.\" -*- nroff -*- .\" .\" Author: Tatu Ylonen .\" Copyright (c) 1995 Tatu Ylonen , Espoo, Finland .\" All rights reserved .\" .\" As far as I am concerned, the code I have written for this software .\" can be used freely for any purpose. Any derived versions of this .\" software must be clearly marked as such, and if the derived work is .\" incompatible with the protocol description in the RFC file, it must be .\" called by a name other than "ssh" or "Secure Shell". .\" .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. .\" Copyright (c) 1999 Aaron Campbell. All rights reserved. .\" Copyright (c) 1999 Theo de Raadt. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. .\" .\" $OpenBSD: sshd.8,v 1.121 2001/05/04 14:21:56 stevesk Exp $ .Dd September 25, 1999 .Dt SSHD 8 .Os .Sh NAME .Nm sshd .Nd OpenSSH SSH daemon .Sh SYNOPSIS .Nm sshd .Op Fl deiqD46 .Op Fl b Ar bits .Op Fl f Ar config_file .Op Fl g Ar login_grace_time .Op Fl h Ar host_key_file .Op Fl k Ar key_gen_time .Op Fl p Ar port .Op Fl u Ar len .Op Fl V Ar client_protocol_id .Sh DESCRIPTION .Nm (SSH Daemon) is the daemon program for .Xr ssh 1 . Together these programs replace rlogin and rsh, and provide secure encrypted communications between two untrusted hosts over an insecure network. The programs are intended to be as easy to install and use as possible. .Pp .Nm is the daemon that listens for connections from clients. It is normally started at boot from .Pa /etc/rc . It forks a new daemon for each incoming connection. The forked daemons handle key exchange, encryption, authentication, command execution, and data exchange. This implementation of .Nm supports both SSH protocol version 1 and 2 simultaneously. .Nm works as follows. .Pp .Ss SSH protocol version 1 .Pp Each host has a host-specific RSA key (normally 1024 bits) used to identify the host. Additionally, when the daemon starts, it generates a server RSA key (normally 768 bits). This key is normally regenerated every hour if it has been used, and is never stored on disk. .Pp Whenever a client connects the daemon responds with its public host and server keys. The client compares the RSA host key against its own database to verify that it has not changed. The client then generates a 256 bit random number. It encrypts this random number using both the host key and the server key, and sends the encrypted number to the server. Both sides then use this random number as a session key which is used to encrypt all further communications in the session. The rest of the session is encrypted using a conventional cipher, currently Blowfish or 3DES, with 3DES being used by default. The client selects the encryption algorithm to use from those offered by the server. .Pp Next, the server and the client enter an authentication dialog. The client tries to authenticate itself using .Pa .rhosts authentication, .Pa .rhosts authentication combined with RSA host authentication, RSA challenge-response authentication, or password based authentication. .Pp Rhosts authentication is normally disabled because it is fundamentally insecure, but can be enabled in the server configuration file if desired. System security is not improved unless .Xr rshd 8 , .Xr rlogind 8 , .Xr rexecd 8 , and .Xr rexd 8 are disabled (thus completely disabling .Xr rlogin 1 and .Xr rsh 1 into the machine). .Pp .Ss SSH protocol version 2 .Pp Version 2 works similarly: Each host has a host-specific DSA key used to identify the host. However, when the daemon starts, it does not generate a server key. Forward security is provided through a Diffie-Hellman key agreement. This key agreement results in a shared session key. .Pp 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-sha1 or hmac-md5). .Pp Protocol version 2 provides a public key based user (PubkeyAuthentication) or client host (HostbasedAuthentication) authentication method, conventional password authentication and challenge response based methods. .Pp .Ss Command execution and data forwarding .Pp 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/IP connections, or forwarding the authentication agent connection over the secure channel. .Pp Finally, 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. .Pp 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. .Pp .Nm can be configured using command-line options or a configuration file. Command-line options override values specified in the configuration file. .Pp .Nm rereads its configuration file when it receives a hangup signal, .Dv SIGHUP , by executing itself with the name it was started as, ie. .Pa /usr/sbin/sshd . .Pp The options are as follows: .Bl -tag -width Ds .It Fl b Ar bits Specifies the number of bits in the ephemeral protocol version 1 server key (default 768). .Pp .It Fl d Debug mode. The server sends verbose debug output to the system log, 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. .It Fl e When this option is specified, .Nm will