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SSHD(8)			  BSD System Manager's Manual		       SSHD(8)

     sshd -- OpenSSH SSH daemon

     sshd [-46DdeiqTt] [-C connection_spec] [-c host_certificate_file]
	  [-E log_file] [-f config_file] [-g login_grace_time]
	  [-h host_key_file] [-o option] [-p port] [-u len]

     sshd (OpenSSH Daemon) is the daemon program for ssh(1).  Together these
     programs replace rlogin and rsh, and provide secure encrypted communica-
     tions between two untrusted hosts over an insecure network.

     sshd listens for connections from clients.	 It is normally started at
     boot from /etc/rc.	 It forks a new daemon for each incoming connection.
     The forked daemons handle key exchange, encryption, authentication, com-
     mand execution, and data exchange.

     sshd can be configured using command-line options or a configuration file
     (by default sshd_config(5)); command-line options override values speci-
     fied in the configuration file.  sshd rereads its configuration file when
     it receives a hangup signal, SIGHUP, by executing itself with the name
     and options it was started with, e.g. /usr/sbin/sshd.

     The options are as follows:

     -4	     Forces sshd to use IPv4 addresses only.

     -6	     Forces sshd to use IPv6 addresses only.

     -C connection_spec
	     Specify the connection parameters to use for the -T extended test
	     mode.  If provided, any Match directives in the configuration
	     file that would apply to the specified user, host, and address
	     will be set before the configuration is written to standard out-
	     put.  The connection parameters are supplied as keyword=value
	     pairs.  The keywords are ``user'', ``host'', ``laddr'',
	     ``lport'', and ``addr''.  All are required and may be supplied in
	     any order, either with multiple -C options or as a comma-sepa-
	     rated list.

     -c host_certificate_file
	     Specifies a path to a certificate file to identify sshd during
	     key exchange.  The certificate file must match a host key file
	     specified using the -h option or the HostKey configuration direc-

     -D	     When this option is specified, sshd will not detach and does not
	     become a daemon.  This allows easy monitoring of sshd.

     -d	     Debug mode.  The server sends verbose debug output to standard
	     error, and does not put itself in the background.	The server
	     also will not fork and will only process one connection.  This
	     option is only intended for debugging for the server.  Multiple
	     -d options increase the debugging level.  Maximum is 3.

     -E log_file
	     Append debug logs to log_file instead of the system log.

     -e	     Write debug logs to standard error instead of the system log.

     -f config_file
	     Specifies the name of the configuration file.  The default is
	     /etc/ssh/sshd_config.  sshd refuses to start if there is no con-
	     figuration file.

     -g login_grace_time
	     Gives the grace time for clients to authenticate themselves
	     (default 120 seconds).  If the client fails to authenticate the
	     user within this many seconds, the server disconnects and exits.
	     A value of zero indicates no limit.

     -h host_key_file
	     Specifies a file from which a host key is read.  This option must
	     be given if sshd is not run as root (as the normal host key files
	     are normally not readable by anyone but root).  The default is
	     /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key,
	     /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key.  It
	     is possible to have multiple host key files for the different
	     host key algorithms.

     -i	     Specifies that sshd is being run from inetd(8).

     -o option
	     Can be used to give options in the format used in the configura-
	     tion file.	 This is useful for specifying options for which there
	     is no separate command-line flag.	For full details of the
	     options, and their values, see sshd_config(5).

     -p port
	     Specifies the port on which the server listens for connections
	     (default 22).  Multiple port options are permitted.  Ports speci-
	     fied in the configuration file with the Port option are ignored
	     when a command-line port is specified.  Ports specified using the
	     ListenAddress option override command-line ports.

     -q	     Quiet mode.  Nothing is sent to the system log.  Normally the
	     beginning, authentication, and termination of each connection is

     -T	     Extended test mode.  Check the validity of the configuration
	     file, output the effective configuration to stdout and then exit.
	     Optionally, Match rules may be applied by specifying the connec-
	     tion parameters using one or more -C options.

     -t	     Test mode.	 Only check the validity of the configuration file and
	     sanity of the keys.  This is useful for updating sshd reliably as
	     configuration options may change.

