/etc/ssh/sshd_config
DESCRIPTION
sshd reads configuration data from /etc/ssh/sshd_config (or
the file
specified with -f on the command line). The file contains
keyword-argu-
ment pairs, one per line. Lines starting with `#' and empty
lines are
interpreted as comments.
The possible keywords and their meanings are as follows
(note that key-
words are case-insensitive and arguments are case-sensi-
tive):
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 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 sep-
arated by
whitespace or spread across multiple AcceptEnv di-
rectives. 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. Valid ar-
guments are ``any'', ``inet'' (use IPv4 only) or
``inet6'' (use
IPv6 only). The default is ``any''.
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
they can al-
ways 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 us-
er names that match one of the patterns. `*' and
`?' can be used
as wildcards in the patterns. Only user names are
valid; a nu-
merical user ID is not recognized. By default, lo-
gin is allowed
for all users. If the pattern takes the form US-
ER@HOST then USER
and HOST are separately checked, restricting logins
to particular
users from particular hosts.
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 connec-
tion set-up.
The following tokens are defined: %% is replaced by
a literal
'%', %h is replaced by the home directory of the us-
er being au-
thenticated and %u is replaced by the username of
that user. Af-
ter expansion, AuthorizedKeysFile is taken to be an
absolute path
or one relative to the user's home directory. The
default is
``.ssh/authorized_keys''.
Banner In some jurisdictions, sending a warning message be-
fore authenti-
cation may be relevant for getting legal protection.
The con-
tents of the specified file are sent to the remote
user before
authentication is allowed. This option is only
available for
protocol version 2. By default, no banner is dis-
played.
ChallengeResponseAuthentication
Specifies whether challenge response authentication
``arcfour256'', ``arcfour'', ``blowfish-cbc'', and
``cast128-cbc''. The default is
``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,ar-
cfour128,
arcfour256,arc-
four,aes192-cbc,aes256-cbc,aes128-ctr,
aes192-ctr,aes256-ctr''
ClientAliveCountMax
Sets the number of client alive messages (see above)
which may be
sent without sshd receiving any messages back from
the client.
If this threshold is reached while client alive mes-
sages are be-
ing sent, sshd will disconnect the client, terminat-
ing the ses-
sion. It is important to note that the use of
client alive mes-
sages is very different from TCPKeepAlive (below).
The client
alive messages are sent through the encrypted chan-
nel and there-
fore will not be spoofable. The TCP keepalive op-
tion enabled by
TCPKeepAlive is spoofable. The client alive mecha-
nism is valu-
able when the client or server depend on knowing
when a connec-
tion has become inactive.
The default value is 3. If ClientAliveInterval
(above) is set to
15, and ClientAliveCountMax is left at the default,
unresponsive
ssh clients will be disconnected after approximately
45 seconds.
ClientAliveInterval
Sets a timeout interval in seconds after which if no
data has
been received from the client, sshd will send a mes-
sage 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
`*' and `?' can be used as wildcards in the pat-
terns. Only group
names are valid; a numerical group ID is not recog-
nized. By de-
fault, login is allowed for all groups.
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. `*' and `?' can be used
as wildcards
in the patterns. Only user names are valid; a nu-
merical 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.
GatewayPorts
Specifies whether remote hosts are allowed to con-
nect to ports
forwarded for the client. By default, sshd binds
remote port
forwardings to the loopback address. This prevents
other remote
hosts from connecting to forwarded ports. Gateway-
Ports 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 re-
mote port forwardings to bind to the wildcard ad-
dress, 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 GSS-
API is allowed.
The default is ``no''. Note that this option ap-
plies to protocol
allowed (hostbased authentication). This option is
similar to
RhostsRSAAuthentication and applies to protocol ver-
sion 2 only.
The default is ``no''.
HostKey
Specifies a file containing a private host key used
by SSH. The
default is /etc/ssh/ssh_host_key for protocol ver-
sion 1, and
/etc/ssh/ssh_host_rsa_key and
/etc/ssh/ssh_host_dsa_key for pro-
tocol version 2. Note that sshd 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/shosts.equiv are still
used. The de-
fault is ``yes''.
IgnoreUserKnownHosts
Specifies whether sshd 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 Ker-
beros servtab
which allows the verification of the KDC's identity.
Default is
``no''.
KerberosGetAFSToken
If AFS is active and the user has a Kerberos 5 TGT,
attempt to
aquire an AFS token before accessing the user's home
directory.
Default is ``no''.
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 steal-
ing 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 should listen on.
The follow-
ing 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 op-
tion for non
port qualified addresses.
LoginGraceTime
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.
LogLevel
Gives the verbosity level that is used when logging
messages from
sshd. The possible values are: QUIET, FATAL, ERROR,
INFO, VER-
BOSE, 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 recommend-
ed.
half this
value, additional failures are logged. The default
is 6.
