sshd(8) — Linux manual page


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

NAME         top

       sshd — OpenSSH daemon

SYNOPSIS         top

       sshd [-46DdeGiqTtV] [-C connection_spec]
       [-c host_certificate_file] [-E log_file] [-f config_file]
       [-g login_grace_time] [-h host_key_file] [-o option] [-p port]
       [-u len]

DESCRIPTION         top

       sshd (OpenSSH Daemon) is the daemon program for ssh(1).  It
       provides secure encrypted communications between two untrusted
       hosts over an insecure network.

       sshd listens for connections from clients.  It is normally
       started at boot from /etc/rc.  It forks a new daemon for each
       incoming connection.  The forked daemons handle key exchange,
       encryption, authentication, command execution, and data exchange.

       sshd can be configured using command-line options or a
       configuration file (by default sshd_config(5)); command-line
       options override values specified in the configuration file.
       sshd rereads its configuration file when it receives a hangup
       signal, SIGHUP, by executing itself with the name and options it
       was started with, e.g. /usr/sbin/sshd.

       The options are as follows:

       -4      Forces sshd to use IPv4 addresses only.

       -6      Forces sshd to use IPv6 addresses only.

       -C connection_spec
               Specify the connection parameters to use for the -T
               extended test mode.  If provided, any Match directives in
               the configuration file that would apply are applied
               before the configuration is written to standard output.
               The connection parameters are supplied as keyword=value
               pairs and may be supplied in any order, either with
               multiple -C options or as a comma-separated list.  The
               keywords are “addr”, “user”, “host”, “laddr”, “lport”,
               and “rdomain” and correspond to source address, user,
               resolved source host name, local address, local port
               number and routing domain 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 directive.

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

       -d      Debug mode.  The server sends verbose debug output to
               standard error, and does not put itself in the
               background.  The server also will not fork(2) and will
               only process one connection.  This option is only
               intended for debugging for the server.  Multiple -d
               options increase the debugging level.  Maximum is 3.

       -E log_file
               Append debug logs to log_file instead of the system log.

       -e      Write debug logs to standard error instead of the system

       -f config_file
               Specifies the name of the configuration file.  The
               default is /etc/ssh/sshd_config.  sshd refuses to start
               if there is no configuration file.

       -G      Parse and print configuration file.  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.

       -g login_grace_time
               Gives the grace time for clients to authenticate
               themselves (default 120 seconds).  If the client fails to
               authenticate the user within this many seconds, the
               server disconnects and exits.  A value of zero indicates
               no limit.

       -h host_key_file
               Specifies a file from which a host key is read.  This
               option must be given if sshd is not run as root (as the
               normal host key files are normally not readable by anyone
               but root).  The default is /etc/ssh/ssh_host_ecdsa_key,
               /etc/ssh/ssh_host_ed25519_key and
               /etc/ssh/ssh_host_rsa_key.  It is possible to have
               multiple host key files for the different host key

       -i      Specifies that sshd is being run from inetd(8).

       -o option
               Can be used to give options in the format used in the
               configuration file.  This is useful for specifying
               options for which there is no separate command-line flag.
               For full details of the options, and their values, see

       -p port
               Specifies the port on which the server listens for
               connections (default 22).  Multiple port options are
               permitted.  Ports specified in the configuration file
               with the Port option are ignored when a command-line port
               is specified.  Ports specified using the ListenAddress
               option override command-line ports.

       -q      Quiet mode.  Nothing is sent to the system log.  Normally
               the beginning, authentication, and termination of each
               connection is logged.

       -T      Extended test mode.  Check the validity of the
               configuration file, output the effective configuration to
               stdout and then exit.  Optionally, Match rules may be
               applied by specifying the connection parameters using one
               or more -C options.  This is similar to the -G flag, but
               it includes the additional testing performed by the -t

       -t      Test mode.  Only check the validity of the configuration
               file and sanity of the keys.  This is useful for updating
               sshd reliably as configuration options may change.

       -u len  This option is used to specify the size of the field in
               the utmp structure that holds the remote host name.  If
               the resolved host name is longer than len, the dotted
               decimal value will be used instead.  This allows hosts
               with very long host names that overflow this field to
               still be uniquely identified.  Specifying -u0 indicates
               that only dotted decimal addresses should be put into the
               utmp file.  -u0 may also be used to prevent sshd from
               making DNS requests unless the authentication mechanism
               or configuration requires it.  Authentication mechanisms
               that may require DNS include HostbasedAuthentication and
               using a from="pattern-list" option in a key file.
               Configuration options that require DNS include using a
               USER@HOST pattern in AllowUsers or DenyUsers.

