SSH

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SSH (Secure SHell) est un programme de terminal chiffré qui remplace telnet , l'outil classique des systèmes d'exploitation du type Unix.

SSH (Secure SHell) is the ubiquitous tool for logging into and working on remote machines securely. All sensitive information is strongly encrypted, and in addition to the remote shell, SSH supports file transfer, and port forwarding for arbitrary protocols, allowing secure access to remote services. It replaces the classic telnet, rlogin, and similar non-secure tools - but SSH is not just a remote shell, it is a complete environment for working with remote systems.

En plus de l'accès par terminal distant procuré par le binaire principal ssh la suite de programmes SSH s'est développée pour inclure d'autres outils tels que scp (Secure Copy Program, pour programme de copie sécurisée) et sftp (Secure File Transfer Protocol, pour protocole de transfert de fichier sécurisé).

À l'origine, SSH n'était pas libre. Cependant, de nos jours, la mise en œuvre la plus populaire de SSH, qui constitue un standard de fait, est l'OpenSSH de OpenBSD. Cette version est pré-installée sur Gentoo.

SSH is multi-platform, and is very widely used: OpenSSH is installed by default on most Unix-like OSs, on Windows10, on MacOS, and can be installed on Android or "jailbroken" iOS (SSH clients are available). This makes SSH a great tool for working with heterogeneous systems.

Installation

Vérifier l'installation

La plupart des installations Gentoo Linux ont déjà OpenSSH d'installé. Sa présence peut être vérifiée avec la commande ssh. Dans le cas où SSH est disponible, une aide sur son invocation est affichée :

user $ssh
usage: ssh [-46AaCfGgKkMNnqsTtVvXxYy] [-B bind_interface]
           [-b bind_address] [-c cipher_spec] [-D [bind_address:]port]
           [-E log_file] [-e escape_char] [-F configfile] [-I pkcs11]
           [-i identity_file] [-J [user@]host[:port]] [-L address]
           [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port]
           [-Q query_option] [-R address] [-S ctl_path] [-W host:port]
           [-w local_tun[:remote_tun]] destination [command]

Dans le cas où aucune aide à l'invocation n'est affichée, SSH est soit corrompu, soit non installé. Il est aussi possible qu'un utilisateur soit en cours de recompilation d'OpenSSH afin d'inclure une nouvelle configuration USE. Quoi qu'il en soit, continuez pour voir la liste des options USE disponibles.

If this does not try to install OpenSSH, the package may have been masked, or even listed in package.provided, though this would be unusual.

Options de la variable USE

USE flags for net-misc/openssh Port of OpenBSD's free SSH release

+pie Build programs as Position Independent Executables (a security hardening technique)
+ssl Enable additional crypto algorithms via OpenSSL
audit Enable support for Linux audit subsystem using sys-process/audit
debug Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces
kerberos Add kerberos support
ldns Use LDNS for DNSSEC/SSHFP validation.
legacy-ciphers Enable support for deprecated, soon-to-be-dropped DSA keys. See https://marc.info/?l=openssh-unix-dev>m=170494903207436>w=2.
libedit Use the libedit library (replacement for readline)
livecd Enable root password logins for live-cd environment.
pam Add support for PAM (Pluggable Authentication Modules) - DANGEROUS to arbitrarily flip
security-key Include builtin U2F/FIDO support
selinux !!internal use only!! Security Enhanced Linux support, this must be set by the selinux profile or breakage will occur
static !!do not set this during bootstrap!! Causes binaries to be statically linked instead of dynamically
test Enable dependencies and/or preparations necessary to run tests (usually controlled by FEATURES=test but can be toggled independently)
verify-sig Verify upstream signatures on distfiles
xmss Enable XMSS post-quantum authentication algorithm
Data provided by the Gentoo Package Database · Last update: 2024-10-30 18:38 More information about USE flags

Emerger OpenSSH

Une fois les paramètres USE changés, n'oubliez pas d'installer (ou de recompiler) OpenSSH.

root #emerge --ask --changed-use --oneshot net-misc/openssh

After changing any global USE flags in make.conf that affect the OpenSSH package, emerge world to update to the new USE flags:

root #emerge --ask --verbose --update --deep --newuse @world

Configuration

Créer des clés

Pour vous fournir un shell sécurisé, des clés de chiffrage sont utilisées pour le chiffrage, le déchiffrage et les fonctionnalités de hâchage offertes par SSH.

