ssh

SSH(7)                    Erlang Application Definition                   SSH(7)



NAME
       SSH - The ssh application implements the Secure Shell (SSH) protocol and
         provides an SSH File Transfer Protocol (SFTP) client and server.

DESCRIPTION
       The ssh application is an implementation of the SSH protocol in Erlang.
       ssh offers API functions to write customized SSH clients and servers as
       well as making the Erlang shell available over SSH. An SFTP client,
       ssh_sftp, and server, ssh_sftpd, are also included.

DEPENDENCIES
       The ssh application uses the applications public_key and crypto to handle
       public keys and encryption. Hence, these applications must be loaded for
       the ssh application to work. In an embedded environment this means that
       they must be started with application:start/1,2 before the ssh
       application is started.

CONFIGURATION
       The ssh application does not have an application- specific configuration
       file, as described in application(3). However, by default it use the
       following configuration files from OpenSSH:

         * known_hosts

         * authorized_keys

         * authorized_keys2

         * id_dsa

         * id_rsa

         * id_ecdsa

         * ssh_host_dsa_key

         * ssh_host_rsa_key

         * ssh_host_ecdsa_key

       By default, ssh looks for id_dsa, id_rsa, id_ecdsa_key, known_hosts, and
       authorized_keys in ~/.ssh, and for the host key files in /etc/ssh. These
       locations can be changed by the options user_dir and system_dir.

       Public key handling can also be customized through a callback module that
       implements the behaviors ssh_client_key_api and ssh_server_key_api.

       See also the default callback module documentation in ssh_file.

PUBLIC KEYS
       id_dsa, id_rsa and id_ecdsa are the users private key files. Notice that
       the public key is part of the private key so the ssh application does not
       use the id_<*>.pub files. These are for the user's convenience when it is
       needed to convey the user's public key.

KNOWN HOSTS
       The known_hosts file contains a list of approved servers and their public
       keys. Once a server is listed, it can be verified without user
       interaction.

AUTHORIZED KEYS
       The authorized_key file keeps track of the user's authorized public keys.
       The most common use of this file is to let users log in without entering
       their password, which is supported by the Erlang ssh daemon.

HOST KEYS
       RSA, DSA and ECDSA host keys are supported and are expected to be found
       in files named ssh_host_rsa_key, ssh_host_dsa_key and ssh_host_ecdsa_key.

ERROR LOGGER AND EVENT HANDLERS
       The ssh application uses the default OTP error logger to log unexpected
       errors or print information about special events.

SUPPORTED SPECIFICATIONS AND STANDARDS
       The supported SSH version is 2.0.

ALGORITHMS
       The actual set of algorithms may vary depending on which OpenSSL crypto
       library that is installed on the machine. For the list on a particular
       installation, use the command ssh:default_algorithms/0. The user may
       override the default algorithm configuration both on the server side and
       the client side. See the options preferred_algorithms and
       modify_algorithms in the ssh:daemon/1,2,3 and ssh:connect/3,4 functions.

       Supported algorithms are (in the default order):

         Key exchange algorithms:


           * ecdh-sha2-nistp384

           * ecdh-sha2-nistp521

           * ecdh-sha2-nistp256

           * diffie-hellman-group-exchange-sha256

           * diffie-hellman-group16-sha512

           * diffie-hellman-group18-sha512

           * diffie-hellman-group14-sha256

           * curve25519-sha256

           * curve25519-sha256@libssh.org

           * curve448-sha512

           * diffie-hellman-group14-sha1

           * diffie-hellman-group-exchange-sha1

           * (diffie-hellman-group1-sha1, retired: It can be enabled with the
             preferred_algorithms or modify_algorithms options. Use for example
             the Option value {modify_algorithms, [{append, [{kex,['diffie-
             hellman-group1-sha1']}]}]})

         Public key algorithms:


           * ecdsa-sha2-nistp384

           * ecdsa-sha2-nistp521

           * ecdsa-sha2-nistp256

           * ssh-ed25519

           * ssh-ed448

           * ssh-rsa

           * rsa-sha2-256

           * rsa-sha2-512

           * ssh-dss

         MAC algorithms:


           * hmac-sha2-256

           * hmac-sha2-512

           * hmac-sha1

           * (hmac-sha1-96 It can be enabled with the preferred_algorithms or
             modify_algorithms options. Use for example the Option value
             {modify_algorithms, [{append, [{mac,['hmac-sha1-96']}]}]})

         Encryption algorithms (ciphers):


           * chacha20-poly1305@openssh.com

           * aes256-gcm@openssh.com

           * aes256-ctr

           * aes192-ctr

           * aes128-gcm@openssh.com

           * aes128-ctr

           * aes256-cbc

           * aes192-cbc

           * aes128-cbc

           * 3des-cbc

           * (AEAD_AES_128_GCM, not enabled per default)

           * (AEAD_AES_256_GCM, not enabled per default)

           See the text at the description of the rfc 5647 further down for more
           information regarding AEAD_AES_*_GCM.

