sshd [-deiqtD46] [-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 proM--
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 norM--
mally started at boot from /etc/rc. It forks a new daemon
for each
incoming connection. The forked daemons handle key ex-
change, encryption,
authentication, command execution, and data exchange. This
implementaM--
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 idenM--
tify 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 pub-
lic 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, cur-
rently Blowfish
insecure, but can be enabled in the server configuration
file if desired.
System security is not improved unless rshd(8), rlogind(8),
and rexecd(8)
are disabled (thus completely disabling rlogin(1) and rsh(1)
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 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
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 (Pub-
keyAuthenticaM--
tion) or client host (HostbasedAuthentication) authentica-
tion method,
conventional password authentication and challenge response
based methM--
ods.
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, for-
warding TCP/IP
connections, or forwarding the authentication agent connec-
tion over the
secure channel.
sshd can be configured using command-line options or a con-
figuration
file. Command-line options override values specified in the
configuraM--
tion 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:
-b bits
Specifies the number of bits in the ephemeral proto-
col version 1
server key (default 768).
-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
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 conM--
figuration file.
-g login_grace_time
Gives the grace time for clients to authenticate
themselves
(default 600 seconds). If the client fails to au-
thenticate the
user within this many seconds, the server discon-
nects and exits.
A value of zero indicates no limit.
-h host_key_file
Specifies a file from which a host key is read.
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 motivaM--
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 commu-
nications even
if the machine is cracked into or physically seized.
A value of
zero indicates that the key will never be regenerat-
ed.
-o option
Can be used to give options in the format used in
the configuraM--
tion file. This is useful for specifying options
for which there
is no separate command-line flag.
-p port
Specifies the port on which the server listens for
connections
(default 22). Multiple port options are permitted.
Ports speciM--
fied 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 configu-
ration file and
sanity of the keys. This is useful for updating
be put into
the utmp file. -u0 is 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
include RhostsAuthentication, RhostsRSAAuthentica-
tion,
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.
-D When this option is specified sshd will not detach
and does not
become a daemon. This allows easy monitoring of
sshd.
-4 Forces sshd to use IPv4 addresses only.
-6 Forces sshd to use IPv6 addresses only.
CONFIGURATION FILE
sshd reads configuration data from /etc/ssh/sshd_config (or
the file
specified with -f on the command line). The file contains
keyword-arguM--
ment pairs, one per line. Lines starting with `#' and empty
lines are
interpreted as comments.
The possible keywords and their meanings are as follows
(note that keyM--
words are case-insensitive and arguments are case-sensi-
tive):
AFSTokenPassing
Specifies whether an AFS token may be forwarded to
the server.
Default is ``yes''.
AllowGroups
This keyword can be followed by a list of group name
patterns,
separated by spaces. If specified, login is allowed
only for
users whose primary group or supplementary group
list matches one
of the patterns. `*' and `'? can be used as wild-
cards in the
AllowUsers
This keyword can be followed by a list of user name
patterns,
separated by spaces. If specified, login is allowed
only for
users names that match one of the patterns. `*' and
`'? can be
used as wildcards in the patterns. Only user names
are valid; a
numerical user ID is not recognized. By default,
login is
allowed for all users. If the pattern takes the
form USER@HOST
then USER and HOST are separately checked, restrict-
ing logins to
particular users from particular hosts.
AuthorizedKeysFile
Specifies the file that contains the public keys
that can be used
for user authentication. AuthorizedKeysFile may
contain tokens
of the form %T which are substituted during connec-
tion set-up.
The following tokens are defined: %% is replaced by
a literal
'%', %h is replaced by the home directory of the us-
er being
authenticated and %u is replaced by the username of
that user.
After expansion, AuthorizedKeysFile is taken to be
an absolute
path or one relative to the user's home directory.
The default
is ``.ssh/authorized_keys''.
Banner In some jurisdictions, sending a warning message be-
fore authentiM--
cation may be relevant for getting legal protection.
The conM--
tents of the specified file are sent to the remote
user before
authentication is allowed. This option is only
available for
protocol version 2.
ChallengeResponseAuthentication
Specifies whether challenge response authentication
is allowed.
All authentication styles from login.conf(5) are
supported. The
default is ``yes''.
sage through
the encrypted channel to request a response from the
client. The
default is 0, indicating that these messages will
not be sent to
the client. This option applies to protocol version
2 only.
