Manual Pages for UNIX Darwin command on man sshd

Manual Pages for UNIX Darwin command on man sshd_config

SSHDCONFIG(5) BSD File Formats Manual SSHDCONFIG(5)

NAME

sssshhddccoonnffiigg - OpenSSH SSH daemon configuration file

SYNOPSIS

//eettcc//sssshhddccoonnffiigg

DESCRIPTION

sshd(8) reads configuration data from /etc/sshdconfig (or the file spec-

ified with -ff on the command line). The file contains keyword-argument

pairs, one per line. Lines starting with `#' and empty lines are inter-

preted as comments. Arguments may optionally be enclosed in double quotes (") in order to represent arguments containing spaces.

The possible keywords and their meanings are as follows (note that key-

words are case-insensitive and arguments are case-sensitive):

AAcccceeppttEEnnvv Specifies what environment variables sent by the client will be copied into the session's environ(7). See SSeennddEEnnvv in

sshconfig(5) for how to configure the client. Note that envi-

ronment passing is only supported for protocol 2. Variables are specified by name, which may contain the wildcard characters `*' and `?'. Multiple environment variables may be separated by whitespace or spread across multiple AAcccceeppttEEnnvv 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. AAddddrreessssFFaammiillyy Specifies which address family should be used by sshd(8). Valid arguments are ``any'', ``inet'' (use IPv4 only), or ``inet6'' (use IPv6 only). The default is ``any''. AAlllloowwAAggeennttFFoorrwwaarrddiinngg

Specifies whether ssh-agent(1) forwarding is permitted. The

default is ``yes''. Note that disabling agent forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders. AAlllloowwGGrroouuppss 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: DDeennyyUUsseerrss, AAlllloowwUUsseerrss, DDeennyyGGrroouuppss, and finally AAlllloowwGGrroouuppss. See PATTERNS in sshconfig(5) for more information on patterns. AAlllloowwTTccppFFoorrwwaarrddiinngg Specifies whether TCP forwarding is permitted. The default is ``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. AAlllloowwUUsseerrss 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: DDeennyyUUsseerrss, AAlllloowwUUsseerrss, DDeennyyGGrroouuppss, and finally AAlllloowwGGrroouuppss. See PATTERNS in sshconfig(5) for more information on patterns. AAuutthhoorriizzeeddKKeeyyssFFiillee Specifies the file that contains the public keys that can be used for user authentication. AAuutthhoorriizzeeddKKeeyyssFFiillee 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, AAuutthhoorriizzeeddKKeeyyssFFiillee is taken to be an absolute path or one relative to the user's home directory. The default is ``.ssh/authorizedkeys''. BBaannnneerr The contents of the specified file are sent to the remote user before authentication is allowed. If the argument is ``none'' then no banner is displayed. This option is only available for protocol version 2. By default, no banner is displayed. CChhaalllleennggeeRReessppoonnsseeAAuutthheennttiiccaattiioonn

Specifies whether challenge-response authentication is allowed.

The default is ``yes''. CChhrroooottDDiirreeccttoorryy Specifies a path to chroot(2) to after authentication. This

path, and all its components, must be root-owned directories that

are not writable by any other user or group. The path may contain the following tokens that are expanded at

runtime once the connecting user has been authenticated: %% is

replaced by a literal '%', %h is replaced by the home directory

of the user being authenticated, and %u is replaced by the user-

name of that user.

The CChhrroooottDDiirreeccttoorryy must contain the necessary files and directo-

ries to support the users' session. For an interactive session this requires at least a shell, typically sh(1), and basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4), arandom(4) and tty(4) devices. For file transfer sessions using

``sftp'', no additional configuration of the environment is nec-

essary if the in-process sftp server is used (see SSuubbssyysstteemm for

details). The default is not to chroot(2). CCiipphheerrss Specifies the ciphers allowed for protocol version 2. Multiple

ciphers must be comma-separated. The supported ciphers are

``3des-cbc'', ``aes128-cbc'', ``aes192-cbc'', ``aes256-cbc'',

``aes128-ctr'', ``aes192-ctr'', ``aes256-ctr'', ``arcfour128'',

``arcfour256'', ``arcfour'', ``blowfish-cbc'', and

``cast128-cbc''. The default is:

aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,

arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,

aes192-ctr,aes256-ctr

CClliieennttAAlliivveeCCoouunnttMMaaxx Sets the number of client alive messages (see below) which may be sent without 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 ses-