MaxStartups
Specifies the maximum number of concurrent unauthen-
ticated con-
nections to the sshd daemon. Additional connections
will be
dropped until authentication succeeds or the Login-
GraceTime ex-
pires 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 will refuse connection attempts
with a proba-
bility of ``rate/100'' (30%) if there are currently
``start''
(10) unauthenticated connections. The probability
increases lin-
early and all connection attempts are refused if the
number of
unauthenticated connections reaches ``full'' (60).
PasswordAuthentication
Specifies whether password authentication is al-
lowed. The de-
fault is ``yes''.
PermitEmptyPasswords
When password authentication is allowed, it speci-
fies whether the
server allows login to accounts with empty password
strings. The
default is ``no''.
PermitRootLogin
Specifies whether root can log in using ssh(1). The
argument
must be ``yes'', ``without-password'', ``forced-com-
mands-only''
or ``no''. The default is ``yes''.
If this option is set to ``without-password'' pass-
word authenti-
cation is disabled for root.
If this option is set to ``forced-commands-only''
root login with
~/.ssh/authorized_keys are processed by sshd. The
default is
``no''. Enabling environment processing may enable
users to by-
pass access restrictions in some configurations us-
ing mechanisms
such as LD_PRELOAD.
PidFile
Specifies the file that contains the process ID of
the sshd dae-
mon. The default is /var/run/sshd.pid.
Port Specifies the port number that sshd listens on. The
default is
22. Multiple options of this type are permitted.
See also
ListenAddress.
PrintLastLog
Specifies whether sshd 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 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 supports. The
possible val-
ues 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 se-
lects among multiple protocol versions offered by
the server.
Specifying ``2,1'' is identical to ``1,2''.
PubkeyAuthentication
Specifies whether public key authentication is al-
lowed. The de-
fault is ``yes''. Note that this option applies to
protocol ver-
fault is ``yes''. This option applies to protocol
version 1 on-
ly.
ServerKeyBits
Defines the number of bits in the ephemeral protocol
version 1
server key. The minimum value is 512, and the de-
fault is 768.
StrictModes
Specifies whether 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 ac-
cidentally
leave their directory or files world-writable. The
default is
``yes''.
Subsystem
Configures an external subsystem (e.g., file trans-
fer daemon).
Arguments should be a subsystem name and a command
to execute up-
on subsystem request. The command sftp-server(8)
implements the
``sftp'' file transfer subsystem. By default no
subsystems are
defined. Note that this option applies to protocol
version 2 on-
ly.
SyslogFacility
Gives the facility code that is used when logging
messages from
sshd. The possible values are: DAEMON, USER, AUTH,
LOCAL0, LO-
CAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LO-
CAL7. The de-
fault 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 no-
ticed. However,
this means that connections will die if the route is
down tem-
To disable TCP keepalive messages, the value should
be set to
``no''.
UseDNS Specifies whether sshd 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 ses-
sions. The default is ``no''. Note that login(1)
is never used
for remote command execution. Note also, that if
this is en-
abled, X11Forwarding will be disabled because lo-
gin(1) does not
know how to handle xauth(1) cookies. If UsePrivi-
legeSeparation
is specified, it will be disabled after authentica-
tion.
UsePAM Enables the Pluggable Authentication Module inter-
face. If set to
``yes'' this will enable PAM authentication using
ChallengeResponseAuthentication and PAM account and
session mod-
ule processing for all authentication types.
Because PAM challenge-response authentication usual-
ly serves an
equivalent role to password authentication, you
should disable
either PasswordAuthentication or ChallengeRespon-
seAuthentication.
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 separates privileges by cre-
ating an un-
privileged child process to deal with incoming net-
work traffic.
After successful authentication, another process
will be created
that has the privilege of the authenticated user.
The goal of
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 addi-
tional exposure
to the server and to client displays if the sshd
proxy display is
configured to listen on the wildcard address (see
X11UseLocalhost
below), however this is not the default. Addition-
ally, the au-
thentication spoofing and authentication data veri-
fication and
substitution occur on the client side. The security
risk of us-
ing X11 forwarding is that the client's X11 display
server may be
exposed to attack when the ssh client requests for-
warding (see
the warnings for ForwardX11 in ssh_config(5)). A
system adminis-
trator may have a stance in which they want to pro-
tect 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 dis-
abled if UseLogin
is enabled.
X11UseLocalhost
Specifies whether sshd should bind the X11 forward-
ing server to
the loopback address or to the wildcard address. By
default,
sshd binds the forwarding server to the loopback ad-
dress and sets
the hostname part of the DISPLAY environment vari-
able to
``localhost''. This prevents remote hosts from con-
necting 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
time[qualifier],
where time is a positive integer value and qualifier is one
of the fol-
lowing:
<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. This file
should be
writable by root only, but it is recommended (though
not neces-
sary) 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 fea-
tures and cre-
ated OpenSSH. Markus Friedl contributed the support for SSH
protocol
versions 1.5 and 2.0. Niels Provos and Markus Friedl con-
tributed support
for privilege separation.
OpenBSD 3.8 September 25, 1999
8
Man(1) output converted with man2html