view dbclient.1 @ 1938:77bc00dcc19f default tip main master

Bump version to 2022.82
author Matt Johnston <>
date Fri, 01 Apr 2022 14:43:27 +0800
parents 94323a20e572
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.TH dbclient 1
dbclient \- lightweight SSH client
.B dbclient
[\fIflag arguments\fR] [\-p
.I port\fR] [\-i
.I id\fR] [\-L
.I l\fR:\fIh\fR:\fIp\fR] [\-R
.I l\fR:\fIh\fR:\fIp\fR] [\-l
.IR user ]
.I host
.RI [ \fImore\ flags\fR ]
.RI [ command ]

.B dbclient

.B dbclient
is the client part of Dropbear SSH
.B command
A command to run on the remote host. This will normally be run by the remote host
using the user's shell. The command begins at the first hyphen argument after the 
host argument. If no command is specified an interactive terminal will be opened
(see -t and -T).
.B \-p \fIport
Connect to 
.I port
on the remote host. Alternatively a port can be specified as hostname^port.
Default is 22.
.B \-i \fIidfile
Identity file.
Read the identity key from file
.I idfile
(multiple allowed). This file is created with dropbearkey(1) or converted
from OpenSSH with dropbearconvert(1). The default path ~/.ssh/id_dropbear is used
.B \-L\fR [\fIlistenaddress\fR]:\fIlistenport\fR:\fIhost\fR:\fIport\fR
Local port forwarding.
.I listenport
on the local host through the SSH connection to 
.I port
.IR host .
.B \-R\fR [\fIlistenaddress\fR]:\fIlistenport\fR:\fIhost\fR:\fIport\fR
Remote port forwarding.
.I listenport
on the remote host through the SSH connection to 
.I port
.IR host .
.B \-l \fIuser
Login as
.I user
on the remote host. An alternative is to specify user@host.
.B \-t
Allocate a PTY. This is the default when no command is given, it gives a full
interactive remote session. The main effect is that keystrokes are sent remotely 
immediately as opposed to local line-based editing.
.B \-T
Don't allocate a PTY. This is the default when a command is given. See -t.
.B \-N
Don't request a remote shell or run any commands. Any command arguments are ignored.
.B \-f
Fork into the background after authentication. A command argument (or -N) is required.
This is useful when using password authentication.
.B \-g
Allow non-local hosts to connect to forwarded ports. Applies to -L and -R
forwarded ports, though remote connections to -R forwarded ports may be limited
by the ssh server.
.B \-y
Always accept hostkeys if they are unknown. If a hostkey mismatch occurs the
connection will abort as normal. If specified a second time no host key checking
is performed at all, this is usually undesirable.
.B \-A
Forward agent connections to the remote host. dbclient will use any
OpenSSH-style agent program if available ($SSH_AUTH_SOCK will be set) for
public key authentication.  Forwarding is only enabled if -A is specified.
.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
if 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.

.\" TODO: how to avoid a line break between these two -J arguments?
.B \-J \fIproxy_command
.B \-J \fI&fd
Use the standard input/output of the program \fIproxy_command\fR rather than using
a normal TCP connection. A hostname should be still be provided, as this is used for
comparing saved hostkeys. This command will be executed as "exec proxy_command ..." with the
default shell.

The second form &fd will make dbclient use the numeric file descriptor as a socket. This
can be used for more complex tunnelling scenarios. Example usage with socat is

socat EXEC:'dbclient -J &38 ev',fdin=38,fdout=38

.B \-B \fIendhost:endport
"Netcat-alike" mode, where Dropbear will connect to the given host, then create a
forwarded connection to \fIendhost\fR. This will then be presented as dbclient's
standard input/output.
.B \-c \fIcipherlist
Specify a comma separated list of ciphers to enable. Use \fI-c help\fR to list possibilities.
.B \-m \fIMAClist
Specify a comma separated list of authentication MACs to enable. Use \fI-m help\fR to list possibilities.
.B \-o \fIoption
Can be used to give options in the format used by OpenSSH config file. This is
useful for specifying options for which there is no separate command-line flag.
For full details of the options listed below, and their possible values, see
The following options have currently been implemented:

.B ExitOnForwardFailure
Specifies whether dbclient should terminate the connection if it cannot set up all requested local and remote port forwardings. The argument must be "yes" or "no".  The default is "no".
.B UseSyslog
Send dbclient log messages to syslog in addition to stderr.
.B Port
Specify a listening port, like the \fI-p\fR argument.
.B \-s 
The specified command will be requested as a subsystem, used for sftp. Dropbear doesn't implement sftp itself but the OpenSSH sftp client can be used eg \fIsftp -S dbclient user@host\fR
.B \-b \fI[address][:port]
Bind to a specific local address when connecting to the remote host. This can be used to choose from
multiple outgoing interfaces. Either address or port (or both) can be given.
.B \-V
Print the version

Dropbear will also allow multiple "hops" to be specified, separated by commas. In
this case a connection will be made to the first host, then a TCP forwarded 
connection will be made through that to the second host, and so on. Hosts other than
the final destination will not see anything other than the encrypted SSH stream. 
A port for a host can be specified with a caret (eg matt@martello^44 ).
This syntax can also be used with scp or rsync (specifying dbclient as the 
ssh/rsh command). A file can be "bounced" through multiple SSH hops, eg

scp -S dbclient matt@martello,root@wrt,canyons:/tmp/dump .

Note that hostnames are resolved by the prior hop (so "canyons" would be resolved by the host "wrt")
in the example above, the same way as other -L TCP forwarded hosts are. Host keys are 
checked locally based on the given hostname.

Typing a newline followed by the  key sequence \fI~.\fR (tilde, dot) will terminate a connection.
The sequence \fI~^Z\fR (tilde, ctrl-z) will background the connection. This behaviour only
applies when a PTY is used.

A password to use for remote authentication can be specified in the environment
variable DROPBEAR_PASSWORD. Care should be taken that the password is not
exposed to other users on a multi-user system, or stored in accessible files.
dbclient can use an external program to request a password from a user.
SSH_ASKPASS should be set to the path of a program that will return a password
on standard output. This program will only be used if either DISPLAY is set and
standard input is not a TTY, or the environment variable SSH_ASKPASS_ALWAYS is
If compiled with zlib support and if the server supports it, dbclient will
always use compression.

Matt Johnston ([email protected]).
Mihnea Stoenescu wrote initial Dropbear client support
Gerrit Pape ([email protected]) wrote this manual page.
dropbear(8), dropbearkey(1)