NAME¶
sshd —
OpenSSH SSH daemon
SYNOPSIS¶
| sshd |
[-46DdeiqTt]
[-b bits]
[-C connection_spec]
[-c host_certificate_file]
[-f config_file]
[-g login_grace_time]
[-h host_key_file]
[-k key_gen_time]
[-o option]
[-p port]
[-u len] |
DESCRIPTION¶
sshd (OpenSSH Daemon) is the daemon program for
ssh(1). Together these programs replace
rlogin(1) and
rsh(1), and provide secure
encrypted communications between two untrusted hosts over an insecure network.
sshd listens for connections from clients. It is normally
started at boot from
/etc/init.d/ssh. 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 specified 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.
- -c
host_certificate_file
- Specifies a path to a certificate file to identify
sshd during key exchange. The certificate file must
match a host key file specified using the -h option or
the HostKey configuration directive.
- -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
standard error, and does not put itself in the background. The server also
will not fork and will only process one connection. This option is only
intended for debugging for the server. Multiple -d
options increase the debugging level. Maximum is 3.
- -e
- When this option is specified, sshd will
send the output to the standard error instead of the system log.
- -f
config_file
- Specifies the name of the configuration file. The default
is /etc/ssh/sshd_config. sshd refuses
to start if there is no configuration file.
- -g
login_grace_time
- Gives the grace time for clients to authenticate themselves
(default 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_dsa_key,
/etc/ssh/ssh_host_ecdsa_key and
/etc/ssh/ssh_host_rsa_key for protocol version 2. It is
possible to have multiple host key files for the different protocol
versions and host key algorithms.
- -i
- Specifies that sshd is being run from
inetd(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 regenerated every time. However, with
small key sizes (e.g. 512) using sshd from inetd may be
feasible.
- -k
key_gen_time
- Specifies how often the ephemeral protocol version 1 server
key is regenerated (default 3600 seconds, or one hour). The motivation for
regenerating the key fairly often is that the key is not stored anywhere,
and after about an hour it becomes impossible to recover the key for
decrypting intercepted communications even if the machine is cracked into
or physically seized. A value of zero indicates that the key will never be
regenerated.
- -o
option
- Can be used to give options in the format used in the
configuration file. This is useful for specifying options for which there
is no separate command-line flag. For full details of the options, 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
specified 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
beginning, 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.
Optionally, 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 overflow 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
include 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. The default is to use
protocol 2 only, though this can be changed via the
Protocol
option in
sshd_config(5). Protocol 2 supports DSA, ECDSA and
RSA 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.
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 3DES being used by
default. The client selects the encryption 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 cryptographic message
authentication code (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160,
hmac-sha2-256 or hmac-sha2-512).
Finally, the server and the client enter an authentication dialog. The client
tries to authenticate itself using host-based authentication, public key
authentication, challenge-response authentication, or password authentication.
Regardless of the authentication type, the account is checked to ensure that it
is accessible. An account is not accessible if it is locked, listed in
DenyUsers or its group is listed in
DenyGroups . The definition of a locked account is system
dependant. Some platforms have their own account database (eg AIX) and some
modify the passwd field ( ‘
*LK*’ on
Solaris and UnixWare, ‘
*’ on HP-UX,
containing ‘
Nologin’ on Tru64, a leading
‘
*LOCKED*’ on FreeBSD and a leading
‘
!’ on most Linuxes). If there is a
requirement to disable password authentication for the account while allowing
still public-key, then the passwd field should be set to something other than
these values (eg ‘
NP’ or
‘
*NP*’ ).
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 connections, 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 connections
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:
- 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).
- If the login is on a tty, records login time.
- Checks /etc/nologin; if it exists,
prints contents and quits (unless root).
- Changes to run with normal user privileges.
- Sets up basic environment.
- Reads the file ~/.ssh/environment, if it
exists, and users are allowed to change their environment. See the
PermitUserEnvironment option in
sshd_config(5).
- Changes to user's home directory.
- If ~/.ssh/rc exists, runs it; else if
/etc/ssh/sshrc exists, runs it; otherwise runs xauth.
The “rc” files are given the X11 authentication protocol and
cookie in standard input. See SSHRC,
below.
- Runs user's shell or command.
SSHRC¶
If the file
~/.ssh/rc exists,
sh(1) runs it
after reading the environment files but before starting the user's shell or
command. It must not produce any output on stdout; stderr must be used
instead. If X11 forwarding is in use, it will receive the "proto
cookie" pair in its standard input (and
DISPLAY
in its environment). The script must call
xauth(1) because
sshd will not run xauth automatically to add X11 cookies.
The primary purpose of this file is to run any initialization routines which may
be needed before the user's home directory becomes accessible; AFS is a
particular example of such an environment.
This file will probably contain some initialization code followed by something
similar to:
if read proto cookie && [ -n "$DISPLAY" ]; then
if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
# X11UseLocalhost=yes
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.
