NAME
sshd_config
—
OpenSSH daemon configuration
file
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
andSetEnv
in ssh_config(5) for how to configure the client. TheTERM
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 multipleAcceptEnv
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), orinet6
(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) orall
to allow StreamLocal forwarding,no
to prevent all StreamLocal forwarding,local
to allow local (from the perspective of ssh(1)) forwarding only orremote
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) orall
to allow TCP forwarding,no
to prevent all TCP forwarding,local
to allow local (from the perspective of ssh(1)) forwarding only orremote
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
,pam
, orskey
, depending on the server configuration. For example, "keyboard-interactive:bsdauth" would restrict keyboard interactive authentication to thebsdauth
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 usualAuthorizedKeysFile
files and will not be executed if a matching key is found there. By default, noAuthorizedKeysCommand
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. IfAuthorizedKeysCommand
is specified butAuthorizedKeysCommandUser
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 tonone
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 toAuthorizedPrincipalsCommand
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 eitherAuthorizedPrincipalsCommand
orAuthorizedPrincipalsFile
is specified, then certificates offered by the client for authentication must contain a principal that is listed. By default, noAuthorizedPrincipalsCommand
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. IfAuthorizedPrincipalsCommand
is specified butAuthorizedPrincipalsCommandUser
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 isnone
, 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 inTrustedUserCAKeys
and is not consulted for certification authorities trusted via ~/.ssh/authorized_keys, though theprincipals=
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:
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
Certificates signed using other algorithms will not be accepted for public key or host-based authentication.
ChallengeResponseAuthentication
- Specifies whether challenge-response authentication is allowed. All
authentication styles from
login.conf(5) are supported. 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.
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 byTCPKeepAlive
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, andClientAliveCountMax
is left at the default, unresponsive SSH clients will be disconnected after approximately 45 seconds. Setting a zeroClientAliveCountMax
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 foryes
) orno
. The default isyes
. 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 isno
. FingerprintHash
- Specifies the hash algorithm used when logging key fingerprints. Valid
options are:
md5
andsha256
. The default issha256
. 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 aMatch
block. The command originally supplied by the client is available in theSSH_ORIGINAL_COMMAND
environment variable. Specifying a command ofinternal-sftp
will force the use of an in-process SFTP server that requires no support files when used withChrootDirectory
. The default isnone
. 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 beno
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, orclientspecified
to allow the client to select the address to which the forwarding is bound. The default isno
. 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 tono
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 isyes
. HostbasedAcceptedKeyTypes
- Specifies the key types 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 key types
will be appended to the default set instead of replacing them. If the
specified list begins with a ‘-’ character, then the
specified key types (including wildcards) will be removed from the default
set instead of replacing them. If the specified list begins with a
‘^’ character, then the specified key types will be placed
at the head of the default set. 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, sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, ssh-ed25519-cert-v01@openssh.com, sk-ssh-ed25519-cert-v01@openssh.com, rsa-sha2-512-cert-v01@openssh.com, rsa-sha2-256-cert-v01@openssh.com, ssh-rsa-cert-v01@openssh.com, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, sk-ecdsa-sha2-nistp256@openssh.com, ssh-ed25519,sk-ssh-ed25519@openssh.com, rsa-sha2-512,rsa-sha2-256,ssh-rsa
The list of available key types may also be obtained using "ssh -Q 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 ofyes
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 isno
. 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 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, sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, ssh-ed25519-cert-v01@openssh.com, sk-ssh-ed25519-cert-v01@openssh.com, rsa-sha2-512-cert-v01@openssh.com, rsa-sha2-256-cert-v01@openssh.com, ssh-rsa-cert-v01@openssh.com, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, sk-ecdsa-sha2-nistp256@openssh.com, ssh-ed25519,sk-ssh-ed25519@openssh.com, rsa-sha2-512,rsa-sha2-256,ssh-rsa
The list of available key types 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 orno
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 aMatch
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, ornone
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 isaf21
(Low-Latency Data) for interactive sessions andcs1
(Lower Effort) for non-interactive sessions. KbdInteractiveAuthentication
- Specifies whether to allow keyboard-interactive authentication. The
argument to this keyword must be
yes
orno
. The default is to use whatever valueChallengeResponseAuthentication
is set to (by defaultyes
). 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 isno
. 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
- sntrup4591761x25519-sha512@tinyssh.org
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 allPort
options specified. The default is to listen on all local addresses on the current default routing domain. MultipleListenAddress
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.
