secsh (SSH client) is a program for logging into a remote machine and for executing commands on a remote a machine. You can also call this program as ssh. secsh is intended to replace rlogin and rsh, and provide secure encrypted communications between two untrusted hosts over an insecure network. X11 connections, arbitrary TCP/IP ports and Unix-domain sockets can also be forwarded over the secure channel.
secsh connects and logs into the specified hostname. The user must prove his/her identity to the remote machine using one of several methods depending on the protocol version used.
First, if the machine the user logs in from is listed in $ROOTDIR/etc/hosts.equiv or $ROOTDIR/etc/shosts.equiv on the remote machine, and the user names are the same on both sides, the user is immediately permitted to log in. Second, if .rhosts or .shosts exists in the user's home directory on the remote machine and contains a line containing the name of the client machine and the name of the user on that machine, the user is permitted to log in. This form of authentication alone is normally not allowed by the server because it is not secure.
The second authentication method is the rhosts or hosts.equiv method combined with RSA-based host authentication. It means that if the login would be permitted by ~/.rhosts, ~/.shosts, $ROOTDIR/etc/hosts.equiv, or $ROOTDIR/etc/shosts.equiv, and if additionally the server can verify the client's host key (see $ROOTDIR/etc/ssh_known_hosts and ~/.ssh/known_hosts in the FILES section), only then login is permitted. This authentication method closes security holes due to IP spoofing, DNS spoofing and routing spoofing. [Note to the administrator: $ROOTDIR/etc/hosts.equiv, ~/.rhosts, and the rlogin/rsh protocol in general, are inherently insecure and should be disabled if security is desired.]
As a third authentication method, secsh supports RSA based authentication. The scheme is based on public-key cryptography: there are cryptosystems where encryption and decryption are done using separate keys, and it is not possible to derive the decryption key from the encryption key. RSA is one such system. The idea is that each user creates a public/private key pair for authentication purposes. The server knows the public key, and only the user knows the private key. The file ~/.ssh/authorized_keys lists the public keys that are permitted for logging in. When the user logs in, the secsh program tells the server which key pair it would like to use for authentication. The server checks if this key is permitted, and if so, sends the user (actually the secsh program running on behalf of the user) a challenge, a random number, encrypted by the user's public key. The challenge can only be decrypted using the proper private key. The user's client then decrypts the challenge using the private key, proving that he/she knows the private key but without disclosing it to the server.
secsh implements the RSA authentication protocol automatically. The user creates his/her RSA key pair by running secsh-keygen This stores the private key in $USERPROFILE/.ssh/identity and the public key in $USERPROFILE/.ssh/identity.pub in the user's home directory. The user should then copy the identity.pub to ~/.ssh/authorized_keys in his/her home directory on the remote machine (the authorized_keys file corresponds to the conventional ~/.rhosts file, and has one key per line, though the lines can be very long). After this, the user can log in without giving the password. RSA authentication is much more secure than rhosts authentication.
A variation on public key authentication is available in the form of certificate authentication: instead of a set of public/private keys, signed certificates are used. This has the advantage that a single trusted certification authority can be used in place of many public/private keys. See the CERTIFICATES section of secsh-keygen for more information.
The most convenient way to use public key or certificate authentication may be with an authentication agent. See secsh-agent for more information.
If other authentication methods fail, secsh prompts the user for a password. The password is sent to the remote host for checking; however, since all communications are encrypted, the password cannot be seen by someone listening on the network.
When a user connects using the protocol version 2 different authentication methods are available. Using the default values for PreferredAuthentications, the client will try to authenticate first using the public key method; if this method fails password authentication is attempted, and finally if this method fails keyboard-interactive authentication and password authentication are tried.
The public key method is similar to RSA authentication described in the previous section and allows the RSA, DSA, ECDSA, or ED25519 algorithm to be used: The client uses his private key, $USERPROFILE/.ssh/id_dsa, $USERPROFILE/.ssh/id_ecdsa, $USERPROFILE/.ssh/id_ed25519 or $USERPROFILE/.ssh/id_rsa, to sign the session identifier and sends the result to the server. The server checks whether the matching public key is listed in ~/.ssh/authorized_keys and grants access if both the key is found and the signature is correct. The session identifier is derived from a shared Diffie-Hellman value and is only known to the client and the server.
If public key authentication fails or is not available a password can be sent encrypted to the remote host for proving the user's identity.
Additionally, secsh supports hostbased or challenge response authentication.
Protocol 2 provides additional mechanisms for confidentiality (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and integrity (hmac-md5, hmac-sha1, hmac-sha2-256, hmac-sha2-512, umac-64, umac-128, hmac-ripemd160). Note that protocol 1 lacks a strong mechanism for ensuring the integrity of the connection.
When the user's identity has been accepted by the server, the server either executes the given command, or logs into the machine and gives the user a normal shell on the remote machine. All communication with the remote command or shell will be automatically encrypted.
If a pseudo-terminal has been allocated (normal login session), the user may use the escape characters noted below.
If no pseudo tty has been allocated, the session is transparent and can be used to reliably transfer binary data. On most systems, setting the escape character to none will also make the session transparent even if a tty is used.
The session terminates when the command or shell on the remote machine exits and all X11 and TCP/IP connections have been closed.
When a pseudo terminal has been requested, secsh supports a number of functions through the use of an escape character.
A single tilde character can be sent as ~~
or by following the tilde by a character other than those described below.
