sshd_config.5

OpenSSH 是 SSH （Secure SHell） 协议的免费开源实现。它用安全、加密的网络连接工具代替了 telnet、ftp、 rlogin、rsh 和 rcp 工具。OpenSSH 支持

The default is 120 seconds.
.It Cm LogLevel
Gives the verbosity level that is used when logging messages from
.Nm sshd .
The possible values are:
QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
The default is INFO.
DEBUG and DEBUG1 are equivalent.
DEBUG2 and DEBUG3 each specify higher levels of debugging output.
Logging with a DEBUG level violates the privacy of users and is not recommended.
.It Cm MACs
Specifies the available MAC (message authentication code) algorithms.
The MAC algorithm is used in protocol version 2
for data integrity protection.
Multiple algorithms must be comma-separated.
The default is
.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
.It Cm MaxAuthTries
Specifies the maximum number of authentication attempts permitted per
connection.
Once the number of failures reaches half this value,
additional failures are logged.
The default is 6.
.It Cm MaxStartups
Specifies the maximum number of concurrent unauthenticated connections to the
.Nm sshd
daemon.
Additional connections will be dropped until authentication succeeds or the
.Cm LoginGraceTime
expires for a connection.
The default is 10.
.Pp
Alternatively, random early drop can be enabled by specifying
the three colon separated values
.Dq start:rate:full
(e.g., "10:30:60").
.Nm sshd
will refuse connection attempts with a probability of
.Dq rate/100
(30%)
if there are currently
.Dq start
(10)
unauthenticated connections.
The probability increases linearly and all connection attempts
are refused if the number of unauthenticated connections reaches
.Dq full
(60).
.It Cm PasswordAuthentication
Specifies whether password authentication is allowed.
The default is
.Dq yes .
.It Cm PermitEmptyPasswords
When password authentication is allowed, it specifies whether the
server allows login to accounts with empty password strings.
The default is
.Dq no .
.It Cm PermitRootLogin
Specifies whether root can log in using
.Xr ssh 1 .
The argument must be
.Dq yes ,
.Dq without-password ,
.Dq forced-commands-only
or
.Dq no .
The default is
.Dq yes .
.Pp
If this option is set to
.Dq without-password
password authentication is disabled for root.
.Pp
If this option is set to
.Dq forced-commands-only
root login with public key authentication will be allowed,
but only if the
.Ar command
option has been specified
(which may be useful for taking remote backups even if root login is
normally not allowed).
All other authentication methods are disabled for root.
.Pp
If this option is set to
.Dq no
root is not allowed to log in.
.It Cm PermitUserEnvironment
Specifies whether
.Pa ~/.ssh/environment
and
.Cm environment=
options in
.Pa ~/.ssh/authorized_keys
are processed by
.Nm sshd .
The default is
.Dq no .
Enabling environment processing may enable users to bypass access
restrictions in some configurations using mechanisms such as
.Ev LD_PRELOAD .
.It Cm PidFile
Specifies the file that contains the process ID of the
.Nm sshd
daemon.
The default is
.Pa /var/run/sshd.pid .
.It Cm Port
Specifies the port number that
.Nm sshd
listens on.
The default is 22.
Multiple options of this type are permitted.
See also
.Cm ListenAddress .
.It Cm PrintLastLog
Specifies whether
.Nm sshd
should print the date and time of the last user login when a user logs
in interactively.
The default is
.Dq yes .
.It Cm PrintMotd
Specifies whether
.Nm sshd
should print
.Pa /etc/motd
when a user logs in interactively.
(On some systems it is also printed by the shell,
.Pa /etc/profile ,
or equivalent.)
The default is
.Dq yes .
.It Cm Protocol
Specifies the protocol versions
.Nm sshd
supports.
The possible values are
.Dq 1
and
.Dq 2 .
Multiple versions must be comma-separated.
The default is
.Dq 2,1 .
Note that the order of the protocol list does not indicate preference,
because the client selects among multiple protocol versions offered
by the server.
Specifying
.Dq 2,1
is identical to
.Dq 1,2 .
.It Cm PubkeyAuthentication
Specifies whether public key authentication is allowed.
The default is
.Dq yes .
Note that this option applies to protocol version 2 only.
.It Cm RhostsRSAAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication together
with successful RSA host authentication is allowed.
The default is
.Dq no .
This option applies to protocol version 1 only.
.It Cm RSAAuthentication
Specifies whether pure RSA authentication is allowed.
The default is
.Dq yes .
This option applies to protocol version 1 only.
.It Cm ServerKeyBits
Defines the number of bits in the ephemeral protocol version 1 server key.
The minimum value is 512, and the default is 768.
.It Cm StrictModes
Specifies whether
.Nm sshd
should check file modes and ownership of the
user's files and home directory before accepting login.
This is normally desirable because novices sometimes accidentally leave their
directory or files world-writable.
The default is
.Dq yes .
.It Cm Subsystem
Configures an external subsystem (e.g., file transfer daemon).
Arguments should be a subsystem name and a command to execute upon subsystem
request.
The command
.Xr sftp-server 8
implements the
.Dq sftp
file transfer subsystem.
By default no subsystems are defined.
Note that this option applies to protocol version 2 only.
.It Cm SyslogFacility
Gives the facility code that is used when logging messages from
.Nm sshd .
The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
The default is AUTH.
.It Cm TCPKeepAlive
Specifies whether the system should send TCP keepalive messages to the
other side.
If they are sent, death of the connection or crash of one
of the machines will be properly noticed.
