Kategorie:SSH/Kryptografie: Unterschied zwischen den Versionen

Beschreibung

OpenSSH server

Configuration

Different versions of OpenSSH support different options which are not always compatible.

• This guide shows settings for the most commonly deployed OpenSSH versions at Mozilla - however, using the latest version of OpenSSH is recommended.

Modern (OpenSSH 6.7+)

File: /etc/ssh/sshd_config

 # Supported HostKey algorithms by order of preference.
HostKey /etc/ssh/ssh_host_ed25519_key
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ecdsa_key

KexAlgorithms curve25519-sha256@libssh.org,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256

Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr

MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com

# Password based logins are disabled - only public key based logins are allowed.

AuthenticationMethods publickey

# LogLevel VERBOSE logs user's key fingerprint on login. 

• Needed to have a clear audit track of which key was using to log in.

LogLevel VERBOSE

# 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

# Root login is not allowed for auditing reasons. 

• This is because it's difficult to track which process belongs to which root user:

#
# On Linux, user sessions are tracking using a kernel-side session id, however, this session id is not recorded by OpenSSH.
# Additionally, only tools such as systemd and auditd record the process session id.
# On other OSes, the user session id is not necessarily recorded at all kernel-side.
# Using regular users in combination with /bin/su or /usr/bin/sudo ensure a clear audit track.

PermitRootLogin No

# Use kernel sandbox mechanisms where possible in unprivileged processes
# Systrace on OpenBSD, Seccomp on Linux, seatbelt on MacOSX/Darwin, rlimit elsewhere.

UsePrivilegeSeparation sandbox

File: /etc/ssh/moduli

All Diffie-Hellman moduli in use should be at least 3072-bit-long (they are used for diffie-hellman-group-exchange-sha256) as per our Security/Guidelines/Key_Management recommendations.

• See also man moduli.

To deactivate short moduli in two commands: awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.tmp && mv /etc/ssh/moduli.tmp /etc/ssh/moduli

Intermediate (OpenSSH 5.3)

This is mainly for use by RHEL6, CentOS6, etc.

• which run older versions of OpenSSH.

File: /etc/ssh/sshd_config

# Supported HostKey algorithms by order of preference.

HostKey /etc/ssh/ssh_host_rsa_key HostKey /etc/ssh/ssh_host_ecdsa_key

KexAlgorithms diffie-hellman-group-exchange-sha256 MACs hmac-sha2-512,hmac-sha2-256 Ciphers aes256-ctr,aes192-ctr,aes128-ctr

# Password based logins are disabled - only public key based logins are allowed.

RequiredAuthentications2 publickey

# RequiredAuthentications2 not work on official OpenSSH 5.3 portable.
# In this is your case, use this instead:
#PubkeyAuthentication yes
#PasswordAuthentication no

# LogLevel VERBOSE logs user's key fingerprint on login. 

• Needed to have a clear audit track of which key was using to log in.

LogLevel VERBOSE

# 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

# Root login is not allowed for auditing reasons. 

• This is because it's difficult to track which process belongs to which root user:

#
# On Linux, user sessions are tracking using a kernel-side session id, however, this session id is not recorded by OpenSSH.
# Additionally, only tools such as systemd and auditd record the process session id.
# On other OSes, the user session id is not necessarily recorded at all kernel-side.
# Using regular users in combination with /bin/su or /usr/bin/sudo ensure a clear audit track.

PermitRootLogin No

File: /etc/ssh/moduli

All Diffie-Hellman moduli in use should be at least 2048-bit-long.

• From the structure of moduli files, this means the fifth field of all lines in this file should be greater than or equal to 2047.

To deactivate weak moduli in two commands: awk '$5 >= 2047' /etc/ssh/moduli > /etc/ssh/moduli.tmp && mv /etc/ssh/moduli.tmp /etc/ssh/moduli

Multi-Factor Authentication (OpenSSH 6.3+)

Recent versions of OpenSSH support MFA (Multi-Factor Authentication).

• Using MFA is recommended where possible.

It requires additional setup, such as using the OATH Toolkit or DuoSecurity.

ATTENTION

In order to allow using one time passwords (OTPs) and any other text input, Keyboard-interactive is enabled in OpenSSH. This MAY allow for password authentication to work. It is therefore very important to check your PAM configuration so that PAM disallow password authentication for OpenSSH.

OpenSSH 6.3+ (default)

File: /etc/ssh/sshd_config

# IMPORTANT: you will have to ensure OpenSSH cannot authenticate with passwords with PAM in /etc/pam.d/sshd
# "PasswordAuthentication no" is not sufficient!
PubkeyAuthentication yes
PasswordAuthentication no
AuthenticationMethods publickey,keyboard-interactive:pam
KbdInteractiveAuthentication yes
UsePAM yes
# Ensure /bin/login is not used so that it cannot bypass PAM settings for sshd.
UseLogin no

OpenSSH 5.3+ w/ RedHat/CentOS patch (old)

File: /etc/ssh/sshd_config

# Allow keyboard-interactive.
# IMPORTANT: you will have to ensure OpenSSH cannot authenticate with passwords with PAM in /etc/pam.d/sshd
# "PasswordAuthentication no" is not sufficient!
RequiredAuthentications2 publickey,keyboard-interactive:skey
PasswordAuthentication no
ChallengeResponseAuthentication yes
UsePAM yes
# Ensure /bin/login is not used so that it cannot bypass PAM settings for sshd.
UseLogin no

PAM configuration for use with the OATH Toolkit or DuoSecurity as second authentication factor. 

File: /etc/pam.d/sshd

#%PAM-1.0
auth       required     pam_sepermit.so

# WARNING: make sure any password authentication module is disabled.
# Example: pam_unix.so, or "password-auth", "system-auth", etc.
#auth       include      password-auth

# Options to enable when using OATH toolkit
#auth       requisite     pam_oath.so usersfile=/etc/users.oath digits=6 window=20

# Options to enable when using DuoSecurity
#auth    sufficient      /lib64/security/pam_duo.so

account    required     pam_nologin.so

Ciphers and algorithms choice

• When CHACHA20 (OpenSSH 6.5+) is not available, AES-GCM (OpenSSH 6.1+) and any other algorithm using EtM (Encrypt then MAC) disclose the packet length - giving some information to the attacker.
• Only recent OpenSSH servers and client support CHACHA20.

• NIST curves (ecdh-sha2-nistp512,ecdh-sha2-nistp384,ecdh-sha2-nistp256) are listed for compatibility, but the use of curve25519 is generally preferred.

• SSH protocol 2 supports DH and ECDH key-exchange as well as forward secrecy.
• Regarding group sizes, please refer to Security/Guidelines/Key_Management.

The various algorithms supported by a particular OpenSSH version can be listed with the following commands:

$ ssh -Q cipher
$ ssh -Q cipher-auth
$ ssh -Q mac
$ ssh -Q kex
$ ssh -Q key