sshd.8

OpenSSL Source code for SFTP, SSH, and many others

.\"  -*- nroff -*-
.\"
.\" Author: Tatu Ylonen <[e-mail]>
.\" Copyright (c) 1995 Tatu Ylonen <[e-mail]>, Espoo, Finland
.\"                    All rights reserved
.\"
.\" As far as I am concerned, the code I have written for this software
.\" can be used freely for any purpose.  Any derived versions of this
.\" software must be clearly marked as such, and if the derived work is
.\" incompatible with the protocol description in the RFC file, it must be
.\" called by a name other than "ssh" or "Secure Shell".
.\"
.\" Copyright (c) 1999,2000 Markus Friedl.  All rights reserved.
.\" Copyright (c) 1999 Aaron Campbell.  All rights reserved.
.\" Copyright (c) 1999 Theo de Raadt.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (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 1.184 2002/06/20 19:56:07 stevesk Exp $
.Dd September 25, 1999
.Dt SSHD 8
.Os
.Sh NAME
.Nm sshd
.Nd OpenSSH SSH daemon
.Sh SYNOPSIS
.Nm sshd
.Op Fl deiqtD46
.Op Fl b Ar bits
.Op Fl f Ar config_file
.Op Fl g Ar login_grace_time
.Op Fl h Ar host_key_file
.Op Fl k Ar key_gen_time
.Op Fl o Ar option
.Op Fl p Ar port
.Op Fl u Ar len
.Sh DESCRIPTION
.Nm
(SSH 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.
It is normally started at boot from
.Pa /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 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
.Xr 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
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.,
.Pa /usr/sbin/sshd .
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl b Ar bits
Specifies the number of bits in the ephemeral protocol version 1
server key (default 768).
.It Fl 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.
.It Fl e
When this option is specified,
.Nm
will send the output to the standard error instead of the system log.
.It Fl f Ar configuration_file
Specifies the name of the configuration file.
The default is
.Pa /etc/ssh/sshd_config .
.Nm
refuses to start if there is no configuration file.
.It Fl g Ar 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.
.It Fl h Ar host_key_file
Specifies a file from which a host key is read.
This option must be given if
.Nm
is not run as root (as the normal
host key files are normally not readable by anyone but root).
The default is
.Pa /etc/ssh/ssh_host_key
for protocol version 1, and
.Pa /etc/ssh/ssh_host_rsa_key
and
.Pa /etc/ssh/ssh_host_dsa_key
for protocol version 2.
It is possible to have multiple host key files for
the different protocol versions and host key algorithms.
.It Fl i
Specifies that
.Nm
is being run from inetd.
.Nm
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
.Nm
from inetd may
be feasible.
.It Fl k Ar key_gen_time
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,
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.
.It Fl o Ar option
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.
.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.
.It Fl q
Quiet mode.
Nothing is sent to the system log.
Normally the beginning,
authentication, and termination of each connection is logged.
.It Fl t
Test mode.
Only check the validity of the configuration file and sanity of the keys.
This is useful for updating
.Nm
reliably as configuration options may change.
.It Fl u Ar len
This option is used to specify the size of the field
in the
.Li utmp
structure that holds the remote host name.
If the resolved host name is longer than
.Ar len ,
the dotted decimal value will be used instead.
This allows hosts with very long host names that
overflow this field to still be uniquely identified.
Specifying
.Fl u0
indicates that only dotted decimal addresses
should be put into the
.Pa utmp
file.
.Fl u0
is also be used to prevent
.Nm
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
and using a
.Cm from="pattern-list"
option in a key file.
Configuration options that require DNS include using a
USER@HOST pattern in
.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
.Xr sshd_config 5 .
.Sh LOGIN PROCESS
When a user successfully logs in,
.Nm
does the following:
.Bl -enum -offset indent
.It
If the login is on a tty, and no command has been specified,
prints last login time and
.Pa /etc/motd
(unless prevented in the configuration file or by
.Pa $HOME/.hushlogin ;
see the
.Sx FILES
section).
.It
If the login is on a tty, records login time.
.It
Checks
.Pa /etc/nologin ;
if it exists, prints contents and quits
(unless root).
.It
Changes to run with normal user privileges.
.It
Sets up basic environment.
.It
Reads
.Pa $HOME/.ssh/environment
if it exists.
.It
Changes to user's home directory.
.It
If
.Pa $HOME/.ssh/rc
exists, runs it; else if
.Pa /etc/ssh/sshrc
exists, runs
it; otherwise runs xauth.
The
.Dq rc
files are given the X11
authentication protocol and cookie in standard input.
.It
Runs user's shell or command.
.El
.Sh AUTHORIZED_KEYS FILE FORMAT
.Pa $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.
.Cm AuthorizedKeysFile
may be used to specify an alternative file.
.Pp
Each line of the file contains one
key (empty lines and lines starting with a
.Ql #
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, keytype, 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, modulus and comment fields give the RSA key for
protocol version 1; the