xref: /freebsd-10.2-release/crypto/openssh/sshd.0

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

NAME
     sshd - OpenSSH SSH daemon

SYNOPSIS
     sshd [-46Ddeiqt] [-b bits] [-f config_file] [-g login_grace_time]
          [-h host_key_file] [-k key_gen_time] [-o option] [-p port] [-u len]

DESCRIPTION
     sshd (OpenSSH Daemon) is the daemon program for ssh(1).  Together these
     programs replace rlogin and rsh, and provide secure encrypted communica-
     tions between two untrusted hosts over an insecure network.

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

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

     The options are as follows:

     -4      Forces sshd to use IPv4 addresses only.

     -6      Forces sshd to use IPv6 addresses only.

     -b bits
             Specifies the number of bits in the ephemeral protocol version 1
             server key (default 768).

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

     -d      Debug mode.  The server sends verbose debug output to the system
             log, and does not put itself in the background.  The server also
             will not fork and will only process one connection.  This option
             is only intended for debugging for the server.  Multiple -d op-
             tions increase the debugging 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 configuration_file
             Specifies the name of the configuration file.  The default is
             /etc/ssh/sshd_config.  sshd refuses to start if there is no con-
             figuration file.

     -g login_grace_time
             Gives the grace time for clients to authenticate themselves (de-
             fault 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_rsa_key and /etc/ssh/ssh_host_dsa_key for pro-
             tocol version 2.  It is possible to have multiple host key files
             for the different protocol versions and host key algorithms.

     -i      Specifies that sshd is being run from inetd(8).  sshd is normally
             not run from inetd because it needs to generate the server key
             before it can respond to the client, and this may take tens of
             seconds.  Clients would have to wait too long if the key was re-
             generated every time.  However, with small key sizes (e.g. 512)
             using sshd from inetd may be feasible.

     -k key_gen_time
             Specifies how often the ephemeral protocol version 1 server key
             is regenerated (default 3600 seconds, or one hour).  The motiva-
             tion for regenerating the key fairly often is that the key is not
             stored anywhere, and after about an hour it becomes impossible to
             recover the key for decrypting intercepted communications even if
             the machine is cracked into or physically seized.  A value of ze-
             ro indicates that the key will never be regenerated.

     -o option
             Can be used to give options in the format used in the configura-
             tion file.  This is useful for specifying options for which there
             is no separate command-line flag.  For full details of the op-
             tions, and their values, see sshd_config(5).

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

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

     -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 over-
             flow 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 in-
             clude RhostsRSAAuthentication, HostbasedAuthentication, and using
             a from="pattern-list" option in a key file.  Configuration op-
             tions that require DNS include using a USER@HOST pattern in
             AllowUsers or DenyUsers.

AUTHENTICATION
     The OpenSSH SSH daemon supports SSH protocols 1 and 2.  Both protocols
     are supported by default, though this can be changed via the Protocol op-
     tion in sshd_config(5).  Protocol 2 supports both RSA and DSA keys; pro-
     tocol 1 only supports RSA keys.  For both protocols, each host has a
     host-specific key, normally 2048 bits, used to identify the host.

     Forward security for protocol 1 is provided through an additional server
     key, normally 768 bits, generated when the server starts.  This key is
     normally regenerated every hour if it has been used, and is never stored
     on disk.  Whenever a client connects, the daemon responds with its public
     host and server keys.  The client compares the RSA host key against its
     own database to verify that it has not changed.  The client then gener-
     ates a 256-bit random number.  It encrypts this random number using both
     the host key and the server key, and sends the encrypted number to the
     server.  Both sides then use this random number as a session key which is
     used to encrypt all further communications in the session.  The rest of
     the session is encrypted using a conventional cipher, currently Blowfish
     or 3DES, with 3DES being used by default.  The client selects the encryp-
     tion algorithm to use from those offered by the server.

     For protocol 2, forward security is provided through a Diffie-Hellman key
     agreement.  This key agreement results in a shared session key.  The rest
     of the session is encrypted using a symmetric cipher, currently 128-bit
     AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES.  The
     client selects the encryption algorithm to use from those offered by the
     server.  Additionally, session integrity is provided through a crypto-
     graphic message authentication code (hmac-sha1 or hmac-md5).

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

     Regardless of the authentication type, the account is checked to ensure
     that it is accessible.  An account is not accessible if it is locked,
     listed in DenyUsers or its group is listed in DenyGroups .  The defini-
     tion of a locked account is system dependant. Some platforms have their
     own account database (eg AIX) and some modify the passwd field ( `*LK*'
     on Solaris and UnixWare, `*' on HP-UX, containing `Nologin' on Tru64, a
     leading `*LOCKED*' on FreeBSD and a leading `!!' on Linux).  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 some-
     thing other than these values (eg `NP' or `*NP*' ).

     If the client successfully authenticates itself, a dialog for preparing
     the session is entered.  At this time the client may request things like
     allocating a pseudo-tty, forwarding X11 connections, forwarding TCP con-
     nections, or forwarding the authentication agent connection over the se-
     cure 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 connec-
     tions have been closed, the server sends command exit status to the
     client, and both sides exit.

LOGIN PROCESS
     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, runs it; else if /etc/ssh/sshrc exists,
                runs it; otherwise runs xauth.  The ``rc'' files are given the
                X11 authentication protocol and cookie in standard input.  See
                SSHRC, below.

           9.   Runs user's shell or command.

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

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

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

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

     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.

