man--5--sshd_config
SSHD_CONFIG(5) BSD File Formats Manual SSHD_CONFIG(5)
NAME
sshd_config - OpenSSH SSH daemon configuration file
SYNOPSIS
/etc/ssh/sshd_config
DESCRIPTION
sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file specified with
-f on the command line). The file contains keyword-argument pairs, one per line. Lines
starting with ‘
be enclosed in double quotes (") in order to represent arguments containing spaces.
The possible keywords and their meanings are as follows (note that keywords are case-
insensitive and arguments are case-sensitive):
AcceptEnv
Specifies what environment variables sent by the client will be copied into the
session’s environ(7). See SendEnv in ssh_config(5) for how to configure the
client. Note that environment passing is only supported for protocol 2. Vari-
ables are specified by name, which may contain the wildcard characters ‘*’ and
‘?’. Multiple environment variables may be separated by whitespace or spread
The possible keywords and their meanings are as follows (note that keywords are case-
insensitive and arguments are case-sensitive):
AcceptEnv
Specifies what environment variables sent by the client will be copied into the
session’s environ(7). See SendEnv in ssh_config(5) for how to configure the
client. Note that environment passing is only supported for protocol 2. Vari-
ables are specified by name, which may contain the wildcard characters ‘*’ and
‘?’. Multiple environment variables may be separated by whitespace or spread
across multiple 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.
AddressFamily
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”.
AllowAgentForwarding
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.
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:
DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
See PATTERNS in ssh_config(5) for more information on patterns.
AllowTcpForwarding
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.
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 par-
ticular users from particular hosts. The allow/deny directives are processed in
the following order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
See PATTERNS in ssh_config(5) for more information on patterns.
AuthorizedKeysFile
Specifies the file that contains the public keys that can be used for user
authentication. 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, AuthorizedKeysFile is taken to be an absolute path or one relative to
the user’s home directory. The default is “.ssh/authorized_keys”.
AuthorizedPrincipalsFile
Specifies a file that lists principal names that are accepted for certificate
authentication. When using certificates signed by a key listed in
TrustedUserCAKeys, this file lists names, one of which must appear in the cer-
tificate for it to be accepted for authentication. Names are listed one per
line; empty lines and comments starting with ‘#’ are ignored.
AuthorizedPrincipalsFile 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 authenti-
cated, and %u is replaced by the username of that user. After expansion,
AuthorizedPrincipalsFile is taken to be an absolute path or one relative to the
user’s home directory.
The default is not to use a principals file – in this case, the username of the
user must appear in a certificate’s principals list for it to be accepted. Note
that AuthorizedPrincipalsFile is only used when authentication proceeds using a
CA listed in TrustedUserCAKeys and is not consulted for certification authori-
ties trusted via ~/.ssh/authorized_keys, though the principals= key option
offers a similar facility (see sshd(8) for details).
Banner The contents of the specified file are sent to the remote user before authenti-
cation 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 dis-
played.
ChallengeResponseAuthentication
Specifies whether challenge-response authentication is allowed (e.g. via PAM or
though authentication styles supported in login.conf(5)) The default is “yes”.
ChrootDirectory
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. After the chroot, sshd(8) changes the working directory to the
user’s home directory.
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 username of that user.
The ChrootDirectory must contain the necessary files and directories to support
the user’s 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 necessary if the
in-process sftp server is used, though sessions which use logging do require
/dev/log inside the chroot directory (see sftp-server(8) for details).
The default is not to chroot(2).
Ciphers
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-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
aes256-cbc,arcfour
ClientAliveCountMax
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 session. It is important to note that the use of client
alive messages is very different from TCPKeepAlive (below). The client alive
messages are sent through the encrypted channel and therefore will not be
spoofable. The TCP keepalive option enabled by TCPKeepAlive is spoofable. The
client alive mechanism is valuable when the client or server depend on knowing
when a connection has become inactive.
The default value is 3. If ClientAliveInterval (see below) is set to 15, and
ClientAliveCountMax is left at the default, unresponsive SSH clients will be
disconnected after approximately 45 seconds. This option applies to protocol
version 2 only.
ClientAliveInterval
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 version 2 only.
Compression
Specifies whether compression is allowed, or delayed until the user has authen-
ticated successfully. The argument must be “yes”, “delayed”, or “no”. The
default is “delayed”.
