SSHD(8) System Manager's Manual SSHD(8)
NAME
sshd – OpenSSH daemon
SYNOPSIS
sshd [-46DdeGiqTtV] [-C connection_spec] [-c host_certificate_file]
[-E log_file] [-f config_file] [-g login_grace_time]
[-h host_key_file] [-o option] [-p port] [-u len]
DESCRIPTION
sshd (OpenSSH Daemon) is the daemon program for ssh(1). It provides
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/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
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.
-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 are applied before the configuration is
written to standard output. The connection parameters are
supplied as keyword=value pairs and may be supplied in any order,
either with multiple -C options or as a comma-separated list.
The keywords are “addr”, “user”, “host”, “laddr”, “lport”, and
“rdomain” and correspond to source address, user, resolved source
host name, local address, local port number and routing domain
respectively. Additionally the “invalid-user” flag (which does
not take a value argument) may be specified to simulate a
connection from an unrecognised username.
-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(2) 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 log_file
Append debug logs to log_file instead of the system log.
-e Write debug logs to 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 Parse and print configuration file. 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.
-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_ecdsa_key, /etc/ssh/ssh_host_ed25519_key and
/etc/ssh/ssh_host_rsa_key. It is possible to have multiple host
key files for the different host key algorithms.
-i Specifies that sshd is being run from inetd(8).
-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. This is
similar to the -G flag, but it includes the additional testing
performed by the -t flag.
-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 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.
-V Display the version number and exit.
AUTHENTICATION
The OpenSSH SSH daemon supports SSH protocol 2 only. Each host has a
host-specific key, used to identify the host. Whenever a client
connects, the daemon responds with its public host key. The client
compares the host key against its own database to verify that it has not
changed. Forward secrecy 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. 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 (MAC).
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 dependent. 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 an interactive shell or execution
of a non-interactive command, which sshd will execute via the user's
shell using its -c option. 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:
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 ~/.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 ~/.ssh/rc exists and the sshd_config(5) PermitUserRC option
is set, runs it; else if /etc/ssh/sshrc exists, runs it;
otherwise runs xauth(1). The “rc” files are given the X11
authentication protocol and cookie in standard input. See
SSHRC, below.
9. Runs user's shell or command. All commands are run under the
user's login shell as specified in the system password
database.
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.
AUTHORIZED_KEYS FILE FORMAT
AuthorizedKeysFile specifies the files containing public keys for public
key authentication; if this option is not 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). Public keys consist of the following space-
separated fields: options, keytype, base64-encoded key, comment. The
options field is optional. The supported key types are:
sk-ecdsa-sha2-nistp256@openssh.com
ecdsa-sha2-nistp256
ecdsa-sha2-nistp384
ecdsa-sha2-nistp521
sk-ssh-ed25519@openssh.com
ssh-ed25519
ssh-rsa
The comment field is not used for anything (but may be convenient for the
user to identify the key).
Note that lines in this file can be several hundred bytes long (because
of the size of the public key encoding) up to a limit of 8 kilobytes,
which permits RSA keys up to 16 kilobits. You don't want to type them
in; instead, copy the id_ecdsa.pub, id_ecdsa_sk.pub, id_ed25519.pub,
id_ed25519_sk.pub, or the id_rsa.pub file and edit it.
sshd enforces a minimum RSA key modulus size of 1024 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):
agent-forwarding
Enable authentication agent forwarding previously disabled by the
restrict option.
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, e.g. using the restrict key option.
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 a sshd_config(5) ForceCommand
directive.
If a command is specified and a forced-command is embedded in a
certificate used for authentication, then the certificate will be
accepted only if the two commands are identical.
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.
expiry-time="timespec"
Specifies a time after which the key will not be accepted. The
time may be specified as a YYYYMMDD[Z] date or a
YYYYMMDDHHMM[SS][Z] time. Dates and times will be interpreted in
the system time zone unless suffixed by a Z character, in which
case they will be interpreted in the UTC time zone.
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.
permitlisten="[host:]port"
Limit remote port forwarding with the ssh(1) -R option such that
it may only listen on the specified host (optional) and port.
