Configuration de SSH

Générer de l'entropie

RNG-Tools

Avant de générer des clefs, il est nécessaire de créer de l'entropie sur le système. La solution est d'installer le paquet rng-tools (sous Debian).

sudo apt update && sudo apt install rng-tools

Pour configuer rng-tools il faut éditer le fichier /etc/default/rng-tools, mais c'est normalement inutile celui-ci pouvant reconnaître seul les sources d'entropie. Il faudra surtout éviter de le configurer avec /dev/urandom comme source (voir cet article).

Si vous avez un module TPM vous pouvez aussi ajouter tpm-rng dans le fichier de configuration /etc/initramfs-tools/modules :

sudo vi /etc/initramfs-tools/modules
sudo update-initramfs -u
sudo systemctl reboot

Installer Open-SSH Server

apt install openssh-server

Après l'installation de SSH

cd /etc/ssh/
rm ssh_host_*key*
ssh-keygen -t ed25519 -f ssh_host_ed25519_key -N "" < /dev/null
ssh-keygen -t rsa -b 4096 -f ssh_host_rsa_key -N "" < /dev/null
ssh-keygen -M generate -O bits=4096 moduli-4096.candidates
ssh-keygen -M screen -f moduli-4096.candidates moduli-4096
mv moduli moduli-`date +"%Y%m%d"`.bak
mv moduli-4096 moduli
# copier les configurations personnalisées comme ci-dessous dans /etc/ssh/sshd_config.d/ et /etc/ssh/ssh_config.d/
systemctl restart ssh.service

Fichier de configuration du serveur

Cette configuration désactive la connexion par l'utilisateur root, elle interdit l'authentification par un mot de passe et la remplace par une clef. Copiez ce fichier dans /etc/ssh/sshd_config.d/

# $OpenBSD: sshd_config,v 1.103 2018/04/09 20:41:22 tj Exp $ # This is the sshd server system-wide configuration file. See # sshd_config(5) for more information. # This sshd was compiled with PATH=/usr/bin:/bin:/usr/sbin:/sbin # The strategy used for options in the default sshd_config shipped with # OpenSSH is to specify options with their default value where # possible, but leave them commented. Uncommented options override the # default value. Port 1234 #AddressFamily any ListenAddress 0.0.0.0 #ListenAddress :: HostKey /etc/ssh/ssh_host_ed25519_key HostKey /etc/ssh/ssh_host_rsa_key #HostKey /etc/ssh/ssh_host_ecdsa_key # Ciphers and keying #RekeyLimit default none #https://www.securiteinfo.com/cryptographie/renforcer-cryptage-ssh.shtml Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-gcm@openssh.com,chacha20-poly1305@openssh.com,aes256-gcm@openssh.com HostKeyAlgorithms rsa-sha2-512,rsa-sha2-256,ssh-ed25519 KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512 MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com # Logging SyslogFacility AUTH LogLevel VERBOSE # Authentication: LoginGraceTime 30 PermitRootLogin no StrictModes yes AllowUsers user1 user2 #MaxAuthTries 6 #MaxSessions 10 # Password based logins are disabled - only public key based logins are allowed. AuthenticationMethods publickey PubkeyAuthentication yes # Expect .ssh/authorized_keys2 to be disregarded by default in future. AuthorizedKeysFile .ssh/authorized_keys .ssh/authorized_keys2 #AuthorizedPrincipalsFile none #AuthorizedKeysCommand none #AuthorizedKeysCommandUser nobody # For this to work you will also need host keys in /etc/ssh/ssh_known_hosts #HostbasedAuthentication no # Change to yes if you don't trust ~/.ssh/known_hosts for # HostbasedAuthentication #IgnoreUserKnownHosts no # Don't read the user's ~/.rhosts and ~/.shosts files #IgnoreRhosts yes # To disable tunneled clear text passwords, change to no here! PasswordAuthentication no PermitEmptyPasswords no # Change to yes to enable challenge-response passwords (beware issues with # some PAM modules and threads) ChallengeResponseAuthentication no # Kerberos options #KerberosAuthentication no #KerberosOrLocalPasswd yes #KerberosTicketCleanup yes #KerberosGetAFSToken no # GSSAPI options #GSSAPIAuthentication no #GSSAPICleanupCredentials yes #GSSAPIStrictAcceptorCheck yes #GSSAPIKeyExchange no # Set this to 'yes' to enable PAM authentication, account processing, # and session processing. If this is enabled, PAM authentication will # be allowed through the ChallengeResponseAuthentication and # PasswordAuthentication. Depending on your PAM configuration, # PAM authentication via ChallengeResponseAuthentication may bypass # the setting of "PermitRootLogin without-password". # If you just want the PAM account and session checks to run without # PAM authentication, then enable this but set PasswordAuthentication # and ChallengeResponseAuthentication to 'no'. UsePAM no #AllowAgentForwarding yes #AllowTcpForwarding yes #GatewayPorts no # Activer X11 suivant l'usage de la machine # https://www.skyminds.net/serveur-dedie-activer-x11-forwarding-pour-ssh/ X11Forwarding no #X11DisplayOffset 10 #X11UseLocalhost yes #PermitTTY yes PrintMotd no PrintLastLog yes TCPKeepAlive yes #PermitUserEnvironment no #Compression delayed #ClientAliveInterval 0 #ClientAliveCountMax 3 #UseDNS no #PidFile /var/run/sshd.pid #MaxStartups 10:30:100 #PermitTunnel no #ChrootDirectory none #VersionAddendum none # no default banner path #Banner none # Allow client to pass locale environment variables AcceptEnv LANG LC_* # override default of no subsystems #Subsystem sftp /usr/lib/openssh/sftp-server # Log sftp level file access (read/write/etc.) that would not be easily logged otherwise. #Subsystem sftp /usr/lib/ssh/sftp-server -f AUTHPRIV -l INFO # Example of overriding settings on a per-user basis #Match User anoncvs # X11Forwarding no # AllowTcpForwarding no # PermitTTY no # ForceCommand cvs server

Fichier de configuration du client

Copiez la configuration suivante dans un fichier se terminant par .conf dans le répertoire : /etc/ssh/ssh_config.d/

Host * PasswordAuthentication yes PubkeyAuthentication yes ChallengeResponseAuthentication no SendEnv LANG LC_* HashKnownHosts yes ## faster connexion GSSAPIAuthentication no GSSAPIKeyExchange no GSSAPIRenewalForcesRekey no GSSAPIDelegateCredentials no ## Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-gcm@openssh.com,aes128-ctr KexAlgorithms sntrup761x25519-sha512@openssh.com,gss-curve25519-sha256-,curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256,gss-group16-sha512-,diffie-hellman-group16-sha512 MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com RequiredRSASize 4096 HostKeyAlgorithms sk-ssh-ed25519-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256 CASignatureAlgorithms sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256 GSSAPIKexAlgorithms gss-curve25519-sha256-,gss-group16-sha512- HostbasedAcceptedAlgorithms sk-ssh-ed25519-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256 PubkeyAcceptedAlgorithms sk-ssh-ed25519-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256

Redémarrer SSH

systemctl restart ssh.service