AuthorizedKeysFile specifies the files containing public keys
for public key authentication; if none is specified, the default is
~/.ssh/authorized_keys and
~/.ssh/authorized_keys2. Each line of the file contains one
key (empty lines and lines starting with a
‘
#’ are ignored as comments). Protocol 1
public keys consist 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
options 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 “ecdsa-sha2-nistp256”,
“ecdsa-sha2-nistp384”, “ecdsa-sha2-nistp521”,
“ssh-dss” or “ssh-rsa”.
Note that lines in this file are usually several hundred bytes long (because of
the size of the public key encoding) up to a limit of 8 kilobytes, which
permits DSA keys up to 8 kilobits and RSA keys up to 16 kilobits. You don't
want to type them in; instead, copy the
identity.pub,
id_dsa.pub,
id_ecdsa.pub, or the
id_rsa.pub file and edit it.
sshd enforces a minimum RSA key modulus size for protocol 1
and protocol 2 keys of 768 bits.
The options (if present) consist of comma-separated option specifications. No
spaces are permitted, except within double quotes. The following option
specifications are supported (note that option keywords are case-insensitive):
- cert-authority
- Specifies that the listed key is a certification authority
(CA) that is trusted to validate signed certificates for user
authentication.
Certificates may encode access restrictions similar to these key options. If
both certificate restrictions and key options are present, the most
restrictive union of the two is applied.
- 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 explicitly prohibited. The command originally
supplied by the client is available in the
SSH_ORIGINAL_COMMAND environment variable. Note
that this option applies to shell, command or subsystem execution. Also
note that this command may be superseded by either a
sshd_config(5) ForceCommand directive
or a command embedded in a certificate.
- 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 hostnames 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 ~/.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 by enclosing the address in square
brackets. Multiple permitopen options may be applied
separated by commas. No pattern matching is performed on the specified
hostnames, they must be literal domains or addresses. A port specification
of * matches any port.
- principals="principals"
- On a cert-authority line, specifies
allowed principals for certificate authentication as a comma-separated
list. At least one name from the list must appear in the certificate's
list of principals for the certificate to be accepted. This option is
ignored for keys that are not marked as trusted certificate signers using
the cert-authority option.
- 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 tunnel.
An example authorized_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
The
/etc/ssh/ssh_known_hosts and
~/.ssh/known_hosts files contain host public keys for all
known hosts. The global file should be prepared by the administrator
(optional), and the per-user file is maintained automatically: whenever the
user connects from an unknown host, its key is added to the per-user file.
Each line in these files contains the following fields: markers (optional),
hostnames, bits, exponent, modulus, comment. The fields are separated by
spaces.
The marker is optional, but if it is present then it must be one of
“@cert-authority”, to indicate that the line contains a
certification authority (CA) key, or “@revoked”, to indicate that
the key contained on the line is revoked and must not ever be accepted. Only
one marker should be used on a key line.
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. A hostname or
address may optionally be enclosed within
‘
[’ and
‘
]’ brackets 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 hostnames 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; either one that matches exactly or, if the server has
presented a certificate for authentication, the key of the certification
authority that signed the certificate. For a key to be trusted as a
certification authority, it must use the “@cert-authority” marker
described above.
The known hosts file also provides a facility to mark keys as revoked, for
example when it is known that the associated private key has been stolen.
Revoked keys are specified by including the “@revoked” marker at
the beginning of the key line, and are never accepted for authentication or as
certification authorities, but instead will produce a warning from
ssh(1) when they are encountered.
It is 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,
ssh-keyscan(1) or by taking
/etc/ssh/ssh_host_key.pub and adding the host names at the
front.
ssh-keygen(1) also offers some basic automated
editing for
~/.ssh/known_hosts including removing hosts
matching a host name and converting all host names to their hashed
representations.
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.....=
# A revoked key
@revoked * ssh-rsa AAAAB5W...
# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
FILES¶
- ~/.hushlogin
- This file is used to suppress printing the last login time
and /etc/motd, if PrintLastLog and
PrintMotd, respectively, are enabled. 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
partition, 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.
- ~/.ssh/
- This directory is the default location for all
user-specific configuration 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.
- ~/.ssh/authorized_keys
- Lists the public keys (DSA/ECDSA/RSA) 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 ~/.ssh directory, or the user's home
directory are writable by other users, then the file could be modified or
replaced by unauthorized users. In this case, sshd will
not allow it to be used unless the StrictModes option
has been set to “no”.
- ~/.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.
- ~/.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.
- ~/.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
defined 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/ssh/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/ssh/shosts.equiv
- This file is used in exactly the same way as
hosts.equiv, but allows host-based authentication
without permitting login with rlogin/rsh.
- /etc/ssh/ssh_host_key
-
- /etc/ssh/ssh_host_dsa_key
-
- /etc/ssh/ssh_host_ecdsa_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_ecdsa_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 configuration options are described in
sshd_config(5).
- /etc/ssh/sshrc
- Similar to ~/.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/run/sshd
- 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 ALSO¶
scp(1),
sftp(1),
ssh(1),
ssh-add(1),
ssh-agent(1),
ssh-keygen(1),
ssh-keyscan(1),
ssh-vulnkey(1),
chroot(2),
hosts_access(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 disabled (thus
completely disabling
rlogin and
rsh into
the machine).