The escape character must always follow a newline to be interpreted as
The escape character can be changed in configuration files using the
configuration directive or on the command line by the
The supported escapes (assuming the default ~) are:
List forwarded connections
Background secsh at logout when waiting for forwarded connection / X11 sessions to terminate
Display a list of escape characters
Send a BREAK to the remote system (only useful for SSH protocol version 2 and if the peer supports it).
Open command line. Currently this allows the addition of port forwardings using the
-L, -Rand -Doptions (see above). It also allows the cancellation of existing port-forwardings with -KL[bind_address:]portfor local, -KR[bind_address:]portfor remote and -KD[bind_address:]portfor dynamic port-forwardings. !command allows the user to execute a local command if the PermitLocalCommand option is enabled. Basic help is available, using the -h option.
Request rekeying of the connection (only useful for SSH protocol version 2 and if the peer supports it)
Decrease the verbosity (LogLevel) when errors are being written to stderr.
Increase the verbosity (LogLevel) when errors are being written to stderr.
If the ForwardX11 variable is set to yes
(or, see the description of the
The DISPLAY value set by secsh will point to the server machine, but with a display number greater than zero. This is normal, and happens because secsh creates a proxy X server on the server machine for forwarding the connections over the encrypted channel.
secsh will also automatically set up Xauthority data on the server machine. For this purpose, it will generate a random authorization cookie, store it in Xauthority on the server, and verify that any forwarded connections carry this cookie and replace it by the real cookie when the connection is opened. The real authentication cookie is never sent to the server machine (and no cookies are sent in the plain).
If the ForwardAgent variable is set to yes
(or, see the description of the
Forwarding of arbitrary TCP/IP connections over the secure channel can be specified either on the command line or in a configuration file. One possible application of TCP/IP forwarding is a secure connection to an electronic purse; another is going through firewalls.
secsh automatically maintains and checks a database containing identifications for all hosts it has ever been used with. Host keys are stored in $USERPROFILE/.ssh/known_hosts in the user's home directory Additionally, the $ROOTDIR/etc/ssh_known_hosts file is automatically checked for known hosts. Any new hosts are automatically added to the user's file. If a host's identification ever changes, secsh warns about this and disables password authentication to prevent a trojan horse from getting the user's password. Another purpose of this mechanism is to prevent man-in-the-middle attacks which could otherwise be used to circumvent the encryption. The StrictHostKeyChecking option (see below) can be used to prevent logins to machines whose host key is not known or has changed.
Disables forwarding of the authentication agent connection.
Enables forwarding of the authentication agent connection. This can also be specified on a per-host basis in a configuration file.
Agent forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the agent's UNIX-domain socket) can access the local agent through the forwarded connection. An attacker cannot obtain key material from the agent, however they can perform operations on the keys that enable them to authenticate using the identities loaded into the agent.
Specifies the interface to transmit from on machines with multiple interfaces or aliased addresses.
Requests compression of all data (including standard input, standard output, standard error, and data for forwarded X11, TCP/IP, and Unix-domain connections). The compression algorithm is the same used by gzip and the compression level can be controlled by the CompressionLevel option (see below) for protocol version 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 Compress option below.
Selects the cipher specification for encrypting the session.
Protocol version 1 allows specification of a single cipher. The supported values are 3des, blowfish, and des.
For protocol version 2, cipher_spec is a comma-separated list of ciphers listed in order of preference. See the Ciphers keyword for more information.
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 a 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 secsh will act as a SOCKS server.
IPv6 addresses can be specified by enclosing the address in square brackets. Only the superuser can forward privileged ports. By default, the local port is bound in accordance with the GatewayPorts setting. However, an explicit bind_address may be used to bind the connection to a specific address. The bind_address of localhost indicates that the listening port be bound for local use only, while an empty address or * indicates that the port should be available from all interfaces.
Append debug logs to log_file instead of standard error.
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.
Specifies an alternative per-user configuration file. If a configuration file is given on the command line, the system-wide configuration information in the registry is ignored. The default for the per-user configuration file is stored in the registry.
Requests secsh to go to background just before command execution. This is useful if secsh 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.
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.
Selects the file from which the identity (private key) for RSA, DSA, ECDSA, or ED25519 authentication is read. Default is $USERPROFILE/.ssh/identity in the user's home directory. Identity files may also be specified on a per-host basis in the configuration file. It is possible to have multiple
-ioptions (and multiple identities specified in configuration files).
secsh will also try to load certificate information from the filename obtained by appending -cert.pub to identity filenames.
Specifies which smartcard device to use. The argument is the device secsh should use to communicate with a smartcard used for storing the user's private RSA key.
Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI credentials to the server.
Disables forwarding of GSSAPI credentials to the server.
Specifies that the given port on the local (client) host is to be forwarded to the given host and port on the remote side. This works by allocating a socket to listen to port on the local side, and whenever a connection is made to this port, the connection is forwarded over the secure channel, and a connection is made to host port hostport from the remote machine. Port forwardings can also be specified in the configuration file. IPv6 addresses can be specified by enclosing the address in square brackets. Only root can forward privileged ports.
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.
Additionally, for protocol version 2 a comma-separated list of MAC (message authentication code) algorithms can be specified in order of preference. See the MACs keyword for more information.
Redirects stdin from /dev/null (actually, prevents reading from stdin). This must be used when secsh 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 secsh program will be put in the background. (This does not work if secsh needs to ask for a password or passphrase; see also the
Does not execute a remote command. This is useful for just forwarding ports (protocol version 2 only).
