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
     communications between two untrusted hosts over an insecure network.

     sshd listens for connections from clients.  It is normally started at
     boot from /etc/init.d/ssh (or /etc/init/ssh.conf on systems using the
     Upstart init daemon).  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 to the specified user, host, and address
         will be set before the configuration is written to standard
         output.  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-separated 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

     -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
         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_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
         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 sshd_config(5).

     -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

     -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 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,
     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,
     public key authentication, challenge-response authentication, or password

     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 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
     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.


     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.  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


     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

     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-
     separated 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,,, or the file and edit it.

     sshd enforces a minimum RSA key modulus size of 768 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

         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

         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
         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

         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
         pattern 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
         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

         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

         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==
    from="*,!" ssh-rsa
    command="dump /home",no-pty,no-port-forwarding ssh-dss
    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
     automatically: whenever the user connects from an unknown host, its key
     is added to the per-user file.

     Each line in these files contains the following fields: markers
     (optional), hostnames, 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 canonical host
     name (when authenticating a client) or against the user-supplied name
     (when authenticating a server).  A pattern may also be preceded by '!' to
     indicate negation: if the host name matches a negated pattern, it is not
     accepted (by that line) even if it matched another pattern on the line.
     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

     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
     ssh(1) 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

     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 representations.

     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...


         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

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

         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 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
         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

         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
         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-readable.

         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-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-


     scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
     ssh-keyscan(1), chroot(2), hosts_access(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
     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.


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