556 lines
28 KiB
Plaintext
556 lines
28 KiB
Plaintext
SSHD(8) OpenBSD System Manager's Manual SSHD(8)
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NAME
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sshd - OpenSSH SSH daemon
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SYNOPSIS
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sshd [-46Ddeiqt] [-b bits] [-f config_file] [-g login_grace_time]
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[-h host_key_file] [-k key_gen_time] [-o option] [-p port] [-u len]
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DESCRIPTION
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sshd (OpenSSH Daemon) is the daemon program for ssh(1). Together these
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programs replace rlogin(1) and rsh(1), and provide secure encrypted com-
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munications between two untrusted hosts over an insecure network.
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sshd listens for connections from clients. It is normally started at
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boot from /etc/rc. It forks a new daemon for each incoming connection.
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The forked daemons handle key exchange, encryption, authentication, com-
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mand execution, and data exchange.
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sshd can be configured using command-line options or a configuration file
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(by default sshd_config(5)); command-line options override values speci-
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fied in the configuration file. sshd rereads its configuration file when
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it receives a hangup signal, SIGHUP, by executing itself with the name
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and options it was started with, e.g. /usr/sbin/sshd.
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The options are as follows:
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-4 Forces sshd to use IPv4 addresses only.
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-6 Forces sshd to use IPv6 addresses only.
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-b bits
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Specifies the number of bits in the ephemeral protocol version 1
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server key (default 768).
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-D When this option is specified, sshd will not detach and does not
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become a daemon. This allows easy monitoring of sshd.
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-d Debug mode. The server sends verbose debug output to the system
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log, and does not put itself in the background. The server also
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will not fork and will only process one connection. This option
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is only intended for debugging for the server. Multiple -d op-
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tions increase the debugging level. Maximum is 3.
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-e When this option is specified, sshd will send the output to the
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standard error instead of the system log.
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-f config_file
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Specifies the name of the configuration file. The default is
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/etc/ssh/sshd_config. sshd refuses to start if there is no con-
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figuration file.
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-g login_grace_time
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Gives the grace time for clients to authenticate themselves (de-
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fault 120 seconds). If the client fails to authenticate the user
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within this many seconds, the server disconnects and exits. A
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value of zero indicates no limit.
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-h host_key_file
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Specifies a file from which a host key is read. This option must
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be given if sshd is not run as root (as the normal host key files
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are normally not readable by anyone but root). The default is
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/etc/ssh/ssh_host_key for protocol version 1, and
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/etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for pro-
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tocol version 2. It is possible to have multiple host key files
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for the different protocol versions and host key algorithms.
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-i Specifies that sshd is being run from inetd(8). sshd is normally
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not run from inetd because it needs to generate the server key
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before it can respond to the client, and this may take tens of
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seconds. Clients would have to wait too long if the key was re-
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generated every time. However, with small key sizes (e.g. 512)
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using sshd from inetd may be feasible.
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-k key_gen_time
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Specifies how often the ephemeral protocol version 1 server key
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is regenerated (default 3600 seconds, or one hour). The motiva-
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tion for regenerating the key fairly often is that the key is not
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stored anywhere, and after about an hour it becomes impossible to
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recover the key for decrypting intercepted communications even if
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the machine is cracked into or physically seized. A value of ze-
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ro indicates that the key will never be regenerated.
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-o option
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Can be used to give options in the format used in the configura-
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tion file. This is useful for specifying options for which there
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is no separate command-line flag. For full details of the op-
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tions, and their values, see sshd_config(5).
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-p port
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Specifies the port on which the server listens for connections
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(default 22). Multiple port options are permitted. Ports speci-
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fied in the configuration file with the Port option are ignored
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when a command-line port is specified. Ports specified using the
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ListenAddress option override command-line ports.
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-q Quiet mode. Nothing is sent to the system log. Normally the be-
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ginning, authentication, and termination of each connection is
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logged.
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-t Test mode. Only check the validity of the configuration file and
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sanity of the keys. This is useful for updating sshd reliably as
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configuration options may change.
