[-h host_key_file] [-k key_gen_time] [-p port] [-u len]
[-V client_protocol_id]
DESCRIPTION
sshd (Secure Shell Daemon) is the daemon program for ssh(1). Together
these programs replace rlogin and rsh, and provide secure encrypted com-
munications 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
incoming 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 simultaneously.
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
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 data-
base 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 ses-
sion 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.
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(8), rlogind(8), rexecd(8),
and rexd(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 DSA key 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 Blowfish,
3DES or CAST128 in CBC mode or Arcfour. The client selects the encryp-
tion algorithm to use from those offered by the server. Additionally,
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 configuration
file. Command-line options override values specified in the configura-
tion file.
sshd rereads its configuration file when it receives a hangup signal,
SIGHUP.
The options are as follows:
-b bits
Specifies the number of bits in the 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. Multiple -d
options increases the debugging level. Maximum is 3.
-f configuration_file
Specifies the name of the configuration file. The default is
/etc/sshd_config. sshd refuses to start if there is no configu-
ration file.
-g login_grace_time
Gives the grace time for clients to authenticate themselves
(default 300 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 the file from which the RSA host key is read (default
/etc/ssh_host_key). This option must be given if sshd is not run
as root (as the normal host file is normally not readable by any-
one but root).
-i Specifies that sshd is being run from inetd. 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 server key is regenerated (default 3600
-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.
-Q Do not print an error message if RSA support is missing.
-V client_protocol_id
SSH-2 compatibility mode. When this option is specified sshd
assumes the client has sent the supplied version string and skips
the Protocol Version Identification Exchange. This option is not
intended to be called directly.
-4 Forces sshd to use IPv4 addresses only.
-6 Forces sshd to use IPv6 addresses only.
CONFIGURATION FILE
sshd reads configuration data from /etc/sshd_config (or the file speci-
fied with -f on the command line). The file contains keyword-value
pairs, one per line. Lines starting with `#' and empty lines are inter-
preted as comments.
The following keywords are possible.
AFSTokenPassing
Specifies whether an AFS token may be forwarded to the server.
Default is ``yes''.
AllowGroups
This keyword can be followed by a number of group names, sepa-
rated by spaces. If specified, login is allowed only for users
whose primary group matches one of the patterns. `*' and `'?
can be used as wildcards in the patterns. Only group names are
valid; a numerical group ID isn't recognized. By default login
is allowed regardless of the primary group.
AllowTcpForwarding
Specifies whether TCP forwarding is permitted. The default is
``yes''. Note that disabling TCP forwarding does not improve
security unless users are also denied shell access, as they can
always install their own forwarders.
AllowUsers
This keyword can be followed by a number of user names, 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 numeri-
rated by spaces. Users whose primary group matches one of the
patterns aren't allowed to log in. `*' and `'? can be used as
wildcards in the patterns. Only group names are valid; a numeri-
cal group ID isn't recognized. By default login is allowed
regardless of the primary group.
DenyUsers
This keyword can be followed by a number of user names, separated
by spaces. Login is disallowed for user names that match one of
the patterns. `*' and `'? can be used as wildcards in the pat-
terns. Only user names are valid; a numerical user ID isn't rec-
ognized. By default login is allowed regardless of the user
name.
DSAAuthentication
Specifies whether DSA authentication is allowed. The default is
``yes''. Note that this option applies to protocol version 2
only.
GatewayPorts
Specifies whether remote hosts are allowed to connect to ports
forwarded for the client. The argument must be ``yes'' or
``no''. The default is ``no''.
HostDSAKey
Specifies the file containing the private DSA host key (default
/etc/ssh_host_dsa_key) used by SSH protocol 2.0. Note that sshd
disables protocol 2.0 if this file is group/world-accessible.
HostKey
Specifies the file containing the private RSA host key (default
/etc/ssh_host_key) used by SSH protocols 1.3 and 1.5. Note that
sshd disables protocols 1.3 and 1.5 if this file is group/world-
accessible.
