.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
-.\" $OpenBSD: sshd.8,v 1.198 2003/06/10 09:12:12 jmc Exp $
+.\" $OpenBSD: sshd.8,v 1.215 2006/02/01 09:11:41 jmc Exp $
.Dd September 25, 1999
.Dt SSHD 8
.Os
.Sh SYNOPSIS
.Nm sshd
.Bk -words
-.Op Fl deiqtD46
+.Op Fl 46Ddeiqt
.Op Fl b Ar bits
.Op Fl f Ar config_file
.Op Fl g Ar login_grace_time
.Ek
.Sh DESCRIPTION
.Nm
-(SSH Daemon) is the daemon program for
+(OpenSSH Daemon) is the daemon program for
.Xr ssh 1 .
Together these programs 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.
.Pp
.Nm
-is the daemon that listens for connections from clients.
+listens for connections from clients.
It is normally started at boot from
.Pa /etc/rc .
It forks a new
The forked daemons handle
key exchange, encryption, authentication, command execution,
and data exchange.
-This implementation of
-.Nm
-supports both SSH protocol version 1 and 2 simultaneously.
-.Nm
-works as follows:
-.Pp
-.Ss SSH protocol version 1
-.Pp
-Each host has a host-specific RSA key
-(normally 1024 bits) used to identify 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.
-.Pp
-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 encryption algorithm
-to use from those offered by the server.
-.Pp
-Next, the server and the client enter an authentication dialog.
-The client tries to authenticate itself using
-.Pa .rhosts
-authentication,
-.Pa .rhosts
-authentication combined with RSA host
-authentication, RSA challenge-response authentication, or password
-based authentication.
-.Pp
-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
-.Nm rshd ,
-.Nm rlogind ,
-and
-.Nm rexecd
-are disabled (thus completely disabling
-.Xr rlogin
-and
-.Xr rsh
-into the machine).
-.Pp
-.Ss SSH protocol version 2
-.Pp
-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.
-.Pp
-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).
-.Pp
-Protocol version 2 provides a public key based
-user (PubkeyAuthentication) or
-client host (HostbasedAuthentication) authentication method,
-conventional password authentication and challenge response based methods.
-.Pp
-.Ss Command execution and data forwarding
-.Pp
-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.
-.Pp
-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.
-.Pp
-When the user program terminates and all forwarded X11 and other
-connections have been closed, the server sends command exit status to
-the client, and both sides exit.
.Pp
.Nm
-can be configured using command-line options or a configuration
-file.
-Command-line options override values specified in the
+can be configured using command-line options or a configuration file
+(by default
+.Xr sshd_config 5 ) ;
+command-line options override values specified in the
configuration file.
-.Pp
.Nm
rereads its configuration file when it receives a hangup signal,
.Dv SIGHUP ,
-by executing itself with the name it was started as, i.e.,
+by executing itself with the name and options it was started with, e.g.,
.Pa /usr/sbin/sshd .
.Pp
The options are as follows:
.Bl -tag -width Ds
+.It Fl 4
+Forces
+.Nm
+to use IPv4 addresses only.
+.It Fl 6
+Forces
+.Nm
+to use IPv6 addresses only.
.It Fl b Ar bits
Specifies the number of bits in the ephemeral protocol version 1
server key (default 768).
+.It Fl D
+When this option is specified,
+.Nm
+will not detach and does not become a daemon.
+This allows easy monitoring of
+.Nm sshd .
.It Fl d
Debug mode.
The server sends verbose debug output to the system
Specifies how often the ephemeral protocol version 1 server key is
regenerated (default 3600 seconds, or one hour).
The motivation for regenerating the key fairly
-often is that the key is not stored anywhere, and after about an hour,
+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.
Can be used to give options in the format used in the configuration 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
+.Xr sshd_config 5 .
.It Fl p Ar port
Specifies the port on which the server listens for connections
(default 22).
Multiple port options are permitted.
-Ports specified in the configuration file are ignored when a
-command-line port is specified.
+Ports specified in the configuration file with the
+.Cm Port
+option are ignored when a command-line port is specified.
+Ports specified using the
+.Cm ListenAddress
+option override command-line ports.
.It Fl q
Quiet mode.
Nothing is sent to the system log.
from making DNS requests unless the authentication
mechanism or configuration requires it.
Authentication mechanisms that may require DNS include
-.Cm RhostsAuthentication ,
.Cm RhostsRSAAuthentication ,
-.Cm HostbasedAuthentication
+.Cm HostbasedAuthentication ,
and using a
.Cm from="pattern-list"
option in a key file.
