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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 option preferred_algorithms in the ssh:daemon/1,2,3 and
ssh:connect/3,4 functions.
Supported algorithms are:
Key exchange algorithms
-
- ecdh-sha2-nistp256
- ecdh-sha2-nistp384
- ecdh-sha2-nistp521
- diffie-hellman-group-exchange-sha1
- diffie-hellman-group-exchange-sha256
- diffie-hellman-group14-sha1
- diffie-hellman-group14-sha256
- diffie-hellman-group16-sha512
- diffie-hellman-group18-sha512
- (diffie-hellman-group1-sha1, retired: can be enabled with the preferred_algorithms option)
Public key algorithms
-
- ecdsa-sha2-nistp256
- ecdsa-sha2-nistp384
- ecdsa-sha2-nistp521
- ssh-rsa
- ssh-dss
- rsa-sha2-256
- rsa-sha2-512
MAC algorithms
-
- hmac-sha2-256
- hmac-sha2-512
- hmac-sha1
Encryption algorithms (ciphers)
-
- aes128-gcm@openssh.com
- aes256-gcm@openssh.com
- aes128-ctr
- aes192-ctr
- aes256-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 option.
- 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 option preferred_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 option preferred-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 option preferred-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