SSHD(8)SSHD(8)NAMEsshd - OpenSSH SSH daemon
SYNOPSISsshd [-46Ddeiqt] [-b bits] [-f config_file] [-g login_grace_time] [-h
host_key_file] [-k key_gen_time] [-o option] [-p port] [-u len]
DESCRIPTIONsshd (OpenSSH Daemon) is the daemon program for ssh(1). Together these
programs replace rlogin and rsh, and provide secure encrypted communi-
cations 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.
-b bits
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.
-f configuration_file
Specifies the name of the configuration file. The default is
/etc/ssh/sshd_config. sshd refuses to start if there is no con-
figuration 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_key for protocol version 1, and
/etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for pro-
tocol version 2. It is possible to have multiple host key files
for the different protocol versions and host key algorithms.
-i Specifies that sshd is being run from inetd(8). sshd is nor-
mally 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.
-k key_gen_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 impossi-
ble 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.
-o option
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).
-p port
Specifies the port on which the server listens for connections
(default 22). Multiple port options are permitted. Ports spec-
ified 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 Test mode. Only check the validity of the configuration file
and sanity of the keys. This is useful for updating sshd reli-
ably 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 over-
flow 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 configura-
tion requires it. Authentication mechanisms that may require
DNS include RhostsRSAAuthentication, HostbasedAuthentication,
and using a from="pattern-list" option in a key file. Configu-
ration options that require DNS include using a USER@HOST pat-
tern in AllowUsers or DenyUsers.
AUTHENTICATION
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, cur-
rently 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 pro-
vided through a cryptographic message authentication code (hmac-sha1 or
hmac-md5).
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 pass-
word 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 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 Linux). 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 com-
mand. 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 con-
nections 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 PermitUserEnviron-
ment 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
SSHRC , below.
9. Runs user's shell or command.
SSHRC
If the file ~/.ssh/rc exists, sh(1) runs it after reading the environ-
ment 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 cook-
ies.
The primary purpose of this file is to run any initialization routines
which may be needed before the user's home directory becomes accessi-
ble; 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.
AUTHORIZED_KEYS FILE FORMAT
AuthorizedKeysFile specifies the file containing public keys for public
key authentication; if none is specified, the default is ~/.ssh/autho-
rized_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, expo-
nent, 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 iden-
tity.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 proto-
col 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 case-insensitive):
command="command"
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 spec-
ify 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 forward-
ing unless they are explicitly prohibited. The command origi-
nally supplied by the client is available in the SSH_ORIGI-
NAL_COMMAND environment variable. Note that this option applies
to shell, command or subsystem execution.
environment="NAME=value"
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.
from="pattern-list"
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.
no-agent-forwarding
Forbids authentication agent forwarding when this key is used
for authentication.
no-port-forwarding
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).
no-X11-forwarding
Forbids X11 forwarding when this key is used for authentication.
Any X11 forward requests by the client will return an error.
permitopen="host:port"
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 per-
mitopen options may be applied separated by commas. No pattern
matching is performed on the specified hostnames, they must be
literal domains or addresses.
tunnel="n"
Force a tun(4) device on the server. Without this option, the
next available device will be used if the client requests a tun-
nel.
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
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: 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 rec-
ommended) 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
|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
AAAA1234.....=
FILES
~/.hushlogin
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
by Banner.
~/.rhosts
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 parti-
tion, 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 others.
~/.shosts
This file is used in exactly the same way as .rhosts, but allows
host-based authentication without permitting login with
rlogin/rsh.
~/.ssh/authorized_keys
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
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 ``no''. The recommended permissions can be set by executing
``chmod go-w ~/ ~/.ssh ~/.ssh/authorized_keys''.
~/.ssh/environment
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.
~/.ssh/known_hosts
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.
~/.ssh/rc
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.
/etc/hosts.allow
/etc/hosts.deny
Access controls that should be enforced by tcp-wrappers are
defined here. Further details are described in hosts_access(5).
/etc/hosts.equiv
This file is for host-based authentication (see ssh(1)) . It
should only be writable by root.
/etc/moduli
Contains Diffie-Hellman groups used for the "Diffie-Hellman
Group Exchange". The file format is described in moduli(5).
/etc/motd
See motd(5).
/etc/nologin
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 world-readable.
/etc/ssh/shosts.equiv
This file is used in exactly the same way as hosts.equiv, but
allows host-based authentication without permitting login with
rlogin/rsh.
/etc/ssh/ssh_known_hosts
Systemwide list of known host keys. This file should be pre-
pared 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.
/etc/ssh/ssh_host_key
/etc/ssh/ssh_host_dsa_key
/etc/ssh/ssh_host_rsa_key
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.
/etc/ssh/ssh_host_key.pub
/etc/ssh/ssh_host_dsa_key.pub
/etc/ssh/ssh_host_rsa_key.pub
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).
/etc/ssh/sshd_config
Contains configuration data for sshd. The file format and con-
figuration options are described in sshd_config(5).
/etc/ssh/sshrc
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.
/etc/ssh/privsep
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-
writable.
/etc/sshd.pid
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-
able.
SEE ALSOscp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
chroot(2), hosts_access(5), login.conf(5), moduli(5), sshd_config(5),
inetd(8), sftp-server(8)AUTHORS
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 contrib-
uted support for privilege separation.
CAVEATS
System security is not improved unless rshd, rlogind, and rexecd are
disabled (thus completely disabling rlogin() and rsh() into the
machine).
September 25, 1999 SSHD(8)