.Cm AllowUsers
or
.Cm DenyUsers .
-.It Fl D
-When this option is specified
-.Nm
-will not detach and does not become a daemon.
-This allows easy monitoring of
-.Nm sshd .
-.It Fl 4
-Forces
-.Nm
-to use IPv4 addresses only.
-.It Fl 6
-Forces
-.Nm
-to use IPv6 addresses only.
.El
-.Sh CONFIGURATION FILE
-.Nm
-reads configuration data from
-.Pa /etc/ssh/sshd_config
-(or the file specified with
-.Fl f
-on the command line).
-The file format and configuration options are described in
+.Sh AUTHENTICATION
+The OpenSSH SSH daemon supports SSH protocols 1 and 2.
+Both protocols are supported by default,
+though this can be changed via the
+.Cm Protocol
+option in
.Xr 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.
+.Pp
+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 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 encryption algorithm
+to use from those offered by the server.
+.Pp
+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 cryptographic message authentication code
+(hmac-sha1 or hmac-md5).
+.Pp
+Finally, the server and the client enter an authentication dialog.
+The client tries to authenticate itself using
+host-based authentication,
+public key authentication,
+challenge-response authentication,
+or password authentication.
+.Pp
+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
+.Cm DenyUsers
+or its group is listed in
+.Cm DenyGroups
+\&. The definition of a locked account is system dependant. Some platforms
+have their own account database (eg AIX) and some modify the passwd field (
+.Ql \&*LK\&*
+on Solaris and UnixWare,
+.Ql \&*
+on HP-UX, containing
+.Ql Nologin
+on Tru64,
+a leading
+.Ql \&*LOCKED\&*
+on FreeBSD and a leading
+.Ql \&!!
+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 something other than these values (eg
+.Ql NP
+or
+.Ql \&*NP\&*
+).
+.Pp
+System security is not improved unless
+.Nm rshd ,
+.Nm rlogind ,
+and
+.Nm rexecd
+are disabled (thus completely disabling
+.Xr rlogin
+and
+.Xr rsh
+into the machine).
+.Sh 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 connections, or forwarding the authentication agent
+connection over the secure channel.
+.Pp
+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.
+.Pp
+When the user program terminates and all forwarded X11 and other
+connections have been closed, the server sends command exit status to
+the client, and both sides exit.
.Sh LOGIN PROCESS
When a user successfully logs in,
.Nm
prints last login time and
.Pa /etc/motd
(unless prevented in the configuration file or by
-.Pa $HOME/.hushlogin ;
+.Pa ~/.hushlogin ;
see the
.Sx FILES
section).
.It
Sets up basic environment.
.It
-Reads
-.Pa $HOME/.ssh/environment
-if it exists and users are allowed to change their environment.
+Reads the file
+.Pa ~/.ssh/environment ,
+if it exists, and users are allowed to change their environment.
See the
.Cm PermitUserEnvironment
option in
Changes to user's home directory.
.It
If
-.Pa $HOME/.ssh/rc
+.Pa ~/.ssh/rc
exists, runs it; else if
.Pa /etc/ssh/sshrc
exists, runs
Runs user's shell or command.
.El
.Sh AUTHORIZED_KEYS FILE FORMAT
-.Pa $HOME/.ssh/authorized_keys
+.Pa ~/.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)
.Dq ssh-rsa .
.Pp
Note that lines in this file are usually several hundred bytes long
-(because of the size of the public key encoding).
+(because of the size of the public key encoding) up to a limit of
+8 kilobytes, 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
.Pa identity.pub ,
.Pa id_dsa.pub
This option might be useful
to restrict certain public 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
+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.
.It Cm environment="NAME=value"
.Cm UseLogin
is enabled.
.It Cm no-port-forwarding
-Forbids TCP/IP forwarding when this key is used for authentication.
+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
.Cm command
port forwarding such that it may only connect to the specified host and
port.
IPv6 addresses can be specified with an alternative syntax:
-.Ar host/port .
+.Ar host Ns / Ns Ar port .
Multiple
.Cm permitopen
options may be applied separated by commas.
No pattern matching is performed on the specified hostnames,
they must be literal domains or addresses.
+.It Cm tunnel="n"
+Force a
+.Xr tun 4
+device on the server.
+Without this option, the next available device will be used if
+the client requests a tunnel.