Au premier démarrage du service SSH, des clés systèmes seront générées. Les clés peuvent être (ré)générées via la commande ssh-keygen.

root #/usr/bin/ssh-keygen -t ed25519 -a 100 -f /etc/ssh/ssh_host_ed25519_key -N ""
root #/usr/bin/ssh-keygen -t rsa -b 4096 -o -a 100 -f /etc/ssh/ssh_host_rsa_key -N ""

Configuration du serveur

Le serveur SSH est ordinairement configuré dans le fichier /etc/ssh/sshd_config, bien qu'il soit aussi possible de compléter la configuration dans le fichier /etc/conf.d/sshd d'OpenRC, y compris le changement de l'emplacement du fichier de configuration. Pour une information détaillée sur la manière de configurer le serveur, reportez-vous à la page de manuel de sshd_config.

The server provides means to validate its configuration using test mode:

root #/usr/sbin/sshd -t
Important
Always validate the configuration changes prior restarting the service in order to keep the remote login available.

Configuration du client

Le client ssh et les programmes apparentés (scp, sftp, etc.) peuvent être configurés grâce aux fichiers suivants :

  • ~/.ssh/config
  • /etc/ssh/ssh_config

Pour une information plus complète reportez-vous à la page de manuel ssh_config:

Intrusion prevention

SSH is a commonly attacked service. Tools such as sshguard and fail2ban monitor logs and black list remote users who have repeatedly attempted, yet failed to login. Utilize them as needed to secure a frequently attacked system.

Services

Commands to run the SSH server will depend on active init system.

OpenRC

Add the OpenSSH daemon to the default runlevel:

root #rc-update add sshd default

Start the sshd daemon with:

root #rc-service sshd start

The OpenSSH server can be controlled like any other OpenRC-managed service:

root #rc-service sshd start
root #rc-service sshd stop
root #rc-service sshd restart
Remarque
Active SSH connections to the server remain unaffected when issuing rc-service sshd restart.

systemd

To have the OpenSSH daemon start when the system starts:

root #systemctl enable sshd.service
Created symlink from /etc/systemd/system/multi-user.target.wants/sshd.service to /usr/lib64/systemd/system/sshd.service.

To start the OpenSSH daemon now:

root #systemctl start sshd.service

To check if the service has started:

root #systemctl status sshd.service

Utilisation

Commandes

OpenSSH provides several commands, see each command's man page for usage information:

Séquences d'échappement

During an active SSH session, pressing the tilde (~) key starts an escape sequence. Enter the following for a list of options:

ssh>~?

Note that escapes are only recognized immediately after a newline. They may not always work with some shells, such as fish.

Passwordless authentication to a distant SSH server

Pratique pour la gestion du serveur git.

See also
For more details see the Security Handbook.
Attention !
Leaving the passphrase empty implies the private key file will not be encrypted. An attacker having access to the local filesystem will be able to read the private key.
Client

Sur votre client, exécutez :

user $ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/larry/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/larry/.ssh/id_rsa.
Your public key has been saved in /home/larry/.ssh/id_rsa.pub.
The key fingerprint is:
de:ad:be:ef:15:g0:0d:13:37:15:ad:cc:dd:ee:ff:61 larry@client
The key's randomart image is:
+--[ RSA 2048]----+
|                 |
|     .           |
| . .. n   .      |
|   . (: . .      |
|  o   . . : .    |
| . ..: >.> .     |
|  * ?. .         |
| o.. .. ..       |
| :. .  ! .       |
+-----------------+

The default file names of the keys must not be changed, or the server may persist in asking for a password even after running ssh-copy-id as explained below. The file name will be one of:

  • id_rsa
  • id_ecdsa
  • id_ed25519

depending on the key algorithm used.

