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).  Together these
     programs replace rlogin and rsh, and provide secure encrypted communi‐
     cations 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 configura‐
             tion 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 respectively.

     -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_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 differ‐
             ent 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 configu‐
             ration 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 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 reli‐
             ably 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 pre‐
             vent sshd from making DNS requests unless the authentication
             mechanism or configuration requires it.  Authentication mecha‐
             nisms that may require DNS include HostbasedAuthentication and
             using a from="pattern-list" option in a key file.  Configura‐
             tion 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 defini‐
     tion 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 com‐
     mand.  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

     When the user program terminates and all forwarded X11 and other con‐
     nections 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

           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

SSHRC         top

     If the file ~/.ssh/rc exists, sh(1) runs it after reading the environ‐
     ment 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 cook‐

     The primary purpose of this file is to run any initialization routines
     which may be needed before the user's home directory becomes accessi‐
     ble; 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 pub‐
     lic key authentication; if this option is not specified, the default is
     ~/.ssh/authorized_keys and ~/.ssh/authorized_keys2.  Each line of the
     file contains one key (empty lines and lines starting with a ‘#’ are
     ignored as comments).  Public keys consist of the following space-sepa‐
     rated fields: options, keytype, base64-encoded key, comment.  The
     options field is optional.  The 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 specifica‐
     tions.  No spaces are permitted, except within double quotes.  The fol‐
     lowing 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 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 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 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 authentica‐
             tion.  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

             Disables execution of ~/.ssh/rc.

             Forbids X11 forwarding when this key is used for authentica‐
             tion.  Any X11 forward requests by the client will return an

             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 cer‐
             tificate 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 ed25519-sk.

             Enable all restrictions, i.e. disable port, agent and X11 for‐
             warding, as well as disabling PTY allocation and execution of
             ~/.ssh/rc.  If any future restriction capabilities are added to
             authorized_keys files they will be included in this set.

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

             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-rsa
        permitopen="",permitopen="" ssh-rsa
        permitlisten="localhost:8080",permitopen="localhost:22000" ssh-rsa
        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==
        no-touch-required AAAAInN...Ko==


     The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host
     public keys for all known hosts.  The global file should be prepared by
     the administrator (optional), and the per-user file is maintained auto‐
     matically: whenever the user connects to 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 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 spec‐
     ified, 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 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 authenti‐
     cation 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 dif‐
     ferent 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; authen‐
     tication 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...

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 parti‐
             tion, because sshd reads it as root.  Additionally, this file
             must be owned by the user, and must not have write permissions
             for anyone else.  The recommended permission for most machines
             is read/write for the user, and not accessible by others.

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

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

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

             If this file, the ~/.ssh directory, or the user's home direc‐
             tory are writable by other users, then the file could be modi‐
             fied 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

             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

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

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

             Similar to ~/.ssh/rc, it can be used to specify machine-spe‐
             cific 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 differ‐
             ent ports, this contains the process ID of the one started
             last).  The content of this file is not sensitive; it can be

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.  Informa‐
     tion 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-8.3p1.tar.gz fetched from
     ⟨⟩ on
     2020-08-13.  If you discover any rendering problems in this HTML ver‐
     sion of the page, or you believe there is a better or more up-to-date
     source for the page, or you have corrections or improvements to the
     information in this COLOPHON (which is not part of the original manual
     page), send a mail to

BSD                           January 25, 2020                           BSD

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