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-u len This option is used to specify the size of the field in the utmp
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structure that holds the remote host name. If the resolved host
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name is longer than len, the dotted decimal value will be used
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instead. This allows hosts with very long host names that over-
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flow this field to still be uniquely identified. Specifying -u0
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indicates that only dotted decimal addresses should be put into
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the utmp file. -u0 may also be used to prevent sshd from making
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DNS requests unless the authentication mechanism or configuration
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requires it. Authentication mechanisms that may require DNS in-
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clude RhostsRSAAuthentication, HostbasedAuthentication, and using
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a from="pattern-list" option in a key file. Configuration op-
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tions that require DNS include using a USER@HOST pattern in
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AllowUsers or DenyUsers.
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AUTHENTICATION
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The OpenSSH SSH daemon supports SSH protocols 1 and 2. Both protocols
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are supported by default, though this can be changed via the Protocol op-
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tion in sshd_config(5). Protocol 2 supports both RSA and DSA keys; pro-
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tocol 1 only supports RSA keys. For both protocols, each host has a
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host-specific key, normally 2048 bits, used to identify the host.
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Forward security for protocol 1 is provided through an additional server
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key, normally 768 bits, generated when the server starts. This key is
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normally regenerated every hour if it has been used, and is never stored
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on disk. Whenever a client connects, the daemon responds with its public
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host and server keys. The client compares the RSA host key against its
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own database to verify that it has not changed. The client then gener-
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ates a 256-bit random number. It encrypts this random number using both
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the host key and the server key, and sends the encrypted number to the
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server. Both sides then use this random number as a session key which is
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used to encrypt all further communications in the session. The rest of
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the session is encrypted using a conventional cipher, currently Blowfish
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or 3DES, with 3DES being used by default. The client selects the encryp-
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tion algorithm to use from those offered by the server.
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For protocol 2, forward security is provided through a Diffie-Hellman key
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agreement. This key agreement results in a shared session key. The rest
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of the session is encrypted using a symmetric cipher, currently 128-bit
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AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. The
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client selects the encryption algorithm to use from those offered by the
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server. Additionally, session integrity is provided through a crypto-
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graphic message authentication code (hmac-md5, hmac-sha1, umac-64 or
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hmac-ripemd160).
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Finally, the server and the client enter an authentication dialog. The
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client tries to authenticate itself using host-based authentication, pub-
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lic key authentication, challenge-response authentication, or password
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authentication.
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Regardless of the authentication type, the account is checked to ensure
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that it is accessible. An account is not accessible if it is locked,
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listed in DenyUsers or its group is listed in DenyGroups . The defini-
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tion of a locked account is system dependant. Some platforms have their
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own account database (eg AIX) and some modify the passwd field ( `*LK*'
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on Solaris and UnixWare, `*' on HP-UX, containing `Nologin' on Tru64, a
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leading `*LOCKED*' on FreeBSD and a leading `!' on most Linuxes). If
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there is a requirement to disable password authentication for the account
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while allowing still public-key, then the passwd field should be set to
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something other than these values (eg `NP' or `*NP*' ).
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If the client successfully authenticates itself, a dialog for preparing
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the session is entered. At this time the client may request things like
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allocating a pseudo-tty, forwarding X11 connections, forwarding TCP con-
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nections, or forwarding the authentication agent connection over the se-
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cure channel.
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After this, the client either requests a shell or execution of a command.
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The sides then enter session mode. In this mode, either side may send
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data at any time, and such data is forwarded to/from the shell or command
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on the server side, and the user terminal in the client side.
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When the user program terminates and all forwarded X11 and other connec-
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tions have been closed, the server sends command exit status to the
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client, and both sides exit.
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LOGIN PROCESS
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When a user successfully logs in, sshd does the following:
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1. If the login is on a tty, and no command has been specified,
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prints last login time and /etc/motd (unless prevented in the
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configuration file or by ~/.hushlogin; see the FILES section).
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2. If the login is on a tty, records login time.
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3. Checks /etc/nologin; if it exists, prints contents and quits
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(unless root).
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4. Changes to run with normal user privileges.
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5. Sets up basic environment.
