SSH(Application)

📦
version 29.1.1 · 4 versions
, in arch rolling

🌸 SSH Application 🌸

ok so bestie, welcome to SSH(Application) — brought to you by Ericsson AB, the whole thing lives in the ssh application and it implements the Secure Shell (SSH) protocol. it also hands you an SSH File Transfer Protocol (SFTP) client and server. so much, right? 💖

✨ DESCRIPTION ✨

the ssh application is, like, an implementation of the SSH protocol in Erlang. no cap. it gives you API functions so you can write your own customized SSH clients and servers, and it'll even make the Erlang shell available over SSH. an SFTP client (ssh_sftp) and server (ssh_sftpd) come along too — the SFTP server can be switched on via the subsystems daemon option. iconic behavior honestly 🫶

🎀 Dependencies

the ssh application leans on the public_key and crypto applications to handle public keys and encryption. so yes, those applications must be loaded or ssh won't work, periodt. the call ssh:start/0 does the necessary application:start/1,2 calls before it starts ssh itself, so it's got you. 💅

🦋 Configuration

the SSH application uses Configuration Parameters (yes that's a whole thing). where to set them is described in the config User's Guide, with the SSH-specific deets in Configuration in SSH.

some special config files from OpenSSH get used too, and here's the full squad:

  • known_hosts
  • authorized_keys
  • authorized_keys2
  • id_dsa (supported but disabled by default)
  • id_rsa (SHA1 sign/verify are supported but disabled by default from OTP-24)
  • id_ecdsa
  • id_ed25519
  • id_ed448
  • ssh_host_dsa_key (supported but disabled by default)
  • ssh_host_rsa_key (SHA1 sign/verify are supported but 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. you can move these locations with 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. fancy.

see also the default callback module documentation in ssh_file.

and listen — disabled public key algorithms can be enabled with the preferred_algorithms or modify_algorithms options. see Example 9 in Configuring algorithms in SSH for a description. 🔥

🔑 Public Keys

id_* are the user's private key files. heads up: the public key is part of the private key, so the ssh application does not use the id_*.pub files. those exist purely for the user's convenience when it's needed to convey the user's public key. thoughtful!

see ssh_file for details.

👑 Known Hosts

the known_hosts file has a list of approved servers and their public keys. once a server is listed, it can be verified without any user interaction. slay, no prompts. ✨

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 — and yes, the Erlang ssh daemon supports that. main character energy fr.

see ssh_file for details.

🧚 Host Keys

RSA, DSA (if enabled), ECDSA, ED25519 and ED448 host keys are all 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. so if something goes sideways, that's where you'll hear about it. 😭

⭐ Supported Specifications and Standards

the supported SSH version is 2.0. that's the vibe check and it passed.

💅 Algorithms

ok so the actual set of algorithms may vary depending on which OpenSSL crypto library is installed on the machine. for the list on a particular installation, run 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

  • mlkem768x25519-sha256
  • curve25519-sha256
  • curve25519-sha256@libssh.org
  • curve448-sha512
  • ecdh-sha2-nistp521
  • ecdh-sha2-nistp384
  • ecdh-sha2-nistp256
  • diffie-hellman-group-exchange-sha256
  • diffie-hellman-group16-sha512
  • diffie-hellman-group18-sha512
  • diffie-hellman-group14-sha256

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_algorithms options. use for example the Option value {modify_algorithms, [{append, [{kex,['diffie-hellman-group1-sha1']}]}]}) ⚠️

🗝️ Public key algorithms

  • ssh-ed25519
  • ssh-ed448
  • ecdsa-sha2-nistp521
  • ecdsa-sha2-nistp384
  • ecdsa-sha2-nistp256
  • rsa-sha2-512
  • rsa-sha2-256

the following unsecure SHA1 algorithms are supported but disabled by default:

  • (ssh-dss)
  • (ssh-rsa)

disabled public key algorithms can be enabled with the preferred_algorithms or modify_algorithms options. see Example 9 in Configuring algorithms in SSH for a description.

🍓 MAC algorithms

  • hmac-sha2-512-etm@openssh.com
  • hmac-sha2-256-etm@openssh.com
  • hmac-sha2-512
  • hmac-sha2-256
  • hmac-sha1-etm@openssh.com
  • hmac-sha1

the following unsecure SHA1 algorithm is disabled by default:

  • (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)

  • aes256-gcm@openssh.com
  • aes256-ctr
  • aes192-ctr
  • aes128-gcm@openssh.com
  • aes128-ctr
  • chacha20-poly1305@openssh.com
  • aes256-cbc
  • aes192-cbc
  • aes128-cbc
  • 3des-cbc

the following unsecure algorithms are disabled by default:

  • (AEAD_AES_128_GCM)
  • (AEAD_AES_256_GCM)

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. iykyk ✨

🦋 Compression algorithms

  • none
  • zlib@openssh.com

the following compression algorithm is deprecated in SSH and disabled by default:

  • (zlib)

it can be enabled with the preferred_algorithms or modify_algorithms options.

🎀 Unicode support

unicode filenames are supported if the emulator and the underlying OS supports it. see the file manual page in Kernel for information about this subject. 📁

the shell and the cli both support unicode. we love to see it.

📜 RFCs

the following RFCs 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
    • ssh-rsa

they are disabled by default as they now are regarded insecure, but they can be enabled with the preferred_algorithms or modify_algorithms options. see Example 8 (diffie-hellman-group1-sha1) and Example 9 (ssh-dss) in Configuring algorithms in SSH for descriptions.

  • 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 as it now is regarded insecure, but it can be enabled with the preferred_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 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:

  1. If the negotiated cipher is AEAD_AES_128_GCM, the mac algorithm is set to AEAD_AES_128_GCM.
  2. If the negotiated cipher is AEAD_AES_256_GCM, the mac algorithm is set to AEAD_AES_256_GCM.
  3. If the mac algorithm is AEAD_AES_128_GCM, the cipher is set to AEAD_AES_128_GCM.
  4. 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:

  • diffie-hellman-group1-sha1
  • diffie-hellman-group-exchange-sha1
  • diffie-hellman-group14-sha1

okay so these ones are not enabled by default anymore — they're regarded as insecure now (as they should be, honestly 💀) — but they're still supported, and you can absolutely turn them back on if you know what you're doing. How? With the preferred_algorithms or modify_algorithms options. That's it, that's the whole thing 🫶

  • 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.

bestie, what's actually implemented here is:

  • The Extension Negotiation Mechanism
  • The extension server-sig-algs
  • Secure Shell (SSH) Key Exchange Method Using Curve25519 and Curve448
  • RFC 8709 Ed25519 and Ed448 public key algorithms for the Secure Shell (SSH) protocol
  • draft-kampanakis-curdle-ssh-pq-ke, Post-Quantum Traditional Hybrid Key Exchange for SSH.

Comment: it implements mlkem768x25519-sha256. The ML-KEM-768 component is defined by NIST FIPS 203. future-proofing your little ssh sessions, we love to see it ✨

🌈 See Also 🌈

  • application

and that's the whole page, bestie, you made it to the end 🎀 this absolute icon of a manual page is from source ssh 6.0.6, with that little 6 sitting in the date spot, and it lives under the title SSH(Application) — go forth and ssh responsibly 💅

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