dropbear(8)

NAME
dropbear - lightweight SSH server

SYNOPSIS
dropbear [flag arguments] [-b banner] [-r hostkeyfile] [-p [address:]port]

DESCRIPTION
dropbear is a small SSH server

OPTIONS

-b banner
    bannerfile. Display the contents of the file banner before user login (default: none).

-r hostkey
    Use the contents of the file hostkey for the SSH hostkey. This file is generated with dropbearkey(1) or automatically with the '-R' option. See "Host Key Files" below.

-R  Generate hostkeys automatically. See "Host Key Files" below.

-F  Don't fork into background.

-E  Log to standard error rather than syslog.

-e  Pass on the server environment to all child processes. This is required, for example, if Dropbear is launched on the fly from a SLURM workload manager. The environment is not passed by default. Note that this could expose secrets in environment variables from the calling process - use with caution.

-m  Don't display the message of the day on login.

-w  Disallow root logins.

-s  Disable password logins.

-g  Disable password logins for root.

-t  Enable two-factor authentication. Both password login and public key authentication are required. Should not be used with the '-s' option.

-j  Disable local port forwarding. This includes unix stream forwards.

-k  Disable remote port forwarding.

-p [address:]port
    Listen on specified address and TCP port. If just a port is given listen on all addresses. Up to 10 can be specified (default 22 if none specified).

-l interface
    Listen on the specified interface

-i  Service program mode. Use this option to run dropbear under TCP/IP servers like inetd, tcpsvd, or tcpserver. In program mode the -F option is implied, and -p options are ignored.

-P pidfile
    Specify a pidfile to create when running as a daemon. If not specified, the default is /var/run/dropbear.pid

-a  Allow remote hosts to connect to forwarded ports.

-W windowsize
    Specify the per-channel receive window buffer size. Increasing this may improve network performance at the expense of memory use. Use -h to see the default buffer size.

-K timeout_seconds
    Ensure that traffic is transmitted at a certain interval in seconds. This is useful for working around firewalls or routers that drop connections after a certain period of inactivity. The trade-off is that a session may be closed if there is a temporary lapse of network connectivity. A setting of 0 disables keepalives. If no response is received for 3 consecutive keepalives the connection will be closed.

-I idle_timeout
    Disconnect the session if no traffic is transmitted or received for idle_timeout seconds.

-M  Set maximum session duration in seconds. All sessions will close after this time. If a compile-time setting was set (default is none), then setting -M 0 will disable the session duration timeout.

-z  By default Dropbear will send network traffic with the AF21 setting for QoS, letting network devices give it higher priority. Some devices may have problems with that, -z can be used to disable it.

-T max_authentication_attempts
    Set the number of authentication attempts allowed per connection. If unspecified the default is 10 (MAX_AUTH_TRIES)

-c forced_command
    Disregard the command/shell provided by the user and always run forced_command. This also overrides any authorized_keys command= option. The original command is saved in the SSH_ORIGINAL_COMMAND environment variable (see below).

    The forced command is started using the user's normal shell, the same as occurs for normal commands.

    Unix stream forwarding is disabled when a forced command is set, since that may allow a bypass of the forced command. Other SSH features such as TCP forwarding are still allowed by default - it may be desirable to disable those using -j or -k options when using forced command.

-D authorized_keys_dir
    Specify the directory to use for authorized_keys files. The default is ~/.ssh , paths with a leading ~/ will be home directory expanded.

-Q algo
    Query supported algorithms. Options are kex, sig, cipher, mac, compress

-V  Print the version

FILES

Authorized Keys

    ~/.ssh/authorized_keys can be set up to allow remote login with a Ed25519, RSA, or ECDSA key. The authorized_keys file and its containing ~/.ssh directory must only be writable by the user, otherwise Dropbear will not allow a login using public key authentication.

    Each line is of the form

[restrictions] ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAIgAsp... [comment]

    and can be extracted from a Dropbear private host key with "dropbearkey -y". This is the same format as used by OpenSSH, though the restrictions are a subset (keys with unknown restrictions are ignored). Restrictions are comma separated, with double quotes around spaces in arguments. Available restrictions are:

    no-port-forwarding
        Don't allow port forwarding for this connection, including unix streams.

    no-agent-forwarding
        Don't allow agent forwarding for this connection

    no-X11-forwarding
        Don't allow X11 forwarding for this connection

    no-pty
        Disable PTY allocation. Note that a user can still obtain most of the same functionality with other means even if no-pty is set.

    restrict
        Applies all the no- restrictions listed above.

    permitopen="host:port"
        Restrict local port forwarding so that connection is allowed only to the specified host and port. Multiple permitopen options separated by commas can be set in authorized_keys. Wildcard character ('*') may be used in port specification for matching any port. Hosts must be literal domain names or IP addresses.

    permitlisten="port"
        Restrict remote port forwarding listeners to the given port. Multiple permitlisten="port" options separated by commas can be set in authorized_keys. Port can be 0 to allow the listen port to be dynamically allocated on the server.

    command="forced_command"
        Disregard the command provided by the user and always run forced_command. The -c command line option overrides this. See -c documentation for behaviour of forced commands.

        When using command= it is often desirable to also use restrict option to avoid other SSH features that may allow additional access.

Host Key Files

    Host key files are read at startup from a standard location, by default /etc/dropbear/dropbear_dss_host_key, /etc/dropbear/dropbear_rsa_host_key, /etc/dropbear/dropbear_ecdsa_host_key and /etc/dropbear/dropbear_ed25519_host_key

    If the -r command line option is specified the default files are not loaded. Host key files are of the form generated by dropbearkey. The -R option can be used to automatically generate keys in the default location - keys will be generated after startup when the first connection is established. This had the benefit that the system /dev/urandom random number source has a better chance of being securely seeded.

Message Of The Day

    By default the file /etc/motd will be printed for any login shell (unless disabled at compile-time). This can also be disabled per-user by creating a file ~/.hushlogin .

ENVIRONMENT VARIABLES
Dropbear sets the standard variables USER, LOGNAME, HOME, SHELL, PATH, and TERM.

The variables below are set for sessions as appropriate.

SSH_TTY
    This is set to the allocated TTY if a PTY was used.

SSH_CONNECTION
    Contains "<remote_ip> <remote_port> <local_ip> <local_port>".

DISPLAY
    Set X11 forwarding is used.

SSH_ORIGINAL_COMMAND
    If a 'command=' authorized_keys option was used, the original command is specified in this variable. If a shell was requested this is set to an empty value.

SSH_AUTH_SOCK
    Set to a forwarded ssh-agent connection.

NOTES
Dropbear only supports SSH protocol version 2.

AUTHOR
Matt Johnston (matt@ucc.asn.au).
Gerrit Pape (pape@smarden.org) wrote this manual page.

SEE ALSO
dropbearkey(1), dbclient(1), dropbearconvert(1)

https://matt.ucc.asn.au/dropbear/dropbear.html