SSHD_CONFIG

SSHD_CONFIG(5) BSD File Formats Manual 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-
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_config(5) for how to configure the client. Note that envi‐
ronment passing is only supported for protocol 2. Variables
are specified by name, which may contain the wildcard charac‐
ters ‘*’ and ‘?’. Multiple environment variables may be sepa‐
rated by whitespace or spread across multiple AcceptEnv direc‐
tives. 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
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 StreamLocal
forwarding does not improve security unless users are also
denied shell access, as they can always install their own for‐
warders.

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, restrict‐
ing 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,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 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 interac‐
tive 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 explic‐
itly 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 stan‐
dard output zero or more lines of authorized_keys output (see
AUTHORIZED_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 Authorized‐
KeysCommand is run.

AuthorizedKeysCommandUser
Specifies the user under whose account the AuthorizedKeysCom‐
mand is run. It is recommended to use a dedicated user that
has no other role on the host than running authorized keys com‐
mands.

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).
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 username of that user. After expansion,
AuthorizedKeysFile is taken to be an absolute path or one rela‐
tive to the user’s home directory. Multiple files may be
listed, separated by whitespace. The default is
“.ssh/authorized_keys .ssh/authorized_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 expan‐
sion, 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 con‐
sulted 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 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. 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 envi‐
ronment 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).

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
[email protected]
[email protected]
arcfour
arcfour128
arcfour256
blowfish-cbc
cast128-cbc
[email protected]

The default is:

aes128-ctr,aes192-ctr,aes256-ctr,
[email protected],[email protected],
[email protected]

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, termi‐
nating 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 mechanism 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,
unresponsive SSH clients will be disconnected after approxi‐
mately 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
version 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 numer‐
ical 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 direc‐
tives are processed in the following order: DenyUsers,
AllowUsers, DenyGroups, 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
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.

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 for‐
wardings 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”. 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
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 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
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
“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/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 con‐
nection. 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 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
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 direc‐
tory. 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. Multi‐
ple algorithms must be comma-separated. The supported algo‐
rithms are:

[email protected]
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:

[email protected],
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group14-sha1

KeyRegenerationInterval
In protocol version 1, the ephemeral server key is automati‐
cally regenerated after this many seconds (if it has been
used). The purpose of regeneration is to prevent decrypting
captured sessions by later breaking into the machine and steal‐
ing the keys. The key is never stored anywhere. If the value
is 0, the key is never regenerated. The default is 3600 (sec‐
onds).

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 per‐
mitted. 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 supported MACs are:

hmac-md5
hmac-md5-96
hmac-ripemd160
hmac-sha1
hmac-sha1-96
hmac-sha2-256
hmac-sha2-512
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]

The default is:

[email protected],[email protected],
[email protected],[email protected],
[email protected],[email protected],
hmac-sha2-256,hmac-sha2-512

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 avail‐
able 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,
AllowAgentForwarding, AllowGroups, AllowTcpForwarding,
AllowUsers, AuthenticationMethods, AuthorizedKeysCommand,
AuthorizedKeysCommandUser, AuthorizedKeysFile,
AuthorizedPrincipalsFile, Banner, ChrootDirectory, DenyGroups,
DenyUsers, ForceCommand, GatewayPorts, GSSAPIAuthentication,
HostbasedAuthentication, HostbasedUsesNameFromPacketOnly,
KbdInteractiveAuthentication, KerberosAuthentication,
MaxAuthTries, MaxSessions, PasswordAuthentication,
PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY,
PermitTunnel, PermitUserRC, PubkeyAuthentication, RekeyLimit,
RhostsRSAAuthentication, 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
network 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 fol‐
lowing 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 authenti‐
cation 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), “ethernet”
(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 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 dae‐
mon. 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”.

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 proto‐
col list does not indicate preference, because the client
selects among multiple protocol versions offered by the server.
Specifying “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 doc‐
umented 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. 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 Revoca‐
tion List (KRL) as generated by ssh-keygen(1). For more infor‐
mation 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.

StreamLocalBindMask
Sets the octal file creation mode mask (umask) used when creat‐
ing a Unix-domain socket file for local or remote port forward‐
ing. 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 acciden‐
tally 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. 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. How‐
ever, 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 indefi‐
nitely 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 cer‐
tificates 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 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
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 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 pro‐
tocol 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 expo‐
sure 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
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. 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 con‐
necting 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/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:

⟨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.

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