secshd

OpenSSH Secure Shell service 

Service


SYNOPSIS

secshd [-install] [-remove] [-debug]


DESCRIPTION

secshd (Secure Shell service) is a 7/2008R2/8/2012/10/2016 service that provides the server side for secsh. Together these programs replace rlogin and rsh, and provide secure encrypted communications between two untrusted hosts over an insecure network. The programs are intended to be as easy to install and use as possible.

Like any other 7/2008R2/8/2012/10/2016 service, you can use the service utility to start and stop secshd.

secshd listens for connections from clients. It is normally started at boot by the 7/2008R2/8/2012/10/2016 service control manager. It spawns a new process for each incoming connection. The spawned processes handle key exchange, encryption, authentication, command execution, and data exchange. This implementation of secshd supports both SSH protocol version 1 and 2 simultaneously. secshd works as follows.

SSH protocol version 1

Each host has a host-specific RSA key (normally 1024 bits) used to identify the host. Additionally, when the daemon starts, it generates a server RSA key (normally 768 bits). This key is normally regenerated every hour if it has been used, and is never stored on disk.

Whenever a client connects the daemon responds with its public host and server keys. The client compares the RSA host key against its own database to verify that it has not changed. The client then generates a 256 bit random number. It encrypts this random number using both the host key and the server key, and sends the encrypted number to the server. Both sides then use this random number as a session key which is used to encrypt all further communications in the session. The rest of the session is encrypted using a conventional cipher, currently Blowfish or 3DES, with 3DES being used by default. The client selects the encryption algorithm to use from those offered by the server.

Next, the server and the client enter an authentication dialog. The client tries to authenticate itself using .rhosts authentication, .rhosts authentication combined with RSA host authentication, RSA challenge-response authentication, or password based authentication.

Rhosts authentication is not supported because it is fundamentally insecure. RhostsRSAAuthentication should be used instead because it performs RSA-based host authentication in addition to normal rhosts or $ROOTDIR/etc/hosts_equiv authentication. System security is not improved unless rshd, rlogind, and rexecd are disabled (thus completely disabling rlogin and rsh into the machine).

SSH protocol version 2

Version 2 works similarly: Each host has a host-specific key (RSA, DSA, ECDSA, or ED25519) used to identify the host. However, when the daemon starts, it does not generate a server key. Forward security is provided through a Diffie-Hellman key agreement. This key agreement results in a shared session key.

The rest of the session is encrypted using a symmetric cipher, currently 128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES. The client selects the encryption algorithm to use from those offered by the server. Additionally, session integrity is provided through a cryptographic message authentication code (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160, hmac-sha2-256 or hmac-sha2-512).

Protocol version 2 provides a Kerberos (GSSAPIAuthentication) public key based user (PubkeyAuthentication) or client host (HostbasedAuthentication) authentication method, conventional password authentication and challenge response based methods.

Command execution and data forwarding

If the client successfully authenticates itself, a dialog for preparing the session is entered. At this time the client may request things like allocating a pseudo-tty, forwarding X11 connections, forwarding TCP/IP connections, or forwarding the authentication agent connection over the secure channel.

Finally, the client either requests a shell or execution of a command. The sides then enter session mode. In this mode, either side may send data at any time, and such data is forwarded to/from the shell or command on the server side, and the user terminal in the client side.

When the user program terminates and all forwarded X11 and other connections have been closed, the server sends command exit status to the client, and both sides exit.

secshd can be configured using the PTC MKS Toolkit control pranel applet.

secshd automatically rereads its configuration file when it detects that the configuration has been changed.

Options

-install 

installs and starts the secshd service. To start the service without installing it, use the service command:

service start mkssecuresh
-remove 

stops and removes the secshd service. To stop the service without removing it, use the service command:

service stop mkssecuresh
-debug 

runs secshd as a normal program in the current console for debugging purposes.

Note:

This option is for debugging the service framework. To help debug secshd, use the PTC MKS Toolkit control panel applet to set the logging level for the service to one of the supported levels. Logging messages will then appear in the event log. When you have finished debugging, reset the logging level to its original value.


CONFIGURATION FILE

secshd reads configuration data from HKEY_LOCAL_MACHINE/Software/Mortice Kern Systems/etc/sshd_config, although under normal circumstances, you should use the PTC MKS Toolkit control panel applet to change the configuration settings.

The possible keywords and their meanings are as follows (note that keywords are case-insensitive and arguments are case-sensitive):

AcceptEnv 

Specifies what environment variables sent by the client are copied into the session's environment. See SendEnv on the secsh reference page for how to configure the client. Environment passing is only supported for protocol 2. Variables are specified by name, which may contain the wildcard characters * and ?. Multiple environment variables may be separated by whitespace or spread across multiple AcceptEnv directives. Be warned that some environment variables could be used to bypass restricted user environments. For this reason, care should be taken in the use of this directive. The default is not to accept any environment variables.

