SSHD(8) BSD System Manager's Manual SSHD(8)NAMEsshd - OpenSSH SSH daemon
SYNOPSISsshd [-deiqD46] [-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]
DESCRIPTIONsshd (SSH Daemon) is the daemon program for ssh(1). Together these pro-
grams replace rlogin and rsh, and provide secure encrypted communications
between two untrusted hosts over an insecure network. The programs are
intended to be as easy to install and use as possible.
sshd is the daemon that listens for connections from clients. It is nor-
mally started at boot from /etc/rc. It forks a new daemon for each incom-
ing connection. The forked daemons handle key exchange, encryption, au-
thentication, 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 iden-
tify the host. Additionally, when the daemon starts, it generates a
server RSA key (normally 768 bits). This key is normally regenerated ev-
ery hour if it has been used, and is never stored on disk.
Whenever a client connects the daemon responds with its public host and
server keys. The client compares the RSA host key against its own
database to verify that it has not changed. The client then generates a
256 bit random number. It encrypts this random number using both the
host key and the server key, and sends the encrypted number to the serv-
er. Both sides then use this random number as a session key which is
used to encrypt all further communications in the session. The rest of
the session is encrypted using a conventional cipher, currently Blowfish
or 3DES, with 3DES being used by default. The client selects the encryp-
tion 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-re-
sponse 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
128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit
AES. The client selects the encryption algorithm to use from those of-
fered by the server. Additionally, session integrity is provided through
a cryptographic message authentication code (hmac-sha1 or hmac-md5).
Protocol version 2 provides a public key based user (PubkeyAuthentica-
tion) or client host (HostbasedAuthentication) authentication method,
conventional password authentication and challenge response based meth-
ods.
Command execution and data forwarding
If the client successfully authenticates itself, a dialog for preparing
the session is entered. At this time the client may request things like
allocating a pseudo-tty, forwarding X11 connections, 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 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, 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 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 op-
tions 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/sshd_config. sshd refuses to start if there is no configura-
tion file.
-g login_grace_time
Gives the grace time for clients to authenticate themselves (de-
fault 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 any-
one but root). 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 re-
generated every time. However, with small key sizes (e.g., 512)
using sshd from inetd may be feasible.
-k key_gen_time
Specifies how often the ephemeral protocol version 1 server key
is regenerated (default 3600 seconds, or one hour). The motiva-
tion for regenerating the key fairly often is that the key is not
stored anywhere, and after about an hour, it becomes impossible
to recover the key for decrypting intercepted 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 be-
ginning, 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 over-
flow 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.
-4 Forces sshd to use IPv4 addresses only.
-6 Forces sshd to use IPv6 addresses only.
CONFIGURATION FILEsshd 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 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 pat-
terns. `*' and `?' can be used as wildcards in the patterns.
Only group names are valid; a numerical group ID isn't recog-
nized. 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 se-
curity unless users are also denied shell access, as they can al-
ways 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 wild-
cards 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 authenti-
cation may be relevant for getting legal protection. The con-
tents of the specified file are sent to the remote user before
authentication is allowed. This option is only available for
protocol version 2.
ChallengeResponseAuthentication
Specifies whether challenge response authentication is allowed.
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''.
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. You want to use the client alive mechanism when you
are basing something important on clients having an active con-
nection to the server.
The default value is 3. If you set ClientAliveInterval (above) to
15, and leave this value at the default, unresponsive ssh clients
will be disconnected after approximately 45 seconds.
DenyGroups
This keyword can be followed by a number of group names, separat-
ed 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 de-
fault 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 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.
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''.
HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication to-
gether 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 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-accessi-
ble. 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 de-
fault 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 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 tem-
porarily, 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 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
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 ses-
sions 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 follow-
ing forms may be used:
ListenAddress host|IPv4_addr|IPv6_addr
ListenAddress host|IPv4_addr:port
ListenAddress [host|IPv6_addr]:port
If port is not specified, sshd will listen on the address and all
prior Port options specified. The default is to listen on all lo-
cal addresses. Multiple ListenAddress options are permitted. Ad-
ditionally, any Port options must precede this option for non
port qualified addresses.
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, VERBOSE
and DEBUG. The default is INFO. Logging with level DEBUG vio-
lates the privacy of users and is not recommended.
MACs Specifies the available MAC (message authentication code) algo-
rithms. The MAC algorithm is used in protocol version 2 for data
integrity protection. Multiple algorithms must be comma-separat-
ed. 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 con-
nections to the sshd daemon. Additional connections will be
dropped until authentication succeeds or the LoginGraceTime ex-
pires 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-
bility of ``rate/100'' (30%) if there are currently ``start''
(10) unauthenticated connections. The probability increases lin-
early and all connection attempts are refused if the number of
unauthenticated connections reaches ``full'' (60).
