sshd_config — OpenSSH SSH daemon configuration file at Linux.org

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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 contain‐
ing spaces.

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, and that the TERM environment variable is always sent whenever the client
requests a pseudo-terminal as it is required by the protocol. Variables are specified by name, which
may contain the wildcard characters ‘*’ and ‘?’. Multiple environment variables may be separated by
whitespace or spread across multiple AcceptEnv directives. Be warned that some environment variables
could be used to bypass restricted user environments. For this reason, care should be taken in the use
of this directive. The default is not to accept any environment variables.

AddressFamily
Specifies which address family should be used by sshd(8). Valid arguments are “any”, “inet” (use IPv4
only), or “inet6” (use IPv6 only). The default is “any”.

AllowAgentForwarding
Specifies whether ssh-agent(1) forwarding is permitted. The default is “yes”. Note that disabling
agent forwarding does not improve security unless users are also denied shell access, as they can always
install their own forwarders.

AllowGroups
This keyword can be followed by a list of group name patterns, separated by spaces. If specified, login
is allowed only for users whose primary group or supplementary group list matches one of the patterns.
Only group names are valid; a numerical group ID is not recognized. By default, login is allowed for
all groups. The allow/deny directives are processed in the following order: DenyUsers, AllowUsers,
DenyGroups, and finally AllowGroups. All of the specified user and group tests must succeed, before
user is allowed to log in.

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 partic‐
ular hosts. The allow/deny directives are processed in the following order: DenyUsers, AllowUsers,
DenyGroups, and finally AllowGroups. All of the specified user and group tests must succeed, before
to complete public key authentication, followed by either password or keyboard interactive authentica‐
tion. 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 explicitly enabled in the
configuration. The default “any” is not to require multiple authentication; successful completion of a
single authentication method is sufficient.

AuthorizedKeysCommand
Specifies a program to be used to look up the user's public keys. The program must be owned by root 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 authenti‐
cate 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 pre‐
ceded 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 set‐
up. 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).

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 ses‐
sions 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”,
“[email protected]”, “[email protected]”, “arcfour128”, “arcfour256”, “arcfour”,
“blowfish-cbc”, “cast128-cbc”, and “[email protected]”.

The default is:

aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
[email protected],[email protected],
[email protected],
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 ver‐
sion 2 only.

Compression
Specifies whether compression is allowed, or delayed until the user has authenticated successfully. The
argument must be “yes”, “delayed”, or “no”. The default is “delayed”.
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. All of the specified user and group tests must succeed, before user is allowed to
log in.

See PATTERNS in ssh_config(5) for more information on patterns.

ExposeAuthenticationMethods
When using SSH2, this option controls the exposure of the list of successful authentication methods to
PAM during the authentication and to the shell environment via the SSH_USER_AUTH variable. See the
description of this variable for more details. Valid options are: “never” (Do not expose successful
authentication methods), “pam-only” (Only expose them to PAM during authentication, not afterwards),
“pam-and-env” (Expose them to PAM and keep them in the shell environment). The default is “never”.

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 environ‐
ment 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.

GSSAPIEnablek5users
Specifies whether to look at .k5users file for GSSAPI authentication access control. Further details are
described in ksu(1). The default is “no”.

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

gss-gex-sha1-,
gss-group1-sha1-,
gss-group14-sha1-

The default is “gss-gex-sha1-,gss-group1-sha1-,gss-group14-sha1-”. This option only applies to protocol
version 2 connections using GSSAPI.

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 pri‐
vate host key already specified by HostKey. The default behaviour of sshd(8) is not to load any cer‐
tificates.

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 opera‐
tions 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 speci‐
fied, it is used as the packet class unconditionally. If two values are specified, the first is auto‐
matically selected for interactive sessions and the second for non-interactive sessions. The default is

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”.

KerberosUseKuserok
Specifies whether to look at .k5login file for user's aliases. The default is “yes”.

KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms must be comma-separated.
The default is

[email protected],
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. Addition‐
ally, 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 pro‐

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 nega‐
tion 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” respec‐
tively.

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, KerberosUseKuserok, MaxAuthTries, MaxSessions, PasswordAuthentication,
PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY, PermitTunnel, PubkeyAuthentication,
RekeyLimit, RhostsRSAAuthentication, RSAAuthentication, X11DisplayOffset, X11MaxDisplays, 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 connec‐
tion. 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
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 back‐
ups 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 ver‐
sions 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.

RhostsRSAAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication together with successful RSA host authenti‐
cation 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.

ShowPatchLevel
Specifies whether sshd will display the patch level of the binary in the identification string. The
patch level is set at compile-time. The default is “no”. This option applies to protocol version 1
only.

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 configu‐
rations 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: DAE‐
MON, USER, AUTH, AUTHPRIV, 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”.

TrustedUserCAKeys
Specifies a file containing public keys of certificate authorities that are trusted to sign user cer‐
tificates for authentication. Keys are listed one per line; empty lines and comments starting with ‘#’
UsePrivilegeSeparation is specified, it will be disabled after authentication.

UsePAM Enables the Pluggable Authentication Module interface. If set to “yes” this will enable PAM authentica‐
tion 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 authentica‐
tion, 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 escala‐
tion 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 con‐
nection. 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.

X11MaxDisplays
Specifies the maximum number of displays available for sshd(8)'s X11 forwarding. This prevents sshd
from exhausting local ports. The default is 1000.

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 veri‐
fication 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 wild‐
card address. By default, sshd binds the forwarding server to the loopback address and sets the host‐
name part of the DISPLAY environment variable to “localhost”. This prevents remote hosts from connect‐
ing 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”.

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 recom‐
mended (though not necessary) that it be world-readable.