SSHD_CONFIG(5) File Formats Manual SSHD_CONFIG(5)
NAME
sshd_config M-bM-^@M-^S OpenSSH SSH daemon configuration file
SYNOPSIS
/etc/ssh/sshd_config
DESCRIPTION
sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file
specified with -f on the command line). The file contains keyword-
argument pairs, one per line. Lines starting with M-bM-^@M-^X#M-bM-^@M-^Y 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 in
ssh_config(5) for how to configure the client. The TERM
environment variable is always sent whenever the client requests
a pseudo-terminal as it is required by the protocol. Variables
are specified by name, which may contain the wildcard characters
M-bM-^@M-^X*M-bM-^@M-^Y and M-bM-^@M-^X?M-bM-^@M-^Y. 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 directives are processed in the following
order: DenyUsers, AllowUsers, DenyGroups, and finally
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 directives are processed
in the following order: DenyUsers, AllowUsers, DenyGroups, and
finally AllowGroups.
See PATTERNS in ssh_config(5) for more information on patterns.
AuthenticationMethods
Specifies the authentication methods that must be successfully
completed for a user to be granted access. This option must be
followed by one or more comma-separated lists of authentication
method names, 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, or skey,
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.
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)). If a
key supplied by AuthorizedKeysCommand does not successfully
authenticate and authorize the user then public key
authentication continues using the usual AuthorizedKeysFile
files. By default, no AuthorizedKeysCommand is run.
AuthorizedKeysCommandUser
Specifies the user under whose account the AuthorizedKeysCommand
is run. It is recommended to use a dedicated user that has no
other role on the host than running authorized keys commands. 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 M-bM-^@M-^X#M-bM-^@M-^Y 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 M-bM-^@M-^S 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.
ChallengeResponseAuthentication
Specifies whether challenge-response authentication is allowed
(e.g. via PAM or through authentication styles supported in
login.conf(5)) The default is yes.
ChrootDirectory
Specifies the pathname of a directory to chroot(2) to after
authentication. At session startup sshd(8) checks that all
components of the pathname are root-owned directories which are
not writable by any other user or group. After the chroot,
sshd(8) changes the working directory to the user's home
directory. 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 value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character,
then the specified ciphers will be appended to the default set
instead of replacing them. If the specified value begins with a
M-bM-^@M-^X-M-bM-^@M-^Y character, then the specified ciphers (including wildcards)
will be removed from the default set instead of replacing them.
The supported ciphers are:
3des-cbc
aes128-cbc
aes192-cbc
aes256-cbc
aes128-ctr
aes192-ctr
aes256-ctr
aes128-gcm@openssh.com
aes256-gcm@openssh.com
arcfour
arcfour128
arcfour256
blowfish-cbc
cast128-cbc
chacha20-poly1305@openssh.com
The default is:
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
inactive.
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.
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 directives are processed in the following order:
DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
See PATTERNS in ssh_config(5) for more information on patterns.
DenyUsers
This keyword can be followed by a list of user name patterns,
separated by spaces. Login is disallowed for user names that
match one of the patterns. Only user names are valid; a
numerical user ID is not recognized. By default, login is
allowed for all users. If the pattern takes the form USER@HOST
then USER and HOST are separately checked, restricting logins to
particular users from particular hosts. HOST criteria may
additionally contain addresses to match in CIDR address/masklen
format. The allow/deny directives are processed in the following
order: DenyUsers, AllowUsers, DenyGroups, and finally
AllowGroups.
See PATTERNS in ssh_config(5) for more information on patterns.
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.
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.
HostbasedAcceptedKeyTypes
Specifies the key types that will be accepted for hostbased
authentication as a comma-separated pattern list. Alternately if
the specified value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the
specified key types will be appended to the default set instead
of replacing them. If the specified value begins with a M-bM-^@M-^X-M-bM-^@M-^Y
character, then the specified key types (including wildcards)
will be removed from the default set instead of replacing them.
The default for this option is:
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa
The list of available key types may also be obtained using "ssh
-Q key".
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_dsa_key,
/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,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa
The list of available key types may also be obtained using "ssh
-Q key".
IgnoreRhosts
Specifies that .rhosts and .shosts files will not be used in
HostbasedAuthentication.
/etc/hosts.equiv and /etc/shosts.equiv are still used. The
default is yes.
IgnoreUserKnownHosts
Specifies whether sshd(8) should ignore the user's
~/.ssh/known_hosts during HostbasedAuthentication. The default
is no.