AddressFamily 

Specifies which address family to use when connecting. Valid arguments are any, inet (use IPv4 only), or inet6 (use IPv6 only). The default is any.

AllowAgentForwarding 

Specifies whether secsh-agent forwarding is permitted. The default is yes. Disabling agent forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders.

AllowGroups 

This keyword can be followed by a list of group names, separated by spaces. If specified, login is allowed only for users whose primary group or supplementary group list matches one of the patterns. * and ? can be used as wildcards in the patterns. Only group names are valid; a numerical group ID isn't recognized. By default login is allowed regardless of the group list.

AllowTcpForwarding 

Specifies whether TCP forwarding is permitted. The available options are yes or all to allow TCP forwarding, no to prevent all TCP forwarding, local to allow local (from the perspective of secsh) forwarding only or remote to allow remote forwarding only. The default is yes. Note that disabling TCP forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders.

AllowStreamLocalForwarding 

Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. The available options are yes or all to allow StreamLocal forwarding, no to prevent all StreamLocal forwarding, local to allow local (from the perspective of secsh) forwarding only or remote to allow remote forwarding only. The default is yes. Note that disabling StreamLocal forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders.

AllowUsers 

This keyword can be followed by a list of user names, separated by spaces. If specified, login is allowed only for users names that match one of the patterns. * and ? can be used as wildcards in the patterns. Only user names are valid; a numerical user ID isn't recognized. By default login is allowed regardless of the user name. If the pattern takes the form user@host then user and host are separately checked, restricting logins to particular users from particular hosts.

AuthenticationMethods 

Specifies the authentication methods that must be successfully completed for a user to be granted access. This option must be set to one or more comma-separated lists of authentication method names. Successful authentication requires completion of every method in at least one of these lists.

For example, an argument of publickey,password publickey,keyboard-interactive would require the user to complete public key authentication, followed by either password or keyboard interactive authentication. Only methods that are next in one or more lists are offered at each stage, so for this example, it would not be possible to attempt password or keyboard-interactive authentication before public key.

This option is only available for SSH protocol 2 and will yield a fatal error if enabled if protocol 1 is also enabled. Note that each authentication method listed should also be explicitly enabled in the configuration. The default is not to require multiple authentication; successful completion of a single authentication method is sufficient.

AuthorizedKeysFile 

Specifies the file that contains the public keys that can be used for user authentication. AuthorizedKeysFile may contain tokens of the form %T which are substituted during connection set-up. The following tokens are defined: %% is replaced by a literal %, %h is replaced by the home directory of the user being authenticated and %u is replaced by the username of that user. After expansion, AuthorizedKeysFile is taken to be an absolute path or one relative to the user's home directory. The default is .ssh/authorized_keys

AuthorizedPrincipalsFile 

Specifies a file that lists principal names that are accepted for certificate authentication. When using certificates signed by a key listed in TrustedUserCAKeys, this file lists names, one of which must appear in the certificate for it to be accepted for authentication. Names are listed one per line preceded by key options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)). Empty lines and comments starting with # are ignored.

AuthorizedPrincipalsFile may contain tokens of the form %T which are substituted during connection setup. The following tokens are defined: %% is replaced by a literal '%', %h is replaced by the home directory of the user being authenticated, and %u is replaced by the username of that user. After expansion, AuthorizedPrincipalsFile is taken to be an absolute path or one relative to the user's home directory.

The default is none, i.e. not to use a principals file - in this case, the username of the user must appear in a certificate's principals list for it to be accepted. Note that AuthorizedPrincipalsFile is only used when authentication proceeds using a CA listed in TrustedUserCAKeys and is not consulted for certification authorities trusted via ~/.ssh/authorized_keys, though the principals= key option offers a similar facility (see the AUTHORIZED_KEYS FILE FORMAT section for details).

Banner 

In some jurisdictions, sending a warning message before authentication may be relevant for getting legal protection. The contents of the specified file are sent to the remote user before authentication is allowed. This option is only available for protocol version 2.

ChallengeResponseAuthentication 

Specifies whether challenge response authentication is allowed. All authentication styles from login.conf are supported. The default is yes.

Ciphers 

Specifies the ciphers allowed for protocol version 2. Multiple ciphers must be comma-separated. The supported ciphers are

3des-cbc
aes128-cbc
aes192-cbc
aes256-cbc
aes128-ctr
aes192-ctr
aes256-ctr
aes128-gcm@openssh.com
aes256-gcm@openssh.com
arcfour
arcfour128
arcfour256
blowfish-cbc
cast128-cbc
chacha20-poly1305@openssh.com

The default is

aes128-ctr,aes192-ctr,aes256-ctr,
aes128-gcm@openssh.com,aes256-gcm@openssh.com,
chacha20-poly1305@openssh.com
ClientAliveInterval 

Sets a timeout interval in seconds after which if no data has been received from the client, secshd will send a message through the encrypted channel to request a response from the client. The default is 0, indicating that these messages will not be sent to the client. This option applies to protocol version 2 only.

