secshd -- OpenSSH Secure Shell service

SYNOPSIS

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

secshd (Secure Shell service) is a 10/2016/2019/11/2022/2025 service that provides the server side for secsh. It provides 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 10/2016/2019/11/2022/2025 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 10/2016/2019/11/2022/2025 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. secshd works as follows.

SSH protocol version 2

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 and SetEnv on the secsh reference page for how to configure the client. The TERM environment variable is always accepted whenever the client requests a pseudo-terminal as it is required by the protocol. 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 for all groups. The allow/deny groups directives are processed in the following order: DenyGroups, AllowGroups.

See PATTERNS for more information on patterns.

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.

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.

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. HOST criteria may additionally contain addresses to match in CIDR address/masklen format. The allow/deny directives are processed in the following order: DenyUsers, AllowUsers.

See PATTERNS for more information on patterns.

AuthenticationMethods

Specifies the authentication methods that must be successfully completed for a user to be granted access. This option must be followed by one or more lists of comma-separated authentication method names, or by the single string any to indicate the default behaviour of accepting any single authentication method. If the default is overridden, then successful authentication requires completion of every method in at least one of these lists.

For example, 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.

For keyboard interactive authentication it is also possible to restrict authentication to a specific device by appending a colon followed by the device identifier bsdauth or pam. depending on the server configuration. For example, keyboard-interactive:bsdauth would restrict keyboard interactive authentication to the bsdauth device.

If the publickey method is listed more than once, sshd(8) verifies that keys that have been used successfully are not reused for subsequent authentications. For example, publickey,publickey requires successful authentication using two different public keys.

Note that each authentication method listed should also be explicitly enabled in the configuration.

The available authentication methods are: gssapi-with-mic, hostbased, keyboard-interactive, none (used for access to password-less accounts when PermitEmptyPasswords is enabled), password and publickey.

AuthorizedKeysCommand

Specifies a program to be used to look up the user's public keys. The program must be owned by root, not writable by group or others and specified by an absolute path. Arguments to AuthorizedKeysCommand accept the tokens described in the TOKENS section. If no arguments are specified then the username of the target user is used.

The program should produce on standard output zero or more lines of authorized_keys output (see AUTHORIZED_KEYS. AuthorizedKeysCommand is tried after the usual AuthorizedKeysFile files and will not be executed if a matching key is found there. By default, no AuthorizedKeysCommand is run.

AuthorizedKeysCommandUser

Specifies the user under whose account the AuthorizedKeysCommand is run. It is recommended to use a dedicated user that has no other role on the host than running authorized keys commands. If AuthorizedKeysCommand is specified but AuthorizedKeysCommandUser is not, then sshd(8) will refuse to start.

AuthorizedKeysFile

Specifies the file that contains the public keys used for user authentication. The format is described in the AUTHORIZED_KEYS FILE FORMAT section of sshd(8). Arguments to AuthorizedKeysFile accept the tokens described in the TOKENS section. After expansion, AuthorizedKeysFile is taken to be an absolute path or one relative to the user's home directory. Multiple files may be listed, separated by whitespace. Alternately this option may be set to none to skip checking for user keys in files. The default is ".ssh/authorized_keys .ssh/authorized_keys2".

AuthorizedKeysFile

Specifies the file that contains the public keys used for user authentication. The format is described in the AUTHORIZED_KEYS FILE FORMAT section of secshd. Arguments to AuthorizedKeysFile accept the tokens described in the TOKENS section. After expansion, AuthorizedKeysFile is taken to be an absolute path or one relative to the user's home directory. Multiple files may be listed, separated by whitespace. Alternately this option may be set to none to skip checking for user keys in files. The default is .ssh/authorized_keys .ssh/authorized_keys2.

AuthorizedPrincipalsCommand

Specifies a program to be used to generate the list of allowed certificate principals as per AuthorizedPrincipalsFile. The program must be owned by root, not writable by group or others and specified by an absolute path. Arguments to AuthorizedPrincipalsCommand accept the tokens described in the TOKENS section. If no arguments are specified then the username of the target user is used.