Serveur

Assurez-vous qu'un compte pour le client existe sur le serveur, et placez le contenu du fichier id_rsa.pub dans ~/.ssh/authorized_keys file.

user $ssh-copy-id <server>
/usr/bin/ssh-copy-id: INFO: Source of key(s) to be installed: "/home/larry/.ssh/id_ed25519.pub"
/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys
larry@<server>'s password: 
 
Number of key(s) added: 1
 
Now try logging into the machine, with:   "ssh '<server>'"
and check to make sure that only the key(s) you wanted were added.

Afterwards a passwordless login should be possible doing:

user $ssh <server>
larry@<server>

Then on the server, the file /etc/ssh/sshd_config should be set to PasswordAuthentication no.

Test sur une machine unique

La procédure mentionnée ci-dessus peut être testée localement :

user $ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/larry/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
...
user $mv ~/.ssh/id_rsa.pub ~/.ssh/authorized_keys
user $ssh localhost

Remote services over ssh

SSH may be used to access remote services, such as HTTP, HTTPS, fileshares, etc., through an encrypted "tunnel". Remote service access is detailed in the SSH tunneling and SSH jump host articles.

Copying files to a remote host

The SFTP command, a part of SSH, uses the SSH File Transfer Protocol to copy files to a remote host. rsync is also an alternative for this.

Remarque
The OpenSSH 8.0 release notes, from 2019, state "The scp protocol is outdated, inflexible and not readily fixed. We recommend the use of more modern protocols like sftp and rsync for file transfer instead.". The OpenSSH 8.8 release notes, from 2021, state "A near-future release of OpenSSH will switch scp(1) from using the legacy scp/rcp protocol to using SFTP by default.".

ssh-agent

OpenSSH comes with ssh-agent, a daemon to cache and prevent from frequent ssh password entries. When run, the environment variable SSH_AUTH_SOCK is used to point to ssh-agent's communication socket. The normal way to setup ssh-agent is to run it as the top most process of the user's session. Otherwise the environment variables will not be visible inside the session.

Depending on the way the graphical user session is configured to launch, it can be tricky to find a suitable way to launch ssh-agent. As an example for the lightdm display manager, edit and change /etc/lightdm/Xsession from:

user $exec $commande

into:

user $exec ssh-agent $commande

To tell ssh-agent the password once per session, either run ssh-add manually or make use of the AddKeysToAgent option.

Recent Xfce will start ssh-agent (and gpg-agent) automatically. If both are installed both will be started which makes identity management especially with SmartCards more complicated. Either stop XFCE from autostarting at least SSH's agent or disable both and use the shell, X-session or similar.

user $xfconf-query -c xfce4-session -p /startup/ssh-agent/enabled -n -t bool -s false
user $xfconf-query -c xfce4-session -p /startup/gpg-agent/enabled -n -t bool -s false

Tips

Terminal multiplexers to preserve sessions

It is possible to use a terminal multiplexer to resume a session after a dropped connection. Tmux and Screen are two popular multiplexers that can be used to be able to reconnect to a session, even if a command was running when the connection dropped out.

SSH over intermittent connections

When on unstable Internet connections, or when roaming between networks (such as when moving wifi networks), mosh can help avoid dropping SSH sessions.

Open new tabs for session with Kitty terminal

By using the SSH kitten for the Kitty terminal emulator, it is possible to open new "tabs", or windows, on the current SSH session without having log in again.

Kitty also provides other practical SSH functionality.

Benchmark the optimal rounds for an ed25519 key

Conseil
It is highly advisable to conduct benchmarking for the ed25519 key generation process, particularly considering the default value of 16 rounds and the potential performance improvements achievable with higher round values.
FILE ssh-benchmark.shBenchmark SSH Ciphers
#!/bin/sh

