файл конфигурации демона OpenSSH (OpenSSH daemon configuration file)
Имя (Name)
sshd_config — OpenSSH daemon configuration file
Описание (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. For each keyword, the first
obtained value will be used. 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.
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
and SetEnv in ssh_config(5) for how to configure the
client. The TERM environment variable is always accepted
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
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 (the default), inet (use IPv4
only), or inet6 (use IPv6 only).
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
groups directives are processed in the following order:
DenyGroups, AllowGroups.
See PATTERNS in ssh_config(5) for more information on
patterns.
AllowStreamLocalForwarding
Specifies whether StreamLocal (Unix-domain socket)
forwarding is permitted. The available options are yes
(the default) 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. Note that disabling
StreamLocal forwarding does not improve security unless
users are also denied shell access, as they can always
install their own forwarders.
AllowTcpForwarding
Specifies whether TCP forwarding is permitted. The
available options are yes (the default) 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. 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.
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 users directives
are processed in the following order: DenyUsers,
AllowUsers.
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 lists of comma-
separated 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, "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
or pam. depending on the server configuration. For
example, "keyboard-interactive:bsdauth" would restrict
keyboard interactive authentication to the bsdauth device.
If the publickey method is listed more than once, sshd(8)
verifies that keys that have been used successfully are not
reused for subsequent authentications. For example,
"publickey,publickey" requires successful authentication
using two different public keys.
Note that each authentication method listed should also be
explicitly enabled in the configuration.
The available authentication methods are:
"gssapi-with-mic", "hostbased", "keyboard-interactive",
"none" (used for access to password-less accounts when
PermitEmptyPasswords is enabled), "password" and
"publickey".
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 absolute path.
Arguments to AuthorizedKeysCommand accept the tokens
described in the TOKENS section. If no arguments are
specified then the username of the target user is used.
The program should produce on standard output zero or more
lines of authorized_keys output (see AUTHORIZED_KEYS in
sshd(8)). AuthorizedKeysCommand is tried after the usual
AuthorizedKeysFile files and will not be executed if a
matching key is found there. 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. If AuthorizedKeysCommand
is specified but AuthorizedKeysCommandUser is not, then
sshd(8) will refuse to start.
AuthorizedKeysFile
Specifies the file that contains the public keys used for
user authentication. The format is described in the
AUTHORIZED_KEYS FILE FORMAT section of sshd(8). Arguments
to AuthorizedKeysFile accept the tokens described in the
TOKENS section. 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. 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
AuthorizedPrincipalsFile. The program must be owned by
root, not writable by group or others and specified by an
absolute path. Arguments to AuthorizedPrincipalsCommand
accept the tokens described in the TOKENS section. If no
arguments are specified then the username of the target
user is used.
The program should produce on standard output zero or more
lines of AuthorizedPrincipalsFile output. If either
AuthorizedPrincipalsCommand or AuthorizedPrincipalsFile 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
AuthorizedPrincipalsCommand 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
AuthorizedPrincipalsCommandUser 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 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.
Arguments to AuthorizedPrincipalsFile accept the tokens
described in the TOKENS section. 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. By default, no banner is
displayed.
CASignatureAlgorithms
Specifies which algorithms are allowed for signing of
certificates by certificate authorities (CAs). The default
is:
ssh-ed25519,ecdsa-sha2-nistp256,
ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
rsa-sha2-512,rsa-sha2-256
If the specified list begins with a '+' character, then the
specified algorithms will be appended to the default set
instead of replacing them. If the specified list begins
with a '-' character, then the specified algorithms
(including wildcards) will be removed from the default set
instead of replacing them.
Certificates signed using other algorithms will not be
accepted for public key or host-based authentication.
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. Arguments to ChrootDirectory accept the
tokens described in the TOKENS section.
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 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).
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 environments 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 list begins with a '+'
character, then the specified ciphers will be appended to
the default set instead of replacing them. If the
specified list begins with a '-' character, then the
specified ciphers (including wildcards) will be removed
from the default set instead of replacing them. If the
specified list begins with a '^' character, then the
specified ciphers will be placed at the head of the default
set.
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
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
"ssh -Q cipher".
ClientAliveCountMax
Sets the number of client alive messages 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. 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 unresponsive.
