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| == SSH ==
| | #WEITERLEITUNG [[SSH/Kryptografie]] |
| 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.
| |
| SSH is defined by the following Internet Standards (RFCs):
| |
| Table 2. SSH Standards
| |
| | |
| {| class="wikitable sortable options"
| |
| |-
| |
| ! align=center| RFC
| |
| ! align=center| Title
| |
| ! align=center| Link
| |
| |-
| |
| || '''4251'''
| |
| || The Secure Shell (SSH) Protocol Architecture
| |
| || [https://tools.ietf.org/html/rfc4251 https://tools.ietf.org/html/rfc4251]
| |
| |-
| |
| || '''4252'''
| |
| || The Secure Shell (SSH) Authentication Protocol
| |
| || [https://tools.ietf.org/html/rfc4252 https://tools.ietf.org/html/rfc4252]
| |
| |-
| |
| || '''4253'''
| |
| || The Secure Shell (SSH) Transport Layer Protocol
| |
| || [https://tools.ietf.org/html/rfc4253 https://tools.ietf.org/html/rfc4253]
| |
| |-
| |
| || '''6668'''
| |
| || SHA-2 Data Integrity Verification for the Secure Shell (SSH) Transport Layer Protocol
| |
| || [https://tools.ietf.org/html/rfc6668 https://tools.ietf.org/html/rfc6668]
| |
| |-
| |
| || '''8268'''
| |
| || More Modular Exponentiation (MODP) Diffie-Hellman (DH) Key Exchange (KEX) Groups for Secure Shell (SSH)
| |
| || [https://tools.ietf.org/html/rfc8268 https://tools.ietf.org/html/rfc8268]
| |
| |-
| |
| || '''8308'''
| |
| || Extension Negotiation in the Secure Shell (SSH) Protocol
| |
| || [https://tools.ietf.org/html/rfc8308 https://tools.ietf.org/html/rfc8308]
| |
| |-
| |
| || '''8332'''
| |
| || Use of RSA Keys with SHA-256 and SHA-512 in the Secure Shell (SSH) Protocol
| |
| || [https://tools.ietf.org/html/rfc8332 https://tools.ietf.org/html/rfc8332]
| |
| |-
| |
| |}
| |
| | |
| === OpenSSH ===
| |
| * [https://www.openssh.com/ OpenSSH] is the most popular implementation of the SSH protocol.
| |
| * It is maintained by the [https://openbsd.org/ 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
| |
| | |
| {| class="wikitable sortable options" style="border-spacing:0;width:7.874cm;"
| |
| |-
| |
| ||
| |
| || 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
| |
| | |
| {| class="wikitable sortable options"
| |
| |-
| |
| ||
| |
| || 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 [[https://bettercrypto.org/#_footnotedef_5 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 [https://www.openssh.org/cgi-bin/man.cgi?query=sshd_config 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
| |
| | |
| {| class="wikitable sortable options"
| |
| |-
| |
| ||
| |
| || 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 ====
| |
| [https://www.cisco.com/en/US/docs/security/asa/asa91/configuration/general/admin_management.html CLI Book 1: Cisco ASA Series General Operations CLI Configuration Guide, 9.1]
| |
| | |
| ==== 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
| |
| | |
| {| class="wikitable sortable options" style="border-spacing:0;width:9.259cm;"
| |
| |-
| |
| || 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
| |
| | |
| {| class="wikitable sortable options"
| |
| |-
| |
| ||
| |
| || 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 ====
| |
| [https://www.cisco.com/c/en/us/support/docs/security-vpn/secure-shell-ssh/4145-ssh.html Cisco SSH]
| |
| | |
| {| class="wikitable sortable options"
| |
| |-
| |
| ||
| |
| || 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.
| |
| | |
| | |
| [[Kategorie:Verschlüsselung:Best Practice]]
| |
| [[Kategorie:SSH]]
| |