if you are sure of what you are doing, and then store some unique information about the remote server in theknown_hosts
file (details).
PS C:\Users\my_login> ssh ciclad.ipsl.jussieu.fr The authenticity of host 'ciclad.ipsl.jussieu.fr (134.157.176.129)' can't be established. RSA key fingerprint is SHA256:n6wFvMaJuyInd0LNhp78dfMd04Dr751lEekcU7X2UfU. Are you sure you want to continue connecting (yes/no)? yes Warning: Permanently added 'ciclad.ipsl.jussieu.fr,134.157.176.129' (RSA) to the list of known hosts. my_login@ciclad.ipsl.jussieu.fr: Permission denied (publickey,hostbased).
ssh
will automatically check this security information each time you connect to the same server, and warn you if something seems wrong.
Most common options
-X
: enable X11 forwarding. This option will allow you to use graphical programs on the remote server- If
-X
does not work, use-Y
instead (Enable trusted X11 forwarding) - More details in the Using an X server to display graphics section
-A
: enable agent forwarding. This is useful when you use ssh keys and an ssh agent
-t command
: this option allows you to execute a specific command on the remote server (without displaying the output of the initialssh
). We use this mostly to chain ssh connections, when we want to automatically go through a specific gateway server to access another server
e.g.ssh -A -X my_login@ssh1.lsce.ipsl.fr -t ssh -A -X obelix
-v
: verbose mode. Use this option only when you can't connect, or things don't seem to work correctly. Analyzing the verbose output when you startssh
should allow you, or the system administrators, to find out what is wrong
Connecting to the LSCE servers, IPSL servers, TGCC, ...
There are several ways to use ssh to connect to the LSCE obelixNN
servers (more details about the available LSCE servers)
- If your computer is on the LSCE ethernet/wired network:
- If your computer is outside LSCE, or on the LSCE WiFi network, you have to:
- Ask your advisor to send a mail to help-lsce, and request an access to the
ssh1
server - Go first through the
ssh1
gateway server
ssh -A -X my_LSCE_login@ssh1.lsce.ipsl.fr -t ssh -A -X obelix
If you want to connect to the IPSL servers:
- Connecting to
ciclad
:
ssh -A -X my_ciclad_login@ciclad.ipsl.jussieu.fr
If you want to connect to the the TGCC servers:
- Connecting to
irene
:
ssh -A -X my_LSCE_login@ssh1.lsce.ipsl.fr -t ssh -A -X my_TGCC_login@irene-ccrt.ccc.cea.fr
- Note: the TGCC connection details may vary, depending on your login type
If you have to use ssh
regularly (with the appropriate options), you should define the following aliases in the ~/.bashrc
configuration file of your local Linux account, or properly configure and use PuTTY on Windows
# Connecting to LSCE from a computer on the LSCE network alias obelix='ssh -A -X my_LSCE_login@obelix' # Connecting to LSCE from outside the LSCE network alias sobelix='ssh -A -X my_LSCE_login@ssh1.lsce.ipsl.fr -t ssh -A -X obelix' # Connecting to ciclad @ IPSL alias ciclad='ssh -A -X my_ciclad_login@ciclad.ipsl.jussieu.fr' # Connnecting to irene @ TGCC alias sirene='ssh -A -X my_LSCE_login@ssh1.lsce.ipsl.fr -t ssh -A -X my_TGCC_login@irene-ccrt.ccc.cea.fr'
If your connection shell is tcsh
instead of bash
, use the appropriate alias syntax in your ~/.cshrc
configuration file,
e.g. alias obelix 'ssh -A -X my_LSCE_login@obelix'
Using an X server to display graphics
A terminal can be used to display text information
e.g. the output of ls
and top
, the vi
editor, etc…
but also to start programs that will open new (graphical) windows outside of the initial terminal
e.g. evince
to display pdf files, eog
to display png/jpg images, the emacs
editor, ferret
, etc…
If you want to use ssh
to start graphical programs on a remote server, you need to:
- use
ssh -X
(orssh -Y
if-X
does not work) to connect to the remote server-X
: enable X11 forwarding-Y
: enable trusted X11 forwarding (low security, but you trust the remote server)- Using the
-X
/-Y
option will automatically define theDISPLAY
environment variable that is required by graphical programs on the remote server to determine where to display the graphical windows.