sion. It is important to note that the use of client alive mes-

sages is very different from TTCCPPKKeeeeppAAlliivvee (below). The client

alive messages are sent through the encrypted channel and there-

fore will not be spoofable. The TCP keepalive option enabled by

TTCCPPKKeeeeppAAlliivvee is spoofable. The client alive mechanism is valu-

able when the client or server depend on knowing when a connec-

tion has become inactive. The default value is 3. If CClliieennttAAlliivveeIInntteerrvvaall (see below) is

set to 15, and CClliieennttAAlliivveeCCoouunnttMMaaxx is left at the default, unre-

sponsive SSH clients will be disconnected after approximately 45 seconds. This option applies to protocol version 2 only. CClliieennttAAlliivveeIInntteerrvvaall Sets a timeout interval in seconds after which if no data has been received from the client, 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 ver-

sion 2 only. CCoommpprreessssiioonn Specifies whether compression is allowed, or delayed until the user has authenticated successfully. The argument must be ``yes'', ``delayed'', or ``no''. The default is ``delayed''. DDeennyyGGrroouuppss 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 recog-

nized. By default, login is allowed for all groups. The allow/deny directives are processed in the following order: DDeennyyUUsseerrss, AAlllloowwUUsseerrss, DDeennyyGGrroouuppss, and finally AAlllloowwGGrroouuppss. See PATTERNS in sshconfig(5) for more information on patterns. DDeennyyUUsseerrss 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 numeri-

cal 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 pro-

cessed in the following order: DDeennyyUUsseerrss, AAlllloowwUUsseerrss, DDeennyyGGrroouuppss, and finally AAlllloowwGGrroouuppss. See PATTERNS in sshconfig(5) for more information on patterns. FFoorrcceeCCoommmmaanndd Forces the execution of the command specified by FFoorrcceeCCoommmmaanndd, ignoring any command supplied by the client and ~/.ssh/rc if present. 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 MMaattcchh block. The command originally supplied by the client is available in the SSHORIGINALCOMMAND environment variable. Specifying a command

of ``internal-sftp'' will force the use of an in-process sftp

server that requires no support files when used with CChhrroooottDDiirreeccttoorryy. GGaatteewwaayyPPoorrttss Specifies whether remote hosts are allowed to connect to ports forwarded for the client. By default, sshd(8) binds remote port forwardings to the loopback address. This prevents other remote hosts from connecting to forwarded ports. GGaatteewwaayyPPoorrttss can be used to specify that sshd should allow remote port forwardings to

bind to non-loopback addresses, thus allowing other hosts to con-

nect. The argument may be ``no'' to force remote port forward-

ings to be available to the local host only, ``yes'' to force remote port forwardings to bind to the wildcard address, or ``clientspecified'' to allow the client to select the address to which the forwarding is bound. The default is ``no''. GGSSSSAAPPIIAAuutthheennttiiccaattiioonn Specifies whether user authentication based on GSSAPI is allowed.

The default is ``yes''. Note that this option applies to proto-

col version 2 only. GGSSSSAAPPIIKKeeyyEExxcchhaannggee Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange doesn't rely on ssh keys to verify host identity.

The default is ``yes''. Note that this option applies to proto-

col version 2 only. GGSSSSAAPPIICClleeaannuuppCCrreeddeennttiiaallss Specifies whether to automatically destroy the user's credentials cache on logout. The default is ``yes''. Note that this option applies to protocol version 2 only. GGSSSSAAPPIISSttrriiccttAAcccceeppttoorrCChheecckk Determines whether to be strict about the identity of the GSSAPI acceptor a client authenticates against. If ``yes'' then the client must authenticate against the host service on the current hostname. If ``no'' then the client may authenticate against any service key stored in the machine's default store. This facility is provided to assist with operation on multi homed machines. The default is ``yes''. Note that this option applies only to protocol version 2 GSSAPI connections, and setting it to ``no'' may only work with recent Kerberos GSSAPI libraries. HHoossttbbaasseeddAAuutthheennttiiccaattiioonn 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

RRhhoossttssRRSSAAAAuutthheennttiiccaattiioonn and applies to protocol version 2 only. The default is ``no''. HHoossttbbaasseeddUUsseessNNaammeeFFrroommPPaacckkeettOOnnllyy Specifies whether or not the server will attempt to perform a reverse name lookup when matching the name in the ~/.shosts, ~/.rhosts, and /etc/hosts.equiv files during HHoossttbbaasseeddAAuutthheennttiiccaattiioonn. A setting of ``yes'' means that sshd(8) uses the name supplied by the client rather than attempting to resolve the name from the TCP connection itself. The default is ``no''. HHoossttKKeeyy Specifies a file containing a private host key used by SSH. The default is /etc/sshhostkey for protocol version 1, and

