||FreeBSD System Manager's Manual
sshd (OpenSSH Daemon) is the daemon program for
provides secure encrypted communications between two untrusted hosts over an
sshd listens for connections from clients.
It is normally started at boot from /etc/rc.d/sshd.
It forks a new daemon for each incoming connection. The forked daemons
handle key exchange, encryption, authentication, command execution, and data
sshd can be configured using command-line
options or a configuration file (by default
command-line options override values specified in the configuration file.
sshd rereads its configuration file when it receives
a hangup signal,
SIGHUP, by executing itself with
the name and options it was started with, e.g.
The options are as follows:
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 secrecy 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. 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 (MAC).
sshd to use IPv4 addresses only.
sshd to use IPv6 addresses only.
- 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
option or the
- When this option is specified,
sshd will not
detach and does not become a daemon. This allows easy monitoring of
- Debug mode. The server sends verbose debug output to standard error, and
does not put itself in the background. The server also will not
and will only process one connection. This option is only intended for
debugging for the server. Multiple
increase the debugging level. Maximum is 3.
- Append debug logs to log_file instead of the system
- Write debug logs to standard error instead of the system log.
- Specifies the name of the configuration file. The default is
refuses to start if there is no configuration file.
- 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_rsa_key. It is possible to have
multiple host key files for the different host key algorithms.
- 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
Port option are
ignored when a command-line port is specified. Ports specified using the
ListenAddress option override command-line
- 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
reliably as configuration options may change.
- 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.
-u0 indicates that only dotted decimal
addresses should be put into the utmp file.
-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
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
Regardless of the authentication type, the account is checked to
ensure that it is accessible. An account is not accessible if it is locked,
DenyUsers or its group is listed in
DenyGroups . The definition of a locked account is
system dependent. Some platforms have their own account database (eg AIX)
and some modify the passwd field (
*LK*’ on Solaris and UnixWare,
*’ on HP-UX, containing
Nologin’ on Tru64, a leading
*LOCKED*’ 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
After this, the client either requests an interactive shell or
execution or a non-interactive command, which
will execute via the user's shell using its
option. 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 sides exit.
When a user successfully logs in,
sshd does the
If the file ~/.ssh/rc exists,
sh(1) 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
- 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; otherwise runs
The “rc” files are given the X11 authentication protocol and
cookie in standard input. See SSHRC,
- Runs user's shell or command. All commands are run under the user's login
shell as specified in the system password database.
in its environment). The script must call
sshd will not run xauth automatically to add
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 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 cookie.
AuthorizedKeysFile specifies the files containing public
keys for public key authentication; if this option is not specified, the
default is ~/.ssh/authorized_keys and
~/.ssh/authorized_keys2. 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 supported key
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 RSA keys up to 16 kilobits. You don't want to type
them in; instead, copy the id_dsa.pub,
id_ed25519_sk.pub, or the
id_rsa.pub file and edit it.
sshd 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
- 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
SSH_ORIGINAL_COMMAND environment 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 addresses, a
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 key).
- 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
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
options may be applied separated by commas. No pattern matching or name
lookup is performed on the specified hostnames, they must be literal host
names and/or addresses. A port specification of
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 the
- Permits tty allocation previously disabled by the
- Do not require demonstration of user presence for signatures made using
this key. This option only makes sense for the FIDO authenticator
- Require that signatures made using this key attest that they verified the
user, e.g. via a PIN. This option only makes sense for the FIDO
- 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 this set.
- 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
- Permits X11 forwarding previously disabled by the
An example authorized_keys file:
The /etc/ssh/ssh_known_hosts and
~/.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 to an unknown host, its key is added to the per-user file.
# Comments are allowed at start of line. Blank lines are allowed.
# Plain key, no restrictions
# Forced command, disable PTY and all forwarding
restrict,command="dump /home" ssh-rsa ...
# Restriction of ssh -L forwarding destinations
permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-rsa ...
# Restriction of ssh -R forwarding listeners
permitlisten="localhost:8080",permitlisten="[::1]:22000" ssh-rsa ...
# Configuration for tunnel forwarding
tunnel="0",command="sh /etc/netstart tun0" ssh-rsa ...
# Override of restriction to allow PTY allocation
restrict,pty,command="nethack" ssh-rsa ...
# Allow FIDO key without requiring touch
no-touch-required firstname.lastname@example.org ...
# Require user-verification (e.g. PIN or biometric) for FIDO key
verify-required email@example.com ...
# Trust CA key, allow touch-less FIDO if requested in certificate
cert-authority,no-touch-required,principals="user_a" ssh-rsa ...
Each line in these files contains the following fields: marker
(optional), hostnames, keytype, base64-encoded key, comment. The fields are
separated by spaces.
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
authenticating a client, such as when using
HostbasedAuthentication, this will be the canonical
client host name. When
authenticating a server, this will be the host name given by the user, the
value of the
HostkeyAlias if it was specified, or the canonical
server hostname if the
CanonicalizeHostname option was used.
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 the line. A hostname or address may optionally be
enclosed within ‘
]’ brackets then followed by
:’ and a non-standard port
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 ‘
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
/etc/ssh/ssh_host_rsa_key.pub. The optional comment
field continues to the end of the line, and is not used.
Lines starting with ‘
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,
/etc/ssh/ssh_host_rsa_key.pub and adding the host
names at the front.
also offers some basic automated editing for
~/.ssh/known_hosts 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...
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 separation.
- 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 unless the
StrictModes option has been set to
- 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
- 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
sshd. 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 world-writable.
- 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-readable.
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