Control an active connection multiplexing master process. When the
-Ooption 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). -ooption
Specifies options in the format used in the configuration file. This is useful for specifying options for which there is no separate command-line flag.
Specifies the port to connect to on the remote host.
-Qcipher | cipher-auth | mac | kex | key
Queries ssh for the algorithms supported for the specified version 2. The available features are: cipher (supported symmetric ciphers), cipher-auth (supported symmetric ciphers that support authenticated encryption), mac (supported message integrity codes), kex (key exchange algorithms), key (key types).
Quiet mode. Causes all warning and diagnostic messages to be suppressed. Only fatal errors are displayed.
Specifies that the given port on the remote (server) host is to be forwarded to the given host and port on the local side. This works by allocating a socket to listen to port on the remote side, and whenever a connection is made to this port, the connection is forwarded over the secure channel, and a connection is made to host port hostport from the local machine. Port forwardings can also be specified in the configuration file. Privileged ports can be forwarded only when logging in as root on the remote machine. IPv6 addresses can be specified by enclosing the address in square brackets.
By default, the listening socket on the server will be bound to the loopback interface only. This may be overridden by specifying a bind_address. An empty bind_address, or the address `*', 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 secshd.
If the port argument is 0, the listen port will be dynamically allocated on the server and reported to the client at run time. When used together with
-Oforward the allocated port will be printed to the standard output. -Sctl_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 for details.
May be used to request invocation of a subsystem on the remote system. Subsystems are a feature of the SSH2 protocol which facilitate the use of SSH as a secure transport for other application (eg. sftp). The subsystem is specified as the remote command.
Force pseudo-tty allocation. This can be used to execute arbitrary screen-based programs on a remote machine, which can be very useful, for example, when implementing menu services. Multiple
-toptions force tty allocation, even if secsh has no local tty. -T
Disable pseudo-tty allocation.
Verbose mode. Causes secsh to print debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems. Multiple
-voptions increases the verbosity. Maximum is 3. -Whost: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. Works with Protocol version 2 only. -x
Disables X11 forwarding.
Enables X11 forwarding. This can also be specified on a per-host basis in a configuration file.
X11 forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the user's X authorization database) can access the local X11 display through the forwarded connection. An attacker may then be able to perform activities such as keystroke monitoring.
Enables trusted X11 forwarding. Trusted X11 forwardings are not subjected to the X11 SECURITY extension controls.
Writes log information to the application event log. By default, this information is sent to the standard error stream.
Forces secsh to try protocol version 1 only.
Forces secsh to try protocol version 2 only.
Forces secsh to use IPv4 addresses only.
Forces secsh to use IPv6 addresses only.
secsh obtains configuration data from the following sources in the following order: command line options, user's configuration in the Windows registry, and system-wide configuration (stored in the Windows registry and configured with the PTC MKS Toolkit control panel applet). For each parameter, the first obtained value will be used. The configuration files contain sections bracketed by Host specifications, and that section is only applied for hosts that match one of the patterns given in the specification. The matched host name is the one given on the command line.
Since the first obtained value for each parameter is used, more host-specific declarations should be given near the beginning of the file, and general defaults at the end.
The configuration file has the following format:
Empty lines and lines starting with # are comments.
Otherwise a line is of the format keyword arguments.
Configuration options may be separated by whitespace or optional whitespace
and exactly one =; the latter format is useful to avoid the
need to quote whitespace when specifying configuration options using the
secsh, scp, and sftp
The possible keywords and their meanings are as follows (note that the configuration files are case-sensitive):
Restricts the following declarations (up to the next Host keyword) to be only for those hosts that match one of the patterns given after the keyword. * and ? can be used as wildcards in the patterns. A single * as a pattern can be used to provide global defaults for all hosts. The host is the hostname argument given on the command line (that is, the name is not converted to a canonicalized host name before matching).
Specifies which address family to use when connecting. Valid arguments are any, inet (use IPv4 only), or inet6 (use IPv6 only).
If set to yes, passphrase/password querying will be disabled. This option is useful in scripts and other batch jobs where you have no user to supply the password. The argument must be yes or no. The default is no.
Specifies the interface to transmit from on machines with multiple interfaces or aliased addresses. Note that this option does not work if UsePrivilegedPort is set to yes.
When CanonicalizeHostname is enabled, this option specifies the list of domain suffixes in which to search for the specified destination host.
Specifies whether to fail with an error when hostname canonicalization fails. The default, yes, will attempt to look up the unqualified hostname using the system resolver's search rules. A value of no will cause secsh to fail instantly if CanonicalizeHostname is enabled and the target hostname cannot be found in any of the domains specified by CanonicalDomains.
Controls whether explicit hostname canonicalization is performed. The default, no, is not to perform any name rewriting and let the system resolver handle all hostname lookups. If set to yes then, for connections that do not use a ProxyCommand, secsh will attempt to canonicalize the hostname specified on the command line using the CanonicalDomains suffixes and CanonicalizePermittedCNAMEs rules. If CanonicalizeHostname is set to always, then canonicalization is applied to proxied connections too.
If this option is enabled and canonicalisation results in the target hostname changing, then the configuration files are processed again using the new target name to pick up any new configuration in matching Host stanzas.
Specifies the maximum number of dot characters in a hostname before canonicalization is disabled. The default, 1, allows a single dot (i.e. hostname.subdomain).