     -u len  This option is used to specify the size of the field in the utmp
	     structure that holds the remote host name.	 If the resolved host
	     name is longer than len, the dotted decimal value will be used
	     instead.  This allows hosts with very long host names that over-
	     flow this field to still be uniquely identified.  Specifying -u0
	     indicates that only dotted decimal addresses should be put into
	     the utmp file.  -u0 may also be used to prevent sshd from making
	     DNS requests unless the authentication mechanism or configuration
	     requires it.  Authentication mechanisms that may require DNS
	     include HostbasedAuthentication and using a from="pattern-list"
	     option in a key file.  Configuration options that require DNS
	     include using a USER@HOST pattern in AllowUsers or DenyUsers.

     The OpenSSH SSH daemon supports SSH protocol 2 only.  Each host has a
     host-specific key, used to identify the host.  Whenever a client con-
     nects, the daemon responds with its public host key.  The client compares
     the host key against its own database to verify that it has not changed.
     Forward security is provided through a Diffie-Hellman key agreement.
     This key agreement results in a shared session key.  The rest of the ses-
     sion is encrypted using a symmetric cipher, currently 128-bit AES, Blow-
     fish, 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 mes-
     sage authentication code (hmac-md5, hmac-sha1, umac-64, umac-128, hmac-
     ripemd160, hmac-sha2-256 or hmac-sha2-512).

     Finally, the server and the client enter an authentication dialog.	 The
     client tries to authenticate itself using host-based authentication, pub-
     lic key authentication, GSSAPI authentication, challenge-response authen-
     tication, or password authentication.

     Regardless of the authentication type, the account is checked to ensure
     that it is accessible.  An account is not accessible if it is locked,
     listed in DenyUsers or its group is listed in DenyGroups .	 The defini-
     tion of a locked account is system dependant. Some platforms have their
     own account database (eg AIX) and some modify the passwd field ( `*LK*'
     on Solaris and UnixWare, `*' on HP-UX, containing `Nologin' on Tru64, a
     leading `*LOCKED*' on FreeBSD and a leading `!' on most Linuxes).	If
     there is a requirement to disable password authentication for the account
     while allowing still public-key, then the passwd field should be set to
     something other than these values (eg `NP' or `*NP*' ).

     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 con-
     nections, or forwarding the authentication agent connection over the
     secure channel.

     After this, 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 connec-
     tions have been closed, the server sends command exit status to the
     client, and both sides exit.

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

	   1.	If the login is on a tty, and no command has been specified,
		prints last login time and /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 /etc/nologin; if it exists, prints contents and quits
		(unless root).

	   4.	Changes to run with normal user privileges.

	   5.	Sets up basic environment.

	   6.	Reads the file ~/.ssh/environment, if it exists, and users are
		allowed to change their environment.  See the
		PermitUserEnvironment option in sshd_config(5).

	   7.	Changes to user's home directory.

	   8.	If ~/.ssh/rc exists and the sshd_config(5) PermitUserRC option
		is set, runs it; else if /etc/ssh/sshrc exists, runs it; oth-
		erwise runs xauth.  The ``rc'' files are given the X11 authen-
		tication protocol and cookie in standard input.	 See SSHRC,

	   9.	Runs user's shell or command.  All commands are run under the
		user's login shell as specified in the system password data-

     If the file ~/.ssh/rc exists, sh(1) runs it after reading the environment
     files but before starting the user's shell or command.  It must not pro-
     duce any output on stdout; stderr must be used instead.  If X11 forward-
     ing is in use, it will receive the "proto cookie" pair in its standard
     input (and DISPLAY in its environment).  The script must call xauth(1)
     because sshd will not run xauth automatically to add X11 cookies.

     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
			# X11UseLocalhost=no
			echo add $DISPLAY $proto $cookie
		fi | xauth -q -

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

     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-sepa-
     rated fields: options, keytype, base64-encoded key, comment.  The options
     field is optional.	 The keytype is ``ecdsa-sha2-nistp256'',
     ``ecdsa-sha2-nistp384'', ``ecdsa-sha2-nistp521'', ``ssh-ed25519'',
     ``ssh-dss'' or ``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 DSA keys up to 8 kilobits and RSA keys up to 16 kilobits.
     You don't want to type them in; instead, copy the id_dsa.pub,
     id_ecdsa.pub, id_ed25519.pub, or the id_rsa.pub file and edit it.

     sshd enforces a minimum RSA key modulus size of 768 bits.

