Sortix nightly manual
This manual documents Sortix nightly, a development build that has not been officially released. You can instead view this document in the latest official manual.
NAME
ssh — OpenSSH remote login clientSYNOPSIS
ssh | [-46AaCfGgKkMNnqsTtVvXxYy] [-B bind_interface] [-b bind_address] [-c cipher_spec] [-D [bind_address:]port] [-E log_file] [-e escape_char] [-F configfile] [-I pkcs11] [-i identity_file] [-J destination] [-L address] [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-P tag] [-p port] [-R address] [-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]] destination [command [argument ...]] |
ssh | [-Q query_option] |
DESCRIPTION
ssh (SSH client) is a program for logging into a remote machine and for executing commands on a remote machine. It is intended to provide secure encrypted communications between two untrusted hosts over an insecure network. X11 connections, arbitrary TCP ports and UNIX-domain sockets can also be forwarded over the secure channel.ssh://
[user@]hostname[:port]. The user must prove their identity to the remote machine using one of several methods (see below).- -4
-
Forces ssh to use IPv4 addresses only.
- -6
-
Forces ssh to use IPv6 addresses only.
- -A
-
Enables forwarding of connections from an authentication agent such as ssh-agent(1). This can also be specified on a per-host basis in a configuration file.
- -a
-
Disables forwarding of the authentication agent connection.
- -B bind_interface
-
Bind to the address of bind_interface before attempting to connect to the destination host. This is only useful on systems with more than one address.
- -b bind_address
-
Use bind_address on the local machine as the source address of the connection. Only useful on systems with more than one address.
- -C
-
Requests compression of all data (including stdin, stdout, stderr, and data for forwarded X11, TCP and UNIX-domain connections). The compression algorithm is the same used by gzip(1). Compression is desirable on modem lines and other slow connections, but will only slow down things on fast networks. The default value can be set on a host-by-host basis in the configuration files; see the Compression option in ssh_config(5).
- -c cipher_spec
-
Selects the cipher specification for encrypting the session. cipher_spec is a comma-separated list of ciphers listed in order of preference. See the Ciphers keyword in ssh_config(5) for more information.
- -D [bind_address:]port
-
Specifies a local “dynamic” application-level port forwarding. This works by allocating a socket to listen to port on the local side, optionally bound to the specified bind_address. Whenever a connection is made to this port, the connection is forwarded over the secure channel, and the application protocol is then used to determine where to connect to from the remote machine. Currently the SOCKS4 and SOCKS5 protocols are supported, and ssh will act as a SOCKS server. Only root can forward privileged ports. Dynamic port forwardings can also be specified in the configuration file.
- -E log_file
-
Append debug logs to log_file instead of standard error.
- -e escape_char
-
Sets the escape character for sessions with a pty (default: ‘
~
’). The escape character is only recognized at the beginning of a line. The escape character followed by a dot (‘.
’) closes the connection; followed by control-Z suspends the connection; and followed by itself sends the escape character once. Setting the character to “none” disables any escapes and makes the session fully transparent. - -F configfile
-
Specifies an alternative per-user configuration file. If a configuration file is given on the command line, the system-wide configuration file (/etc/ssh_config) will be ignored. The default for the per-user configuration file is ~/.ssh/config. If set to “none”, no configuration files will be read.
- -f
-
Requests ssh to go to background just before command execution. This is useful if ssh is going to ask for passwords or passphrases, but the user wants it in the background. This implies -n. The recommended way to start X11 programs at a remote site is with something like ssh -f host xterm.
- -G
-
Causes ssh to print its configuration after evaluating Host and Match blocks and exit.
- -g
-
Allows remote hosts to connect to local forwarded ports. If used on a multiplexed connection, then this option must be specified on the master process.
- -I pkcs11
-
Specify the PKCS#11 shared library ssh should use to communicate with a PKCS#11 token providing keys for user authentication.
- -i identity_file
-
Selects a file from which the identity (private key) for public key authentication is read. You can also specify a public key file to use the corresponding private key that is loaded in ssh-agent(1) when the private key file is not present locally. The default is ~/.ssh/id_rsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519 and ~/.ssh/id_ed25519_sk. Identity files may also be specified on a per-host basis in the configuration file. It is possible to have multiple -i options (and multiple identities specified in configuration files). If no certificates have been explicitly specified by the CertificateFile directive, ssh will also try to load certificate information from the filename obtained by appending -cert.pub to identity filenames.
