SSHD_CONFIG(5) FILE FORMATS SSHD_CONFIG(5)
NAME
sshd_config - OpenSSH SSH daemon configuration file
SYNOPSIS
/etc/ssh/sshd_config
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 com-
ments. Arguments may optionally be enclosed in double
quotes (") in order to represent arguments containing
spaces.
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 Sen-
dEnv in ssh_config(5) for how to configure the client.
The TERM environment variable is always sent whenever
the client requests a pseudo-terminal as it is required
by the protocol. Variables are specified by name,
which may contain the wildcard characters `*' and `?'.
Multiple environment variables may be separated by whi-
tespace or spread across multiple AcceptEnv directives.
Be warned that some environment variables could be used
to bypass restricted user environments. For this rea-
son, 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 for-
warding 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 supple-
mentary group list matches one of the patterns. Only
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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) for-
warding 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)) for-
warding 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 pat-
terns. 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. HOST
criteria may additionally contain addresses to match in
CIDR address/masklen format. 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 suc-
cessfully completed for a user to be granted access.
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This option must be followed by one or more comma-
separated lists of authentication method names, or by
the single string ``any'' to indicate the default
behaviour of accepting any single authentication
method. if the default is overridden, then 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 pos-
sible 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 interac-
tive authentication to the ``bsdauth'' device.
If the ``publickey'' method is listed more than once,
sshd(8) verifies that keys that have been used success-
fully are not reused for subsequent authentications.
For example, an AuthenticationMethods of
``publickey,publickey'' will require successful authen-
tication using two different public keys.
This option will yield a fatal error if enabled if pro-
tocol 1 is also enabled. Note that each authentication
method listed should also be explicitly enabled in the
configuration. The default ``any'' 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, not
writable by group or others and specified by an abso-
lute path.
Arguments to AuthorizedKeysCommand may be provided
using the following tokens, which will be expanded at
runtime: %% is replaced by a literal '%', %u is
replaced by the username being authenticated, %h is
replaced by the home directory of the user being
authenticated, %t is replaced with the key type offered
for authentication, %f is replaced with the fingerprint
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SSHD_CONFIG(5) FILE FORMATS SSHD_CONFIG(5)
of the key, and %k is replaced with the key being
offered for authentication. If no arguments are speci-
fied then the username of the target user will be sup-
plied.
The program 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 authenti-
cate and authorize the user then public key authentica-
tion continues using the usual AuthorizedKeysFile
files. By default, no AuthorizedKeysCommand is run.
AuthorizedKeysCommandUser
Specifies the user under whose account the Author-
izedKeysCommand is run. It is recommended to use a
dedicated user that has no other role on the host than
running authorized keys commands. If AuthorizedKeys-
Command is specified but AuthorizedKeysCommandUser is
not, then sshd(8) will refuse to start.
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, Author-
izedKeysFile is taken to be an absolute path or one
relative to the user's home directory. Multiple files
may be listed, separated by whitespace. Alternately
this option may be set to ``none'' to skip checking for
user keys in files. The default is
``.ssh/authorized_keys''.ssh/authorized_keys2 .
AuthorizedPrincipalsCommand
Specifies a program to be used to generate the list of
allowed certificate principals as per AuthorizedPrinci-
palsFile. The program must be owned by root, not writ-
able by group or others and specified by an absolute
path.
Arguments to AuthorizedPrincipalsCommand may be pro-
vided using the following tokens, which will be
expanded at runtime: %% is replaced by a literal '%',
%u is replaced by the username being authenticated and
%h is replaced by the home directory of the user being
authenticated.
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The program should produce on standard output zero or
more lines of AuthorizedPrincipalsFile output. If
either AuthorizedPrincipalsCommand or AuthorizedPrinci-
palsFile is specified, then certificates offered by the
client for authentication must contain a principal that
is listed. By default, no AuthorizedPrincipalsCommand
is run.
AuthorizedPrincipalsCommandUser
Specifies the user under whose account the Author-
izedPrincipalsCommand is run. It is recommended to use
a dedicated user that has no other role on the host
than running authorized principals commands. If
AuthorizedPrincipalsCommand is specified but Author-
izedPrincipalsCommandUser is not, then sshd(8) will
refuse to start.