The program should produce on standard output zero or more lines of AuthorizedPrincipalsFile output. If either AuthorizedPrincipalsCommand or AuthorizedPrincipalsFile is specified, then certificates offered by the client for authentication must contain a principal that is listed. By default, no AuthorizedPrincipalsCommand is run.

AuthorizedPrincipalsCommandUser

Specifies the user under whose account the AuthorizedPrincipalsCommand is run. It is recommended to use a dedicated user that has no other role on the host than running authorized principals commands. If AuthorizedPrincipalsCommand is specified but AuthorizedPrincipalsCommandUser is not, then sshd(8) will refuse to start.

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 secshd). Empty lines and comments starting with # are ignored.

Arguments to AuthorizedPrincipalsFile accept the tokens described in the TOKENS section. 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 secshd for details).

Banner

The contents of the specified file are sent to the remote user before authentication is allowed. If the argument is none then no banner is displayed. By default, no banner is displayed.

CASignatureAlgorithms

Specifies which algorithms are allowed for signing of certificates by certificate authorities (CAs). The default is:

ssh-ed25519,ecdsa-sha2-nistp256,
ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
rsa-sha2-512,rsa-sha2-256

If the specified list begins with a + character, then the specified algorithms will be appended to the default set instead of replacing them. If the specified list begins with a - character, then the specified algorithms (including wildcards) will be removed from the default set instead of replacing them.

Certificates signed using other algorithms will not be accepted for public key or host-based authentication.

ChannelTimeout

Specifies whether and how quickly secshd should close inactive channels. Timeouts are specified as one or more type=interval pairs separated by whitespace, where the type must be the special keyword global or a channel type name from the list below, optionally containing wildcard characters.

The timeout value interval is specified in seconds or may use any of the units documented in the TIME FORMATS section. For example, session=5m would cause interactive sessions to terminate after five minutes of inactivity. Specifying a zero value disables the inactivity timeout.

The special timeout global applies to all active channels, taken together. Traffic on any active channel will reset the timeout, but when the timeout expires then all open channels will be closed. Note that this global timeout is not matched by wildcards and must be specified explicitly.

The available channel type names include:

agent-connection: Open connections to secsh-agent.
direct-tcpip
direct-streamlocal@openssh.com: Open TCP or Unix socket (respectively) connections that have been established from a secsh local forwarding, i.e. LocalForward or DynamicForward.
forwarded-tcpip
forwarded-streamlocal@openssh.com: Open TCP or Unix socket (respectively) connections that have been established to a secshd listening on behalf of a secsh remote forwarding, i.e. RemoteForward.
session: The interactive main session, including shell session, command execution, scp, sftp, etc.
tun-connection: Open TunnelForward connections.
x11-connection: Open X11 forwarding sessions.

Note that in all the above cases, terminating an inactive session does not guarantee to remove all resources associated with the session, e.g. shell processes or X11 clients relating to the session may continue to execute.

Moreover, terminating an inactive channel or session does not necessarily close the SSH connection, nor does it prevent a client from requesting another channel of the same type. In particular, expiring an inactive forwarding session does not prevent another identical forwarding from being subsequently created.

The default is not to expire channels of any type for inactivity.

Ciphers

Specifies the ciphers allowed. Multiple ciphers must be comma- separated. If the specified list begins with a + character, then the specified ciphers will be appended to the default set instead of replacing them. If the specified list begins with a - character, then the specified ciphers (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a ^ character, then the specified ciphers will be placed at the head of the default set.

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
chacha20-poly1305@openssh.com

The default is:

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

The list of available ciphers may also be obtained using secsh -Q cipher.

ClientAliveCountMax

Sets the number of client alive messages 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 TCPKeepAlive. The client alive messages are sent through the encrypted channel and therefore will not be spoofable. The TCP keepalive option enabled by TCPKeepAlive is spoofable. The client alive mechanism is valuable when the client or server depend on knowing when a connection has become unresponsive.

The default value is 3. If ClientAliveInterval is set to 15, and ClientAliveCountMax is left at the default, unresponsive SSH clients will be disconnected after approximately 45 seconds. Setting a zero ClientAliveCountMax disables connection termination.

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.

Compression

Specifies whether compression is enabled after the user has authenticated successfully. The argument must be yes, delayed (a legacy synonym for yes) or no. The default is yes.

