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man pages section 7: Standards, Environments, Macros, Character Sets, and Miscellany

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Updated: Thursday, June 13, 2019
 
 

ssh (7erl)

Name

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

Synopsis

Please see following description for synopsis

Description

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 han-
       dle 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 configura-
       tion 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.

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 pub-
       lic  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 mod-
       ify_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

           * 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)

         Public key algorithms:


           * ecdsa-sha2-nistp384

           * ecdsa-sha2-nistp521

           * ecdsa-sha2-nistp256

           * ssh-rsa

           * rsa-sha2-256

           * rsa-sha2-512

           * ssh-dss

         MAC algorithms:


           * hmac-sha2-256

           * hmac-sha2-512

           * hmac-sha1

         Encryption algorithms (ciphers):


           * aes256-gcm@openssh.com

           * aes256-ctr

           * aes192-ctr

           * aes128-gcm@openssh.com

           * aes128-ctr

           * 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  algo-
           rithm    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 imple-
           mented. 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 inter-
         pretation 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  pre-
             ferred_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

SEE ALSO
       application(3)



Ericsson AB                         ssh 4.7                             SSH(7)