OpenSSH SSH daemon
(OpenSSH Daemon) is the daemon program
Together these programs replace rlogin and rsh, and provide secure encrypted
communications between two untrusted hosts over an insecure network.
listens for connections from clients. It
is normally started at boot from
. It forks a new daemon for
each incoming connection. The forked daemons handle key exchange, encryption,
authentication, command execution, and data exchange.
can be configured using command-line
options or a configuration file (by default
command-line options override values specified in the configuration file.
rereads its configuration file when it
receives a hangup signal,
executing itself with the name and options it was started with, e.g.
The options are as follows:
sshd to use IPv4 addresses
sshd to use IPv6 addresses
- Specify the connection parameters to use for the
-T extended test mode. If provided, any
Match directives in the configuration
file that would apply are applied before the configuration is written to
standard output. The connection parameters are supplied as keyword=value
pairs and may be supplied in any order, either with multiple
-C options or as a comma-separated
list. The keywords are “addr,” “user”,
“host”, “laddr”, “lport”, and
“rdomain” and correspond to source address, user, resolved
source host name, local address, local port number and routing domain
- Specifies a path to a certificate file to identify
sshd during key exchange. The
certificate file must match a host key file specified using the
-h option or the
HostKey configuration directive.
- When this option is specified,
will not detach and does not become a daemon. This allows easy monitoring
- Debug mode. The server sends verbose debug output to standard error, 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
options increase the debugging level. Maximum is 3.
- Append debug logs to log_file instead of
the system log.
- Write debug logs to standard error instead of the system log.
- Specifies the name of the configuration file. The default is
sshd refuses to start if there is no
- Gives the grace time for clients to authenticate themselves (default 120
seconds). If the client fails to authenticate the user within this many
seconds, the server disconnects and exits. A value of zero indicates no
- Specifies a file from which a host key is read. This option must be given
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_ecdsa_key,
/etc/ssh/ssh_host_rsa_key. It is
possible to have multiple host key files for the different host key
- Specifies that
sshd is being run from
- 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
- Specifies the port on which the server listens for connections (default
22). Multiple port options are permitted. Ports specified in the
configuration file with the
are ignored when a command-line port is specified. Ports specified using
ListenAddress option override
- Quiet mode. Nothing is sent to the system log. Normally the beginning,
authentication, and termination of each connection is logged.
- Extended test mode. Check the validity of the configuration file, output
the effective configuration to stdout and then exit. Optionally,
Match rules may be applied by
specifying the connection parameters using one or more
- 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
- 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 overflow
this field to still be uniquely identified. Specifying
-u0 indicates that only dotted decimal
addresses should be put into the utmp
-u0 may also be used to prevent
sshd from making DNS requests unless
the authentication mechanism or configuration 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 pattern in
The OpenSSH SSH daemon supports SSH protocol 2 only. Each host has a
host-specific key, used to identify the host. Whenever a client connects, the
daemon responds with its public host key. The client compares the host key
against its own database to verify that it has not changed. 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-md5,
hmac-sha1, umac-64, umac-128, hmac-sha2-256 or hmac-sha2-512).
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.
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
or its group is listed in
. The definition of a locked
account is system dependent. Some platforms have their own account database
(eg AIX) and some modify the passwd field (
’ on Solaris and UnixWare,
’ on HP-UX, containing
’ on Tru64, a leading
’ on FreeBSD and a leading
’ on most Linuxes). 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 ‘
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.
After this, 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 connections
have been closed, the server sends command exit status to the client, and both
When a user successfully logs in,
- 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
~/.hushlogin; see the
- If the login is on a tty, records login time.
- Checks /etc/nologin and
/var/run/nologin; if one exists, it
prints the contents and quits (unless root).
- Changes to run with normal user privileges.
- Sets up basic environment.
- Reads the file ~/.ssh/environment, if
it exists, and users are allowed to change their environment. See the
PermitUserEnvironment option in
- Changes to user's home directory.
- If ~/.ssh/rc exists and the
PermitUserRC option is set, runs it;
else if /etc/ssh/sshrc exists, runs it;
The “rc” files are given the X11 authentication protocol and
cookie in standard input. See
- Runs user's shell or command. All commands are run under the user's login
shell as specified in the system password database.
If the file ~/.ssh/rc
runs it after reading the environment files but before starting the user's
shell or command. 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
in its environment). The script
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 becomes accessible; AFS is a
particular example of such an environment.