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. The allow/deny directives are processed in the following order: DenyGroups, AllowGroups.

See PATTERNS for more information on patterns.

DenyUsers

This keyword can be followed by a number of user names, separated by spaces. Login is disallowed for user names that match one of the patterns. Only user names are valid; a numerical user ID is not recognized. By default, login is allowed for all users. If the pattern takes the form USER@HOST then USER and HOST are separately checked, restricting logins to particular users from particular hosts. HOST criteria may additionally contain addresses to match in CIDR address/masklen format. The allow/deny directives are processed in the following order: DenyUsers, AllowUsers.

See PATTERNS for more information on patterns.

DisableForwarding

Disables all forwarding features, including X11, secsh-agent, TCP and StreamLocal. This option overrides all other forwarding-related options and may simplify restricted configurations.

ExposeAuthInfo

Writes a temporary file containing a list of authentication methods and public credentials (e.g. keys) used to authenticate the user. The location of the file is exposed to the user session through the SSH_USER_AUTH environment variable. The default is no.

FingerprintHash

Specifies the hash algorithm used when logging key fingerprints. Valid options are: md5 and sha256. The default is sha256.

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. The default is none.

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.

GSSAPICleanupCredentials

Specifies whether to automatically destroy the user's credentials cache on logout. The default is yes.

GSSAPIStrictAcceptorCheck

Determines whether to be strict about the identity of the GSSAPI acceptor a client authenticates against. If set to yes then the client must authenticate against the host service on the current hostname. If set to no then the client may authenticate against any service key stored in the machine's default store. This facility is provided to assist with operation on multi homed machines. The default is yes.

HostbasedAcceptedAlgorithms

Specifies the signature algorithms that will be accepted for hostbased authentication as a list of comma-separated patterns. Alternately if the specified list begins with a + character, then the specified signature algorithms will be appended to the default set instead of replacing them. If the specified list begins with a - character, then the specified signature algorithms (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a ^ character, then the specified signature algorithms will be placed at the head of the default set. The default for this option is:

ssh-ed25519-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
rsa-sha2-512,rsa-sha2-256

The list of available signature algorithms may also be obtained using secsh -Q HostbasedAcceptedAlgorithms. This was formerly named HostbasedAcceptedKeyTypes.

HostbasedAuthentication

Specifies whether rhosts or $ROOTDIR/etc/hosts.equiv authentication together with successful public key client host authentication is allowed (hostbased authentication). 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 $ROOTDIR/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.

HostCertificate

Specifies a file containing a public host certificate. The certificate's public key must match a private host key already specified by HostKey. The default behaviour of sshd(8) is not to load any certificates.

HostKey

Specifies a file containing a private host key used by SSH. The defaults are $ROOTDIR/etc/ssh/ssh_host_ecdsa_key, $ROOTDIR/etc/ssh/ssh_host_ed25519_key and $ROOTDIR/etc/ssh/ssh_host_rsa_key.

Note that secshd will refuse to use a file if it is group/world- accessible and that the HostKeyAlgorithms option restricts which of the keys are actually used by secshd.

It is possible to have multiple host key files. It is also possible to specify public host key files instead. In this case operations on the private key will be delegated to an secsh-agent.

HostKeyAgent

Identifies the UNIX-domain socket used to communicate with an agent that has access to the private host keys. If the string "SSH_AUTH_SOCK" is specified, the location of the socket will be read from the SSH_AUTH_SOCK environment variable.

HostKeyAlgorithms

Specifies the host key signature algorithms that the server offers. The default for this option is:

ssh-ed25519-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
rsa-sha2-512,rsa-sha2-256

The list of available signature algorithms may also be obtained using ssh -Q HostKeyAlgorithms.

IgnoreRhosts

Specifies whether to ignore per-user .rhosts and .shosts files during HostbasedAuthentication. The system-wide $ROOTDIR/etc/hosts.equiv and $ROOTDIR/etc/shosts.equiv are still used regardless of this setting.

Accepted values are yes (the default) to ignore all per-user files, shosts-only to allow the use of .shosts but to ignore .rhosts or no to allow both .shosts and .rhosts.