IgnoreRhosts
Specifies that .rhosts and .shosts files will not be used in
authentication. /etc/hosts.equiv and /etc/shosts.equiv are still
used. The default is ``yes''.
IgnoreUserKnownHosts
Specifies whether sshd should ignore the user's
$HOME/.ssh/known_hosts during RhostsRSAAuthentication. The
default is ``no''.
KeepAlive
Specifies whether the system should send keepalive messages to
the other side. If they are sent, death of the connection or
crash of one of the machines will be properly noticed. However,
this means that connections will die if the route is down tempo-
rarily, and some people find it annoying. On the other hand, if
keepalives are not sent, sessions may hang indefinitely on the
Kerberos servtab which allows the verification of the KDC's iden-
tity. Default is ``yes''.
KerberosOrLocalPasswd
If set then if password authentication through Kerberos fails
then the password will be validated via any additional 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 Kerberos KDC is
actually an AFS kaserver.
KerberosTicketCleanup
Specifies whether to automatically destroy the user's ticket
cache file on logout. Default is ``yes''.
KeyRegenerationInterval
The server key is automatically regenerated after this many sec-
onds (if it has been used). The purpose of regeneration is to
prevent decrypting captured sessions by later breaking into the
machine and stealing the keys. The key is never stored anywhere.
If the value is 0, the key is never regenerated. The default is
3600 (seconds).
ListenAddress
Specifies what local address sshd should listen on. The default
is to listen to all local addresses. Multiple options of this
type are permitted. Additionally, the Ports options must precede
this option.
LoginGraceTime
The server disconnects after this time if the user has not suc-
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, VER-
BOSE and DEBUG. The default is INFO. Logging with level DEBUG
violates the privacy of users and is not recommended.
MaxStartups
Specifies the maximum number of concurrent unauthenticated con-
nections to the sshd daemon. Additional connections will be
dropped until authentication succeeds or the LoginGraceTime
expires for a connection. The default is 10.
Alternatively, random early drop can be enabled by specifying the
three colon separated values ``start:rate:full'' (e.g.,
"10:30:60"). sshd will refuse connection attempts with a proba-
billity of ``rate/100'' (30%) if there are currently ``start''
PermitRootLogin
Specifies whether the root can log in using ssh(1). The argument
must be ``yes'', ``without-password'' or ``no''. The default is
``yes''. If this options is set to ``without-password'' only
password authentication is disabled for root.
Root login with RSA authentication when the command option has
been specified will be allowed regardless of the value of this
setting (which may be useful for taking remote backups even if
root login is normally not allowed).
PidFile
Specifies the file that contains the process identifier of the
sshd daemon. The default is /var/run/sshd.pid.
Port Specifies the port number that sshd listens on. The default is
22. Multiple options of this type are permitted.
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 possi-
ble values are ``1'' and ``2''. Multiple versions must be comma-
separated. The default is ``1''.
RandomSeed
Obsolete. Random number generation uses other techniques.
RhostsAuthentication
Specifies whether authentication using rhosts or /etc/hosts.equiv
files is sufficient. Normally, this method should not be permit-
ted because it is insecure. RhostsRSAAuthentication should be
used instead, because it performs RSA-based host authentication
in addition to normal rhosts or /etc/hosts.equiv authentication.
The default is ``no''.
RhostsRSAAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication
together with successful RSA host authentication is allowed. The
default is ``no''.
RSAAuthentication
Specifies whether pure RSA authentication is allowed. The
default is ``yes''. Note that this option applies to protocol
version 1 only.
ServerKeyBits
Defines the number of bits in the server key. The minimum value
is 512, and the default is 768.
Configures an external subsystem (e.g., file transfer 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 protocol 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, LOCAL7. The
default is AUTH.
UseLogin
Specifies whether login(1) is used for interactive login ses-
sions. Note that login(1) is never used for remote command exe-
cution. The default is ``no''.
X11DisplayOffset
Specifies the first display number available for sshd's X11 for-
warding. This prevents sshd from interfering with real X11
servers. The default is 10.