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6. Reads the file ~/.ssh/environment, if it exists, and users are
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allowed to change their environment. See the
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PermitUserEnvironment option in sshd_config(5).
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7. Changes to user's home directory.
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8. If ~/.ssh/rc exists, runs it; else if /etc/ssh/sshrc exists,
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runs it; otherwise runs xauth. The ``rc'' files are given the
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X11 authentication protocol and cookie in standard input. See
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SSHRC, below.
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9. Runs user's shell or command.
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SSHRC
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If the file ~/.ssh/rc exists, sh(1) runs it after reading the environment
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files but before starting the user's shell or command. It must not pro-
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duce any output on stdout; stderr must be used instead. If X11 forward-
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ing is in use, it will receive the "proto cookie" pair in its standard
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input (and DISPLAY in its environment). The script must call xauth(1)
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because sshd will not run xauth automatically to add X11 cookies.
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The primary purpose of this file is to run any initialization routines
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which may be needed before the user's home directory becomes accessible;
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AFS is a particular example of such an environment.
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This file will probably contain some initialization code followed by
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something similar to:
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if read proto cookie && [ -n "$DISPLAY" ]; then
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if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
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# X11UseLocalhost=yes
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echo add unix:`echo $DISPLAY |
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cut -c11-` $proto $cookie
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else
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# X11UseLocalhost=no
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echo add $DISPLAY $proto $cookie
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fi | xauth -q -
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fi
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If this file does not exist, /etc/ssh/sshrc is run, and if that does not
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exist either, xauth is used to add the cookie.
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AUTHORIZED_KEYS FILE FORMAT
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AuthorizedKeysFile specifies the file containing public keys for public
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key authentication; if none is specified, the default is
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~/.ssh/authorized_keys. Each line of the file contains one key (empty
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lines and lines starting with a `#' are ignored as comments). Protocol 1
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public keys consist of the following space-separated fields: options,
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bits, exponent, modulus, comment. Protocol 2 public key consist of: op-
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tions, keytype, base64-encoded key, comment. The options field is op-
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tional; its presence is determined by whether the line starts with a num-
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ber or not (the options field never starts with a number). The bits, ex-
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ponent, modulus, and comment fields give the RSA key for protocol version
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1; the comment field is not used for anything (but may be convenient for
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the user to identify the key). For protocol version 2 the keytype is
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``ssh-dss'' or ``ssh-rsa''.
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Note that lines in this file are usually several hundred bytes long (be-
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cause of the size of the public key encoding) up to a limit of 8 kilo-
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bytes, which permits DSA keys up to 8 kilobits and RSA keys up to 16
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kilobits. You don't want to type them in; instead, copy the
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identity.pub, id_dsa.pub, or the id_rsa.pub file and edit it.
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sshd enforces a minimum RSA key modulus size for protocol 1 and protocol
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2 keys of 768 bits.
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The options (if present) consist of comma-separated option specifica-
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tions. No spaces are permitted, except within double quotes. The fol-
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lowing option specifications are supported (note that option keywords are
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case-insensitive):
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command="command"
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Specifies that the command is executed whenever this key is used
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for authentication. The command supplied by the user (if any) is
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ignored. The command is run on a pty if the client requests a
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pty; otherwise it is run without a tty. If an 8-bit clean chan-
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nel is required, one must not request a pty or should specify no-
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pty. A quote may be included in the command by quoting it with a
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backslash. This option might be useful to restrict certain pub-
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lic keys to perform just a specific operation. An example might
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be a key that permits remote backups but nothing else. Note that
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the client may specify TCP and/or X11 forwarding unless they are
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explicitly prohibited. The command originally supplied by the
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client is available in the SSH_ORIGINAL_COMMAND environment vari-
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able. Note that this option applies to shell, command or subsys-
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tem execution.
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environment="NAME=value"
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Specifies that the string is to be added to the environment when
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logging in using this key. Environment variables set this way
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override other default environment values. Multiple options of
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this type are permitted. Environment processing is disabled by
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default and is controlled via the PermitUserEnvironment option.
