Provided by: dropbear-bin_2022.82-4_amd64
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. -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. -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. -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
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. 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 .
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.
Dropbear only supports SSH protocol version 2.
Matt Johnston (email@example.com). Gerrit Pape (firstname.lastname@example.org) wrote this manual page.
dropbearkey(1), dbclient(1), dropbearconvert(1) https://matt.ucc.asn.au/dropbear/dropbear.html dropbear(8)