SSHD(8) System Manager's Manual SSHD(8)

NAME
sshd – OpenSSH SSH daemon

SYNOPSIS
sshd [-deiqtD46] [-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/rc. 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.

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), 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 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.

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. 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 it was started as, i.e.,
/usr/sbin/sshd.

The options are as follows:

-b bits
Specifies the number of bits in the ephemeral protocol 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. Multiple -d
options increase the debugging level. Maximum is 3.

-e When this option is specified, sshd will send the output to the
standard error instead of the system log.

-f configuration_file
Specifies the name of the configuration file. The default is
/etc/ssh/sshd_config. sshd refuses to start if there is no
configuration file.

-g login_grace_time
Gives the grace time for clients to authenticate themselves
(default 600 seconds). If the client fails to authenticate the
user within this many seconds, the server disconnects and exits.
A value of zero indicates no limit.

-h host_key_file
Specifies a file from which a host key is read. This option must
be given if sshd is not run as root (as the normal host key files
are normally not readable by anyone but root). The default is
/etc/ssh/ssh_host_key for protocol version 1, and
/etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for
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. 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.

-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 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 is 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 RhostsAuthentication,
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.

-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-
argument 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
keywords are case-insensitive and arguments are case-sensitive):

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 wildcards in the
patterns. Only group names are valid; a numerical group ID is
not recognized. By default, login is allowed for all groups.

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 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, restricting 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 connection set-up.
The following tokens are defined: %% is replaced by a literal
'%', %h is replaced by the home directory of the user 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 before
authentication may be relevant for getting legal protection. The
contents of the specified file are sent to the remote user before
authentication is allowed. This option is only available for
protocol version 2. By default, no banner is displayed.

ChallengeResponseAuthentication
Specifies whether challenge response authentication is allowed.
All authentication styles from login.conf(5) are supported. The
default is “yes”.

Ciphers
Specifies the ciphers allowed for protocol version 2. Multiple
ciphers must be comma-separated. The default is

``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
aes192-cbc,aes256-cbc''

ClientAliveInterval
Sets a timeout interval in seconds after which if no data has
been received from the client, sshd will send a message 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 messages 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
inactive.

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 patterns. Only
group names are valid; a numerical group ID is not recognized.
By default, login is allowed for all groups.

DenyUsers
This keyword can be followed by a list of user name patterns,
separated by spaces. Login is disallowed for user 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, restricting logins to particular users from
particular hosts.

GatewayPorts
Specifies whether remote hosts are allowed to connect 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. GatewayPorts 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 authentication
together with successful public key client host authentication is
allowed (hostbased authentication). This option is similar to
RhostsRSAAuthentication and applies to protocol version 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 version 1, and
/etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for
protocol 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 used. The
default is “yes”.

IgnoreUserKnownHosts
Specifies whether sshd should ignore the user's
$HOME/.ssh/known_hosts during RhostsRSAAuthentication or
HostbasedAuthentication. The default is “no”.

KeepAlive
Specifies whether the system should send TCP 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
temporarily, and some people find it annoying. On the other
hand, if keepalives are not sent, sessions may hang indefinitely
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 allowed. This can
be in the form of a Kerberos ticket, or if PasswordAuthentication
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
identity. 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
In protocol version 1, the ephemeral server key is automatically
regenerated after this many seconds (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 the local addresses sshd should listen on. The
following 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 listen on all
local addresses. Multiple ListenAddress options are permitted.
Additionally, any Port options must precede this option for non
port qualified addresses.

LoginGraceTime
The server disconnects after this time if the user has not
successfully 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,
VERBOSE, 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 recommended.

MACs Specifies the available MAC (message authentication code)
algorithms. The MAC algorithm is used in protocol version 2 for
data integrity protection. Multiple algorithms must be comma-
separated. The default is
“hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96”.