Specifies rules to determine whether CNAMEs should be followed when canonicalizing hostnames. The rules consist of one or more arguments of source_domain_list:target_domain_list, where source_domain_list is a pattern-list of domains that may follow CNAMEs in canonicalization, and target_domain_list is a pattern- list of domains that they may resolve to. For example, *.a.example.com:*.b.example.com,*.c.example.com will allow hostnames matching *.a.example.com to be canonicalized to names in the *.b.example.com or *.c.example.com domains.
Specifies whether to use challenge-response authentication. The argument to this keyword must be yes or no. The default is yes.
If this flag is set to yes, secsh will additionally check the host IP address in the known_hosts file. This allows secsh to detect if a host key changed due to DNS spoofing. If the option is set to no, the check will not be executed. The default is yes.
Specifies the cipher to use for encrypting the session in protocol version 1. Currently, blowfish, 3des, and des are supported. des is only supported in the secsh client for interoperability with legacy protocol 1 implementations that do not support the 3des cipher. Its use is strongly discouraged due to crytographic weakness. The default is 3des.
Specifies the ciphers allowed for protocol version 2 in order of preference. Multiple ciphers must be comma-separated. The supported ciphers are
3des-cbc, aes128-cbc, aes192-cbc, aes256-cbc, aes128-ctr, aes192-ctr, aes256-ctr, firstname.lastname@example.org, email@example.com, arcfour, arcfour128, arcfour256, blowfish-cbc, cast128-cbc, firstname.lastname@example.org
The default is
aes128-ctr,aes192-ctr,aes256-ctr, email@example.com,firstname.lastname@example.org, email@example.com, arcfour256,arcfour128, aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc, aes192-cbc,aes256-cbc,arcfour
The list of available ciphers may also be obtained using the
-Qoption of secsh.
Specifies that all local, remote and dynamic port forwardings specified in the configuration files or on the command line be cleared. This option is primarily useful when used from the secsh command line to clear port forwardings set in configuration files, and is automatically set by scp and sftp. The argument must be yes or no. The default is no.
Specifies whether to use compression. The argument must be yes or no. The default is no.
Specifies the compression level to use if compression is enabled. The argument must be an integer from 1 (fast) to 9 (slow, best). The default level is 6, which is good for most applications. The meaning of the values is the same as in gzip Note that this option applies to protocol version 1 only.
Specifies the number of tries (one per second) to make before falling back to rsh or exiting. The argument must be an integer. This may be useful in scripts if the connection sometimes fails. The default is 1.
Specifies the timeout (in seconds) used when connecting to the SSH server, instead of using the default system TCP timeout. This value is used only when the target is down or really unreachable, not when it refuses the connection.
Enables the sharing of multiple sessions over a single network connection. When set to yes, secsh will listen for connections on a control socket specified using the ControlPath argument. Additional sessions can connect to this socket using the same ControlPath with ControlMaster set to no (the default). These sessions will try to reuse the master instance's network connection rather than initiating new ones, but will fall back to connecting normally if the control socket does not exist, or is not listening.
Setting this to ask will cause ssh to listen for control connections, but require confirmation using the SSH_ASKPASS program before they are accepted (see secsh-add for details). If the ControlPath cannot be opened, ssh will continue without connecting to a master instance.
X11 and secsh-agent forwarding is supported over these multiplexed connections, however the display and agent forwarded will be the one belonging to the master connection i.e. it is not possible to forward multiple displays or agents.
Two additional options allow for opportunistic multiplexing: try to use a master connection but fall back to creating a new one if one does not already exist. These options are: auto and autoask. The latter requires confirmation like the ask option.
Specify the path to the control socket used for connection sharing as described in the ControlMaster section above or the string none to disable connection sharing. In the path, `%L' will be substituted by the first component of the local host name, `%l' will be substituted by the local host name (including any domain name), `%h' will be substituted by the target host name, `%n' will be substituted by the original target host name specified on the command line, `%p' the destination port, `%r' by the remote login username, `%u' by the username of the user running secsh, and `%C' by a hash of the concatenation `%l%h%p%r'. It is recommended that any ControlPath used for opportunistic connection sharing include at least %h, %p, and %r (or alternatively `%C'). This ensures that shared connections are uniquely identified.
When used in conjunction with ControlMaster, specifies that the master connection should remain open in the background (waiting for future client connections) after the initial client connection has been closed. If set to no, then the master connection will not be placed into the background, and will close as soon as the initial client connection is closed. If set to yes, then the master connection will remain in the background indefinitely (until killed or closed via a mechanism such as the secsh -O exit option). If set to a time in seconds, or a time in any of the formats documented in secshd, then the backgrounded master connection will automatically terminate after it has remained idle (with no client connections) for the specified time.
Specifies that a TCP/IP port on the local machine be forwarded over the secure channel, and the application protocol is then used to determine where to connect to from the remote machine. The argument must be a port number. Currently the SOCKS4 protocol is supported, and ssh will act as a SOCKS4 server. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the superuser can forward privileged ports.
Setting this option to yes in the global client configuration enables the use of the helper program secsh-keysign during host-based authentication. The argument must be yes or no. The default is no. This option should be placed in the non-host-specific section. See the secsh-keysign reference page for more information.
Sets the escape character (default: ~). The escape character can also be set on the command line. The argument should be a single character, ^ followed by a letter, or none to disable the escape character entirely (making the connection transparent for binary data).
Specifies whether secsh should terminate the connection if it cannot set up all requested dynamic, tunnel, local, and remote port forwardings. The argument must be yes or no. The default is no.
Specifies whether the connection to the authentication agent (if any) will be forwarded to the remote machine. The argument must be yes or no. The default is no.