AuthorizedPrincipalsFile
Specifies a file that lists principal names that are
accepted for certificate authentication. When using
certificates signed by a key listed in TrustedUserCA-
Keys, this file lists names, one of which must appear
in the certificate for it to be accepted for authenti-
cation. 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, Author-
izedPrincipalsFile 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 certifica-
tion 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. By
default, no banner is displayed.
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ChallengeResponseAuthentication
Specifies whether challenge-response authentication is
allowed (e.g. via PAM or through authentication styles
supported in login.conf(5)) The default is ``yes''.
ChrootDirectory
Specifies the pathname of a directory to chroot(2) to
after authentication. At session startup sshd(8)
checks that all components of the pathname are root-
owned directories which 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), and tty(4)
devices. For file transfer sessions using ``sftp'', no
additional configuration of the environment is neces-
sary if the in-process sftp server is used, though ses-
sions which use logging may require /dev/log inside the
chroot directory on some operating systems (see sftp-
server(8) for details).
For safety, it is very important that the directory
hierarchy be prevented from modification by other
processes on the system (especially those outside the
jail). Misconfiguration can lead to unsafe environ-
ments which sshd(8) cannot detect.
The default is ``none'', indicating not to chroot(2).
Ciphers
Specifies the ciphers allowed. Multiple ciphers must
be comma-separated. If the specified value begins with
a `+' character, then the specified ciphers will be
appended to the default set instead of replacing them.
The supported ciphers are:
3des-cbc
aes128-cbc
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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:
chacha20-poly1305@openssh.com,
aes128-ctr,aes192-ctr,aes256-ctr,
aes128-gcm@openssh.com,aes256-gcm@openssh.com
The list of available ciphers may also be obtained
using the -Q option of ssh(1) with an argument of
``cipher''.
ClientAliveCountMax
Sets the number of client alive messages (see below)
which may be sent without sshd(8) receiving any mes-
sages back from the client. If this threshold is
reached while client alive messages are being sent,
sshd will disconnect the client, terminating the ses-
sion. 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 spoofa-
ble. 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 con-
nection has become inactive.
The default value is 3. If ClientAliveInterval (see
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below) is set to 15, and ClientAliveCountMax is left at
the default, unresponsive SSH clients will be discon-
nected after approximately 45 seconds.
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, indicat-
ing that these messages will not be sent to the client.
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''.
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. HOST criteria may addi-
tionally contain addresses to match in CIDR
address/masklen format. 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.
FingerprintHash
Specifies the hash algorithm used when logging key
fingerprints. Valid options are: ``md5'' and
``sha256''. The default is ``md5''.
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ForceCommand
Forces the execution of the command specified by For-
ceCommand, 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 ori-
ginally 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. The default is
``none''.
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''.
GSSAPICleanupCredentials
Specifies whether to automatically destroy the user's
credentials cache on logout. The default is ``yes''.
GSSAPIStrictAcceptorCheck
Determines whether to be strict about the identity of
the GSSAPI acceptor a client authenticates against. If
set to ``yes'' then the client must authenticate
against the host service on the current hostname. If
set to ``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''.
HostbasedAcceptedKeyTypes
Specifies the key types that will be accepted for host-
based authentication as a comma-separated pattern list.
Alternately if the specified value begins with a `+'
character, then the specified key types will be
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SSHD_CONFIG(5) FILE FORMATS SSHD_CONFIG(5)
appended to the default set instead of replacing them.
The default for this option is:
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-
nistp521,
ssh-ed25519,ssh-rsa
The -Q option of ssh(1) may be used to list supported
key types.
HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authenti-
cation together with successful public key client host
authentication is allowed (host-based authentication).
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 and that the HostKeyAlgorithms
option restricts which of the keys are actually used by
sshd(8).
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
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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 the string ``SSH_AUTH_SOCK'' is specified, the loca-
tion of the socket will be read from the SSH_AUTH_SOCK
environment variable.
HostKeyAlgorithms
Specifies the host key algorithms that the server
offers. The default for this option is:
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-
nistp521,
ssh-ed25519,ssh-rsa
The list of available key types may also be obtained
using the -Q option of ssh(1) with an argument of
``key''.
IgnoreRhosts
Specifies that .rhosts and .shosts files will not be
used in RhostsRSAAuthentication or HostbasedAuthentica-
tion.
/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
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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 ses-
sions.
