|SSHD(8)||System Manager's Manual||SSHD(8)|
sshd — OpenSSH
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
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.
its configuration file when it receives a hangup signal,
SIGHUP, by executing itself with the name and
options it was started with, e.g.
The options are as follows:
sshdto use IPv4 addresses only.
sshdto use IPv6 addresses only.
- Specify the connection parameters to use for the
-Textended test mode. If provided, any
Matchdirectives 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
-Coptions 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.
- Specifies a path to a certificate file to identify
sshdduring key exchange. The certificate file must match a host key file specified using the
-hoption or the
- When this option is specified,
sshdwill not detach and does not become a daemon. This allows easy monitoring of
- 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
-doptions increase the debugging level. Maximum is 3.
- Append debug logs to log_file instead of the system log.
- Write debug logs to standard error instead of the system log.
- Specifies the name of the configuration file. The default is
sshdrefuses to start if there is no configuration file.
- Parse and print configuration file. Check the validity of the
configuration file, output the effective configuration to stdout and then
Matchrules may be applied by specifying the connection parameters using one or more
- 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.
- Specifies a file from which a host key is read. This option must be given
sshdis 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.
- Specifies that
sshdis being run from inetd(8).
- 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).
- 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
Portoption are ignored when a command-line port is specified. Ports specified using the
ListenAddressoption override command-line ports.
- Quiet mode. Nothing is sent to the system log. Normally the beginning, authentication, and termination of each connection is logged.
- Extended test mode. Check the validity of the configuration file, output
the effective configuration to stdout and then exit. Optionally,
Matchrules may be applied by specifying the connection parameters using one or more
-Coptions. This is similar to the
-Gflag, but it includes the additional testing performed by the
- Test mode. Only check the validity of the configuration file and sanity of
the keys. This is useful for updating
sshdreliably as configuration options may change.
- 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.
-u0indicates that only dotted decimal addresses should be put into the utmp file.
-u0may also be used to prevent
sshdfrom making DNS requests unless the authentication mechanism or configuration requires it. Authentication mechanisms that may require DNS include
HostbasedAuthenticationand using a
from="pattern-list"option in a key file. Configuration options that require DNS include using a USER@HOST pattern in
- Display the version number and exit.
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,
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 ‘
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 or a non-interactive command, which
will execute via the user's shell using its
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.
When a user successfully logs in,
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
PermitUserEnvironmentoption in sshd_config(5).
- Changes to user's home directory.
- If ~/.ssh/rc exists and the
PermitUserRCoption 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.
- Runs user's shell or command. All commands are run under the user's login shell as specified in the system password database.
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
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:
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):
- Enable authentication agent forwarding previously disabled by the
- 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.
- 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
The command originally supplied by the client is available in the
SSH_ORIGINAL_COMMANDenvironment 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)
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.
- 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
- 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.
- 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
fromstanza 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).
- Forbids authentication agent forwarding when this key is used for authentication.
- 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
- Prevents tty allocation (a request to allocate a pty will fail).
- Disables execution of ~/.ssh/rc.
- Forbids X11 forwarding when this key is used for authentication. Any X11 forward requests by the client will return an error.
- Limit remote port forwarding with the ssh(1)
-Roption 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
permitlistenoptions 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
GatewayPortsmay 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”.
- Limit local port forwarding with the ssh(1)
-Loption 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
permitopenoptions 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.
- Enable port forwarding previously disabled by the
- On a
cert-authorityline, 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
- Permits tty allocation previously disabled by the
- Do not require demonstration of user presence for signatures made using
this key. This option only makes sense for the FIDO authenticator
- 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
- 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.
- Force a tun(4) device on the server. Without this option, the next available device will be used if the client requests a tunnel.
- Enables execution of ~/.ssh/rc previously disabled
- Permits X11 forwarding previously disabled by the
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 firstname.lastname@example.org ... # Require user-verification (e.g. PIN or biometric) for FIDO key verify-required email@example.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
?’ act as wildcards); each pattern in
turn is matched against the host name. When
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
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 ‘
]’ brackets then followed by
:’ and a non-standard port
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 ‘
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 ‘
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...
- This file is used to suppress printing the last login time and
PrintMotd, respectively, are enabled. It does not suppress printing of the banner specified by
- 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
sshdreads 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.
- This file is used in exactly the same way as .rhosts, but allows host-based authentication without permitting login with rlogin/rsh.
- 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.
- 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,
sshdwill not allow it to be used unless the
StrictModesoption has been set to “no”.
- 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
- 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.
- 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.
- Access controls that should be enforced by tcp-wrappers are defined here. Further details are described in hosts_access(5).
- This file is for host-based authentication (see ssh(1)). It should only be writable by root.
- 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.
- See motd(5).
- If this file exists,
sshdrefuses 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.
- This file is used in exactly the same way as hosts.equiv, but allows host-based authentication without permitting login with rlogin/rsh.
- 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
sshddoes not start if these files are group/world-accessible.
- 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).
- 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.
- Contains configuration data for
sshd. The file format and configuration options are described in sshd_config(5).
- 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.
- chroot(2) directory used by
sshdduring 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.
- Contains the process ID of the
sshdlistening 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.
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.
|February 10, 2023||Debian|