sshd_config(5)
NAME
sshd_config -- OpenSSH SSH daemon configuration file
SYNOPSIS
/etc/ssh/sshd_config
DESCRIPTION
sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file
specified with -f on the command line). The file contains keyword-argu-
ment pairs, one per line. Lines starting with `#' and empty lines are
interpreted as comments. Arguments may optionally be enclosed in double
quotes (") in order to represent arguments containing spaces.
The possible keywords and their meanings are as follows (note that key-
words 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 environ-
ment variable is always sent whenever the client requests a
pseudo-terminal as it is required by the protocol. Variables are
specified by name, which may contain the wildcard characters `*'
and `?'. Multiple environment variables may be separated by
whitespace or spread across multiple AcceptEnv directives. Be
warned that some environment variables could be used to bypass
restricted user environments. For this reason, care should be
taken in the use of this directive. The default is not to accept
any environment variables.
AddressFamily
Specifies which address family should be used by sshd(8). Valid
arguments are ``any'', ``inet'' (use IPv4 only), or ``inet6''
(use IPv6 only). The default is ``any''.
AllowAgentForwarding
Specifies whether ssh-agent(1) forwarding is permitted. The
default is ``yes''. Note that disabling agent forwarding does
not improve security unless users are also denied shell access,
as they can always install their own forwarders.
AllowGroups
This keyword can be followed by a list of group name patterns,
separated by spaces. If specified, login is allowed only for
users whose primary group or supplementary group list matches one
of the patterns. Only group names are valid; a numerical group
ID is not recognized. By default, login is allowed for all
groups. The allow/deny directives are processed in the following
order: DenyUsers, AllowUsers, DenyGroups, and finally
AllowGroups.
See PATTERNS in ssh_config(5) for more information on patterns.
AllowTcpForwarding
Specifies whether TCP forwarding is permitted. The available
options are ``yes'' or ``all'' to allow TCP forwarding, ``no'' to
prevent all TCP forwarding, ``local'' to allow local (from the
perspective of ssh(1)) forwarding only or ``remote'' to allow
remote forwarding only. The default is ``yes''. Note that dis-
abling TCP forwarding does not improve security unless users are
also denied shell access, as they can always install their own
forwarders.
AllowStreamLocalForwarding
Specifies whether StreamLocal (Unix-domain socket) forwarding is
permitted. The available options are ``yes'' or ``all'' to allow
StreamLocal forwarding, ``no'' to prevent all StreamLocal for-
warding, ``local'' to allow local (from the perspective of
ssh(1)) forwarding only or ``remote'' to allow remote forwarding
only. The default is ``yes''. Note that disabling StreamLocal
forwarding does not improve security unless users are also denied
shell access, as they can always install their own forwarders.
AllowUsers
This keyword can be followed by a list of user name patterns,
separated by spaces. If specified, login is allowed only for
user names that match one of the patterns. Only user names are
valid; a numerical user ID is not recognized. By default, login
is allowed for all users. If the pattern takes the form
USER@HOST then USER and HOST are separately checked, restricting
logins to particular users from particular hosts. 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, an argument of ``publickey,password
publickey,keyboard-interactive'' would require the user to com-
plete 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 exam-
ple, 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 interac-
tive authentication to the ``bsdauth'' device.
If the ``publickey'' method is listed more than once, sshd(8)
verifies that keys that have been used successfully are not
reused for subsequent authentications. For example, an
AuthenticationMethods of ``publickey,publickey'' will require
successful authentication using two different public keys.
This option will yield a fatal error if enabled if protocol 1 is
also enabled. Note that each authentication method listed should
also be explicitly enabled in the configuration. The default
``any'' is not to require multiple authentication; successful
completion of a single authentication method is sufficient.
AuthorizedKeysCommand
Specifies a program to be used to look up the user's public keys.
The program must be owned by root, not writable by group or oth-
ers and specified by an absolute path.
Arguments to AuthorizedKeysCommand may be provided using the fol-
lowing tokens, which will be expanded at runtime: %% is replaced
by a literal '%', %u is replaced by the username being authenti-
cated, %h is replaced by the home directory of the user being
authenticated, %t is replaced with the key type offered for
authentication, %f is replaced with the fingerprint of the key,
and %k is replaced with the key being offered for authentication.
If no arguments are specified then the username of the target
user will be supplied.