send the output to the standard error instead of the system log. .It Fl f Ar configuration_file Specifies the name of the configuration file. The default is .Pa /etc/sshd_config . .Nm refuses to start if there is no configuration file. .It Fl g Ar login_grace_time Gives the grace time for clients to authenticate themselves (default 600 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. .It Fl h Ar host_key_file Specifies the file from which the host key is read (default .Pa /etc/ssh_host_key ) . This option must be given if .Nm is not run as root (as the normal host file is normally not readable by anyone but root). It is possible to have multiple host key files for the different protocol versions and host key algorithms. .It Fl i Specifies that .Nm is being run from inetd. .Nm is normally not run from inetd because it needs to generate the server key before it can respond to the client, and this may take tens of seconds. Clients would have to wait too long if the key was regenerated every time. However, with small key sizes (e.g., 512) using .Nm from inetd may be feasible. .It Fl k Ar key_gen_time Specifies how often the ephemeral protocol version 1 server key is regenerated (default 3600 seconds, or one hour). The motivation for regenerating the key fairly often is that the key is not stored anywhere, and after about an hour, it becomes impossible to recover the key for decrypting intercepted communications even if the machine is cracked into or physically seized. A value of zero indicates that the key will never be regenerated. .It Fl p Ar port Specifies the port on which the server listens for connections (default 22). .It Fl q Quiet mode. Nothing is sent to the system log. Normally the beginning, authentication, and termination of each connection is logged. .It Fl u Ar len This option is used to specify the size of the field in the .Li utmp structure that holds the remote host name. If the resolved host name is longer than .Ar 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 .Fl u0 indicates that only dotted decimal addresses should be put into the .Pa utmp file. .It Fl D When this option is specified .Nm will not detach and does not become a daemon. This allows easy monitoring of .Nm sshd . .It Fl 4 Forces .Nm to use IPv4 addresses only. .It Fl 6 Forces .Nm to use IPv6 addresses only. .El .Sh CONFIGURATION FILE .Nm reads configuration data from .Pa /etc/sshd_config (or the file specified with .Fl f on the command line). The file contains keyword-value pairs, one per line. Lines starting with .Ql # and empty lines are interpreted as comments. .Pp The following keywords are possible. .Bl -tag -width Ds .It Cm AFSTokenPassing Specifies whether an AFS token may be forwarded to the server. Default is .Dq yes . .It Cm AllowGroups This keyword can be followed by a list of group names, separated by spaces. If specified, login is allowed only for users whose primary group or supplementary group list matches one of the patterns. .Ql \&* and .Ql ? can be used as wildcards in the patterns. Only group names are valid; a numerical group ID isn't recognized. By default login is allowed regardless of the group list. .Pp .It Cm AllowTcpForwarding Specifies whether TCP forwarding is permitted. The default is .Dq yes . Note that disabling TCP forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders. .Pp .It Cm AllowUsers This keyword can be followed by a list of user names, separated by spaces. If specified, login is allowed only for users names that match one of the patterns. .Ql \&* and .Ql ? can be used as wildcards in the patterns. Only user names are valid; a numerical user ID isn't recognized. By default login is allowed regardless of the user name. .Pp .It Cm Banner In some jurisdictions, sending a warning message before authentication may be relevant for getting legal protection. The contents of the specified file are sent to the remote user before authentication is allowed. This option is only available for protocol version 2. .Pp .It Cm ChallengeResponseAuthentication Specifies whether challenge response authentication is allowed. Currently there is only support for .Xr skey 1 authentication. The default is .Dq yes . .It Cm Ciphers Specifies the ciphers allowed for protocol version 2. Multiple ciphers must be comma-separated. The default is .Dq aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour. .It Cm CheckMail Specifies whether .Nm should check for new mail for interactive logins. The default is .Dq no . .It Cm ClientAliveInterval Sets a timeout interval in seconds after which if no data has been received from the client, .Nm will send a message through the encrypted channel to request a response from the client. The default is 0, indicating that these messages will not be sent to the client. This option applies to protocol version 2 only. .It Cm ClientAliveCountMax Sets the number of client alive messages (see