dbclient(1) General Commands Manual dbclient(1)
NAME
dbclient - lightweight SSH client
SYNOPSIS
dbclient [flag arguments] [-p port] [-i id] [-L l:h:p] [-R l:h:p] [-l
user] host [more flags] [command]
dbclient [args] [user1]@host1[^port1],[user2]@host2[^port2],...
DESCRIPTION
dbclient is the client part of Dropbear SSH
OPTIONS
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).
-p port
Connect to port on the remote host. Alternatively a port can be
specified as hostname^port. Default is 22.
-i idfile
Identity file. Read the identity key from file 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
-L [listenaddress]:listenport:host:port
Local port forwarding. Forward listenport on the local host
through the SSH connection to port on host.
-R [listenaddress]:listenport:host:port
Remote port forwarding. Forward listenport on the remote host
through the SSH connection to port on host.
-l user
Username. Login as user on the remote host. An alternative is
to specify user@host.
-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.
-T Don't allocate a PTY. This is the default when a command is
given. See -t.
-N Don't request a remote shell or run any commands. Any command
arguments are ignored.
-f Fork into the background after authentication. A command argu-
ment (or -N) is required. This is useful when using password
authentication.
-g Allow non-local hosts to connect to forwarded ports. Applies to
-L and -R forwarded ports, though remote connections to -R for-
warded ports may be limited by the ssh server.
-y Always accept hostkeys if they are unknown. If a hostkey mis-
match occurs the connection will abort as normal. If specified a
second time no host key checking is performed at all, this is
usually undesirable.
-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.
Beware that a forwarded agent connection will allow the remote
server to have the same authentication credentials as you have
used locally. A compromised remote server could use that to log
in to other servers.
In many situations Dropbear's multi-hop mode is a better and
more secure alternative to agent forwarding, avoiding having to
trust the intermediate server.
If the SSH agent program is set to prompt when a key is used,
the -o DisableTrivialAuth option can prevent UI confusion.
-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 sec-
onds. 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 tempo-
rary 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.
-z By default Dropbear will send network traffic with the AF21 set-
ting for QoS, letting network devices give it higher priority.
Some devices may have problems with that, -z can be used to dis-
able it.
-J proxy_command
-J &fd
Use the standard input/output of the program proxy_command
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 de-
scriptor as a socket. This can be used for more complex tun-
nelling scenarios. Example usage with socat is
socat EXEC:'dbclient -J &38 ev',fdin=38,fdout=38 TCP4:host.exam-
ple.com:22
-B endhost:endport
"Netcat-alike" mode, where Dropbear will connect to the given
host, then create a forwarded connection to endhost. This will
then be presented as dbclient's standard input/output.
-c cipherlist
Specify a comma separated list of ciphers to enable. Use -c help
to list possibilities.
-m MAClist
Specify a comma separated list of authentication MACs to enable.
Use -m help to list possibilities.
-o option
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 ssh_config(5). The
following options have currently been implemented:
ExitOnForwardFailure
Specifies whether dbclient should terminate the connec-
tion if it cannot set up all requested local and remote
port forwardings. The argument must be "yes" or "no".
The default is "no".
UseSyslog
Send dbclient log messages to syslog in addition to
stderr.
Port Specify a listening port, like the -p argument.
DisableTrivialAuth
Disallow a server immediately giving successful authenti-
cation (without presenting any password/pubkey prompt).
This avoids a UI confusion issue where it may appear that
the user is accepting a SSH agent prompt from their local
machine, but are actually accepting a prompt sent immedi-
ately by the remote server.
-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 sftp -S dbclient user@host
-b [address][:port]
Bind to a specific local address when connecting to the remote
host. This can be used to choose from multiple outgoing inter-
faces. Either address or port (or both) can be given.
-V Print the version
MULTI-HOP
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.
ESCAPE CHARACTERS
Typing a newline followed by the key sequence ~. (tilde, dot) will
terminate a connection. The sequence ~^Z (tilde, ctrl-z) will back-
ground the connection. This behaviour only applies when a PTY is used.
ENVIRONMENT
DROPBEAR_PASSWORD
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.
SSH_ASKPASS
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 set.
NOTES
If compiled with zlib support and if the server supports it, dbclient
will always use compression.
AUTHOR
Matt Johnston (matt@ucc.asn.au).
Mihnea Stoenescu wrote initial Dropbear client support
Gerrit Pape (pape@smarden.org) wrote this manual page.
SEE ALSO
dropbear(8), dropbearkey(1)
https://matt.ucc.asn.au/dropbear/dropbear.html
dbclient(1)
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