SSHD_CONFIG(5) | File Formats Manual | SSHD_CONFIG(5) |
sshd_config
—
OpenSSH SSH daemon configuration file
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. For each keyword, the first obtained
value will be used. 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):
ChallengeResponseAuthentication
noX11Forwarding
yesPrintMotd
noAcceptEnv
LANG LC_*Subsystem
sftp /usr/lib/openssh/sftp-serverUsePAM
yesThe possible keywords and their meanings are as follows (note that keywords are case-insensitive and arguments are case-sensitive):
AcceptEnv
SendEnv
and SetEnv
in
ssh_config(5) for how to configure the client. The
TERM
environment variable is always accepted
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
any
(the default), inet
(use IPv4 only), or
inet6
(use IPv6 only).AllowAgentForwarding
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
DenyUsers
,
AllowUsers
, DenyGroups
,
and finally AllowGroups
.
See PATTERNS in ssh_config(5) for more information on patterns.
AllowStreamLocalForwarding
yes
(the
default) or all
to allow StreamLocal forwarding,
no
to prevent all StreamLocal forwarding,
local
to allow local (from the perspective of
ssh(1)) forwarding only or
remote
to allow remote forwarding only. Note that
disabling StreamLocal forwarding does not improve security unless users
are also denied shell access, as they can always install their own
forwarders.AllowTcpForwarding
yes
(the default) 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. 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
DenyUsers
,
AllowUsers
, DenyGroups
,
and finally AllowGroups
.
See PATTERNS in ssh_config(5) for more information on patterns.
AuthenticationMethods
any
to indicate the default
behaviour of accepting any single authentication method. If the default is
overridden, then successful authentication requires completion of every
method in at least one of these lists.
For example, "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
or
pam
. depending on the server configuration. For
example, "keyboard-interactive:bsdauth" would restrict
keyboard interactive authentication to the
bsdauth
device.
If the publickey method is listed more than once, sshd(8) verifies that keys that have been used successfully are not reused for subsequent authentications. For example, "publickey,publickey" requires successful authentication using two different public keys.
Note that each authentication method listed should also be explicitly enabled in the configuration.
The available authentication methods are:
"gssapi-with-mic", "hostbased",
"keyboard-interactive", "none" (used for access to
password-less accounts when PermitEmptyPasswords
is enabled), "password" and "publickey".
AuthorizedKeysCommand
AuthorizedKeysCommand
accept the tokens described
in the TOKENS section. If no arguments
are specified then the username of the target user is used.
The program 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
AuthorizedKeysCommand
is run. It is recommended to
use a dedicated user that has no other role on the host than running
authorized keys commands. If AuthorizedKeysCommand
is specified but AuthorizedKeysCommandUser
is not,
then sshd(8) will refuse to start.AuthorizedKeysFile
AuthorizedKeysFile
accept the tokens described in
the TOKENS section. 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. Alternately this option may be set to
none
to skip checking for user keys in files. The
default is ".ssh/authorized_keys .ssh/authorized_keys2".AuthorizedPrincipalsCommand
AuthorizedPrincipalsFile
. The
program must be owned by root, not writable by group or others and
specified by an absolute path. Arguments to
AuthorizedPrincipalsCommand
accept the tokens
described in the TOKENS section. If no
arguments are specified then the username of the target user is used.
The program should produce on standard output zero or more
lines of AuthorizedPrincipalsFile
output. If
either AuthorizedPrincipalsCommand
or
AuthorizedPrincipalsFile
is specified, then
certificates offered by the client for authentication must contain a
principal that is listed. By default, no
AuthorizedPrincipalsCommand
is run.
AuthorizedPrincipalsCommandUser
AuthorizedPrincipalsCommand
is run. It is
recommended to use a dedicated user that has no other role on the host
than running authorized principals commands. If
AuthorizedPrincipalsCommand
is specified but
AuthorizedPrincipalsCommandUser
is not, then
sshd(8) will refuse to start.AuthorizedPrincipalsFile
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.
