SSHD(8) UNIX System V (September 25, 1999) SSHD(8)
NAME
sshd - OpenSSH SSH daemon
SYNOPSIS
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]
DESCRIPTION
sshd (SSH Daemon) is the daemon program for ssh(1).
Together these programs replace rlogin and rsh, and provide
secure encrypted communications between two untrusted hosts
over an insecure network. The programs are intended to be
as easy to install and use as possible.
sshd is the daemon that listens for connections from
clients. It is normally started at boot from
/etc/init.d/sshd. It forks a new daemon for each incoming
connection. The forked daemons handle key exchange,
encryption, authentication, command execution, and data
exchange. This implementation of sshd supports both SSH
protocol version 1 and 2 simultaneously. sshd works as
follows:
SSH protocol version 1
Each host has a host-specific RSA key (normally 1024 bits)
used to identify the host. Additionally, when the daemon
starts, it generates a server RSA key (normally 768 bits).
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.
Next, the server and the client enter an authentication
dialog. The client tries to authenticate itself using
.rhosts authentication, .rhosts authentication combined with
RSA host authentication, RSA challenge-response
authentication, or password based authentication.
Regardless of the authentication type, the account is
checked to ensure that it is accessible. An account is not
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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, `*' on HP-UX, containing `Nologin' on
Tru64 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*' ).
rhosts authentication is normally disabled because it is
fundamentally insecure, but can be enabled in the server
configuration file if desired. System security is not
improved unless rshd, rlogind, and rexecd are disabled (thus
completely disabling rlogin() and rsh() into the machine).
SSH protocol version 2
Version 2 works similarly: Each host has a host-specific
key (RSA or DSA) used to identify the host. However, when
the daemon starts, it does not generate a server key.
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-sha1 or
hmac-md5).
Protocol version 2 provides a public key based user
(PubkeyAuthentication) or client host
(HostbasedAuthentication) authentication method,
conventional password authentication and challenge response
based methods.
Command execution and data forwarding
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/IP connections,
or forwarding the authentication agent connection over the
secure channel.
Finally, 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.
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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.
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/openssh/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
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normal host key files are normally not readable by
anyone but root). The default is
/etc/openssh/ssh_host_key for protocol version 1, and
/etc/openssh/ssh_host_rsa_key and
/etc/openssh/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 algorithms.
-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.
-k key_gen_time
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.
-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
are ignored when a command-line port is specified.
-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 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
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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.
CONFIGURATION FILE
sshd reads configuration data from /etc/openssh/sshd_config
(or the file specified with -f on the command line). The
file format and configuration options are described in
sshd_config(5).
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
$HOME/.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 $HOME/.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 $HOME/.ssh/rc exists, runs it; else if
/etc/openssh/sshrc exists, runs it; otherwise runs
xauth. The ``rc'' files are given the X11
authentication protocol and cookie in standard input.
9. Runs user's shell or command.
AUTHORIZED_KEYS FILE FORMAT
$HOME/.ssh/authorized_keys is the default file that lists
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the public keys that are permitted for RSA authentication in
protocol version 1 and for public key authentication
(PubkeyAuthentication) in protocol version 2.
AuthorizedKeysFile may be used to specify an alternative
file.
Each line of the file contains one key (empty lines and
lines starting with a `#' are ignored as comments). Each
RSA public key consists of the following fields, separated
by spaces: options, bits, exponent, modulus, comment. Each
protocol version 2 public key consists 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).
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):
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
(`*' and `?' serve as wildcards). The list may also
contain patterns negated by prefixing them with `!' ;
if the canonical host name matches a negated pattern,
the key is not accepted. 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).
command="command"
Specifies that the command is executed whenever this
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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/IP and/or X11 forwarding unless they
are explicitly prohibited. 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.
no-port-forwarding
Forbids TCP/IP 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-X11-forwarding
Forbids X11 forwarding when this key is used for
authentication. Any X11 forward requests by the client
will return an error.
no-agent-forwarding
Forbids authentication agent forwarding when this key
is used for authentication.
no-pty
Prevents tty allocation (a request to allocate a pty
will fail).
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 permitopen options may be applied
separated by commas. No pattern matching is performed
on the specified hostnames, they must be literal
domains or addresses.
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Examples
1024 33 12121...312314325 ylo@foo.bar
from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334
ylo@niksula
command="dump /home",no-pty,no-port-forwarding 1024 33
23...2323 backup.hut.fi
permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33
23...2323
SSH_KNOWN_HOSTS FILE FORMAT
The /etc/openssh/ssh_known_hosts and $HOME/.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.
Bits, exponent, and modulus are taken directly from the RSA
host key; they can be obtained, e.g., from
/etc/openssh/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
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the host keys by hand. Rather, generate them by a script or
by taking /etc/openssh/ssh_host_key.pub and adding the host
names at the front.
