SSHD(8) BSD System Manager's Manual SSHD(8)NAMEsshd - OpenSSH SSH daemon
SYNOPSISsshd [-46DdeiqTt] [-b bits] [-C connection_spec] [-f config_file]
[-g login_grace_time] [-h host_key_file] [-k key_gen_time]
[-o option] [-p port] [-u len]
DESCRIPTIONsshd (OpenSSH Daemon) is the daemon program for ssh(1). Together these
programs replace rlogin(1) and rsh(1), and provide secure encrypted com-
munications between two untrusted hosts over an insecure network.
sshd 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.
sshd can be configured using command-line options or a configuration file
(by default sshd_config(5)); command-line options override values speci-
fied in the configuration file. sshd rereads its configuration file when
it receives a hangup signal, SIGHUP, by executing itself with the name
and options it was started with, e.g. /usr/sbin/sshd.
The options are as follows:
-4 Forces sshd to use IPv4 addresses only.
-6 Forces sshd to use IPv6 addresses only.
-b bits
Specifies the number of bits in the ephemeral protocol version 1
server key (default 1024).
-C connection_spec
Specify the connection parameters to use for the -T extended test
mode. If provided, any Match directives in the configuration file
that would apply to the specified user, host, and address will be
set before the configuration is written to standard output. The
connection parameters are supplied as keyword=value pairs. The
keywords are "user", "host", and "addr". All are required and may
be supplied in any order, either with multiple -C options or as a
comma-separated list.
-D When this option is specified, sshd will not detach and does not
become a daemon. This allows easy monitoring of sshd.
-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 config_file
Specifies the name of the configuration file. The default is
/etc/ssh/sshd_config. sshd refuses to start if there is no confi-
guration file.
-g login_grace_time
Gives the grace time for clients to authenticate themselves (de-
fault 120 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 pro-
tocol 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(8). 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.
-o option
Can be used to give options in the format used in the configura-
tion file. This is useful for specifying options for which there
is no separate command-line flag. For full details of the op-
tions, and their values, see sshd_config(5).
-p port
Specifies the port on which the server listens for connections
(default 22). Multiple port options are permitted. Ports speci-
fied in the configuration file with the Port option are ignored
when a command-line port is specified. Ports specified using the
ListenAddress option override command-line ports.
-q Quiet mode. Nothing is sent to the system log. Normally the be-
ginning, authentication, and termination of each connection is
logged.
-T Extended test mode. Check the validity of the configuration file,
output the effective configuration to stdout and then exit. Op-
tionally, Match rules may be applied by specifying the connection
parameters using one or more -C options.
-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 over-
flow this field to still be uniquely identified. Specifying -u0
indicates that only dotted decimal addresses should be put into
the utmp file. -u0 may also be used to prevent sshd from making
DNS requests unless the authentication mechanism or configuration
requires it. Authentication mechanisms that may require DNS in-
clude 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.
AUTHENTICATION
The OpenSSH SSH daemon supports SSH protocols 1 and 2. Both protocols are
supported by default, though this can be changed via the Protocol option
in sshd_config(5). Protocol 2 supports both RSA and DSA keys; protocol 1
only supports RSA keys. For both protocols, each host has a host-specific
key, normally 2048 bits, used to identify the host. Much longer RSA keys
should be used, but this makes logins very slow if less powerful machines
are involved. DSA keys cannot be longer than 2048 bits due to misdesign,
though. On MirOS, only protocol 2 RSA is enabled by default to improve
security.
Forward security for protocol 1 is provided through an additional server
key, normally 768 bits, generated when the server starts. 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 Blowfish being used by default. The client selects the en-
cryption algorithm to use from those offered by the server.
For protocol 2, 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 crypto-
graphic message authentication code (umac-64, hmac-ripemd160, hmac-sha1
or hmac-md5).
Finally, the server and the client enter an authentication dialog. The
client tries to authenticate itself using host-based authentication, pub-
lic key authentication, challenge-response authentication, or password
authentication.
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 con-
nections, or forwarding the authentication agent connection over the
secure channel.
After this, 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.