       -V      Display the version number and exit.


       The OpenSSH SSH daemon supports SSH protocol 2 only.  Each host
       has a host-specific key, used to identify the host.  Whenever a
       client connects, the daemon responds with its public host key.
       The client compares the host key against its own database to
       verify that it has not changed.  Forward secrecy is provided
       through a Diffie-Hellman key agreement.  This key agreement
       results in a shared session key.  The rest of the session is
       encrypted using a symmetric cipher.  The client selects the
       encryption algorithm to use from those offered by the server.
       Additionally, session integrity is provided through a
       cryptographic message authentication code (MAC).

       Finally, the server and the client enter an authentication
       dialog.  The client tries to authenticate itself using host-based
       authentication, public key authentication, challenge-response
       authentication, or password authentication.

       Regardless of the authentication type, the account is checked to
       ensure that it is accessible.  An account is not accessible if it
       is locked, listed in DenyUsers or its group is listed in
       DenyGroups .  The definition of a locked account is system
       dependent. Some platforms have their own account database (eg
       AIX) and some modify the passwd field ( ‘*LK*’ on Solaris and
       UnixWare, ‘*’ on HP-UX, containing ‘Nologin’ on Tru64, a leading
       ‘*LOCKED*’ on FreeBSD and a leading ‘!’ on most Linuxes).  If
       there is a requirement to disable password authentication for the
       account while allowing still public-key, then the passwd field
       should be set to something other than these values (eg ‘NP’ or
       ‘*NP*’ ).

       If the client successfully authenticates itself, a dialog for
       preparing the session is entered.  At this time the client may
       request things like allocating a pseudo-tty, forwarding X11
       connections, forwarding TCP connections, or forwarding the
       authentication agent connection over the secure channel.

       After this, the client either requests an interactive shell or
       execution of a non-interactive command, which sshd will execute
       via the user's shell using its -c option.  The sides then enter
       session mode.  In this mode, either side may send data at any
       time, and such data is forwarded to/from the shell or command on
       the server side, and the user terminal in the client side.

       When the user program terminates and all forwarded X11 and other
       connections have been closed, the server sends command exit
       status to the client, and both sides exit.

LOGIN PROCESS         top

       When a user successfully logs in, sshd does the following:

             1.   If the login is on a tty, and no command has been
                  specified, prints last login time and /etc/motd
                  (unless prevented in the configuration file or by
                  ~/.hushlogin; see the “FILES” section).

             2.   If the login is on a tty, records login time.

             3.   Checks /etc/nologin; if it exists, prints contents and
                  quits (unless root).

             4.   Changes to run with normal user privileges.

             5.   Sets up basic environment.

             6.   Reads the file ~/.ssh/environment, if it exists, and
                  users are allowed to change their environment.  See
                  the PermitUserEnvironment option in sshd_config(5).

             7.   Changes to user's home directory.

             8.   If ~/.ssh/rc exists and the sshd_config(5)
                  PermitUserRC option is set, runs it; else if
                  /etc/ssh/sshrc exists, runs it; otherwise runs
                  xauth(1).  The “rc” files are given the X11
                  authentication protocol and cookie in standard input.
                  See “SSHRC”, below.

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

SSHRC         top

       If the file ~/.ssh/rc exists, sh(1) runs it after reading the
       environment files but before starting the user's shell or
       command.  It must not produce any output on stdout; stderr must
       be used instead.  If X11 forwarding is in use, it will receive
       the "proto cookie" pair in its standard input (and DISPLAY in its
       environment).  The script must call xauth(1) because sshd will
       not run xauth automatically to add X11 cookies.

       The primary purpose of this file is to run any initialization
       routines which may be needed before the user's home directory
       becomes accessible; AFS is a particular example of such an

       This file will probably contain some initialization code followed
       by something similar to:

          if read proto cookie && [ -n "$DISPLAY" ]; then
                  if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
                          # X11UseLocalhost=yes
                          echo add unix:`echo $DISPLAY |
                              cut -c11-` $proto $cookie
                          # X11UseLocalhost=no
                          echo add $DISPLAY $proto $cookie
                  fi | xauth -q -

       If this file does not exist, /etc/ssh/sshrc is run, and if that
       does not exist either, xauth is used to add the cookie.