KbdInteractiveAuthentication
Specifies whether to allow keyboard-interactive authen-
tication. The argument to this keyword must be ``yes''
or ``no''. The default is to use whatever value Chal-
lengeResponseAuthentication 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. Alter-
nately if the specified value begins with a `+' charac-
ter, then the specified methods will be appended to the
default set instead of replacing them. The supported
algorithms are:
curve25519-sha256@libssh.org
diffie-hellman-group1-sha1
diffie-hellman-group14-sha1
diffie-hellman-group-exchange-sha1
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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
The list of available key exchange algorithms may also
be obtained using the -Q option of ssh(1) with an argu-
ment of ``kex''.
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:
ListenAddress host | ArIPv4_addr|IPv6_addr
ListenAddress host | ArIPv4_addr:port
ListenAddress [host | ArIPv6_addr]:port
If port is not specified, sshd will listen on the
address and all Port options specified. The default is
to listen on all local addresses. Multiple ListenAd-
dress options are permitted.
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
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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 for data
integrity protection. Multiple algorithms must be
comma-separated. If the specified value begins with a
`+' character, then the specified algorithms will be
appended to the default set instead of replacing them.
The algorithms that contain ``-etm'' calculate the MAC
after encryption (encrypt-then-mac). These are con-
sidered 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
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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,
hmac-sha1-etm@openssh.com,
umac-64@openssh.com,umac-128@openssh.com,
hmac-sha2-256,hmac-sha2-512,hmac-sha1
The list of available MAC algorithms may also be
obtained using the -Q option of ssh(1) with an argument
of ``mac''.
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 sec-
tion 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 satisfied, 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 cri-
teria. The available criteria are User, Group, Host,
LocalAddress, LocalPort, and Address. The match pat-
terns 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 for-
mat, 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 fol-
lowing a Match keyword. Available keywords are Accep-
tEnv, AllowAgentForwarding, AllowGroups, AllowStreamLo-
calForwarding, AllowTcpForwarding, AllowUsers, Authen-
ticationMethods, AuthorizedKeysCommand, AuthorizedKeys-
CommandUser, AuthorizedKeysFile, AuthorizedPrincipal-
sCommand, AuthorizedPrincipalsCommandUser, Author-
izedPrincipalsFile, Banner, ChrootDirectory, Deny-
Groups, DenyUsers, ForceCommand, GatewayPorts,
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SSHD_CONFIG(5) FILE FORMATS SSHD_CONFIG(5)
GSSAPIAuthentication, HostbasedAcceptedKeyTypes, Host-
basedAuthentication, HostbasedUsesNameFromPacketOnly,
IPQoS, KbdInteractiveAuthentication, KerberosAuthenti-
cation, MaxAuthTries, MaxSessions, PasswordAuthentica-
tion, PermitEmptyPasswords, PermitOpen, PermitRootLo-
gin, PermitTTY, PermitTunnel, PermitUserRC, PubkeyAc-
ceptedKeyTypes, PubkeyAuthentication, RekeyLimit,
RevokedKeys, RhostsRSAAuthentication, RSAAuthentica-
tion, StreamLocalBindMask, StreamLocalBindUnlink,
TrustedUserCAKeys, 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 shell, login or
subsystem (e.g. sftp) sessions permitted per network
connection. Multiple sessions may be established by
clients that support connection multiplexing. Setting
MaxSessions to 1 will effectively disable session mul-
tiplexing, whereas setting it to 0 will prevent all
shell, login and subsystem sessions while still permit-
ting forwarding. The default is 10.
MaxStartups
Specifies the maximum number of concurrent unauthenti-
cated connections to the SSH daemon. Additional con-
nections 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
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SSHD_CONFIG(5) FILE FORMATS SSHD_CONFIG(5)
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:
PermitOpen host:port
PermitOpen IPv4_addr:port
PermitOpen [IPv6_addr]:port
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. The wildcard ``*''
can be used for host or port to allow all hosts or
ports, respectively. By default all port forwarding
requests are permitted.
PermitRootLogin
Specifies whether root can log in using ssh(1). The
argument must be ``yes'', ``prohibit-password'',
``without-password'', ``forced-commands-only'', or
``no''. The default is ``prohibit-password''.
If this option is set to ``prohibit-password'' or
``without-password'', password and keyboard-interactive
authentication are 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 authen-
tication 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 ``ether-
net''. The default is ``no''.
