sshd.0

OpenSSL Source code for SFTP, SSH, and many others

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

NAME
     sshd - OpenSSH SSH daemon

SYNOPSIS
     sshd [-deiqtD46] [-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 (SSH Daemon) is the daemon program for ssh(1).  Together these proM--
     grams replace rlogin and rsh, and provide secure encrypted communications
     between two untrusted hosts over an insecure network.  The programs are
     intended to be as easy to install and use as possible.

     sshd is the daemon that listens for connections from clients.  It is norM--
     mally 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.  This implementaM--
     tion of sshd supports both SSH protocol version 1 and 2 simultaneously.
     sshd works as follows.

   SSH protocol version 1

     Each host has a host-specific RSA key (normally 1024 bits) used to idenM--
     tify the host.  Additionally, when the daemon starts, it generates a
     server RSA key (normally 768 bits).  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 generates 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 encrypM--
     tion algorithm to use from those offered by the server.

     Next, the server and the client enter an authentication dialog.  The
     client tries to authenticate itself using .rhosts authentication, .rhosts
     authentication combined with RSA host authentication, RSA challenge-
     response authentication, or password based authentication.

     Rhosts authentication is normally disabled because it is fundamentally
     insecure, but can be enabled in the server configuration file if desired.
     System security is not improved unless rshd, rlogind, and rexecd are disM--
     abled (thus completely disabling rlogin and rsh into the machine).

   SSH protocol version 2

     Version 2 works similarly: Each host has a host-specific key (RSA or DSA)
     used to identify the host.  However, when the daemon starts, it does not
     generate a server key.  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 cryptographic message authentication code (hmac-sha1 or hmac-
     md5).

     Protocol version 2 provides a public key based user (PubkeyAuthenticaM--
     tion) or client host (HostbasedAuthentication) authentication method,
     conventional password authentication and challenge response based methM--
     ods.

   Command execution and data forwarding

     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/IP
     connections, 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 connecM--
     tions have been closed, the server sends command exit status to the
     client, and both sides exit.

     sshd can be configured using command-line options or a configuration
     file.  Command-line options override values specified in the configuraM--
     tion file.

     sshd rereads its configuration file when it receives a hangup signal,
     SIGHUP, by executing itself with the name it was started as, i.e.,
     /usr/sbin/sshd.

     The options are as follows:

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

     -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 conM--
             figuration file.

     -g login_grace_time
             Gives the grace time for clients to authenticate themselves
             (default 600 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 proM--
             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.  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 motivaM--
             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 configuraM--
             tion file.  This is useful for specifying options for which there
             is no separate command-line flag.

     -p port
             Specifies the port on which the server listens for connections
             (default 22).  Multiple port options are permitted.  Ports speciM--
             fied in the configuration file are ignored when a command-line
             port is specified.

     -q      Quiet mode.  Nothing is sent to the system log.  Normally the
             beginning, 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 overM--
             flow this field to still be uniquely identified.  Specifying -u0
             indicates that only dotted decimal addresses should be put into
             the utmp file.  -u0 is 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 RhostsAuthentication, 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.

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

     -4      Forces sshd to use IPv4 addresses only.

     -6      Forces sshd to use IPv6 addresses only.

CONFIGURATION FILE
     sshd reads configuration data from /etc/ssh/sshd_config (or the file
     specified with -f on the command line).  The file format and configuraM--
     tion options are described in sshd_config(5).

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 $HOME/.hushlogin; see the FILES secM--
                tion).

           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 $HOME/.ssh/environment if it exists.

           7.   Changes to user's home directory.

           8.   If $HOME/.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 user's shell or command.

AUTHORIZED_KEYS FILE FORMAT
     $HOME/.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, keyM--
     type, base64 encoded key, comment.  The options fields are optional; its
     presence is determined by whether the line starts with a number or not
     (the option field never starts with a number).  The bits, exponent, moduM--
     lus and comment fields give the RSA key for protocol version 1; the comM--
     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 ``ssh-rsa''.

     Note that lines in this file are usually several hundred bytes long
     (because of the size of the RSA key modulus).  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 specificaM--
     tions.  No spaces are permitted, except within double quotes.  The folM--
     lowing option specifications are supported (note that option keywords are
     case-insensitive):

     from="pattern-list"
             Specifies that in addition to RSA 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: RSA 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).

     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