20122017 Ericsson AB. All Rights Reserved. Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License. SSH ssh_app.xml
SSH The ssh application implements the Secure Shell (SSH) protocol and provides an SSH File Transfer Protocol (SFTP) client and server.

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.

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 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 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.

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

SEE ALSO

application(3)