Specifies whether X11 connections will be automatically redirected over the secure channel and DISPLAY set. The argument must be yes or no. The default is no.
Specify a timeout for untrusted X11 forwarding using the format described in the TIME FORMATS section of secshd. X11 connections received by secsh after this time will be refused. The default is to disable untrusted X11 forwarding after twenty minutes has elapsed.
When this option is set to yes, remote X11 clients have full access to the original X11 display.
When this option is set to no, remote X11 clients are considered untrusted and prevented from stealing or tampering with data belonging to trusted X11 clients. Furthermore, the xauth token used for the session are set to expire after 20 minutes. Remote clients are refused access after this time.
The default is no.
See the X11 SECURITY extension specification for full details on the restrictions imposed on untrusted clients.
Specifies whether remote hosts are allowed to connect to local forwarded ports. By default, secsh binds local port forwardings to the loopback address. This prevents other remote hosts from connecting to forwarded ports. GatewayPorts can be used to specify that secsh should bind local port forwardings to the wildcard address, thus allowing remote hosts to connect to forwarded ports. The argument must be yes or no. The default is no.
Specifies a file to use for the host key database instead of $ROOTDIR/etc/ssh_known_hosts.
Specifies whether user authentication based on GSSAPI is allowed. The default is no. This option applies to protocol version 2 only.
Forwards (delegates) credentials to the server. The default is no. This option applies to protocol version 2 only.
Indicates that secsh should hash host names and addresses when they are added to $USERPROFILE/.ssh/known_hosts. These hashed names may be used normally by secsh and secshd, but they do not reveal identifying information should the file's contents be disclosed. The default is no. Existing names and addresses in known hosts files are not converted automatically, but may be manually hashed using secsh-keygen.
Specifies whether to try rhosts based authentication with public key authentication. The argument must be yes or no. The default is yes. This option applies to protocol version 2 only and is similar to RhostsRSAAuthentication.
Specifies the protocol version 2 host key algorithms that the client wants to use in order of preference. The default for this option is:
firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org,email@example.com, firstname.lastname@example.org,email@example.com, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, ssh-ed25519,ssh-rsa,ssh-dss
Specifies an alias that should be used instead of the real host name when looking up or saving the host key in the host key database files. This option is useful for tunneling ssh connections or if you have multiple servers running on a single host.
Specifies the real host name to log into. This can be used to specify nicknames or abbreviations for hosts. If the hostname contains the character sequence `%h', then this will be replaced with the host name specified on the command line (this is useful for manipulating unqualified names). The character sequence `%%' will be replaced by a single `%' character, which may be used when specifying IPv6 link-local addresses.
Default is the name given on the command line. Numeric IP addresses are also permitted (both on the command line and in HostName specifications).
Specifies that secsh should only use the authentication identity files configured in the secsh_configuration, even if secsh-agent offers more identities. The argument to this keyword must be yes or no. This option is intended for situations where secsh-agent offers many different identities. The default is no.
Specifies the IPv4 type-of-service or DSCP class for connections. Accepted values are af11, af12, af13, af21, af22, af23, af31, af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3, cs4, cs5, cs6, cs7, ef, lowdelay, throughput, reliability, or a numeric value. This option may take one or two arguments, separated by whitespace. If one argument is specified, it is used as the packet class unconditionally. If two values are specified, the first is automatically selected for interactive sessions and the second for non-interactive sessions. The default is lowdelay for interactive sessions and throughput for non-interactive sessions.
Specifies the file from which the user's RSA, DSA, ECDSA, or ED25519 authentication identity is read (default $USERPROFILE/.ssh/identity in the user's profile directory for protocol version 1 and $USERPRFILE/.ssh/id_dsa, $USERPRFILE/.ssh/id_ecdsa, $USERPRFILE/.ssh/id_ed25519 and $USERPRFILE/.ssh/id_rsa for protocol version 2). Additionally, any identities represented by the authentication agent will be used for authentication unless IdentitiesOnly is set. secsh will try to load certificate information from the filename obtained by appending -cert.pub to the path of a specified IdentityFile.
The file name may use the tilde syntax to refer to a user's home directory or one of the following escape characters: %d (local user's home directory), %u (local user name), %l (local host name), %h (remote host name) or %r (remote user name).
It is possible to have multiple identity files specified in configuration files; all these identities will be tried in sequence. Multiple IdentityFile directives will add to the list of identities tried (this behaviour differs from that of other configuration directives).
IdentityFile may be used in conjunction with IdentitiesOnly to select which identities in an agent are offered during authentication.
Specifies a pattern-list of unknown options to be ignored if they are encountered in configuration parsing. This may be used to suppress errors if the configuration contains options that are unrecognised by secsh.
Specifies wheter to use keyboard-interactive authentication. The argument to this keyword must be yes or no. The default is yes.
Specifies the list of methods to use in keyboard-interactive authentication. Multiple method names must be comma-separated. The default is to use the server specified list. The methods available vary depending on what the server supports. For an OpenSSH server, it may be zero or more of: bsdauth, pam, and skey.
Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms must be comma-separated. The default is:
firstname.lastname@example.org, ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, diffie-hellman-group-exchange-sha256, diffie-hellman-group14-sha1, diffie-hellman-group-exchange-sha1, diffie-hellman-group1-sha1
Specifies a command to execute on the local machine after successfully connecting to the server. The command string extends to the end of the line, and is executed with the user's shell. The following escape character substitutions are performed: %d (local user's home directory), %h (remote host name), %l (local host name), %n (host name as provided on the command line), %p (remote port), %r (remote user name), %u (local user name), or %C by a hash of the concatenation %l%h%p%r.
