view dropbear.8 @ 1938:77bc00dcc19f default tip main master

Bump version to 2022.82
author Matt Johnston <>
date Fri, 01 Apr 2022 14:43:27 +0800
parents e9854650d45b
line wrap: on
line source
.TH dropbear 8
dropbear \- lightweight SSH server
.B dropbear
[\fIflag arguments\fR] [\-b
.I banner\fR] 
.I hostkeyfile\fR] [\-p [\fIaddress\fR:]\fIport\fR]
.B dropbear
is a small SSH server 
.B \-b \fIbanner
Display the contents of the file
.I banner
before user login (default: none).
.B \-r \fIhostkey
Use the contents of the file
.I hostkey
for the SSH hostkey.
This file is generated with
.BR dropbearkey (1) 
or automatically with the '-R' option. See "Host Key Files" below.
.B \-R
Generate hostkeys automatically. See "Host Key Files" below.
.B \-F
Don't fork into background.
.B \-E
Log to standard error rather than syslog.
.B \-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.
.B \-m
Don't display the message of the day on login.
.B \-w
Disallow root logins.
.B \-s
Disable password logins.
.B \-g
Disable password logins for root.
.B \-j
Disable local port forwarding.
.B \-k
Disable remote port forwarding.
.B \-p\fR [\fIaddress\fR:]\fIport
Listen on specified 
.I address
and TCP
.I port.
If just a port is given listen
on all addresses.
Up to 10 can be specified (default 22 if none specified).
.B \-i
Service program mode.
Use this option to run
.B dropbear
under TCP/IP servers like inetd, tcpsvd, or tcpserver.
In program mode the \-F option is implied, and \-p options are ignored.
.B \-P \fIpidfile
Specify a pidfile to create when running as a daemon. If not specified, the 
default is /var/run/
.B \-a
Allow remote hosts to connect to forwarded ports.
.B \-W \fIwindowsize
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.
.B \-K \fItimeout_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.
.B \-I \fIidle_timeout
Disconnect the session if no traffic is transmitted or received for \fIidle_timeout\fR seconds.
.B \-T \fImax_authentication_attempts
Set the number of authentication attempts allowed per connection. If unspecified the default is 10 (MAX_AUTH_TRIES)
.B \-c \fIforced_command
Disregard the command provided by the user and always run \fIforced_command\fR. This also
overrides any authorized_keys command= option. The original command is saved in the 
SSH_ORIGINAL_COMMAND environment variable (see below).
.B \-V
Print the version


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:

.B no-port-forwarding
Don't allow port forwarding for this connection

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

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

.B 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.

.B restrict
Applies all the no- restrictions listed above.

.B command=\fR"\fIforced_command\fR"
Disregard the command provided by the user and always run \fIforced_command\fR.
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 .

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 ([email protected]).
Gerrit Pape ([email protected]) wrote this manual page.
dropbearkey(1), dbclient(1), dropbearconvert(1)