DISPLAY
will not be defined if you forget to use-X
/-Y
- Example:
my_login@my_local_computer:~$ echo $DISPLAY localhost:0.0 my_login@my_local_computer:~$ ssh ssh1.lsce.ipsl.fr Last login: Wed Jul 8 14:45:31 2020 from [...some address...] [my_login@ssh1 ~]$ echo $DISPLAY DISPLAY: Undefined variable. [my_login@ssh1 ~]$ logout Connection to ssh1.lsce.ipsl.fr closed. my_login@my_local_computer:~$ ssh -X ssh1.lsce.ipsl.fr [my_login@ssh1 ~]$ echo $DISPLAY localhost:43.0
- and have a local X server running!
An X server is basically a program running on your computer that understands the X Windows System protocol used by the remote server to display graphics- Linux computer: nothing to do, an X server is already running !
- Windows: install, configure and launch VcXsrv
- Mac:
Configuration files
ssh
will store all its configuration text files in the ~/.ssh/
directory (C:\Users\your_windows_login\.ssh
on Windows 10)
known_hosts
: the text file weressh
stores one line of security information about each server you have connected to from this computer
e.g.ciclad.ipsl.jussieu.fr,134.157.176.253 ssh-rsa AAAAB3NzaC1y[a long identifier…]
config
: an optional configuration text file, e.g.
# Empty lines and lines starting with '#' are "comments" # More details => man ssh_config ServerAliveInterval=120 ServerAliveCountMax=90
authorized_keys
: the public keys of the accounts authorized to connect to this account
- the private and public ssh keys used on this account
A recommended ssh client for Windows
PuTTY is a convenient and user-friendly ssh client for Windows
Solving common problems
- You want to start a graphical program on a remote server, but get a
Can't open display: [NO VALUE DISPLAYED HERE]
error
$ xterm & $ xterm: Xt error: Can't open display: xterm: DISPLAY is not set $ echo $DISPLAY
TheDISPLAY
variable is probably not defined because you have not specified the-X
(or-Y
) option when connecting to the remote server. See Using an X server to display graphics
- You want to start a graphical program on a remote server, but get a
Can't open display: localhost:[SOME VALUE]
error
$ xterm & $ connect localhost port 6000: Connection refused xterm: Xt error: Can't open display: localhost:12.0
TheDISPLAY
variable is defined correctly, but you probably don't have a local X server running. See Using an X server to display graphics
- Other types of errors: remember that you can run
ssh
in verbose mode to help you determine what is wrong (-v
option)
Copying files between servers/computers
Sometimes you just need to copy files from one remote server (or your desktop) to the other. The files can be securely copied over ssh with the scp
command
Note: if you work with big data files, you should keep the files were they are instead of duplicating them, and move the data processing (your scripts, etc…) to the server where the files are located (e.g. the ciclad
server at IPSL)
Copying files with scp
Note: the following will work in a Linux terminal, but can also work in a terminal on a Mac or on a Windows 10 computer (scp
is directly available in Windows Powershell
, Windows Terminal
or the old cmd
, but it is not the most user-friendly way to use ssh
on Windows)
If you have a Windows computer, it is much easier to use WinSCP for copying files
scp [options] local_path_or_file [my_login@]remote_server:remote_path
orscp [options] [my_login@]remote_server:remote_path_or_file local_path
- If your login is the same on the local and remote computer, you can omit the optional
my_login@
part
- Most common options:
-p
: preserves modification times, access times, and modes from the original file.
This option is very useful if you want the copied file(s) to have the same date/time as the original file(s). Otherwise, the time will be the time when you copy the file(s)-r
: recursively copy entire directories.
You have to use this option if the source location is a directory.scp -r
will copy the complete content of the directory (including sub-directories)
A recommended graphical scp client for Windows
WinSCP is a convenient and user-friendly scp client for Windows
Synchronizing directories
In some cases, you may want to synchronize the content of directories:
- because you are creating a backup
- because you have lots of files, possibly (very) big, and you don't want to start copying everything again if the copy fails due to temporary network problems
- …
In that case, you should use the rsync
command, that will only copy files that are not already in the destination (and that have not changed since the previous copy).
rsync
has lots of complex options and rules, and should be used carefully if you do not want to lose files. This page does not cover this topic. Use man rsync
or ask somebody
Using ssh keys
What are ssh keys and why use them?
ssh keys are a combination of two specific (and unique) text files, the private key file and the public key file, linked by a special kind of password called the passphrase, that can be used instead of a standard password to connect securely from one server to another server
ssh keys have to be configured properly (a few easy steps), and are very convenient because:
- They usually don't expire!
