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(OpenSSH Daemon) is the daemon program for 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, by executing itself
with the name and options it was started with, e.g. The options are as
follows: Forces to use IPv4 addresses only. Forces 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 extended test mode. If provided, any 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 output. The connection
parameters are supplied as keyword=value pairs. The keywords are and
All are required and may be supplied in any order, either with multiple
options or as a comma-separated list. Specifies a path to a certificate
file to identify during key exchange. The certificate file must match a
host key file specified using the option or the configuration directive.
When this option is specified, 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 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 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 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 limit. Specifies a file from which
a host key is read. This option must be given if is not run as root (as
the normal host key files are normally not readable by anyone but root).
The default is for protocol version 1, and and for protocol version
2. It is possible to have multiple host key files for the different protocol
versions and host key algorithms. Specifies that is being run from 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 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 regenerated. 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 option are ignored when a command-line port is specified. Ports specified
using the option override command-line ports. 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, rules may be applied by specifying the connection
parameters using one or more options. 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 structure that holds the remote host name.
If the resolved host name is longer than 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 indicates that only
dotted decimal addresses should be put into the file. may also be used
to prevent from making DNS requests unless the authentication mechanism
or configuration requires it. Authentication mechanisms that may require
DNS include and using a option in a key file. Configuration options that
require DNS include using a USER@HOST pattern in or
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 option in Protocol
2 supports DSA, ECDSA, ED25519 and RSA 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, 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, 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 dependant. 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 or ). 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 sides exit.
When a user successfully
logs in, does the following: If the login is on a tty, and no command
has been specified, prints last login time and (unless prevented in the
configuration file or by see the section). If the login is on a tty,
records login time. Checks if it exists, prints contents and quits (unless
root). Changes to run with normal user privileges. Sets up basic environment.
Reads the file if it exists, and users are allowed to change their environment.
See the option in Changes to user’s home directory. If exists and the
option is set, runs it; else if exists, runs it; otherwise runs xauth.
The files are given the X11 authentication protocol and cookie in standard
input. See below. Runs user’s shell or command.
If the file exists,
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
must call because 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
# X11UseLocalhost=yes
echo add unix:‘echo $DISPLAY |
cut -c11-‘ $proto $cookie
else
# X11UseLocalhost=no
echo add $DISPLAY $proto $cookie
fi | xauth -q -
fi If this file does not exist, 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 none
is specified, the default is and 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 or 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 or the file and edit it. 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 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 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 environment
variable. Note that this option applies to shell, command or subsystem execution.
Also note that this command may be superseded by either a directive or
a command embedded in a certificate. 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 option. This option is automatically disabled
if 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 for more
information on patterns. In addition to the wildcard matching that may
be applied to hostnames or addresses, a 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 authentication. 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 option. Prevents tty allocation
(a request to allocate a pty will fail). Disables execution of 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
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 is performed
on the specified hostnames, they must be literal domains or addresses. A
port specification of matches any port. On a 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 option. Force
a device on the server. Without this option, the next available device
will be used if the client requests a tunnel. An example authorized_keys
file: # Comments allowed at start of line ssh-rsa AAAAB3Nza...LiPk== user@example.net
from="*.sales.example.net,!pc.sales.example.net" ssh-rsa AAAAB2...19Q== john@example.net
command="dump /home",no-pty,no-port-forwarding ssh-dss AAAAC3...51R== example.net
permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss AAAAB5...21S== tunnel="0",command="sh
/etc/netstart tun0" ssh-rsa AAAA...== jane@example.net
The and 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: 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 to indicate that
the line contains a certification authority (CA) key, or 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 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 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 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 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 and adding 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 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 |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM=
ssh-rsa AAAA1234.....= # 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 if and 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,
because 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 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 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 sensitive,
but the recommended permissions are read/write for the user, and not accessible
by others. If this file, the 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, will not allow it to be used unless the
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 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 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. 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". The file format is described in See If this file exists,
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 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 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 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 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 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.
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.
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