Examples
closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
FILES
/etc/openssh/sshd_config
Contains configuration data for sshd. The file format
and configuration options are described in
sshd_config(5).
/etc/openssh/ssh_host_rsa_key
/etc/openssh/ssh_host_key, /etc/openssh/ssh_host_dsa_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 this file is
group/world-accessible.
/etc/openssh/ssh_host_rsa_key.pub
/etc/openssh/ssh_host_key.pub, /etc/openssh/ssh_host_dsa_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/openssh/moduli
Contains Diffie-Hellman groups used for the "Diffie-
Hellman Group Exchange".
/var/empty
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/openssh/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-readable.
$HOME/.ssh/authorized_keys
Lists the public keys (RSA or DSA) that can be used to
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log into the user's account. This file must be
readable by root (which may on some machines imply it
being world-readable if the user's home directory
resides on an NFS volume). It is recommended that it
not be accessible by others. The format of this file
is described above. Users will place the contents of
their identity.pub, id_dsa.pub and/or id_rsa.pub files
into this file, as described in ssh-keygen(1).
/etc/openssh/ssh_known_hosts , $HOME/.ssh/known_hosts
These files are consulted when using rhosts with RSA
host authentication or protocol version 2 hostbased
authentication to check the public key of the host.
The key must be listed in one of these files to be
accepted. The client uses the same files to verify
that it is connecting to the correct remote host.
These files should be writable only by root/the owner.
/etc/openssh/ssh_known_hosts should be world-readable,
and $HOME/.ssh/known_hosts can, but need not be,
world-readable.
/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.
$HOME/.rhosts
This file contains host-username pairs, separated by a
space, one per line. The given user on the
corresponding host is permitted to log in without a
password. The same file is used by rlogind and rshd.
The file must be writable only by the user; it is
recommended that it not be accessible by others.
It is also possible to use netgroups in the file.
Either host or user name may be of the form +@groupname
to specify all hosts or all users in the group.
$HOME/.shosts
For ssh, this file is exactly the same as for .rhosts.
However, this file is not used by rlogin and rshd, so
using this permits access using SSH only.
/etc/hosts.equiv
This file is used during rhosts authentication. In the
simplest form, this file contains host names, one per
line. Users on those hosts are permitted to log in
without a password, provided they have the same user
name on both machines. The host name may also be
followed by a user name; such users are permitted to
log in as any user on this machine (except root).
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Additionally, the syntax ``+@group'' can be used to
specify netgroups. Negated entries start with `-'.
If the client host/user is successfully matched in this
file, login is automatically permitted provided the
client and server user names are the same.
Additionally, successful RSA host authentication is
normally required. This file must be writable only by
root; it is recommended that it be world-readable.
Warning: It is almost never a good idea to use user
names in hosts.equiv. Beware that it really means that
the named user(s) can log in as anybody, which includes
bin, daemon, adm, and other accounts that own critical
binaries and directories. Using a user name
practically grants the user root access. The only
valid use for user names that I can think of is in
negative entries.
Note that this warning also applies to rsh/rlogin.
/etc/openssh/shosts.equiv
This is processed exactly as /etc/hosts.equiv.
However, this file may be useful in environments that
want to run both rsh/rlogin and ssh.
$HOME/.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.
$HOME/.ssh/rc
If this file exists, it is run with /bin/sh after
reading the environment files but before starting the
user's shell or command. It must not produce any
output on stdout; stderr must be used instead. If X11
forwarding is in use, it will receive the "proto
cookie" pair in its standard input (and DISPLAY in its
environment). The script must call xauth(1) because
sshd will not run xauth automatically to add X11
cookies.
The primary purpose of this file is to run any
initialization routines which may be needed before the
user's home directory becomes accessible; AFS is a
particular example of such an environment.
This file will probably contain some initialization
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code followed by something similar to:
if read proto cookie && [ -n "$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/openssh/sshrc is run,
and if that does not exist either, xauth is used to add
the cookie.
This file should be writable only by the user, and need
not be readable by anyone else.
/etc/openssh/sshrc
Like $HOME/.ssh/rc. This can be used to specify
machine-specific login-time initializations globally.
This file should be writable only by root, and should
be world-readable.
SEE ALSO
scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-
keygen(1), chroot(2), hosts_access(5), sshd_config(5),
inetd(8), sftp-server(8)
S. Lehtinen, T. Rinne, M. Saarinen, T. Kivinen and T.
Ylonen, SSH Protocol Architecture, draft-ietf-secsh-
architecture-12.txt, January 2002, work in progress
material.
W. A. Simpson, N. Provos and M. Friedl, Diffie-Hellman Group
Exchange for the SSH Transport Layer Protocol, draft-ietf-
secsh-dh-group-exchange-02.txt, January 2002, work in
progress material.
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
contributed support for privilege separation.
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