sshd(8) — Linux manual page


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

NAME         top

     sshd — OpenSSH daemon

SYNOPSIS         top

     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]

DESCRIPTION         top

     sshd (OpenSSH Daemon) is the daemon program for ssh(1).  It
     provides secure encrypted communications 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, command 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 specified 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 are applied before
             the configuration is written to standard output.  The
             connection parameters are supplied as keyword=value pairs
             and may be supplied in any order, either with multiple -C
             options or as a comma-separated list.  The keywords are
             “addr”, “user”, “host”, “laddr”, “lport”, and “rdomain” and
             correspond to source address, user, resolved source host
             name, local address, local port number and routing domain

     -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 directive.

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

     -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(2) 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

     -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 configuration 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_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
             configuration 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

     -p port
             Specifies the port on which the server listens for
             connections (default 22).  Multiple port options are
             permitted.  Ports specified 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 logged.

     -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 connection 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 overflow 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
     connects, 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 secrecy 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.  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 (MAC).

     Finally, the server and the client enter an authentication dialog.
     The client tries to authenticate itself using host-based
     authentication, public key authentication, challenge-response
     authentication, 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 definition of a locked account is system dependent. 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 connections, 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
     connections have been closed, the server sends command exit status
     to the client, and both sides exit.

LOGIN PROCESS         top

     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; otherwise runs xauth(1).  The “rc” files are
                given the X11 authentication protocol and cookie in
                standard input.  See SSHRC, below.

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

SSHRC         top

     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 produce any output on stdout; stderr must be used
     instead.  If X11 forwarding 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

     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-separated fields: options, keytype,
     base64-encoded key, comment.  The options field is optional.  The
     supported key types are:


     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,,,,, or the 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):

             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 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.

             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 sshd_config(5) 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

             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 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.

             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
             hostnames 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 authentication.

             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

             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 remote port forwarding with the ssh(1) -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
             ssh_config(5).  A port specification of * matches any port.
             Note that the setting of GatewayPorts may further restrict
             listen addresses.  Note that ssh(1) 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 “”
             and “::1”.

             Limit local port forwarding with the ssh(1) -L option 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 or name
             lookup is performed on the specified hostnames, they must
             be literal host names and/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
             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

             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

             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.

             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.

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

             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 are allowed at start of line. Blank lines are allowed.
        # Plain key, no restrictions
        ssh-rsa ...
        # Forced command, disable PTY and all forwarding
        restrict,command="dump /home" ssh-rsa ...
        # Restriction of ssh -L forwarding destinations
        permitopen="",permitopen="" ssh-rsa ...
        # Restriction of ssh -R forwarding listeners
        permitlisten="localhost:8080",permitlisten="[::1]:22000" ssh-rsa ...
        # Configuration for tunnel forwarding
        tunnel="0",command="sh /etc/netstart tun0" ssh-rsa ...
        # Override of restriction to allow PTY allocation
        restrict,pty,command="nethack" ssh-rsa ...
        # Allow FIDO key without requiring touch
        no-touch-required ...
        # Require user-verification (e.g. PIN or biometric) for FIDO key
        verify-required ...
        # Trust CA key, allow touch-less FIDO if requested in certificate
        cert-authority,no-touch-required,principals="user_a" ssh-rsa ...


     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 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 sshd is authenticating a client, such as when using
     HostbasedAuthentication, this will be the canonical client host
     name.  When ssh(1) is authenticating a server, this will be the
     host name given by the user, the value of the ssh(1) HostkeyAlias
     if it was specified, or the canonical server hostname if the ssh(1)
     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
     /etc/ssh/  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

     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 ssh(1) when they are

     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, ssh-keyscan(1) or
     by taking, for example, /etc/ssh/ 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 converting all host names to their hashed

     An example ssh_known_hosts file:

        # Comments allowed at start of line
        closenet,..., 1024 37 159...93, 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 * or *
        @cert-authority *,* ssh-rsa AAAAB5W...

FILES         top

             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 partition, 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 rlogin/rsh.

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

             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 sensitive, 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 “no”.

             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.

             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 world-readable.

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

             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 convenience 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-

             Contains configuration data for sshd.  The file format and
             configuration 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-

             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-writable.

             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-readable.

SEE ALSO         top

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

AUTHORS         top

     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.

COLOPHON         top

     This page is part of the openssh (Portable OpenSSH) project.
     Information about the project can be found at  If you have a bug report for
     this manual page, see ⟨⟩.  This
     page was obtained from the tarball openssh-8.7p1.tar.gz fetched
     from ⟨⟩ on
     2021-08-27.  If you discover any rendering problems in this HTML
     version of the page, or you believe there is a better or more up-
     to-date source for the page, or you have corrections or
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     of the original manual page), send a mail to

BSD                           July 30, 2021                          BSD

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