/etc/sshhostrsakey and /etc/sshhostdsakey for protocol ver-

sion 2. Note that sshd(8) will refuse to use a file if it is

group/world-accessible. It is possible to have multiple host key

files. ``rsa1'' keys are used for version 1 and ``dsa'' or ``rsa'' are used for version 2 of the SSH protocol. IIggnnoorreeRRhhoossttss Specifies that .rhosts and .shosts files will not be used in RRhhoossttssRRSSAAAAuutthheennttiiccaattiioonn or HHoossttbbaasseeddAAuutthheennttiiccaattiioonn. /etc/hosts.equiv and /etc/shosts.equiv are still used. The default is ``yes''. IIggnnoorreeUUsseerrKKnnoowwnnHHoossttss Specifies whether sshd(8) should ignore the user's ~/.ssh/knownhosts during RRhhoossttssRRSSAAAAuutthheennttiiccaattiioonn or HHoossttbbaasseeddAAuutthheennttiiccaattiioonn. The default is ``no''. KKeerrbbeerroossAAuutthheennttiiccaattiioonn Specifies whether the password provided by the user for PPaasssswwoorrddAAuutthheennttiiccaattiioonn 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 ``no''. KKeerrbbeerroossGGeettAAFFSSTTookkeenn 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 ``no''. KKeerrbbeerroossOOrrLLooccaallPPaasssswwdd

If password authentication through Kerberos fails then the pass-

word will be validated via any additional local mechanism such as /etc/passwd. The default is ``yes''. KKeerrbbeerroossTTiicckkeettCClleeaannuupp Specifies whether to automatically destroy the user's ticket cache file on logout. The default is ``yes''. KKeeyyRReeggeenneerraattiioonnIInntteerrvvaall 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 ses-

sions 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). LLiisstteennAAddddrreessss

Specifies the local addresses sshd(8) should listen on. The fol-

lowing forms may be used: LLiisstteennAAddddrreessss host|IPv4addr|IPv6addr LLiisstteennAAddddrreessss host|IPv4addr:port LLiisstteennAAddddrreessss [host|IPv6addr]:port If port is not specified, sshd will listen on the address and all prior PPoorrtt options specified. The default is to listen on all local addresses. Multiple LLiisstteennAAddddrreessss options are permitted.

Additionally, any PPoorrtt options must precede this option for non-

port qualified addresses. LLooggiinnGGrraacceeTTiimmee

The server disconnects after this time if the user has not suc-

cessfully logged in. If the value is 0, there is no time limit. The default is 120 seconds. LLooggLLeevveell Gives the verbosity level that is used when logging messages from

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.

MMAACCss Specifies the available MAC (message authentication code) algo-

rithms. The MAC algorithm is used in protocol version 2 for data

integrity protection. Multiple algorithms must be comma-sepa-

rated. The default is:

hmac-md5,hmac-sha1,umac-64@openssh.com,

hmac-ripemd160,hmac-sha1-96,hmac-md5-96

MMaattcchh Introduces a conditional block. If all of the criteria on the MMaattcchh line are satisfied, the keywords on the following lines override those set in the global section of the config file, until either another MMaattcchh line or the end of the file.

The arguments to MMaattcchh are one or more criteria-pattern pairs.

The available criteria are UUsseerr, GGrroouupp, HHoosstt, and AAddddrreessss. The

match patterns may consist of single entries or comma-separated

lists and may use the wildcard and negation operators described in the PATTERNS section of sshconfig(5). The patterns in an AAddddrreessss criteria may additionally contain addresses to match in CIDR address/masklen format, e.g. ``192.0.2.0/24'' or ``3ffe:ffff::/32''. Note that the mask

length provided must be consistent with the address - it is an

error to specify a mask length that is too long for the address or one with bits set in this host portion of the address. For example, ``192.0.2.0/33'' and ``192.0.2.0/8'' respectively. Only a subset of keywords may be used on the lines following a MMaattcchh keyword. Available keywords are AAlllloowwTTccppFFoorrwwaarrddiinngg, BBaannnneerr, CChhrroooottDDiirreeccttoorryy, FFoorrcceeCCoommmmaanndd, GGaatteewwaayyPPoorrttss, GGSSSSAAPPIIAAuutthheennttiiccaattiioonn, HHoossttbbaasseeddAAuutthheennttiiccaattiioonn, KKbbddIInntteerraaccttiivveeAAuutthheennttiiccaattiioonn, KKeerrbbeerroossAAuutthheennttiiccaattiioonn, MMaaxxAAuutthhTTrriieess, MMaaxxSSeessssiioonnss, PPaasssswwoorrddAAuutthheennttiiccaattiioonn, PPeerrmmiittOOppeenn, PPeerrmmiittRRoooottLLooggiinn, RRhhoossttssRRSSAAAAuutthheennttiiccaattiioonn, RRSSAAAAuutthheennttiiccaattiioonn, XX1111DDiissppllaayyOOffffsseett, XX1111FFoorrwwaarrddiinngg, and XX1111UUsseeLLooccaallHHoosstt. MMaaxxAAuutthhTTrriieess 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. MMaaxxSSeessssiioonnss

