sshd - OpenSSH SSH daemon
sshd [-46Ddeiqt] [-b bits] [-f config_file] [-g
[-h host_key_file] [-k key_gen_time] [-o option] [-p
port] [-u len]
sshd (SSH Daemon) is the daemon program for ssh(1). Together these programs
replace rlogin and rsh, and provide secure encrypted
between two untrusted hosts over an insecure network. The
intended to be as easy to install and use as possible.
sshd is the daemon that listens for connections from
clients. It is normally
started at boot from /etc/rc. It forks a new daemon
for each incoming
connection. The forked daemons handle key exchange,
authentication, command execution, and data exchange. This
of sshd supports both SSH protocol version 1 and 2 simultaneously.
sshd works as follows:
SSH protocol version 1 [Toc] [Back]
Each host has a host-specific RSA key (normally 1024 bits)
used to identify
the host. Additionally, when the daemon starts, it
server RSA key (normally 768 bits). This key is normally
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
database to verify that it has not changed. The client then
256-bit random number. It encrypts this random number using
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
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.
Next, the server and the client enter an authentication dialog. The
client tries to authenticate itself using rhosts authentication combined
with RSA host authentication, RSA challenge-response authentication, or
password based authentication.
System security is not improved unless rshd, rlogind, and
rexecd are disabled
(thus completely disabling rlogin and rsh into the machine).
SSH protocol version 2 [Toc] [Back]
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
key agreement. This key agreement results in a
The rest of the session is encrypted using a symmetric cipher, currently
128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES,
AES. The client selects the encryption algorithm to use
from those offered
by the server. Additionally, session integrity is
a cryptographic message authentication code (hmac-sha1 or
Protocol version 2 provides a public key based user (PubkeyAuthentication)
or client host (HostbasedAuthentication) authentication method,
conventional password authentication and challenge response
Command execution and data forwarding [Toc] [Back]
If the client successfully authenticates itself, a dialog
the session is entered. At this time the client may request
allocating a pseudo-tty, forwarding X11 connections, forwarding TCP/IP
connections, or forwarding the authentication agent connection over the
Finally, the client either requests a shell or execution of
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
When the user program terminates and all forwarded X11 and
have been closed, the server sends command exit status
client, and both sides exit.
sshd can be configured using command-line options or a configuration file
(by default sshd_config(5)). Command-line options override
in the configuration file.
sshd rereads its configuration file when it receives a
SIGHUP, by executing itself with the name and options it was
with, e.g., /usr/sbin/sshd.
The options are as follows:
-4 Forces sshd to use IPv4 addresses only.
-6 Forces sshd to use IPv6 addresses only.
Specifies the number of bits in the ephemeral protocol version 1
server key (default 768).
-D When this option is specified, sshd will not detach
and does not
become a daemon. This allows easy monitoring of
-d Debug mode. The server sends verbose debug output
to the system
log, and does not put itself in the background. The
will not fork and will only process one connection.
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.
Specifies the name of the configuration file. The
/etc/ssh/sshd_config. sshd refuses to start if
there is no configuration
Gives the grace time for clients to authenticate
120 seconds). If the client fails to authenticate the user
within this many seconds, the server disconnects and
value of zero indicates no limit.
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
/etc/ssh/ssh_host_key for protocol version 1, and
/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.
-i Specifies that sshd is being run from inetd(8).
sshd is normally
not run from inetd because it needs to generate the
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
using sshd from inetd may be feasible.
Specifies how often the ephemeral protocol version 1
is regenerated (default 3600 seconds, or one hour).
for regenerating the key fairly often is that
the key is not
stored anywhere, and after about an hour it becomes
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.
Can be used to give options in the format used in
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 sshd_config(5).
Specifies the port on which the server listens for
(default 22). Multiple port options are permitted.
in the configuration file are ignored when a
port is specified.
-q Quiet mode. Nothing is sent to the system log.
Normally the beginning,
authentication, and termination of each
-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
name is longer than 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.
indicates that only dotted decimal addresses should
be put into
the utmp file. -u0 may also be used to prevent sshd
DNS requests unless the authentication mechanism or
requires it. Authentication mechanisms that may require DNS include
HostbasedAuthentication and using
a from="pattern-list" option in a key file. Configuration options
that require DNS include using a USER@HOST
AllowUsers or DenyUsers.
sshd reads configuration data from /etc/ssh/sshd_config (or
specified with -f on the command line). The file format and
options are described in sshd_config(5).