IgnoreUserKnownHosts

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

IPQoS

Specifies the IPv4 type-of-service or DSCP class for the connection. Accepted values are af11, af12, af13, af21, af22, af23, af31, af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3, cs4, cs5, cs6, cs7, ef, le, lowdelay, throughput, reliability, a numeric value, or none to use the operating system default. 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 af21 (Low-Latency Data) for interactive sessions and cs1 (Lower Effort) for non- interactive sessions.

KbdInteractiveAuthentication

Specifies whether to allow keyboard-interactive authentication. All authentication styles from login.conf(5) are supported. The default is yes. The argument to this keyword must be yes or no. ChallengeResponseAuthentication is a deprecated alias for this.

KexAlgorithms

Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms must be comma-separated. Alternately if the specified list begins with a + character, then the specified algorithms will be appended to the default set instead of replacing them. If the specified list begins with a - character, then the specified algorithms (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a ^ character, then the specified algorithms will be placed at the head of the default set. The supported algorithms are:

curve25519-sha256
curve25519-sha256@libssh.org
diffie-hellman-group1-sha1
diffie-hellman-group14-sha1
diffie-hellman-group14-sha256
diffie-hellman-group16-sha512
diffie-hellman-group18-sha512
diffie-hellman-group-exchange-sha1
diffie-hellman-group-exchange-sha256
ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521
sntrup761x25519-sha512@openssh.com

The default is:

sntrup761x25519-sha512@openssh.com,
curve25519-sha256,curve25519-sha256@libssh.org,
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
diffie-hellman-group14-sha256

The list of available key exchange algorithms may also be obtained using secsh -Q KexAlgorithms.

ListenAddress

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

ListenAddress hostname|address
ListenAddress hostname:port
ListenAddress IPv4_address:port
ListenAddress [hostname|address]:port

If port is not specified, sshd will listen on the address and all Port options specified. The default is to listen on all local addresses. Multiple ListenAddress options are permitted.

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.

LogVerbose

Specify one or more overrides to LogLevel. An override consists of a pattern lists that matches the source file, function and line number to force detailed logging for. For example, an override pattern of:

kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*

would enable detailed logging for line 1000 of kex.c, everything in the kex_exchange_identification() function, and all code in the packet.c file. This option is intended for debugging and no overrides are enabled by default.

MACs

Specifies the available MAC (message authentication code) algorithms. The MAC algorithm is used for data integrity protection. Multiple algorithms must be comma-separated. If the specified value begins with a + character, then the specified algorithms will be appended to the default set instead of replacing them. If the specified value begins with a - character, then the specified algorithms (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a `^' character, then the specified algorithms will be placed at the head of the default set.

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-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-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,
hmac-sha1-etm@openssh.com,
umac-64@openssh.com,umac-128@openssh.com,
hmac-sha2-256,hmac-sha2-512,hmac-sha1

The list of available MAC algorithms may also be obtained using secsh -Q mac.

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 shell, login or subsystem (e.g. sftp) sessions permitted per network connection. Multiple sessions may be established by clients that support connection multiplexing. Setting MaxSessions to 1 will effectively disable session multiplexing, whereas setting it to 0 will prevent all shell, login and subsystem sessions while still permitting forwarding. The default is 10.

MaxStartups

Specifies the maximum number of concurrent unauthenticated connections to secshd. 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 (e.g. "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).

ModuliFile

Specifies the moduli file that contains the Diffie-Hellman groups used for the diffie-hellman-group-exchange-sha1 and diffie-hellman-group-exchange-sha256 key exchange methods. The default is $ROOTDIR/etc/moduli.

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.

PermitListen

Specifies the addresses/ports on which a remote TCP port forwarding may listen. The listen specification must be one of the following forms:

PermitListen port
PermitListen host:port

Multiple permissions may be specified by separating them with whitespace. An argument of any can be used to remove all restrictions and permit any listen requests. An argument of none can be used to prohibit all listen requests. The host name may contain wildcards as described in the PATTERNS section in secsh. The wildcard * can also be used in place of a port number to allow all ports. By default all port forwarding listen requests are permitted. Note that the GatewayPorts option may further restrict which addresses may be listened on. Note also that secsh will request a listen host of localhost if no listen host was specifically requested, and this this name is treated differently to explicit localhost addresses of 127.0.0.1 and ::1.