Specifies the maximum number of open sessions permitted per net-

work connection. The default is 10. MMaaxxSSttaarrttuuppss

Specifies the maximum number of concurrent unauthenticated con-

nections to the SSH daemon. Additional connections will be dropped until authentication succeeds or the LLooggiinnGGrraacceeTTiimmee expires for a connection. The default is 10. Alternatively, random early drop can be enabled by specifying the three colon separated values ``start:rate:full'' (e.g. "10:30:60"). sshd(8) will refuse connection attempts with a

probability of ``rate/100'' (30%) if there are currently

``start'' (10) unauthenticated connections. The probability increases linearly and all connection attempts are refused if the number of unauthenticated connections reaches ``full'' (60). PPaasssswwoorrddAAuutthheennttiiccaattiioonn Specifies whether password authentication is allowed. The default is ``yes''. PPeerrmmiittEEmmppttyyPPaasssswwoorrddss When password authentication is allowed, it specifies whether the server allows login to accounts with empty password strings. The default is ``no''. PPeerrmmiittOOppeenn

Specifies the destinations to which TCP port forwarding is per-

mitted. The forwarding specification must be one of the follow-

ing forms: PPeerrmmiittOOppeenn host:port PPeerrmmiittOOppeenn IPv4addr:port PPeerrmmiittOOppeenn [IPv6addr]:port

Multiple forwards may be specified by separating them with white-

space. An argument of ``any'' can be used to remove all restric-

tions and permit any forwarding requests. By default all port forwarding requests are permitted. PPeerrmmiittRRoooottLLooggiinn Specifies whether root can log in using ssh(1). The argument

must be ``yes'', ``without-password'', ``forced-commands-only'',

or ``no''. The default is ``yes''.

If this option is set to ``without-password'', password authenti-

cation is disabled for root.

If this option is set to ``forced-commands-only'', root login

with public key authentication will be allowed, but only if the 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. If this option is set to ``no'', root is not allowed to log in. PPeerrmmiittTTuunnnneell

Specifies whether tun(4) device forwarding is allowed. The argu-

ment must be ``yes'', ``point-to-point'' (layer 3), ``ethernet''

(layer 2), or ``no''. Specifying ``yes'' permits both

``point-to-point'' and ``ethernet''. The default is ``no''.

PPeerrmmiittUUsseerrEEnnvviirroonnmmeenntt Specifies whether ~/.ssh/environment and eennvviirroonnmmeenntt== options in ~/.ssh/authorizedkeys are processed by sshd(8). The default is ``no''. Enabling environment processing may enable users to

bypass access restrictions in some configurations using mecha-

nisms such as LDPRELOAD. PPiiddFFiillee

Specifies the file that contains the process ID of the SSH dae-

mon. The default is /var/run/sshd.pid. PPoorrtt Specifies the port number that sshd(8) listens on. The default is 22. Multiple options of this type are permitted. See also LLiisstteennAAddddrreessss. PPrriinnttLLaassttLLoogg Specifies whether sshd(8) should print the date and time of the last user login when a user logs in interactively. The default is ``yes''. PPrriinnttMMoottdd Specifies whether sshd(8) should print /etc/motd when a user logs in interactively. (On some systems it is also printed by the shell, /etc/profile, or equivalent.) The default is ``yes''. PPrroottooccooll Specifies the protocol versions sshd(8) supports. The possible

values are `1' and `2'. Multiple versions must be comma-sepa-

rated. The default is ``2,1''. Note that the order of the pro-

tocol list does not indicate preference, because the client selects among multiple protocol versions offered by the server. Specifying ``2,1'' is identical to ``1,2''. PPuubbkkeeyyAAuutthheennttiiccaattiioonn Specifies whether public key authentication is allowed. The default is ``yes''. Note that this option applies to protocol version 2 only. RRhhoossttssRRSSAAAAuutthheennttiiccaattiioonn Specifies whether rhosts or /etc/hosts.equiv authentication together with successful RSA host authentication is allowed. The default is ``no''. This option applies to protocol version 1 only. RRSSAAAAuutthheennttiiccaattiioonn Specifies whether pure RSA authentication is allowed. The default is ``yes''. This option applies to protocol version 1 only. SSAACCLLSSuuppppoorrtt Enables use of Service ACLs on Mac OS X. SSeerrvveerrKKeeyyBBiittss Defines the number of bits in the ephemeral protocol version 1 server key. The minimum value is 512, and the default is 1024. SSttrriiccttMMooddeess Specifies whether 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

``yes''. SSuubbssyysstteemm 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 sftp-server(8) implements the ``sftp'' file transfer

subsystem.