       AuthorizedKeysFile specifies the files containing public keys for
       public key authentication; if this option is not specified, the
       default is ~/.ssh/authorized_keys and ~/.ssh/authorized_keys2.
       Each line of the file contains one key (empty lines and lines
       starting with a ‘#’ are ignored as comments).  Public keys
       consist of the following space-separated fields: options,
       keytype, base64-encoded key, comment.  The options field is
       optional.  The supported key types are:


       The comment field is not used for anything (but may be convenient
       for the user to identify the key).

       Note that lines in this file can be several hundred bytes long
       (because of the size of the public key encoding) up to a limit of
       8 kilobytes, which permits RSA keys up to 16 kilobits.  You don't
       want to type them in; instead, copy the,,,,, or the file and edit it.

       sshd enforces a minimum RSA key modulus size of 1024 bits.

       The options (if present) consist of comma-separated option
       specifications.  No spaces are permitted, except within double
       quotes.  The following option specifications are supported (note
       that option keywords are case-insensitive):

               Enable authentication agent forwarding previously
               disabled by the restrict option.

               Specifies that the listed key is a certification
               authority (CA) that is trusted to validate signed
               certificates for user authentication.

               Certificates may encode access restrictions similar to
               these key options.  If both certificate restrictions and
               key options are present, the most restrictive union of
               the two is applied.

               Specifies that the command is executed whenever this key
               is used for authentication.  The command supplied by the
               user (if any) is ignored.  The command is run on a pty if
               the client requests a pty; otherwise it is run without a
               tty.  If an 8-bit clean channel is required, one must not
               request a pty or should specify no-pty.  A quote may be
               included in the command by quoting it with a backslash.

               This option might be useful to restrict certain public
               keys to perform just a specific operation.  An example
               might be a key that permits remote backups but nothing
               else.  Note that the client may specify TCP and/or X11
               forwarding unless they are explicitly prohibited, e.g.
               using the restrict key option.

               The command originally supplied by the client is
               available in the SSH_ORIGINAL_COMMAND environment
               variable.  Note that this option applies to shell,
               command or subsystem execution.  Also note that this
               command may be superseded by a sshd_config(5)
               ForceCommand directive.

               If a command is specified and a forced-command is
               embedded in a certificate used for authentication, then
               the certificate will be accepted only if the two commands
               are 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[Z]
               date or a YYYYMMDDHHMM[SS][Z] time.  Dates and times will
               be interpreted in the system time zone unless suffixed by
               a Z character, in which case they will be interpreted in
               the UTC time zone.

               Specifies that in addition to public key authentication,
               either the canonical name of the remote host or its IP
               address must be present in the comma-separated list of
               patterns.  See PATTERNS in ssh_config(5) for more
               information on patterns.

               In addition to the wildcard matching that may be applied
               to hostnames or addresses, a from stanza may match IP
               addresses using CIDR address/masklen notation.

               The purpose of this option is to optionally increase
               security: public key authentication by itself does not
               trust the network or name servers or anything (but the
               key); however, if somebody somehow steals the key, the
               key permits an intruder to log in from anywhere in the
               world.  This additional option makes using a stolen key
               more difficult (name servers and/or routers would have to
               be compromised in addition to just the key).

               Forbids authentication agent forwarding when this key is
               used for authentication.

               Forbids TCP forwarding when this key is used for
               authentication.  Any port forward requests by the client
               will return an error.  This might be used, e.g. in
               connection with the command option.

       no-pty  Prevents tty allocation (a request to allocate a pty will

               Disables execution of ~/.ssh/rc.

               Forbids X11 forwarding when this key is used for
               authentication.  Any X11 forward requests by the client
               will return an error.

               Limit remote port forwarding with the ssh(1) -R option
               such that it may only listen on the specified host
               (optional) and port.  IPv6 addresses can be specified by
               enclosing the address in square brackets.  Multiple
               permitlisten options may be applied separated by commas.
               Hostnames may include wildcards as described in the
               PATTERNS section in ssh_config(5).  A port specification
               of * matches any port.  Note that the setting of
               GatewayPorts may further restrict listen addresses.  Note
               that ssh(1) will send a hostname of “localhost” if a
               listen host was not specified when the forwarding was
               requested, and that this name is treated differently to
               the explicit localhost addresses “” and “::1”.