AUTHORIZED_KEYS FILE FORMAT
     AuthorizedKeysFile specifies the file containing public keys for public
     key authentication; if none is specified, the default is
     ~/.ssh/authorized_keys.  Each line of the file contains one key (empty
     lines and lines starting with a `#' are ignored as comments).  Protocol 1
     public keys consist of the following space-separated fields: options,
     bits, exponent, modulus, comment.  Protocol 2 public key consist of: op-
     tions, keytype, base64-encoded key, comment.  The options field is op-
     tional; its presence is determined by whether the line starts with a num-
     ber or not (the options field never starts with a number).  The bits, ex-
     ponent, modulus, and comment fields give the RSA key for protocol 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
     ``ssh-dss'' or ``ssh-rsa''.

     Note that lines in this file are usually several hundred bytes long (be-
     cause of the size of the public key encoding) up to a limit of 8 kilo-
     bytes, which permits DSA keys up to 8 kilobits and RSA keys up to 16
     kilobits.  You don't want to type them in; instead, copy the
     identity.pub, id_dsa.pub, or the id_rsa.pub file and edit it.

     sshd enforces a minimum RSA key modulus size for protocol 1 and protocol
     2 keys of 768 bits.

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

     command="command"
             Specifies that the command is executed whenever this key is used
             for authentication.  The command supplied by the user (if any) is
             ignored.  The command is run on a pty if the client requests a
             pty; otherwise it is run without a tty.  If an 8-bit clean chan-
             nel is required, one must not request a pty or should specify no-
             pty.  A quote may be included in the command by quoting it with a
             backslash.  This option might be useful to restrict certain pub-
             lic 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 vari-
             able.  Note that this option applies to shell, command or subsys-
             tem execution.

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

     from="pattern-list"
             Specifies that in addition to public key authentication, the
             canonical name of the remote host must be present in the comma-
             separated list of patterns.  The purpose of this option is to op-
             tionally 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 per-
             mits an intruder to log in from anywhere in the world.  This ad-
             ditional option makes using a stolen key more difficult (name
             servers and/or routers would have to be compromised in addition
             to just the key).

             See PATTERNS in ssh_config(5) for more information on patterns.

     no-agent-forwarding
             Forbids authentication agent forwarding when this key is used for
             authentication.

     no-port-forwarding
             Forbids TCP forwarding when this key is used for authentication.
             Any port forward requests by the client will return an error.
             This might be used, e.g. in connection with the command option.

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

     no-X11-forwarding
             Forbids X11 forwarding when this key is used for authentication.
             Any X11 forward requests by the client will return an error.

     permitopen="host:port"
             Limit local ``ssh -L'' port forwarding such that it may only con-
             nect to the specified host and port.  IPv6 addresses can be spec-
             ified with an alternative syntax: host/port.  Multiple permitopen
             options may be applied separated by commas.  No pattern matching
             is performed on the specified hostnames, they must be literal do-
             mains or addresses.

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

     An example authorized_keys file:

        # Comments allowed at start of line
        ssh-rsa AAAAB3Nza...LiPk== user@example.net
        from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
        AAAAB2...19Q== john@example.net
        command="dump /home",no-pty,no-port-forwarding ssh-dss
        AAAAC3...51R== example.net
        permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
        AAAAB5...21S==
        tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
        jane@example.net

SSH_KNOWN_HOSTS FILE FORMAT
     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 from an unknown host, its key is
     added to the per-user file.

     Each line in these files contains the following fields: hostnames, bits,
     exponent, modulus, comment.  The fields are separated by spaces.

     Hostnames is a comma-separated list of patterns (`*' and `?' act as wild-
     cards); each pattern in turn is matched against the canonical host name
     (when authenticating a client) or against the user-supplied name (when
     authenticating a server).  A pattern may also be preceded by `!' to indi-
     cate negation: if the host name matches a negated pattern, it is not ac-
     cepted (by that line) even if it matched another pattern on the line.  A
     hostname or address may optionally be enclosed within `[' and `]' brack-
     ets 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 ap-
     pear on a single line and none of the above negation or wildcard opera-
     tors may be applied.

     Bits, exponent, and modulus are taken directly from the RSA host key;
     they can be obtained, for example, from /etc/ssh/ssh_host_key.pub.  The
     optional comment field continues to the end of the line, and is not used.

     Lines starting with `#' and empty lines are ignored as comments.

     When performing host authentication, authentication is accepted if any
     matching line has the proper key.  It is thus permissible (but not recom-
     mended) 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 con-
     flicting 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 or by taking /etc/ssh/ssh_host_key.pub
     and adding the host names at the front.

     An example ssh_known_hosts file:

        # Comments allowed at start of line
        closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
        cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
        # A hashed hostname
        |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
        AAAA1234.....=

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

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

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

     ~/.ssh/authorized_keys
             Lists the public keys (RSA/DSA) 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 al-
             low it to be used unless the StrictModes option has been set to
             ``no''.  The recommended permissions can be set by executing
             ``chmod go-w ~/ ~/.ssh ~/.ssh/authorized_keys''.

     ~/.ssh/environment
             This file is read into the environment at login (if it exists).
             It can only contain empty lines, comment lines (that start with
             `#'), and assignment lines of the form name=value.  The file
             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.

     ~/.ssh/known_hosts
             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.

     ~/.ssh/rc
             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.

     /etc/hosts.allow
     /etc/hosts.deny
             Access controls that should be enforced by tcp-wrappers are de-
             fined here.  Further details are described in hosts_access(5).

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

     /etc/moduli
             Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
             Exchange".  The file format is described in moduli(5).

     /etc/motd
             See motd(5).

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

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

     /etc/ssh/ssh_known_hosts
             Systemwide list of known host keys.  This file should be prepared
             by the system administrator to contain the public host keys of
             all machines in the organization.  The format of this file is de-
             scribed above.  This file should be writable only by root/the
             owner and should be world-readable.

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

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

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

     /etc/ssh/sshrc
             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.

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

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

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

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

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

OpenBSD 4.1                   September 25, 1999                             9