- -J destination
-
Connect to the target host by first making an ssh connection to the jump host described by destination and then establishing a TCP forwarding to the ultimate destination from there. Multiple jump hops may be specified separated by comma characters. IPv6 addresses can be specified by enclosing the address in square brackets. This is a shortcut to specify a ProxyJump configuration directive. Note that configuration directives supplied on the command-line generally apply to the destination host and not any specified jump hosts. Use ~/.ssh/config to specify configuration for jump hosts.
- -K
-
Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI credentials to the server.
- -k
-
Disables forwarding (delegation) of GSSAPI credentials to the server.
- -L [bind_address:]port:host:hostport
- -L [bind_address:]port:remote_socket
- -L local_socket:host:hostport
- -L local_socket:remote_socket
-
Specifies that connections to the given TCP port or Unix socket on the local (client) host are to be forwarded to the given host and port, or Unix socket, on the remote side. This works by allocating a socket to listen to either a TCP port on the local side, optionally bound to the specified bind_address, or to a Unix socket. Whenever a connection is made to the local port or socket, the connection is forwarded over the secure channel, and a connection is made to either host port hostport, or the Unix socket remote_socket, from the remote machine.
- -l login_name
-
Specifies the user to log in as on the remote machine. This also may be specified on a per-host basis in the configuration file.
- -M
-
Places the ssh client into “master” mode for connection sharing. Multiple -M options places ssh into “master” mode but with confirmation required using ssh-askpass(1) before each operation that changes the multiplexing state (e.g. opening a new session). Refer to the description of ControlMaster in ssh_config(5) for details.
- -m mac_spec
-
A comma-separated list of MAC (message authentication code) algorithms, specified in order of preference. See the MACs keyword in ssh_config(5) for more information.
- -N
-
Do not execute a remote command. This is useful for just forwarding ports. Refer to the description of SessionType in ssh_config(5) for details.
- -n
-
Redirects stdin from /dev/null (actually, prevents reading from stdin). This must be used when ssh is run in the background. A common trick is to use this to run X11 programs on a remote machine. For example, ssh -n shadows.cs.hut.fi emacs & will start an emacs on shadows.cs.hut.fi, and the X11 connection will be automatically forwarded over an encrypted channel. The ssh program will be put in the background. (This does not work if ssh needs to ask for a password or passphrase; see also the -f option.) Refer to the description of StdinNull in ssh_config(5) for details.
- -O ctl_cmd
-
Control an active connection multiplexing master process. When the -O option is specified, the ctl_cmd argument is interpreted and passed to the master process. Valid commands are: “check” (check that the master process is running), “forward” (request forwardings without command execution), “cancel” (cancel forwardings), “exit” (request the master to exit), and “stop” (request the master to stop accepting further multiplexing requests).
- -o option
-
Can be used to give options in the format used in the configuration 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).
- AddKeysToAgent
- AddressFamily
- BatchMode
- BindAddress
- CanonicalDomains
- CanonicalizeFallbackLocal
- CanonicalizeHostname
- CanonicalizeMaxDots
- CanonicalizePermittedCNAMEs
- CASignatureAlgorithms
- CertificateFile
- CheckHostIP
- Ciphers
- ClearAllForwardings
- Compression
- ConnectionAttempts
- ConnectTimeout
- ControlMaster
- ControlPath
- ControlPersist
- DynamicForward
- EnableEscapeCommandline
- EscapeChar
- ExitOnForwardFailure
- FingerprintHash
- ForkAfterAuthentication
- ForwardAgent
- ForwardX11
- ForwardX11Timeout
- ForwardX11Trusted
- GatewayPorts
- GlobalKnownHostsFile
- GSSAPIAuthentication
- GSSAPIDelegateCredentials
- HashKnownHosts
- Host
- HostbasedAcceptedAlgorithms
- HostbasedAuthentication
- HostKeyAlgorithms
- HostKeyAlias
- Hostname
- IdentitiesOnly
- IdentityAgent
- IdentityFile
- IPQoS
- KbdInteractiveAuthentication
- KbdInteractiveDevices
- KexAlgorithms
- KnownHostsCommand
- LocalCommand
- LocalForward
- LogLevel
- MACs
- Match
- NoHostAuthenticationForLocalhost
- NumberOfPasswordPrompts
- PasswordAuthentication
- PermitLocalCommand
- PermitRemoteOpen
- PKCS11Provider
- Port
- PreferredAuthentications
- ProxyCommand
- ProxyJump
- ProxyUseFdpass
- PubkeyAcceptedAlgorithms
- PubkeyAuthentication
- RekeyLimit
- RemoteCommand
- RemoteForward
- RequestTTY
- RequiredRSASize
- SendEnv
- ServerAliveInterval
- ServerAliveCountMax
- SessionType
- SetEnv
- StdinNull
- StreamLocalBindMask
- StreamLocalBindUnlink
- StrictHostKeyChecking
- TCPKeepAlive
- Tunnel
- TunnelDevice
- UpdateHostKeys
- User
- UserKnownHostsFile
- VerifyHostKeyDNS
- VisualHostKey
- XAuthLocation
- -P tag
- Specify a tag name that may be used to select configuration in ssh_config(5). Refer to the Tag and Match keywords in ssh_config(5) for more information.