ClientAliveCountMax 

Sets the number of client alive messages (see above) which may be sent without secshd receiving any messages back from the client. If this threshold is reached while client alive messages are being sent, secshd will disconnect the client, terminating the session. It is important to note that the use of client alive messages is very different from Keepalive (below). The client alive messages are sent through the encrypted channel and therefore will not be spoofable. The TCP keepalive option enabled by Keepalive is spoofable. The client alive mechanism is valuable when the client of server depend on knowing when a connection has become inactive.

The default value is 3. If you set ClientAliveInterval (above) to 15, and ClientAliveCountMax is left at the default, unresponsive secsh clients will be disconnected after approximately 45 seconds.

Compression 

Specifies whether compression is allowed or delayed until the user has authenticated successfully. The argument must be yes, delayed, or no. The default is delayed.

DenyGroups 

This keyword can be followed by a number of group names, separated by spaces. Users whose primary group or supplementary group list matches one of the patterns aren't allowed to log in. * and ? can be used as wildcards in the patterns. Only group names are valid; a numerical group ID is not recognized. By default login is allowed regardless of the group list.

DenyUsers 

This keyword can be followed by a number of user names, separated by commas. Login is disallowed for user names that match one of the patterns. * and ? can be used as wildcards in the patterns. Only user names are valid; a numerical user ID is not recognized. By default login is allowed regardless of the user name.

ForceCommand 

Forces the execution of the command specified by ForceCommand, ignoring any command supplied by the client and ~/.ssh/rc if present. The command is invoked by using the user's login shell with the -c option. This applies to shell, command, or subsystem execution. The command originally supplied by the client is available in the SSH_ORIGINAL_COMMAND environment variable. Specifying a command of internal-sftp forces the use of an in-process sftp server that requires no support files when used with ChrootDirectory.

GatewayPorts 

Specifies whether remote hosts are allowed to connect to ports forwarded for the client. By default, secshd binds remote port forwardings to the loopback address. This prevents other remote hosts from connecting to forwarded ports. GatewayPorts can be used to specify that secshd should bind remote 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.

GSSAPIAuthentication 

Specifies whether user authentication based on GSSAPI is allowed. The default is no. This option applies to protocol version 2 only.

GSSAPICleanupCredentials 

Specifies whether to automatically destroy the user's credentials cache on logout. The default is yes. This option applies to protocol version 2 only.

HostbasedAuthentication 

Specifies whether rhosts or $ROOTDIR/etc/hosts.equiv authentication together with successful public key client host authentication is allowed (hostbased authentication). This option is similar to RhostsRSAAuthentication and applies to protocol version 2 only. The default is no.

HostbasedUsesNameFromPacketOnly 

Specifies whether or not the server attempts to perform a reverse name lookup when matching the name in the ~/.shosts, ~/.rhosts, and /etc/hosts.equiv files during HostbasedAuthentication. A setting of yes means that secshd uses the name supplied by the client rather than attempting to resolve the name from the TCP connection itself. The default is no.

HostKey 

Specifies the file containing the private host keys (default $ROOTDIR/etc/ssh_host_key) used by SSH protocol versions 1 and 2. Note that secshd will refuse to use a file if it is group/world-accessible. It is possible to have multiple host key files. rsa1 keys are used for version 1 and dsa, ecdsa, ed25519 or rsa are used for version 2 of the SSH protocol.

IgnoreRhosts 

Specifies that .rhosts and .shosts files will not be used in RhostsAuthentication, RhostsRSAAuthentication, or HostbasedAuthentication.

$ROOTDIR/etc/hosts.equiv and $ROOTDIR/etc/shosts.equiv are still used. The default is yes.

IgnoreUserKnownHosts 

Specifies whether secshd should ignore the user's ~/.ssh/known_hosts during RhostsRSAAuthentication or HostbasedAuthentication. The default is no.

KexAlgorithms 

Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms must be comma-separated. The supported algorithms are:

curve25519-sha256@libssh.org
diffie-hellman-group1-sha1
diffie-hellman-group14-sha1
diffie-hellman-group-exchange-sha1
diffie-hellman-group-exchange-sha256
ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521

The default is:

curve25519-sha256@libssh.org,
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group14-sha1
KeyRegenerationInterval 

In protocol version 1, the ephemeral server key is automatically regenerated after this many seconds (if it has been used). The purpose of regeneration is to prevent decrypting captured sessions by later breaking into the machine and stealing the keys. The key is never stored anywhere. If the value is 0, the key is never regenerated. The default is 3600 (seconds).

