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SSHD_CONFIG


Section: File Formats (5)
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BSD mandoc

 

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
`#’

and empty lines are interpreted 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
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_config5

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
`*’

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-agent1

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
Sx PATTERNS

in
ssh_config5

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 [email protected] 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
Sx PATTERNS

in
ssh_config5

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 certificate 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 authenticated, 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 authorities 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
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.

ChallengeResponseAuthentication


Specifies whether challenge-response authentication is allowed (e.g. via
PAM or though authentication styles supported in
login.conf5)

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-server8

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 authenticated 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
Sx PATTERNS

in
ssh_config5

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 [email protected] 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
Sx PATTERNS

in
ssh_config5

for more information on patterns.

ForceCommand


Forces the execution of the command specified by
ForceCommand

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
Match

block.
The command 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 forwardings 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 version 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 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.

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 successful public key client host authentication is allowed
(host-based authentication).
This option is similar to
RhostsRSAAuthentication

and applies to protocol 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 validated 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 precede 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
Sx PATTERNS

section of
ssh_config5.

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
connection.
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 succeeds 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 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).

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 forwarding 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 argument 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 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.

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 interactively.
(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 program will be invoked with its first
argument the name of the user being authorized, and should produce
on standard output AuthorizedKeys lines (see AUTHORIZED_KEYS
in sshd(8)). By default (or when set to the empty string) there is no
AuthorizedKeysCommand run. If the AuthorizedKeysCommand does not successfully
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 authorizing 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 successful 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 identification 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 subsystem request.

The command
sftp-server8

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
Sx CERTIFICATES

section in
ssh-keygen1.

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)Ns’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
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
ForwardX11

in
ssh_config5).

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

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.
X11UseLocalhost

may be set to
“no”

to specify that the forwarding 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 integer value and
qualifier

is one of the following:


Aq 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 contributed the support for SSH
protocol versions 1.5 and 2.0.
Niels Provos and Markus Friedl contributed support
for privilege separation.



 

Index



NAME

SYNOPSIS

DESCRIPTION

TIME FORMATS

FILES

SEE ALSO

AUTHORS



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