X11Forwarding
Specifies whether X11 forwarding is permitted. The default is
``no''. Note that disabling X11 forwarding does not improve
security in any way, as users can always install their own for-
warders.
XAuthLocation
Specifies the location of the xauth(1) program. The default is
/usr/X11R6/bin/xauth.
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 sec-
tion).
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 $HOME/.ssh/environment if it exists.
comments). Each line consists of the following fields, separated by spa-
ces: options, bits, exponent, modulus, comment. The options field is
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, modulus and comment fields give the RSA key; the comment field
is not used for anything (but may be convenient for the user to identify
the key).
Note that lines in this file are usually several hundred bytes long
(because of the size of the RSA key modulus). You don't want to type
them in; instead, copy the identity.pub file and edit it.
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:
from="pattern-list"
Specifies that in addition to RSA 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: 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 user (if any) is
ignored. The command is run on a pty if the connection requests
a pty; otherwise it is run without a tty. A quote may be
included in the command by quoting it with a backslash. This
option might be useful to restrict certain RSA keys to perform
just a specific operation. An example might be a key that per-
mits remote backups but nothing else. Note that the client may
specify TCP/IP and/or X11 forwarding unless they are explicitly
prohibited.
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.
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
error. This might be used, e.g., in connection with the command
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
SSH_KNOWN_HOSTS FILE FORMAT
The /etc/ssh_known_hosts, /etc/ssh_known_hosts2, $HOME/.ssh/known_hosts,
and $HOME/.ssh/known_hosts2 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 wild-
cards); 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/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 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_host_key.pub and
adding the host names at the front.
Examples
closenet,closenet.hut.fi,...,130.233.208.41 1024 37 159...93
closenet.hut.fi
FILES
/etc/sshd_config
Contains configuration data for sshd. This file should be
writable by root only, but it is recommended (though not neces-
sary) that it be world-readable.
/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 con-
tent of this file is not sensitive; it can be world-readable.
$HOME/.ssh/authorized_keys
Lists the RSA keys 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 direc-
tory 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 files
into this file, as described in ssh-keygen(1).
$HOME/.ssh/authorized_keys2
Lists the DSA keys 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 direc-
tory 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 id_dsa.pub files
into this file, as described in ssh-keygen(1).
/etc/ssh_known_hosts and $HOME/.ssh/known_hosts
These files are consulted when using rhosts with RSA host authen-
tication 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 the remote host is the one it intended
to connect. These files should be writable only by root/the
owner. /etc/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.
/etc/hosts.allow, /etc/hosts.deny
If compiled with LIBWRAP support, tcp-wrappers access controls
may be defined here as 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 recom-
mended that it not be accessible by others.
If is also possible to use netgroups in the file. Either host or
in as any user on this machine (except root). 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/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.
$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 com-
mand. If X11 spoofing is in use, this will receive the "proto
cookie" pair in standard input (and DISPLAY in environment).
This must call xauth(1) in that case.
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 envi-
ronment.
This file will probably contain some initialization code followed
by something similar to: "if read proto cookie; then echo add
$DISPLAY $proto $cookie | xauth -q -; fi".
If this file does not exist, /etc/sshrc is run, and if that does
not exist either, xauth is used to store the cookie.
This file should be writable only by the user, and need not be
o has all components of a restrictive nature (i.e., patents, see
crypto(3)) directly removed from the source code; any licensed or
patented components are chosen from external libraries.
o has been updated to support SSH protocol 1.5 and 2, making it compat-
ible with all other SSH clients and servers.
o contains added support for kerberos(8) authentication and ticket
passing.
o supports one-time password authentication with skey(1).
OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
Niels Provos, Theo de Raadt, and Dug Song.
The support for SSH protocol 2 was written by Markus Friedl.
SEE ALSO
scp(1), sftp-server(8), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
crypto(3), rlogin(1), rsh(1)
BSD September 25, 1999 BSD
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