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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/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.
-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.
-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-dss
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_dsa.pub, 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 (DSA, 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.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" 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.
/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.
/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),
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.