ClientAliveCountMax
Sets the number of client alive messages (see above)
which may be
sent without sshd receiving any messages back from
the client. If
this threshold is reached while client alive mes-
sages are being
sent, sshd will disconnect the client, terminating
the session.
It is important to note that the use of client alive
messages is
very different from KeepAlive (below). The client
alive messages
are sent through the encrypted channel and therefore
will not be
spoofable. The TCP keepalive option enabled by
KeepAlive is
spoofable. The client alive mechanism is valuable
when the client
or server depend on knowing when a connection has
become inacM--
tive.
The default value is 3. If ClientAliveInterval
(above) is set to
15, and ClientAliveCountMax is left at the default,
unresponsive
ssh clients will be disconnected after approximately
45 seconds.
DenyGroups
This keyword can be followed by a list of group name
patterns,
separated by spaces. Login is disallowed for users
whose primary
group or supplementary group list matches one of the
patterns.
`*' and `'? can be used as wildcards in the pat-
terns. Only
group names are valid; a numerical group ID is not
recognized.
By default, login is allowed for all groups.
DenyUsers
particular hosts.
GatewayPorts
Specifies whether remote hosts are allowed to con-
nect to ports
forwarded for the client. By default, sshd binds
remote port
forwardings to the loopback addresss. This prevents
other remote
hosts from connecting to forwarded ports. Gateway-
Ports can be
used to specify that sshd should bind remote port
forwardings to
the wildcard address, thus allowing remote hosts to
connect to
forwarded ports. The argument must be ``yes'' or
``no''. The
default is ``no''.
HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authen-
tication
together with successful public key client host au-
thentication is
allowed (hostbased authentication). This option is
similar to
RhostsRSAAuthentication and applies to protocol ver-
sion 2 only.
The default is ``no''.
HostKey
Specifies a file containing a private host key used
by SSH. The
default is /etc/ssh/ssh_host_key for protocol ver-
sion 1, and
/etc/ssh/ssh_host_rsa_key and
/etc/ssh/ssh_host_dsa_key for proM--
tocol version 2. Note that sshd will refuse to use
a file if it
is group/world-accessible. It is possible to have
multiple host
key files. ``rsa1'' keys are used for version 1 and
``dsa'' or
``rsa'' are used for version 2 of the SSH protocol.
IgnoreRhosts
Specifies that .rhosts and .shosts files will not be
used in
RhostsAuthentication, RhostsRSAAuthentication or
HostbasedAuthentication.
/etc/hosts.equiv and /etc/shosts.equiv are still
crash of one of the machines will be properly no-
ticed. However,
this means that connections will die if the route is
down temM--
porarily, and some people find it annoying. On the
other hand,
if keepalives are not sent, sessions may hang indef-
initely on the
server, leaving ``ghost'' users and consuming server
resources.
The default is ``yes'' (to send keepalives), and the
server will
notice if the network goes down or the client host
crashes. This
avoids infinitely hanging sessions.
To disable keepalives, the value should be set to
``no''.
KerberosAuthentication
Specifies whether Kerberos authentication is al-
lowed. This can
be in the form of a Kerberos ticket, or if Passwor-
dAuthentication
is yes, the password provided by the user will be
validated
through the Kerberos KDC. To use this option, the
server needs a
Kerberos servtab which allows the verification of
the KDC's idenM--
tity. Default is ``yes''.
KerberosOrLocalPasswd
If set then if password authentication through Ker-
beros fails
then the password will be validated via any addi-
tional local
mechanism such as /etc/passwd. Default is ``yes''.
KerberosTgtPassing
Specifies whether a Kerberos TGT may be forwarded to
the server.
Default is ``no'', as this only works when the Ker-
beros KDC is
actually an AFS kaserver.
KerberosTicketCleanup
Specifies whether to automatically destroy the us-
er's ticket
cache file on logout. Default is ``yes''.
Specifies the local addresses sshd should listen on.
The followM--
ing forms may be used:
ListenAddress host|IPv4_addr|IPv6_addr
ListenAddress host|IPv4_addr:port
ListenAddress [host|IPv6_addr]:port
If port is not specified, sshd will listen on the
address and all
prior Port options specified. The default is to lis-
ten on all
local addresses. Multiple ListenAddress options are
permitted.
Additionally, any Port options must precede this op-
tion for non
port qualified addresses.
LoginGraceTime
The server disconnects after this time if the user
has not sucM--
cessfully logged in. If the value is 0, there is no
time limit.