.El
.Ss Examples
-1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
+1024 33 12121...312314325 ylo@foo.bar
.Pp
-from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
+from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula
.Pp
-command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
+command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi
.Pp
-permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
+permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23...2323
+.Pp
+tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== reyk@openbsd.org
.Sh SSH_KNOWN_HOSTS FILE FORMAT
The
.Pa /etc/ssh/ssh_known_hosts
and
-.Pa $HOME/.ssh/known_hosts
+.Pa ~/.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
pattern, it is not accepted (by that line) even if it matched another
pattern on the line.
.Pp
+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
+.Ql |
+character.
+Only one hashed hostname may appear on a single line and none of the above
+negation or wildcard operators may be applied.
+.Pp
Bits, exponent, and modulus are taken directly from the RSA host key; they
can be obtained, e.g., from
.Pa /etc/ssh/ssh_host_key.pub .
and adding the host names at the front.
.Ss Examples
.Bd -literal
-closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
+closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
.Ed
+.Bd -literal
+# A hashed hostname
+|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
+AAAA1234.....=
+.Ed
.Sh FILES
.Bl -tag -width Ds
.It Pa /etc/ssh/sshd_config
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-readable.
-.It Pa $HOME/.ssh/authorized_keys
+.It Pa ~/.ssh/authorized_keys
Lists the public keys (RSA or DSA) that can be used to log into the user's account.
This file must be readable by root (which may on some machines imply
it being world-readable if the user's home directory resides on an NFS
.Pa id_rsa.pub
files into this file, as described in
.Xr ssh-keygen 1 .
-.It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
+.It Pa "/etc/ssh/ssh_known_hosts", "~/.ssh/known_hosts"
These files are consulted when using rhosts with RSA host
authentication or protocol version 2 hostbased authentication
to check the public key of the host.
These files should be writable only by root/the owner.
.Pa /etc/ssh/ssh_known_hosts
should be world-readable, and
-.Pa $HOME/.ssh/known_hosts
+.Pa ~/.ssh/known_hosts
can, but need not be, world-readable.
+.It Pa /etc/motd
+See
+.Xr motd 5 .
+.It Pa ~/.hushlogin
+This file is used to suppress printing the last login time and
+.Pa /etc/motd ,
+if
+.Cm PrintLastLog
+and
+.Cm PrintMotd ,
+respectively,
+are enabled.
+It does not suppress printing of the banner specified by
+.Cm Banner .
.It Pa /etc/nologin
If this file exists,
.Nm
Access controls that should be enforced by tcp-wrappers are defined here.
Further details are described in
.Xr hosts_access 5 .
-.It Pa $HOME/.rhosts
-This file contains host-username pairs, separated by a space, one per
+.It Pa ~/.rhosts
+This file is used during
+.Cm RhostsRSAAuthentication
+and
+.Cm HostbasedAuthentication
+and contains host-username pairs, separated by a space, one per
line.
The given user on the corresponding host is permitted to log in
without a password.
be writable only by the user; it is recommended that it not be
accessible by others.
.Pp
-If is also possible to use netgroups in the file.
+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.
-.It Pa $HOME/.shosts
+.It Pa ~/.shosts
For ssh,
this file is exactly the same as for
.Pa .rhosts .
not used by rlogin and rshd, so using this permits access using SSH only.
.It Pa /etc/hosts.equiv
This file is used during
-.Pa .rhosts
+.Cm RhostsRSAAuthentication
+and
+.Cm HostbasedAuthentication
authentication.
In the simplest form, this file contains host names, one per line.
Users on
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 RSA host authentication is normally required.
+Additionally, successful client host key authentication is required.
This file must be writable only by root; it is recommended
that it be world-readable.
.Pp
.Pa /etc/hosts.equiv .
However, this file may be useful in environments that want to run both
rsh/rlogin and ssh.
-.It Pa $HOME/.ssh/environment
+.It Pa ~/.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
.Ql # ) ,
controlled via the
.Cm PermitUserEnvironment
option.
-.It Pa $HOME/.ssh/rc
+.It Pa ~/.ssh/rc
If this file exists, it is run with
.Pa /bin/sh
after reading the
readable by anyone else.
.It Pa /etc/ssh/sshrc
Like
-.Pa $HOME/.ssh/rc .
+.Pa ~/.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.
.Xr ssh-add 1 ,
.Xr ssh-agent 1 ,
.Xr ssh-keygen 1 ,
+.Xr chroot 2 ,
+.Xr hosts_access 5 ,
.Xr login.conf 5 ,
.Xr moduli 5 ,
.Xr sshd_config 5 ,
+.Xr inetd 8 ,
.Xr sftp-server 8
.Rs
.%A T. Ylonen
andersk / openssh.git / blobdiff