SSHD_CONFIG(5)SSHD_CONFIG(5)NAMEsshd_config - OpenSSH SSH daemon configuration file
SYNOPSIS
/etc/ssh/sshd_config
DESCRIPTIONsshd(8) reads configuration data from /etc/ssh/sshd_config (or the file
specified with -f on the command line). The file contains keyword-
argument pairs, one per line. Lines starting with `#' and empty lines
are interpreted as 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_con‐
fig(5) for how to configure the client. Note that environment
passing is only supported for protocol 2. Variables are speci‐
fied by name, which may contain the wildcard characters `*' and
`?'. Multiple environment variables may be separated by white‐
space 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 fol‐
lowing order: DenyUsers, AllowUsers, DenyGroups, and finally
AllowGroups.
See PATTERNS in ssh_config(5) for more information on patterns.
AllowTcpForwarding
Specifies whether TCP forwarding is permitted. The available
options are ``yes'' or ``all'' to allow TCP forwarding, ``no''
to prevent all TCP forwarding, ``local'' to allow local (from
the perspective of ssh(1)) forwarding only or ``remote'' to
allow remote forwarding only. The default is ``yes''. Note
that disabling TCP forwarding does not improve security unless
users are also denied shell access, as they can always install
their own forwarders.
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. 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. Successful authentication requires completion of
every method in at least one of these lists.
For example, an argument of ``publickey,password publickey,key‐
board-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-interac‐
tive 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 inter‐
active authentication to the ``bsdauth'' device.
This option is only available for SSH protocol 2 and 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 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 and not writable by
group or others. It will be invoked with a single argument of
the username being authenticated, and should produce on standard
output zero or more lines of authorized_keys output (see AUTHO‐
RIZED_KEYS in sshd(8)) . If a key supplied by Authorized‐
KeysCommand 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.
AuthorizedKeysFile
Specifies the file that contains the public keys that can be
used for user authentication. The format is described in the
AUTHORIZED_KEYS FILE FORMAT section of sshd(8). Authorized‐
KeysFile 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 direc‐
tory of the user being authenticated, and %u is replaced by the
username of that user. 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 white‐
space. The default is ``.ssh/authorized_keys''.ssh/autho‐
rized_keys2 .
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, Autho‐
rizedPrincipalsFile is taken to be an absolute path or one rela‐
tive 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 Autho‐
rizedPrincipalsFile 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. This option is only available for
protocol version 2. By default, no banner is displayed.
ChallengeResponseAuthentication
Specifies whether challenge-response authentication is allowed
(e.g. via PAM or though authentication styles supported in
login.conf(5)) The default is ``yes''.
ChrootDirectory
Specifies the pathname of a directory to chroot(2) to after
authentication. All components of the pathname must be root-
owned directories that 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 direc‐
tory of the user being authenticated, and %u is replaced by the
username of that user.
The ChrootDirectory must contain the necessary files and direc‐
tories to support the user's session. For an interactive ses‐
sion 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), arandom(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 do require /dev/log inside the
chroot directory (see sftp-server(8) for details).
The default is not to chroot(2).
Ciphers
Specifies the ciphers allowed for protocol version 2. Multiple
ciphers must be comma-separated. 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'', ``arc‐
four128'', ``arcfour256'', ``arcfour'', ``blowfish-cbc'',
``cast128-cbc'', and ``chacha20-poly1305@openssh.com''.
The default is:
aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
aes128-gcm@openssh.com,aes256-gcm@openssh.com,
chacha20-poly1305@openssh.com,
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
aes256-cbc,arcfour
The list of available ciphers may also be obtained using the -Q
option of ssh(1).
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 mes‐
sages are being sent, sshd will disconnect the client, terminat‐
ing the session. It is important to note that the use of client
alive messages is very different from TCPKeepAlive (below). The
client alive messages are sent through the encrypted channel and
therefore will not be spoofable. The TCP keepalive option
enabled by TCPKeepAlive is spoofable. The client alive mecha‐
nism is valuable when the client or server depend on knowing
when a connection 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. This option applies to protocol version 2 only.
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. This option applies to protocol ver‐
sion 2 only.
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 pri‐
mary group or supplementary group list matches one of the pat‐
terns. 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 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 particu‐
lar users from particular hosts. The allow/deny directives are
processed in the following order: DenyUsers, AllowUsers, Deny‐
Groups, and finally AllowGroups.
See PATTERNS in ssh_config(5) for more information on patterns.
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 subsys‐
tem 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 ChrootDi‐
rectory.
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 for‐
wardings 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''. Note that this option applies
to protocol version 2 only.
GSSAPICleanupCredentials
Specifies whether to automatically destroy the user's creden‐
tials cache on logout. The default is ``yes''. Note that this
option applies to protocol version 2 only.
HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication
together with successful public key client host authentication
is allowed (host-based authentication). This option is similar
to RhostsRSAAuthentication and applies to protocol version 2
only. 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 HostbasedAuthenti‐
cation. 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
specified by HostKey. The default behaviour of sshd(8) is not
to load any certificates.
HostKey
Specifies a file containing a private host key used by SSH. The
default is /etc/ssh/ssh_host_key for protocol version 1, and
/etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key,
/etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key for
protocol version 2. Note that sshd(8) will refuse to use a file
if it is group/world-accessible. It is possible to have multi‐
ple 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
``SSH_AUTH_SOCK'' is specified, the location of the socket will
be read from the SSH_AUTH_SOCK environment variable.
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 Host‐
basedAuthentication. 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'', ``lowde‐
lay'', ``throughput'', ``reliability'', or a numeric value.
