Ubuntu Manpage: SSH - The ssh application implements the Secure Shell (SSH) protocol and

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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.  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(3erl).  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)

NAME

DESCRIPTION

DEPENDENCIES

CONFIGURATION

PUBLIC KEYS

KNOWN HOSTS

AUTHORIZED KEYS

HOST KEYS

ERROR LOGGER AND EVENT HANDLERS

SUPPORTED SPECIFICATIONS AND STANDARDS

ALGORITHMS

UNICODE SUPPORT

RFCS

SEE ALSO