Provided by: openssh-server_6.6p1-2ubuntu2.13_amd64 
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.
Note that the Debian openssh-server package sets several options as standard in
/etc/ssh/sshd_config which are not the default in sshd(8). The exact list depends on
whether the package was installed fresh or upgraded from various possible previous versions,
but includes at least the following:
• ChallengeResponseAuthentication no
• X11Forwarding yes
• PrintMotd no
• AcceptEnv LANG LC_*
• Subsystem sftp /usr/lib/openssh/sftp-server
• UsePAM yes
The possible keywords and their meanings are as follows (note that keywords are case-
insensitive and arguments are case-sensitive):
AcceptEnv
Specifies what environment variables sent by the client will be copied into the
session's environ(7). See SendEnv in ssh_config(5) for how to configure the client.
Note that environment passing is only supported for protocol 2. 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 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,keyboard-interactive”
would require the user to complete public key authentication, followed by either
password or keyboard interactive authentication. Only methods that are next in one
or more lists are offered at each stage, so for this example, it would not be
possible to attempt password or keyboard-interactive authentication before public
key.
For keyboard interactive authentication it is also possible to restrict
authentication to a specific device by appending a colon followed by the device
identifier “bsdauth”, “pam”, or “skey”, depending on the server configuration. For
example, “keyboard-interactive:bsdauth” would restrict keyboard interactive
authentication to the “bsdauth” device.
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 AUTHORIZED_KEYS in
sshd(8)). If a key supplied by AuthorizedKeysCommand does not successfully
authenticate and authorize the user then public key authentication continues using
the usual AuthorizedKeysFile files. By default, no AuthorizedKeysCommand is run.
AuthorizedKeysCommandUser
Specifies the user under whose account the AuthorizedKeysCommand is run. It is
recommended to use a dedicated user that has no other role on the host than running
authorized keys commands.
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 relative 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 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 certificate's principals list for it to be
accepted. Note that AuthorizedPrincipalsFile is only used when authentication
proceeds using a CA listed in TrustedUserCAKeys and is not consulted for
certification authorities trusted via ~/.ssh/authorized_keys, though the principals=
key option offers a similar facility (see sshd(8) for details).
Banner The contents of the specified file are sent to the remote user before authentication
is allowed. If the argument is “none” then no banner is displayed. 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). 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 directory of the user being authenticated, and %u is replaced
by the username of that user.
The ChrootDirectory must contain the necessary files and directories to support the
user's session. For an interactive session this requires at least a shell,
typically sh(1), and basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4),
stderr(4), 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”, “arcfour128”,
“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 messages are being sent, sshd will disconnect the client,
terminating the session. It is important to note that the use of client alive
messages is very different from TCPKeepAlive (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 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 version 2 only.
Compression
Specifies whether compression is enabled after the user has authenticated
successfully. The argument must be “yes”, “delayed” (a legacy synonym for “yes”) or
“no”. The default is “yes”.
DebianBanner
Specifies whether the distribution-specified extra version suffix is included during
initial protocol handshake. The default is “yes”.
DenyGroups
This keyword can be followed by a list of group name patterns, separated by spaces.
Login is disallowed for users whose primary group or supplementary group list
matches one of the patterns. Only group names are valid; a numerical group ID is
not recognized. By default, login is allowed for all groups. The allow/deny
directives are processed in the following order: DenyUsers, AllowUsers, DenyGroups,
and finally AllowGroups.
See PATTERNS in ssh_config(5) for more information on patterns.
DenyUsers
This keyword can be followed by a list of user name patterns, separated by spaces.
Login is disallowed for user names that match one of the patterns. Only user names
are valid; a numerical user ID is not recognized. By default, login is allowed for
all users. If the pattern takes the form USER@HOST then USER and HOST are
separately checked, restricting logins to particular users from particular hosts.
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.
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 forwardings to bind to
non-loopback addresses, thus allowing other hosts to connect. The argument may be
“no” to force remote port forwardings to be available to the local host only, “yes”
to force remote port forwardings to bind to the wildcard address, or
“clientspecified” to allow the client to select the address to which the forwarding
is bound. The default is “no”.
GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is allowed. The default is
“no”. Note that this option applies to protocol version 2 only.
GSSAPIKeyExchange
Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange
doesn't rely on ssh keys to verify host identity. The default is “no”. Note that
this option applies to protocol version 2 only.
GSSAPICleanupCredentials
Specifies whether to automatically destroy the user's credentials cache on logout.
The default is “yes”. Note that this option applies to protocol version 2 only.
GSSAPIStrictAcceptorCheck
Determines whether to be strict about the identity of the GSSAPI acceptor a client
authenticates against. If “yes” then the client must authenticate against the host
service on the current hostname. If “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”. Note that
this option applies only to protocol version 2 GSSAPI connections, and setting it to
“no” may only work with recent Kerberos GSSAPI libraries.
GSSAPIStoreCredentialsOnRekey
Controls whether the user's GSSAPI credentials should be updated following a
successful connection rekeying. This option can be used to accepted renewed or
updated credentials from a compatible client. The default is “no”.