This option may take one or two arguments, separated by white‐
space. 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 sec‐
ond 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 Passwor‐
dAuthentication 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. The default is
curve25519-sha256@libssh.org,
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group-exchange-sha1,
diffie-hellman-group14-sha1,
diffie-hellman-group1-sha1
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
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 prior Port options specified. The default is to listen on
all local addresses. Multiple ListenAddress options are permit‐
ted. Additionally, any Port options must precede this option
for non-port qualified addresses.
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 recom‐
mended.
MACs Specifies the available MAC (message authentication code) algo‐
rithms. The MAC algorithm is used in protocol version 2 for
data integrity protection. Multiple algorithms must be comma-
separated. The algorithms that contain ``-etm'' calculate the
MAC after encryption (encrypt-then-mac). These are considered
safer and their use recommended. The default is:
hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
umac-64-etm@openssh.com,umac-128-etm@openssh.com,
hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,
hmac-md5-96-etm@openssh.com,
hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com,
hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,
hmac-sha1-96,hmac-md5-96
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 satisified,
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, 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, AllowAgentFor‐
warding, AllowGroups, AllowTcpForwarding, AllowUsers, Authenti‐
cationMethods, AuthorizedKeysCommand, AuthorizedKeysCommandUser,
AuthorizedKeysFile, AuthorizedPrincipalsFile, Banner, ChrootDi‐
rectory, DenyGroups, DenyUsers, ForceCommand, GatewayPorts, GSS‐
APIAuthentication, HostbasedAuthentication, HostbasedUsesName‐
FromPacketOnly, KbdInteractiveAuthentication, KerberosAuthenti‐
cation, MaxAuthTries, MaxSessions, PasswordAuthentication, Per‐
mitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY, Per‐
mitTunnel, PubkeyAuthentication, RekeyLimit, RhostsRSAAuthenti‐
cation, RSAAuthentication, X11DisplayOffset, X11Forwarding and
X11UseLocalHost.
MaxAuthTries
Specifies the maximum number of authentication attempts permit‐
ted 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 sessions permitted per net‐
work connection. 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
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. By
default all port forwarding requests are permitted.
PermitRootLogin
Specifies whether root can log in using ssh(1). The argument
must be ``yes'', ``without-password'', ``forced-commands-only'',
or ``no''. The default is ``yes''.
If this option is set to ``without-password'', password authen‐
tication is 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 tak‐
ing 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
argument must be ``yes'', ``point-to-point'' (layer 3), ``ether‐
net'' (layer 2), or ``no''. Specifying ``yes'' permits both
``point-to-point'' and ``ethernet''. The default is ``no''.
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.
PidFile
Specifies the file that contains the process ID of the SSH dae‐
mon. The default is /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''.
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'. Note that the order of the protocol
list does not indicate preference, because the client selects
among multiple protocol versions offered by the server. Speci‐
fying ``2,1'' is identical to ``1,2''.
PubkeyAuthentication
Specifies whether public key authentication is allowed. The
default is ``yes''. Note that this option applies to protocol
version 2 only.
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
performed after the cipher's default amount of data has been
sent or received and no time based rekeying is done. This
option applies to protocol version 2 only.
RevokedKeys
Specifies revoked public keys. Keys listed in this file will be
refused for public key authentication. Note that if this file
is not readable, then public key authentication will be refused
for all users. Keys may be specified as a text file, listing
one public key per line, or as an OpenSSH Key Revocation List
(KRL) as generated by ssh-keygen(1). For more information on
KRLs, see the KEY REVOCATION LISTS section in ssh-keygen(1).
RhostsRSAAuthentication
Specifies whether rhosts or /etc/hosts.equiv 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 minimum value is 512, and the default is 1024.
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 acciden‐
tally leave their directory or files world-writable. The
default is ``yes''. Note that this does not apply to ChrootDi‐
rectory, whose permissions and ownership are checked uncondi‐
tionally.
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 Chroot‐
Directory to force a different filesystem root on clients.
By default no subsystems are defined. Note that this option
applies to protocol version 2 only.
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. 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 authen‐
tication using TrustedUserCAKeys. For more details on certifi‐
cates, see the CERTIFICATES section in ssh-keygen(1).
UseDNS Specifies whether sshd(8) should look up the remote host name
and check that the resolved host name for the remote IP address
maps back to the very same IP address. The default is ``yes''.
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 UsePrivilegeSepara‐
tion 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 Challen‐
geResponseAuthentication and PasswordAuthentication in addition
to PAM account and session module processing for all authentica‐
tion types.
Because PAM challenge-response authentication usually serves an
equivalent role to password authentication, you should disable
either PasswordAuthentication or ChallengeResponseAuthentica‐
tion.
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 traf‐
fic. 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 containing any corruption within the unprivileged processes.
The default is ``yes''. If UsePrivilegeSeparation is set to
``sandbox'' then the pre-authentication unprivileged process is
subject to additional restrictions.
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)Ns '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 dis‐
play 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_con‐
fig(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 UseLo‐
gin 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 wildcard address. The argument must be ``yes'' or ``no''.
The default is ``yes''.
XAuthLocation
Specifies the full pathname of the xauth(1) program. The
default is /usr/bin/xauth.
TIME FORMATSsshd(8) command-line arguments and configuration file options that
specify time may be expressed using a sequence of the form: time[quali‐
fier,] where time is a positive integer value and qualifier is one of
the following:
<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 ALSOsshd(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 contrib‐
uted support for privilege separation.
February 27 2014 SSHD_CONFIG(5)