This file will probably contain some initialization code followed by something
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 cookie.
specifies the files
containing public keys for public key authentication; if this option is not
specified, the default is
. Each line of the
file contains one key (empty lines and lines starting with a
’ are ignored as comments). Public
keys consist of the following space-separated fields: options, keytype,
base64-encoded key, comment. The options field is optional. The keytype is
“ssh-dss” or “ssh-rsa”; the comment field is not
used for anything (but may be convenient for the user to identify the key).
Note that lines in this file can be several hundred bytes long (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 id_dsa.pub
, or the
file and edit it.
enforces a minimum RSA key modulus size
of 1024 bits.
The options (if present) consist of comma-separated option specifications. No
spaces are permitted, except within double quotes. The following option
specifications are supported (note that option keywords are case-insensitive):
- Enable authentication agent forwarding previously disabled by the
- Specifies that the listed key is a certification authority (CA) that is
trusted to validate signed certificates for user authentication.
Certificates may encode access restrictions similar to these key options. If
both certificate restrictions and key options are present, the most
restrictive union of the two is applied.
- 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 an 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 backslash.
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 and/or X11
forwarding unless they are explicitly prohibited, e.g. using the
restrict key option.
The command originally supplied by the client is available in the
variable. Note that this option applies to shell, command or subsystem
execution. Also note that this command may be superseded by a
If a command is specified and a forced-command is embedded in a certificate
used for authentication, then the certificate will be accepted only if the
two commands are identical.
- 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.
Environment processing is disabled by default and is controlled via the
- Specifies a time after which the key will not be accepted. The time may be
specified as a YYYYMMDD date or a YYYYMMDDHHMM[SS] time in the system
- Specifies that in addition to public key authentication, either the
canonical name of the remote host or its IP address must be present in the
comma-separated list of patterns. See PATTERNS in
for more information on patterns.
In addition to the wildcard matching that may be applied to hostnames or
from stanza may match IP
addresses using CIDR address/masklen notation.
The purpose of this option is to optionally 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
additional option makes using a stolen key more difficult (name servers
and/or routers would have to be compromised in addition to just the
- Forbids authentication agent forwarding when this key is used for
- 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
- Prevents tty allocation (a request to allocate a pty will fail).
- Disables execution of ~/.ssh/rc.
- Forbids X11 forwarding when this key is used for authentication. Any X11
forward requests by the client will return an error.
- Limit remote port forwarding with the
-R option such that it may only listen
on the specified host (optional) and port. IPv6 addresses can be specified
by enclosing the address in square brackets. Multiple
permitlisten options may be applied
separated by commas. Hostnames may include wildcards as described in the
PATTERNS section in
A port specification of
* matches any
port. Note that the setting of
GatewayPorts may further restrict
listen addresses. Note that
will send a hostname of “localhost” if a listen host was not
specified when the forwarding was requested, and that this name is treated
differently to the explicit localhost addresses “127.0.0.1”
- Limit local port forwarding with the
-L option such that it may only connect
to the specified host and port. IPv6 addresses can be specified by
enclosing the address in square brackets. Multiple
permitopen options may be applied
separated by commas. No pattern matching is performed on the specified
hostnames, they must be literal domains or addresses. A port specification
* matches any port.
- Enable port forwarding previously disabled by the
- On a
cert-authority line, specifies
allowed principals for certificate authentication as a comma-separated
list. At least one name from the list must appear in the certificate's
list of principals for the certificate to be accepted. This option is
ignored for keys that are not marked as trusted certificate signers using
- Permits tty allocation previously disabled by the
- Enable all restrictions, i.e. disable port, agent and X11 forwarding, as
well as disabling PTY allocation and execution of
~/.ssh/rc. If any future restriction
capabilities are added to authorized_keys files they will be included in
- Force a
device on the server. Without this option, the next available device will
be used if the client requests a tunnel.
- Enables execution of ~/.ssh/rc
previously disabled by the
- Permits X11 forwarding previously disabled by the
An example authorized_keys file:
# Comments allowed at start of line
ssh-rsa AAAAB3Nza...LiPk== firstname.lastname@example.org
command="dump /home",no-pty,no-port-forwarding ssh-rsa
tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
restrict,command="uptime" ssh-rsa AAAA1C8...32Tv==
restrict,pty,command="nethack" ssh-rsa AAAA1f8...IrrC5==
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 to an unknown host, its key is added to the
Each line in these files contains the following fields: markers (optional),
hostnames, keytype, base64-encoded key, comment. The fields are separated by
The marker is optional, but if it is present then it must be one of
“@cert-authority”, to indicate that the line contains a
certification authority (CA) key, or “@revoked”, to indicate
that the key contained on the line is revoked and must not ever be accepted.