When a user successfully logs in, sshd does the following:
1. If the login is on a tty, and no command has been
prints last login time and /etc/motd (unless prevented in the
configuration file or by $HOME/.hushlogin; see
the FILES section).
2. If the login is on a tty, records login time.
3. Checks /etc/nologin; if it exists, prints contents and quits
4. Changes to run with normal user privileges.
5. Sets up basic environment.
6. Reads the file $HOME/.ssh/environment, if it exists, and users
are allowed to change their environment. See the
PermitUserEnvironment option in sshd_config(5).
7. Changes to user's home directory.
8. If $HOME/.ssh/rc exists, runs it; else if
runs it; otherwise runs xauth. The ``rc''
given the X11 authentication protocol and cookie
9. Runs user's shell or command.
AUTHORIZED_KEYS FILE FORMAT [Toc] [Back]
$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
AuthorizedKeysFile may be used to specify an alternative
Each line of the file contains one key (empty lines and
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:
base64 encoded key, comment. The options field is optional; its
presence is determined by whether the line starts with a
number or not
(the options field never starts with a number). The bits,
and comment fields give the RSA key for protocol version 1; the comment
field is not used for anything (but may be convenient
for the user
to identify the key). For protocol version 2 the keytype is
Note that lines in this file are usually several hundred
bytes long (because
of the size of the public key encoding). You don't
want to type
them in; instead, copy the identity.pub, id_dsa.pub or the
file and edit it.
sshd enforces a minimum RSA key modulus size for protocol 1
2 keys of 768 bits.
The options (if present) consist of comma-separated option
No spaces are permitted, except within double
quotes. The following
option specifications are supported (note that option
Specifies that in addition to public key authentication, the
canonical name of the remote host must be present in
list of patterns (`*' and `?' serve as
list may also contain patterns negated by prefixing
`!'; if the canonical host name matches a negated
key is not accepted. The purpose of this option is
increase security: public key 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
makes using a stolen key more difficult (name
routers would have to be compromised in addition to
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
pty; otherwise it is run without a tty. If an 8-bit
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
backslash. This option might be useful to restrict
keys to perform just a specific operation. An
be a key that permits remote backups but nothing
else. Note that
the client may specify TCP/IP and/or X11 forwarding
are explicitly prohibited. Note that this option
shell, command or subsystem execution.
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
this type are permitted. Environment processing is
default and is controlled via the
This option is automatically disabled if UseLogin is
Forbids TCP/IP forwarding when this key is used for
Any port forward requests by the client will
return an error.
This might be used, e.g., in connection with
Forbids X11 forwarding when this key is used for authentication.
Any X11 forward requests by the client will return
Forbids authentication agent forwarding when this
key is used for
no-pty Prevents tty allocation (a request to allocate a pty
Limit local ``ssh -L'' port forwarding such that it
may only connect
to the specified host and port. IPv6 addresses
can be specified
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 domains
Examples [Toc] [Back]
1024 33 12121...312314325 firstname.lastname@example.org
from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334
command="dump /home",no-pty,no-port-forwarding 1024 33
permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33
SSH_KNOWN_HOSTS FILE FORMAT [Toc] [Back]
The /etc/ssh/ssh_known_hosts and $HOME/.ssh/known_hosts
host public keys for all known hosts. The global file
should be prepared
by the administrator (optional), and the per-user file is
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:
exponent, modulus, comment. The fields are separated by
Hostnames is a comma-separated list of patterns (`*' and `?'
act as wildcards);
each pattern in turn is matched against the canonical host name
(when authenticating a client) or against the user-supplied
authenticating a server). A pattern may also be preceded by
`!' to indicate
negation: if the host name matches a negated pattern,
it is not accepted
(by that line) even if it matched another pattern on
Bits, exponent, and modulus are taken directly from the RSA
they can be obtained, e.g., from /etc/ssh/ssh_host_key.pub.
comment field continues to the end of the line, and is not
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 recommended)
to have several lines or different host keys for the
This will inevitably happen when short forms of host names
domains are put in the file. It is possible that the files
information; authentication is accepted if valid
be found from either file.