Arguments to AuthorizedPrincipalsFile
accept the tokens described in the
TOKENS section. 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
none
then no banner is displayed. By default, no banner is displayed.CASignatureAlgorithms
ecdsa-sha2-nistp256.ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
Certificates signed using other algorithms will not be accepted for public key or host-based authentication.
ChallengeResponseAuthentication
yes
.ChrootDirectory
ChrootDirectory
accept the
tokens described in the TOKENS section.
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), 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 may require /dev/log
inside the chroot directory on some operating systems (see
sftp-server(8) for details).
For safety, it is very important that the directory hierarchy be prevented from modification by other processes on the system (especially those outside the jail). Misconfiguration can lead to unsafe environments which sshd(8) cannot detect.
The default is none
, indicating not to
chroot(2).
Ciphers
The supported ciphers are:
The default is:
chacha20-poly1305@openssh.com, aes128-ctr,aes192-ctr,aes256-ctr, aes128-gcm@openssh.com,aes256-gcm@openssh.com
The list of available ciphers may also be obtained using "ssh -Q cipher".
ClientAliveCountMax
TCPKeepAlive
. 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
is set to 15, and
ClientAliveCountMax
is left at the default,
unresponsive SSH clients will be disconnected after approximately 45
seconds.
ClientAliveInterval
Compression
yes
,
delayed
(a legacy synonym for
yes
) or no
. The default is
yes
.DebianBanner
yes
.DenyGroups
DenyUsers
,
AllowUsers
, DenyGroups
,
and finally AllowGroups
.
See PATTERNS in ssh_config(5) for more information on patterns.
DenyUsers
DenyUsers
, AllowUsers
,
DenyGroups
, and finally
AllowGroups
.
See PATTERNS in ssh_config(5) for more information on patterns.
DisableForwarding
ExposeAuthInfo
SSH_USER_AUTH
environment variable. The default is
no
.FingerprintHash
md5
and
sha256
. The default is
sha256
.ForceCommand
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
. The default is
none
.GatewayPorts
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
no
.GSSAPIKeyExchange
no
.GSSAPICleanupCredentials
yes
.GSSAPIStrictAcceptorCheck
yes
then
the client must authenticate against the host service on the current
hostname. If set to 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
.GSSAPIStoreCredentialsOnRekey
no
.HostbasedAcceptedKeyTypes
ecdsa-sha2-nistp256-cert-v01@openssh.com, ecdsa-sha2-nistp384-cert-v01@openssh.com, ecdsa-sha2-nistp521-cert-v01@openssh.com, ssh-ed25519-cert-v01@openssh.com, rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, ssh-rsa-cert-v01@openssh.com, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
The list of available key types may also be obtained using "ssh -Q key".
HostbasedAuthentication
no
.HostbasedUsesNameFromPacketOnly
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
HostKey
. The default behaviour of
sshd(8) is not to load any certificates.HostKey
Note that sshd(8) will refuse to use a file
if it is group/world-accessible and that the
HostKeyAlgorithms
option restricts which of the
keys are actually used by sshd(8).
It is possible to have multiple host key files. 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
SSH_AUTH_SOCK
environment
variable.HostKeyAlgorithms
ecdsa-sha2-nistp256-cert-v01@openssh.com, ecdsa-sha2-nistp384-cert-v01@openssh.com, ecdsa-sha2-nistp521-cert-v01@openssh.com, ssh-ed25519-cert-v01@openssh.com, rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, ssh-rsa-cert-v01@openssh.com, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
The list of available key types may also be obtained using "ssh -Q key".
IgnoreRhosts
HostbasedAuthentication
.
/etc/hosts.equiv and
/etc/ssh/shosts.equiv are still used. The
default is yes
.
IgnoreUserKnownHosts
HostbasedAuthentication
and use only the
system-wide known hosts file /etc/ssh/known_hosts.