ListenAddress 

Specifies the local addresses secshd should listen on. The following forms may be used:

ListenAddress host|IPv4_addr|IPv6_addr
ListenAddress host|IPv4_addr:port
ListenAddress [host|IPv6_addr]:port

If port is not specified, secshd will listen on the address and all prior Port options specified. The default is to listen on all local addresses. Multiple ListenAddress options are permitted. Additionally, any Port options must precede this option for non port qualified addresses.

LoginGraceTime 

The server disconnects after this time if the user has not successfully logged in. If the value is 0, there is no time limit. The default is 600 (seconds).

LogLevel 

Gives the verbosity level that is used when logging messages from secshd. The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE, and DEBUG. The default is INFO. Logging with level DEBUG violates the privacy of users and is not recommended.

MACs 

Specifies the available MAC (message authentication code) algorithms. The MAC algorithm is used in protocol version 2 for data integrity protection. Multiple algorithms must be comma-separated. The algorithms that contain ``-etm'' calculate the MAC after encryption (encrypt-then-mac). These are considered safer and their use recommended. The supported MACs are:

hmac-md5
hmac-md5-96
hmac-ripemd160
hmac-sha1
hmac-sha1-96
hmac-sha2-256
hmac-sha2-512
umac-64@openssh.com
umac-128@openssh.com
hmac-md5-etm@openssh.com
hmac-md5-96-etm@openssh.com
hmac-ripemd160-etm@openssh.com
hmac-sha1-etm@openssh.com
hmac-sha1-96-etm@openssh.com
hmac-sha2-256-etm@openssh.com
hmac-sha2-512-etm@openssh.com
umac-64-etm@openssh.com
umac-128-etm@openssh.com

The default is

umac-64-etm@openssh.com,umac-128-etm@openssh.com,
hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
umac-64@openssh.com,umac-128@openssh.com,
hmac-sha2-256,hmac-sha2-512
MaxAuthTries 

Specifies the maximum number of authentication attempts permitted per connection. Once the number of failures reaches half this value, additional failures are logged. The default is 6.

MaxSessions 

Specifies the maximum number of open sessions permitted per connection. The default is 10.

MaxStartups 

Specifies the maximum number of concurrent unauthenticated connections to the secshd daemon. Additional connections will be dropped until authentication succeeds or the LoginGraceTime expires for a connection. The default is 10:30:100.

Alternatively, random early drop can be enabled by specifying the three colon separated values start:rate:full (for example, "10:30:60"). secshd will refuse connection attempts with a probability of rate/100 (30%) if there are currently start (10) unauthenticated connections. The probability increases linearly and all connection attempts are refused if the number of unauthenticated connections reaches full (60).

PasswordAuthentication 

Specifies whether password authentication is allowed. The default is yes.

PermitEmptyPasswords 

When password authentication is allowed, it specifies whether the server allows login to accounts with empty password strings. The default is no.

PermitOpen 

Specifies the destinations to which TCP port forwarding is permitted. The forwarding specification must be one of the following forms:

PermitOpen host:port
PermitOpen IPv4_addr:port
PermitOpen [IPv6_addr]:port

Multiple forwards may be specified by separating them with whitespace. An argument of any can be used to remove all restrictions and permit any forwarding requests. An argument of none can be used to prohibit all forwarding requests. By default, all port forwarding requests are permitted.

PermitRootLogin 

Specifies whether Administrator can login using secsh The argument must be yes, without-password, forced-commands-only, or no. The default is yes.

If this option is set to without-password password authentication is disabled for Administrator.

If this option is set to forced-commands-only Administrator login with public key authentication is allowed, but only if the command option has been specified (which may be useful for taking remote backups even if Administrator login is normally not allowed). All other authentication methods are disabled for Administrator.

If this option is set to no, Administrator is not allowed to login.

PermitTTY 

Specifies whether pty allocation is permitted. The default is yes.

PermitUserEnvironment 

Specifies whether ~/.ssh/environment and environment= options in ~/.ssh/authorized_keys are processed by secshd. The default is no. Enabling environment processing may enable users to bypass access restrictions in some configurations using mechanisms such as LD_PRELOAD.

PermitUserRC 

Specifies whether any ~/.ssh/rc file is executed. The default is yes.

Port 

Specifies the port number that secshd listens on. The default is 22. Multiple options of this type are permitted. See also ListenAddress.

PrintLastLog 

Specifies whether secshd should print the date and time when the user last logged in. The default is yes.

PrintMotd 

Specifies whether secshd should print $ROOTDIR/etc/motd when a user logs in interactively. (On some systems it is also printed by the shell, $ROOTDIR/etc/profile, or equivalent.) The default is yes.

Protocol 

Specifies the protocol versions secshd should support. The possible values are 1 and 2. Multiple versions must be comma-separated. The default is 2.

PubkeyAuthentication 

Specifies whether public key authentication is allowed. The default is yes. Note that this option applies to protocol version 2 only.