However, this means that
connections will die if the route is down temporarily, and some people
find it annoying.
On the other hand, if TCP keepalives are not sent,
sessions may hang indefinitely on the server, leaving
.Dq ghost
users and consuming server resources.
.Pp
The default is
.Dq yes
(to send TCP keepalive messages), and the server will notice
if the network goes down or the client host crashes.
This avoids infinitely hanging sessions.
.Pp
To disable TCP keepalive messages, the value should be set to
.Dq no .
.It Cm UseDNS
Specifies whether
.Nm sshd
should lookup the remote host name and check that
the resolved host name for the remote IP address maps back to the
very same IP address.
The default is
.Dq yes .
.It Cm UseLogin
Specifies whether
.Xr login 1
is used for interactive login sessions.
The default is
.Dq no .
Note that
.Xr login 1
is never used for remote command execution.
Note also, that if this is enabled,
.Cm X11Forwarding
will be disabled because
.Xr login 1
does not know how to handle
.Xr xauth 1
cookies.
If
.Cm UsePrivilegeSeparation
is specified, it will be disabled after authentication.
.It Cm UsePAM
Enables the Pluggable Authentication Module interface.
If set to
.Dq yes
this will enable PAM authentication using
.Cm ChallengeResponseAuthentication
and PAM account and session module processing for all authentication types.
.Pp
Because PAM challenge-response authentication usually serves an equivalent
role to password authentication, you should disable either
.Cm PasswordAuthentication
or
.Cm ChallengeResponseAuthentication.
.Pp
If
.Cm UsePAM
is enabled, you will not be able to run
.Xr sshd 8
as a non-root user.
The default is
.Dq no .
.It Cm UsePrivilegeSeparation
Specifies whether
.Nm sshd
separates privileges by creating an unprivileged child process
to deal with incoming network traffic.
After successful authentication, another process will be created that has
the privilege of the authenticated user.
The goal of privilege separation is to prevent privilege
escalation by containing any corruption within the unprivileged processes.
The default is
.Dq yes .
.It Cm X11DisplayOffset
Specifies the first display number available for
.Nm sshd Ns 's
X11 forwarding.
This prevents
.Nm sshd
from interfering with real X11 servers.
The default is 10.
.It Cm X11Forwarding
Specifies whether X11 forwarding is permitted.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.Pp
When X11 forwarding is enabled, there may be additional exposure to
the server and to client displays if the
.Nm sshd
proxy display is configured to listen on the wildcard address (see
.Cm X11UseLocalhost
below), however this is not the default.
Additionally, the authentication spoofing and authentication data
verification and substitution occur on the client side.
The security risk of using X11 forwarding is that the client's X11
display server may be exposed to attack when the ssh client requests
forwarding (see the warnings for
.Cm ForwardX11
in
.Xr ssh_config 5 ) .
A system administrator may have a stance in which they want to
protect clients that may expose themselves to attack by unwittingly
requesting X11 forwarding, which can warrant a
.Dq no
setting.
.Pp
Note that disabling X11 forwarding does not prevent users from
forwarding X11 traffic, as users can always install their own forwarders.
X11 forwarding is automatically disabled if
.Cm UseLogin
is enabled.
.It Cm X11UseLocalhost
Specifies whether
.Nm sshd
should bind the X11 forwarding server to the loopback address or to
the wildcard address.
By default,
.Nm sshd
binds the forwarding server to the loopback address and sets the
hostname part of the
.Ev DISPLAY
environment variable to
.Dq localhost .
This prevents remote hosts from connecting to the proxy display.
However, some older X11 clients may not function with this
configuration.
.Cm X11UseLocalhost
may be set to
.Dq no
to specify that the forwarding server should be bound to the wildcard
address.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq yes .
.It Cm XAuthLocation
Specifies the full pathname of the
.Xr xauth 1
program.
The default is
.Pa /usr/X11R6/bin/xauth .
.El
.Ss Time Formats
.Nm sshd
command-line arguments and configuration file options that specify time
may be expressed using a sequence of the form:
.Sm off
.Ar time Op Ar qualifier ,
.Sm on
where
.Ar time
is a positive integer value and
.Ar qualifier
is one of the following:
.Pp
.Bl -tag -width Ds -compact -offset indent
.It Cm <none>
seconds
.It Cm s | Cm S
seconds
.It Cm m | Cm M
minutes
.It Cm h | Cm H
hours
.It Cm d | Cm D
days
.It Cm w | Cm W
weeks
.El
.Pp
Each member of the sequence is added together to calculate
the total time value.
.Pp
Time format examples:
.Pp
.Bl -tag -width Ds -compact -offset indent
.It 600
600 seconds (10 minutes)
.It 10m
10 minutes
.It 1h30m
1 hour 30 minutes (90 minutes)
.El
.Sh FILES
.Bl -tag -width Ds
.It Pa /etc/ssh/sshd_config
Contains configuration data for
.Nm sshd .
This file should be writable by root only, but it is recommended
(though not necessary) that it be world-readable.
.El
.Sh SEE ALSO
.Xr sshd 8
.Sh AUTHORS
OpenSSH is a derivative of the original and free
ssh 1.2.12 release by Tatu Ylonen.
Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
Theo de Raadt and Dug Song
removed many bugs, re-added newer features and
created OpenSSH.
Markus Friedl contributed the support for SSH
protocol versions 1.5 and 2.0.
Niels Provos and Markus Friedl contributed support
for privilege separation.