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. The wildcard * can be used for host or port to allow all hosts orports, respectively. Otherwise, no pattern matching or address lookups are performed on supplied names. By default all port forwarding requests are permitted.

PermitRootLogin

Specifies whether Administrator can login using secsh The argument must be yes, prohibit-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. Valid options are yes, no or a pattern-list specifying which environment variable names to accept (for example LANG,LC_*). 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.

PerSourceMaxStartups

Specifies the number of unauthenticated connections allowed from a given source address, or ``none'' if there is no limit. This limit is applied in addition to MaxStartups, whichever is lower. The default is none.

PerSourceNetBlockSize

Specifies the number of bits of source address that are grouped together for the purposes of applying PerSourceMaxStartups limits. Values for IPv4 and optionally IPv6 may be specified, separated by a colon. The default is 32:128, which means each address is considered individually.

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.

PubkeyAcceptedAlgorithms

Specifies the signature algorithms that will be accepted for public key authentication as a list of comma-separated patterns. Alternately if the specified list begins with a + character, then the specified algorithms will be appended to the default set instead of replacing them. If the specified list begins with a - character, then the specified algorithms (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a ^ character, then the specified algorithms will be placed at the head of the default set. The default for this option is:

ssh-ed25519-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
rsa-sha2-512,rsa-sha2-256

The list of available signature algorithms may also be obtained using secsh -Q PubkeyAcceptedAlgorithms.

PubkeyAuthOptions

Sets one or more public key authentication options. The supported keywords are: none (the default; indicating no additional options are enabled), touch-required and verify-required.

The touch-required option causes public key authentication using a FIDO authenticator algorithm (i.e. ecdsa-sk or ed25519-sk) to always require the signature to attest that a physically present user explicitly confirmed the authentication (usually by touching the authenticator). By default, secshd requires user presence unless overridden with an authorized_keys option. The touch-required flag disables this override.

The verify-required option requires a FIDO key signature attest that the user was verified, e.g. via a PIN.

Neither the touch-required or verify-required options have any effect for other, non-FIDO, public key types.

PubkeyAuthentication

Specifies whether public key authentication is allowed. The default is yes.

RekeyLimit

Specifies the maximum amount of data that may be transmitted or received before the session key is renegotiated, optionally followed by 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.

RequiredRSASize

Specifies the minimum RSA key size (in bits) that secshd will accept. User and host-based authentication keys smaller than this limit will be refused. The default is 1024 bits. Note that this limit may only be raised from the default.

RevokedKeys

Specifies revoked public keys file, or none to not use one. 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.

RDomain

Specifies an explicit routing domain that is applied after authentication has completed. The user session, as well as any forwarded or listening IP sockets, will be bound to this rdomain. If the routing domain is set to %D, then the domain in which the incoming connection was received will be applied.

SecurityKeyProvider

Specifies a path to a library that will be used when loading FIDO authenticator-hosted keys, overriding the default of using the built-in USB HID support.

SetEnv

Specifies one or more environment variables to set in child sessions started by secshd as NAME=VALUEM. The environment value may be quoted (e.g. if it contains whitespace characters). Environment variables set by SetEnv override the default environment and any variables specified by the user via AcceptEnv or PermitUserEnvironment.

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.

Alternately the name internal-sftp implements an in-process SFTP server. This may simplify configurations using ChrootDirectory to force a different filesystem root on clients. It accepts the same command line arguments as sftp-server and even though it is in-process, settings such as LogLevel or SyslogFacility do not apply to it and must be set explicitly via command line arguments.

The command sftp-server implements the sftp file transfer subsystem.

By default no subsystems are defined.

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.

TrustedUserCAKeys

Specifies a file containing public keys of certificate authorities that are trusted to sign user certificates for authentication, or none to not use one. Keys are listed one per line; empty lines and comments starting with # are allowed. If a certificate is presented for authentication and has its signing CA key listed in this file, then it may be used for authentication for any user listed in the certificate's principals list. Note that certificates that lack a list of principals will not be permitted for authentication using TrustedUserCAKeys. For more details on certificates, see the CERTIFICATES section in secsh-keygen.