The default value is 3. If ClientAliveInterval is set to
15, and ClientAliveCountMax is left at the default,
unresponsive SSH clients will be disconnected after
approximately 45 seconds. Setting a zero
ClientAliveCountMax disables connection termination.
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.
Compression
Specifies whether compression is enabled after the user has
authenticated successfully. The argument must be yes,
delayed (a legacy synonym for yes) or no. The default is
yes.
DenyGroups
This keyword can be followed by a list of group name
patterns, separated by spaces. Login is disallowed for
users whose primary group or supplementary group list
matches one of the patterns. Only group names are valid; a
numerical group ID is not recognized. By default, login is
allowed for all groups. The allow/deny groups directives
are processed in the following order: DenyGroups,
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 additionally contain
addresses to match in CIDR address/masklen format. The
allow/deny users directives are processed in the following
order: DenyUsers, AllowUsers.
See PATTERNS in ssh_config(5) for more information on
patterns.
DisableForwarding
Disables all forwarding features, including X11,
ssh-agent(1), TCP and StreamLocal. This option overrides
all other forwarding-related options and may simplify
restricted configurations.
ExposeAuthInfo
Writes a temporary file containing a list of authentication
methods and public credentials (e.g. keys) used to
authenticate the user. The location of the file is exposed
to the user session through the SSH_USER_AUTH environment
variable. The default is no.
FingerprintHash
Specifies the hash algorithm used when logging key
fingerprints. Valid options are: md5 and sha256. The
default is sha256.
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. 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.
HostbasedAcceptedAlgorithms
Specifies the signature algorithms that will be accepted
for hostbased authentication as a list of comma-separated
patterns. Alternately if the specified list begins with a
'+' character, then the specified signature algorithms will
be appended to the default set instead of replacing them.
If the specified list begins with a '-' character, then the
specified signature algorithms (including wildcards) will
be removed from the default set instead of replacing them.
If the specified list begins with a '^' character, then the
specified signature algorithms will be placed at the head
of the default set. The default for this option is:
ssh-ed25519-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
rsa-sha2-512,rsa-sha2-256,ssh-rsa
The list of available signature algorithms may also be
obtained using "ssh -Q HostbasedAcceptedAlgorithms". This
was formerly named HostbasedAcceptedKeyTypes.
HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication
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 defaults are /etc/ssh/ssh_host_ecdsa_key,
/etc/ssh/ssh_host_ed25519_key and
/etc/ssh/ssh_host_rsa_key.
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. 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 location of the
socket will be read from the SSH_AUTH_SOCK environment
variable.
HostKeyAlgorithms
Specifies the host key signature algorithms that the server
offers. The default for this option is:
ssh-ed25519-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
rsa-sha2-512,rsa-sha2-256,ssh-rsa
The list of available signature algorithms may also be
obtained using "ssh -Q HostKeyAlgorithms".
IgnoreRhosts
Specifies whether to ignore per-user .rhosts and .shosts
files during HostbasedAuthentication. The system-wide
/etc/hosts.equiv and /etc/shosts.equiv are still used
regardless of this setting.
Accepted values are yes (the default) to ignore all per-
user files, shosts-only to allow the use of .shosts but to
ignore .rhosts or no to allow both .shosts and rhosts.
IgnoreUserKnownHosts
Specifies whether sshd(8) should ignore the user's
~/.ssh/known_hosts during HostbasedAuthentication and use
only the system-wide known hosts file /etc/ssh/known_hosts.
The default is 'no'.
Include
Include the specified configuration file(s). Multiple
pathnames may be specified and each pathname may contain
glob(7) wildcards that will be expanded and processed in
lexical order. Files without absolute paths are assumed to
be in /etc/ssh. An Include directive may appear inside a
Match block to perform conditional inclusion.
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, le, lowdelay, throughput,
reliability, a numeric value, or none to use the operating
system default. 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 af21 (Low-Latency Data) for
interactive sessions and cs1 (Lower Effort) for non-
interactive sessions.
KbdInteractiveAuthentication
Specifies whether to allow keyboard-interactive
authentication. All authentication styles from
login.conf(5) are supported. The default is yes. The
argument to this keyword must be yes or no.
ChallengeResponseAuthentication is a deprecated alias for
this.
