SSH

Callers can reach the text BBS over SSH, encrypted, and log in with their BBS password or with a key. Modern BBS terminals (SyncTERM, NetRunner, MagiTerm) and a plain ssh client all speak SSH, and unlike telnet and rlogin it keeps passwords off the wire.

[listen ssh]
type = ssh
address = :2222

ssh -p 2222 alice@bbs.example.org

Logging in¶

For a name with spaces, use _ or . in their place (Alice_Smith). Names are not case-sensitive. Wrong passwords count against the caller's address just as they do on a terminal (fail_delay and throttling), and a locked account cannot log in with either a password or a key.

Keys¶

Users add their own keys in one of these ways:

• on the text BBS, in Your profile (K), after giving their password again (for an account modemmenu made, its modemmenu password or a sign-in code);
• through the HTTP API, with POST /api/v1/me/keys and a login's session token (not an API key).

The sysop can also add them:

bbs sshkey add alice ~alice/.ssh/id_ed25519.pub   # each line of the file (- : stdin)
bbs sshkey list [USER]
bbs sshkey del ID

The BBS accepts ed25519, ECDSA (including the security-key sk- kinds) and RSA keys of 2048 bits or more, but not DSA. A key belongs to one user only, a user can have up to 20, and keys carry no options (command=, from=). The log records each key added, deleted or used to log in, by its SHA-256 fingerprint.

The session¶

The listener offers only a terminal session: a shell request, with a pty if the client asks for one. The pty's terminal type and size go to the text BBS just as telnet's do (xterm-256color means ANSI and UTF-8, ansi and syncterm mean ANSI and CP437, and for anything else the caller is asked). ansi, utf8 and sixel on the listener set them as they do on telnet. Commands (ssh host ls), SFTP and SCP, port and agent forwarding, X11 and a second session on the same connection are all refused. Resizing the window does not change the size the session began with.

A client has 30 seconds after connecting to ask for its shell. max_per_ip limits connections from one address, as on telnet.

Host keys¶

host_keys = /etc/bbs/ssh_host_ed25519_key /etc/bbs/ssh_host_rsa_key

By default the listener uses an ed25519 key and an RSA key beside the database, ssh_host_ed25519_key and ssh_host_rsa_key. If they are not there, it creates them (mode 0600) the first time it starts, and the log and bbs check show their fingerprints. Keys made by ssh-keygen (OpenSSH or PEM format, without a passphrase) work too. A missing key is created according to its name: RSA (3072 bits) if the name says rsa, ECDSA P-256 if it says ecdsa, and ed25519 otherwise. Like sshd, the BBS refuses a key file that other users can read.

Old clients¶

legacy = yes

This adds the AES-CBC and 3DES-CBC ciphers and the 1024-bit diffie-hellman-group1-sha1 key exchange to the usual algorithms, for old clients that have nothing newer (old SyncTERM and PuTTY builds, cryptlib). These algorithms are weak, and bbs check warns about them. Without legacy, the server offers curve25519, ECDH and DH of 2048 bits or more, with AES-GCM, ChaCha20-Poly1305 and AES-CTR.

See also: Configuration, Security, HTTP API