Quick ?s
Cheat Sheets
Man Pages
The Lynx
SSHD(8) 		  BSD System Managers Manual		      SSHD(8)

     sshd - OpenSSH SSH daemon

     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]

     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
	     options 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
	     (default 120 seconds).  If the client fails to authenticate the
	     user within this many seconds, the server disconnects and exits.
	     A value of zero indicates no limit.

     -h host_key_file
	     Specifies a file from which a host key is read.  This option must
	     be given if sshd is not run as root (as the normal host key files
	     are normally not readable by anyone but root).  The default is
	     /etc/ssh/ssh_host_key for protocol version 1, and
	     /etc/ssh/ssh_host_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
	     regenerated every time.  However, with small key sizes (e.g.,
	     512) using sshd from inetd may be feasible.

     -k key_gen_time
	     Specifies how often the ephemeral protocol version 1 server key
	     is regenerated (default 3600 seconds, or one hour).  The 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
	     zero 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
	     options, and their values, see sshd_config(5).

     -p port
	     Specifies the port on which the server listens for connections
	     (default 22).  Multiple port options are permitted.  Ports 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.  Only fatal errors are sent to the system log.  Nor
	     mally the beginning, authentication, and termination of each con
	     nection is logged.  If a second -q is given then nothing is sent
	     to the system log.

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

     The OpenSSH SSH daemon supports SSH protocols 1 and 2.  Both protocols
     are supported by default, though this can be changed via the Protocol
     option in sshd_config(5).	Protocol 2 supports both RSA and DSA keys;
     protocol 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

     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* ).

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

     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
     secure channel.

     Finally, 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.

     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 users 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.

	   9.	Runs users shell or command.

     ~/.ssh/authorized_keys is the default file that lists the public keys
     that are permitted for RSA authentication in protocol version 1 and for
     public key authentication (PubkeyAuthentication) in protocol version 2.
     AuthorizedKeysFile may be used to specify an alternative file.

     Each line of the file contains one key (empty lines and lines starting
     with a # are ignored as comments).  Each RSA public key consists of the
     following fields, separated by spaces: options, bits, exponent, modulus,
     comment.  Each protocol version 2 public key consists of: options, key
     type, base64 encoded key, comment.  The options field is optional; its
     presence is determined by whether the line starts with a number or not
     (the options field never starts with a number).  The bits, exponent, mod
     ulus and comment fields give the RSA key for protocol version 1; the com
     ment 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

     Note that lines in this file are usually several hundred bytes long
     (because of the size of the public key encoding) up to a limit of 8 kilo
     bytes, which permits DSA keys up to 8 kilobits and RSA keys up to 16
     kilobits.	You dont 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

	     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 (* and ? serve as wildcards).  The
	     list may also contain patterns negated by prefixing them with
	     !; if the canonical host name matches a negated pattern, the
	     key is not accepted.  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); how
	     ever, 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).

	     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 cer
	     tain public keys to perform just a specific operation.  An exam
	     ple 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.  Note that this option applies to
	     shell, command or subsystem execution.

	     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.

	     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.

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

	     Forbids authentication agent forwarding when this key is used for

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

	     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
	     domains or addresses.

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

     1024 33 12121...312314325 ylo@foo.bar

     from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula

     command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323

     permitopen="",permitopen="" 1024 33 23...2323

     tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==

     The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host
     public keys for all known hosts.  The global file should be prepared by
     the administrator (optional), and the per-user file is maintained 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
     accepted (by that line) even if it matched another pattern on the line.

     Alternately, hostnames may be stored in a hashed form which hides host
     names and addresses should the files 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 opera
     tors may be applied.

     Bits, exponent, and modulus are taken directly from the RSA host key;
     they can be obtained, e.g., 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 dont 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.

     closenet,..., 1024 37 159...93 closenet.hut.fi
     cvs.openbsd.org, ssh-rsa AAAA1234.....=

     # A hashed hostname
     |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa

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

     /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_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
	     this file is group/world-accessible.