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. Alternately
if the specified list begins with a '+' character, then the
specified methods will be appended to the default set
instead of replacing them. If the specified list begins
with a '-' character, then the specified methods (including
wildcards) will be removed from the default set instead of
replacing them. If the specified list begins with a '^'
character, then the specified methods will be placed at the
head of the default set. The supported algorithms are:
curve25519-sha256
curve25519-sha256@libssh.org
diffie-hellman-group1-sha1
diffie-hellman-group14-sha1
diffie-hellman-group14-sha256
diffie-hellman-group16-sha512
diffie-hellman-group18-sha512
diffie-hellman-group-exchange-sha1
diffie-hellman-group-exchange-sha256
ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521
sntrup761x25519-sha512@openssh.com
The default is:
curve25519-sha256,curve25519-sha256@libssh.org,
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
diffie-hellman-group14-sha256
The list of available key exchange algorithms may also be
obtained using "ssh -Q KexAlgorithms".
ListenAddress
Specifies the local addresses sshd(8) should listen on.
The following forms may be used:
ListenAddress hostname|address [rdomain domain]
ListenAddress hostname:port [rdomain domain]
ListenAddress IPv4_address:port [rdomain domain]
ListenAddress [hostname|address]:port [rdomain
domain]
The optional rdomain qualifier requests sshd(8) listen in
an explicit routing domain. 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 on the
current default routing domain. Multiple ListenAddress
options are permitted. For more information on routing
domains, see rdomain(4).
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.
LogVerbose
Specify one or more overrides to LogLevel. An override
consists of a pattern lists that matches the source file,
function and line number to force detailed logging for.
For example, an override pattern of:
kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
would enable detailed logging for line 1000 of kex.c,
everything in the kex_exchange_identification() function,
and all code in the packet.c file. This option is intended
for debugging and no overrides are enabled by default.
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 list begins with a '+' character, then the
specified algorithms will be appended to the default set
instead of replacing them. If the specified list begins
with a '-' character, then the specified algorithms
(including wildcards) will be removed from the default set
instead of replacing them. If the specified list begins
with a '^' character, then the specified algorithms will be
placed at the head of the default set.
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-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-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,
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 "ssh -Q 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 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 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 criteria.
The available criteria are User, Group, Host, LocalAddress,
LocalPort, RDomain, and Address (with RDomain representing
the rdomain(4) on which the connection was received).
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,
such as 192.0.2.0/24 or 2001:db8::/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,
AllowStreamLocalForwarding, AllowTcpForwarding, AllowUsers,
AuthenticationMethods, AuthorizedKeysCommand,
AuthorizedKeysCommandUser, AuthorizedKeysFile,
AuthorizedPrincipalsCommand,
AuthorizedPrincipalsCommandUser, AuthorizedPrincipalsFile,
Banner, ChrootDirectory, ClientAliveCountMax,
ClientAliveInterval, DenyGroups, DenyUsers,
DisableForwarding, ForceCommand, GatewayPorts,
GSSAPIAuthentication, HostbasedAcceptedAlgorithms,
HostbasedAuthentication, HostbasedUsesNameFromPacketOnly,
IgnoreRhosts, Include, IPQoS, KbdInteractiveAuthentication,
KerberosAuthentication, LogLevel, MaxAuthTries,
MaxSessions, PasswordAuthentication, PermitEmptyPasswords,
PermitListen, PermitOpen, PermitRootLogin, PermitTTY,
PermitTunnel, PermitUserRC, PubkeyAcceptedAlgorithms,
PubkeyAuthentication, RekeyLimit, RevokedKeys, RDomain,
SetEnv, 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
multiplexing, whereas setting it to 0 will prevent all
shell, login and subsystem sessions while still permitting
forwarding. 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).
ModuliFile
Specifies the moduli(5) file that contains the Diffie-
Hellman groups used for the
'diffie-hellman-group-exchange-sha1' and
'diffie-hellman-group-exchange-sha256' key exchange
methods. The default is /etc/moduli.
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.
PermitListen
Specifies the addresses/ports on which a remote TCP port
forwarding may listen. The listen specification must be
one of the following forms:
PermitListen port
PermitListen host:port
Multiple permissions may be specified by separating them
with whitespace. An argument of any can be used to remove
all restrictions and permit any listen requests. An
argument of none can be used to prohibit all listen
requests. The host name may contain wildcards as described
in the PATTERNS section in ssh_config(5). The wildcard '*'
can also be used in place of a port number to allow all
ports. By default all port forwarding listen requests are
permitted. Note that the GatewayPorts option may further
restrict which addresses may be listened on. Note also
that ssh(1) will request a listen host of 'localhost' if no
listen host was specifically requested, and this name is
treated differently to explicit localhost addresses of
'127.0.0.1' and '::1'.