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 ~/.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 the file ~/.etc/ssh/environment, if it exists, and users
are allowed to change their environment. See the
PermitUserEnvironment option in sshd_config(5).
7. Changes to user's home directory.
8. If ~/.etc/ssh/rc exists, runs it; else if /etc/ssh/sshrc ex-
ists, runs it; otherwise runs xauth. The "rc" files are given
the X11 authentication protocol and cookie in standard input.
See SSHRC, below.
9. Runs user's shell or command.
SSHRC
If the file ~/.etc/ssh/rc exists, sh(1) runs it after reading the en-
vironment 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 cook-
ies.
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
echo add unix:`echo $DISPLAY |
cut -c11-` $proto $cookie
else
# X11UseLocalhost=no
echo add $DISPLAY $proto $cookie
fi | xauth -q -
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.
AUTHORISED_KEYS FILE FORMAT
AuthorisedKeysFile specifies the file containing public keys for public
key authentication; if none is specified, the default is
~/.etc/ssh/authorised_keys. If the user is the superuser and his home
directory the root or unset, /etc/ssh/root:authorised_keys is used in-
stead. Each line of the file contains one key (empty lines and lines
starting with a '#' are ignored as comments). Protocol 1 public keys con-
sist of the following space-separated fields: options, bits, exponent,
modulus, comment. Protocol 2 public key consist of: options, keytype,
base64-encoded key, comment. The options field is optional; its presence
is determined by whether the line starts with a number or not (the op-
tions 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 (be-
cause of the size of the public key encoding) up to a limit of 8 kilo-
bytes, which permits DSA keys up to 8 kilobits and RSA keys up to 16 ki-
lobits. 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 specifica-
tions. No spaces are permitted, except within double quotes. The follow-
ing option specifications are supported (note that option keywords are
case-insensitive):
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 an 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 public
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 and/or X11 forwarding unless they are ex-
plicitly prohibited. The command originally supplied by the
client is available in the SSH_ORIGINAL_COMMAND environment vari-
able. Note that this option applies to shell, command or subsys-
tem 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. Environment processing is disabled by
default and is controlled via the PermitUserEnvironment option.
This option is automatically disabled if UseLogin is enabled.
from="pattern-list"
Specifies that in addition to public key authentication, either
the canonical name of the remote host or its IP address must be
present in the comma-separated list of patterns. See PATTERNS in
ssh_config(5) for more information on patterns.
In addition to the wildcard matching that may be applied to host-
names or addresses, a from stanza may match IP addresses using
CIDR address/masklen notation.
The purpose of this option is to optionally increase security:
public key 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).
no-agent-forwarding
Forbids authentication agent forwarding when this key is used for
authentication.
no-port-forwarding
Forbids TCP 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-pty Prevents tty allocation (a request to allocate a pty will fail).
no-user-rc
Disables execution of ~/.etc/ssh/rc.
no-X11-forwarding
Forbids X11 forwarding when this key is used for authentication.
Any X11 forward requests by the client will return an error.
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 op-
tions may be applied separated by commas. No pattern matching is
performed on the specified hostnames, they must be literal
domains or addresses.
tunnel="n"
Force a tun(4) device on the server. Without this option, the
next available device will be used if the client requests a tun-
nel.
An example authorised_keys file:
# Comments allowed at start of line
ssh-rsa AAAAB3Nza...LiPk== user@example.net
from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
AAAAB2...19Q== john@example.net
command="dump /home",no-pty,no-port-forwarding ssh-dss
AAAAC3...51R== example.net
permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
AAAAB5...21S==
tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
jane@example.net
SSH_KNOWN_HOSTS FILE FORMAT
The /etc/ssh/ssh_known_hosts and ~/.etc/ssh/known_hosts and
/etc/ssh/root:known_hosts, if the user is the superuser and his home
directory the root or unset, 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. A
hostname or address may optionally be enclosed within '[' and ']' brack-
ets then followed by ':' and a non-standard port number.
Alternately, hostnames may be stored in a hashed form which hides host
names and addresses should the file's contents be disclosed. Hashed host-
names start with a '|' character. Only one hashed hostname may appear on
a single line and none of the above negation or wildcard operators may be
applied.