IPQoS Specifies the IPv4 type-of-service or DSCP class for the
connection. Accepted values are af11, af12, af13, af21, af22,
af23, af31, af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3,
cs4, cs5, cs6, cs7, ef, lowdelay, throughput, reliability, or a
numeric value. This option may take one or two arguments,
separated by whitespace. If one argument is specified, it is
used as the packet class unconditionally. If two values are
specified, the first is automatically selected for interactive
sessions and the second for non-interactive sessions. The
default is lowdelay for interactive sessions and throughput for
non-interactive sessions.
KbdInteractiveAuthentication
Specifies whether to allow keyboard-interactive authentication.
The argument to this keyword must be yes or no. The default is
to use whatever value ChallengeResponseAuthentication is set to
(by default yes).
KerberosAuthentication
Specifies whether the password provided by the user for
PasswordAuthentication will be validated through the Kerberos
KDC. To use this option, the server needs a Kerberos servtab
which allows the verification of the KDC's identity. The default
is no.
KerberosGetAFSToken
If AFS is active and the user has a Kerberos 5 TGT, attempt to
acquire an AFS token before accessing the user's home directory.
The default is no.
KerberosOrLocalPasswd
If password authentication through Kerberos fails then the
password will be validated via any additional local mechanism
such as /etc/passwd. The default is yes.
KerberosTicketCleanup
Specifies whether to automatically destroy the user's ticket
cache file on logout. The default is yes.
KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms. Multiple
algorithms must be comma-separated. Alternately if the specified
value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the specified methods
will be appended to the default set instead of replacing them.
If the specified value begins with a M-bM-^@M-^X-M-bM-^@M-^Y character, then the
specified methods (including wildcards) will be removed from the
default set instead of replacing them. The supported algorithms
are:
curve25519-sha256
curve25519-sha256@libssh.org
diffie-hellman-group1-sha1
diffie-hellman-group14-sha1
diffie-hellman-group-exchange-sha1
diffie-hellman-group-exchange-sha256
ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521
The default is:
curve25519-sha256,curve25519-sha256@libssh.org,
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group14-sha1
The list of available key exchange algorithms may also be
obtained using "ssh -Q kex".
ListenAddress
Specifies the local addresses sshd(8) should listen on. The
following forms may be used:
ListenAddress host|IPv4_addr|IPv6_addr
ListenAddress host|IPv4_addr:port
ListenAddress [host|IPv6_addr]:port
If port is not specified, sshd will listen on the address and all
Port options specified. The default is to listen on all local
addresses. Multiple ListenAddress options are permitted.
LoginGraceTime
The server disconnects after this time if the user has not
successfully logged in. If the value is 0, there is no time
limit. The default is 120 seconds.
LogLevel
Gives the verbosity level that is used when logging messages from
sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO,
VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO.
DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
higher levels of debugging output. Logging with a DEBUG level
violates the privacy of users and is not recommended.
MACs Specifies the available MAC (message authentication code)
algorithms. The MAC algorithm is used for data integrity
protection. Multiple algorithms must be comma-separated. If the
specified value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the specified
algorithms will be appended to the default set instead of
replacing them. If the specified value begins with a M-bM-^@M-^X-M-bM-^@M-^Y
character, then the specified algorithms (including wildcards)
will be removed from the default set instead of replacing them.
The algorithms that contain "-etm" calculate the MAC after
encryption (encrypt-then-mac). These are considered safer and
their use recommended. The supported MACs are:
hmac-md5
hmac-md5-96
hmac-ripemd160
hmac-sha1
hmac-sha1-96
hmac-sha2-256
hmac-sha2-512
umac-64@openssh.com
umac-128@openssh.com
hmac-md5-etm@openssh.com
hmac-md5-96-etm@openssh.com
hmac-ripemd160-etm@openssh.com
hmac-sha1-etm@openssh.com
hmac-sha1-96-etm@openssh.com
hmac-sha2-256-etm@openssh.com
hmac-sha2-512-etm@openssh.com
umac-64-etm@openssh.com
umac-128-etm@openssh.com
The default is:
umac-64-etm@openssh.com,umac-128-etm@openssh.com,
hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
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, and
Address. The match patterns may consist of single entries or
comma-separated lists and may use the wildcard and negation
operators described in the PATTERNS section of ssh_config(5).