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 authentica-
tion 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 that can be used
for user authentication. The format is described in the AUTHO-
RIZED_KEYS FILE FORMAT section of sshd(8). AuthorizedKeysFile
may contain tokens of the form %T which are substituted during
connection setup. The following tokens are defined: %% is
replaced by a literal '%', %h is replaced by the home directory
of the user being authenticated, and %u is replaced by the user-
name of that user. After expansion, AuthorizedKeysFile is taken
to be an absolute path or one relative to the user's home direc-
tory. 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 pro-
gram must be owned by root, not writable by group or others and
specified by an absolute path.
Arguments to AuthorizedPrincipalsCommand may be provided using
the following tokens, which will be expanded at runtime: %% is
replaced by a literal '%', %u is replaced by the username being
authenticated and %h is replaced by the home directory of the
user being authenticated.
The program should produce on standard output zero or more lines
of AuthorizedPrincipalsFile output. If either
AuthorizedPrincipalsCommand or AuthorizedPrincipalsFile is speci-
fied, then certificates offered by the client for authentication
must contain a principal that is listed. By default, no Autho-
rizedPrincipalsCommand is run.
AuthorizedPrincipalsCommandUser
Specifies the user under whose account the AuthorizedPrinci-
palsCommand is run. It is recommended to use a dedicated user
that has no other role on the host than running authorized prin-
cipals 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.
AuthorizedPrincipalsFile may contain tokens of the form %T which
are substituted during connection setup. The following tokens
are defined: %% is replaced by a literal '%', %h is replaced by
the home directory of the user being authenticated, and %u is
replaced by the username of that user. After expansion,
AuthorizedPrincipalsFile is taken to be an absolute path or one
relative to the user's home directory.
The default is ``none'', i.e. not to use a principals file - in
this case, the username of the user must appear in a certifi-
cate's principals list for it to be accepted. Note that
AuthorizedPrincipalsFile is only used when authentication pro-
ceeds 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 com-
ponents of the pathname are root-owned directories which are not
writable by any other user or group. After the chroot, sshd(8)
changes the working directory to the user's home directory.
The pathname may contain the following tokens that are expanded
at runtime once the connecting user has been authenticated: %% is
replaced by a literal '%', %h is replaced by the home directory
of the user being authenticated, and %u is replaced by the user-
name of that user.
The ChrootDirectory must contain the necessary files and directo-
ries 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 `+' character,
then the specified ciphers will be appended to 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 the -Q
option of ssh(1) with an argument of ``cipher''.
ClientAliveCountMax
Sets the number of client alive messages (see below) which may be
sent without sshd(8) receiving any messages back from the client.
If this threshold is reached while client alive messages are
being sent, sshd will disconnect the client, terminating the ses-
sion. It is important to note that the use of client alive mes-
sages is very different from TCPKeepAlive (below). The client
alive messages are sent through the encrypted channel and there-
fore will not be spoofable. The TCP keepalive option enabled by
TCPKeepAlive is spoofable. The client alive mechanism is valu-
able when the client or server depend on knowing when a connec-
tion has become inactive.
The default value is 3. If ClientAliveInterval (see below) is
set to 15, and ClientAliveCountMax is left at the default, unre-
sponsive 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 allowed, or delayed until the
user has authenticated successfully. The argument must be
``yes'', ``delayed'', or ``no''. The default is ``delayed''.
DenyGroups
This keyword can be followed by a list of group name patterns,
separated by spaces. Login is disallowed for users whose primary
group or supplementary group list matches one of the patterns.
Only group names are valid; a numerical group ID is not recog-
nized. 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 numeri-
cal 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 con-
tain addresses to match in CIDR address/masklen format. The
allow/deny directives are processed in the following order:
DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
See PATTERNS in ssh_config(5) for more information on patterns.
FingerprintHash
Specifies the hash algorithm used when logging key fingerprints.
Valid options are: ``md5'' and ``sha256''. The default is
``md5''.
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 con-
nect. The argument may be ``no'' to force remote port forward-
ings 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 cur-
rent 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 `+' character, then the speci-
fied key types will be appended to the default set instead of
replacing them. The default for this option is:
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa
The -Q option of ssh(1) may be used to list supported key types.
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 cer-
tificate's public key must match a private host key already spec-
ified by HostKey. The default behaviour of sshd(8) is not to
load any certificates.
HostKey
Specifies a file containing a private host key used by SSH. The
default is /etc/ssh/ssh_host_key for protocol version 1, and
/etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key,
/etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key for
protocol version 2.
Note that sshd(8) will refuse to use a file if it is group/world-
accessible and that the HostKeyAlgorithms option restricts which
of the keys are actually used by sshd(8).
