dropbear (8) - Linux Manuals

dropbear: lightweight SSH server


dropbear - lightweight SSH server


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


dropbear is a small SSH server


-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.
Generate hostkeys automatically. See "Host Key Files" below.
Don't fork into background.
Log to standard error rather than syslog.
Don't display the message of the day on login.
Disallow root logins.
Disable password logins.
Disable password logins for root.
Disable local port forwarding.
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).
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
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 if 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.
Print the version


Authorized Keys

~/.ssh/authorized_keys can be set up to allow remote login with a RSA, ECDSA, 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:

Don't allow port forwarding for this connection

Don't allow agent forwarding for this connection

Don't allow X11 forwarding for this connection

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

Disregard the command provided by the user and always run forced_command.

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, and /etc/dropbear/dropbear_ecdsa_host_key or specified on the commandline with -r. These 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 .


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

The variables below are set for sessions as appropriate.

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

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

Set X11 forwarding is used.

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.

Set to a forwarded ssh-agent connection.


Dropbear only supports SSH protocol version 2.


Matt Johnston (matt [at] ucc.asn.au).
Gerrit Pape (pape [at] smarden.org) wrote this manual page.