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 anotherMatch
line or the end of the file. If a keyword appears in multipleMatch
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 tokenAll
which matches all criteria. The available criteria areUser
,Group
,Host
,LocalAddress
,LocalPort
,RDomain
, andAddress
(withRDomain
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 areAcceptEnv
,AllowAgentForwarding
,AllowGroups
,AllowStreamLocalForwarding
,AllowTcpForwarding
,AllowUsers
,AuthenticationMethods
,AuthorizedKeysCommand
,AuthorizedKeysCommandUser
,AuthorizedKeysFile
,AuthorizedPrincipalsCommand
,AuthorizedPrincipalsCommandUser
,AuthorizedPrincipalsFile
,Banner
,ChrootDirectory
,ClientAliveCountMax
,ClientAliveInterval
,DenyGroups
,DenyUsers
,ForceCommand
,GatewayPorts
,GSSAPIAuthentication
,HostbasedAcceptedKeyTypes
,HostbasedAuthentication
,HostbasedUsesNameFromPacketOnly
,IgnoreRhosts
,Include
,IPQoS
,KbdInteractiveAuthentication
,KerberosAuthentication
,LogLevel
,MaxAuthTries
,MaxSessions
,PasswordAuthentication
,PermitEmptyPasswords
,PermitListen
,PermitOpen
,PermitRootLogin
,PermitTTY
,PermitTunnel
,PermitUserRC
,PubkeyAcceptedKeyTypes
,PubkeyAuthentication
,RekeyLimit
,RevokedKeys
,RDomain
,SetEnv
,StreamLocalBindMask
,StreamLocalBindUnlink
,TrustedUserCAKeys
,X11DisplayOffset
,X11Forwarding
andX11UseLocalhost
. 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).
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
portPermitListen
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 ofnone
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 theGatewayPorts
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:portPermitOpen
IPv4_addr:portPermitOpen
[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 ofnone
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
, orno
. The default isprohibit-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), orno
. Specifyingyes
permits bothpoint-to-point
andethernet
. The default isno
.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 areyes
,no
or a pattern-list specifying which environment variable names to accept (for example "LANG,LC_*"). The default isno
. Enabling environment processing may enable users to bypass access restrictions in some configurations using mechanisms such asLD_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 /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
. PubkeyAcceptedKeyTypes
- Specifies the key types 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 key types will be appended to the default set instead of
replacing them. If the specified list begins with a ‘-’
character, then the specified key types (including wildcards) will be
removed from the default set instead of replacing them. If the specified
list begins with a ‘^’ character, then the specified key
types will be placed at the head of the default set. 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, sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, ssh-ed25519-cert-v01@openssh.com, sk-ssh-ed25519-cert-v01@openssh.com, rsa-sha2-512-cert-v01@openssh.com, rsa-sha2-256-cert-v01@openssh.com, ssh-rsa-cert-v01@openssh.com, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, sk-ecdsa-sha2-nistp256@openssh.com, ssh-ed25519,sk-ssh-ed25519@openssh.com, rsa-sha2-512,rsa-sha2-256,ssh-rsa
The list of available key types may also be obtained using "ssh -Q PubkeyAcceptedKeyTypes".
PubkeyAuthOptions
- Sets one or more public key authentication options. The supported keywords
are:
none
(the default; indicating no additional options are enabled),touch-required
andverify-required
.The
touch-required
option causes public key authentication using a FIDO authenticator algorithm (i.e.ecdsa-sk
ored25519-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. Thetouch-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
orverify-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 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
isdefault 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 and 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 viaAcceptEnv
orPermitUserEnvironment
. 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
orno
. The default isno
. 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 toChrootDirectory
, 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 usingChrootDirectory
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 usingTrustedUserCAKeys
. 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_keysfrom
andsshd_config
Match
Host
directives. 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
orno
. The default isno
.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 forForwardX11
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 ano
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 tolocalhost
. 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 tono
to specify that the forwarding server should be bound to the wildcard address. The argument must beyes
orno
. The default isyes
. XAuthLocation
- Specifies the full pathname of the
xauth(1) program, or
none
to not use one. The default is /usr/X11R6/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:
Each member of the sequence is added together to calculate the total time value.
Time format examples:
- 600
- 600 seconds (10 minutes)
- 10m
- 10 minutes
- 1h30m
- 1 hour 30 minutes (90 minutes)
TOKENS
Arguments to some keywords can make use of tokens, which are expanded at runtime:
- %%
- A literal ‘%’.
- %D
- The routing domain in which the incoming connection was received.
- %F
- The fingerprint of the CA key.
- %f
- The fingerprint of the key or certificate.
- %h
- The home directory of the user.
- %i
- The key ID in the certificate.
- %K
- The base64-encoded CA key.
- %k
- The base64-encoded key or certificate for authentication.
- %s
- The serial number of the certificate.
- %T
- The type of the CA key.
- %t
- The key or certificate type.
- %U
- The numeric user ID of the target user.
- %u
- The username.
AuthorizedKeysCommand
accepts the tokens
%%, %f, %h, %k, %t, %U, and %u.
AuthorizedKeysFile
accepts the tokens %%,
%h, %U, and %u.
AuthorizedPrincipalsCommand
accepts the
tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.
AuthorizedPrincipalsFile
accepts the
tokens %%, %h, %U, and %u.
ChrootDirectory
accepts the tokens %%, %h,
%U, and %u.
RoutingDomain
accepts the token %D.
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
AUTHORS
OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer features and created OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support for privilege separation.