Only one marker should be used on a key line.
Hostnames is a comma-separated list of patterns
’ act as wildcards); each pattern in
turn is matched against the host name. When
is authenticating a client, such as
will be the canonical client host name. When
is authenticating a server, this will be the host name given by the user, the
value of the
if it was specified, or the
canonical server hostname if the
option was used.
A pattern may also be preceded by ‘
indicate negation: if the host name matches a negated pattern, it is not
accepted (by that line) even if it matched another pattern on the line. A
hostname or address may optionally be enclosed within
’ brackets then followed by
’ and a non-standard port number.
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
’ character. Only one hashed
hostname may appear on a single line and none of the above negation or
wildcard operators may be applied.
The keytype and base64-encoded key are taken directly from the host key; they
can be obtained, for example, from
. 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; either one that matches exactly or, if the server has
presented a certificate for authentication, the key of the certification
authority that signed the certificate. For a key to be trusted as a
certification authority, it must use the “@cert-authority”
marker described above.
The known hosts file also provides a facility to mark keys as revoked, for
example when it is known that the associated private key has been stolen.
Revoked keys are specified by including the “@revoked” marker at
the beginning of the key line, and are never accepted for authentication or as
certification authorities, but instead will produce a warning from
when they are encountered.
It is permissible (but not recommended) 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 conflicting 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, for example,
the host names at the front.
also offers some basic automated editing for
including removing hosts
matching a host name and converting all host names to their hashed
An example ssh_known_hosts file:
# Comments allowed at start of line
closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
# A hashed hostname
# A revoked key
@revoked * ssh-rsa AAAAB5W...
# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
- This file is used to suppress printing the last login time and
PrintMotd, respectively, are enabled.
It does not suppress printing of the banner specified by
- This file is used for host-based authentication (see
for more information). On some machines this file may need to be
world-readable if the user's home directory is on an NFS partition,
sshd reads it as root.
Additionally, this file must be owned by the user, and must not have write
permissions for anyone else. The recommended permission for most machines
is read/write for the user, and not accessible by others.
- This file is used in exactly the same way as
.rhosts, but allows host-based
authentication without permitting login with rlogin/rsh.
- This directory is the default location for all user-specific configuration
and authentication information. There is no general requirement to keep
the entire contents of this directory secret, but the recommended
permissions are read/write/execute for the user, and not accessible by
- Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used for
logging in as this user. The format of this file is described above. The
content of the file is not highly sensitive, but the recommended
permissions are read/write for the user, and not accessible by others.
If this file, the ~/.ssh directory, or
the user's home directory are writable by other users, then the file could
be modified or replaced by unauthorized users. In this case,
sshd will not allow it to be used
StrictModes option has been
set to “no”.
- 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. Environment processing is disabled by default
and is controlled via the
- Contains a list of host keys for all hosts the user has logged into that
are not already in the systemwide list of known host keys. The format of
this file is described above. This file should be writable only by
root/the owner and can, but need not be, world-readable.
- Contains initialization routines to be run before the user's home
directory becomes accessible. This file should be writable only by the
user, and need not be readable by anyone else.
- Access controls that should be enforced by tcp-wrappers are defined here.
Further details are described in
- This file is for host-based authentication (see
It should only be writable by root.
- Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
Exchange" key exchange method. The file format is described in
If no usable groups are found in this file then fixed internal groups will
- 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.
- This file is used in exactly the same way as
hosts.equiv, but allows host-based
authentication without permitting login with rlogin/rsh.
- These 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 these files are group/world-accessible.
- These 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
- Systemwide list of known host keys. This file should be prepared by the
system administrator to contain the public host keys of all machines in
the organization. The format of this file is described above. This file
should be writable only by root/the owner and should be world-readable.
- Contains configuration data for
The file format and configuration options are described in
- Similar to ~/.ssh/rc, it can be used to
specify machine-specific login-time initializations globally. This file
should be writable only by root, and should be world-readable.
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
- Contains the process ID of the
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
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 created OpenSSH.
Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0.
Niels Provos and Markus Friedl contributed support for privilege