.\" -*- nroff -*- .\" .\" Author: Tatu Ylonen <firstname.lastname@example.org> .\" Copyright (c) 1995 Tatu Ylonen <email@example.com>, 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_config.5,v 1.29 2004/03/08 10:18:57 dtucker Exp $ .Dd September 25, 1999 .Dt SSHD_CONFIG 5 .Os .Sh NAME .Nm sshd_config .Nd OpenSSH SSH daemon configuration file .Sh SYNOPSIS .Bl -tag -width Ds -compact .It Pa /etc/ssh/sshd_config .El .Sh DESCRIPTION .Nm sshd reads configuration data from .Pa /etc/ssh/sshd_config (or the file specified with .Fl f on the command line). The file contains keyword-argument pairs, one per line. Lines starting with .Ql # and empty lines are interpreted as comments. .Pp The possible keywords and their meanings are as follows (note that keywords are case-insensitive and arguments are case-sensitive): .Bl -tag -width Ds .It Cm AllowGroups This keyword can be followed by a list of group name patterns, 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 is not recognized. By default, login is allowed for all groups. .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 name patterns, separated by spaces. If specified, login is allowed only 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 is not recognized. By default, login is allowed for all users. If the pattern takes the form USER@HOST then USER and HOST are separately checked, restricting logins to particular users from particular hosts. .Pp .It Cm AuthorizedKeysFile Specifies the file that contains the public keys that can be used for user authentication. .Cm AuthorizedKeysFile may contain tokens of the form %T which are substituted during connection set-up. The following tokens are defined: %% is replaced by a literal '%', %h is replaced by the home directory of the user being authenticated and %u is replaced by the username of that user. After expansion, .Cm AuthorizedKeysFile is taken to be an absolute path or one relative to the user's home directory. The default is .Dq .ssh/authorized_keys . .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. By default, no banner is displayed. .Pp .It Cm ChallengeResponseAuthentication Specifies whether challenge response authentication is allowed. All authentication styles from .Xr login.conf 5 are supported. 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 .Pp .Bd -literal ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour, aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr'' .Ed .It Cm ClientAliveInterval Sets a timeout interval in seconds after which if no data has been received from the client, .Nm sshd 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 sshd receiving any messages back from the client. If this threshold is reached while client alive messages are being sent, .Nm sshd will disconnect the client, terminating the session. It is important to note that the use of client alive messages is very different from .Cm TCPKeepAlive (below). The client alive messages are sent through the encrypted channel and therefore will not be spoofable. The TCP keepalive option enabled by .Cm TCPKeepAlive is spoofable. The client alive mechanism is valuable when the client or server depend on knowing when a connection has become inactive. .Pp The default value is 3. If .Cm ClientAliveInterval (above) is set to 15, and .Cm ClientAliveCountMax is left at the default, unresponsive ssh clients will be disconnected after approximately 45 seconds. .It Cm Compression Specifies whether compression is allowed. The argument must be .Dq yes or .Dq no . The default is .Dq yes . .It Cm DenyGroups This keyword can be followed by a list of group name patterns, separated by spaces. Login is disallowed 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 is not recognized. By default, login is allowed for all groups. .Pp .It Cm DenyUsers This keyword can be followed by a list of user name patterns, 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 is not recognized. By default, login is allowed for all users. If the pattern takes the form USER@HOST then USER and HOST are separately checked, restricting logins to particular users from particular hosts. .It Cm GatewayPorts Specifies whether remote hosts are allowed to connect to ports forwarded for the client. By default, .Nm sshd binds remote port forwardings to the loopback address. This prevents other remote hosts from connecting to forwarded ports. .Cm GatewayPorts can be used to specify that .Nm sshd should bind remote port forwardings to the wildcard address, thus allowing remote hosts to connect to forwarded ports. The argument must be .Dq yes or .Dq no . The default is .Dq no . .It Cm GSSAPIAuthentication Specifies whether user authentication based on GSSAPI is allowed. The default is .Dq no . Note that this option applies to protocol version 2 only. .It Cm GSSAPICleanupCredentials Specifies whether to automatically destroy the user's credentials cache on logout. The default is .Dq yes . Note that this option applies to protocol version 2 only. .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 a file containing a private host key used by SSH. The default is .Pa /etc/ssh/ssh_host_key for protocol version 1, and .Pa /etc/ssh/ssh_host_rsa_key and .Pa /etc/ssh/ssh_host_dsa_key for protocol version 2. Note that .Nm sshd 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 GssapiAuthentication Specifies whether authentication based on GSSAPI may be used, either using the result of a successful key exchange, or using GSSAPI user authentication. The default is .Dq yes . Note that this option applies to protocol version 2 only. .It Cm GssapiKeyExchange Specifies whether key exchange based on GSSAPI may be used. When using GSSAPI key exchange the server need not have a host key. The default is .Dq yes . Note that this option applies to protocol version 2 only. .It Cm GssapiUseSessionCredCache Specifies whether a unique credentials cache name should be generated per session for storing delegated credentials. The default is .Dq yes . Note that this option applies to protocol version 2 only. .It Cm IgnoreRhosts Specifies that .Pa .rhosts and .Pa .shosts files will not be used in .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 sshd should ignore the user's .Pa $HOME/.ssh/known_hosts during .Cm RhostsRSAAuthentication or .Cm HostbasedAuthentication . The default is .Dq no . .It Cm KerberosAuthentication Specifies whether the password provided by the user for .Cm PasswordAuthentication 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 no . .It Cm KerberosGetAFSToken If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire an AFS token before accessing the user's home directory. Default is .Dq no . .