sshd [-deiqtD46] [-b bits] [-f config_file] [-g lo-
gin_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). Togeth-
er 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 ex-
change, encryption,
authentication, command execution, and data exchange. This
implementaM--
tion of sshd supports both SSH protocol version 1 and 2 si-
multaneously.
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 pub-
lic 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, cur-
rently Blowfish
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 ma-
chine).
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 ci-
pher, 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 (Pub-
keyAuthenticaM--
tion) or client host (HostbasedAuthentication) authentica-
tion 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, for-
warding TCP/IP
connections, or forwarding the authentication agent connec-
tion over the
secure channel.
Finally, the client either requests a shell or execution of
sshd can be configured using command-line options or a con-
figuration
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 proto-
col 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. Mul-
tiple -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 au-
thenticate the
user within this many seconds, the server discon-
nects 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
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 commu-
nications even
if the machine is cracked into or physically seized.
A value of
zero indicates that the key will never be regenerat-
ed.
-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 configu-
ration file and
sanity of the keys. This is useful for updating
sshd reliably as
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 re-
quire DNS
include RhostsAuthentication, RhostsRSAAuthentica-
tion,
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 pre-
vented 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 con-
tents 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.
$HOME/.ssh/authorized_keys is the default file that lists
the public keys
that are permitted for RSA authentication in protocol ver-
sion 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, expo-
nent, 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"
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 us-
er (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 backM--
slash. This option might be useful to restrict cer-
tain 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 un-
less they are
explicitly prohibited. Note that this option ap-
plies to shell,
command or subsystem execution.
environment="NAME=value"
Specifies that the string is to be added to the en-
vironment 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 automati-
cally disabled
if UseLogin is enabled.
no-port-forwarding
Forbids TCP/IP forwarding when this key is used for
authenticaM--
tion. Any port forward requests by the client will
return an
error. This might be used, e.g., in connection with
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 conM--
nect to the specified host and port. IPv6 addresses
can be specM--
ified with an alternative syntax: host/port. Multi-
ple permitopen
options may be applied separated by commas. No pat-
tern matching
is performed on the specified hostnames, they must
be literal
domains 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
backup.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
automatically: 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 wildM--
cards); each pattern in turn is matched against the canoni-
cal host name
(when authenticating a client) or against the user-supplied
name (when
authenticating a server). A pattern may also be preceded by
ments.
When performing host authentication, authentication is ac-
cepted if any
matching line has the proper key. It is thus permissible
(but not recomM--
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 conM--
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 for-
mat and conM--
figuration 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
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-readM--
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
described above. Users will place the contents of
their
identity.pub, id_dsa.pub and/or id_rsa.pub files in-
to 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 authenM--
tication or protocol version 2 hostbased authentica-
tion 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.
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 recomM--
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 pass-
word, provided
they have the same user name on both machines. The
host name may
also be followed by a user name; such users are per-
mitted to log
in as any user on this machine (except root). Addi-
tionally, 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
RSA host
authentication is normally required. This file must
be writable
only by root; it is recommended that it be world-
entries.
Note that this warning also applies to rsh/rlogin.
/etc/shosts.equiv
This is processed exactly as /etc/hosts.equiv. How-
ever, 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.
$HOME/.ssh/rc
If this file exists, it is run with /bin/sh after
reading the
environment files but before starting the user's
shell or comM--
mand. It must not produce any output on stdout;
stderr must be
used instead. If X11 forwarding is in use, it will
receive the
"proto cookie" pair in its standard input (and DIS-
PLAY in its
environment). The script must call xauth(1) because
sshd will
not run xauth automatically to add X11 cookies.
The primary purpose of this file is to run any ini-
tialization
routines which may be needed before the user's home
directory
becomes accessible; AFS is a particular example of
such an enviM--
ronment.
This file will probably contain some initialization
code followed
by something similar to:
if read proto cookie && [ -n "$DISPLAY" ]; then
if [ `echo $DISPLAY | cut -c1-10` = 'local-
host:' ]; then
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 ma-
chine-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 fea-
tures and creM--
ated OpenSSH. Markus Friedl contributed the support for SSH
protocol
versions 1.5 and 2.0. Niels Provos and Markus Friedl con-
tributed support
for privilege separation.
SEE ALSO
scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-key-
gen(1),
login.conf(5), moduli(5), sshd_config(5), sftp-server(8)
T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehti-
nen, 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.
BSD September 25, 1999
BSD
Man(1) output converted with man2html