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    Command:

    sshd

    
              [-c host_certificate_file] [-f config_file] [-g login_grace_time]
              [-h host_key_file] [-k key_gen_time] [-o option] [-p port] [-u len]
    
    
    

    DESCRIPTION

         sshd (OpenSSH Daemon) is the daemon program for ssh(1).  Together these
         programs replace rlogin(1) and rsh(1), and provide secure encrypted com-
         munications 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.
    
         -b bits
                 Specifies the number of bits in the ephemeral protocol version 1
                 server key (default 1024).
    
         -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.
    
         -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", and "addr".  All are
                 required and may be supplied in any order, either with multiple
                 -C options or as a comma-separated list.
    
         -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 the system
                 log, 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
                 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_key for protocol version 1, and
                 /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key and
                 /etc/ssh/ssh_host_rsa_key for protocol version 2.  It is possible
                 to have multiple host key files for the different protocol ver-
                 sions and host key algorithms.
    
         -i      Specifies that sshd is being run from inetd(8).  sshd is normally
                 not run from inetd because it needs to generate the server key
                 before it can respond to the client, and this may take tens of
                 seconds.  Clients would have to wait too long if the key was
                 regenerated every time.  However, with small key sizes (e.g. 512)
                 using sshd from inetd may be feasible.
    
         -k key_gen_time
                 Specifies how often the ephemeral protocol version 1 server key
                 is regenerated (default 3600 seconds, or one hour).  The motiva-
                 tion for regenerating the key fairly often is that the key is not
                 stored anywhere, and after about an hour it becomes impossible to
                 recover the key for decrypting intercepted communications even if
                 the machine is cracked into or physically seized.  A value of
                 zero indicates that the key will never be regenerated.
    
         -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
                 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 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
                 AllowUsers or DenyUsers.
    
    
    

    AUTHENTICATION

         The OpenSSH SSH daemon supports SSH protocols 1 and 2.  Both protocols
         are supported by default, though this can be changed via the Protocol
         option in sshd_config(5).  Protocol 2 supports DSA, ECDSA and RSA keys;
         protocol 1 only supports RSA keys.  For both protocols, each host has a
         host-specific key, normally 2048 bits, used to identify the host.
    
         Forward security for protocol 1 is provided through an additional server
         key, normally 768 bits, generated when the server starts.  This key is
         normally regenerated every hour if it has been used, and is never stored
         on disk.  Whenever a client connects, the daemon responds with its public
         host and server keys.  The client compares the RSA host key against its
         own database to verify that it has not changed.  The client then gener-
         ates a 256-bit random number.  It encrypts this random number using both
         the host key and the server key, and sends the encrypted number to the
         server.  Both sides then use this random number as a session key which is
         used to encrypt all further communications in the session.  The rest of
         the session is encrypted using a conventional cipher, currently Blowfish
         or 3DES, with 3DES being used by default.  The client selects the encryp-
         tion algorithm to use from those offered by the server.
    
         For protocol 2, 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 crypto-
         graphic message authentication code (hmac-md5, hmac-sha1, umac-64, 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, 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 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
               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, 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.
    
    
    

    SSHRC

         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
                    else
                            # X11UseLocalhost=no
                            echo add $DISPLAY $proto $cookie
                    fi | xauth -q -
            fi
    
         sion 1; the comment field is not used for anything (but may be convenient
         for the user to identify the key).  For protocol version 2 the keytype is
         "ecdsa-sha2-nistp256", "ecdsa-sha2-nistp384", "ecdsa-sha2-nistp521",
         "ssh-dss" or "ssh-rsa".
    
         Note that lines in this file are usually several hundred bytes long
         (because of the size of the public key encoding) up to a limit of 8 kilo-
         bytes, 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
         identity.pub, id_dsa.pub, id_ecdsa.pub, or the id_rsa.pub file and edit
         it.
    
         sshd enforces a minimum RSA key modulus size for protocol 1 and protocol
         2 keys 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
         case-insensitive):
    
         cert-authority
                 Specifies that the listed key is a certification authority (CA)
                 that is trusted to validate signed certificates for user authen-
                 tication.
    