rounds="16 32 64 100 150"
num_runs=20

for r in $rounds; do
    printf "Benchmarking 'ssh-keygen -t ed25519 -a %s' on average:\n" "$r"
    total_time=0
    i=1
    while [ $i -le $num_runs ]; do
        start_time=$(date +%s.%N)
        ssh-keygen -t ed25519 -a "$r" -f test -N test >/dev/null 2>&1
        end_time=$(date +%s.%N)
        runtime=$(echo "$end_time - $start_time" | bc)
        total_time=$(echo "$total_time + $runtime" | bc)
        rm test{,.pub} >/dev/null 2>&1
        printf "Run %s: %s seconds\n" "$i" "$runtime"
        i=$((i + 1))
    done
    average_time=$(echo "scale=3; $total_time / $num_runs"| bc)
    printf "Average execution time: %s seconds\n\n" "$average_time"
done
Remarque
It is highly recommended to perform benchmarking for ed25519 key generation, considering both security and performance aspects. While the highest recommended round may offer superior security, it's essential to find the optimal balance based on individual requirements and time constraints. By conducting benchmark tests, one can evaluate the trade-off between security and performance, ensuring the selection of an appropriate round value for ed25519 keys.

Benchmarking is a crucial process to measure the performance and efficiency of a system or a specific component, such as cryptographic algorithms. In the context of SSH (Secure Shell) ciphers, it is important to determine the optimal number of rounds for generating ed25519 keys.

The provided script, ssh-benchmark.sh, conducts benchmarking on the ssh-keygen command with different round values for ed25519 keys. The script executes the ssh-keygen command multiple times with varying round values and measures the execution time for each run. It then calculates the average execution time for each round value.

By benchmarking different round values, system administrators and security professionals can identify the optimal round value that strikes a balance between security and performance. Higher round values generally provide stronger security but can result in increased computational overhead. Finding the right balance ensures that ed25519 keys are generated efficiently without compromising security.

Benchmarking helps identify potential bottlenecks, vulnerabilities, or areas that require improvement in security systems. It assists in selecting the most suitable algorithms and configurations for a particular use case, ensuring that security measures are robust and effective.

Dépannage

Il y a trois niveaux de débogage. -v fait que ssh imprime des messages de débogage sur sa progression. Ceci est utile pour déboguer une connexion, l'authentification, et les problèmes de configuration. De multiples options -v augmentent la loquacité. Le maximum est 3.

user $ssh example.org -v
user $ssh example.org -vv
user $ssh example.org -vvv

Permissions are too open

An ssh connection will only work if the file permissions of the ~/.ssh directory and contents are correct.

  • The ~/.ssh directory permissions should be 700 (drwx------), i.e. the owner has full access and no one else has any access.
  • Under ~/.ssh:
    • public key files' permissions should be 644 (-rw-r--r--), i.e. anyone may read the file, only the owner can write.
    • all other files' permissions should be 600 (-rw-------), i.e. only the owner may read or write the file.

These permissions need to be correct on the client and server.

Extinction des connexions à longue durée de vie

Beaucoup de périphériques d'accès à Internet effectuent de la translation d'adresses (NAT), un processus qui autorise des matériels sur un réseau privé, tels que ceux rencontrés à la maison ou au bureau, à accéder à des réseaux externes, comme l'Internet, tout en n'ayant qu'une adresse IP unique sur ces réseaux. Malheureusement, tous les périphériques NAT ne se valent pas, et quelques uns d'entre-eux ferment inadéquatement des connexions à longue durée de vie qui utilisent occasionnellement TCP comme celles qui utilisent SSH. Ceci est généralement observable comme une soudaine incapacité à interagir avec le serveur distant, alors que le programme client ssh n'a pas été interrompu.

Pour résoudre ce problème, les clients OpenSSH et les serveurs peuvent être configurés pour envoyer un message invisible , ou 'keep alive (maintenir en vie)', qui vise à maintenir et à confirmer l'état vivant du lien.

  • Pour activer keep alive pour tous les clients se connectant à votre serveur local , définissez ClientAliveInterval 30 (ou une autre valeur en secondes) dans le fichier /etc/ssh/sshd_config.
  • Pour activer keep alive pour tous les serveurs auxquels se connecte votre client local, définissez ServerAliveInterval 30 (ou une autre valeur en secondes) dans le fichier /etc/ssh/ssh_config.
  • To enable keep alive for all clients connecting to the local server, set ClientAliveInterval 30 (or some other value, in seconds) within the /etc/ssh/sshd_config file.
  • To enable keep alive for all servers connected to by the local client, set ServerAliveInterval 30 (or some other value, in seconds) within the /etc/ssh/ssh_config or ~/.ssh/config file.
  • Set TCPKeepAlive no to help eliminate disconnections.