RekeyLimit 

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 specified in bytes and may have a suffix of K, M, or G to indicate Kilobytes, 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. 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.

RevokedKeys 

Specifies revoked public keys. Keys listed in this file will be refused for public key authentication. Note that if this file is not readable, then public key authentication will be refused for all users. Keys may be specified as a text file, listing one public key per line, or as an OpenSSH Key Revocation List (KRL) as generated by secsh-keygen. For more information on KRLs, see the KEY REVOCATION LISTS section in secsh-keygen.

RhostsRSAAuthentication 

Specifies whether rhosts or $ROOTDIR/etc/hosts.equiv authentication together with successful RSA host authentication is allowed. The default is no. This option applies to protocol version 1 only.

RSAAuthentication 

Specifies whether pure RSA authentication is allowed. The default is yes. This option applies to protocol version 1 only.

ServerKeyBits 

Defines the number of bits in the ephemeral protocol version 1 server key. The minimum value is 512, and the default is 768.

StreamLocalBindMask 

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.

StreamLocalBindUnlink 

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, sshd 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.

StrictModes 

Specifies whether secshd should check file modes and ownership of the user's files and home directory before accepting login. This is normally desirable because novices sometimes accidentally leave their directory or files writable by everyone. The default is yes.

Subsystem 

Configures an external subsystem (for example, file transfer daemon). Arguments should be a subsystem name and a command to execute upon subsystem request. The command sftp-server implements the sftp file transfer subsystem. By default no subsystems are defined. Note that this option applies to protocol version 2 only.

SyslogFacility 

Gives the facility code that is used when logging messages from secshd. The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is AUTH.

TCPKeepAlive 

Specifies whether the system should send TCP keepalive messages to the other side. When they are sent, death of the connection or crash of one of the machines is properly noticed. However, this means that connections die if the route is down temporarily, and some people find it annoying. On the other hand, if TCP keepalives are not sent, sessions may hang indefinitely on the server, leaving "ghost" users and consuming server resources.

The default is yes (to send TCP keepalive messages), and the server notices when the network goes down or the client host crashes. This avoids infinitely hanging sessions.

To disable TCP keepalive messages, the value should be set to no.

UseDNS 

Specifies whether secshd should look up the remote host name and check that the resolved host name for the remote IP address maps back to the very same IP address. The default is yes.

UsePrivilegeSeparation 

Specifies whether secshd separates privileges by creating an unprivileged child process to deal with incoming network traffic. After successful authentication, another process is created that has the privilege of the authenticated user. The goal of privilege separation is to prevent privilege escalation by containing any corruption within the unprivileged processes. The default is yes.

X11DisplayOffset 

Specifies the first display number available for secshd's X11 forwarding. This prevents secshd from interfering with real X11 servers. The default is 10.

VersionAddendum 

Optionally specifies additional text to append to the SSH protocol banner sent by the server upon connection. The default is none.

X11Forwarding 

Specifies whether X11 forwarding is permitted. The default is no. Note that disabling X11 forwarding does not improve security in any way, as users can always install their own forwarders. X11 forwarding is automatically disabled if UseLogin is enabled.

X11UseLocalhost 

Specifies whether secshd should bind the X11 forwarding server to the loopback address or to the wildcard address. By default, secshd binds the forwarding server to the loopback address and sets the hostname part of the DISPLAY environment variable to localhost. This prevents remote hosts from connecting to the proxy display. However, some older X11 clients may not function with this configuration. X11UseLocalhost may be set to no to specify that the forwarding server should be bound to the wildcard address. The argument must be yes or no. The default is yes.

XAuthLocation 

Specifies the location of the xauth program. The default is /usr/X11R6/bin/xauth.

Time Formats

secshd command-line arguments and configuration file options that specify time may be expressed using a sequence of the form: time[qualifier], where time is a positive integer value and qualifier is one of the following:

<none>    seconds
s | S     seconds
m | M     minutes
h | H     hours
d | D     days
w | W     weeks

Each member of the sequence is added together to calculate the total time value.

Time format examples:

600     600 seconds (10 minutes)
10m     10 minutes
1h30m   1 hour 30 minutes (90 minutes)

LOGIN PROCESS

When a user successfully logs in, secshd does the following:

  1. If the login is on a tty, and no command has been specified, prints last login time and $ROOTDIR/etc/motd (unless prevented in the configuration file or by ~/.hushlogin; see the FILES section).

  2. If the login is on a tty, records login time.

  3. Checks $ROOTDIR/etc/nologin; if it exists, prints contents and quits (unless Administrator).

  4. Changes to run with normal user privileges.

  5. Sets up basic environment.

  6. Reads ~/.ssh/environment if it exists and users are allowed to change their environment.

  7. Changes to user's home directory.

  8. If ~/.ssh/rc exists, runs it; else if $ROOTDIR/etc/sshrc exists, runs it; otherwise runs xauth. The rc files are given the X11 authentication protocol and cookie in standard input.