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. Otherwise, no
pattern matching or address lookups are performed on
supplied names. 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,
forced-commands-only, or no. The default is
prohibit-password.
If this option is set to prohibit-password (or its
deprecated alias, 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 authentication methods are
disabled for root.
If this option is set to no, root is not allowed to log in.
PermitTTY
Specifies whether pty(4) allocation is permitted. The
default is yes.
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.
Independent of this setting, the permissions of the
selected tun(4) device must allow access to the user.
PermitUserEnvironment
Specifies whether ~/.ssh/environment and environment=
options in ~/.ssh/authorized_keys are processed by sshd(8).
Valid options are yes, no or a pattern-list specifying
which environment variable names to accept (for example
"LANG,LC_*"). 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.
PerSourceMaxStartups
Specifies the number of unauthenticated connections allowed
from a given source address, or 'none' if there is no
limit. This limit is applied in addition to MaxStartups,
whichever is lower. The default is none.
PerSourceNetBlockSize
Specifies the number of bits of source address that are
grouped together for the purposes of applying
PerSourceMaxStartups limits. Values for IPv4 and
optionally IPv6 may be specified, separated by a colon.
The default is 32:128, which means each address is
considered individually.
PidFile
Specifies the file that contains the process ID of the SSH
daemon, or none to not write one. 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.
PubkeyAcceptedAlgorithms
Specifies the signature algorithms that will be accepted
for public key authentication as a list of comma-separated
patterns. Alternately if the specified list begins with a
'+' character, then the specified algorithms will be
appended to the default set instead of replacing them. If
the specified list begins with a '-' character, then the
specified algorithms (including wildcards) will be removed
from the default set instead of replacing them. If the
specified list begins with a '^' character, then the
specified algorithms will be placed at the head of the
default set. The default for this option is:
ssh-ed25519-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
rsa-sha2-512,rsa-sha2-256,ssh-rsa
The list of available signature algorithms may also be
obtained using "ssh -Q PubkeyAcceptedAlgorithms".
PubkeyAuthOptions
Sets one or more public key authentication options. The
supported keywords are: none (the default; indicating no
additional options are enabled), touch-required and
verify-required.
The touch-required option causes public key authentication
using a FIDO authenticator algorithm (i.e. ecdsa-sk or
ed25519-sk) to always require the signature to attest that
a physically present user explicitly confirmed the
authentication (usually by touching the authenticator). By
default, sshd(8) requires user presence unless overridden
with an authorized_keys option. The touch-required flag
disables this override.
The verify-required option requires a FIDO key signature
attest that the user was verified, e.g. via a PIN.
Neither the touch-required or verify-required options have
any effect for other, non-FIDO, public 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 by 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).
RDomain
Specifies an explicit routing domain that is applied after
authentication has completed. The user session, as well as
any forwarded or listening IP sockets, will be bound to
this rdomain(4). If the routing domain is set to %D, then
the domain in which the incoming connection was received
will be applied.
SecurityKeyProvider
Specifies a path to a library that will be used when
loading FIDO authenticator-hosted keys, overriding the
default of using the built-in USB HID support.
SetEnv Specifies one or more environment variables to set in child
sessions started by sshd(8) as 'NAME=VALUE'. The
environment value may be quoted (e.g. if it contains
whitespace characters). Environment variables set by
SetEnv override the default environment and any variables
specified by the user via AcceptEnv or
PermitUserEnvironment.
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 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.
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.
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 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 to check that the resolved host name for the
remote IP address maps back to the very same IP address.
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.
UsePAM Enables the Pluggable Authentication Module interface. If
set to yes this will enable PAM authentication using
KbdInteractiveAuthentication and PasswordAuthentication in
addition to PAM account and session module processing for
all authentication types.
Because PAM keyboard-interactive authentication usually
serves an equivalent role to password authentication, you
should disable either PasswordAuthentication or
KbdInteractiveAuthentication.
If UsePAM is enabled, you will not be able to run sshd(8)
as a non-root user. The default is no.
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), 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.
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, or
none to not use one. The default is /usr/X11R6/bin/xauth.