above) which may be sent without .Nm receiving any messages back from the client. If this threshold is reached while client alive messages are being sent, .Nm will disconnect the client, terminating the session. It is important to note that the use of client alive messages is very different from .Cm Keepalive (below). The client alive messages are sent through the encrypted channel and therefore will not be spoofable. The TCP keepalive option enabled by .Cm Keepalive is spoofable. You want to use the client alive mechanism when you are basing something important on clients having an active connection to the server. .Pp The default value is 3. If you set .Cm ClientAliveInterval (above) to 15, and leave this value at the default, unresponsive ssh clients will be disconnected after approximately 45 seconds. .It Cm DenyGroups This keyword can be followed by a number of group names, separated by spaces. Users whose primary group or supplementary group list matches one of the patterns aren't allowed to log in. .Ql \&* and .Ql ? can be used as wildcards in the patterns. Only group names are valid; a numerical group ID isn't recognized. By default login is allowed regardless of the group list. .Pp .It Cm DenyUsers This keyword can be followed by a number of user names, separated by spaces. Login is disallowed for user names that match one of the patterns. .Ql \&* and .Ql ? can be used as wildcards in the patterns. Only user names are valid; a numerical user ID isn't recognized. By default login is allowed regardless of the user name. .It Cm GatewayPorts Specifies whether remote hosts are allowed to connect to ports forwarded for the client. The argument must be .Dq yes or .Dq no . The default is .Dq no . .It Cm HostbasedAuthentication Specifies whether rhosts or /etc/hosts.equiv authentication together with successful public key client host authentication is allowed (hostbased authentication). This option is similar to .Cm RhostsRSAAuthentication and applies to protocol version 2 only. The default is .Dq no . .It Cm HostKey Specifies the file containing the private host keys (default .Pa /etc/ssh_host_key ) used by SSH protocol versions 1 and 2. Note that .Nm will refuse to use a file if it is group/world-accessible. It is possible to have multiple host key files. .Dq rsa1 keys are used for version 1 and .Dq dsa or .Dq rsa are used for version 2 of the SSH protocol. .It Cm IgnoreRhosts Specifies that .Pa .rhosts and .Pa .shosts files will not be used in .Cm RhostsAuthentication , .Cm RhostsRSAAuthentication or .Cm HostbasedAuthentication . .Pp .Pa /etc/hosts.equiv and .Pa /etc/shosts.equiv are still used. The default is .Dq yes . .It Cm IgnoreUserKnownHosts Specifies whether .Nm should ignore the user's .Pa $HOME/.ssh/known_hosts during .Cm RhostsRSAAuthentication or .Cm HostbasedAuthentication . The default is .Dq no . .It Cm KeepAlive Specifies whether the system should send keepalive messages to the other side. If they are sent, death of the connection or crash of one of the machines will be properly noticed. However, this means that connections will die if the route is down temporarily, and some people find it annoying. On the other hand, if keepalives are not sent, sessions may hang indefinitely on the server, leaving .Dq ghost users and consuming server resources. .Pp The default is .Dq yes (to send keepalives), and the server will notice if the network goes down or the client host reboots. This avoids infinitely hanging sessions. .Pp To disable keepalives, the value should be set to .Dq no in both the server and the client configuration files. .It Cm KerberosAuthentication Specifies whether Kerberos authentication is allowed. This can be in the form of a Kerberos ticket, or if .Cm PasswordAuthentication is yes, the password provided by the user will be validated through the Kerberos KDC. To use this option, the server needs a Kerberos servtab which allows the verification of the KDC's identity. Default is .Dq yes . .It Cm KerberosOrLocalPasswd If set then if password authentication through Kerberos fails then the password will be validated via any additional local mechanism such as .Pa /etc/passwd . Default is .Dq yes . .It Cm KerberosTgtPassing Specifies whether a Kerberos TGT may be forwarded to the server. Default is .Dq no , as this only works when the Kerberos KDC is actually an AFS kaserver. .It Cm KerberosTicketCleanup Specifies whether to automatically destroy the user's ticket cache file on logout. Default is .Dq yes . .It Cm KeyRegenerationInterval In protocol version 1, the ephemeral server key is automatically regenerated after this many seconds (if it has been used). The purpose of regeneration is to prevent decrypting captured sessions by later breaking into the machine and stealing the keys. The key is never stored anywhere. If the value is 0, the key is never regenerated. The default is 3600 (seconds). .It Cm ListenAddress