NAME¶
sshd_config
—
OpenSSH SSH daemon configuration file
SYNOPSIS¶
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.
Note that the Debian
openssh-server
package
sets several options as standard in
/etc/ssh/sshd_config which are not the
default in
sshd(8). The exact list depends on
whether the package was installed fresh or upgraded from various possible
previous versions, but includes at least the following:
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_config(5) 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-agent(1) 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 PATTERNS in ssh_config(5) for more
information on patterns.
AllowTcpForwarding
- Specifies whether TCP forwarding is permitted. The available options are
“yes” or “all” to allow TCP forwarding,
“no” to prevent all TCP forwarding, “local” to
allow local (from the perspective of ssh(1))
forwarding only or “remote” to allow remote forwarding only.
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.
AllowStreamLocalForwarding
- Specifies whether StreamLocal (Unix-domain socket) forwarding is
permitted. The available options are “yes” or
“all” to allow StreamLocal forwarding, “no” to
prevent all StreamLocal forwarding, “local” to allow local
(from the perspective of ssh(1)) forwarding
only or “remote” to allow remote forwarding only. The
default is “yes”. Note that disabling StreamLocal 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 USER@HOST 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 PATTERNS in ssh_config(5) for more
information on patterns.
AuthenticationMethods
- Specifies the authentication methods that must be successfully completed
for a user to be granted access. This option must be followed by one or
more comma-separated lists of authentication method names. Successful
authentication requires completion of every method in at least one of
these lists.
For example, an argument of “publickey,password
publickey,keyboard-interactive” would require the user to complete
public key authentication, followed by either password or keyboard
interactive authentication. Only methods that are next in one or more
lists are offered at each stage, so for this example, it would not be
possible to attempt password or keyboard-interactive authentication before
public key.
For keyboard interactive authentication it is also possible to restrict
authentication to a specific device by appending a colon followed by the
device identifier “bsdauth”, “pam”, or
“skey”, depending on the server configuration. For example,
“keyboard-interactive:bsdauth” would restrict keyboard
interactive authentication to the “bsdauth” device.
This option is only available for SSH protocol 2 and will yield a fatal
error if enabled if protocol 1 is also enabled. Note that each
authentication method listed should also be explicitly enabled in the
configuration. The default is not to require multiple authentication;
successful completion of a single authentication method is
sufficient.
AuthorizedKeysCommand
- Specifies a program to be used to look up the user's public keys. The
program must be owned by root and not writable by group or others. It will
be invoked with a single argument of the username being authenticated, and
should produce on standard output zero or more lines of authorized_keys
output (see AUTHORIZED_KEYS in sshd(8)). If a
key supplied by AuthorizedKeysCommand does not successfully authenticate
and authorize the user then public key authentication continues using the
usual
AuthorizedKeysFile
files. By
default, no AuthorizedKeysCommand is run.
AuthorizedKeysCommandUser
- Specifies the user under whose account the AuthorizedKeysCommand is run.
It is recommended to use a dedicated user that has no other role on the
host than running authorized keys commands.
AuthorizedKeysFile
- Specifies the file that contains the public keys that can be used for user
authentication. The format is described in the AUTHORIZED_KEYS FILE FORMAT
section of sshd(8).
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. Multiple files
may be listed, separated by whitespace. The default is
“.ssh/authorized_keys .ssh/authorized_keys2”.
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 preceded by key options
(as described in AUTHORIZED_KEYS FILE FORMAT in
sshd(8)). 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 “none”, i.e. 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). The default is “yes”.
ChrootDirectory
- Specifies the pathname of a directory to
chroot(2) to after authentication. All
components of the pathname 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 pathname 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 may require /dev/log inside
the chroot directory on some operating systems (see
sftp-server(8) 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
- aes128-gcm@openssh.com
- aes256-gcm@openssh.com
- arcfour
- arcfour128
- arcfour256
- blowfish-cbc
- cast128-cbc
- chacha20-poly1305@openssh.com
The default is:
aes128-ctr,aes192-ctr,aes256-ctr,
aes128-gcm@openssh.com,aes256-gcm@openssh.com,
chacha20-poly1305@openssh.com
The list of available ciphers may also be obtained using the
-Q
option of
ssh(1).
