SSH/Kryptografie/Konfiguration

Disabling Weak MAC Algorithms on a Secure Shell Server

SSH-Server soll nur starke MAC-Algorithmen verwenden

Sie können die verfügbaren MAC-Algorithmen mit dem Befehl ermitteln

sudo sshd -T | grep mac 

MD5- und 96-Bit-MAC-Algorithmen werden als schwache Algorithmen betrachtet

• Sie müssen entfernen weden

Konfiguration

Schwachen MAC-Algorithmen entfernen

1. Melden Sie sich mit Root-Zugangsdaten an
2. Öffnen Sie die Datei /etc/ssh/sshd_config und suchen Sie nach "macs"
3. Entfernen Sie die schwachen MAC-Algorithmen, die aufgeführt sind

   Der Eintrag wird ähnlich wie die folgende Zeile aussehen und kann zusätzliche starke MAC-Algorithmen enthalten:

   macs hmac-sha1,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,hmac-ripemd160@openssh.com,umac-64@openssh.com,umac-128@openssh.com,hmac-sha1-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-ripemd160-etm@openssh.com,umac-64-etm@openssh.com,umac-128-etm@openssh.com

4. Speichern Sie die Datei
5. Starten Sie den sshd-Dienst neu, indem Sie den Befehl service sshd restart verwenden
6. Starten Sie eine neue SSH-Sitzung, bevor Sie die bestehende Sitzung schließen

   Dadurch wird die Verbindung verifiziert und Sie können sich mit dem Root-Konto am Server anmelden

7. Wenn die Verbindung zum Server fehlschlägt, machen Sie die Änderungen an der Datei sshd_config rückgängig

Anhang

Siehe auch

• SSH/Kryptografie/Konfiguration

TMP

/etc/ssh/sshd_config

# Ports to listen for
Port 22

# Restrict interfaces/protocols
#ListenAddress ::
#ListenAddress 0.0.0.0
Protocol 2

# HostKeys for protocol version 2
HostKey /etc/ssh/ssh_host_rsa_key
#HostKey /etc/ssh/ssh_host_dsa_key
#HostKey /etc/ssh/ssh_host_ecdsa_key
HostKey /etc/ssh/ssh_host_ed25519_key

# Privilege Separation is turned on for security
UsePrivilegeSeparation yes

# Lifetime and size of ephemeral version 1 server key
KeyRegenerationInterval 3600
ServerKeyBits 1024

# Logging
SyslogFacility AUTH
LogLevel INFO

# Authentication:
LoginGraceTime 120
PermitRootLogin no # or 'without-password' to allow SSH key based login
StrictModes yes
RSAAuthentication yes
PubkeyAuthentication yes
#AuthorizedKeysFile %h/.ssh/authorized_keys
# Don't read the user's ~/.rhosts and ~/.shosts files
IgnoreRhosts yes

# For this to work you will also need host keys in /etc/ssh_known_hosts
RhostsRSAAuthentication no
# similar for protocol version 2
HostbasedAuthentication no

# Uncomment if you don't trust ~/.ssh/known_hosts for RhostsRSAAuthentication
#IgnoreUserKnownHosts yes

# To enable empty passwords, change to yes (NOT RECOMMENDED)
PermitEmptyPasswords no
# Change to yes to enable challenge-response passwords (beware issues with
# some PAM modules and threads)
ChallengeResponseAuthentication no

# Change to no to disable tunnelled clear text passwords
#PasswordAuthentication yes

# Kerberos options
#KerberosAuthentication no
#KerberosGetAFSToken no
#KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes

# GSSAPI options
#GSSAPIAuthentication no
#GSSAPICleanupCredentials yes

# Cipher selection
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-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,hmac-ripemd160

KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1,diffie-hellman-group-exchange-sha1

# X11
X11Forwarding yes
X11DisplayOffset 10

PrintMotd no
PrintLastLog yes
TCPKeepAlive yes
#UseLogin no
#MaxStartups 10:30:60
#Banner /etc/issue.net

# Allow client to pass locale environment variables
AcceptEnv LANG LC_*
Subsystem sftp /usr/lib/openssh/sftp-server

# 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 yes

Curve25519

OpenSSH 6.6p1 supports Curve25519

Tested Version

OpenSSH 6.5 (Debian Jessie)

Kompatibilität

• Older Linux systems won’t support SHA2
• PuTTY (Windows) does not support RIPE-MD160
• Curve25519, AES-GCM and UMAC are only available upstream (OpenSSH 6.6p1)
• DSA host keys have been removed on purpose, the DSS standard does not support for DSA keys stronger than 1024bit [5] which is far below current standards (see section #section:keylengths)
• Legacy systems can use this configuration and simply omit unsupported ciphers, key exchange algorithms and MACs

References

• Cisco SSH is a basic SSH reference for all routers and switches
• Refer to the specific documentation of the device and IOS version that you are configuring

How to test

Connect a client with verbose logging enabled to the SSH server

$ ssh -vvv myserver.com

and observe the key exchange in the output.