sshd [-46DdeiqTt] [-b bits] [-C connection_spec]
[-c host_certificate_file] [-E log_file] [-f config_file]
[-g login_grace_time] [-h host_key_file] [-k key_gen_time]
[-o option] [-p port] [-u len]
sshd (OpenSSH Daemon) is the daemon program for ssh(1). Together these
programs replace rlogin and rsh, and provide secure encrypted communica-
tions between two untrusted hosts over an insecure network.
sshd listens for connections from clients. It is normally started at
boot from /etc/init.d/ssh (or /etc/init/ssh.conf on systems using the
Upstart init daemon). It forks a new daemon for each incoming connec-
tion. The forked daemons handle key exchange, encryption, authentica-
tion, command execution, and data exchange.
sshd can be configured using command-line options or a configuration file
(by default sshd_config(5)); command-line options override values speci-
fied 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. /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 1024).
Specify the connection parameters to use for the -T extended test
mode. If provided, any Match directives in the configuration
file that would apply to the specified user, host, and address
will be set before the configuration is written to standard out-
put. The connection parameters are supplied as keyword=value
pairs. The keywords are ``user'', ``host'', ``laddr'',
``lport'', and ``addr''. All are required and may be supplied in
any order, either with multiple -C options or as a comma-sepa-
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 direc-
-D When this option is specified, sshd will not detach and does not
become a daemon. This allows easy monitoring of sshd.
-d Debug mode. The server sends verbose debug output to standard
error, and does not put itself in the background. The server
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 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 default is
/etc/ssh/ssh_host_key for protocol version 1, and
/etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key for
protocol version 2. It is possible to have multiple host key
files for the different protocol versions and host key algo-
-i Specifies that sshd is being run from inetd(8). If SSH protocol
1 is enabled, sshd should not normally be run from inetd because
it needs to generate the server key before it can respond to the
client, and this may take some time. Clients may have to wait
too long if the key was regenerated every time.
Specifies how often the ephemeral protocol version 1 server key
is regenerated (default 3600 seconds, or one hour). The motiva-
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 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 the configura-
tion 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 connections
(default 22). Multiple port options are permitted. Ports speci-
fied 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 ports.
-q Quiet mode. Nothing is sent to the system log. Normally the
beginning, authentication, and termination of each connection is
-T 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 connec-
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 RhostsRSAAuthentication, 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.
The OpenSSH SSH daemon supports SSH protocols 1 and 2. The default is to
use protocol 2 only, though this can be changed via the Protocol option
in sshd_config(5). Protocol 1 should not be used and is only offered to
support legacy devices.
Each host has a host-specific key, used to identify the host. Partial
forward security for protocol 1 is provided through an additional server
key, normally 1024 bits, generated when the server starts. 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 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 gener-
ates 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 encryp-
tion algorithm to use from those offered by the server.
For protocol 2, 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 crypto-
graphic message authentication code (hmac-md5, hmac-sha1, umac-64,
umac-128, hmac-ripemd160, 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, pub-
lic 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,
listed in DenyUsers or its group is listed in DenyGroups . The defini-
tion of a locked account is system dependant. 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 'NP' or '*NP*' ).
tions 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 following:
1. 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 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 ~/.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 ~/.ssh/rc exists and the sshd_config(5) PermitUserRC option
is set, runs it; else if /etc/ssh/sshrc exists, runs it; oth-
erwise runs xauth. The ``rc'' files are given the X11 authen-
tication protocol and cookie in standard input. See SSHRC,
9. Runs user's shell or command. All commands are run under the
user's login shell as specified in the system password data-
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 pro-
duce any output on stdout; stderr must be used instead. If X11 forward-
ing is in use, it will receive the "proto cookie" pair in its standard
input (and DISPLAY 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 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 |
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). Protocol 1 public keys consist of the following
space-separated fields: options, bits, exponent, modulus, comment. Pro-
tocol 2 public key consist of: options, keytype, base64-encoded key, com-
ment. 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, exponent, modulus, 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 ``ecdsa-sha2-nistp256'',
``ecdsa-sha2-nistp384'', ``ecdsa-sha2-nistp521'', ``ssh-ed25519'',
``ssh-dss'' or ``ssh-rsa''.
Note that lines in this file are usually several hundred bytes long
(because of the size of the public key encoding) up to a limit of 8 kilo-
bytes, 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
identity.pub, id_dsa.pub, id_ecdsa.pub, id_ed25519.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 specifica-
tions. No spaces are permitted, except within double quotes. The fol-
lowing 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 authen-
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 chan-
nel 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 cer-
tain public keys to perform just a specific operation. An exam-
ple might be a key that permits remote backups but nothing else.
Note that the client may specify TCP and/or X11 forwarding unless
default and is controlled via the PermitUserEnvironment option.
This option is automatically disabled if UseLogin is enabled.
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
ssh_config(5) for more information on patterns.
In addition to the wildcard matching that may be applied to host-
names 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 command option.
no-pty 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 local port forwarding with ssh(1) -L 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 pat-
tern matching is performed on the specified hostnames, they must
be literal domains or addresses. A port specification of *
matches any port.
Enable port forwarding previously disabled by the restrict
On a cert-authority line, specifies allowed principals for cer-
tificate authentication as a comma-separated list. At least one
authorized_keys files they will be included in this set.
Force a tun(4) device on the server. Without this option, the
next available device will be used if the client requests a tun-
Enables execution of ~/.ssh/rc previously disabled by the
Permits X11 forwarding previously disabled by the restrict
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-dss
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==
SSH_KNOWN_HOSTS FILE FORMAT
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 auto-
matically: 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: markers
(optional), hostnames, bits, exponent, modulus, 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 ('*' and '?' act as wild-
cards); each pattern in turn is matched against the canonical host name
(when authenticating a client) or against the user-supplied name (when
authenticating a server). A pattern may also be preceded by '!' to indi-
cate negation: if the host name matches a negated pattern, it is not
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 authentica-
tion or as certification authorities, but instead will produce a warning
from ssh(1) when they are encountered.
It is permissible (but not recommended) to have several lines or differ-
ent 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, ssh-keyscan(1) or by taking
/etc/ssh/ssh_host_key.pub and adding the host names at the front.
ssh-keygen(1) also offers some basic automated editing for
~/.ssh/known_hosts including removing hosts matching a host name and con-
verting all host names to their hashed representations.
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
/etc/motd, if PrintLastLog and PrintMotd, respectively, are
enabled. It does not suppress printing of the banner specified
This file is used for host-based authentication (see ssh(1) for
This directory is the default location for all user-specific con-
figuration 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 others.
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 sensi-
tive, 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 PermitUserEnvironment option.
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
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 hosts_access(5).
This file is for host-based authentication (see ssh(1)). 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
moduli(5). If no usable groups are found in this file then fixed
internal groups will be used.
allows host-based authentication without permitting login with
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 con-
venience of the user so their contents can be copied to known
hosts files. These files are created using ssh-keygen(1).
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 con-
figuration options are described in sshd_config(5).
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.
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-
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-read-
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