Ubuntu Manpage: dropbear - lightweight SSH server

Provided by: dropbear-bin_2022.83-4_amd64

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.

-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).

-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.

-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 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).

-V Print the version

FILES

Authorized Keys

~/.ssh/authorized_keys can be set up to allow remote login with a RSA, ECDSA, Ed25519 or DSS key.
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

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.

command="forced_command"
Disregard the command provided by the user and always run forced_command. The -c command line
option overrides this.

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.

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.