Manual Pages: sshd

## Manual Pages

    SSHD(8)                 OpenBSD 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 pro-
         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 nor-
         mally started at boot from /etc/rc. It forks a new daemon for each incom-
         ing connection.  The forked daemons handle key exchange, encryption, au-
         thentication, command execution, and data exchange.  This implementation
         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 iden-
         tify the host.  Additionally, when the daemon starts, it generates a
         server RSA key (normally 768 bits).  This key is normally regenerated ev-
         ery 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 serv-
         er.  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.

         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-re-
         sponse 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 dis-
         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 of-
         fered 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 (PubkeyAuthentica-
         tion) or client host (HostbasedAuthentication) authentication method,
         conventional password authentication and challenge response based meth-
         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 connec-
         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 configura-
         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 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 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 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.  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
                 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.

         -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 are ignored when a command-line
                 port is specified.

         -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 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 in-
                 clude 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 configura-
         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 sec-

                    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 ex-
                    ists, 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, key-
         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, modu-
         lus 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 ``ssh-rsa''.

         Note that lines in this file are usually several hundred bytes long (be-
         cause 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 specifica-
         tions.  No spaces are permitted, except within double quotes.  The fol-
         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 ac-
                 cepted.  The purpose of this option is to optionally increase se-
                 curity: 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
                 pty; otherwise it is run without a tty.  If a 8-bit clean channel
                 is required, one must not request a pty or should specify no-pty.
                 A quote may be included in the command by quoting it with a back-
                 slash.  This option might be useful to restrict certain RSA 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/IP and/or X11 forwarding unless they are
                 explicitly prohibited.  Note that this option applies to shell,
                 command or subsystem 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.  This option is automatically disabled
                 if UseLogin is enabled.

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

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

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

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

         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.

       Examples
         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 back-
         up.hut.fi

         permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323

    SSH_KNOWN_HOSTS FILE FORMAT
         The /etc/ssh/ssh_known_hosts, and $HOME/.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 au-
         tomatically: 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.

         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 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.

       Examples

         closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
         cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=

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

         /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
                 this file is 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/moduli
                 Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
                 Exchange".

         /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.

         $HOME/.ssh/authorized_keys
                 Lists the public keys (RSA or DSA) that can be used to log into
                 the user's account.  This file must be readable by root (which
                 may on some machines imply it being world-readable if the user's
                 home directory resides on an NFS volume).  It is recommended that
                 it not be accessible by others.  The format of this file is de-
                 scribed 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 and $HOME/.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
                 $HOME/.ssh/known_hosts can but need not be world-readable.

         /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/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).

         $HOME/.rhosts
                 This file contains host-username pairs, separated by a space, one
                 per line.  The given user on the corresponding host is permitted
                 to log in without password.  The same file is used by rlogind and
                 rshd.  The file must be writable only by the user; it is recom-
                 mended that it not be accessible by others.

                 If 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.

         $HOME/.shosts
                 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.

         /etc/hosts.equiv
                 This file is used during .rhosts 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 en-
                 tries start with `-'.

                 If the client host/user is successfully matched in this file, lo-
                 gin is automatically permitted provided the client and server us-
                 er names are the same.  Additionally, successful RSA host authen-
                 tication is normally 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 us-
                 er name practically grants the user root access.  The only valid
                 use for user names that I can think of is in negative entries.

                 Note that this warning also applies to rsh/rlogin.

         /etc/shosts.equiv
                 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.

         $HOME/.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.

         $HOME/.ssh/rc
                 If this file exists, it is run with /bin/sh after reading the en-
                 vironment files but before starting the user's shell or command.
                 It must not produce any output on stdout; stderr must be used in-
                 stead.  If X11 forwarding is in use, it will receive the "proto
                 cookie" pair in its standard input (and DISPLAY in its environ-
                 ment).  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 be-
                 comes accessible; AFS is a particular example of such an environ-
                 ment.

                 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
                                 xauth add unix:`echo $DISPLAY |
                                     cut -c11-` $proto $cookie
                         else
                                 # X11UseLocalhost=no
                                 xauth add $DISPLAY $proto $cookie
                         fi
                 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.

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

         /etc/ssh/sshrc
                 Like $HOME/.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.

    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.

    SEE ALSO
         scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
         login.conf(5), moduli(5), sshd_config(5), 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.

    OpenBSD 3.1                   September 25, 1999                             9

NAME | SYNOPSIS | DESCRIPTION | CONFIGURATION FILE | LOGIN PROCESS | FILES | AUTHORS | SEE ALSO