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+SSHD(8) System Manager's Manual SSHD(8)
+
+NAME
+ sshd M-bM-^@M-^S OpenSSH SSH daemon
+
+SYNOPSIS
+ sshd [-46DdeiqTt] [-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). Together these
+ programs replace rlogin and rsh, 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/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 M-bM-^@M-^\addr,M-bM-^@M-^] M-bM-^@M-^\userM-bM-^@M-^], M-bM-^@M-^\hostM-bM-^@M-^], M-bM-^@M-^\laddrM-bM-^@M-^], M-bM-^@M-^\lportM-bM-^@M-^], and
+ M-bM-^@M-^\rdomainM-bM-^@M-^] 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 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 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.
+
+ -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.
+
+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 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,
+ umac-128, 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 dependent. Some platforms have
+ their own account database (eg AIX) and some modify the passwd field (
+ M-bM-^@M-^X*LK*M-bM-^@M-^Y on Solaris and UnixWare, M-bM-^@M-^X*M-bM-^@M-^Y on HP-UX, containing M-bM-^@M-^XNologinM-bM-^@M-^Y on
+ Tru64, a leading M-bM-^@M-^X*LOCKED*M-bM-^@M-^Y on FreeBSD and a leading M-bM-^@M-^X!M-bM-^@M-^Y 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 M-bM-^@M-^XNPM-bM-^@M-^Y or M-bM-^@M-^X*NP*M-bM-^@M-^Y ).
+
+ 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:
+
+ 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. The M-bM-^@M-^\rcM-bM-^@M-^] 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 M-bM-^@M-^X#M-bM-^@M-^Y 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 keytype is M-bM-^@M-^\ecdsa-sha2-nistp256M-bM-^@M-^],
+ M-bM-^@M-^\ecdsa-sha2-nistp384M-bM-^@M-^], M-bM-^@M-^\ecdsa-sha2-nistp521M-bM-^@M-^], M-bM-^@M-^\ssh-ed25519M-bM-^@M-^], M-bM-^@M-^\ssh-dssM-bM-^@M-^] or
+ M-bM-^@M-^\ssh-rsaM-bM-^@M-^]; 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 DSA keys up to 8 kilobits and RSA keys up to 16 kilobits.
+ You don't want to type them in; instead, copy the id_dsa.pub,
+ id_ecdsa.pub, id_ed25519.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 date or a YYYYMMDDHHMM[SS]
+ time in the system 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 M-bM-^@M-^\localhostM-bM-^@M-^] if a listen host was
+ not specified when the forwarding was requested, and that this
+ name is treated differently to the explicit localhost addresses
+ M-bM-^@M-^\127.0.0.1M-bM-^@M-^] and M-bM-^@M-^\::1M-bM-^@M-^].
+
+ 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 is performed on the specified
+ hostnames, they must be literal domains 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.
+
+ 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 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-rsa
+ AAAAC3...51R== example.net
+ permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-rsa
+ AAAAB5...21S==
+ permitlisten="localhost:8080",permitopen="localhost:22000" ssh-rsa
+ AAAAB5...21S==
+ tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
+ jane@example.net
+ restrict,command="uptime" ssh-rsa AAAA1C8...32Tv==
+ user@example.net
+ restrict,pty,command="nethack" ssh-rsa AAAA1f8...IrrC5==
+ user@example.net
+
+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: markers
+ (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
+ M-bM-^@M-^\@cert-authorityM-bM-^@M-^], to indicate that the line contains a certification
+ authority (CA) key, or M-bM-^@M-^\@revokedM-bM-^@M-^], 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 (M-bM-^@M-^X*M-bM-^@M-^Y and M-bM-^@M-^X?M-bM-^@M-^Y 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 M-bM-^@M-^X!M-bM-^@M-^Y 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 M-bM-^@M-^X[M-bM-^@M-^Y and M-bM-^@M-^X]M-bM-^@M-^Y brackets then followed by M-bM-^@M-^X:M-bM-^@M-^Y
+ 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 M-bM-^@M-^X|M-bM-^@M-^Y 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 M-bM-^@M-^X#M-bM-^@M-^Y 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 M-bM-^@M-^\@cert-authorityM-bM-^@M-^]
+ 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 M-bM-^@M-^\@revokedM-bM-^@M-^] 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
+ 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, 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
+ M-bM-^@M-^\noM-bM-^@M-^].
+
+ ~/.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
+ M-bM-^@M-^X#M-bM-^@M-^Y), 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/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/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.
+
+ /var/empty
+ chroot(2) directory used by sshd during privilege separation in
+ the pre-authentication phase. The directory should not contain
+ any files and must be owned by root and not group or world-
+ writable.
+
+ /var/run/sshd.pid
+ Contains the process ID of the sshd listening for connections (if
+ there are several daemons running concurrently for different
+ ports, this contains the process ID of the one started last).
+ The content of this file is not sensitive; it can be world-
+ readable.
+
+SEE 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.
+
+OpenBSD 6.4 July 22, 2018 OpenBSD 6.4