                 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 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 cer-
                 tain public keys to perform just a specific operation.  An exam-
                 ple might be a key that permits remote backups but nothing else.
                 Note that the client may specify TCP and/or X11 forwarding unless
                 they are explicitly prohibited.  The command originally supplied
                 by the client is available in the SSH_ORIGINAL_COMMAND environ-
                 ment variable.  Note that this option applies to shell, command
                 or subsystem execution.  Also note that this command may be
                 superseded by either a sshd_config(5) ForceCommand directive or a
                 command embedded in a certificate.
    
         environment="NAME=value"
                 Specifies that the string is to be added to the environment when
                 logging in using this key.  Environment variables set this way
                 override other default environment values.  Multiple options of
                 this type are permitted.  Environment processing is disabled by
                 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 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).
    
         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.
    
         permitopen="host:port"
                 Limit local ''ssh -L'' port forwarding such that it may only con-
                 nect to the specified host and port.  IPv6 addresses can be spec-
                 ified with an alternative syntax: host/port.  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 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.
    
         tunnel="n"
                 Force a tun(4) device on the server.  Without this option, the
                 next available device will be used if the client requests a tun-
                 nel.
    
         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
    
         Each line in these files contains the following fields: markers
         (optional), hostnames, bits, exponent, modulus, comment.  The fields are
         separated by spaces.
    
         The marker is optional, but if it is present then it must be one of
         "@cert-authority", to indicate that the line contains a certification
         authority (CA) key, or "@revoked", to indicate that the key contained on
         the line is revoked and must not ever be accepted.  Only one marker
         should be used on a key line.
    
         Hostnames is a comma-separated list of patterns ('*' and '?' act as 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.
    
         Bits, exponent, and modulus are taken directly from the RSA host key;
         they can be obtained, for example, from /etc/ssh/ssh_host_key.pub.  The
         optional comment field continues to the end of the line, and is not used.
    
         Lines starting with '#' and empty lines are ignored as comments.
    
         When performing host authentication, authentication is accepted if any
         matching line has the proper key; either one that matches exactly or, if
         the server has presented a certificate for authentication, the key of the
         certification authority that signed the certificate.  For a key to be
         trusted as a certification authority, it must use the "@cert-authority"
         marker described above.
    
         The known hosts file also provides a facility to mark keys as revoked,
         for example when it is known that the associated private key has been
         stolen.  Revoked keys are specified by including the "@revoked" marker at
         the beginning of the key line, and are never accepted for authentication
         or as certification authorities, but instead will produce a warning from
         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.
    
            |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...
    
    
    

    FILES

         ~/.hushlogin
                 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.
    
         ~/.rhosts
                 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.
    
         ~/.shosts
                 This file is used in exactly the same way as .rhosts, but allows
                 host-based authentication without permitting login with
                 rlogin/rsh.
    
         ~/.ssh/
                 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.
    
         ~/.ssh/authorized_keys
                 Lists the public keys (RSA/ECDSA/DSA) that can be used for log-
                 ging 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".
    
         ~/.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
                 by the user, and need not be readable by anyone else.
    
         /etc/hosts.allow
         /etc/hosts.deny
                 Access controls that should be enforced by tcp-wrappers are
                 defined here.  Further details are described in hosts_access(5).
    
         /etc/hosts.equiv
                 This file is for host-based authentication (see ssh(1)).  It
                 should only be writable by root.
    
         /etc/ssh/moduli
                 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
                 Exchange".  The file format is described in moduli(5).
    
         /etc/motd
                 See motd(5).
    
         /etc/nologin
                 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.
    
         /etc/ssh/shosts.equiv
                 This file is used in exactly the same way as hosts.equiv, but
                 allows host-based authentication without permitting login with
                 rlogin/rsh.
    
         /etc/ssh/ssh_host_key
         /etc/ssh/ssh_host_dsa_key
         /etc/ssh/ssh_host_ecdsa_key
         /etc/ssh/ssh_host_rsa_key
                 These three 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.
    
         /etc/ssh/ssh_host_key.pub
         /etc/ssh/ssh_host_dsa_key.pub
         /etc/ssh/ssh_host_ecdsa_key.pub
         /etc/ssh/ssh_host_rsa_key.pub
                 These three 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).
    
         /etc/ssh/ssh_known_hosts
                 Systemwide list of known host keys.  This file should be prepared
                 by the system administrator to contain the public host keys of
                 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.
    
         /var/run/sshd.pid
                 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-
                 able.
    
    
    

    ENVIRONMENT

         SSH_USE_STRONG_RNG
                 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

         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.
    
    
    

    SEE ALSO

         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)
    
    
    

    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 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.
    
    
    

    CAVEATS

         System security is not improved unless rshd, rlogind, and rexecd are dis-
         abled (thus completely disabling rlogin and rsh into the machine).
    
    
    

    BSD March 28, 2017 BSD

    
    
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