Note that the lines in these files are typically hundreds of
long, and you definitely don't want to type in the host keys
Rather, generate them by a script or by taking
and adding the host names at the front.
Examples [Toc] [Back]
closenet,...,18.104.22.168 1024 37 159...93 closenet.hut.fi
cvs.openbsd.org,22.214.171.124 ssh-rsa AAAA1234.....=
Contains configuration data for sshd. The file format and configuration
options are described in sshd_config(5).
These three files contain the private parts of the
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.
These three files contain the public parts of the
These files should be world-readable but writable
only by root.
Their contents should match the respective private
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).
Contains Diffie-Hellman groups used for the "DiffieHellman Group
Exchange". The file format is described in moduli(5).
chroot(2) directory used by sshd during privilege
the pre-authentication phase. The directory should
any files and must be owned by root and not group or
Contains the process ID of the sshd listening for
there are several daemons running concurrently for
ports, this contains the process ID of the one
The content of this file is not sensitive; it can be
Lists the public keys (RSA or DSA) that can be used
to log into
the user's account. This file must be readable by
may on some machines imply it being world-readable
if the user's
home directory resides on an NFS volume). It is
it not be accessible by others. The format of this
file is described
above. Users will place the contents of
identity.pub, id_dsa.pub and/or id_rsa.pub files into this file,
as described in ssh-keygen(1).
These files are consulted when using rhosts with RSA
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
files should be writable only by root/the owner.
/etc/ssh/ssh_known_hosts should be world-readable,
$HOME/.ssh/known_hosts can, but need not be, worldreadable.
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
Access controls that should be enforced by tcp-wrappers are defined
here. Further details are described in
This file is used during RhostsRSAAuthentication and
HostbasedAuthentication and contains host-username
by a space, one per line. The given user on
host is permitted to log in without a password. The
same file is used by rlogind and rshd. The file
must be writable
only by the user; it is recommended that it not be
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.
For ssh, this file is exactly the same as for
this file is not used by rlogin and rshd, so using
access using SSH only.
This file is used during RhostsRSAAuthentication and
HostbasedAuthentication authentication. In the simplest form,
this file contains host names, one per line. Users
hosts are permitted to log in without a password,
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.
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
client host key
authentication is 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
hosts.equiv. Beware that it really means that the
can log in as anybody, which includes bin, daemon,
adm, and other
accounts that own critical binaries and directories.
Using a user
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.
This is processed exactly as /etc/hosts.equiv. However, this
file may be useful in environments that want to run
rsh/rlogin and ssh.
This file is read into the environment at login (if
It can only contain empty lines, comment lines (that
`#'), and assignment lines of the form name=value.
should be writable only by the user; it need not be
anyone else. Environment processing is disabled by
is controlled via the PermitUserEnvironment option.
If this file exists, it is run with /bin/sh after
reading the environment
files but before starting the user's shell
It must not produce any output on stdout; stderr
must be used instead.
If X11 forwarding is in use, it will receive
cookie" pair in its standard input (and DISPLAY in
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
accessible; AFS is a particular example of
such an environment.
This file will probably contain some initialization
by something similar to:
if read proto cookie && [ -n "$DISPLAY" ]; then
if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
echo add unix:`echo $DISPLAY |
cut -c11-` $proto $cookie
echo add $DISPLAY $proto $cookie
fi | xauth -q -
If this file does not exist, /etc/ssh/sshrc is run,
and if that
does not exist either, xauth is used to add the
This file should be writable only by the user, and
need not be
readable by anyone else.
Like $HOME/.ssh/rc. This can be used to specify machine-specific
login-time initializations globally. This file
writable only by root, and should be world-readable.
scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
chroot(2), hosts_access(5), login.conf(5), moduli(5),
T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, SSH
Protocol Architecture, draft-ietf-secsh-architecture-12.txt,
2002, work in progress material.
M. Friedl, N. Provos, and W. A. Simpson, Diffie-Hellman
for the SSH Transport Layer Protocol, draft-ietf-secsh-dhgroupexchange-02.txt,
January 2002, work in progress material.
OpenSSH is a derivative of the original and free ssh 1.2.12
Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels
de Raadt and Dug Song removed many bugs, re-added newer features and
created OpenSSH. Markus Friedl contributed the support for
versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
for privilege separation.
OpenBSD 3.6 September 25, 1999
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