You don't have to change them (except in some extra secure computing centers like TGCC) and you can keep them for years - They don't depend on the accounts and the passwords of the servers where you use them
- You can (and should!) use the same set of ssh keys on several servers, and you can then connect to these servers just using the same passphrase, rather than memorizing different passwords
e.g. if you have your private key onaccount_A
ofserver_A
and install the matching public key onaccount_B
ofserver_B
, etc… you can then usessh
to accessaccount_B@server_B
,account_C@server_C
, … with the same passphrase ! - You can give your public key to somebody and then access their account using your own passphrase (no need to know the password of the other person)
- The IPSL Mésocentre ESPRI servers can only be accessed with a public key and passphrase (the password is not used)
- By default,
ssh
will ask you to type your passphrase each time you connect to a server, but you can use an ssh agent to securely store your passphrase for you
Once you have typed your passphrase in the ssh agent, you can connect to all the servers that have your public key without having to type your passphrase!scp
(andWinSCP
) and the tools usingssh
on your local computer will not ask your passphrase, if they find the passphrase in a running local ssh agent- if you use the
-A
option (agent forwarding), the remote will also know (securely) your passphrase, and you will not have to type the passphrase when usingssh
,scp
and tools running over ssh on the remote server(s)
- the ssh agent is terminated when you log out of your local computer (or reboot it)
Generating ssh keys
Some common sense advice
- Generate only one pair of private/public keys and use the same pair of keys everywhere!
Put differently, do not generate a different pair of key on each computer/server you use! - Do not use an empty passphrase!
If you do that, somebody gaining access to your private key will be able to access all the accounts where you have installed your public key - Keep a backup of your your keys outside of the computer where they were generated
- Useful if you erase or overwrite the keys by mistake, or if you move to another lab and use a new computer/account, but still need to access the accounts where you have installed your public key
- The keys can't be used easily by somebody else to gain access to your accounts if you have not used an empty passhrase
- Do not forget your passphrase!
Do not write your passphrase on a postit taped to your computer. When you create your keys and type your passphrase, choose something that you will be able to remember during several years
Generating keys in a terminal
Remember that if you already have a pair of keys, you probably don't want to generate a new pair, unless you have been asked to, or have lost one of the keys, or forgotten your passphrase. If you generate a new pair of keys, you will probably have to replace the old keys that you were using on all the remote servers
There are several ways to generate pairs of ssh keys with ssh-keygen
. The following one is the one recommended for opening an account on IPSL Mésocentre ESPRI. If you open an account on ciclad, but already have a public key, just send the existing key!
- Type
ssh-keygen -t rsa -b 4096
- Accept the default path and key name
- Do not specify an empty passphrase!
- This will generate two text key files in a sub-directory of your account (
~/.ssh
on Linux,C:\Users\my_login/.ssh/
on Windows 10):- The private key, that has to be readable only by you:
id_rsa
> cd ~/.ssh > ls -l id_dsa -rw------- 1 my_login my_group some_date id_rsa > cat id_rsa -----BEGIN RSA PRIVATE KEY----- Proc-Type: 4,ENCRYPTED DEK-Info: AES-128-CBC,906569054A4C58A28AD23CBA28771EDE C/Aacy+qcSWIG56eWc3XQhm2oyfAVKFKVm54pwoCmIZ5nmLx/8kV8XcDcMHxoWIz xgc3cPwxNczIS/i4A0AOk3uI8JiT8RVLELVbn+B5T0ewbvMjln4Ec/7W9+aNe/NF [ lots of literally cryptic lines ] v/rj1Ze/PEQ+nVX3dh3FB1TaL/aNm48PBP9WQQXm011PY6isZJklyWANGJ6jtOf9 -----END RSA PRIVATE KEY-----
- The public key:
id_rsa.pub
This is the information that you can share. Note that themy_login@my_machine
at the end of the line is just some information about who generated the keys, and where, and can be removed or replaced by something more informative
> cat id_rsa.pub ssh-rsa AAAAB3NzaC1yc2EAAAADAQ [ lots of cryptic characters ] 8WPbpreOOrIbNw== my_login@my_machine
Generating or importing keys with PuTTY on a Windows computer
Read the Importing or creating ssh keys with PuTTYgen section
Installing ssh keys
Using the keys
Using an ssh agent
More...
- If you want to know more (options, etc…), check the man(ual) page on Linux:
man ssh
- emacs
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