     /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_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).

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

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

	     Contains the process ID of the sshd listening for connections (if
	     there are several daemons running concurrently for 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

	     Lists the public keys (RSA or DSA) that can be used to log into
	     the users account.  This file must be readable by root (which
	     may on some machines imply it being world-readable if the users
	     home directory resides on an NFS volume).	It is recommended that
	     it not be accessible by others.  The format of this file is
	     described above.  Users will place the contents of their
	     identity.pub, id_dsa.pub and/or id_rsa.pub files into this file,
	     as described in ssh-keygen(1).

     /etc/ssh/ssh_known_hosts, ~/.ssh/known_hosts
	     These files are consulted when using rhosts with RSA host authen
	     tication or protocol version 2 hostbased authentication to check
	     the public key of the host.  The key must be listed in one of
	     these files to be accepted.  The client uses the same files to
	     verify that it is connecting to the correct remote host.  These
	     files should be writable only by root/the owner.
	     /etc/ssh/ssh_known_hosts should be world-readable, and
	     ~/.ssh/known_hosts can, but need not be, world-readable.

	     See motd(5).

	     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.

	     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

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

	     This file is used during RhostsRSAAuthentication and
	     HostbasedAuthentication and contains host-username pairs, sepa
	     rated by a space, one per line.  The given user on the corre
	     sponding host is permitted to log in without a password.  The
	     same file is used by rlogind and rshd.  The file must be writable
	     only by the user; it is recommended that it not be accessible by

	     It is also possible to use netgroups in the file.	Either host or
	     user name may be of the form +@groupname to specify all hosts or
	     all users in the group.

	     For ssh, this file is exactly the same as for .rhosts.  However,
	     this file is not used by rlogin and rshd, so using this permits
	     access using SSH only.

	     This file is used during RhostsRSAAuthentication and
	     HostbasedAuthentication authentication.  In the simplest form,
	     this file contains host names, one per line.  Users on those
	     hosts are permitted to log in without a password, provided they
	     have the same user name on both machines.	The host name may also
	     be followed by a user name; such users are permitted to log in as
	     any user on this machine (except root).  Additionally, the syntax
	     +@group can be used to specify netgroups.	Negated entries
	     start with -.

	     If the client host/user is successfully matched in this file,
	     login is automatically permitted provided the client and server
	     user names are the same.  Additionally, successful client host
	     key authentication is required.  This file must be writable only
	     by root; it is recommended that it be world-readable.

	     Warning: It is almost never a good idea to use user names in
	     hosts.equiv.  Beware that it really means that the named user(s)
	     can log in as anybody, which includes bin, daemon, adm, and other
	     accounts that own critical binaries and directories.  Using a
	     user name practically grants the user root access.  The only
	     valid use for user names that I can think of is in negative

	     Note that this warning also applies to rsh/rlogin.

	     This is processed exactly as /etc/hosts.equiv.  However, this
	     file may be useful in environments that want to run both
	     rsh/rlogin and ssh.

	     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.

	     If this file exists, it is run with /bin/sh after reading the
	     environment files but before starting the users shell or com
	     mand.  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 users home directory
	     becomes accessible; AFS is a particular example of such an envi

	     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.

	     This file should be writable only by the user, and need not be
	     readable by anyone else.

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

     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)

     T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, SSH
     Protocol Architecture, draft-ietf-secsh-architecture-12.txt, January
     2002, work in progress material.

     M. Friedl, N. Provos, and W. A. Simpson, Diffie-Hellman Group Exchange
     for the SSH Transport Layer Protocol, draft-ietf-secsh-dh-group-
     exchange-02.txt, January 2002, work in progress material.

     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.

BSD			      September 25, 1999			   BSD

Yals.net is © 1999-2009 Crescendo Communications
Sharing tech info on the web for more than a decade!
This page was generated Thu Apr 30 17:05:32 2009