- -p port
-
Port to connect to on the remote host. This can be specified on a per-host basis in the configuration file.
- -Q query_option
-
Queries for the algorithms supported by one of the following features: cipher (supported symmetric ciphers), cipher-auth (supported symmetric ciphers that support authenticated encryption), help (supported query terms for use with the -Q flag), mac (supported message integrity codes), kex (key exchange algorithms), key (key types), key-ca-sign (valid CA signature algorithms for certificates), key-cert (certificate key types), key-plain (non-certificate key types), key-sig (all key types and signature algorithms), protocol-version (supported SSH protocol versions), and sig (supported signature algorithms). Alternatively, any keyword from ssh_config(5) or sshd_config(5) that takes an algorithm list may be used as an alias for the corresponding query_option.
- -q
-
Quiet mode. Causes most warning and diagnostic messages to be suppressed.
- -R [bind_address:]port:host:hostport
- -R [bind_address:]port:local_socket
- -R remote_socket:host:hostport
- -R remote_socket:local_socket
- -R [bind_address:]port
-
Specifies that connections to the given TCP port or Unix socket on the remote (server) host are to be forwarded to the local side.
*
’, indicates that the remote socket should listen on all interfaces. Specifying a remote bind_address will only succeed if the server's GatewayPorts option is enabled (see sshd_config(5)).0
’, the listen port will be dynamically allocated on the server and reported to the client at run time. When used together with -O forward, the allocated port will be printed to the standard output. - -S ctl_path
-
Specifies the location of a control socket for connection sharing, or the string “none” to disable connection sharing. Refer to the description of ControlPath and ControlMaster in ssh_config(5) for details.
- -s
-
May be used to request invocation of a subsystem on the remote system. Subsystems facilitate the use of SSH as a secure transport for other applications (e.g. sftp(1)). The subsystem is specified as the remote command. Refer to the description of SessionType in ssh_config(5) for details.
- -T
-
Disable pseudo-terminal allocation.
- -t
-
Force pseudo-terminal allocation. This can be used to execute arbitrary screen-based programs on a remote machine, which can be very useful, e.g. when implementing menu services. Multiple -t options force tty allocation, even if ssh has no local tty.
- -V
-
Display the version number and exit.
- -v
-
Verbose mode. Causes ssh to print debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems. Multiple -v options increase the verbosity. The maximum is 3.
- -W host:port
-
Requests that standard input and output on the client be forwarded to host on port over the secure channel. Implies -N, -T, ExitOnForwardFailure and ClearAllForwardings, though these can be overridden in the configuration file or using -o command line options.
- -w local_tun[:remote_tun]
-
Requests tunnel device forwarding with the specified tun(4) devices between the client (local_tun) and the server (remote_tun).
- -X
-
Enables X11 forwarding. This can also be specified on a per-host basis in a configuration file.
- -x
-
Disables X11 forwarding.
- -Y
-
Enables trusted X11 forwarding. Trusted X11 forwardings are not subjected to the X11 SECURITY extension controls.
- -y
- Send log information using the syslog(3) system module. By default this information is sent to stderr.
AUTHENTICATION
The OpenSSH SSH client supports SSH protocol 2.ESCAPE CHARACTERS
When a pseudo-terminal has been requested, ssh supports a number of functions through the use of an escape character.~
’) are:- ~.
- Disconnect.
