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diff --git a/crypto/openssh/sshd_config.5 b/crypto/openssh/sshd_config.5 new file mode 100644 index 0000000..2bcaf22 --- /dev/null +++ b/crypto/openssh/sshd_config.5 @@ -0,0 +1,962 @@ +.\" -*- nroff -*- +.\" +.\" Author: Tatu Ylonen <ylo@cs.hut.fi> +.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, 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 1.70 2006/08/21 08:14:01 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 +.Xr sshd 8 +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. +Arguments may optionally be enclosed in double quotes +.Pq \&" +in order to represent arguments containing spaces. +.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 AcceptEnv +Specifies what environment variables sent by the client will be copied into +the session's +.Xr environ 7 . +See +.Cm SendEnv +in +.Xr ssh_config 5 +for how to configure the client. +Note that environment passing is only supported for protocol 2. +Variables are specified by name, which may contain the wildcard characters +.Ql * +and +.Ql \&? . +Multiple environment variables may be separated by whitespace or spread +across multiple +.Cm AcceptEnv +directives. +Be warned that some environment variables could be used to bypass restricted +user environments. +For this reason, care should be taken in the use of this directive. +The default is not to accept any environment variables. +.It Cm AddressFamily +Specifies which address family should be used by +.Xr sshd 8 . +Valid arguments are +.Dq any , +.Dq inet +(use IPv4 only), or +.Dq inet6 +(use IPv6 only). +The default is +.Dq any . +.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. +Only group names are valid; a numerical group ID is not recognized. +By default, login is allowed for all groups. +The allow/deny directives are processed in the following order: +.Cm DenyUsers , +.Cm AllowUsers , +.Cm DenyGroups , +and finally +.Cm AllowGroups . +.Pp +See +.Sx PATTERNS +in +.Xr ssh_config 5 +for more information on patterns. +.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. +.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. +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. +The allow/deny directives are processed in the following order: +.Cm DenyUsers , +.Cm AllowUsers , +.Cm DenyGroups , +and finally +.Cm AllowGroups . +.Pp +See +.Sx PATTERNS +in +.Xr ssh_config 5 +for more information on patterns. +.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 +setup. +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. +.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 supported ciphers are +.Dq 3des-cbc , +.Dq aes128-cbc , +.Dq aes192-cbc , +.Dq aes256-cbc , +.Dq aes128-ctr , +.Dq aes192-ctr , +.Dq aes256-ctr , +.Dq arcfour128 , +.Dq arcfour256 , +.Dq arcfour , +.Dq blowfish-cbc , +and +.Dq cast128-cbc . +The default is: +.Bd -literal -offset 3n +aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, +arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, +aes192-ctr,aes256-ctr +.Ed +.It Cm ClientAliveCountMax +Sets the number of client alive messages (see below) which may be +sent without +.Xr sshd 8 +receiving any messages back from the client. +If this threshold is reached while client alive messages are being sent, +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 +(see below) is set to 15, and +.Cm ClientAliveCountMax +is left at the default, unresponsive SSH clients +will be disconnected after approximately 45 seconds. +This option applies to protocol version 2 only. +.It Cm ClientAliveInterval +Sets a timeout interval in seconds after which if no data has been received +from the client, +.Xr sshd 8 +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 Compression +Specifies whether compression is allowed, or delayed until +the user has authenticated successfully. +The argument must be +.Dq yes , +.Dq delayed , +or +.Dq no . +The default is +.Dq delayed . +.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. +Only group names are valid; a numerical group ID is not recognized. +By default, login is allowed for all groups. +The allow/deny directives are processed in the following order: +.Cm DenyUsers , +.Cm AllowUsers , +.Cm DenyGroups , +and finally +.Cm AllowGroups . +.Pp +See +.Sx PATTERNS +in +.Xr ssh_config 5 +for more information on patterns. +.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. +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. +The allow/deny directives are processed in the following order: +.Cm DenyUsers , +.Cm AllowUsers , +.Cm DenyGroups , +and finally +.Cm AllowGroups . +.Pp +See +.Sx PATTERNS +in +.Xr ssh_config 5 +for more information on patterns. +.It Cm ForceCommand +Forces the execution of the command specified by +.Cm ForceCommand , +ignoring any command supplied by the client. +The command is invoked by using the user's login shell with the -c option. +This applies to shell, command, or subsystem execution. +It is most useful inside a +.Cm Match +block. +The command originally supplied by the client is available in the +.Ev SSH_ORIGINAL_COMMAND +environment variable. +.It Cm GatewayPorts +Specifies whether remote hosts are allowed to connect to ports +forwarded for the client. +By default, +.Xr sshd 8 +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 sshd +should allow remote port forwardings to bind to non-loopback addresses, thus +allowing other hosts to connect. +The argument may be +.Dq no +to force remote port forwardings to be available to the local host only, +.Dq yes +to force remote port forwardings to bind to the wildcard address, or +.Dq clientspecified +to allow the client to select the address to which the forwarding is bound. +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 +(host-based authentication). +This option is similar to +.Cm RhostsRSAAuthentication +and applies to protocol version 2 only. +The default is +.Dq no . +.It Cm HostbasedUsesNameFromPacketOnly +Specifies whether or not the server will attempt to perform a reverse +name lookup when matching the name in the +.Pa ~/.shosts , +.Pa ~/.rhosts , +and +.Pa /etc/hosts.equiv +files during +.Cm HostbasedAuthentication . +A setting of +.Dq yes +means that +.Xr sshd 8 +uses the name supplied by the client rather than +attempting to resolve the name from the TCP connection itself. +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 +.Xr sshd 8 +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 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 +.Xr sshd 8 +should ignore the user's +.Pa ~/.