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: DenyUsers,
AllowUsers, DenyGroups, and finally AllowGroups.
See PATTERNS in ssh_config(5) for more information on patterns.
DenyUsers
See PATTERNS in ssh_config(5) for more information on patterns.
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: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
See PATTERNS in ssh_config(5) for more information on patterns.
ForceCommand
Forces the execution of the command specified by ForceCommand, ignoring any com-
mand 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, com-
mand, or subsystem execution. It is most useful inside a Match block. The com-
mand originally supplied by the client is available in the SSH_ORIGINAL_COMMAND
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
ChrootDirectory.
GatewayPorts
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.
GatewayPorts can be used to specify that sshd should allow remote port forward-
ings to bind to non-loopback addresses, thus allowing other hosts to connect.
The argument may be “no” to force remote port forwardings 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”.
GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is allowed. The default
is “no”. Note that this option applies to protocol version 2 only.
GSSAPIKeyExchange
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 “no”. Note
that this option applies to protocol version 2 only.
GSSAPICleanupCredentials
Specifies whether to automatically destroy the user’s credentials cache on
logout. The default is “yes”. Note that this option applies to protocol ver-
sion 2 only.
GSSAPIStrictAcceptorCheck
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 authenti-
cate against any service key stored in the machine’s default store. This facil-
ity 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 con-
nections, and setting it to “no” may only work with recent Kerberos GSSAPI
libraries.
GSSAPIStoreCredentialsOnRekey
Controls whether the user’s GSSAPI credentials should be updated following a
successful connection rekeying. This option can be used to accepted renewed or
updated credentials from a compatible client. The default is “no”.
HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication together with suc-
cessful public key client host authentication is allowed (host-based authentica-
tion). This option is similar to RhostsRSAAuthentication and applies to proto-
col version 2 only. The default is “no”.
HostbasedUsesNameFromPacketOnly
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 HostbasedAuthentication. 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”.
HostCertificate
Specifies a file containing a public host certificate. The certificate’s public
key must match a private host key already specified by HostKey. The default
behaviour of sshd(8) is not to load any certificates.
HostKey
Specifies a file containing a private host key used by SSH. The default is
/etc/ssh/ssh_host_key for protocol version 1, and /etc/ssh/ssh_host_rsa_key and
/etc/ssh/ssh_host_dsa_key for protocol version 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.
IgnoreRhosts
Specifies that .rhosts and .shosts files will not be used in
RhostsRSAAuthentication or HostbasedAuthentication.
/etc/hosts.equiv and /etc/ssh/shosts.equiv are still used. The default is
“yes”.
IgnoreUserKnownHosts
Specifies whether sshd(8) should ignore the user’s ~/.ssh/known_hosts during
RhostsRSAAuthentication or HostbasedAuthentication. The default is “no”.
KerberosAuthentication
Specifies whether the password provided by the user for 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 “no”.
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 “no”.
KerberosOrLocalPasswd
If password authentication through Kerberos fails then the password will be val-
idated via any additional local mechanism such as /etc/passwd. The default is
“yes”.
KerberosTicketCleanup
Specifies whether to automatically destroy the user’s ticket cache file on
logout. The default is “yes”.
KerberosUseKuserok
Specifies whether to look at .k5login file for user’s aliases. The default is
“yes”.
KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms must
be comma-separated. The default is “diffie-hellman-group-exchange-sha256”,
“diffie-hellman-group-exchange-sha1”, “diffie-hellman-group14-sha1”,
“diffie-hellman-group1-sha1”.
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).
ListenAddress
Specifies the local addresses sshd(8) should listen on. The following forms may
be used:
ListenAddress host|IPv4_addr|IPv6_addr
ListenAddress host|IPv4_addr:port
ListenAddress [host|IPv6_addr]:port
If port is not specified, sshd will listen on the address and all prior Port
options specified. The default is to listen on all local addresses. Multiple
ListenAddress options are permitted. Additionally, any Port options must pre-
cede this option for non-port qualified addresses.
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.
LogLevel
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.
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:
hmac-md5,hmac-sha1,[email protected],
hmac-ripemd160,hmac-sha1-96,hmac-md5-96,
hmac-sha2-256,hmac-sha2-512
Match Introduces a conditional block. If all of the criteria on the Match line are
satisfied, the keywords on the following lines override those set in the global
section of the config file, until either another Match line or the end of the
file.