IPv6 addresses can be specified by enclosing the address in
square brackets. Multiple permitlisten options may be applied
separated by commas. Hostnames may include wildcards as
described in the PATTERNS section in ssh_config(5). A port
specification of * matches any port. Note that the setting of
GatewayPorts may further restrict listen addresses. Note that
ssh(1) will send a hostname of “localhost” if a listen host was
not specified when the forwarding was requested, and that this
name is treated differently to the explicit localhost addresses
“127.0.0.1” and “::1”.
permitopen="host:port"
Limit local port forwarding with the ssh(1) -L option 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 or name lookup is performed on
the specified hostnames, they must be literal host names and/or
addresses. A port specification of * matches any port.
port-forwarding
Enable port forwarding previously disabled by the restrict
option.
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.
pty Permits tty allocation previously disabled by the restrict
option.
no-touch-required
Do not require demonstration of user presence for signatures made
using this key. This option only makes sense for the FIDO
authenticator algorithms ecdsa-sk and ed25519-sk.
verify-required
Require that signatures made using this key attest that they
verified the user, e.g. via a PIN. This option only makes sense
for the FIDO authenticator algorithms ecdsa-sk and ed25519-sk.
restrict
Enable all restrictions, i.e. disable port, agent and X11
forwarding, as well as disabling PTY allocation and execution of
~/.ssh/rc. If any future restriction capabilities are added to
authorized_keys files, they will be included in this set.
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.
user-rc
Enables execution of ~/.ssh/rc previously disabled by the
restrict option.
X11-forwarding
Permits X11 forwarding previously disabled by the restrict
option.
An example authorized_keys file:
# Comments are allowed at start of line. Blank lines are allowed.
# Plain key, no restrictions
ssh-rsa ...
# Forced command, disable PTY and all forwarding
restrict,command="dump /home" ssh-rsa ...
# Restriction of ssh -L forwarding destinations
permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-rsa ...
# Restriction of ssh -R forwarding listeners
permitlisten="localhost:8080",permitlisten="[::1]:22000" ssh-rsa ...
# Configuration for tunnel forwarding
tunnel="0",command="sh /etc/netstart tun0" ssh-rsa ...
# Override of restriction to allow PTY allocation
restrict,pty,command="nethack" ssh-rsa ...
# Allow FIDO key without requiring touch
no-touch-required sk-ecdsa-sha2-nistp256@openssh.com ...
# Require user-verification (e.g. PIN or biometric) for FIDO key
verify-required sk-ecdsa-sha2-nistp256@openssh.com ...
# Trust CA key, allow touch-less FIDO if requested in certificate
cert-authority,no-touch-required,principals="user_a" ssh-rsa ...
SSH_KNOWN_HOSTS FILE FORMAT
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 to an unknown host, its key is
added to the per-user file.
Each line in these files contains the following fields: marker
(optional), hostnames, keytype, base64-encoded key, 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 host name. When
sshd is authenticating a client, such as when using
HostbasedAuthentication, this will be the canonical client host name.
When ssh(1) is authenticating a server, this will be the host name given
by the user, the value of the ssh(1) HostkeyAlias if it was specified, or
the canonical server hostname if the ssh(1) CanonicalizeHostname option
was used.
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.
The keytype and base64-encoded key are taken directly from the host key;
they can be obtained, for example, from /etc/ssh/ssh_host_rsa_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, for
example, /etc/ssh/ssh_host_rsa_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
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 (ECDSA, Ed25519, 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.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" key exchange method. The file format is described in
moduli(5). If no usable groups are found in this file then fixed
internal groups will be used.
/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_ecdsa_key
/etc/ssh/ssh_host_ed25519_key
/etc/ssh/ssh_host_rsa_key
These 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_ecdsa_key.pub
/etc/ssh/ssh_host_ed25519_key.pub
/etc/ssh/ssh_host_rsa_key.pub
These 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.
/usr/share/empty.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.
/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), chroot(2), 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.
Debian September 15, 2024 Debian
No description available