демон OpenSSH (OpenSSH daemon)
Имя (Name)
sshd — OpenSSH daemon
Синопсис (Synopsis)
sshd [-46DdeiqTt] [-C connection_spec] [-c host_certificate_file]
[-E log_file] [-f config_file] [-g login_grace_time]
[-h host_key_file] [-o option] [-p port] [-u len]
Описание (Description)
sshd (OpenSSH Daemon) is the daemon program for ssh(1). It
provides 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.
-C connection_spec
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
respectively.
-c host_certificate_file
Specifies a path to a certificate file to identify sshd
during key exchange. The certificate file must match a
host key file specified using the -h option or the HostKey
configuration directive.
-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 also will not fork(2) 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 log_file
Append debug logs to log_file instead of the system log.
-e Write debug logs to standard error instead of the system
log.
-f config_file
Specifies the name of the configuration file. The default
is /etc/ssh/sshd_config. sshd refuses to start if there is
no configuration file.
-g login_grace_time
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.
-h host_key_file
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_ecdsa_key,
/etc/ssh/ssh_host_ed25519_key and
/etc/ssh/ssh_host_rsa_key. It is possible to have multiple
host key files for the different host key algorithms.
-i Specifies that sshd is being run from inetd(8).
-o option
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).
-p port
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. Nothing is sent to the system log. Normally
the beginning, authentication, and termination of each
connection is logged.
-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 connection parameters using one
or more -C options.
-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 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.
Аутентификация (Authentication)
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).
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 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 '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 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.
LOGIN PROCESS
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 (unless root).
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; otherwise runs xauth(1). The 'rc' files are
given the X11 authentication protocol and cookie in
standard input. See SSHRC, below.
9. Runs user's shell or command. All commands are run
under the user's login shell as specified in the system
password database.
SSHRC
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 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, /etc/ssh/sshrc is run, and if that
does not exist either, xauth is used to add the cookie.