Specifies the local addresses .Nm should listen on. The following forms may be used: .Pp .Bl -item -offset indent -compact .It .Cm ListenAddress .Sm off .Ar host No | Ar IPv4_addr No | Ar IPv6_addr .Sm on .It .Cm ListenAddress .Sm off .Ar host No | Ar IPv4_addr No : Ar port .Sm on .It .Cm ListenAddress .Sm off .Oo .Ar host No | Ar IPv6_addr Oc : Ar port .Sm on .El .Pp If .Ar port is not specified, .Nm will listen on the address and all prior .Cm Port options specified. The default is to listen on all local addresses. Multiple .Cm ListenAddress options are permitted. Additionally, any .Cm Port options must precede this option for non port qualified addresses. .It Cm LoginGraceTime The server disconnects after this time if the user has not successfully logged in. If the value is 0, there is no time limit. The default is 600 (seconds). .It Cm LogLevel Gives the verbosity level that is used when logging messages from .Nm sshd . The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG. The default is INFO. Logging with level DEBUG violates the privacy of users and is not recommended. .It Cm MACs Specifies the available MAC (message authentication code) algorithms. The MAC algorithm is used in protocol version 2 for data integrity protection. Multiple algorithms must be comma-separated. The default is .Pp .Bd -literal ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com, hmac-sha1-96,hmac-md5-96'' .Ed .It Cm MaxStartups Specifies the maximum number of concurrent unauthenticated connections to the .Nm daemon. Additional connections will be dropped until authentication succeeds or the .Cm LoginGraceTime expires for a connection. The default is 10. .Pp Alternatively, random early drop can be enabled by specifying the three colon separated values .Dq start:rate:full (e.g., "10:30:60"). .Nm will refuse connection attempts with a probability of .Dq rate/100 (30%) if there are currently .Dq start (10) unauthenticated connections. The probability increases linearly and all connection attempts are refused if the number of unauthenticated connections reaches .Dq full (60). .It Cm PAMAuthenticationViaKbdInt Specifies whether PAM challenge response authentication is allowed. This allows the use of most PAM challenge response authentication modules, but it will allow password authentication regardless of whether .Cm PasswordAuthentication is disabled. The default is .Dq no . .It Cm PasswordAuthentication Specifies whether password authentication is allowed. The default is .Dq yes . .It Cm PermitEmptyPasswords When password authentication is allowed, it specifies whether the server allows login to accounts with empty password strings. The default is .Dq no . .It Cm PermitRootLogin Specifies whether root can login using .Xr ssh 1 . The argument must be .Dq yes , .Dq without-password , .Dq forced-commands-only or .Dq no . The default is .Dq yes . .Pp If this option is set to .Dq without-password password authentication is disabled for root. .Pp If this option is set to .Dq forced-commands-only root login with public key authentication will be allowed, but only if the .Ar command option has been specified (which may be useful for taking remote backups even if root login is normally not allowed). All other authentication methods are disabled for root. .Pp If this option is set to .Dq no root is not allowed to login. .It Cm PidFile Specifies the file that contains the process identifier of the .Nm daemon. The default is .Pa /var/run/sshd.pid . .It Cm Port Specifies the port number that .Nm listens on. The default is 22. Multiple options of this type are permitted. See also .Cm ListenAddress . .It Cm PrintLastLog Specifies whether .Nm should print the date and time when the user last logged in. The default is .Dq yes . .It Cm PrintMotd Specifies whether .Nm should print .Pa /etc/motd when a user logs in interactively. (On some systems it is also printed by the shell, .Pa /etc/profile , or equivalent.) The default is .Dq yes . .It Cm Protocol Specifies the protocol versions .Nm should support. The possible values are .Dq 1 and .Dq 2 . Multiple versions must be comma-separated. The default is .Dq 2,1 . .It Cm PubkeyAuthentication Specifies whether public key authentication is allowed. The default is .Dq yes . Note that this option applies to protocol version 2 only. .It Cm ReverseMappingCheck Specifies whether .Nm should try to verify the remote host name and check that the resolved host name for the remote IP address maps back to the very same IP address. The default is .Dq no . .It Cm RhostsAuthentication Specifies whether authentication using rhosts or /etc/hosts.equiv files is sufficient. Normally, this method should not be permitted because it is insecure. .Cm RhostsRSAAuthentication should be used instead, because it performs RSA-based host authentication in addition to normal rhosts or /etc/hosts.equiv authentication. The default is .Dq no . This option