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 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 sshd 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 sshd 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 120 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, DEBUG, DEBUG1, DEBUG2 and DEBUG3. The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify higher levels of debugging output. Logging with a DEBUG level 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 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . .It Cm MaxStartups Specifies the maximum number of concurrent unauthenticated connections to the .Nm sshd 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 sshd 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 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. Note that other authentication methods (e.g., keyboard-interactive/PAM) may still allow root to login using a password. .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 PermitUserEnvironment Specifies whether .Pa ~/.ssh/environment and .Cm environment= options in .Pa ~/.ssh/authorized_keys are processed by .Nm sshd . The default is .Dq no . Enabling environment processing may enable users to bypass access restrictions in some configurations using mechanisms such as .Ev LD_PRELOAD . .It Cm PidFile Specifies the file that contains the process ID of the .Nm sshd daemon. The default is .Pa /var/run/sshd.pid . .It Cm Port Specifies the port number that .Nm sshd listens on. The default is 22. Multiple options of this type are permitted. See also .Cm ListenAddress . .It Cm PrintLastLog Specifies whether .Nm sshd should print the date and time when the user last logged in. The default is .Dq yes . .It Cm PrintMotd Specifies whether .Nm sshd 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 sshd supports. The possible values are .Dq 1 and .Dq 2 . Multiple versions must be comma-separated. The default is .Dq 2,1 . Note that the order of the protocol list does not indicate preference, because the client selects among multiple protocol versions offered by the server. Specifying .Dq 2,1 is identical to .Dq 1,2 . .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 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 sshd 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 TCPKeepAlive Specifies whether the system should send TCP 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 TCP 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 TCP keepalive messages), and the server will notice if the network goes down or the client host crashes. This avoids infinitely hanging sessions. .Pp To disable TCP keepalive messages, the value should be set to .Dq no . .It Cm UseDNS Specifies whether .Nm sshd should lookup 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 yes . .It Cm UseLogin Specifies whether .Xr login 1 is used for interactive login sessions. The default is .Dq no . Note that .Xr login 1 is never used for remote command execution. Note also, that if this is enabled, .Cm X11Forwarding will be disabled because .Xr login 1 does not know how to handle .Xr xauth 1 cookies. If .Cm UsePrivilegeSeparation is specified, it will be disabled after authentication. .It Cm UsePAM Enables PAM authentication (via challenge-response) and session set up. If you enable this, you should probably disable .Cm PasswordAuthentication . If you enable .CM UsePAM then you will not be able to run sshd as a non-root user. The default is .Dq no . .It Cm UsePrivilegeSeparation Specifies whether .Nm sshd separates privileges by creating an unprivileged child process to deal with incoming network traffic. After successful authentication, another process will be created that has the privilege of the authenticated user. The goal of privilege separation is to prevent privilege escalation by containing any corruption within the unprivileged processes. The default is .Dq yes . .It Cm X11DisplayOffset Specifies the first display number available for .Nm sshd Ns 's X11 forwarding. This prevents .Nm sshd from interfering with real X11 servers. The default is 10. .It Cm X11Forwarding Specifies whether X11 forwarding is permitted. The argument must be .Dq yes or .Dq no . The default is .Dq no . .Pp When X11 forwarding is enabled, there may be additional exposure to the server and to client displays if the .Nm sshd proxy display is configured to listen on the wildcard address (see .Cm X11UseLocalhost below), however this is not the default. Additionally, the authentication spoofing and authentication data verification and substitution occur on the client side. The security risk of using X11 forwarding is that the client's X11 display server may be exposed to attack when the ssh client requests forwarding (see the warnings for .Cm ForwardX11 in .Xr ssh_config 5 ) . A system administrator may have a stance in which they want to protect clients that may expose themselves to attack by unwittingly requesting X11 forwarding, which can warrant a .Dq no setting. .Pp Note that disabling X11 forwarding does not prevent users from forwarding X11 traffic, as users can always install their own forwarders. X11 forwarding is automatically disabled if .Cm UseLogin is enabled. .It Cm X11UseLocalhost Specifies whether .Nm sshd should bind the X11 forwarding server to the loopback address or to the wildcard address. By default, .Nm sshd binds the forwarding server to the loopback address and sets the hostname part of the .Ev DISPLAY environment variable to .Dq localhost . This prevents remote hosts from connecting to the proxy display. However, some older X11 clients may not function with this configuration. .Cm X11UseLocalhost may be set to .Dq no to specify that the forwarding server should be bound to the wildcard address. The argument must be .Dq yes or .Dq no . The default is .Dq yes . .It Cm XAuthLocation Specifies the full pathname of the .Xr xauth 1 program. The default is .Pa /usr/X11R6/bin/xauth . .El .Ss Time Formats .Nm sshd command-line arguments and configuration file options that specify time may be expressed using a sequence of the form: .Sm off .Ar time Op Ar qualifier , .Sm on where .Ar time is a positive integer value and .Ar qualifier is one of the following: .Pp .Bl -tag -width Ds -compact -offset indent .It Cm <none> seconds .It Cm s | Cm S seconds .It Cm m | Cm M minutes .It Cm h | Cm H hours .It Cm d | Cm D days .It Cm w | Cm W weeks .El .Pp Each member of the sequence is added together to calculate the total time value. .Pp Time format examples: .Pp .Bl -tag -width Ds -compact -offset indent .It 600 600 seconds (10 minutes) .It 10m 10 minutes .It 1h30m 1 hour 30 minutes (90 minutes) .El .Sh FILES .Bl -tag -width Ds .It Pa /etc/ssh/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. .El .Sh SEE ALSO .Xr sshd 8 .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. Niels Provos and Markus Friedl contributed support for privilege separation.