MaxStartups
Specifies the maximum number of concurrent unauthenticated
connections 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
probability of “rate/100” (30%) if there are currently “start”
(10) unauthenticated connections. The probability increases
linearly and all connection attempts are refused if the number of
unauthenticated connections reaches “full” (60).

PAMAuthenticationViaKbdInt
Specifies whether PAM challenge response authentication is
allowed. This allows the use of most PAM challenge response
authentication modules, but it will allow password authentication
regardless of whether PasswordAuthentication is disabled. The
default is “no”.

PasswordAuthentication
Specifies whether password authentication is allowed. The
default is “yes”.

PermitEmptyPasswords
When password authentication is allowed, it specifies 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” password
authentication 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 useful 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.

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. See also
ListenAddress.

PrintLastLog
Specifies whether sshd should print the date and 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
possible values are “1” and “2”. Multiple versions must be
comma-separated. The default is “2,1”.

PubkeyAuthentication
Specifies whether public key authentication is allowed. 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
permitted 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”. This option applies to
protocol version 1 only.

RhostsRSAAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication
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 allowed. The
default is “yes”. This option applies to protocol version 1
only.

ServerKeyBits
Defines the number of bits in the ephemeral protocol version 1
server key. The minimum value is 512, and the default is 768.

StrictModes
Specifies whether sshd should check file modes and ownership of
the user's files and home directory before accepting login. This
is normally desirable because novices sometimes accidentally
leave their directory or files world-writable. The default is
“yes”.

Subsystem
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
sessions. 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 login(1) does not
know how to handle xauth(1) cookies. If UsePrivilegeSeparation
is specified, it will be disabled after authentication.

UsePrivilegeSeparation
Specifies whether sshd separated privileges by creating an
unprivileged child process to deal with incoming network traffic.
After successful authentication, another process will be created
that has the privilege of the authenticated user. The goal of
privilege separation is to prevent privilege escalation by
containing any corruption within the unprivileged processes. The
default is “no”.

VerifyReverseMapping
Specifies whether sshd should try to verify the remote 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
forwarding. 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
forwarders. X11 forwarding is automatically disabled if UseLogin
is enabled.

X11UseLocalhost
Specifies whether sshd should bind the X11 forwarding server to
the loopback address or to the wildcard address. By default,
sshd binds the forwarding server to the loopback address and sets
the hostname part of the DISPLAY environment variable to
“localhost”. This prevents remote hosts from connecting 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
wildcard 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
following:

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)

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 $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 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 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,
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 RSA key modulus). 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 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 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
backslash. This option might be useful to restrict certain 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 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. 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.

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 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/ssh/ssh_host_key.pub. The optional
comment field continues to the end of the line, and is not used.

Lines starting with ‘#’ and empty lines are ignored as comments.

When performing host authentication, authentication is accepted if any
matching line has the proper key. It is thus permissible (but not
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 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
writable by root only, but it is recommended (though not
necessary) that it be world-readable.

/etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key,
/etc/ssh/ssh_host_rsa_key
These three files contain the private parts of the host keys.
These files should only be owned by root, readable only by root,
and not accessible to others. Note that sshd does not start if
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-keygen(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
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 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/ssh/ssh_known_hosts and $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/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 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
Access controls that should be enforced by tcp-wrappers 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
recommended 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 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). 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
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 code followed
by something similar to:

if read proto cookie && [ -n "$DISPLAY" ]; then
if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
# X11UseLocalhost=yes
xauth add unix:`echo $DISPLAY |
cut -c11-` $proto $cookie
else
# X11UseLocalhost=no
xauth add $DISPLAY $proto $cookie
fi
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 machine-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 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.

SEE ALSO
scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
login.conf(5), moduli(5), sftp-server(8)

T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, 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 Group Exchange
for the SSH Transport Layer Protocol, draft-ietf-secsh-dh-group-
exchange-02.txt, January 2002, work in progress material.

BSD September 25, 1999 BSD