SSH/Server

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TMP

SSH-Server

Im Gegensatz zum SSH-Klienten ist der SSH-Server unter Ubuntu standardmäßig nicht installiert.

# apt-get install openssh-server 

installieren .

Die Konfiguration des SSH-Servers sshd findet über die Datei /etc/ssh/sshd_config statt.

• Die Voreinstellungen sind aber durchweg akzeptabel.

Wer den sshd auf einem Gateway oder Router betreibt oder aus einem anderen Grund mehrere Netzwerkschnittstellen verwendet (bspw.

• WLAN), der möchte dort vielleicht die ListenAddress-Direktive benutzen, um den Server nur an bestimmten Netzwerk-Schnittstellen laufen zu lassen.

Außerdem kann es sinnvoll sein, PermitRootLogin auf no zu setzen.

• Dann kann sich niemand direkt als root einloggen, sondern man meldet sich unter seinem Benutzernamen an und ruft dann su oder sudo -s auf.
• Dies ist aber unter Ubuntu nur relevant, sollte man dem "root"-Benutzerkonto ein Passwort zugewiesen haben.

Mit den Direktiven AllowUsers und AllowGroups oder DenyUsers und DenyGroups lässt sich noch genauer festlegen, welche Benutzer sich anmelden dürfen und welche nicht.

• Dies empfiehlt sich besonders bei Servern. AllowGroups admin verbietet bspw.
• allen Benutzern, die keine Mitglieder der Gruppe admin sind, den Zugriff.

Wer sich ausschließlich über das noch sicherere anmelden will, der sollte die Benutzung von Passwörtern mit PasswordAuthentication no abschalten.

Falls lange Wartezeiten bei der Anmeldung am SSH-Server auftreten, könnte das an einer fehlgeschlagenen Namensauflösung liegen.

• Da man SSH normalerweise sowieso über die IP benutzt, können diese DNS-Anfragen in der sshd_config deaktiviert werden.
• Der dafür nötige Eintrag wäre UseDNS no.

Nach erfolgter Änderung der Datei sshd_config muss der Server mit dem Befehl:

sudo reload ssh 

neugestartet werden, damit die Änderungen wirksam werden.

Hinweis

Standardmäßig wird der SSH-Server beim Booten geladen.

• Ab ist Upstart für den Autostart des SSH-Servers zuständig.
• Wie man den Autostart deaktiviert, wird im beschrieben.

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

Appendixes

Key material handling

Key material identifies the cryptographic secrets that compose a key. All key material must be treated as RESTRICTED data, meaning that: * Only individual with specific training and need-to-know should have access to key material.

• Key material must be encrypted on transmission.
• Key material can be stored in clear text, but only with proper access control (limited access).

This includes: * OpenSSH server keys (/etc/ssh/ssh_host_*key)

• Client keys (~/.ssh/id_{rsa,dsa,ecdsa,ed25519} and ~/.ssh/identity).

Client key size and login latency

In order to figure out the impact on performance of using larger keys - such as RSA 4096 bytes keys - on the client side, we have run a few tests:

On an idle, i7 4500 intel CPU using OpenSSH_6.7p1, OpenSSL 1.0.1l and ed25519 server keys the following command is ran 10 times:

time ssh localhost -i .ssh/id_thekey exit 

Results:

Client key	Minimum	Maximum	Average
RSA 4096	120ms	145ms	127ms
RSA 2048	120ms	129ms	127ms
ed25519	117ms	138ms	120ms

Keep in mind that these numbers may differ on a slower machine, and that this contains the complete login sequence and therefore is subject to variations. However, it seems safe to say that the latency differences are not significant and do not impact performance sufficiently to cause any concern regardless of the type of key used.

Reference documents

• Key Management
• Server Side TLS
• RFC4418 (umac)
• umac draft
• Safe curves
• DJM blog
• Stribika blog
• AES-GCM performance study
• CHACHA20 vs AES-GCM performance study
• PROTOCOL.certkeys
• rfc44880bis from GnuPG
• Weak Diffie-Hellman and the Logjam Attack
• On OpenSSH and Logjam, by Jethro Beekman