The arguments to Match are one or more criteria-pattern pairs. The available
criteria are User, Group, Host, and Address. 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 ssh_config(5).
The patterns in an Address 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 Match keyword.
Available keywords are AllowAgentForwarding, AllowTcpForwarding, Banner,
ChrootDirectory, ForceCommand, GatewayPorts, GSSAPIAuthentication,
HostbasedAuthentication, KbdInteractiveAuthentication, KerberosAuthentication,
KerberosUseKuserok, MaxAuthTries, MaxSessions, PubkeyAuthentication,
AuthorizedKeysCommand, AuthorizedKeysCommandRunAs, PasswordAuthentication,
PermitEmptyPasswords, PermitOpen, PermitRootLogin, RequiredAuthentications1,
RequiredAuthentications2, RhostsRSAAuthentication, RSAAuthentication,
X11DisplayOffset, X11Forwarding and X11UseLocalHost.
MaxAuthTries
Specifies the maximum number of authentication attempts permitted per connec-
tion. Once the number of failures reaches half this value, additional failures
are logged. The default is 6.
MaxSessions
Specifies the maximum number of open sessions permitted per network connection.
The default is 10.
MaxStartups
Specifies the maximum number of concurrent unauthenticated connections to the
SSH daemon. Additional connections will be dropped until authentication suc-
ceeds or the LoginGraceTime expires for a connection. The default is 10:30:100.
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 con-
nection 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 connec-
tions reaches “full” (60).
PasswordAuthentication
Specifies whether password authentication is allowed. The default is “yes”.
PermitEmptyPasswords
When password authentication is allowed, it specifies whether the server allows
login to accounts with empty password strings. The default is “no”.
PermitOpen
Specifies the destinations to which TCP port forwarding is permitted. The for-
warding specification must be one of the following forms:
PermitOpen host:port
PermitOpen IPv4_addr:port
PermitOpen [IPv6_addr]:port
Multiple forwards may be specified by separating them with whitespace. An argu-
ment of “any” can be used to remove all restrictions and permit any forwarding
requests. By default all port forwarding requests are permitted.
PermitRootLogin
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 authentication 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 speci-
fied (which may be useful for taking remote backups even if root login is nor-
mally not allowed). All other authentication methods are disabled for root.
If this option is set to “no”, root is not allowed to log in.
PermitTunnel
Specifies whether tun(4) device forwarding is allowed. The argument 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”.
PermitUserEnvironment
Specifies whether ~/.ssh/environment and environment= options in
~/.ssh/authorized_keys are processed by sshd(8). The default is “no”. Enabling
environment processing may enable users to bypass access restrictions in some
configurations using mechanisms such as LD_PRELOAD.
PidFile
Specifies the file that contains the process ID of the SSH daemon. The default
is /var/run/sshd.pid.
Port Specifies the port number that sshd(8) listens on. The default is 22. Multiple
options of this type are permitted. See also ListenAddress.
PrintLastLog
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”.
PrintMotd
Specifies whether sshd(8) should print /etc/motd when a user logs in interac-
tively. (On some systems it is also printed by the shell, /etc/profile, or
equivalent.) The default is “yes”.
Protocol
Specifies the protocol versions sshd(8) supports. The possible values are ‘1’
and ‘2’. Multiple versions must be comma-separated. The default is “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 “2,1” is identical to “1,2”.
PubkeyAuthentication
Specifies whether public key authentication is allowed. The default is “yes”.
Note that this option applies to protocol version 2 only.
AuthorizedKeysCommand
Specifies a program to be used for lookup of the user’s public keys. The pro-
gram will be invoked with its first argument the name of the user being autho-
rized, and should produce on standard output AuthorizedKeys lines (see AUTHO-
RIZED_KEYS in sshd(8)). By default (or when set to the empty string) there is
no AuthorizedKeysCommand run. If the AuthorizedKeysCommand does not success-
fully authorize the user, authorization falls through to the AuthorizedKeysFile.
Note that this option has an effect only with PubkeyAuthentication turned on.
AuthorizedKeysCommandRunAs
Specifies the user under whose account the AuthorizedKeysCommand is run. Empty
string (the default value) means the user being authorized is used.
RequiredAuthentications[12]
Specifies required methods of authentications that has to succeed before autho-
rizing the connection. (RequiredAuthentication1 for Protocol version 1, and
RequiredAuthentication2 for v2)
RequiredAuthentications1 method[,method...]
