dropbear8

Usage

Overview

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(1). 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

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.