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sshd - OpenSSH SSH daemon
sshd [-46Ddeiqt] [-b bits] [-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(1) and rsh(1), and provide secure encrypted
communications between two untrusted hosts over an insecure network.
sshd 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, encryption, authentication,
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
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. /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 sshd.
-d Debug mode. The server sends verbose debug output to the system
log, and does not put itself in the background. The server also
will not fork and will only process one connection. This option
is only intended for debugging for the server. 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 default is
/etc/ssh/sshd_config. 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 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_rsa_key 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
-i Specifies that sshd is being run from inetd(8). sshd is normally
not run from inetd because it needs to generate the server key
before it can respond to the client, and this may take tens of
seconds. Clients would have to wait too long if the key was
regenerated every time. However, with small key sizes (e.g. 512)
using sshd from inetd may be feasible.
Specifies how often the ephemeral protocol version 1 server key
is regenerated (default 3600 seconds, or one hour). The
motivation 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
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 sshd_config(5).
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 ports.
-q Quiet mode. Only fatal errors are sent to the system log.
Normally the beginning, authentication, and termination of each
connection is logged. If a second -q is given then nothing is
sent to the system log.
-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 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 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. Both protocols
are supported by default, though this can be changed via the Protocol
option in sshd_config(5). Protocol 2 supports both RSA and DSA keys;
protocol 1 only supports RSA keys. For both protocols, each host has a
host-specific key, normally 2048 bits, used to identify the host.
Forward security for protocol 1 is provided through an additional server
key, normally 768 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
generates a 256-bit random number. It encrypts this random number using
both the host key and the server key, and sends the encrypted number to
the server. Both sides then use this random number as a session key
which is used to encrypt all further communications in the session. The
rest of the session is encrypted using a conventional cipher, currently
Blowfish or 3DES, with 3DES being used by default. The client selects
the encryption 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
cryptographic message authentication code (hmac-md5, hmac-sha1, umac-64
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,
listed in DenyUsers or its group is listed in DenyGroups . The
definition 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*’ ).
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 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 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, runs it; else if /etc/ssh/sshrc exists,
runs it; otherwise runs xauth. The “rc” files are given the
X11 authentication protocol and cookie in standard input. See
9. Runs user’s shell or command.
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 DISPLAY in its environment). The script must call
xauth(1) because sshd will not run xauth automatically to add X11
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.
AUTHORIZED_KEYS FILE FORMAT
AuthorizedKeysFile specifies the file containing public keys for public
key authentication; if none is specified, the default is
~/.ssh/authorized_keys. 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. Protocol 2 public key consist of:
options, keytype, 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,
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 “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
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
identity.pub, id_dsa.pub, or the id_rsa.pub file and edit it.
sshd enforces a minimum RSA key modulus size for protocol 1 and protocol
2 keys of 768 bits.
The options (if present) consist of comma-separated option
specifications. No spaces are permitted, except within double quotes.
The following option specifications are supported (note that option
keywords are case-insensitive):
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. The command originally
supplied by the client is available in the SSH_ORIGINAL_COMMAND
environment variable. Note that this option applies to 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 options of
this type are permitted. Environment processing is disabled by
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, the
canonical name of the remote host must be present in the comma-
separated list of patterns. 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).
See PATTERNS in ssh_config(5) for more information on patterns.
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 ‘‘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 or addresses.
Force a tun(4) device on the server. Without this option, the
next available device will be used if the client requests a
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...==
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
automatically: whenever the user connects from an unknown host, its key
is added to the per-user file.
Each line in these files contains the following fields: hostnames, bits,
exponent, modulus, comment. The fields are separated by spaces.
Hostnames is a comma-separated list of patterns (‘*’ and ‘?’ act as
wildcards); 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
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 ‘[’ and ‘]’
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 a ‘|’ character. Only one hashed hostname may
appear on a single line and none of the above negation or wildcard
operators may be applied.
Bits, exponent, and modulus are taken directly from the RSA host key;
they can be obtained, for example, from /etc/ssh/ssh_host_key.pub. 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. It is thus 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 /etc/ssh/ssh_host_key.pub
and adding the host names at the front.
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
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
more information). On some machines this file may need to be
world-readable if the user’s home directory is on an NFS
partition, because 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
This file is used in exactly the same way as .rhosts, but allows
host-based authentication without permitting login with
Lists the public keys (RSA/DSA) 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
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
“no”. The recommended permissions can be set by executing “chmod
go-w ~/ ~/.ssh ~/.ssh/authorized_keys”.
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". The file format is described in moduli(5).
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
These three files contain the private parts of the host keys.
These files should only be owned by root, readable only by root,
and not accessible to others. Note that sshd does not start if
these files are group/world-accessible.
These three files contain the public parts of the host keys.
These files should be world-readable but writable only by root.
Their contents should match the respective private 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 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
configuration 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-
scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
ssh-keyscan(1), chroot(2), hosts_access(5), login.conf(5), moduli(5),
sshd_config(5), inetd(8), sftp-server(8)
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.
System security is not improved unless rshd, rlogind, and rexecd are
disabled (thus completely disabling rlogin and rsh into the machine).