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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.  The
       call ssh:start/0 will do the necessary calls to application:start/1,2 before it starts the
       ssh itself.

CONFIGURATION

       The SSH application uses Configuration Parameters. Where to  set  them  are  described  in
       config User's Guide with SSH details in Configuration in SSH.

       Some special configuration files from OpenSSH are also used:

         * known_hosts

         * authorized_keys

         * authorized_keys2

         * id_dsa(supported but disabled by default)

         * id_rsa(SHA1 sign/verify are supported but disabled by default from OTP-24)

         * id_ecdsa

         * id_ed25519

         * id_ed448

         * ssh_host_dsa_key(supported but disabled by default)

         * ssh_host_rsa_key(SHA1 sign/verify are supported but disabled by default from OTP-24)

         * ssh_host_ecdsa_key

         * ssh_host_ed25519_key

         * ssh_host_ed448_key

       By  default,  ssh  looks for id_*, known_hosts, and authorized_keys in ~/.ssh, and for the
       ssh_host_*_key files in /etc/ssh. These locations can be changed by the  options  user_dir
       and system_dir. More about where to set them is described in Configuration in SSH.

       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.

       Disabled  public  key  algorithms  can  be  enabled  with  the   preferred_algorithms   or
       modify_algorithms  options.  See  Example  9  in  Configuring  algorithms  in  SSH  for  a
       description.

PUBLIC KEYS

       id_* 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.

       See ssh_file for details.

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.

       See ssh_file for details.

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.

       See ssh_file for details.

HOST KEYS

       RSA,  DSA  (if enabled), ECDSA, ED25519 and ED448 host keys are supported and are expected
       to  be  found  in  files  named  ssh_host_rsa_key,  ssh_host_dsa_key,  ssh_host_ecdsa_key,
       ssh_host_ed25519_key and ssh_host_ed448_key.

       See ssh_file for details.

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

           The following unsecure SHA1 algorithms are now disabled by default:

           * (diffie-hellman-group14-sha1)

           * (diffie-hellman-group-exchange-sha1)

           * (diffie-hellman-group1-sha1)

           They 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

           * rsa-sha2-256

           * rsa-sha2-512

           The following unsecure SHA1 algorithms are supported but disabled by default:

           * (ssh-dss)

           * (ssh-rsa)

           See Disabled public key algorithms can be enabled  with  the  preferred_algorithms  or
           modify_algorithms  options.  See  Example  9  in  Configuring  algorithms in SSH for a
           description.

         MAC algorithms:

           * hmac-sha2-256-etm@openssh.com

           * hmac-sha2-512-etm@openssh.com

           * hmac-sha1-etm@openssh.com

           * hmac-sha2-256

           * hmac-sha2-512

           * hmac-sha1

           The following unsecure SHA1 algorithm is disabled by default:

           * (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 underlying 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

           * 6.6. Public Key Algorithms

             * ssh-dss

             * ssh-rsa

           They are disabled by default as they now  are  regarded  insecure,  but  they  can  be
           enabled  with  the  preferred_algorithms  or  modify_algorithms options. See Example 8
           (diffie-hellman-group1-sha1) and Example 9 (ssh-dss) in Configuring algorithms in  SSH
           for descriptions.

         * 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.

           Except

           * 4.1. diffie-hellman-group-exchange-sha1

           It is disabled by defaultas as it now is regarded insecure, but it can be enabled with
           the preferred_algorithms or modify_algorithms options.

         * 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:

           * diffie-hellman-group1-sha1

           * diffie-hellman-group-exchange-sha1

           * diffie-hellman-group14-sha1

           are not enabled by default as they now are regarded insecure, but are still  supported
           and can be enabled with the options preferred_algorithms or modify_algorithms.

         * 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

         * RFC 8709 Ed25519 and Ed448 public key algorithms for the Secure Shell (SSH) protocol

SEE ALSO

       application(3erl)