Alternately the name ``internal-sftp'' implements an in-process

``sftp'' server. This may simplify configurations using CChhrroooottDDiirreeccttoorryy to force a different filesystem root on clients. By default no subsystems are defined. Note that this option applies to protocol version 2 only. SSyyssllooggFFaacciilliittyy Gives the facility code that is used when logging messages from sshd(8). The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is AUTH. TTCCPPKKeeeeppAAlliivvee 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 tempo-

rarily, and some people find it annoying. On the other hand, if TCP keepalives are not sent, sessions may hang indefinitely on the server, leaving ``ghost'' users and consuming server resources. The default is ``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. To disable TCP keepalive messages, the value should be set to ``no''. UUsseeDDNNSS Specifies whether 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 ``yes''. UUsseeLLooggiinn

Specifies whether login(1) is used for interactive login ses-

sions. The default is ``no''. Note that login(1) is never used for remote command execution. Note also, that if this is enabled, XX1111FFoorrwwaarrddiinngg will be disabled because login(1) does not know how to handle xauth(1) cookies. If UUsseePPrriivviilleeggeeSSeeppaarraattiioonn is specified, it will be disabled after authentication. UUsseePPAAMM Enables the Pluggable Authentication Module interface. If set to ``yes'' this will enable PAM authentication using CChhaalllleennggeeRReessppoonnsseeAAuutthheennttiiccaattiioonn and PPaasssswwoorrddAAuutthheennttiiccaattiioonn in addition to PAM account and session module processing for all authentication types.

Because PAM challenge-response authentication usually serves an

equivalent role to password authentication, you should disable either PPaasssswwoorrddAAuutthheennttiiccaattiioonn or CChhaalllleennggeeRReessppoonnsseeAAuutthheennttiiccaattiioonn.. If UUsseePPAAMM is enabled, you will not be able to run sshd(8) as a

non-root user. The default is ``no''.

UUsseePPrriivviilleeggeeSSeeppaarraattiioonn Specifies whether 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 con-

taining any corruption within the unprivileged processes. The default is ``yes''. XX1111DDiissppllaayyOOffffsseett Specifies the first display number available for sshd(8)'s X11 forwarding. This prevents sshd from interfering with real X11 servers. The default is 10. XX1111FFoorrwwaarrddiinngg Specifies whether X11 forwarding is permitted. The argument must be ``yes'' or ``no''. The default is ``no''. When X11 forwarding is enabled, there may be additional exposure to the server and to client displays if the sshd(8) proxy display is configured to listen on the wildcard address (see

XX1111UUsseeLLooccaallhhoosstt below), though this is not the default. Addi-

tionally, 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 FFoorrwwaarrddXX1111 in sshconfig(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 war-

rant a ``no'' setting. 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 UUsseeLLooggiinn is enabled. XX1111UUsseeLLooccaallhhoosstt Specifies whether 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 DISPLAY environment variable to ``localhost''. This prevents remote hosts from connecting to the proxy display. However, some older X11 clients may not function with this configuration. XX1111UUsseeLLooccaallhhoosstt may be set to ``no'' to

specify that the forwarding server should be bound to the wild-

card address. The argument must be ``yes'' or ``no''. The default is ``yes''. XXAAuutthhLLooccaattiioonn Specifies the full pathname of the xauth(1) program. The default is /usr/X11R6/bin/xauth. TTIIMMEE FFOORRMMAATTSS

sshd(8) command-line arguments and configuration file options that spec-

ify time may be expressed using a sequence of the form: time[qualifier],

where time is a positive integer value and qualifier is one of the fol-

lowing: seconds ss | SS seconds mm | MM minutes hh | HH hours dd | DD days ww | WW weeks Each member of the sequence is added together to calculate the total time value. Time format examples: 600 600 seconds (10 minutes) 10m 10 minutes 1h30m 1 hour 30 minutes (90 minutes) FILES /etc/sshdconfig Contains configuration data for sshd(8). This file should be

writable by root only, but it is recommended (though not neces-

sary) that it be world-readable.