The command is run synchronously and does not have access to the session of the secsh that spawned it. It should not be used for interactive commands.
This directive is ignored unless PermitLocalCommand has been enabled.
Specifies that a TCP/IP port on the local machine be forwarded over the secure channel to ths specified host and post from the remote machine. The first argument must be a port number, and the second must be host:port. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the superuser can forward privileged ports.
Gives the verbosity level that is used when logging messages from secsh. The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG. The default is INFO.
Specifies the MAC (message authentication code) algorithms in order of preference. The MAC algorithm is used in protocol version 2 for data integrity protection. Multiple algorithms must be comma-separated. The default is
email@example.com,firstname.lastname@example.org, email@example.com,firstname.lastname@example.org, email@example.com,firstname.lastname@example.org, hmac-sha2-256,hmac-sha2-512, email@example.com,firstname.lastname@example.org, email@example.com, firstname.lastname@example.org,email@example.com, hmac-md5,hmac-sha1,hmac-ripemd160, hmac-sha1-96,hmac-md5-96
This option can be used if the home directory is shared across machines. In this case localhost will refer to a different machine on each of the machines and the user will get many warnings about changed host keys. However, this option disables host authentication for localhost. The argument to this keyword must be yes or no. The default is to check the host key for localhost.
Specifies the number of password prompts before giving up. The argument to this keyword must be an integer. Default is 3.
Specifies whether to use password authentication. The argument to this keyword must be yes or no. The default is yes.
Allows local command execution via the LocalCommand option or using the !command sequence in secsh. The argument must be yes or no. The default is no.
Specifies the port number to connect on the remote host. Default is 22.
Specifies the order in which the client should try protocol 2 authentication methods. This allows a client to prefer one method (for example keyboard-interactive) over another method (for example password) The default for this option is: hostbased,publickey,password,keyboard-interactive
Specifies the protocol versions secsh should support in order of preference. The possible values are 1 and 2. Multiple versions must be comma-separated. The default is 2,1. This means that secsh tries version 2 and falls back to version 1 if version 2 is not available. The default is 2.
Specifies the command to use to connect to the server. The command string extends to the end of the line, and is executed using the user's shell exec directive to avoid a lingering shell process.
In the command string, %h will be substituted by the host name to connect and %p by the port, and %r by the remote user name. The command can be basically anything, and should read from its standard input and write to its standard output. It should eventually connect an secshd server running on some machine, or execute sshd -i somewhere. Host key management will be done using the HostName of the host being connected (defaulting to the name typed by the user). Note that CheckHostIP is not available for connects with a proxy command.
Specifies that ProxyCommand will pass a connected file descriptor back to secsh instead of continuing to execute and pass data. The default is no.
Specifies whether to try public key authentication. The argument to this keyword must be yes or no. The default is yes. This option applies to protocol version 2 only.
Specifies the maximum amount of data that may be transmitted before the session key is renegotiated, optionally followed a maximum amount of ! time that may pass before the session key is renegotiated. The first argument is the number of bytes, with an optional suffix of K, M, or G to indicated Kilobyes, Megabytes, or Gigabytes, respectively. The default is between 1G and 4G, depending on the cipher. The optional second value is specified in seconds and may use any of the units documented in the TIME FORMATS section of secshd. The default value for RekeyLimit is default none, which means that rekeying is performed after the cipher's default amount of data has been sent or received and no time based rekeying is done. This option applies to protocol version 2 only.
Specifies that a TCP/IP port on the remote machine be forwarded over the secure channel to the specified host and port from the local machine. The first argument must be a port number, and the second must be host:port. IPv6 addresses can be specified with an alternative syntax: host/port. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the superuser can forward privileged ports.
Specifies whether to request a pseudo-tty for the session. The argument may be one of: no (never request a TTY), yes (always request a TTY when standard input is a TTY), force (always request a TTY) or auto (request a TTY when opening a login session). This option mirrors the
-tand -Tflags for secsh.
Specifies whether to try rhosts based authentication with RSA host authentication. The argument must be yes or no. The default is yes. This option applies to protocol version 1 only.
Specifies whether to try RSA authentication. The argument to this keyword must be yes or no. RSA authentication will only be attempted if the identity file exists, or an authentication agent is running. The default is yes. Note that this option applies to protocol version 1 only.
Specifies what variables from the local environment should be sent to the server. Environment passing is only supported for protocol 2. The server must also support it, and the server must be configured to accept these environment variables. Refer to AcceptEnv in secshd for how to configure the server. Variables are specified by name, which may contain wildcard characters. Multiple environment variables may be separated by whitespace or spread across multiple SendEnv directives. The default is not to send any environment variables.
Sets the number of server alive messages (see below) which may be sent without secsh receiving any messages back from the server. When this threshold is reached while server alive messages are being sent, secsh disconnects from the server, terminating the session. The use of server alive messages is very different from TCPKeepAlive (below). The server alive messages are sent through the encrypted channel and therefore are not spoofable. The TCP keepalive option enabled by TCPKeepAlive is spoofable. The server alive mechanism is valuable when the client or server depend on knowing when a connection has become inactive. The default value is 3. If, for example, ServerAliveInterval (see below) is set to 15 and ServerAliveCountMax is left at the default, and the server becomes unresponsive, secsh disconnects after approximately 45 seconds. This option applies to protocol version 2 only.