The default is 600 (seconds).
LogLevel
Gives the verbosity level that is used when logging
messages from
sshd. The possible values are: QUIET, FATAL, ERROR,
INFO, VERM--
BOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3. The default
is INFO.
DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3
each specify
higher levels of debugging output. Logging with a
DEBUG level
violates the privacy of users and is not recommend-
ed.
MACs Specifies the available MAC (message authentication
code) algoM--
rithms. The MAC algorithm is used in protocol ver-
sion 2 for data
integrity protection. Multiple algorithms must be
comma-sepaM--
rated. The default is
``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-
sha1-96,hmac-md5-96''.
MaxStartups
Specifies the maximum number of concurrent unauthen-
ticated conM--
(10) unauthenticated connections. The probability
increases linM--
early and all connection attempts are refused if the
number of
unauthenticated connections reaches ``full'' (60).
PAMAuthenticationViaKbdInt
Specifies whether PAM challenge response authentica-
tion is
allowed. This allows the use of most PAM challenge
response
authentication modules, but it will allow password
authentication
regardless of whether PasswordAuthentication is dis-
abled. The
default is ``no''.
PasswordAuthentication
Specifies whether password authentication is al-
lowed. The
default is ``yes''.
PermitEmptyPasswords
When password authentication is allowed, it speci-
fies whether the
server allows login to accounts with empty password
strings. The
default is ``no''.
PermitRootLogin
Specifies whether root can login using ssh(1). The
argument must
be ``yes'', ``without-password'', ``forced-commands-
only'' or
``no''. The default is ``yes''.
If this option is set to ``without-password'' pass-
word authentiM--
cation is disabled for root.
If this option is set to ``forced-commands-only''
root login with
public key authentication will be allowed, but only
if the
command option has been specified (which may be use-
ful for taking
remote backups even if root login is normally not
allowed). All
other authentication methods are disabled for root.
If this option is set to ``no'' root is not allowed
to login.
time when the
user last logged in. The default is ``yes''.
PrintMotd
Specifies whether sshd should print /etc/motd when a
user logs in
interactively. (On some systems it is also printed
by the shell,
/etc/profile, or equivalent.) The default is
``yes''.
Protocol
Specifies the protocol versions sshd should support.
The possiM--
ble values are ``1'' and ``2''. Multiple versions
must be comma-
separated. The default is ``2,1''.
PubkeyAuthentication
Specifies whether public key authentication is al-
lowed. The
default is ``yes''. Note that this option applies
to protocol
version 2 only.
RhostsAuthentication
Specifies whether authentication using rhosts or
/etc/hosts.equiv
files is sufficient. Normally, this method should
not be permitM--
ted because it is insecure. RhostsRSAAuthentication
should be
used instead, because it performs RSA-based host au-
thentication
in addition to normal rhosts or /etc/hosts.equiv au-
thentication.
The default is ``no''. This option applies to pro-
tocol version 1
only.
RhostsRSAAuthentication
Specifies whether rhosts or /etc/hosts.equiv authen-
tication
together with successful RSA host authentication is
allowed. The
default is ``no''. This option applies to protocol
version 1
only.
RSAAuthentication
Specifies whether pure RSA authentication is al-
lowed. The
login. This
is normally desirable because novices sometimes ac-
cidentally
leave their directory or files world-writable. The
default is
``yes''.
Subsystem
Configures an external subsystem (e.g., file trans-
fer daemon).
Arguments should be a subsystem name and a command
to execute
upon subsystem request. The command sftp-server(8)
implements
the ``sftp'' file transfer subsystem. By default no
subsystems
are defined. Note that this option applies to pro-
tocol version 2
only.
SyslogFacility
Gives the facility code that is used when logging
messages from
sshd. The possible values are: DAEMON, USER, AUTH,
LOCAL0,
LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LO-
CAL7. The
default is AUTH.
UseLogin
Specifies whether login(1) is used for interactive
login sesM--
sions. The default is ``no''. Note that login(1)
is never used
for remote command execution. Note also, that if
this is
enabled, X11Forwarding will be disabled because lo-
gin(1) does not
know how to handle xauth(1) cookies.
VerifyReverseMapping
Specifies whether sshd should try to verify the re-
mote host name
and check that the resolved host name for the remote
IP address
maps back to the very same IP address. The default
is ``no''.