- ~^Z
- Background ssh.
- ~#
- List forwarded connections.
- ~&
- Background ssh at logout when waiting for forwarded connection / X11 sessions to terminate.
- ~?
- Display a list of escape characters.
- ~B
- Send a BREAK to the remote system (only useful if the peer supports it).
- ~C
- Open command line. Currently this allows the addition of port forwardings using the -L, -R and -D options (see above). It also allows the cancellation of existing port-forwardings with -KL[bind_address:]port for local, -KR[bind_address:]port for remote and -KD[bind_address:]port for dynamic port-forwardings. !command allows the user to execute a local command if the PermitLocalCommand option is enabled in ssh_config(5). Basic help is available, using the -h option.
- ~R
- Request rekeying of the connection (only useful if the peer supports it).
- ~V
- Decrease the verbosity (LogLevel) when errors are being written to stderr.
- ~v
- Increase the verbosity (LogLevel) when errors are being written to stderr.
TCP FORWARDING
Forwarding of arbitrary TCP connections over a secure channel can be specified either on the command line or in a configuration file. One possible application of TCP forwarding is a secure connection to a mail server; another is going through firewalls.$ ssh -f -L 6667:localhost:6667 server.example.com sleep 10 $ irc -c '#users' pinky IRC/127.0.0.1
X11 FORWARDING
If the ForwardX11 variable is set to “yes” (or see the description of the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY environment variable is set), the connection to the X11 display is automatically forwarded to the remote side in such a way that any X11 programs started from the shell (or command) will go through the encrypted channel, and the connection to the real X server will be made from the local machine. The user should not manually set DISPLAY. Forwarding of X11 connections can be configured on the command line or in configuration files.VERIFYING HOST KEYS
When connecting to a server for the first time, a fingerprint of the server's public key is presented to the user (unless the option StrictHostKeyChecking has been disabled). Fingerprints can be determined using ssh-keygen(1):$ ssh-keygen -l -f /etc/ssh_host_rsa_key
$ ssh-keygen -lv -f ~/.ssh/known_hosts
$ ssh-keygen -r host.example.com.
$ dig -t SSHFP host.example.com
$ ssh -o "VerifyHostKeyDNS ask" host.example.com [...] Matching host key fingerprint found in DNS. Are you sure you want to continue connecting (yes/no)?
SSH-BASED VIRTUAL PRIVATE NETWORKS
ssh contains support for Virtual Private Network (VPN) tunnelling using the tun(4) network pseudo-device, allowing two networks to be joined securely. The sshd_config(5) configuration option PermitTunnel controls whether the server supports this, and at what level (layer 2 or 3 traffic).# ssh -f -w 0:1 192.168.1.15 true # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 # route add 10.0.99.0/24 10.1.1.2
# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 # route add 10.0.50.0/24 10.1.1.1
tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
ENVIRONMENT
ssh will normally set the following environment variables:- DISPLAY
- The DISPLAY variable indicates the location of the X11 server. It is automatically set by ssh to point to a value of the form “hostname:n”, where “hostname” indicates the host where the shell runs, and ‘n’ is an integer ≥ 1. ssh uses this special value to forward X11 connections over the secure channel. The user should normally not set DISPLAY explicitly, as that will render the X11 connection insecure (and will require the user to manually copy any required authorization cookies).
- HOME
- Set to the path of the user's home directory.
- LOGNAME
- Synonym for USER; set for compatibility with systems that use this variable.
- Set to the path of the user's mailbox.
- PATH
- Set to the default PATH, as specified when compiling ssh.
- SSH_ASKPASS
- If ssh needs a passphrase, it will read the passphrase from the current terminal if it was run from a terminal. If ssh does not have a terminal associated with it but DISPLAY and SSH_ASKPASS are set, it will execute the program specified by SSH_ASKPASS and open an X11 window to read the passphrase. This is particularly useful when calling ssh from a .xsession or related script. (Note that on some machines it may be necessary to redirect the input from /dev/null to make this work.)
- SSH_ASKPASS_REQUIRE
- Allows further control over the use of an askpass program. If this variable is set to “never” then ssh will never attempt to use one. If it is set to “prefer”, then ssh will prefer to use the askpass program instead of the TTY when requesting passwords. Finally, if the variable is set to “force”, then the askpass program will be used for all passphrase input regardless of whether DISPLAY is set.