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”.
DebianBanner
- Specifies whether the distribution-specified extra version suffix is
included during initial protocol handshake. 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. 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 PATTERNS in ssh_config(5) 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
USER@HOST 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 PATTERNS in ssh_config(5) 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_dsa_key,
/etc/ssh/ssh_host_ecdsa_key,
/etc/ssh/ssh_host_ed25519_key and
/etc/ssh/ssh_host_rsa_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”, “ecdsa”, “ed25519” or
“rsa” are used for version 2 of the SSH protocol. It is also
possible to specify public host key files instead. In this case operations
on the private key will be delegated to an
ssh-agent(1).
HostKeyAgent
- Identifies the UNIX-domain socket used to communicate with an agent that
has access to the private host keys. If “SSH_AUTH_SOCK” is
specified, the location of the socket will be read from the
SSH_AUTH_SOCK
environment
variable.
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”.
IPQoS
- Specifies the IPv4 type-of-service or DSCP class for the connection.
Accepted values are “af11”, “af12”,
“af13”, “af21”, “af22”,
“af23”, “af31”, “af32”,
“af33”, “af41”, “af42”,
“af43”, “cs0”, “cs1”,
“cs2”, “cs3”, “cs4”,
“cs5”, “cs6”, “cs7”,
“ef”, “lowdelay”, “throughput”,
“reliability”, or a numeric value. This option may take one
or two arguments, separated by whitespace. If one argument is specified,
it is used as the packet class unconditionally. If two values are
specified, the first is automatically selected for interactive sessions
and the second for non-interactive sessions. The default is
“lowdelay” for interactive sessions and
“throughput” for non-interactive sessions.
KbdInteractiveAuthentication
- Specifies whether to allow keyboard-interactive authentication. The
argument to this keyword must be “yes” or
“no”. The default is to use whatever value
ChallengeResponseAuthentication
is set
to (by default “yes”).
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”.
KexAlgorithms
- Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms
must be comma-separated. The supported algorithms are:
- curve25519-sha256@libssh.org
- diffie-hellman-group1-sha1
- diffie-hellman-group14-sha1
- diffie-hellman-group-exchange-sha1
- diffie-hellman-group-exchange-sha256
- ecdh-sha2-nistp256
- ecdh-sha2-nistp384
- ecdh-sha2-nistp521
The default is:
curve25519-sha256@libssh.org,
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group14-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:
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 algorithms that contain
“-etm” calculate the MAC after encryption
(encrypt-then-mac). These are considered safer and their use recommended.
The supported MACs are:
- hmac-md5
- hmac-md5-96
- hmac-ripemd160
- hmac-sha1
- hmac-sha1-96
- hmac-sha2-256
- hmac-sha2-512
- umac-64@openssh.com
- umac-128@openssh.com
- hmac-md5-etm@openssh.com
- hmac-md5-96-etm@openssh.com
- hmac-ripemd160-etm@openssh.com
- hmac-sha1-etm@openssh.com
- hmac-sha1-96-etm@openssh.com
- hmac-sha2-256-etm@openssh.com
- hmac-sha2-512-etm@openssh.com
- umac-64-etm@openssh.com
- umac-128-etm@openssh.com
The default is:
umac-64-etm@openssh.com,umac-128-etm@openssh.com,
hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
umac-64@openssh.com,umac-128@openssh.com,
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. If a keyword appears in multiple
Match
blocks that are satisified, only
the first instance of the keyword is applied.