applies to protocol version 1 only. .It Cm RhostsRSAAuthentication Specifies whether rhosts or /etc/hosts.equiv authentication together with successful RSA host authentication is allowed. The default is .Dq no . This option applies to protocol version 1 only. .It Cm RSAAuthentication Specifies whether pure RSA authentication is allowed. The default is .Dq yes . This option applies to protocol version 1 only. .It Cm ServerKeyBits Defines the number of bits in the ephemeral protocol version 1 server key. The minimum value is 512, and the default is 768. .It Cm StrictModes Specifies whether .Nm should check file modes and ownership of the user's files and home directory before accepting login. This is normally desirable because novices sometimes accidentally leave their directory or files world-writable. The default is .Dq yes . .It Cm Subsystem Configures an external subsystem (e.g., file transfer daemon). Arguments should be a subsystem name and a command to execute upon subsystem request. The command .Xr sftp-server 8 implements the .Dq sftp file transfer subsystem. By default no subsystems are defined. Note that this option applies to protocol version 2 only. .It Cm SyslogFacility Gives the facility code that is used when logging messages from .Nm sshd . The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is AUTH. .It Cm UseLogin Specifies whether .Xr login 1 is used for interactive login sessions. Note that .Xr login 1 is never used for remote command execution. The default is .Dq no . .It Cm X11DisplayOffset Specifies the first display number available for .Nm sshd Ns 's X11 forwarding. This prevents .Nm from interfering with real X11 servers. The default is 10. .It Cm X11Forwarding Specifies whether X11 forwarding is permitted. The default is .Dq no . Note that disabling X11 forwarding does not improve security in any way, as users can always install their own forwarders. .It Cm XAuthLocation Specifies the location of the .Xr xauth 1 program. The default is .Pa /usr/X11R6/bin/xauth . .El .Sh LOGIN PROCESS When a user successfully logs in, .Nm does the following: .Bl -enum -offset indent .It If the login is on a tty, and no command has been specified, prints last login time and .Pa /etc/motd (unless prevented in the configuration file or by .Pa $HOME/.hushlogin ; see the .Sx FILES section). .It If the login is on a tty, records login time. .It Checks .Pa /etc/nologin ; if it exists, prints contents and quits (unless root). .It Changes to run with normal user privileges. .It Sets up basic environment. .It Reads .Pa $HOME/.ssh/environment if it exists. .It Changes to user's home directory. .It If .Pa $HOME/.ssh/rc exists, runs it; else if .Pa /etc/sshrc exists, runs it; otherwise runs xauth. The .Dq rc files are given the X11 authentication protocol and cookie in standard input. .It Runs user's shell or command. .El .Sh AUTHORIZED_KEYS FILE FORMAT The .Pa $HOME/.ssh/authorized_keys file lists the RSA keys that are permitted for RSA authentication in protocol version 1. Similarly, the .Pa $HOME/.ssh/authorized_keys2 file lists the DSA and RSA keys that are permitted for public key authentication (PubkeyAuthentication) in protocol version 2. .Pp Each line of the file contains one key (empty lines and lines starting with a .Ql # are ignored as comments). Each RSA public key consists of the following fields, separated by spaces: options, bits, exponent, modulus, comment. Each protocol version 2 public key consists of: options, keytype, base64 encoded key, comment. The options fields are optional; its presence is determined by whether the line starts with a number or not (the option field never starts with a number). The bits, exponent, modulus and comment fields give the RSA key for protocol version 1; the comment field is not used for anything (but may be convenient for the user to identify the key). For protocol version 2 the keytype is .Dq ssh-dss or .Dq ssh-rsa . .Pp Note that lines in this file are usually several hundred bytes long (because of the size of the RSA key modulus). You don't want to type them in; instead, copy the .Pa identity.pub , .Pa id_dsa.pub or the .Pa id_rsa.pub file and edit it. .Pp The options (if present) consist of comma-separated option specifications. No spaces are permitted, except within double quotes. The following option specifications are supported: .Bl -tag -width Ds .It Cm from="pattern-list" Specifies that in addition to RSA authentication, the canonical name of the remote host must be present in the comma-separated list of patterns .Pf ( Ql * and .Ql ? serve as wildcards). The list may also contain patterns negated by prefixing them with .Ql ! ; if the canonical host name matches a negated pattern, the key is not accepted. The purpose of this option is to optionally increase security: RSA 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). .It Cm 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 