     The options (if present) consist of comma-separated option specifica-
     tions.  No spaces are permitted, except within double quotes.  The fol-
     lowing option specifications are supported (note that option keywords are

	     Enable authentication agent forwarding previously disabled by the
	     restrict option.

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

	     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.

	     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 chan-
	     nel 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 sshd_config(5) ForceCommand

	     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.

	     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.

	     Specifies that in addition to public key authentication, either
	     the canonical name of the remote host or its IP address must be
	     present in the comma-separated list of patterns.  See PATTERNS in
	     ssh_config(5) for more information on patterns.

	     In addition to the wildcard matching that may be applied to host-
	     names or addresses, a from stanza may match IP addresses using
	     CIDR address/masklen notation.

	     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).

	     Forbids authentication agent forwarding when this key is used for

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

     no-pty  Prevents tty allocation (a request to allocate a pty will fail).

	     Disables execution of ~/.ssh/rc.

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

	     Limit local port forwarding with ssh(1) -L 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 pat-
	     tern matching is performed on the specified hostnames, they must
	     be literal domains or addresses.  A port specification of *
	     matches any port.

	     Enable port forwarding previously disabled by the restrict

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

     pty     Permits tty allocation previously disabled by the restrict

	     Enable all restrictions, i.e. disable port, agent and X11 for-
	     warding, 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.

	     Force a tun(4) device on the server.  Without this option, the
	     next available device will be used if the client requests a tun-

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

	     Permits X11 forwarding previously disabled by the restrict

     An example authorized_keys file:

	# Comments allowed at start of line
	ssh-rsa AAAAB3Nza...LiPk== user@example.net
	from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
	AAAAB2...19Q== john@example.net
	command="dump /home",no-pty,no-port-forwarding ssh-dss
	AAAAC3...51R== example.net
	permitopen="",permitopen="" ssh-dss
	tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
	restrict,command="uptime" ssh-rsa AAAA1C8...32Tv==
	restrict,pty,command="nethack" ssh-rsa AAAA1f8...IrrC5==

     The /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 auto-
     matically: 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, 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 wild-
     cards); 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 indi-
     cate 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 opera-
     tors may be applied.

     The keytype and base64-encoded key are taken directly from the host key;
     they can be obtained, for example, from /etc/ssh/ssh_host_rsa_key.pub.
     The optional comment field continues to the end of the line, and is not

     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 authentica-
     tion or as certification authorities, but instead will produce a warning
     from ssh(1) when they are encountered.

     It is permissible (but not recommended) to have several lines or differ-
     ent 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, ssh-keyscan(1) or by taking, for exam-
     ple, /etc/ssh/ssh_host_rsa_key.pub and adding the host names at the
     front.  ssh-keygen(1) also offers some basic automated editing for
     ~/.ssh/known_hosts including removing hosts matching a host name and con-
     verting all host names to their hashed representations.

     An example ssh_known_hosts file:

	# Comments allowed at start of line
	closenet,..., 1024 37 159...93 closenet.example.net
	cvs.example.net, ssh-rsa AAAA1234.....=
	# A hashed hostname
	|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
	# 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...

	     This file is used to suppress printing the last login time and
	     /etc/motd, if PrintLastLog and PrintMotd, respectively, are
	     enabled.  It does not suppress printing of the banner specified
	     by Banner.

	     This file is used for host-based authentication (see ssh(1) for
	     more information).	 On some machines this file may need to be
	     world-readable if the user's home directory is on an NFS parti-
	     tion, because sshd reads it as root.  Additionally, this file
	     must be owned by the user, and must not have write permissions
	     for anyone else.  The recommended permission for most machines is
	     read/write for the user, and not accessible by others.