For example, to modify the server's configuration, add following file:

FILE /etc/ssh/sshd_config/01_ClientAlive.confHelp disconnects occur less often (server)
# The following ClientAlive values will keep an inactive session open for 30 minutes
ClientAliveCountMax 60
ClientAliveInterval 30
#
# Deactivate TCPKeepAlive
TCPKeepAlive no

To modify the client's configuration, add following file:

FILE /etc/ssh/ssh_config/01_ServerAlive.confHelp disconnects occur less often (client)
# The following ServerAlive values will keep an inactive session open for 2 hours
ServerAliveInterval 60
ServerAliveCountMax 120

New key does not get used

This scenario covers the case when a key to access a remote system has been created, the public key installed on the remote system, but the remote system is (for some reason) not accessible via ssh. This can happen if the name of the keyfile is not known to ssh.

Confirm which key files ssh is trying by running it with one of the verbose options, as described at the start of the Troubleshooting section. The verbose output will include the names of the keyfiles it is trying, and the one (if any) that actually gets used.

The default key files for the system are listed in the /etc/ssh/ssh_config, see the commented-out lines containing IdentityFile directives.

There are several ways to use a key with a non-default name.

The key name can be specified on the command line every time:

user $ssh -i ~/.ssh/my_keyfile user@remotesys

Alternatively, add following ssh configuration file to add a special case for ssh to the remote system:

FILE /etc/ssh/ssh_config/02_remotesys.confDefine keyfiles to use for host remotesys
Host remotesys
    IdentityFile ~/.ssh/id_rsa
    IdentityFile ~/.ssh/my_keyfile

If any are specified, it appears to be necessary to specify all the desired keys on a remote host. Read up on the ssh IdentityFile.

X11 Forwarding, Not Forwarding, ou Tunneling

Problème: Après avoir effectué les changements nécessaires aux fichiers de configuration pour autoriser le transfert X11 ( X11 Forwarding), vous vous rendrez compte que les applications X sont exécutées sur le serveur et ne sont pas transmises au client.

Solution: Ce qui se passe très vraissemblablement lors de la connexion SSH au serveur ou hôte distant, c'est que la variable DISPLAY est, soit non définie, soit définie après que la session SSH l'ait définie.

Vérifiez ce scénario après vous être connecté à distance de la manière suivante :

user $echo $DISPLAY
localhost:10.0

Vous devriez obtenir quelque chose qui ressemble à "localhost:10.0" ou "localhost2.local:10.0" en utilisant le règlage coté serveur X11UseLocalhost no . Si vous obtenez le ":0.0" habituel, assurez-vous que vous n'effacez pas, ou ne réinitialisez pas, la variable $DISPLAY dans $HOME/.bash_profile. Si c'est le cas, retirez, ou mettez en commentaire, votre initialisation personnalisée de $DISPLAY ou empêchez bash_profile d'exécuter SSH lors de la connexion :

user $ssh -t larry@localhost2 bash --noprofile

Be sure to substitute larry in the command above with the proper username.

Une astuce consiste à créer un alias pour cela dans bashrc.

Voir aussi

  • Autossh — a command that detects when SSH connections drop and automatically reconnects them.
  • dropbear — a lightweight SSH server. It runs on a variety of POSIX-based platforms.
  • Keychain
  • Mosh — a SSH client server that is aware of connectivity problems of the original SSH implementation.
  • SCP — an interactive file transfer program, similar to the copy command, that copies files over an encrypted SSH transport.
  • SFTP — an interactive file transfer program, similar to FTP, which performs all operations over an encrypted SSH transport.
  • SSHFS — a secure shell client used to mount remote filesystems to local machines.
  • SSH tunneling — a method of connecting to machines on the other side of a gateway machine.
  • SSH jump host — employed as an alternative to SSH tunneling to access internal machines through a gateway.
  • rsync — a powerful file sync program capable of efficient file transfers and directory synchronization.
  • Securing the SSH service (Security Handbook)
  • Starting the SSH daemon — Gentoo Handbook — Installation

Ressources externes

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