Bits, exponent, and modulus are taken directly from the RSA host key;
they can be obtained, for example, 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 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/ssh_host_key.pub
and adding the host names at the front.
An example ssh_known_hosts file:
# Comments allowed at start of line
closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
# A hashed hostname
|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
AAAA1234.....=
FILES
~/.hushlogin
This file is used to suppress printing the last login time and
/etc/motd, if PrintLastLog and PrintMotd, respectively, are en-
abled. It does not suppress printing of the banner specified by
Banner.
~/.rhosts
This file is used for host-based authentication (see ssh(1) for
more information). On some machines this file may need to be
world-readable if the user's home directory is on an NFS parti-
tion, because sshd reads it as root. Additionally, this file must
be owned by the user, and must not have write permissions for
anyone else. The recommended permission for most machines is
read/write for the user, and not accessible by others.
~/.shosts
This file is used in exactly the same way as .rhosts, but allows
host-based authentication without permitting login with
rlogin/rsh.
~/.etc/ssh/
This directory is the default location for all user-specific con-
figuration and authentication information. There is no general
requirement to keep the entire contents of this directory secret,
but the recommended permissions are read/write/execute for the
user, and not accessible by others.
~/.etc/ssh/authorised_keys
Lists the public keys (RSA/DSA) that can be used for logging in
as this user. The format of this file is described above. The
content of the file is not highly sensitive, but the recommended
permissions are read/write for the user, and not accessible by
others. If this file, the ~/.etc/ssh directory, or the user's
home directory are writable by other users, then the file could
be modified or replaced by unauthorised users. In this case, sshd
will not allow it to be used unless the StrictModes option has
been set to "no".
/etc/ssh/root:authorised_keys
The same, for the superuser, if his home is the root directory or
unset.
~/.ssh/authorized_keys
Portable name for the above file.
~/.etc/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. Environment processing is disabled by default and is
controlled via the PermitUserEnvironment option.
~/.etc/ssh/known_hosts
Contains a list of host keys for all hosts the user has logged
into that are not already in the systemwide list of known host
keys. The format of this file is described above. This file
should be writable only by root/the owner and can, but need not
be, world-readable.
/etc/ssh/root:known_hosts
The same, for the superuser, if his home is the root directory or
unset.
~/.etc/ssh/rc
Contains initialization routines to be run before the user's home
directory becomes accessible. This file should be writable only
by the user, and need not be readable by anyone else.
/etc/hosts.allow
/etc/hosts.deny
Access controls that should be enforced by tcp-wrappers are de-
fined here. Further details are described in hosts_access(5).
/etc/hosts.equiv
This file is for host-based authentication (see ssh(1)). It
should only be writable by root.
/etc/moduli
Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
Exchange". The file format is described in moduli(5).
/etc/motd
See motd(5).
/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/shosts.equiv
This file is used in exactly the same way as hosts.equiv, but al-
lows host-based authentication without permitting login with
rlogin/rsh.
/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
these files are 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/ssh/ssh_known_hosts
Systemwide list of known host keys. This file should be prepared
by the system administrator to contain the public host keys of
all machines in the organization. The format of this file is
described above. This file should be writable only by root/the
owner and should be world-readable.
/etc/ssh/sshd_config
Contains configuration data for sshd. The file format and confi-
guration options are described in sshd_config(5).
/etc/ssh/sshrc
Similar to ~/.etc/ssh/rc, it can be used to specify machine-
specific login-time initializations globally. This file should be
writable only by root, and should be world-readable.
/var/empty
chroot(2) directory used by sshd during privilege separation in
the pre-authentication phase. The directory should not contain
any files and must be owned by root and not group or world-
writable.
/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 process ID of the one started last). The
content of this file is not sensitive; it can be world-readable.
SEE ALSOscp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-
keyscan(1), chroot(2), hosts_access(5), login.conf(5), moduli(5),
sshd_config(5), inetd(8), sftp-server(8)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.
CAVEATS
System security is not improved unless rshd, rlogind, and rexecd are dis-
abled (thus completely disabling rlogin and rsh into the machine).
MirOS BSD #10-current January 15, 2011 8
