Ubuntu Manpage: sshd_config — OpenSSH SSH daemon configuration file

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.

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.

Debian                                          February 27, 2014                                 SSHD_CONFIG(5)

NAME

SYNOPSIS

DESCRIPTION

TIME FORMATS

FILES

SEE ALSO

AUTHORS