  9. Runs user's shell or command.


AUTHORIZED_KEYS FILE FORMAT

~/.ssh/authorized_keys is the default file that lists the public keys that are permitted for RSA authentication in protocol version 1 and for public key authentication (PubkeyAuthentication) in protocol version 2.

Each line of the file contains one key (empty lines and lines starting with a # are ignored as comments). Each RSA public key consists of the following fields, separated by spaces: options, bits, exponent, modulus, comment. Each protocol version 2 public key consists of: options, keytype, base64 encoded key, comment. The options field is optional; its presence is determined by whether the line starts with a number or not (the option field never starts with a number). The bits, exponent, modulus, and comment fields give the RSA key for protocol version 1; the comment field is not used for anything (but may be convenient for the user to identify the key). For protocol version 2, the keytype is ecdsa-sha2-nistp256, ecdsa-sha2-nistp384 ecdsa-sha2-nistp521 ssh-dss or ssh-rsa.

Note that lines in this file are usually several hundred bytes long (because of the size of the public key encoding). You don't want to type them in; instead, copy the identity.pub, id_dsa.pub, id_ecdsa.pub, id_ed25519.pub, or the id_rsa.pub file and edit it.

The options (if present) consist of comma-separated option specifications. No spaces are permitted, except within double quotes. The following option specifications are supported (note that option keywords are case-insensitive):

from="pattern-list" 

Specifies that in addition to public key authentication, the canonical name of the remote host must be present in the comma-separated list of patterns (* and ? serve as wildcards). The list may also contain patterns negated by prefixing them with !; if the canonical host name matches a negated pattern, the key is not accepted. The purpose of this option is to optionally increase security: public key authentication by itself does not trust the network or name servers or anything (but the key); however, if somebody somehow steals the key, the key permits an intruder to log in from anywhere in the world. This additional option makes using a stolen key more difficult (name servers and/or routers would have to be compromised in addition to just the key).

cert-authority 

Specifies that the listed key is a certification authority (CA) that is trusted to validate signed certificates for user authentication.

Certificates may encode access restrictions similar to these key options. If both certificate restrictions and key options are present, the most restrictive union of the two is applied.

command="command" 

Specifies that the command is executed whenever this key is used for authentication. The command supplied by the user (if any) is ignored. The command is run on a pty if the client requests a pty; otherwise it is run without a tty. If an 8-bit clean channel is required, one must not request a pty or should specify no-pty. A quote may be included in the command by quoting it with a backslash. This option might be useful to restrict certain public keys to perform just a specific operation. An example might be a key that permits remote backups but nothing else. Note that the client may specify TCP and/or X11 forwarding unless they are explicitly prohibited. The command originally supplied by the client is available in the SSH_ORIGINAL_COMMAND environment variable. Note that this option applies to shell, command or subsystem execution. Also note that this command may be superseded by either a ForceCommand directive or a command embedded in a certificate.

environment="NAME=value" 

Specifies that the string is to be added to the environment when logging in using this key. Environment variables set this way override other default environment values. Multiple options of this type are permitted. Environment processing is disabled by default and is controlled via the PermitUserEnvironment option. This option is automatically disabled if UseLogin is enabled.

no-port-forwarding 

Forbids TCP/IP forwarding when this key is used for authentication. Any port forward requests by the client will return an error. This might be used, for example, in connection with the command option.

no-X11-forwarding 

Forbids X11 forwarding when this key is used for authentication. Any X11 forward requests by the client will return an error.

no-agent-forwarding 

Forbids authentication agent forwarding when this key is used for authentication.

no-pty 

Prevents tty allocation (a request to allocate a pty will fail).

permitopen="host:port" 

Limit local secsh -L port forwarding such that it may only connect to the specified host and port. IPv6 addresses can be specified by enclosing the address in square brackets. Multiple permitopen options may be applied separated by commas. No pattern matching is performed on the specified hostnames, they must be literal domains or addresses. A port specification of * matches any port.

principals="principals" 

On a cert-authority line, specifies allowed principals for certificate authentication as a comma-separated list. At least one name from the list must appear in the certificate's list of principals for the certificate to be accepted. This option is ignored for keys that are not marked as trusted certificate signers using the cert-authority option.

Examples

1024 33 12121...312314325 ylo@foo.bar

from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula

command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi

permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323

SSH_KNOWN_HOSTS FILE FORMAT

The $ROOTDIR/etc/ssh_known_hosts and ~/.ssh/known_hosts files contain host public keys for all known hosts. The global file should be prepared by the administrator (optional), and the per-user file is maintained automatically: whenever the user connects from an unknown host its key is added to the per-user file.

Each line in these files contains the following fields: markers (optional), hostnames, bits, exponent, modulus, comment. The fields are separated by spaces.