X11DisplayOffset
Specifies the first display number available for
sshd's X11 forM--
warding. This prevents sshd from interfering with
X11UseLocalhost
Specifies whether sshd should bind the X11 forward-
ing server to
the loopback address or to the wildcard address. By
default,
sshd binds the forwarding server to the loopback ad-
dress and sets
the hostname part of the DISPLAY environment vari-
able to
``localhost''. This prevents remote hosts from con-
necting to the
fake display. However, some older X11 clients may
not function
with this configuration. X11UseLocalhost may be set
to ``no'' to
specify that the forwarding server should be bound
to the wildM--
card address. The argument must be ``yes'' or
``no''. The
default is ``yes''.
XAuthLocation
Specifies the location of the xauth(1) program. The
default is
/usr/X11R6/bin/xauth.
Time Formats
sshd command-line arguments and configuration file options
that specify
time may be expressed using a sequence of the form:
time[qualifier],
where time is a positive integer value and qualifier is one
of the folM--
lowing:
<none> seconds
s | S seconds
m | M minutes
h | H hours
d | D days
w | W weeks
Each member of the sequence is added together to calculate
the total time
value.
Time format examples:
600 600 seconds (10 minutes)
10m 10 minutes
1h30m 1 hour 30 minutes (90 minutes)
3. Checks /etc/nologin; if it exists, prints con-
tents and quits
(unless root).
4. Changes to run with normal user privileges.
5. Sets up basic environment.
6. Reads $HOME/.ssh/environment if it exists.
7. Changes to user's home directory.
8. If $HOME/.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.
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, keyM--
type, base64 encoded key, comment. The options fields are
optional; its
presence is determined by whether the line starts with a
number or not
(the option field never starts with a number). The bits,
exponent, moduM--
lus and comment fields give the RSA key for protocol version
1; the comM--
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''
lowing option specifications are supported (note that option
keywords are
case-insensitive):
from="pattern-list"
Specifies that in addition to RSA authentication,
the canonical
name of the remote host must be present in the com-
ma-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 option-
ally increase
security: RSA 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
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 a 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 backM--
slash. This option might be useful to restrict cer-
tain RSA 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 un-
less they are
explicitly prohibited. Note that this option ap-
plies to shell,
Forbids TCP/IP forwarding when this key is used for
authenticaM--
tion. 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 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 conM--
nect to the specified host and port. IPv6 addresses
can be specM--
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
domains 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
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/ssh/ssh_known_hosts, and $HOME/.ssh/known_hosts
files contain
host public keys for all known hosts. The global file
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
indicate negation: if the host name matches a negated pat-
tern, 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/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 recomM--
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 conM--
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
cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
FILES
/etc/ssh/sshd_config
Contains configuration data for sshd. This file
should be
/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".
/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 pid of the one started
last). The conM--
tent 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 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 in-
to this file,
as described in ssh-keygen(1).
/etc/ssh/ssh_known_hosts and $HOME/.ssh/known_hosts
These files are consulted when using rhosts with RSA
host authenM--
tication or protocol version 2 hostbased authentica-
tion to check
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
defined 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 password. The same file is used
by rlogind and
rshd. The file must be writable only by the user;
it is recomM--
mended that it not be accessible by others.
If 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 pass-
word, provided
they have the same user name on both machines. The
host name may
also be followed by a user name; such users are per-
mitted to log
in as any user on this machine (except root). Addi-
tionally, the
syntax ``+@group'' can be used to specify netgroups.
Negated
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/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.
$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 comM--
mand. If X11 spoofing is in use, this will receive
the "proto
cookie" pair in standard input (and DISPLAY in envi-
ronment).
This must call xauth(1) in that case.
The primary purpose of this file is to run any ini-
tialization
routines which may be needed before the user's home
directory
becomes accessible; AFS is a particular example of
such an enviM--
ronment.
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.
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 fea-
tures and creM--
ated OpenSSH. Markus Friedl contributed the support for SSH
protocol
versions 1.5 and 2.0.
SEE ALSO
scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-key-
gen(1),
login.conf(5), moduli(5), sftp-server(8)
T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehti-
nen, SSH
Protocol Architecture, draft-ietf-secsh-architecture-09.txt,
July 2001,
work in progress material.
M. Friedl, N. Provos, and W. A. Simpson, Diffie-Hellman
Group Exchange
for the SSH Transport Layer Protocol, draft-ietf-secsh-dh-
group-
exchange-01.txt, April 2001, work in progress material.
BSD September 25, 1999
BSD
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