UnusedConnectionTimeout

Specifies whether and how quickly secshd should close client connections with no open channels. Open channels include active shell, command execution or subsystem sessions, connected network, socket, agent or X11 forwardings. Forwarding listeners, such as those from the secsh -R flag, are not considered as open channels and do not prevent the timeout. The timeout value is specified in seconds or may use any of the units documented in the TIME FORMATS section.

Note that this timeout starts when the client connection completes user authentication but before the client has an opportunity to open any channels. Caution should be used when using short timeout values, as they may not provide sufficient time for the client to request and open its channels before terminating the connection.

The default none is to never expire connections for having no open channels. This option may be useful in conjunction with ChannelTimeout.

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.

If this option is set to no (the default) then only addresses and not host names may be used in ~/.ssh/authorized_keys from directives.

VersionAddendum

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

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.

X11Forwarding

Specifies whether X11 forwarding is permitted. The default is no.

When X11 forwarding is enabled, there may be additional exposure to the server and to client displays if the secshd proxy display is configured to listen on the wildcard address (see X11UseLocalhost), though this is not the default. Additionally, the authentication spoofing and authentication data verification and substitution occur on the client side. The security risk of using X11 forwarding is that the client's X11 display server may be exposed to attack when the SSH client requests forwarding (see the warnings for ForwardX11 in secsh. A system administrator may have a stance in which they want to protect clients that may expose themselves to attack by unwittingly requesting X11 forwarding, which can warrant a no setting.

Note that disabling X11 forwarding does not prevent users from forwarding X11 traffic, as users can always install their own forwarders.

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 $ROOTDIR/bin/X11/xauth.exe.

LOGIN PROCESS

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

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).

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

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

Changes to run with normal user privileges.

Sets up basic environment.

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

Changes to user's home directory.

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.

Runs user's shell or command.

AUTHORIZED_KEYS FILE FORMAT

AuthorizedKeysFile specifies the files containing public keys for public key authentication; if this option is not specified, the default is ~/.ssh/authorized_keys and ~/.ssh/authorized_keys2. Each line of the file contains one key (empty lines and lines starting with a # are ignored as comments). Public keys consist of the following space- separated fields: options, keytype, base64-encoded key, comment. The options field is optional. The supported key types are:

sk-ecdsa-sha2-nistp256@openssh.com
ecdsa-sha2-nistp256
ecdsa-sha2-nistp384
ecdsa-sha2-nistp521
sk-ssh-ed25519@openssh.com
ssh-ed25519
ssh-dss
ssh-rsa

The comment field is not used for anything (but may be convenient for the user to identify the key).

Note that lines in this file can be several hundred bytes long (because of the size of the public key encoding) up to a limit of 8 kilobytes, which permits RSA keys up to 16 kilobits. You don't want to type them in; instead, copy the id_dsa.pub, id_ecdsa.pub, id_ecdsa_sk.pub, id_ed25519.pub, id_ed25519_sk.pub, or the id_rsa.pub file and edit it.

sshd enforces a minimum RSA key modulus size of 1024 bits.

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):

agent-forwarding

Enable authentication agent forwarding previously disabled by the restrict option.

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, e.g. using the restrict key option.

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 a secshd ForceCommand directive.

If a command is specified and a forced-command is embedded in a certificate used for authentication, then the certificate will be accepted only if the two commands are identical.

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.

expiry-time="timespec"

Specifies a time after which the key will not be accepted. The time may be specified as a YYYYMMDD date or a YYYYMMDDHHMM[SS] time in the system time-zone. Dates and times will be interpreted in the system time zone unless suffixed by a Z character, in which case they will be interpreted in the UTC time zone.

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).

no-agent-forwarding

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

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-pty

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

no-user-rc

Disables execution of ~/.ssh/rc.

no-X11-forwarding

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

permitlisten="[host:]port"

Limit remote port forwarding with the secsh -R option such that it may only listen on the specified host (optional) and port. IPv6 addresses can be specified by enclosing the address in square brackets. Multiple permitlisten options may be applied separated by commas. Hostnames may include wildcards as described in the PATTERNS section in secsh. A port specification of * matches any port. Note that the setting of GatewayPorts may further restrict listen addresses. Note that secsh will send a hostname of localhost if a listen host was not specified when the forwarding was requested, and that this name is treated differently to the explicit localhost addresses 127.0.0.1 and ::1.