The marker is optional, but if it is present then it must be one of @cert-authority, to indicate that the line contains a certification authority (CA) key, or @revoked, to indicate that the key contained on the line is revoked and must not ever be accepted. Only one marker should be used on a key line.

Hostnames is a comma-separated list of patterns (* and ? act as wildcards); each pattern in turn is matched against the canonical host name (when authenticating a client) or against the user-supplied name (when authenticating a server). A pattern may also be preceded by ! to indicate negation: if the host name matches a negated pattern, it is not accepted (by that line) even if it matched another pattern on the line.

Bits, exponent, and modulus are taken directly from the RSA host key; they can be obtained, for example, from $ROOTDIR/etc/ssh_host_key.pub. The optional comment field continues to the end of the line, and is not used.

Lines starting with # and empty lines are ignored as comments.

When performing host authentication, authentication is accepted if any matching line has the proper key; either one that matches exactly or, if the server has presented a certificate for authentication, the key of the certification authority that signed the certificate. For a key to be trusted as a certification authority, it must use the cert-authority marker described above.

The known hosts file also provides a facility to mark keys as revoked, for example when it is known that the associated private key has been stolen. Revoked keys are specified by including the @revoked marker at the beginning of the key line, and are never accepted for authentication or as certification authorities, but instead will produce a warning from secsh when they are encountered.

It is permissible (but not recommended) to have several lines or different host keys for the same names. This will inevitably happen when short forms of host names from different domains are put in the file. It is possible that the files contain conflicting information; authentication is accepted if valid information can be found from either file.

Note that the lines in these files are typically hundreds of characters long, and you definitely don't want to type in the host keys by hand. Rather, generate them by a script or by taking $ROOTDIR/etc/ssh_host_key.pub and adding the host names at the front. ssh-keygen also offers some basic automated editing for ~/.ssh/known_hosts including removing hosts matching a host name and converting all host names to their hashed representations.

Examples

closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
# A hashed hostname
|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
AAAA1234.....=
# A revoked key
@revoked * ssh-rsa AAAAB5W...
# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...

TIME FORMATS

secshd command-line arguments and configuration file options that specify time may be expressed using a sequence of the form: time[qualifier], where time is a positive integer value and qualifier is one of the following:

           <none>  seconds
           s | S   seconds
           m | M   minutes
           h | H   hours
           d | D   days
           w | W   weeks

Each member of the sequence is added together to calculate the total time value.

Time format examples:

           600     600 seconds (10 minutes)
           10m     10 minutes
           1h30m   1 hour 30 minutes (90 minutes)


FILES

HKEY_LOCAL_MACHINE/Software/Mortice Kern Systems/etc/sshd_config 

Contains configuration data for secshd.

$ROOTDIR/etc/ssh_host_key 
$ROOTDIR/etc/ssh_host_dsa_key 
$ROOTDIR/etc/ssh_host_ecdsa_key 
$ROOTDIR/etc/ssh_host_ed25519_key 
$ROOTDIR/etc/ssh_host_rsa_key 

These four files contain the private parts of the host keys. These files should only be owned by Administrator, readable only by Administrator, and not accessible to others. Note that secshd does not start if this file is accessible to non-administrative accounts.

$ROOTDIR/etc/ssh_host_key.pub 
$ROOTDIR/etc/ssh_host_dsa_key.pub 
$ROOTDIR/etc/ssh_host_ecdsa_key.pub 
$ROOTDIR/etc/ssh_host_ed25519_key.pub 
$ROOTDIR/etc/ssh_host_rsa_key.pub 

These four files contain the public parts of the host keys. These files should be readable by everyone but writable only by Administrator. Their contents should match the respective private parts. These files are not really used for anything; they are provided for the convenience of the user so their contents can be copied to known hosts files. These files are created using secsh-keygen.

$ROOTDIR/etc/moduli 

Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".

~/.ssh/authorized_keys 

Lists the public keys (RSA, DSA, ECDSA, or ED25519) that can be used to log into the user's account. This file must be readable by Administrator (which may on some machines imply it being readable by everyone if the user's home directory resides on a network sharepoint). It is recommended that it not be accessible by others. The format of this file is described above. Users will place the contents of their id_dsa.pub, id_ecdsa.pub, id_ed25519.pub and/or id_rsa.pub files into this file, as described in secsh-keygen.

$ROOTDIR/etc/ssh_known_hosts 
~/.ssh/known_hosts 

These files are consulted when using rhosts with RSA host authentication to check the public key of the host. The key must be listed in one of these files to be accepted. The client uses the same files to verify that it is connecting to the correct remote host. These files should be writable only by Administrator and the owner. $ROOTDIR/etc/ssh_known_hosts should be readable by everyone, and ~/.ssh/known_hosts can but need not be readable by everyone.