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This option is automatically disabled if UseLogin is enabled.
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from="pattern-list"
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Specifies that in addition to public key authentication, the
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canonical name of the remote host must be present in the comma-
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separated list of patterns. The purpose of this option is to op-
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tionally increase security: public key authentication by itself
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does not trust the network or name servers or anything (but the
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key); however, if somebody somehow steals the key, the key per-
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mits an intruder to log in from anywhere in the world. This ad-
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ditional option makes using a stolen key more difficult (name
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servers and/or routers would have to be compromised in addition
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to just the key).
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See PATTERNS in ssh_config(5) for more information on patterns.
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no-agent-forwarding
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Forbids authentication agent forwarding when this key is used for
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authentication.
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no-port-forwarding
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Forbids TCP forwarding when this key is used for authentication.
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Any port forward requests by the client will return an error.
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This might be used, e.g. in connection with the command option.
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no-pty Prevents tty allocation (a request to allocate a pty will fail).
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no-user-rc
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Disables execution of ~/.ssh/rc.
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no-X11-forwarding
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Forbids X11 forwarding when this key is used for authentication.
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Any X11 forward requests by the client will return an error.
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permitopen="host:port"
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Limit local ``ssh -L'' port forwarding such that it may only con-
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nect to the specified host and port. IPv6 addresses can be spec-
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ified with an alternative syntax: host/port. Multiple permitopen
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options may be applied separated by commas. No pattern matching
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is performed on the specified hostnames, they must be literal do-
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mains or addresses.
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tunnel="n"
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Force a tun(4) device on the server. Without this option, the
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next available device will be used if the client requests a tun-
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nel.
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An example authorized_keys file:
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# Comments allowed at start of line
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ssh-rsa AAAAB3Nza...LiPk== user@example.net
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from="*.sales.example.net,!pc.sales.example.net" ssh-rsa
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AAAAB2...19Q== john@example.net
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command="dump /home",no-pty,no-port-forwarding ssh-dss
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AAAAC3...51R== example.net
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permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss
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AAAAB5...21S==
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tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...==
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jane@example.net
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SSH_KNOWN_HOSTS FILE FORMAT
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The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host
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public keys for all known hosts. The global file should be prepared by
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the administrator (optional), and the per-user file is maintained auto-
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matically: whenever the user connects from an unknown host, its key is
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added to the per-user file.
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Each line in these files contains the following fields: hostnames, bits,
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exponent, modulus, comment. The fields are separated by spaces.
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Hostnames is a comma-separated list of patterns (`*' and `?' act as wild-
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cards); each pattern in turn is matched against the canonical host name
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(when authenticating a client) or against the user-supplied name (when
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authenticating a server). A pattern may also be preceded by `!' to indi-
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cate negation: if the host name matches a negated pattern, it is not ac-
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cepted (by that line) even if it matched another pattern on the line. A
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hostname or address may optionally be enclosed within `[' and `]' brack-
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ets then followed by `:' and a non-standard port number.
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Alternately, hostnames may be stored in a hashed form which hides host
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names and addresses should the file's contents be disclosed. Hashed
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hostnames start with a `|' character. Only one hashed hostname may ap-
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pear on a single line and none of the above negation or wildcard opera-
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tors may be applied.
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Bits, exponent, and modulus are taken directly from the RSA host key;
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they can be obtained, for example, from /etc/ssh/ssh_host_key.pub. The
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optional comment field continues to the end of the line, and is not used.
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Lines starting with `#' and empty lines are ignored as comments.
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When performing host authentication, authentication is accepted if any
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matching line has the proper key. It is thus permissible (but not recom-
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mended) to have several lines or different host keys for the same names.
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This will inevitably happen when short forms of host names from different
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domains are put in the file. It is possible that the files contain con-
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flicting information; authentication is accepted if valid information can
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be found from either file.
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Note that the lines in these files are typically hundreds of characters
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long, and you definitely don't want to type in the host keys by hand.
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Rather, generate them by a script or by taking /etc/ssh/ssh_host_key.pub
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and adding the host names at the front.