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.

pty

Permits tty allocation previously disabled by the restrict option.

no-touch-required

Do not require demonstration of user presence for signatures made using this key. This option only makes sense for the FIDO authenticator algorithms ecdsa-sk and ed25519-sk.

verify-required

Require that signatures made using this key attest that they verified the user, e.g. via a PIN. This option only makes sense for the FIDO authenticator algorithms ecdsa-sk and ed25519-sk.

restrict

Enable all restrictions, i.e. disable port, agent and X11 forwarding, as well as disabling PTY allocation and execution of ~/.ssh/rc. If any future restriction capabilities are added to authorized_keys files they will be included in this set.

user-rc

Enables execution of ~/.ssh/rc previously disabled by the restrict option.

X11-forwarding

Permits X11 forwarding previously disabled by the restrict 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/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 to an unknown host, its key is added to the per-user file.

Each line in these files contains the following fields: marker (optional), hostnames, keytype, base64-encoded key, 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 host name. When secshd is authenticating a client, such as when using HostbasedAuthentication, this will be the canonical client host name. When secsh is authenticating a server, this will be the host name given by the user, the value of the secsh HostkeyAlias if it was specified, or the canonical server hostname if the secsh CanonicalizeHostname option was used.

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. A hostname or address may optionally be enclosed within [ and ] brackets then followed by : and a non-standard port number.

Alternately, hostnames may be stored in a hashed form which hides host names and addresses should the file's contents be disclosed. Hashed hostnames start with a | character. Only one hashed hostname may appear on a single line and none of the above negation or wildcard operators may be applied.

The keytype and base64-encoded key are taken directly from the host key; they can be obtained, for example, from $ROOTDIR/etc/ssh/ssh_host_rsa_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, secsh-keyscan or by taking, for example, $ROOTDIR/etc/ssh/ssh_host_rsa_key.pub and adding the host names at the front. secsh-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

# Comments allowed at start of line
cvs.example.net,192.0.2.10 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)

TOKENS

Arguments to some keywords can make use of tokens, which are expanded at runtime:

%%    A literal %.
%C    Identifies the connection endpoints, containing four space-
      separated values: client address, client port number, server
      address, and server port number.
%D    The routing domain in which the incoming connection was received.
%F    The fingerprint of the CA key.
%f    The fingerprint of the key or certificate.
%h    The home directory of the user.
%i    The key ID in the certificate.
%K    The base64-encoded CA key.
%k    The base64-encoded key or certificate for authentication.
%s    The serial number of the certificate.
%T    The type of the CA key.
%t    The key or certificate type.
%U    The numeric user ID of the target user.
%u    The username.

AuthorizedKeysCommand accepts the tokens %%, %C, %D, %f, %h, %k, %t, %U, and %u.

AuthorizedKeysFile accepts the tokens %%, %h, %U, and %u.

AuthorizedPrincipalsCommand accepts the%C tokens %%, %C, %D, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.

AuthorizedPrincipalsFile accepts the tokens %%, %h, %U, and %u.

FILES

HKEY_LOCAL_MACHINE/Software/Mortice Kern Systems/etc/sshd_config

Contains configuration data for secshd.

$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_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. Niels Provos and Markus Friedl contributed support for privilege separation.

PORTABILITY

All UNIX systems. Windows 10. Windows Server 2016. Windows Server 2019. Windows 11. Windows Server 2022. Windows Server 2025.

NOTES

The client utilities will load keys from the $USERPROFILE directory on 10/2016/2019/11/2022/2025 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 PTC.

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

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

secshd
OpenSSH Secure Shell service

Service

SYNOPSIS

DESCRIPTION

SSH protocol version 2

Command execution and data forwarding

Options

CONFIGURATION FILE

LOGIN PROCESS

AUTHORIZED_KEYS FILE FORMAT

Examples

SSH_KNOWN_HOSTS FILE FORMAT

Examples

TIME FORMATS

TOKENS

FILES

AUTHORS

PORTABILITY

NOTES

AVAILABILITY

SEE ALSO

secshd OpenSSH Secure Shell service

Service

secshd
OpenSSH Secure Shell service