Mac Developer Library Developer


This manual page is for Mac OS X version 10.9

If you are running a different version of Mac OS X, view the documentation locally:

  • In Terminal, using the man(1) command

Reading manual pages

Manual pages are intended as a quick reference for people who already understand a technology.

  • To learn how the manual is organized or to learn about command syntax, read the manual page for manpages(5).

  • For more information about this technology, look for other documentation in the Apple Developer Library.

  • For general information about writing shell scripts, read Shell Scripting Primer.

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

     sshd -- OpenSSH SSH daemon

     sshd [-46DdeiqTt] [-b bits] [-C connection_spec] [-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]

     sshd (OpenSSH Daemon) is the daemon program for ssh(1).  Together these programs replace rlogin(1) and
     rsh(1), and provide secure encrypted communications between two untrusted hosts over an insecure net-work. 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, authenti-cation, authentication,
     cation, 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 configura-tion configuration
     tion 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 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 multi-ple multiple
             ple -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 debug-ging debugging
             ging level.  Maximum is 3.

     -e      When this option is specified, sshd will send the output to the 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_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 possi-ble possible
             ble to have multiple host key files for the different protocol versions and host key algo-rithms. 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 motivation 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 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 logged.

     -T      Extended test mode.  Check the validity of the configuration file, output the effective config-uration configuration
             uration to stdout and then exit.  Optionally, Match rules may be applied by specifying the con-nection connection
             nection 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 RhostsRSAAuthentication, 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 protocols 1 and 2.  The default is to use protocol 2 only, 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, nor-mally normally
     mally 2048 bits, used to identify the host.

     Forward security for protocol 1 is provided through an additional server key, normally 768 bits, gener-ated generated
     ated 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 generates 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 encryption algorithm to use from those offered by the server.

     For protocol 2, forward security is provided through a Diffie-Hellman key agreement.  This key agree-ment agreement
     ment 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 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 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, for-warding forwarding
     warding 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 sec-tion). 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 envi-ronment. environment.
                ronment.  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.

     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 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 -fi -qfi

     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 none is
     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).  Protocol 1 pub-lic public
     lic keys consist of the following space-separated fields: options, bits, exponent, modulus, comment.
     Protocol 2 public key consist of: options, keytype, base64-encoded key, comment.  The options field is
     optional; its presence is determined by whether the line starts with a number or not (the options field
     never starts with a number).  The bits, exponent, modulus, and comment fields give the RSA key for pro-tocol protocol
     tocol version 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 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 for protocol 1 and protocol 2 keys 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 key-words keywords
     words are case-insensitive):

             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 com-mand command
             mand 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.  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 either a sshd_config(5)
             ForceCommand directive or a command embedded in a certificate.

             Specifies that the string is to be added to the environment when logging in using this key.
             Environment variables set this way override other default environment values.  Multiple options
             of this type are permitted.  Environment processing is disabled by default and is controlled
             via the PermitUserEnvironment option.  This option is automatically disabled if UseLogin is

             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 some-body somebody
             body 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

     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 ``ssh -L'' port forwarding such that it may only connect to the specified host and
             port.  IPv6 addresses can be specified by enclosing the address in square brackets.  Multiple
             permitopen options may be applied separated by commas.  No pattern matching is performed on the
             specified hostnames, they must be literal domains or addresses.  A port specification of *
             matches any port.

             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.

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

     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...==

     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, 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 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.

     Bits, exponent, and modulus are taken directly from the RSA host key; they can be obtained, for exam-ple, example,
     ple, 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 cer-tification certification
     tification 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 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
     /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

             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 par-tition, partition,
             tition, 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 others.

             Lists the public keys (DSA/ECDSA/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 rec-ommended recommended
             ommended 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".  The file format
             is described in moduli(5).

             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

             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. worldreadable.

             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-writable. worldwritable.

             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.

     scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), chroot(2),
     hosts_access(5), sshd_config(5), 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.

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

BSD                            October 11, 2013                            BSD

Reporting Problems

The way to report a problem with this manual page depends on the type of problem:

Content errors
Report errors in the content of this documentation to the OpenSSH project.
Bug reports
Report bugs in the functionality of the described tool or API to Apple through Bug Reporter and to the OpenSSH project through their bug reporting page.
Formatting problems
Report formatting mistakes in the online version of these pages with the feedback links below.