sshd [-46Ddeiqt] [-b bits] [-f config_file] [-g lo-
gin_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). Togeth-
er these pro-
grams 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 nor-
mally started at boot from /etc/rc. It forks a new daemon
for each in-
coming connection. The forked daemons handle key exchange,
encryption,
authentication, command execution, and data exchange. This
implementa-
tion of sshd supports both SSH protocol version 1 and 2 si-
multaneously.
sshd works as follows:
SSH protocol version 1
Each host has a host-specific RSA key (normally 1024 bits)
used to iden-
tify the host. Additionally, when the daemon starts, it
generates a
server RSA key (normally 768 bits). This key is normally
regenerated ev-
ery 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 serv-
er. 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, cur-
rently Blowfish
or 3DES, with 3DES being used by default. The client se-
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, `*' on HP-UX, containing `Nologin' on Tru64 and
a leading
`!!' on Linux). If there is a requirement to disable pass-
word authenti-
cation 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 dis-
abled (thus completely disabling rlogin and rsh into the ma-
chine).
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 ci-
pher, 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 of-
fered 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 (Pub-
keyAuthentica-
tion) or client host (HostbasedAuthentication) authentica-
tion method,
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.
When the user program terminates and all forwarded X11 and
other connec-
tions 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 con-
figuration file
(by default sshd_config(5)). Command-line options override
values speci-
fied in the configuration file.
sshd rereads its configuration file when it receives a
hangup signal,
SIGHUP, by executing itself with the name it was started as,
i.e.,
/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 proto-
col 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. Mul-
tiple -d op-
Gives the grace time for clients to authenticate
themselves (de-
fault 120 seconds). If the client fails to authen-
ticate 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 al-
gorithms.
-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 re-
generated 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
impossible to
recover the key for decrypting intercepted communi-
cations even if
the machine is cracked into or physically seized. A
value of ze-
ro indicates that the key will never be regenerated.
-o option
Can be used to give options in the format used in
port is specified.
-q Quiet mode. Nothing is sent to the system log.
Normally the be-
ginning, authentication, and termination of each
connection is
logged.
-t Test mode. Only check the validity of the configu-
ration 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 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
configuration
requires it. Authentication mechanisms that may re-
quire DNS in-
clude RhostsRSAAuthentication, HostbasedAuthentica-
tion and using
a from="pattern-list" option in a key file. Config-
uration op-
tions that require DNS include using a USER@HOST
pattern in
AllowUsers or DenyUsers.
CONFIGURATION FILE
sshd reads configuration data from /etc/ssh/sshd_config (or
the file
specified with -f on the command line). The file format and
configura-
tion 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,
5. Sets up basic environment.
6. Reads the file $HOME/.ssh/environment, if it ex-
ists, 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/ssh/sshrc ex-
ists, 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
the public keys
that are permitted for RSA authentication in protocol ver-
sion 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, expo-
nent, modulus,
comment. Each protocol version 2 public key consists of:
options, key-
type, base64 encoded key, comment. The options field is op-
tional; 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, mod-
ulus and comment fields give the RSA key for protocol ver-
sion 1; the com-
ment 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
quotes. The fol-
lowing option specifications are supported (note that option
keywords are
case-insensitive):
from="pattern-list"
Specifies that in addition to public key authentica-
tion, 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 it-
self does not
trust the network or name servers or anything (but
the key); how-
ever, if somebody somehow steals the key, the key
permits an in-
truder to log in from anywhere in the world. This
additional op-
tion 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
key is used
for authentication. The command supplied by the us-
er (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 chan-
nel 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 pub-
lic 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
default and is controlled via the PermitUserEnviron-
ment option.
This option is automatically disabled if UseLogin is
enabled.
no-port-forwarding
Forbids TCP/IP forwarding when this key is used for
authentica-
tion. Any port forward requests by the client will
return an er-
ror. This might be used, e.g., in connection with
the command
option.
no-X11-forwarding
Forbids X11 forwarding when this key is used for au-
thentication.
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 con-
nect to the specified host and port. IPv6 addresses
can be spec-
ified with an alternative syntax: host/port. Multi-
ple permitopen
options may be applied separated by commas. No pat-
tern matching
is performed on the specified hostnames, they must
be literal do-
mains or addresses.
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 back-
up.hut.fi
permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33
hostnames, bits,
exponent, modulus, comment. The fields are separated by
spaces.
Hostnames is a comma-separated list of patterns (`*' and `?'
act as wild-
cards); each pattern in turn is matched against the canoni-
cal host name
(when authenticating a client) or against the user-supplied
name (when
authenticating a server). A pattern may also be preceded by
`!' to indi-
cate negation: if the host name matches a negated pattern,
it is not ac-
cepted (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/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 com-
ments.
When performing host authentication, authentication is ac-
cepted if any
matching line has the proper key. It is thus permissible
(but not recom-
mended) 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 con-
flicting 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.
Examples
closenet,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
and not accessible to others. Note that sshd does
not start if
this file is 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-key-
gen(1).
/etc/moduli
Contains Diffie-Hellman groups used for the "Diffie-
Hellman Group
Exchange". The file format is described in mod-
uli(5).
/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.
/var/run/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.
$HOME/.ssh/authorized_keys
Lists the public keys (RSA or DSA) that can be used
to 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
tion 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/ssh/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 ex-
cept root log
in. The contents of the file are displayed to any-
one trying to
log in, and non-root connections are refused. The
file should be
world-readable.
/etc/hosts.allow, /etc/hosts.deny
Access controls that should be enforced by tcp-wrap-
pers are de-
fined here. Further details are described in
hosts_access(5).
$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 us-
er; it is rec-
ommended 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.
Negated en-
tries start with `-'.
If the client host/user is successfully matched in
this file, lo-
gin is automatically permitted provided the client
and server us-
er names are the same. Additionally, successful RSA
host authen-
tication is normally required. This file must be
writable only
by root; it is recommended that it be world-read-
able.
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 us-
er name practically grants the user root access.
The only valid
use for user names that I can think of is in nega-
tive entries.
Note that this warning also applies to rsh/rlogin.
/etc/shosts.equiv
This is processed exactly as /etc/hosts.equiv. How-
ever, 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.
$HOME/.ssh/rc
If this file exists, it is run with /bin/sh after
reading the en-
routines which may be needed before the user's home
directory be-
comes accessible; AFS is a particular example of
such an environ-
ment.
This file will probably contain some initialization
code followed
by something similar to:
if read proto cookie && [ -n "$DISPLAY" ]; then
if [ `echo $DISPLAY | cut -c1-10` = 'local-
host:' ]; 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.
This file should be writable only by the user, and
need not be
readable by anyone else.
/etc/ssh/sshrc
Like $HOME/.ssh/rc. This can be used to specify ma-
chine-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-key-
gen(1),
chroot(2), hosts_access(5), login.conf(5), moduli(5),
sshd_config(5),
inetd(8), sftp-server(8)
T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehti-
nen, SSH
Protocol Architecture, draft-ietf-secsh-architecture-12.txt,
January
2002, work in progress material.
M. Friedl, N. Provos, and W. A. Simpson, Diffie-Hellman
versions 1.5 and 2.0. Niels Provos and Markus Friedl con-
tributed support
for privilege separation.
OpenBSD 3.5 September 25, 1999
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