It is possible to have multiple host key files. ``rsa1'' keys
are used for version 1 and ``dsa'', ``ecdsa'', ``ed25519'' or
``rsa'' are used for version 2 of the SSH protocol. It is also
possible to specify public host key files instead. In this case
operations on the private key will be delegated to an
ssh-agent(1).
HostKeyAgent
Identifies the UNIX-domain socket used to communicate with an
agent that has access to the private host keys. If the string
``SSH_AUTH_SOCK'' is specified, the 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 the -Q
option of ssh(1) with an argument of ``key''.
IgnoreRhosts
Specifies that .rhosts and .shosts files will not be used in
RhostsRSAAuthentication or HostbasedAuthentication.
/etc/hosts.equiv and /etc/ssh/shosts.equiv are still used. The
default is ``yes''.
IgnoreUserKnownHosts
Specifies whether sshd(8) should ignore the user's
~/.ssh/known_hosts during RhostsRSAAuthentication or
HostbasedAuthentication. The default is ``no''.
IPQoS Specifies the IPv4 type-of-service or DSCP class for the connec-
tion. 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 uncondi-
tionally. If two values are specified, the first is automati-
cally selected for interactive sessions and the second for non-
interactive sessions. The default is ``lowdelay'' for interac-
tive 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 pass-
word 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 `+' character, then the specified methods
will be appended to the default set instead of replacing them.
The supported algorithms are:
curve25519-sha256@libssh.org
diffie-hellman-group1-sha1
diffie-hellman-group14-sha1
diffie-hellman-group-exchange-sha1
diffie-hellman-group-exchange-sha256
ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521
The default is:
curve25519-sha256@libssh.org,
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group14-sha1
The list of available key exchange algorithms may also be
obtained using the -Q option of ssh(1) with an argument of
``kex''.
KeyRegenerationInterval
In protocol version 1, the ephemeral server key is automatically
regenerated after this many seconds (if it has been used). The
purpose of regeneration is to prevent decrypting captured ses-
sions by later breaking into the machine and stealing the keys.
The key is never stored anywhere. If the value is 0, the key is
never regenerated. The default is 3600 (seconds).
ListenAddress
Specifies the local addresses sshd(8) should listen on. The fol-
lowing 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 suc-
cessfully 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) algo-
rithms. The MAC algorithm is used for data integrity protection.
Multiple algorithms must be comma-separated. If the specified
value begins with a `+' character, then the specified algorithms
will be appended to the default set instead of replacing them.
The algorithms that contain ``-etm'' calculate the MAC after
encryption (encrypt-then-mac). These are 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
the -Q option of ssh(1) with an argument of ``mac''.
Match Introduces a conditional block. If all of the criteria on the
Match line are satisfied, the keywords on the following lines
override those set in the global 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 oper-
ators described in the PATTERNS section of ssh_config(5).
The patterns in an Address criteria may additionally contain
addresses to match in CIDR address/masklen format, e.g.
``192.0.2.0/24'' or ``3ffe:ffff::/32''. Note that the mask
length provided must be consistent with the address - it is an
error to specify a mask length that is too long for the address
or one with bits set in this host portion of the address. For
example, ``192.0.2.0/33'' and ``192.0.2.0/8'' respectively.
Only a subset of keywords may be used on the lines following a
Match keyword. Available keywords are AcceptEnv,
AllowAgentForwarding, AllowGroups, AllowStreamLocalForwarding,
AllowTcpForwarding, AllowUsers, AuthenticationMethods,
AuthorizedKeysCommand, AuthorizedKeysCommandUser,
AuthorizedKeysFile, AuthorizedPrincipalsCommand,
AuthorizedPrincipalsCommandUser, AuthorizedPrincipalsFile,
Banner, ChrootDirectory, DenyGroups, DenyUsers, ForceCommand,
GatewayPorts, GSSAPIAuthentication, HostbasedAcceptedKeyTypes,
HostbasedAuthentication, HostbasedUsesNameFromPacketOnly, IPQoS,
KbdInteractiveAuthentication, KerberosAuthentication,
MaxAuthTries, MaxSessions, PasswordAuthentication,
PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY,
PermitTunnel, PermitUserRC, PubkeyAcceptedKeyTypes,
PubkeyAuthentication, RekeyLimit, RevokedKeys,
RhostsRSAAuthentication, RSAAuthentication, 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 for-
warding. The default is 10.
