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 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-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 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 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.
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 wildcards 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
user names that match one of the patterns. ‘*’ and ‘?’ can be
used as wildcards in 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.
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
set-up. 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”.
Banner In some jurisdictions, sending a warning message before authen-
tication 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 displayed.
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 supported ciphers are
“3des-cbc”, “aes128-cbc”, “aes192-cbc”, “aes256-cbc”,
“aes128-ctr”, “aes192-ctr”, “aes256-ctr”, “arcfour”,
“blowfish-cbc”, and “cast128-cbc”. The default is
‘‘aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr’’
ClientAliveInterval
Sets a timeout interval in seconds after which if no data has
been received from the client, sshd 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.
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 being sent, sshd will disconnect the client, terminat-
ing 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 mecha-
nism is valuable when the client or server depend on knowing
when a connection has become inactive.
The default value is 3. If ClientAliveInterval (above) is set
to 15, and ClientAliveCountMax is left at the default, unrespon-
sive ssh clients will be disconnected after approximately 45
seconds.
Compression
Specifies whether compression is allowed. The argument 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 pri-
mary group or supplementary group list matches one of the pat-
terns. ‘*’ and ‘?’ can be used as wildcards in the patterns.
Only group names are valid; a numerical group ID is not recog-
nized. 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 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.
GatewayPorts
Specifies whether remote hosts are allowed to connect 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. GatewayPorts can be
used to specify that sshd should bind remote port forwardings to
the wildcard address, thus allowing remote hosts to connect to
forwarded ports. The argument must be “yes” or “no”. 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.
GSSAPICleanupCredentials
Specifies whether to automatically destroy the user’s creden-
tials cache on logout. The default is “yes”. Note that this
option applies to protocol version 2 only.
HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication
together with successful public key client host authentication
is allowed (hostbased authentication). This option is similar
to RhostsRSAAuthentication and applies to protocol version 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 version 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/ssh/shosts.equiv are still used. The
default is “yes”.
IgnoreUserKnownHosts
Specifies whether sshd should ignore the user’s
$HOME/.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. 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”.
KerberosOrLocalPasswd
If set then if password authentication through Kerberos fails
then the password will be validated via any additional local
mechanism such as /etc/passwd. Default is “yes”.
KerberosTicketCleanup
Specifies whether to automatically destroy the user’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 ses-
sions 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 should listen on. The fol-
lowing 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 permit-
ted. 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 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,
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) algo-
rithms. 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,hmac-ripemd160,hmac-sha1-96,hmac-md5-96”.
MaxAuthTries
Specifies the maximum number of authentication attempts permit-
ted per connection. Once the number of failures reaches half
this value, additional failures are logged. The default is 6.
MaxStartups
Specifies the maximum number of concurrent unauthenticated con-
nections to the sshd daemon. Additional connections will be
dropped until authentication succeeds or the LoginGraceTime
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 proba-
bility 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 unau-
thenticated 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”.
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” password authentica-
tion is disabled for root. Note that other authentication meth-
ods (e.g., keyboard-interactive/PAM) may still allow root to
login using a password.
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 tak-
ing 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.
PermitUserEnvironment
Specifies whether ~/.ssh/environment and environment= options in
~/.ssh/authorized_keys are processed by sshd. The default is
“no”. Enabling environment processing may enable users to
bypass access restrictions in some configurations using mecha-
nisms 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 when the
user last logged in. 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
values are “1” and “2”. Multiple versions must be comma-sepa-
rated. The default is “2,1”. Note that the order of the proto-
col 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.
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 768.
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 pro-
tocol version 1 only.
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 acciden-
tally 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 to execute
upon 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 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, 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 tem-
porarily, 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”.
UseDNS Specifies whether 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 “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
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 PAM account and session mod-
ule 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 separates privileges by creating an
unprivileged child process to deal with incoming network traf-
fic. 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’s X11 for-
warding. 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 proxy display
is configured to listen on the wildcard address (see
X11UseLocalhost below), however this is not the default. Addi-
tionally, 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_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 war-
rant 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 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 wild-
card 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/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 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 features
and created OpenSSH. Markus Friedl contributed the support for SSH pro-
tocol versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed
support for privilege separation.
BSD September 25, 1999 BSD