Independent of this setting, the permissions of the
selected tun(4) device must allow access to the user.
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SSHD_CONFIG(5) FILE FORMATS SSHD_CONFIG(5)
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 restric-
tions 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, or ``none'' to not write one. The default
is /etc/sshd.pid.
Port Specifies the port number that sshd(8) listens on. The
default is 22. Multiple options of this type are per-
mitted. 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'. Protocol
1 suffers from a number of cryptographic weaknesses and
should not be used. It is only offered to support
legacy devices.
Note that the order of the protocol list does not indi-
cate preference, because the client selects among mul-
tiple protocol versions offered by the server. Speci-
fying ``2,1'' is identical to ``1,2''.
PubkeyAcceptedKeyTypes
Specifies the key types that will be accepted for pub-
lic key authentication as a comma-separated pattern
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SSHD_CONFIG(5) FILE FORMATS SSHD_CONFIG(5)
list. Alternately if the specified value begins with a
`+' character, then the specified key types will be
appended to the default set instead of replacing them.
The default for this option is:
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-
nistp521,
ssh-ed25519,ssh-rsa
The -Q option of ssh(1) may be used to list supported
key types.
PubkeyAuthentication
Specifies whether public key authentication is allowed.
The default is ``yes''.
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.
RevokedKeys
Specifies revoked public keys file, or ``none'' to not
use one. 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 authenti-
cation together with successful RSA host authentication
is allowed. The default is ``no''. This option
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SSHD_CONFIG(5) FILE FORMATS SSHD_CONFIG(5)
applies to protocol version 1 only.
RSAAuthentication
Specifies whether pure RSA authentication is allowed.
The default is ``yes''. This option applies to proto-
col version 1 only.
ServerKeyBits
Defines the number of bits in the ephemeral protocol
version 1 server key. The default and minimum value 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 per-
missions 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 sub-
system request.
The command sftp-server(8) implements the ``sftp'' file
transfer subsystem.
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SSHD_CONFIG(5) FILE FORMATS SSHD_CONFIG(5)
Alternately the name ``internal-sftp'' implements an
in-process ``sftp'' server. This may simplify confi-
gurations using ChrootDirectory to force a different
filesystem root on clients.
By default no subsystems are defined.
SyslogFacility
Gives the facility code that is used when logging mes-
sages 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 mes-
sages), and the server will notice if the network goes
down or the client host crashes. This avoids infin-
itely hanging sessions.
To disable TCP keepalive messages, the value should be
set to ``no''.
TrustedUserCAKeys
Specifies a file containing public keys of certificate
authorities that are trusted to sign user certificates
for authentication, or ``none'' to not use one. 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 authenti-
cation for any user listed in the certificate's princi-
pals list. Note that certificates that lack a list of
principals will not be permitted for authentication
using TrustedUserCAKeys. For more details on certifi-
cates, see the CERTIFICATES section in ssh-keygen(1).
UseDNS
Specifies whether sshd(8) should look up the remote
host name, and to check that the resolved host name for
the remote IP address maps back to the very same IP
address.
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SSHD_CONFIG(5) FILE FORMATS SSHD_CONFIG(5)
If this option is set to ``no'' (the default) then only
addresses and not host names may be used in
~/.ssh/authorized_keys from and sshd_config Match Host
directives.
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 han-
dle 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 PasswordAu-
thentication 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 authentica-
tion, 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 argument must be ``yes'', ``no'', or
``sandbox''. If UsePrivilegeSeparation is set to
``sandbox'' then the pre-authentication unprivileged
process is subject to additional restrictions. The
default is ``sandbox''.
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)Ns 's X11 forwarding. This prevents sshd from
interfering with real X11 servers. The default is 10.
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SSHD_CONFIG(5) FILE FORMATS SSHD_CONFIG(5)
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 authentica-
tion 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 forward-
ing 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 func-
tion 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, or
``none'' to not use one. The default is
/usr/X/bin/xauth.
TIME FORMATS
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>
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SSHD_CONFIG(5) FILE FORMATS SSHD_CONFIG(5)
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 calcu-
late 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 ver-
sions 1.5 and 2.0. Niels Provos and Markus Friedl contri-
buted support for privilege separation.
Last change: July 19 2016 24
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