Kategorie:SSH/Kryptografie
Beschreibung
OpenSSH server
This section documents the Secure Shell (SSH) protocol. SSH is used to remotely manage computer systems, secururly transfer files over untrusted networks and to create "ad-hoc" virtual-private networks.
OpenSSH
- OpenSSH is the most popular implementation of the SSH protocol.
- It is maintained by the OpenBSD project and portable versions are disitributed with many unix-like operating-systems and Windows Server.
Tested with Version
- OpenSSH 6.6p1 (Gentoo)
- OpenSSH 6.6p1-2 on Ubuntu 14.04.2 LTS
- OpenSSH 7.2p2 on Ubuntu 16.04.3 LTS
Settings
- Important OpenSSH 6.6 security settings
# Package generated configuration file # See the sshd_config(5) manpage for details # What ports, IPs and protocols we listen for Port 22 # Use these options to restrict which interfaces/protocols sshd will bind to #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 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 now supports Curve25519. |
Tested with Version
- OpenSSH 6.5 (Debian Jessie)
Settings
- Important OpenSSH 6.5 security settings
# Package generated configuration file # See the sshd_config(5) manpage for details # What ports, IPs and protocols we listen for Port 22 # Use these options to restrict which interfaces/protocols sshd will bind to #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 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 diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1,diffie-hellman-group-exchange-sha1 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
Tested with Version
- OpenSSH 6.0p1 (Debian wheezy)
Settings
- Important OpenSSH 6.0 security settings
# Package generated configuration file # See the sshd_config(5) manpage for details # What ports, IPs and protocols we listen for Port 22 # Use these options to restrict which interfaces/protocols sshd will bind to #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 #Privilege Separation is turned on for security UsePrivilegeSeparation yes # Lifetime and size of ephemeral version 1 server key KeyRegenerationInterval 3600 ServerKeyBits 768 # 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 aes256-ctr,aes128-ctr MACs hmac-sha2-512,hmac-sha2-256,hmac-ripemd160 KexAlgorithms diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1,diffie-hellman-group-exchange-sha1 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
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
The OpenSSH sshd_config — OpenSSH SSH daemon configuration file man page is the best reference:
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.
Cisco ASA
Tested with Versions
- 9.1(3)
Settings
- crypto key generate rsa modulus 2048
- ssh version 2
- ssh key-exchange group dh-group14-sha1
- When the ASA is configured for SSH, by default both SSH versions 1 and 2 are allowed.
- In addition to that, only a group1 DH-key-exchange is used.
- This should be changed to allow only SSH version 2 and to use a key-exchange with group14.
- The generated RSA key should be 2048 bit (the actual supported maximum).
- A non-cryptographic best practice is to reconfigure the lines to only allow SSH-logins.
References
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.
Cisco IOS
Tested with Versions
Table 3. Tested Myservice Versions
| Program Version | OS/Distribution/Version | Comment |
| 15.0 | IOS | |
| 15.1 | IOS | |
| 15.2 | IOS |
Settings
crypto key generate rsa modulus 4096 label SSH-KEYS ip ssh rsa keypair-name SSH-KEYS ip ssh version 2 ip ssh dh min size 2048 line vty 0 15 transport input ssh
| Same as with the ASA, also on IOS by default both SSH versions 1 and 2 are allowed and the DH-key-exchange only use a DH-group of 768 Bit. In IOS, a dedicated Key-pair can be bound to SSH to reduce the usage of individual keys-pairs. From IOS Version 15.0 onwards, 4096 Bit rsa keys are supported and should be used according to the paradigm "use longest supported key". Also, do not forget to disable telnet vty access. |
References
| This guide is a basic SSH reference for all routers and switches. Pleaes 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 switch.example.net
and observe the key exchange in the output.
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.
|
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
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