RequiredAuthentications2 method[,method...]
Example 1:
RequiredAuthentications2 password,hostbased
Example 2:
RequiredAuthentications2 publickey,password
Available methods:
password, keyboard-interactive, publickey, hostbased, gssapi-keyex, gssapi-with-mic
RevokedKeys
Specifies a list of revoked public keys. Keys listed in this file will be
refused for public key authentication. Note that if this file is not readable,
then public key authentication will be refused for all users.
RhostsRSAAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication together with suc-
cessful RSA host authentication is allowed. The default is “no”. This option
applies to protocol version 1 only.
RSAAuthentication
Specifies whether pure RSA authentication is allowed. The default is “yes”.
This option applies to protocol version 1 only.
ServerKeyBits
Defines the number of bits in the ephemeral protocol version 1 server key. The
minimum value is 512, and the default is 1024.
ShowPatchLevel
Specifies whether sshd will display the patch level of the binary in the identi-
fication string. The patch level is set at compile-time. The default is “no”.
This option applies to protocol version 1 only.
StrictModes
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”.
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 sub-
system 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 ChrootDirectory to force a different
filesystem root on clients.
By default no subsystems are defined. Note that this option applies to protocol
version 2 only.
SyslogFacility
Gives the facility code that is used when logging messages from sshd(8). The
possible values are: DAEMON, USER, AUTH, AUTHPRIV, LOCAL0, LOCAL1, LOCAL2,
LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is AUTH.
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 “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”.
TrustedUserCAKeys
Specifies a file containing public keys of certificate authorities that are
trusted sign user certificates for authentication. Keys are listed one per
line, empty lines and comments starting with ‘#’ are allowed. If a certificate
is presented for authentication and has its signing CA key listed in this file,
then it may be used for authentication for any user listed in the certificate’s
principals list. Note that certificates that lack a list of principals will not
be permitted for authentication using TrustedUserCAKeys. For more details in
certificates, please see the CERTIFICATES section in ssh-keygen(1).
UseDNS 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”.
UseLogin
Specifies whether login(1) is used for interactive login sessions. The default
is “no”. Note that login(1) is never used for remote command execution. Note
also, that if this is enabled, X11Forwarding will be disabled because login(1)
does not know how to handle xauth(1) cookies. If UsePrivilegeSeparation is
specified, it will be disabled after authentication.
UsePAM Enables the Pluggable Authentication Module interface. If set to “yes” this
will enable PAM authentication using ChallengeResponseAuthentication and
PasswordAuthentication 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 PasswordAuthentication or
ChallengeResponseAuthentication.
If UsePAM is enabled, you will not be able to run sshd(8) as a non-root user.
The default is “no”.
UsePrivilegeSeparation
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
containing any corruption within the unprivileged processes. The default is
“yes”.
X11DisplayOffset
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.
X11Forwarding
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 X11UseLocalhost below), though this is not the
default. Additionally, the authentication spoofing and authentication data ver-
ification 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 ForwardX11 in
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 “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 UseLogin is enabled.
X11UseLocalhost
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 envi-
ronment variable to “localhost”. This prevents remote hosts from connecting to
the proxy display. However, some older X11 clients may not function with this
configuration. X11UseLocalhost may be set to “no” to specify that the forward-
ing server should be bound to the wildcard address. The argument must be “yes”
or “no”. The default is “yes”.
XAuthLocation
Specifies the full pathname of the xauth(1) program. The default is
/usr/bin/xauth.
TIME FORMATS
sshd(8) command-line arguments and configuration file options that specify time may be
expressed using a sequence of the form: time[qualifier], where time is a positive inte-
ger value and qualifier is one of the following:
〈none〉 seconds
s | S seconds
m | M minutes
h | H hours
d | D days
w | W 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/ssh/sshd_config
Contains configuration data for sshd(8). This file should be writable by root
only, but it is recommended (though not necessary) that it be world-readable.
SEE ALSO
sshd(8)
AUTHORS
OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen.
Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song
removed many bugs, re-added newer features and created OpenSSH. Markus Friedl con-
tributed the support for SSH protocol versions 1.5 and 2.0. Niels Provos and Markus
Friedl contributed support for privilege separation.
BSD July 25, 2016 BSD
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