connection requests a pty; otherwise it is run without a tty. Note that if you want a 8-bit clean channel, you must not request a pty or should specify .Cm no-pty . A quote may be included in the command by quoting it with a backslash. This option might be useful to restrict certain RSA 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/IP and/or X11 forwarding unless they are explicitly prohibited. .It Cm 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. .It Cm no-port-forwarding Forbids TCP/IP 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 .Cm command option. .It Cm no-X11-forwarding Forbids X11 forwarding when this key is used for authentication. Any X11 forward requests by the client will return an error. .It Cm no-agent-forwarding Forbids authentication agent forwarding when this key is used for authentication. .It Cm no-pty Prevents tty allocation (a request to allocate a pty will fail). .It Cm permitopen="host:port" Limit local .Li ``ssh -L'' port forwarding such that it may only connect to the specified host and port. Multiple .Cm permitopen options may be applied separated by commas. No pattern matching is performed on the specified hostnames, they must be literal domains or addresses. .El .Ss Examples 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar .Pp from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula .Pp command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi .Pp permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323 .Sh SSH_KNOWN_HOSTS FILE FORMAT The .Pa /etc/ssh_known_hosts , .Pa /etc/ssh_known_hosts2 , .Pa $HOME/.ssh/known_hosts , and .Pa $HOME/.ssh/known_hosts2 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 from an unknown host its key is added to the per-user file. .Pp Each line in these files contains the following fields: hostnames, bits, exponent, modulus, comment. The fields are separated by spaces. .Pp Hostnames is a comma-separated list of patterns ('*' and '?' act as wildcards); each pattern in turn is matched against the canonical host name (when authenticating a client) or against the user-supplied name (when authenticating a server). A pattern may also be preceded by .Ql ! 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. .Pp Bits, exponent, and modulus are taken directly from the RSA host key; they can be obtained, e.g., from .Pa /etc/ssh_host_key.pub . The optional comment field continues to the end of the line, and is not used. .Pp Lines starting with .Ql # and empty lines are ignored as comments. .Pp When performing host authentication, authentication is accepted if any matching line has the proper key. It is thus 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. .Pp 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 or by taking .Pa /etc/ssh_host_key.pub and adding the host names at the front. .Ss Examples .Bd -literal closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....= .Ed .Sh FILES .Bl -tag -width Ds .It Pa /etc/sshd_config Contains configuration data for .Nm sshd . This file should be writable by root only, but it is recommended (though not necessary) that it be world-readable. .It Pa /etc/ssh_host_key, /etc/ssh_host_dsa_key, /etc/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 .Nm does not start if this file is group/world-accessible. .It Pa /etc/ssh_host_key.pub, /etc/ssh_host_dsa_key.pub, /etc/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 .Xr ssh-keygen 1 . .It Pa /etc/primes Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". .It Pa /var/run/sshd.pid Contains the process ID of the .Nm listening for connections (if there are several daemons running concurrently for different ports, this contains the pid of the one started last). The content of this file is not sensitive; it can be world-readable. .It Pa $HOME/.ssh/authorized_keys Lists the RSA keys that can be used to log into the user's account. This file must be readable by root (which may on some machines imply it being world-readable if the user's home directory resides on an NFS volume). It is recommended that it not be accessible by others. The format of this file is described above. Users will place the contents of their .Pa identity.pub files into this file, as described in .Xr ssh-keygen 1 . .It Pa $HOME/.ssh/authorized_keys2 Lists the public keys (RSA or DSA) that can be used to log into the user's account. This file must be readable by root (which may on some machines imply it being world-readable if the user's home directory resides on an NFS volume). It is recommended that it not be accessible by others. The format of this file is described above. Users will place the contents of their .Pa id_dsa.pub and/or .Pa id_rsa.pub files into this file, as described in .Xr ssh-keygen 1 . .