               Limit local port forwarding with the ssh(1) -L option
               such that it may only connect to the specified host and
               port.  IPv6 addresses can be specified by enclosing the
               address in square brackets.  Multiple permitopen options
               may be applied separated by commas.  No pattern matching
               or name lookup is performed on the specified hostnames,
               they must be literal host names and/or addresses.  A port
               specification of * matches any port.

               Enable port forwarding previously disabled by the
               restrict option.

               On a cert-authority line, specifies allowed principals
               for certificate authentication as a comma-separated list.
               At least one name from the list must appear in the
               certificate's list of principals for the certificate to
               be accepted.  This option is ignored for keys that are
               not marked as trusted certificate signers using the
               cert-authority option.

       pty     Permits tty allocation previously disabled by the
               restrict option.

               Do not require demonstration of user presence for
               signatures made using this key.  This option only makes
               sense for the FIDO authenticator algorithms ecdsa-sk and

               Require that signatures made using this key attest that
               they verified the user, e.g. via a PIN.  This option only
               makes sense for the FIDO authenticator algorithms
               ecdsa-sk and ed25519-sk.

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

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

               Enables execution of ~/.ssh/rc previously disabled by the
               restrict option.

               Permits X11 forwarding previously disabled by the
               restrict option.

       An example authorized_keys file:

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


       The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain
       host public keys for all known hosts.  The global file should be
       prepared by the administrator (optional), and the per-user file
       is maintained automatically: whenever the user connects to an
       unknown host, its key is added to the per-user file.

       Each line in these files contains the following fields: marker
       (optional), hostnames, keytype, base64-encoded key, comment.  The
       fields are separated by spaces.

       The marker is optional, but if it is present then it must be one
       of “@cert-authority”, to indicate that the line contains a
       certification authority (CA) key, or “@revoked”, to indicate that
       the key contained on the line is revoked and must not ever be
       accepted.  Only one marker should be used on a key line.

       Hostnames is a comma-separated list of patterns (‘*’ and ‘?’ act
       as wildcards); each pattern in turn is matched against the host
       name.  When sshd is authenticating a client, such as when using
       HostbasedAuthentication, this will be the canonical client host
       name.  When ssh(1) is authenticating a server, this will be the
       host name given by the user, the value of the ssh(1) HostkeyAlias
       if it was specified, or the canonical server hostname if the
       ssh(1) CanonicalizeHostname option was used.

       A pattern may also be preceded by ‘!’ to indicate negation: if
       the host name matches a negated pattern, it is not accepted (by
       that line) even if it matched another pattern on the line.  A
       hostname or address may optionally be enclosed within ‘[’ and ‘]’
       brackets then followed by ‘:’ and a non-standard port number.

       Alternately, hostnames may be stored in a hashed form which hides
       host names and addresses should the file's contents be disclosed.
       Hashed hostnames start with a ‘|’ character.  Only one hashed
       hostname may appear on a single line and none of the above
       negation or wildcard operators may be applied.

       The keytype and base64-encoded key are taken directly from the
       host key; they can be obtained, for example, from
       /etc/ssh/  The optional comment field
       continues to the end of the line, and is not used.

       Lines starting with ‘#’ and empty lines are ignored as comments.

       When performing host authentication, authentication is accepted
       if any matching line has the proper key; either one that matches
       exactly or, if the server has presented a certificate for
       authentication, the key of the certification authority that
       signed the certificate.  For a key to be trusted as a
       certification authority, it must use the “@cert-authority” marker
       described 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

       It is permissible (but not recommended) to have several lines or
       different host keys for the same names.  This will inevitably
       happen when short forms of host names from different domains are
       put in the file.  It is possible that the files contain
       conflicting information; authentication is accepted if valid
       information can be found from either file.

       Note that the lines in these files are typically hundreds of
       characters long, and you definitely don't want to type in the
       host keys by hand.  Rather, generate them by a script,
       ssh-keyscan(1) or by taking, for example,
       /etc/ssh/ and adding the host names at the
       front.  ssh-keygen(1) also offers some basic automated editing
       for ~/.ssh/known_hosts including removing hosts matching a host
       name and converting all host names to their hashed

       An example ssh_known_hosts file:

          # Comments allowed at start of line
, ssh-rsa AAAA1234.....=
          # A hashed hostname
          |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
          # A revoked key
          @revoked * ssh-rsa AAAAB5W...
          # A CA key, accepted for any host in * or *
          @cert-authority *,* ssh-rsa AAAAB5W...