MaxStartups
Specifies the maximum number of concurrent unauthenticated con-
nections 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 per-
mitted. The forwarding specification must be one of the follow-
ing forms:
PermitOpen host:port
PermitOpen IPv4_addr:port
PermitOpen [IPv6_addr]:port
Multiple forwards may be specified by separating them with white-
space. An argument of ``any'' can be used to remove all restric-
tions and permit any forwarding requests. An argument of
``none'' can be used to prohibit all forwarding requests. The
wildcard ``*'' can be used for host or port to allow all hosts or
ports, respectively. By default all port forwarding requests are
permitted.
PermitRootLogin
Specifies whether root can log in using ssh(1). The argument
must be ``yes'', ``prohibit-password'', ``without-password'',
``forced-commands-only'', or ``no''. The default is
``prohibit-password''.
If this option is set to ``prohibit-password'' or
``without-password'', password and keyboard-interactive authenti-
cation 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.
PermitTunnel
Specifies whether tun(4) device forwarding is allowed. The argu-
ment 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.
PermitTTY
Specifies whether pty(4) allocation is permitted. The default is
``yes''.
PermitUserEnvironment
Specifies whether ~/.ssh/environment and environment= options in
~/.ssh/authorized_keys are processed by sshd(8). The default is
``no''. Enabling environment processing may enable users to
bypass access restrictions in some configurations using mecha-
nisms 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 dae-
mon, or ``none'' to not write one. The default is /etc/sshd.pid.
Port Specifies the port number that sshd(8) listens on. The default
is 22. Multiple options of this type are permitted. See also
ListenAddress.
PrintLastLog
Specifies whether sshd(8) should print the date and time of the
last user login when a user logs in interactively. The default
is ``yes''.
PrintMotd
Specifies whether sshd(8) should print /etc/motd when a user logs
in interactively. (On some systems it is also printed by the
shell, /etc/profile, or equivalent.) The default is ``yes''.
Protocol
Specifies the protocol versions sshd(8) supports. The possible
values are `1' and `2'. Multiple versions must be comma-sepa-
rated. The default is `2'. Protocol 1 suffers from a number of
cryptographic weaknesses and should not be used. It is only
offered to support legacy devices.
Note that the order of the protocol list does not indicate pref-
erence, because the client selects among multiple protocol ver-
sions offered by the server. Specifying ``2,1'' is identical to
``1,2''.
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 `+' character, then the speci-
fied key types will be appended to the default set instead of
replacing them. The default for this option is:
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa
The -Q option of ssh(1) may be used to list supported key types.
PubkeyAuthentication
Specifies whether public key authentication is allowed. The
default is ``yes''.
RekeyLimit
Specifies the maximum amount of data that may be transmitted
before the session key is renegotiated, optionally followed a
maximum amount of time that may pass before the session key is
renegotiated. The first argument is specified in bytes and may
have a suffix of `K', `M', or `G' to indicate Kilobytes,
Megabytes, or Gigabytes, respectively. The default is between
`1G' and `4G', depending on the cipher. The optional second
value is specified in seconds and may use any of the units docu-
mented in the TIME FORMATS section. The default value for
RekeyLimit is ``default none'', which means that rekeying is per-
formed 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 authenti-
cation. Note that if this file is not readable, then public key
authentication will be refused for all users. Keys may be speci-
fied 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 sec-
tion in ssh-keygen(1).
RhostsRSAAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication
together with successful RSA host authentication is allowed. The
default is ``no''. This option applies to protocol version 1
only.
RSAAuthentication
Specifies whether pure RSA authentication is allowed. The
default is ``yes''. This option applies to protocol version 1
only.
ServerKeyBits
Defines the number of bits in the ephemeral protocol version 1
server key. The default and minimum value is 1024.
StreamLocalBindMask
Sets the octal file creation mode mask (umask) used when creating
a Unix-domain socket file for local or remote port forwarding.
This option is only used for port forwarding to a Unix-domain
socket file.
The default value is 0177, which creates a Unix-domain socket
file that is readable and writable only by the owner. Note that
not all operating systems honor the file mode on Unix-domain
socket files.
StreamLocalBindUnlink
Specifies whether to remove an existing Unix-domain socket file
for local or remote port forwarding before creating a new one.
If the socket file already exists and StreamLocalBindUnlink is
not enabled, sshd will be unable to forward the port to the Unix-
domain socket file. This option is only used for port forwarding
to a Unix-domain socket file.