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. +The default is +.Dq no . +.It Cm KerberosGetAFSToken +If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire +an AFS token before accessing the user's home directory. +The default is +.Dq no . +.It Cm KerberosOrLocalPasswd +If password authentication through Kerberos fails then +the password will be validated via any additional local mechanism +such as +.Pa /etc/passwd . +The default is +.Dq yes . +.It Cm KerberosTicketCleanup +Specifies whether to automatically destroy the user's ticket cache +file on logout. +The 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 +.Xr sshd 8 +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, +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 +.Xr sshd 8 . +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 Match +Introduces a conditional block. +If all of the criteria on the +.Cm Match +line are satisfied, the keywords on the following lines override those +set in the global section of the config file, until either another +.Cm Match +line or the end of the file. +The arguments to +.Cm Match +are one or more criteria-pattern pairs. +The available criteria are +.Cm User , +.Cm Group , +.Cm Host , +and +.Cm Address . +Only a subset of keywords may be used on the lines following a +.Cm Match +keyword. +Available keywords are +.Cm AllowTcpForwarding , +.Cm ForceCommand , +.Cm GatewayPorts , +.Cm PermitOpen , +.Cm X11DisplayOffset , +.Cm X11Forwarding , +and +.Cm X11UseLocalHost . +.It Cm MaxAuthTries +Specifies the maximum number of authentication attempts permitted per +connection. +Once the number of failures reaches half this value, +additional failures are logged. +The default is 6. +.It Cm MaxStartups +Specifies the maximum number of concurrent unauthenticated connections to the +SSH 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"). +.Xr sshd 8 +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 PermitOpen +Specifies the destinations to which TCP port forwarding is permitted. +The forwarding specification must be one of the following forms: +.Pp +.Bl -item -offset indent -compact +.It +.Cm PermitOpen +.Sm off +.Ar host : port +.Sm on +.It +.Cm PermitOpen +.Sm off +.Ar IPv4_addr : port +.Sm on +.It +.Cm PermitOpen +.Sm off +.Ar \&[ IPv6_addr \&] : port +.Sm on +.El +.Pp +Multiple forwards may be specified by separating them with whitespace. +An argument of +.Dq any +can be used to remove all restrictions and permit any forwarding requests. +By default all port forwarding requests are permitted. +.It Cm PermitRootLogin +Specifies whether root can log in 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 log in. +.It Cm PermitTunnel +Specifies whether +.Xr tun 4 +device forwarding is allowed. +The argument must be +.Dq yes , +.Dq point-to-point +(layer 3), +.Dq ethernet +(layer 2), or +.Dq no . +Specifying +.Dq yes +permits both +.Dq point-to-point +and +.Dq ethernet . +The default is +.Dq no . +.It Cm PermitUserEnvironment +Specifies whether +.Pa ~/.ssh/environment +and +.Cm environment= +options in +.Pa ~/.ssh/authorized_keys +are processed by +.Xr sshd 8 . +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 +SSH daemon. +The default is +.Pa /var/run/sshd.pid . +.It Cm Port +Specifies the port number that +.Xr sshd 8 +listens on. +The default is 22. +Multiple options of this type are permitted. +See also +.Cm ListenAddress . +.It Cm PrintLastLog +Specifies whether +.Xr sshd 8 +should print the date and time of the last user login when a user logs +in interactively. +The default is +.Dq yes . +.It Cm PrintMotd +Specifies whether +.Xr sshd 8 +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 +.Xr sshd 8 +supports. +The possible values are +.Sq 1 +and +.Sq 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 +.Xr sshd 8 +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 (with optional arguments) +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 +.Xr sshd 8 . +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 +.Xr sshd 8 +should look up 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 the Pluggable Authentication Module interface. +If set to +.Dq yes +this will enable PAM authentication using +.Cm ChallengeResponseAuthentication +and +.Cm PasswordAuthentication +in addition to PAM account and session module processing for all +authentication types. +.Pp +Because PAM challenge-response authentication usually serves an equivalent +role to password authentication, you should disable either +.Cm PasswordAuthentication +or +.Cm ChallengeResponseAuthentication. +.Pp +If +.Cm UsePAM +is enabled, you will not be able to run +.Xr sshd 8 +as a non-root user. +The default is +.Dq no . +.It Cm UsePrivilegeSeparation +Specifies whether +.Xr sshd 8 +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 +.Xr sshd 8 Ns 's +X11 forwarding. +This prevents 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 +.Xr sshd 8 +proxy display is configured to listen on the wildcard address (see +.Cm X11UseLocalhost +below), though 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 +.Xr sshd 8 +should bind the X11 forwarding server to the loopback address or to +the wildcard address. +By default, +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 +.Sh TIME FORMATS +.Xr sshd 8 +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 Aq 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 +.Xr sshd 8 . +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. |