PAMAuthenticationViaKbdInt
Specifies whether PAM challenge response authentication is al-
lowed. This allows the use of most PAM challenge response authen-
tication modules, but it will allow password authentication re-
gardless of whether PasswordAuthentication is disabled. The de-
fault is ``no''.
PasswordAuthentication
Specifies whether password authentication is allowed. The de-
fault 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 authenti-
cation is disabled for root.
If this option is set to ``forced-commands-only'' root login with
public key authentication will be allowed, but only if the
command option has been specified (which may be 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 possi-
ble 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 de-
fault is ``yes''. Note that this option applies to protocol ver-
sion 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 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''. This option applies to protocol version 1
only.
RhostsRSAAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication to-
gether with successful RSA host authentication is allowed. The
default is ``no''. This option applies to protocol version 1 on-
ly.
RSAAuthentication
Specifies whether pure RSA authentication is allowed. The de-
fault 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 up-
on 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 on-
ly.
SyslogFacility
Gives the facility code that is used when logging messages from
sshd. The possible values are: DAEMON, USER, AUTH, LOCAL0, LO-
CAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The de-
fault 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 secu-
rity 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.
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 protocol version 1 Similarly, the
$HOME/.ssh/authorized_keys2 file lists the DSA and RSA keys that are per-
mitted for public key authentication (PubkeyAuthentication) in protocol
version 2.
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, key-
type, base64 encoded key, comment. The options fields are optional; its
presence is determined by whether the line starts with a number or not
(the option field never starts with a number). The bits, exponent, modu-
lus and comment fields give the RSA key for protocol version 1; the com-
ment field is not used for anything (but may be convenient for the user
to identify the key). For protocol version 2 the keytype is ``ssh-dss''
or ``ssh-rsa''.
Note that lines in this file are usually several hundred bytes long (be-
cause 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.
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 ac-
cepted. The purpose of this option is to optionally increase se-
curity: 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 spec-
ify no-pty. A quote may be included in the command by quoting it
with a backslash. This option might be useful to restrict cer-
tain RSA keys to perform just a specific operation. An example
might be a key that permits remote backups but nothing else.
Note that the client may specify TCP/IP and/or X11 forwarding un-
less they are explicitly prohibited.
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 er-
ror. This might be used, e.g., in connection with the command
option.
no-X11-forwarding
Forbids X11 forwarding when this key is used for 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 con-
nect to the specified host and port. Multiple permitopen options
may be applied separated by commas. No pattern matching is per-
formed 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 back-
up.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 (option-
al), 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 indi-
cate negation: if the host name matches a negated pattern, it is not ac-
cepted (by that line) even if it matched another pattern on the line.
Bits, exponent, and modulus are taken directly from the RSA host key;
they can be obtained, e.g., from /etc/ssh_host_key.pub. The optional com-
ment 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,...,130.233.208.41 1024 37 159...93 closenet.hut.fi
cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
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.
/etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/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_host_key.pub, /etc/ssh_host_dsa_key.pub,
/etc/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/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 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 ac-
count. This file must be readable by root (which may on some ma-
chines imply it being world-readable if the user's home directory
resides on an NFS volume). It is recommended that it not be ac-
cessible 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 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 de-
scribed above. Users will place the contents of their id_dsa.pub
and/or id_rsa.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 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/ssh_known_hosts2 and $HOME/.ssh/known_hosts2
These files are consulted when using 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 re-
mote host. These files should be writable only by root/the own-
er. /etc/ssh_known_hosts2 should be world-readable, and
$HOME/.ssh/known_hosts2 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
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 en-
tries start with `-'.
If the client host/user is successfully matched in this file, lo-
gin is automatically permitted provided the client and server us-
er names are the same. Additionally, successful RSA host authen-
tication is normally required. This file must be writable only
by root; it is recommended that it be world-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 us-
er 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 en-
vironment 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 be-
comes accessible; AFS is a particular example of such an environ-
ment.
This file will probably contain some initialization code followed
by something similar to:
if read proto cookie; 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 cre-
ated OpenSSH. Markus Friedl contributed the support for SSH protocol
versions 1.5 and 2.0.
SEE ALSOscp(1), sftp(1), sftp-server(8), ssh(1), ssh-add(1), ssh-agent(1),
ssh-keygen(1), rlogin(1), rsh(1)
T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, SSH
Protocol Architecture, draft-ietf-secsh-architecture-07.txt, January
2001, work in progress material.
M. Friedl, N. Provos, and W. A. Simpson, Diffie-Hellman Group Exchange
for the SSH Transport Layer Protocol, draft-ietf-secsh-dh-group-
exchange-00.txt, January 2001, work in progress material.
BSDI BSD/OS September 25, 1999 14