The argument must be ``yes'' or ``no''. The default is ``no''.
StrictModes
Specifies whether sshd(8) should check file modes and ownership
of the user's files and home directory before accepting login.
This is normally desirable because novices sometimes accidentally
leave their directory or files world-writable. The default is
``yes''. Note that this does not apply to ChrootDirectory, whose
permissions and ownership are checked unconditionally.
Subsystem
Configures an external subsystem (e.g. file transfer daemon).
Arguments should be a subsystem name and a command (with optional
arguments) to execute upon subsystem request.
The command sftp-server(8) implements the ``sftp'' file transfer
subsystem.
Alternately the name ``internal-sftp'' implements an in-process
``sftp'' server. This may simplify configurations using
ChrootDirectory to force a different filesystem root on clients.
By default no subsystems are defined.
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 tempo-
rarily, 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 authori-
ties that are trusted to sign user certificates for authentica-
tion, 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 authentica-
tion 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.
UseLogin
Specifies whether login(1) is used for interactive login ses-
sions. The default is ``no''. Note that login(1) is never used
for remote command execution. Note also, that if this is
enabled, X11Forwarding will be disabled because login(1) does not
know how to handle xauth(1) cookies. If UsePrivilegeSeparation
is specified, it will be disabled after authentication.
UsePAM Enables the Pluggable Authentication Module interface. If set to
``yes'' this will enable PAM authentication using
ChallengeResponseAuthentication and PasswordAuthentication in
addition to PAM account and session module processing for all
authentication types.
Because PAM challenge-response authentication usually serves an
equivalent role to password authentication, you should disable
either PasswordAuthentication or ChallengeResponseAuthentication.
If UsePAM is enabled, you will not be able to run sshd(8) as a
non-root user. The default is ``no''.
UsePrivilegeSeparation
Specifies whether sshd(8) separates privileges by creating an
unprivileged child process to deal with incoming network traffic.
After successful authentication, another process will be created
that has the privilege of the authenticated user. The goal of
privilege separation is to prevent privilege escalation by con-
taining any corruption within the unprivileged processes. The
argument must be ``yes'', ``no'', or ``sandbox''. If
UsePrivilegeSeparation is set to ``sandbox'' then the pre-authen-
tication unprivileged process is subject to additional restric-
tions. The default is ``sandbox''.
VersionAddendum
Optionally specifies additional text to append to the SSH proto-
col banner sent by the server upon connection. The default is
``none''.
X11DisplayOffset
Specifies the first display number available for sshd(8)'s X11
forwarding. This prevents sshd from interfering with real X11
servers. The default is 10.
X11Forwarding
Specifies whether X11 forwarding is permitted. The argument must
be ``yes'' or ``no''. The default is ``no''.
When X11 forwarding is enabled, there may be additional exposure
to the server and to client displays if the sshd(8) proxy display
is configured to listen on the wildcard address (see
X11UseLocalhost below), though this is not the default. Addi-
tionally, 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 war-
rant a ``no'' setting.
Note that disabling X11 forwarding does not prevent users from
forwarding X11 traffic, as users can always install their own
forwarders. X11 forwarding is automatically disabled if UseLogin
is enabled.
X11UseLocalhost
Specifies whether sshd(8) should bind the X11 forwarding server
to the loopback address or to the wildcard address. By default,
sshd binds the forwarding server to the loopback address and sets
the hostname part of the DISPLAY environment variable to
``localhost''. This prevents remote hosts from connecting to the
proxy display. However, some older X11 clients may not function
with this configuration. X11UseLocalhost may be set to ``no'' to
specify that the forwarding server should be bound to the wild-
card 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/bin/X11/xauth.
TIME FORMATS
sshd(8) command-line arguments and configuration file options that spec-
ify 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 fol-
lowing:
<none> seconds
s | S seconds
m | M minutes
h | H hours
d | D days
w | W weeks
Each member of the sequence is added together to calculate the total time
value.
Time format examples:
600 600 seconds (10 minutes)
10m 10 minutes
1h30m 1 hour 30 minutes (90 minutes)
FILES
/etc/ssh/sshd_config
Contains configuration data for sshd(8). This file should be
writable by root only, but it is recommended (though not neces-
sary) that it be world-readable.
SEE ALSO
sshd(8)
AUTHORS
OpenSSH is a derivative of the original and free ssh 1.2.12 release by
Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
de Raadt and Dug Song removed many bugs, re-added newer features and cre-
ated 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.
BSD December 6, 2017 BSD
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