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 han-
dle 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(disabled by default)
* id_rsa(disabled by default from OTP-24)
* id_ecdsa
* id_ed25519
* id_ed448
* ssh_host_dsa_key(disabled by default)
* ssh_host_rsa_key(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.
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 pub-
lic keys. Once a server is listed, it can be verified without user in-
teraction.
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 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
* 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_algo-
rithms options. Use for example the Option value {modify_algo-
rithms, [{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
* ssh-rsa (disabled by default from OTP-24)
The following unsecure sha1 algorithm is now disabled by default:
* (ssh-dss)
It can be enabled with the preferred_algorithms or modify_algo-
rithms options. Use for example the Option value {modify_algo-
rithms, [{append, [{public_key,['ssh-dss']}]}]})
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_algo-
rithms options. Use for example the Option value {modify_algo-
rithms, [{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 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
* 6.6. Public Key Algorithms
* ssh-dss
They are disabled by default, but can be enabled with the pre-
ferred_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.
Except
* 4.1. diffie-hellman-group-exchange-sha1
It is disabled by default, but can be enabled with the pre-
ferred_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 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 re-
sponsibility 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 ap-
plied
* 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, but are still supported and can be en-
abled 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 (work in progress)
* RFC 8709 Ed25519 and Ed448 public key algorithms for the Secure
Shell (SSH) protocol
SEE ALSO
application(3erl)
Ericsson AB ssh 4.10 SSH(7)