The arguments to Match
are one or more
criteria-pattern pairs or the single token
All
which matches all criteria. The
available criteria are User
,
Group
,
Host
,
LocalAddress
,
LocalPort
, 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 PATTERNS section of
ssh_config(5).
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
AcceptEnv
,
AllowAgentForwarding
,
AllowGroups
,
AllowTcpForwarding
,
AllowUsers
,
AuthenticationMethods
,
AuthorizedKeysCommand
,
AuthorizedKeysCommandUser
,
AuthorizedKeysFile
,
AuthorizedPrincipalsFile
,
Banner
,
ChrootDirectory
,
DenyGroups
,
DenyUsers
,
ForceCommand
,
GatewayPorts
,
GSSAPIAuthentication
,
HostbasedAuthentication
,
HostbasedUsesNameFromPacketOnly
,
KbdInteractiveAuthentication
,
KerberosAuthentication
,
MaxAuthTries
,
MaxSessions
,
PasswordAuthentication
,
PermitEmptyPasswords
,
PermitOpen
,
PermitRootLogin
,
PermitTTY
,
PermitTunnel
,
PermitUserRC
,
PubkeyAuthentication
,
RekeyLimit
,
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:
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. An argument of “none” can be
used to prohibit all 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”.
PermitTTY
- Specifies whether pty(4) allocation is
permitted. The default is “yes”.
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
.
PermitUserRC
- Specifies whether any ~/.ssh/rc file is
executed. The default is “yes”.
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’. 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.
RekeyLimit
- Specifies the maximum amount of data that may be transmitted before the
session key is renegotiated, optionally followed a maximum amount of time
that may pass before the session key is renegotiated. The first argument
is specified in bytes and may have a suffix of ‘K’,
‘M’, or ‘G’ to indicate Kilobytes, Megabytes,
or Gigabytes, respectively. The default is between ‘1G’ and
‘4G’, depending on the cipher. The optional second value is
specified in seconds and may use any of the units documented in the
TIME FORMATS section.
The default value for
RekeyLimit
is
“default none”, which means that rekeying is performed after
the cipher's default amount of data has been sent or received and no time
based rekeying is done. This option applies to protocol version 2
only.
RevokedKeys
- Specifies 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. Keys may be
specified as a text file, listing one public key per line, or as an
OpenSSH Key Revocation List (KRL) as generated by
ssh-keygen(1). For more information on KRLs,
see the KEY REVOCATION LISTS section in
ssh-keygen(1).
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.
StreamLocalBindMask
- Sets the octal file creation mode mask (umask) used when creating a
Unix-domain socket file for local or remote port forwarding. This option
is only used for port forwarding to a Unix-domain socket file.
The default value is 0177, which creates a Unix-domain socket file that is
readable and writable only by the owner. Note that not all operating
systems honor the file mode on Unix-domain socket files.
StreamLocalBindUnlink
- Specifies whether to remove an existing Unix-domain socket file for local
or remote port forwarding before creating a new one. If the socket file
already exists and
StreamLocalBindUnlink
is not enabled,
sshd
will be unable to forward the port
to the Unix-domain socket file. This option is only used for port
forwarding to a Unix-domain socket file.
The argument must be “yes” or “no”. The default
is “no”.
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”. Note that this does not apply to
ChrootDirectory
, whose permissions and
ownership are checked unconditionally.
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-server(8) 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, 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”.
This option was formerly called
KeepAlive
.
TrustedUserCAKeys
- Specifies a file containing public keys of certificate authorities that
are trusted to 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 on
certificates, see the CERTIFICATES section in
ssh-keygen(1).
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”. If
UsePrivilegeSeparation
is set to
“sandbox” then the pre-authentication unprivileged process
is subject to additional restrictions.
VersionAddendum
- Optionally specifies additional text to append to the SSH protocol banner
sent by the server upon connection. The default is
“none”.
X11DisplayOffset
- Specifies the first display number available for
sshd(8)'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_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 “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.
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:
- ⟨
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.