--- name: "ssh" section: "7" platform: "linux" package: "erlang-manpages" version: "1:21.2.6+dfsg-1" release: "Debian 10 (Buster)" platforms: ["linux"] url: "https://new.carta.tech/ssh/7" --- # ssh(7) The ssh application implements the Secure Shell (SSH) protocol and provides an SSH File Transfer Protocol (SFTP) client and server. ## 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)](/application/3erl.md)**. 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) - **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 - **Encryption algorithms (ciphers):** - \* chacha20-poly1305@openssh.com - \* 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**. **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) ## SEE ALSO **[application(3erl)](/application/3erl.md)**