	     This file is used in exactly the same way as .rhosts, but allows
	     host-based authentication without permitting login with

	     These files enforce GSSAPI/Kerberos authentication access con-
	     trol.  Further details are described in ksu(1).  The location of
	     the k5login file depends on the configuration option
	     k5login_directory in the krb5.conf(5).

	     This directory is the default location for all user-specific con-
	     figuration and authentication information.	 There is no general
	     requirement to keep the entire contents of this directory secret,
	     but the recommended permissions are read/write/execute for the
	     user, and not accessible by others.

	     Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
	     for logging in as this user.  The format of this file is
	     described above.  The content of the file is not highly sensi-
	     tive, but the recommended permissions are read/write for the
	     user, and not accessible by others.

	     If this file, the ~/.ssh directory, or the user's home directory
	     are writable by other users, then the file could be modified or
	     replaced by unauthorized users.  In this case, sshd will not
	     allow it to be used unless the StrictModes option has been set to

	     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.

	     Contains a list of host keys for all hosts the user has logged
	     into that are not already in the systemwide list of known host
	     keys.  The format of this file is described above.	 This file
	     should be writable only by root/the owner and can, but need not
	     be, world-readable.

	     Contains initialization routines to be run before the user's home
	     directory becomes accessible.  This file should be writable only
	     by the user, and need not be readable by anyone else.

	     Access controls that should be enforced by tcp-wrappers are
	     defined here.  Further details are described in hosts_access(5).

	     This file is for host-based authentication (see ssh(1)).  It
	     should only be writable by root.

	     Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
	     Exchange" key exchange method.  The file format is described in
	     moduli(5).	 If no usable groups are found in this file then fixed
	     internal groups will be used.

	     See motd(5).

	     If this file exists, sshd refuses to let anyone except root log
	     in.  The contents of the file are displayed to anyone trying to
	     log in, and non-root connections are refused.  The file should be

	     This file is used in exactly the same way as hosts.equiv, but
	     allows host-based authentication without permitting login with

	     These files contain the private parts of the host keys.  These
	     files should only be owned by root, readable only by root, and
	     not accessible to others.	Note that sshd does not start if these
	     files are group/world-accessible.

	     These files contain the public parts of the host keys.  These
	     files should be world-readable but writable only by root.	Their
	     contents should match the respective private parts.  These files
	     are not really used for anything; they are provided for the con-
	     venience of the user so their contents can be copied to known
	     hosts files.  These files are created using ssh-keygen(1).

	     Systemwide list of known host keys.  This file should be prepared
	     by the system administrator to contain the public host keys of
	     all machines in the organization.	The format of this file is
	     described above.  This file should be writable only by root/the
	     owner and should be world-readable.

	     Contains configuration data for sshd.  The file format and con-
	     figuration options are described in sshd_config(5).

	     Similar to ~/.ssh/rc, it can be used to specify machine-specific
	     login-time initializations globally.  This file should be
	     writable only by root, and should be world-readable.

	     chroot(2) directory used by sshd during privilege separation in
	     the pre-authentication phase.  The directory should not contain
	     any files and must be owned by root and not group or world-

	     Contains the process ID of the sshd listening for connections (if
	     there are several daemons running concurrently for different
	     ports, this contains the process ID of the one started last).
	     The content of this file is not sensitive; it can be world-read-

	     The reseeding of the OpenSSL random generator is usually done
	     from /dev/urandom.	 If the SSH_USE_STRONG_RNG environment vari-
	     able is set to value other than 0 the OpenSSL random generator is
	     reseeded from /dev/random.	 The number of bytes read is defined
	     by the SSH_USE_STRONG_RNG value.  Minimum is 14 bytes.  This set-
	     ting is not recommended on the computers without the hardware
	     random generator because insufficient entropy causes the connec-
	     tion to be blocked until enough entropy is available.

     IPv6 address can be used everywhere where IPv4 address. In all entries
     must be the IPv6 address enclosed in square brackets. Note: The square
     brackets are metacharacters for the shell and must be escaped in shell.

     scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
     ssh-keyscan(1), chroot(2), hosts_access(5), login.conf(5), moduli(5),
     sshd_config(5), inetd(8), sftp-server(8)

     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 cre-
     ated 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.

BSD				 May 18, 2024				   BSD