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

NAME

     sshd - OpenSSH SSH daemon

SYNOPSIS

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

          [-g login_grace_time] [-h host_key_file] [-k key_gen_time]

          [-o option] [-p port] [-u len]

DESCRIPTION

     sshd (OpenSSH Daemon) is the daemon program for ssh(1).  Together these

     programs replace rlogin(1) and rsh(1), and provide secure encrypted com-

     munications between two untrusted hosts over an insecure network.

     sshd listens for connections from clients.  It is normally started at

     boot from /etc/rc.  It forks a new daemon for each incoming connection.

     The forked daemons handle key exchange, encryption, authentication, com-

     mand execution, and data exchange.

     sshd can be configured using command-line options or a configuration file

     (by default sshd_config(5)); command-line options override values speci-

     fied in the configuration file.  sshd rereads its configuration file when

     it receives a hangup signal, SIGHUP, by executing itself with the name

     and options it was started with, e.g. /usr/sbin/sshd.

     The options are as follows:

     -4      Forces sshd to use IPv4 addresses only.

     -6      Forces sshd to use IPv6 addresses only.

     -b bits

             Specifies the number of bits in the ephemeral protocol version 1

             server key (default 1024).

     -C connection_spec

             Specify the connection parameters to use for the -T extended test

             mode.  If provided, any Match directives in the configuration

             file that would apply to the specified user, host, and address

             will be set before the configuration is written to standard out-

             put.  The connection parameters are supplied as keyword=value

             pairs.  The keywords are ``user'', ``host'', and ``addr''.  All

             are required and may be supplied in any order, either with multi-

             ple -C options or as a comma-separated list.

     -D      When this option is specified, sshd will not detach and does not

             become a daemon.  This allows easy monitoring of sshd.

     -d      Debug mode.  The server sends verbose debug output to the system

             log, and does not put itself in the background.  The server also

             will not fork and will only process one connection.  This option

             is only intended for debugging for the server.  Multiple -d 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 config_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      Extended test mode.  Check the validity of the configuration

             file, output the effective configuration to stdout and then exit.

             Optionally, Match rules may be applied by specifying the connec-

             tion parameters using one or more -C options.

     -t      Test mode.  Only check the validity of the configuration file and

             sanity of the keys.  This is useful for updating sshd reliably as

             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-md5, hmac-sha1, umac-64 or

     hmac-ripemd160).

     Finally, the server and the client enter an authentication dialog.  The

     client tries to authenticate itself using host-based authentication, pub-

     lic key authentication, challenge-response authentication, or password

     authentication.

     Regardless of the authentication type, the account is checked to ensure

     that it is accessible.  An account is not accessible if it is locked,

     listed in DenyUsers or its group is listed in DenyGroups .  The defini-

     tion of a locked account is system dependant. Some platforms have their

     own account database (eg AIX) and some modify the passwd field ( `*LK*'

     on Solaris and UnixWare, `*' on HP-UX, containing `Nologin' on Tru64, a

     leading `*LOCKED*' on FreeBSD and a leading `!' on most Linuxes).  If

     there is a requirement to disable password authentication for the account

     while allowing still public-key, then the passwd field should be set to

     something other than these values (eg `NP' or `*NP*' ).

     If the client successfully authenticates itself, a dialog for preparing

     the session is entered.  At this time the client may request things like

     allocating a pseudo-tty, forwarding X11 connections, forwarding TCP con-

     nections, or forwarding the authentication agent connection over the 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, 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 host-

             names or addresses, a from stanza may match IP addressess 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).

     no-agent-forwarding

             Forbids authentication agent forwarding when this key is used for

             authentication.

     no-port-forwarding

             Forbids TCP forwarding when this key is used for authentication.

             Any port forward requests by the client will return an error.

             This might be used, e.g. in connection with the command option.

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

     no-user-rc

             Disables execution of ~/.ssh/rc.

     no-X11-forwarding

             Forbids X11 forwarding when this key is used for authentication.

             Any X11 forward requests by the client will return an error.

     permitopen="host:port"

             Limit local ``ssh -L'' port forwarding such that it may only con-

             nect to the specified host and port.  IPv6 addresses can be spec-

             ified with an alternative syntax: host/port.  Multiple permitopen

             options may be applied separated by commas.  No pattern matching

             is performed on the specified hostnames, they must be literal 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/

             This directory is the default location for all user-specific con-

             figuration and authentication information.  There is no general

             requirement to keep the entire contents of this directory secret,

             but the recommended permissions are read/write/execute for the

             user, and not accessible by others.

     ~/.ssh/authorized_keys

             Lists the public keys (RSA/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_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/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/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),

     ssh-keyscan(1), chroot(2), hosts_access(5), login.conf(5), moduli(5),

     sshd_config(5), inetd(8), sftp-server(8)

AUTHORS

     OpenSSH is a derivative of the original and free ssh 1.2.12 release by

     Tatu Ylonen.  Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo

     de Raadt and Dug Song removed many bugs, re-added newer features and cre-

     ated OpenSSH.  Markus Friedl contributed the support for SSH protocol

     versions 1.5 and 2.0.  Niels Provos and Markus Friedl contributed support

     for privilege separation.

CAVEATS

     System security is not improved unless rshd, rlogind, and rexecd are dis-

     abled (thus completely disabling rlogin and rsh into the machine).

OpenBSD 4.4                      July 2, 2008                                9