Sets a timeout interval in seconds after which if no data has been received from the server, secsh sends a message through the encrypted channel to request a response from the server. The default is 0, inidicating that these messages are not sent to the server. This option applies to protocol version 2 only.
Specifies which smartcard device to use. The argument to this keyword is the device secsh should use to communicate with a smartcard used for storing the user's private RSA key. By default, no device is specified and smartcard support is not activated.
Sets the octal file creation mode mask (umask) used when creating a Unix-domain socket file for local or remote port forwarding. This option is only used for port forwarding to a Unix-domain socket file.
The default value is 0177, which creates a Unix-domain socket file that is readable and writable only by the owner. Note that not all operating systems honor the file mode on Unix-domain socket files.
Specifies whether to remove an existing Unix-domain socket file for local or remote port forwarding before creating a new one. If the socket file already exists and StreamLocalBindUnlink is not enabled, ssh will be unable to forward the port to the Unix- domain socket file. This option is only used for port forwarding to a Unix-domain socket file.
The argument must be yes or no. The default is no.
If this flag is set to yes, secsh will never automatically add host keys to the $USERPROFILE/.ssh/known_hosts and $USERPROFILE/.ssh/known_hosts2 files, and refuses to connect to hosts whose host key has changed. This provides maximum protection against trojan horse attacks. However, it can be somewhat annoying if you don't have good $ROOTDIR/etc/ssh_known_hosts and $ROOTDIR/etc/ssh_known_hosts2 files installed and frequently connect to new hosts. This option forces the user to manually add all new hosts. If this flag is set to no, secsh will automatically add new host keys to the user known hosts files. If this flag is set to ask, new host keys will be added to the user known host files only after the user has confirmed that is what they really want to do, and secsh will refuse to connect to hosts whose host key has changed. The host keys of known hosts will be verified automatically in all cases. The argument must be yes, no, or ask. The default is ask.
Specifies whether the system should send TCP keepalive messages to the other side. If they are sent, death of the connection or crash of one of the machines will be properly noticed. However, this means that connections die if the route is down temporarily, and some people find it annoying.
The default is yes (to send TCP keepalive messages), and the client notices if the network goes down or the remote host dies. This is important in scripts and many users want it too.
To disable TCP keepalive messages, the value should be set to no.
Specifies whether to use a privileged port for outgoing connections. The argument must be yes or no. The default is no. Note that you need to set this option to yes if you want to use RhostsAuthentication and RhostsRSAAuthentication with older servers.
Specifies the user to log in as. This can be useful when you have a different user name on different machines. This saves the trouble of having to remember to give the user name on the command line.
Specifies a file to use for the user host key database instead of $USERPROFILE/.ssh/known_hosts.
Specifies whether to verify the remote key using DNS and SSHFP resource records. When this option is set to yes, the client implicitly trusts keys that match a secure fingerprint from DNS. Insecure fingerprints are handled as if this option was set to ask. When this option is set to ask, information on fingerprint matches is displayed, but the user stills need to confirm new host keys according to the StrictHostKeyChecking option. The argument must be yes, no, or ask. The default is no. This option applies to protocol version 2 only.
When this flag is set to yes, an ASCII art representation of the remote host key fingerprint is printed in addition to the hex fingerprint string at login and for unknown host keys. If this flag is set to no, no fingerprint strings are printed at login and only the hex fingerprint string are printed for unknown host keys. The default is no.
Specifies the location of the xauth program. The default is /usr/X11R6/bin/xauth.
secsh will normally set the following environment variables:
The DISPLAY variable indicates the location of the X11 server. It is automatically set by secsh 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. secsh 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).
Is set to the path of the user's home directory.
Is a synonym for USER; is set for compatibility with systems that use this variable.
Is set to the path of the user's mailbox.
Is set to the default PATH, as specified when compiling secsh.
If secsh needs a passphrase, it reads the passphrase from the current terminal if it was run from a terminal. If secsh does not have a terminal associated with it but DISPLAY and SSH_ASKPASS are set, it executes the program specified by SSH_ASKPASS and open an X11 window to read the passphrase. This is particularly useful when calling secsh 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.)
Identifies the path of a unix-domain socket used to communicate with the agent.
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.
Contains the original command line if a forced command is executed. It can be used to extract the original arguments.
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.
Is set to indicate the present timezone if it was set when the daemon was started (that is, the daemon passes the value on to new connections).
Is set to the name of the user logging in.
Is set to the location of the user's Windows NT/2000/XP profile. In the user's profile is a roaming profile, this will be saved and restored as the user logs in and out of machines within the domain.
Additionally, secsh reads $HOME/.ssh/environment, and adds lines of the format VARNAME=value to the environment.
Records host keys for all hosts the user has logged into that are not in $ROOTDIR/etc/ssh_known_hosts. See secshd.
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). secsh 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 3DES.
Contains the public key for authentication (public part of the identity file in human-readable form). The contents of the $USERPROFILE/.ssh/identity.pub file should be added to ~/.ssh/authorized_keys on all machines where you wish to log in using protocol version 1 RSA authentication. The contents of the $USERPROFILE/.ssh/id_dsa.pub, $USERPROFILE/.ssh/id_ecdsa.pub, $USERPROFILE/.ssh/id_ed25519.pub, and $USERPROFILE/.ssh/id_rsa.pub file should be added to ~/.ssh/authorized_keys on all machines where the user wishes to log in using protocol version 2 DSA/RSA authentication. These files are not sensitive and can (but need not) be readable by anyone. These files are never used automatically and are not necessary; they are only provided for the convenience of the user.