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/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 connection. Accepted
values are “af11”, “af12”, “af13”, “af21”, “af22”, “af23”, “af31”, “af32”, “af33”,
“af41”, “af42”, “af43”, “cs0”, “cs1”, “cs2”, “cs3”, “cs4”, “cs5”, “cs6”, “cs7”,
“ef”, “lowdelay”, “throughput”, “reliability”, or a numeric value. This option may
take one or two arguments, separated by whitespace. If one argument is specified,
it is used as the packet class unconditionally. If two values are specified, the
first is automatically selected for interactive sessions and the second for non-
interactive sessions. The default is “lowdelay” for interactive sessions and
“throughput” for non-interactive sessions.
KbdInteractiveAuthentication
Specifies whether to allow keyboard-interactive authentication. The argument to
this keyword must be “yes” or “no”. The default is to use whatever value
ChallengeResponseAuthentication is set to (by default “yes”).
KerberosAuthentication
Specifies whether the password provided by the user for PasswordAuthentication will
be validated through the Kerberos KDC. To use this option, the server needs a
Kerberos servtab which allows the verification of the KDC's identity. The default
is “no”.
KerberosGetAFSToken
If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire an AFS token
before accessing the user's home directory. The default is “no”.
KerberosOrLocalPasswd
If password authentication through Kerberos fails then the password will be
validated via any additional local mechanism such as /etc/passwd. The default is
“yes”.
KerberosTicketCleanup
Specifies whether to automatically destroy the user's ticket cache file on logout.
The default is “yes”.
KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms must be
comma-separated. 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 sessions 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 permitted. 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 successfully logged in.
If the value is 0, there is no time limit. The default is 120 seconds.
LogLevel
Gives the verbosity level that is used when logging messages from sshd(8). The
possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and
DEBUG3. The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3
each specify higher levels of debugging output. Logging with a DEBUG level violates
the privacy of users and is not recommended.
MACs Specifies the available MAC (message authentication code) algorithms. The MAC
algorithm is used 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, 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, PubkeyAuthentication, RekeyLimit, RhostsRSAAuthentication,
RSAAuthentication, 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 sessions permitted per network connection. The
default is 10.
MaxStartups
Specifies the maximum number of concurrent unauthenticated connections to the SSH
daemon. Additional connections will be dropped until authentication succeeds or the
LoginGraceTime expires for a connection. The default is 10:30:100.
Alternatively, random early drop can be enabled by specifying the three colon
separated values “start:rate:full” (e.g. "10:30:60"). sshd(8) will refuse
connection attempts with a probability of “rate/100” (30%) if there are currently
“start” (10) unauthenticated connections. The probability increases linearly and
all connection attempts are refused if the number of unauthenticated connections
reaches “full” (60).
PasswordAuthentication
Specifies whether password authentication is allowed. The default is “yes”.
PermitEmptyPasswords
When password authentication is allowed, it specifies whether the server allows
login to accounts with empty password strings. The default is “no”.
PermitOpen
Specifies the destinations to which TCP port forwarding is permitted. The
forwarding specification must be one of the following forms:
PermitOpen host:port
PermitOpen IPv4_addr:port
PermitOpen [IPv6_addr]:port
Multiple forwards may be specified by separating them with whitespace. An argument
of “any” can be used to remove all restrictions and permit any forwarding requests.
An argument of “none” can be used to prohibit all forwarding requests. 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 authentication 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 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 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.
PidFile
Specifies the file that contains the process ID of the SSH daemon. 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-separated. 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. 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
documented in the TIME FORMATS section. The default value for RekeyLimit is
“default none”, which means that rekeying is performed after the cipher's default
amount of data has been sent or received and no time based rekeying is done. 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 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. 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 temporarily, and some people find it annoying. On the other hand, if TCP
keepalives are not sent, sessions may hang indefinitely on the server, leaving
“ghost” users and consuming server resources.
The default is “yes” (to send TCP keepalive messages), and the server will notice if
the network goes down or the client host crashes. This avoids infinitely hanging
sessions.
To disable TCP keepalive messages, the value should be set to “no”.
This option was formerly called KeepAlive.
TrustedUserCAKeys
Specifies a file containing public keys of certificate authorities that are trusted
to sign user certificates for authentication. Keys are listed one per line; empty
lines and comments starting with ‘#’ are allowed. If a certificate is presented for
authentication and has its signing CA key listed in this file, then it may be used
for authentication for any user listed in the certificate's principals list. Note
that certificates that lack a list of principals will not be permitted for
authentication 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 sessions. 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 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 protocol banner sent by
the server upon connection. The default is “none”.
X11DisplayOffset
Specifies the first display number available for sshd(8)'s X11 forwarding. This
prevents sshd from interfering with real X11 servers. The default is 10.
X11Forwarding
Specifies whether X11 forwarding is permitted. The argument must be “yes” or “no”.
The default is “no”.
When X11 forwarding is enabled, there may be additional exposure to the server and
to client displays if the sshd(8) proxy display is configured to listen on the
wildcard address (see X11UseLocalhost below), though this is not the default.
Additionally, the authentication spoofing and authentication data verification and
substitution occur on the client side. The security risk of using X11 forwarding is
that the client's X11 display server may be exposed to attack when the SSH client
requests forwarding (see the warnings for ForwardX11 in ssh_config(5)). A system
administrator may have a stance in which they want to protect clients that may
expose themselves to attack by unwittingly requesting X11 forwarding, which can
warrant a “no” setting.
Note that disabling X11 forwarding does not prevent users from forwarding X11
traffic, as users can always install their own forwarders. 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 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 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 necessary) 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 created OpenSSH. Markus Friedl contributed the support
for SSH protocol versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
for privilege separation.