/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-arguM--
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 keyM--
words are case-insensitive and arguments are case-sensi-
tive):
AFSTokenPassing
Specifies whether an AFS token may be forwarded to
the server.
Default is ``no''.
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. `*' and `'? can be used as wild-
cards in the
patterns. Only group names are valid; a numerical
group ID is
not recognized. By default, login is allowed for
all groups.
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
users names that match one of the patterns. `*' and
`'? can be
used as wildcards in the patterns. Only user names
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
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''.
Banner In some jurisdictions, sending a warning message be-
fore authentiM--
cation may be relevant for getting legal protection.
The conM--
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
is allowed.
All authentication styles from login.conf(5) are
supported. The
default is ``yes''.
Ciphers
Specifies the ciphers allowed for protocol version
2. Multiple
ciphers must be comma-separated. The default is
``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,ar-
cfour,
aes192-cbc,aes256-cbc''
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.
are sent through the encrypted channel and therefore
will not be
spoofable. The TCP keepalive option enabled by
KeepAlive is
spoofable. The client alive mechanism is valuable
when the client
or server depend on knowing when a connection has
become inacM--
tive.
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.
Compression
Specifies whether compression is allowed. The argu-
ment must be
``yes'' or ``no''. The default is ``yes''.
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.
`*' and `'? can be used as wildcards in the pat-
terns. Only
group names are valid; a numerical group ID is not
recognized.
By default, 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.
``no''. The
default is ``no''.
HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authen-
tication
together with successful public key client host au-
thentication is
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 proM--
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
RhostsAuthentication, RhostsRSAAuthentication or
HostbasedAuthentication.
/etc/hosts.equiv and /etc/shosts.equiv are still
used. The
default is ``yes''.
IgnoreUserKnownHosts
Specifies whether sshd should ignore the user's
$HOME/.ssh/known_hosts during RhostsRSAAuthentica-
tion or
HostbasedAuthentication. The default is ``no''.
KeepAlive
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,
To disable keepalives, the value should be set to
``no''.
KerberosAuthentication
Specifies whether Kerberos authentication is al-
lowed. This can
be in the form of a Kerberos ticket, or if Passwor-
dAuthentication
is yes, the password provided by the user 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 idenM--
tity. Default is ``no''.
KerberosOrLocalPasswd
If set then if password authentication through Ker-
beros fails
then the password will be validated via any addi-
tional local
mechanism such as /etc/passwd. Default is ``yes''.
KerberosTgtPassing
Specifies whether a Kerberos TGT may be forwarded to
the server.
Default is ``no'', as this only works when the Ker-
beros KDC is
actually an AFS kaserver.
KerberosTicketCleanup
Specifies whether to automatically destroy the us-
er's ticket
cache file on logout. Default is ``yes''.
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 sesM--
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 followM--
port qualified addresses.
LoginGraceTime
The server disconnects after this time if the user
has not sucM--
cessfully logged in. If the value is 0, there is no
time limit.
The default is 600 (seconds).
LogLevel
Gives the verbosity level that is used when logging
messages from
sshd. The possible values are: QUIET, FATAL, ERROR,
INFO, VERM--
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.
MACs Specifies the available MAC (message authentication
code) algoM--
rithms. The MAC algorithm is used in protocol ver-
sion 2 for data
integrity protection. Multiple algorithms must be
comma-sepaM--
rated. The default is
``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-
sha1-96,hmac-md5-96''.
MaxStartups
Specifies the maximum number of concurrent unauthen-
ticated conM--
nections to the sshd daemon. Additional connections
will be
dropped until authentication succeeds or the Login-
GraceTime
expires for a connection. The default is 10.
Alternatively, random early drop can be enabled by
specifying the
three colon separated values ``start:rate:full''
(e.g.,
"10:30:60"). sshd will refuse connection attempts
with a probaM--
bility of ``rate/100'' (30%) if there are currently
``start''
(10) unauthenticated connections. The probability
increases linM--
PasswordAuthentication
Specifies whether password authentication is al-
lowed. The
default 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 login 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'' pass-
word authentiM--
cation is disabled for root.
If this option is set to ``forced-commands-only''
root login with
public key authentication will be allowed, but only
if the
command option has been specified (which may be use-
ful 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 login.
PidFile
Specifies the file that contains the process ID of
the sshd daeM--
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 when the
user last logged in. The default is ``yes''.
PubkeyAuthentication
Specifies whether public key authentication is al-
lowed. The
default is ``yes''. Note that this option applies
to protocol
version 2 only.
RhostsAuthentication
Specifies whether authentication using rhosts or
/etc/hosts.equiv
files is sufficient. Normally, this method should
not be permitM--
ted because it is insecure. RhostsRSAAuthentication
should be
used instead, because it performs RSA-based host au-
thentication
in addition to normal rhosts or /etc/hosts.equiv au-
thentication.
The default is ``no''. This option applies to pro-
tocol version 1
only.
RhostsRSAAuthentication
Specifies whether rhosts or /etc/hosts.equiv authen-
tication
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 al-
lowed. 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 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
tocol version 2
only.
SyslogFacility
Gives the facility code that is used when logging
messages from
sshd. The possible values are: DAEMON, USER, AUTH,
LOCAL0,
LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LO-
CAL7. The
default is AUTH.
UseLogin
Specifies whether login(1) is used for interactive
login sesM--
sions. 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 lo-
gin(1) does not
know how to handle xauth(1) cookies. If UsePrivi-
legeSeparation
is specified, it will be disabled after authentica-
tion.
UsePrivilegeSeparation
Specifies whether sshd separates privileges by cre-
ating 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 escala-
tion by conM--
taining any corruption within the unprivileged pro-
cesses. The
default is ``yes''.
VerifyReverseMapping
Specifies whether sshd should try to verify the re-
mote 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 ``no''.
X11DisplayOffset
Specifies the first display number available for
sshd's X11 forM--
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
fake 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 wildM--
card address. The argument must be ``yes'' or
``no''. The
default is ``yes''.
XAuthLocation
Specifies the location of the xauth(1) program. The
default is
/usr/X11R6/bin/xauth.
Time Formats
sshd 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 folM--
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
Provos, Theo
de Raadt and Dug Song removed many bugs, re-added newer fea-
tures and creM--
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.
SEE ALSO
sshd(8)
BSD September 25, 1999
BSD
Man(1) output converted with man2html