Lists the public keys (RSA/DSA/ECDSA/ED25519) that can be used for logging in as this user. The format of this file is described in the secshd reference page. In the simplest form the format is the same as the .pub identity files (that is, each line contains the number of bits in modulus, public exponent, modulus, and comment fields, separated by spaces). This file is not highly sensitive, but the recommended permissions are read/write for the user, and not writable by others.
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. This file should be world-readable. This file contains public keys, one per line, in the following format (fields separated by spaces): system name, public key, and optional comment field. When different names are used for the same machine, all such names should be listed, separated by commas. The format is described on the secshd reference page.
The canonical system name (as returned by name servers) is used by secshd to verify the client host when logging in; other names are needed because secsh does not convert the user-supplied name to a canonical name before checking the key, because someone with access to the name servers would then be able to fool host authentication.
- HKEY_LOCAL_MACHINE/Software/Mortice Kern Systems/etc/ssh_config
System-wide configuration. This provides defaults for those values that are not specified in the user's configuration file, and for those users who do not have a configuration file. The values here are normally configured from the PTC MKS Toolkit control panel applet.
These files contain the private parts of the host keys and are used for RhostsRSAAuthentication and HostbasedAuthentication. Since they are readable only by root, secsh must be setuid root if these authentication methods are desired.
This file is used in .rhosts authentication to list the host/user pairs that are permitted to log in. (Note that this file is also used by rlogin and rsh, which makes using this file insecure.) Each line of the file contains a host name (in the canonical form returned by name servers), and then a user name on that host, separated by a space. On some machines this file may need to be world-readable if the user's home directory is on a NFS partition, because secshd 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.
Note that by default secshd will be installed so that it requires successful RSA host authentication before permitting .rhosts authentication. If the server machine does not have the client's host key in $ROOTDIR/etc/ssh_known_hosts, you can store it in ~/.ssh/known_hosts. The easiest way to do this is to connect back to the client from the server machine using secsh; this will automatically add the host key to $USERPROFILE/.ssh/known_hosts.
This file is used during .rhosts authentication. It contains canonical hosts names, one per line (the full format is described on the secshd reference page). If the client host is found in this file, login is automatically permitted provided client and server user names are the same. Additionally, successful RSA host authentication is normally required. This file should only be writable by root.
Commands in this file are executed by secsh when the user logs in just before the user's shell (or command) is started. See the secshd reference page for more information.
Commands in this file are executed by secsh when the user logs in just before the user's shell (or command) is started. See the secshd reference page for more information.
Contains additional definitions for environment variables, see section ENVIRONMENT_VARIABLES above.
OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer features and created OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0.
All UNIX systems. Windows 7. Windows Server 2008 R2. Windows 8. Windows Server 2012. Windows 10. Windows Server 2016.
When you are using the MKS secure shell client on a Windows NT machine,
the default user name will be of the form
Should you be connecting to the secure shell server on another Windows machine,
this may well be exactly what you want.
In the event that you are connecting to a UNIX machine, you must specify
the UNIX username with the
The secsh utility, by default, loads keys from the $USERPROFILE directory on 7/2008R2/8/2012/10/2016 platforms instead of ~/. This is done because the ~/ directory might not be available for the case of a domain machine that cannot contact a domain controller. Note that the secure shell service continues to use the ~/.ssh directory to store configuration files. In particular, note that there are effectively two known_hosts files: one under $USERPROFILE/.ssh/known_hosts, and a second one under ~/.ssh/known_hosts. There is no guarantee that the directory to which the USERPROFILE environment variable points is on an NTFS partition, and thus there is no guarantee that the keys in this location can be properly secured.
Using the secure shell client in a console window where the screen buffer width is larger than the window (that is, the console window has a horizontal scrollbar) does not work well. It is recommended that the secure shell client be used from console windows where these two widths are the same. This limitation may be lifted in a future release.
Password prompting using the program specified in the SSH_ASKPASS environment variable does not work. The application creates a console window in which you are prompted for the password. This will be corrected in a future release.
To use X11 forwarding, you must have an X server running on your client machine.
When you are not using Kerberos authentication (GSSAPI), to enable passwordless logon to machines running the PTC MKS Toolkit version of secshd on a Windows NT machine, you must enable your account for rhosts access. This can be done with the rsetup utility or from the PTC MKS Toolkit control panel applet. There is no requirement that the rhosts or rshd service be running. The only requirement is that you enable your account. In the event that your password expires, the passwordless authentication fails and you are prompted for a password.
To use Kerberos authentication, see the PTC MKS Toolkit Connectivity Solutions Guide for more information about the capabilities and limitations of Kerberos with secure shell.
When connecting to secshd, you may find that neither the backspace or delete key work correctly to erase characters. You can correct this by using either:
stty erase ^H
stty erase ^?
to set the erase character to either backspace or delete, respectively.
PTC MKS Toolkit for System Administrators
PTC MKS Toolkit for Developers
PTC MKS Toolkit for Interoperability
PTC MKS Toolkit for Professional Developers
PTC MKS Toolkit for Professional Developers 64-Bit Edition
PTC MKS Toolkit for Enterprise Developers
PTC MKS Toolkit for Enterprise Developers 64-Bit Edition
PTC MKS Toolkit Connectivity Solutions Guide
T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, SSH Protocol Architecture, draft-ietf-secsh-architecture-09.txt, July 2001, work in progress material.
PTC MKS Toolkit 10.1 patch 1 Documentation Build 2.