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An example ssh_known_hosts file:
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# Comments allowed at start of line
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closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
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cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
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# A hashed hostname
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|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
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AAAA1234.....=
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FILES
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~/.hushlogin
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This file is used to suppress printing the last login time and
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/etc/motd, if PrintLastLog and PrintMotd, respectively, are en-
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abled. It does not suppress printing of the banner specified by
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Banner.
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~/.rhosts
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This file is used for host-based authentication (see ssh(1) for
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more information). On some machines this file may need to be
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world-readable if the user's home directory is on an NFS parti-
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tion, because sshd reads it as root. Additionally, this file
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must be owned by the user, and must not have write permissions
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for anyone else. The recommended permission for most machines is
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read/write for the user, and not accessible by others.
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~/.shosts
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This file is used in exactly the same way as .rhosts, but allows
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host-based authentication without permitting login with
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rlogin/rsh.
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~/.ssh/
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This directory is the default location for all user-specific con-
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figuration and authentication information. There is no general
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requirement to keep the entire contents of this directory secret,
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but the recommended permissions are read/write/execute for the
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user, and not accessible by others.
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~/.ssh/authorized_keys
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Lists the public keys (RSA/DSA) that can be used for logging in
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as this user. The format of this file is described above. The
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content of the file is not highly sensitive, but the recommended
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permissions are read/write for the user, and not accessible by
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others.
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If this file, the ~/.ssh directory, or the user's home directory
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are writable by other users, then the file could be modified or
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replaced by unauthorized users. In this case, sshd will not al-
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low it to be used unless the StrictModes option has been set to
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``no''. The recommended permissions can be set by executing
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``chmod go-w ~/ ~/.ssh ~/.ssh/authorized_keys''.
|
|
|
|
~/.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 de-
|
|
fined 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/moduli
|
|
Contains Diffie-Hellman groups used for the "Diffie-Hellman Group
|
|
Exchange". The file format is described in moduli(5).
|
|
|
|
/etc/motd
|
|
See motd(5).
|
|
|
|
/etc/nologin
|
|
If this file exists, sshd refuses to let anyone except root log
|
|
in. The contents of the file are displayed to anyone trying to
|
|
log in, and non-root connections are refused. The file should be
|
|
world-readable.
|
|
|
|
/etc/shosts.equiv
|
|
This file is used in exactly the same way as hosts.equiv, but al-
|
|
lows host-based authentication without permitting login with
|
|
rlogin/rsh.
|
|
|
|
/etc/ssh/ssh_host_key
|
|
/etc/ssh/ssh_host_dsa_key
|
|
/etc/ssh/ssh_host_rsa_key
|
|
These three files contain the private parts of the host keys.
|
|
These files should only be owned by root, readable only by root,
|
|
and not accessible to others. Note that sshd does not start if
|
|
these files are group/world-accessible.
|
|
|
|
/etc/ssh/ssh_host_key.pub
|
|
/etc/ssh/ssh_host_dsa_key.pub
|
|
/etc/ssh/ssh_host_rsa_key.pub
|
|
These three files contain the public parts of the host keys.
|
|
These files should be world-readable but writable only by root.
|
|
Their contents should match the respective private parts. These
|
|
files are not really used for anything; they are provided for the
|
|
convenience of the user so their contents can be copied to known
|
|
hosts files. These files are created using ssh-keygen(1).
|
|
|
|
/etc/ssh/ssh_known_hosts
|
|
Systemwide list of known host keys. This file should be prepared
|
|
by the system administrator to contain the public host keys of
|
|
all machines in the organization. The format of this file is de-
|
|
scribed 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 con-
|
|
figuration 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-read-
|
|
able.
|
|
|
|
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), 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 cre-
|
|
ated OpenSSH. Markus Friedl contributed the support for SSH protocol
|
|
versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
|
|
for privilege separation.
|
|
|
|
CAVEATS
|
|
System security is not improved unless rshd, rlogind, and rexecd are dis-
|
|
abled (thus completely disabling rlogin and rsh into the machine).
|
|
|
|
OpenBSD 4.3 April 3, 2008 9
|