$ROOTDIR/etc/nologin 

If this file exists, secshd refuses to let anyone except Administrator log in. The contents of the file are displayed to anyone trying to log in, and non-Administrator connections are refused. The file should be readable by everyone.

$ROOTDIR/etc/hosts.allow 
$ROOTDIR/etc/hosts.deny 

Access controls that should be enforced by tecp-wrappers are defined here.

~/.rhosts 

This file contains host-username pairs, separated by a space, one per line. The given user on the corresponding host is permitted to log in without password. The same file is used by rlogind and rshd. The file must be writable only by the user; it is recommended that it not be accessible by others.

If is also possible to use netgroups in the file. Either host or user name may be of the form +@groupname to specify all hosts or all users in the group.

~/.shosts 

For ssh, this file is exactly the same as for .rhosts. However, this file is not used by rlogin and rshd, so using this permits access using SSH only.

$ROOTDIR/etc/hosts.equiv 

This file is used during .rhosts authentication. In the simplest form, this file contains host names, one per line. Users on those hosts are permitted to log in without a password, provided they have the same user name on both machines. The host name may also be followed by a user name; such users are permitted to log in as any user on this machine (except Administrator). Additionally, the syntax +@group can be used to specify netgroups. Negated entries start with -.

If the client host/user is successfully matched in this file, login is automatically permitted provided the client and server user names are the same. Additionally, successful RSA host authentication is normally required. This file must be writable only by Administrator; it is recommended that it be readable by everyone.

Warning

It is almost never a good idea to use user names in hosts.equiv. Beware that it really means that the named user(s) can log in as anybody, which can include accounts that own critical binaries and directories. Using a user name practically grants the user Administrator access. The only valid use for user names that I can think of is in negative entries.

Note that this warning also applies to rsh/rlogin.

$ROOTDIR/etc/shosts.equiv 

This is processed exactly as $ROOTDIR/etc/hosts.equiv. However, this file may be useful in environments that want to run both rsh/rlogin and secure shell.

~/.ssh/environment 

This file is read into the environment at login (if it exists). It can only contain empty lines, comment lines (that start with #), and assignment lines of the form name=value. The file should be writable only by the user; it need not be readable by anyone else. Environment processing is disabled by default and is controlled via the PermitUserEnvironment option.

~/.ssh/rc 

If this file exists, it is run with SHELL after reading the environment files but before starting the user's shell or command. If X11 spoofing is in use, this will receive the "proto cookie" pair in standard input (and DISPLAY in environment). This must call xauth in that case.

The primary purpose of this file is to run any initialization routines which may be needed before the user's home directory becomes accessible; AFS is a particular example of such an environment.

This file will probably contain some initialization code followed by something similar to:

if read proto cookie && [ -n "$DISPLAY" ]; then
        if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
                # X11UseLocalhost=yes
                echo add unix:`echo $DISPLAY |
                    cut -c11-` $proto $cookie
        else
                # X11UseLocalhost=no
                echo add $DISPLAY $proto $cookie
        fi | xauth -q -
fi

If this file does not exist, $ROOTDIR/etc/sshrc is run, and if that does not exist either, xauth is used to store the cookie.

This file should be writable only by the user, and need not be readable by anyone else.

$ROOTDIR/etc/sshrc 

Like ~/.ssh/rc. This can be used to specify machine-specific login-time initializations globally. This file should be writable only by Administrator, and should be readable by everyone.


AUTHORS

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.


PORTABILITY

All UNIX systems. Windows 7. Windows Server 2008 R2. Windows 8. Windows Server 2012. Windows 10. Windows Server 2016.


NOTES

The client utilities will load 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 (such as a laptop connected to the public Internet). 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 used by the secure shell client, and a second one under ~/.ssh/known_hosts that is used by the secure shell service.

Furthermore, the location of the ~/ directory itself is somewhat ambiguous and depends upon whether you have actually logged into the server machine itself. In the event you have logged in, then the ~/ directory will be your normal home directory. In the event that you have not, however, the ~/ directory will appear in your user profile directory. This ambiguity may be resolved in a future release.

Straight rhosts authentication, while supported by OpenSSH is not supported in the version shipped in PTC MKS Toolkit.

Normally, the PTC MKS Toolkit version of secshd comes with a default license that allows only one person connect to secshd at a time. Licenses for multiple users are available from MKS.

When connecting with secsh, 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

or

stty erase ^?

to set the erase character to either backspace or delete, respectively.


AVAILABILITY

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


SEE ALSO

Commands:
rlogin, rsh, scp, secsh, secsh-add, secsh-agent, secsh-keygen, secsh-keysign, sftp, sftp-server, stty

Miscellaneous:
rlogind

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.

M. Friedl, N. Provos, and W. A. Simpson, Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol, draft-ietf-secsh-dh-group-exchange-01.txt, April 2001, work in progress material.


PTC MKS Toolkit 10.1 Documentation Build 15.