           Following the internet de-facto standard, the cipher and mac
           algorithm AEAD_AES_128_GCM is selected when the cipher
           aes128-gcm@openssh.com is negotiated. The cipher and mac algorithm
           AEAD_AES_256_GCM is selected when the cipher aes256-gcm@openssh.com
           is negotiated.

         Compression algorithms:


           * none

           * zlib@openssh.com

           * zlib

UNICODE SUPPORT
       Unicode filenames are supported if the emulator and the underlaying OS
       support it. See section DESCRIPTION in the file manual page in Kernel for
       information about this subject.

       The shell and the cli both support unicode.

RFCS
       The following rfc:s are supported:

         * RFC 4251, The Secure Shell (SSH) Protocol Architecture.

           Except

           * 9.4.6 Host-Based Authentication

           * 9.5.2 Proxy Forwarding

           * 9.5.3 X11 Forwarding

         * RFC 4252, The Secure Shell (SSH) Authentication Protocol.

           Except

           * 9. Host-Based Authentication: "hostbased"

         * RFC 4253, The Secure Shell (SSH) Transport Layer Protocol.

           Except

           * 8.1. diffie-hellman-group1-sha1. Disabled by default, can be
             enabled with the preferred_algorithms or modify_algorithms options.

         * RFC 4254, The Secure Shell (SSH) Connection Protocol.

           Except

           * 6.3. X11 Forwarding

           * 7. TCP/IP Port Forwarding

         * RFC 4256, Generic Message Exchange Authentication for the Secure
           Shell Protocol (SSH).

           Except

           * num-prompts > 1

           * password changing

           * other identification methods than userid-password

         * RFC 4419, Diffie-Hellman Group Exchange for the Secure Shell (SSH)
           Transport Layer Protocol.

         * RFC 4716, The Secure Shell (SSH) Public Key File Format.

         * RFC 5647, AES Galois Counter Mode for the Secure Shell Transport
           Layer Protocol.

           There is an ambiguity in the synchronized selection of cipher and mac
           algorithm. This is resolved by OpenSSH in the ciphers
           aes128-gcm@openssh.com and aes256-gcm@openssh.com which are
           implemented. If the explicit ciphers and macs AEAD_AES_128_GCM or
           AEAD_AES_256_GCM are needed, they could be enabled with the options
           preferred_algorithms or modify_algorithms.

     Warning:
         If the client or the server is not Erlang/OTP, it is the users
         responsibility to check that other implementation has the same
         interpretation of AEAD_AES_*_GCM as the Erlang/OTP SSH before enabling
         them. The aes*-gcm@openssh.com variants are always safe to use since
         they lack the ambiguity.


           The second paragraph in section 5.1 is resolved as:

           * If the negotiated cipher is AEAD_AES_128_GCM, the mac algorithm is
             set to AEAD_AES_128_GCM.

           * If the negotiated cipher is AEAD_AES_256_GCM, the mac algorithm is
             set to AEAD_AES_256_GCM.

           * If the mac algorithm is AEAD_AES_128_GCM, the cipher is set to
             AEAD_AES_128_GCM.

           * If the mac algorithm is AEAD_AES_256_GCM, the cipher is set to
             AEAD_AES_256_GCM.

           The first rule that matches when read in order from the top is
           applied

         * RFC 5656, Elliptic Curve Algorithm Integration in the Secure Shell
           Transport Layer.

           Except

           * 5. ECMQV Key Exchange

           * 6.4. ECMQV Key Exchange and Verification Method Name

           * 7.2. ECMQV Message Numbers

           * 10.2. Recommended Curves

         * RFC 6668, SHA-2 Data Integrity Verification for the Secure Shell
           (SSH) Transport Layer Protocol

           Comment: Defines hmac-sha2-256 and hmac-sha2-512

         * Draft-ietf-curdle-ssh-kex-sha2 (work in progress), Key Exchange (KEX)
           Method Updates and Recommendations for Secure Shell (SSH).

           Deviations:

           * The diffie-hellman-group1-sha1 is not enabled by default, but is
             still supported and can be enabled with the options
             preferred_algorithms or modify_algorithms.

           * The questionable sha1-based algorithms diffie-hellman-group-
             exchange-sha1 and diffie-hellman-group14-sha1 are still enabled by
             default for compatibility with ancient clients and servers. They
             can be disabled with the options preferred_algorithms or
             modify_algorithms. They will be disabled by default when the draft
             is turned into an RFC.

         * RFC 8332, Use of RSA Keys with SHA-256 and SHA-512 in the Secure
           Shell (SSH) Protocol.

         * RFC 8308, Extension Negotiation in the Secure Shell (SSH) Protocol.

           Implemented are:

           * The Extension Negotiation Mechanism

           * The extension server-sig-algs

         * Secure Shell (SSH) Key Exchange Method using Curve25519 and Curve448
           (work in progress)

         * Ed25519 and Ed448 public key algorithms for the Secure Shell (SSH)
           protocol (work in progress)

SEE ALSO
       application(3)



Ericsson AB                        ssh 4.9.1.3                            SSH(7)