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts" These files are consulted when using rhosts with RSA host authentication to check the public key of the host. The key must be listed in one of these files to be accepted. The client uses the same files to verify that it is connecting to the correct remote host. These files should be writable only by root/the owner. .Pa /etc/ssh_known_hosts should be world-readable, and .Pa $HOME/.ssh/known_hosts can but need not be world-readable. .It Pa "/etc/ssh_known_hosts2" and "$HOME/.ssh/known_hosts2" These files are consulted when using protocol version 2 hostbased authentication to check the public key of the host. The key must be listed in one of these files to be accepted. The client uses the same files to verify that it is connecting to the correct remote host. These files should be writable only by root/the owner. .Pa /etc/ssh_known_hosts2 should be world-readable, and .Pa $HOME/.ssh/known_hosts2 can but need not be world-readable. .It Pa /etc/nologin If this file exists, .Nm 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. .It Pa /etc/hosts.allow, /etc/hosts.deny If compiled with .Sy LIBWRAP support, tcp-wrappers access controls may be defined here as described in .Xr hosts_access 5 . .It Pa $HOME/.rhosts This file contains host-username pairs, separated by a space, one per line. The given user on the corresponding host is permitted to log in without password. The same file is used by rlogind and rshd. The file must be writable only by the user; it is recommended that it not be accessible by others. .Pp If is also possible to use netgroups in the file. Either host or user name may be of the form +@groupname to specify all hosts or all users in the group. .It Pa $HOME/.shosts For ssh, this file is exactly the same as for .Pa .rhosts . However, this file is not used by rlogin and rshd, so using this permits access using SSH only. .It Pa /etc/hosts.equiv This file is used during .Pa .rhosts authentication. In the simplest form, this file contains host names, one per line. Users on those hosts are permitted to log in without a password, provided they have the same user name on both machines. The host name may also be followed by a user name; such users are permitted to log in as .Em any user on this machine (except root). Additionally, the syntax .Dq +@group can be used to specify netgroups. Negated entries start with .Ql \&- . .Pp If the client host/user is successfully matched in this file, login is automatically permitted provided the client and server user names are the same. Additionally, successful RSA host authentication is normally required. This file must be writable only by root; it is recommended that it be world-readable. .Pp .Sy "Warning: It is almost never a good idea to use user names in" .Pa hosts.equiv . Beware that it really means that the named user(s) can log in as .Em anybody , which includes bin, daemon, adm, and other accounts that own critical binaries and directories. Using a user name practically grants the user root access. The only valid use for user names that I can think of is in negative entries. .Pp Note that this warning also applies to rsh/rlogin. .It Pa /etc/shosts.equiv This is processed exactly as .Pa /etc/hosts.equiv . However, this file may be useful in environments that want to run both rsh/rlogin and ssh. .It Pa $HOME/.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 .Ql # ) , 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. .It Pa $HOME/.ssh/rc If this file exists, it is run with /bin/sh after reading the environment files but before starting the user's shell or command. If X11 spoofing is in use, this will receive the "proto cookie" pair in standard input (and .Ev DISPLAY in environment). This must call .Xr xauth 1 in that case. .Pp 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. .Pp This file will probably contain some initialization code followed by something similar to: .Bd -literal if read proto cookie; then echo add $DISPLAY $proto $cookie | xauth -q - fi .Ed .Pp If this file does not exist, .Pa /etc/sshrc is run, and if that does not exist either, xauth is used to store the cookie. .Pp This file should be writable only by the user, and need not be readable by anyone else. .It Pa /etc/sshrc Like .Pa $HOME/.ssh/rc . This can be used to specify machine-specific login-time initializations globally. This file should be writable only by root, and should be world-readable. .El .Sh 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. .Sh SEE ALSO .Xr scp 1 , .Xr sftp 1 , .Xr sftp-server 8 , .Xr ssh 1 , .Xr ssh-add 1 , .Xr ssh-agent 1 , .Xr ssh-keygen 1 , .Xr rlogin 1 , .Xr rsh 1 .Rs .%A T. Ylonen .%A T. Kivinen .%A M. Saarinen .%A T. Rinne .%A S. Lehtinen .%T "SSH Protocol Architecture" .%N draft-ietf-secsh-architecture-07.txt .%D January 2001 .%O work in progress material .Re .Rs .%A M. Friedl .%A N. Provos .%A W. A. Simpson .%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol" .%N draft-ietf-secsh-dh-group-exchange-00.txt .%D January 2001 .%O work in progress material .Re