- SSH_AUTH_SOCK
- Identifies the path of a UNIX-domain socket used to communicate with the agent.
- SSH_CONNECTION
- Identifies the client and server ends of the connection. The variable contains four space-separated values: client IP address, client port number, server IP address, and server port number.
- SSH_ORIGINAL_COMMAND
- This variable contains the original command line if a forced command is executed. It can be used to extract the original arguments.
- SSH_TTY
- This is set to the name of the tty (path to the device) associated with the current shell or command. If the current session has no tty, this variable is not set.
- SSH_TUNNEL
- Optionally set by sshd(8) to contain the interface names assigned if tunnel forwarding was requested by the client.
- SSH_USER_AUTH
- Optionally set by sshd(8), this variable may contain a pathname to a file that lists the authentication methods successfully used when the session was established, including any public keys that were used.
- TZ
- This variable is set to indicate the present time zone if it was set when the daemon was started (i.e. the daemon passes the value on to new connections).
- USER
- Set to the name of the user logging in.
FILES
- ~/.rhosts
-
This file is used for host-based authentication (see above). On some machines this file may need to be world-readable if the user's home directory is on an NFS partition, because sshd(8) reads it as root. Additionally, this file must be owned by the user, and must not have write permissions for anyone else. The recommended permission for most machines is read/write for the user, and not accessible by others.
- ~/.shosts
-
This file is used in exactly the same way as .rhosts, but allows host-based authentication without permitting login with rlogin/rsh.
- ~/.ssh/
-
This directory is the default location for all user-specific configuration and authentication information. There is no general requirement to keep the entire contents of this directory secret, but the recommended permissions are read/write/execute for the user, and not accessible by others.
- ~/.ssh/authorized_keys
-
Lists the public keys (ECDSA, Ed25519, RSA) that can be used for logging in as this user. The format of this file is described in the sshd(8) manual page. This file is not highly sensitive, but the recommended permissions are read/write for the user, and not accessible by others.
- ~/.ssh/config
-
This is the per-user configuration file. The file format and configuration options are described in ssh_config(5). Because of the potential for abuse, this file must have strict permissions: read/write for the user, and not writable by others.
- ~/.ssh/environment
-
Contains additional definitions for environment variables; see ENVIRONMENT, above.
- ~/.ssh/id_ecdsa
- ~/.ssh/id_ecdsa_sk
- ~/.ssh/id_ed25519
- ~/.ssh/id_ed25519_sk
- ~/.ssh/id_rsa
-
Contains the private key for authentication. These files contain sensitive data and should be readable by the user but not accessible by others (read/write/execute). ssh will simply ignore a private key file if it is accessible by others. It is possible to specify a passphrase when generating the key which will be used to encrypt the sensitive part of this file using AES-128.
- ~/.ssh/id_ecdsa.pub
- ~/.ssh/id_ecdsa_sk.pub
- ~/.ssh/id_ed25519.pub
- ~/.ssh/id_ed25519_sk.pub
- ~/.ssh/id_rsa.pub
-
Contains the public key for authentication. These files are not sensitive and can (but need not) be readable by anyone.
- ~/.ssh/known_hosts
-
Contains a list of host keys for all hosts the user has logged into that are not already in the systemwide list of known host keys. See sshd(8) for further details of the format of this file.
- ~/.ssh/rc
-
Commands in this file are executed by ssh when the user logs in, just before the user's shell (or command) is started. See the sshd(8) manual page for more information.
- /etc/hosts.equiv
-
This file is for host-based authentication (see above). It should only be writable by root.
- /etc/shosts.equiv
-
This file is used in exactly the same way as hosts.equiv, but allows host-based authentication without permitting login with rlogin/rsh.
- /etc/ssh_config
-
Systemwide configuration file. The file format and configuration options are described in ssh_config(5).
- /etc/ssh_host_ecdsa_key
- /etc/ssh_host_ed25519_key
- /etc/ssh_host_rsa_key
-
These files contain the private parts of the host keys and are used for host-based authentication.
- /etc/ssh_known_hosts
-
Systemwide list of known host keys. This file should be prepared by the system administrator to contain the public host keys of all machines in the organization. It should be world-readable. See sshd(8) for further details of the format of this file.
- /etc/sshrc
- Commands in this file are executed by ssh when the user logs in, just before the user's shell (or command) is started. See the sshd(8) manual page for more information.