The patterns in an Address criteria may additionally contain
addresses to match in CIDR address/masklen format, 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, ForceCommand,
GatewayPorts, GSSAPIAuthentication, HostbasedAcceptedKeyTypes,
HostbasedAuthentication, HostbasedUsesNameFromPacketOnly, IPQoS,
KbdInteractiveAuthentication, KerberosAuthentication,
MaxAuthTries, MaxSessions, PasswordAuthentication,
PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY,
PermitTunnel, PermitUserRC, PubkeyAcceptedKeyTypes,
PubkeyAuthentication, RekeyLimit, RevokedKeys,
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).
PasswordAuthentication
Specifies whether password authentication is allowed. The
default is yes.
PermitEmptyPasswords
When password authentication is allowed, it specifies whether the
server allows login to accounts with empty password strings. The
default is no.
PermitOpen
Specifies the destinations to which TCP port forwarding is
permitted. The forwarding specification must be one of the
following forms:
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 M-bM-^@M-^X*M-bM-^@M-^Y can be used for host or port to allow all hosts or
ports, respectively. By default all port forwarding requests are
permitted.
PermitRootLogin
Specifies whether root can log in using ssh(1). The argument
must be yes, prohibit-password, without-password,
forced-commands-only, or no. The default is prohibit-password.
If this option is set to prohibit-password or without-password,
password and keyboard-interactive authentication are disabled for
root.
If this option is set to forced-commands-only, root login with
public key authentication will be allowed, but only if the
command option has been specified (which may be useful for taking
remote backups even if root login is normally not allowed). All
other 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). The default is
no. Enabling environment processing may enable users to bypass
access restrictions in some configurations using mechanisms such
as LD_PRELOAD.
PermitUserRC
Specifies whether any ~/.ssh/rc file is executed. The default is
yes.
PidFile
Specifies the file that contains the process ID of the SSH
daemon, 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 comma-separated pattern list. Alternately if
the specified value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the
specified key types will be appended to the default set instead
of replacing them. If the specified value begins with a M-bM-^@M-^X-M-bM-^@M-^Y
character, then the specified key types (including wildcards)
will be removed from the default set instead of replacing them.
The default for this option is:
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa
The list of available key types may also be obtained using "ssh
-Q key".
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 M-bM-^@M-^XKM-bM-^@M-^Y, M-bM-^@M-^XMM-bM-^@M-^Y, or M-bM-^@M-^XGM-bM-^@M-^Y to indicate Kilobytes,
Megabytes, or Gigabytes, respectively. The default is between
M-bM-^@M-^X1GM-bM-^@M-^Y and M-bM-^@M-^X4GM-bM-^@M-^Y, 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).
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 M-bM-^@M-^X#M-bM-^@M-^Y 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
ChallengeResponseAuthentication and PasswordAuthentication in
addition to PAM account and session module processing for all
authentication types.
Because PAM challenge-response authentication usually serves an
equivalent role to password authentication, you should disable
either PasswordAuthentication or ChallengeResponseAuthentication.
If UsePAM is enabled, you will not be able to run sshd(8) as a
non-root user. The default is no.
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.
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:
M-bM-^_M-(noneM-bM-^_M-) seconds
s | S seconds
m | M minutes
h | H hours
d | D days
w | W weeks
Each member of the sequence is added together to calculate the total time
value.
Time format examples:
600 600 seconds (10 minutes)
10m 10 minutes
1h30m 1 hour 30 minutes (90 minutes)
TOKENS
Arguments to some keywords can make use of tokens, which are expanded at
runtime:
%% A literal M-bM-^@M-^X%M-bM-^@M-^Y.
%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 username.
AuthorizedKeysCommand accepts the tokens %%, %f, %h, %k, %t, and %u.
AuthorizedKeysFile accepts the tokens %%, %h, and %u.
AuthorizedPrincipalsCommand accepts the tokens %%, %F, %f, %h, %i, %K,
%k, %s, %T, %t, and %u.
AuthorizedPrincipalsFile accepts the tokens %%, %h, and %u.
ChrootDirectory accepts the tokens %%, %h, and %u.
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
sftp-server(8), sshd(8)
AUTHORS
OpenSSH is a derivative of the original and free ssh 1.2.12 release by
Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
de Raadt and Dug Song removed many bugs, re-added newer features and
created OpenSSH. Markus Friedl contributed the support for SSH protocol
versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
for privilege separation.
OpenBSD 6.0 March 14, 2017 OpenBSD 6.0