FILES         top

               This file is used to suppress printing the last login
               time and /etc/motd, if PrintLastLog and PrintMotd,
               respectively, are enabled.  It does not suppress printing
               of the banner specified by Banner.

               This file is used for host-based authentication (see
               ssh(1) for more information).  On some machines this file
               may need to be world-readable if the user's home
               directory is on an NFS partition, because sshd reads it
               as root.  Additionally, this file must be owned by the
               user, and must not have write permissions for anyone
               else.  The recommended permission for most machines is
               read/write for the user, and not accessible by others.

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

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

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

               If this file, the ~/.ssh directory, or the user's home
               directory are writable by other users, then the file
               could be modified or replaced by unauthorized users.  In
               this case, sshd will not allow it to be used unless the
               StrictModes option has been set to “no”.

               This file is read into the environment at login (if it
               exists).  It can only contain empty lines, comment lines
               (that start with ‘#’), and assignment lines of the form
               name=value.  The file should be writable only by the
               user; it need not be readable by anyone else.
               Environment processing is disabled by default and is
               controlled via the PermitUserEnvironment option.

               Contains a list of host keys for all hosts the user has
               logged into that are not already in the systemwide list
               of known host keys.  The format of this file is described
               above.  This file should be writable only by root/the
               owner and can, but need not be, world-readable.

               Contains initialization routines to be run before the
               user's home directory becomes accessible.  This file
               should be writable only by the user, and need not be
               readable by anyone else.

               This file is for host-based authentication (see ssh(1)).
               It should only be writable by root.

               Contains Diffie-Hellman groups used for the "Diffie-
               Hellman Group Exchange" key exchange method.  The file
               format is described in moduli(5).  If no usable groups
               are found in this file then fixed internal groups will be

               See motd(5).

               If this file exists, sshd refuses to let anyone except
               root log in.  The contents of the file are displayed to
               anyone trying to log in, and non-root connections are
               refused.  The file should be world-readable.

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

               These files contain the private parts of the host keys.
               These files should only be owned by root, readable only
               by root, and not accessible to others.  Note that sshd
               does not start if these files are group/world-accessible.

               These files contain the public parts of the host keys.
               These files should be world-readable but writable only by
               root.  Their contents should match the respective private
               parts.  These files are not really used for anything;
               they are provided for the convenience of the user so
               their contents can be copied to known hosts files.  These
               files are created using ssh-keygen(1).

               Systemwide list of known host keys.  This file should be
               prepared by the system administrator to contain the
               public host keys of all machines in the organization.
               The format of this file is described above.  This file
               should be writable only by root/the owner and should be

               Contains configuration data for sshd.  The file format
               and configuration options are described in

               Similar to ~/.ssh/rc, it can be used to specify machine-
               specific login-time initializations globally.  This file
               should be writable only by root, and should be world-

               chroot(2) directory used by sshd during privilege
               separation in the pre-authentication phase.  The
               directory should not contain any files and must be owned
               by root and not group or world-writable.

               Contains the process ID of the sshd listening for
               connections (if there are several daemons running
               concurrently for different ports, this contains the
               process ID of the one started last).  The content of this
               file is not sensitive; it can be world-readable.

SEE ALSO         top

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

AUTHORS         top

       OpenSSH is a derivative of the original and free ssh 1.2.12
       release by Tatu Ylonen.  Aaron Campbell, Bob Beck, Markus Friedl,
       Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-
       added newer features and created OpenSSH.  Markus Friedl
       contributed the support for SSH protocol versions 1.5 and 2.0.
       Niels Provos and Markus Friedl contributed support for privilege

COLOPHON         top

       This page is part of the openssh (Portable OpenSSH) project.
       Information about the project can be found at  If you have a bug report
       for this manual page, see ⟨⟩.
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GNU                        September 19, 2023                    SSHD(8)

Pages that refer to this page: systemd-ssh-proxy(1)pts(4)environment.d(5)user@.service(5)systemd-ssh-generator(8)