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

NAME
sshd – OpenSSH ssh daemon

SYNOPSIS
sshd [-diqD46] [-b bits] [-f config_file] [-g login_grace_time]
[-h host_key_file] [-k key_gen_time] [-p port] [-u len]
[-V client_protocol_id]

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), 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, CAST128, Arcfour, 128 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 authentication method
(PubkeyAuthentication) and conventional password authentication.

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, ie.
/usr/sbin/sshd.

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
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 the file from which the 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
anyone but root). It is possible to have multiple host key files
for the different protocol versions.

-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
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.

-p port
Specifies the port on which the server listens for connections
(default 22).

-q Quiet mode. Nothing is sent to the system log. Normally the
beginning, authentication, and termination of each connection is
logged.

-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.

-D When this option is specified sshd will not detach and does not
become a daemon. This allows easy monitoring of sshd.

-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
specified with -f on the command line). The file contains keyword-value
pairs, one per line. Lines starting with ‘#’ and empty lines are
interpreted 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 list of group names, 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 isn't
recognized. By default login is allowed regardless of the group
list.

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 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
numerical user ID isn't recognized. By default login is allowed
regardless of the user name.

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.

ChallengeResponseAuthentication
Specifies whether challenge response authentication is allowed.
Currently there is only support for skey(1) authentication. 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.”

CheckMail
Specifies whether sshd should check for new mail for interactive
logins. The default is “no”.

DenyGroups
This keyword can be followed by a number of group names,
separated by spaces. Users whose primary group or supplementary
group list 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 numerical group ID isn't recognized. By
default login is allowed regardless of the group list.

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
patterns. Only user names are valid; a numerical user ID isn't
recognized. By default login is allowed regardless of the user
name.

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”.

HostKey
Specifies the file containing the private host keys (default
/etc/ssh_host_key) used by SSH protocol versions 1 and 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
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
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 reboots. This
avoids infinitely hanging sessions.

To disable keepalives, the value should be set to “no” in both
the server and the client configuration files.

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
The 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 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
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 and DEBUG. The default is INFO. Logging with level
DEBUG 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-ripemd160@openssh.com,
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).

PasswordAuthentication
Specifies whether password authentication is allowed. The
default is “yes”. Note that this option applies to both protocol
versions 1 and 2.

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.

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 “1”.

PubkeyAuthentication
Specifies whether public key authentication is allowed. The
default is “yes”. Note that this option applies to protocol
version 2 only.

ReverseMappingCheck
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”.

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”.

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.

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. Note that login(1) is never used for remote command
execution. 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.

XAuthLocation
Specifies the location of the xauth(1) program. The default is
/usr/X11R6/bin/xauth.

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/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
The $HOME/.ssh/authorized_keys file lists the RSA keys that are permitted
for RSA authentication in SSH protocols 1.3 and 1.5 Similarly, the
$HOME/.ssh/authorized_keys2 file lists the DSA and RSA keys that are
permitted for public key authentication (PubkeyAuthentication) in SSH
protocol 2.0.

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 or the id_dsa.pub file and edit
it.

The options (if present) consist of comma-separated option
specifications. No spaces are permitted, except within double quotes.
The following 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. Note that if you want
a 8-bit clean channel, you 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.

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
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-forwading such that it may only
connect to the specified host and port. Multiple permitopen
options may be applied seperated 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_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
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_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_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
necessary) that it be world-readable.

/etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/ssh_host_rsa_key
These three files contain the private parts of the (SSH1, SSH2
DSA, and SSH2 RSA) 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_host_key.pub, /etc/ssh_host_dsa_key.pub,
/etc/ssh_host_rsa_key.pub
These three files contain the public parts of the (SSH1, SSH2
DSA, and SSH2 RSA) 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/primes
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 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
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 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
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
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
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_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
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. 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
environment.

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
readable by anyone else.

/etc/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.

SEE ALSO
scp(1), sftp(1), sftp-server(8), ssh(1), ssh-add(1), ssh-agent(1),
ssh-keygen(1), rlogin(1), rsh(1)

BSD September 25, 1999 BSD