The default is no
.IPQoS
af11
,
af12
, af13
,
af21
, af22
,
af23
, af31
,
af32
, af33
,
af41
, af42
,
af43
, cs0
,
cs1
, cs2
,
cs3
, cs4
,
cs5
, cs6
,
cs7
, ef
,
lowdelay
, throughput
,
reliability
, a numeric value, or
none
to use the operating system default. 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
yes
or
no
. The default is to use whatever value
ChallengeResponseAuthentication
is set to (by
default yes
).KerberosAuthentication
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
no
.KerberosOrLocalPasswd
yes
.KerberosTicketCleanup
yes
.KexAlgorithms
The default is:
curve25519-sha256,curve25519-sha256@libssh.org, ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, diffie-hellman-group-exchange-sha256, diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, diffie-hellman-group14-sha256,diffie-hellman-group14-sha1
The list of available key exchange algorithms may also be obtained using "ssh -Q kex".
ListenAddress
ListenAddress
hostname|address
[rdomain
domain]ListenAddress
hostname:port
[rdomain
domain]ListenAddress
IPv4_address:port
[rdomain
domain]ListenAddress
[hostname|address]:port
[rdomain
domain]The optional rdomain
qualifier
requests sshd(8) listen in an explicit routing domain.
If port is not specified, sshd will listen on the
address and all Port
options specified. The
default is to listen on all local addresses on the current default
routing domain. Multiple ListenAddress
options
are permitted. For more information on routing domains, see
rdomain(4).
LoginGraceTime
LogLevel
MACs
The algorithms that contain "-etm" calculate the MAC after encryption (encrypt-then-mac). These are considered safer and their use recommended. The supported MACs are:
The default is:
umac-64-etm@openssh.com,umac-128-etm@openssh.com, hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, hmac-sha1-etm@openssh.com, umac-64@openssh.com,umac-128@openssh.com, hmac-sha2-256,hmac-sha2-512,hmac-sha1
The list of available MAC algorithms may also be obtained using "ssh -Q mac".
Match
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 satisfied, 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
, RDomain
, and
Address
(with RDomain
representing the rdomain(4) on which the connection
was received.)
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, such as 192.0.2.0/24 or 2001:db8::/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
,
AllowStreamLocalForwarding
,
AllowTcpForwarding
,
AllowUsers
,
AuthenticationMethods
,
AuthorizedKeysCommand
,
AuthorizedKeysCommandUser
,
AuthorizedKeysFile
,
AuthorizedPrincipalsCommand
,
AuthorizedPrincipalsCommandUser
,
AuthorizedPrincipalsFile
,
Banner
, ChrootDirectory
,
ClientAliveCountMax
,
ClientAliveInterval
,
DenyGroups
, DenyUsers
,
ForceCommand
,
GatewayPorts
,
GSSAPIAuthentication
,
HostbasedAcceptedKeyTypes
,
HostbasedAuthentication
,
HostbasedUsesNameFromPacketOnly
,
IPQoS
,
KbdInteractiveAuthentication
,
KerberosAuthentication
,
LogLevel
, MaxAuthTries
,
MaxSessions
,
PasswordAuthentication
,
PermitEmptyPasswords
,
PermitListen
,
PermitOpen
,
PermitRootLogin
,
PermitTTY
, PermitTunnel
,
PermitUserRC
,
PubkeyAcceptedKeyTypes
,
PubkeyAuthentication
,
RekeyLimit
, RevokedKeys
,
RDomain
, SetEnv
,
StreamLocalBindMask
,
StreamLocalBindUnlink
,
TrustedUserCAKeys
,
X11DisplayOffset
,
X11Forwarding
and
X11UseLocalHost
.
MaxAuthTries
MaxSessions
MaxSessions
to 1 will effectively disable session
multiplexing, whereas setting it to 0 will prevent all shell, login and
subsystem sessions while still permitting forwarding. The default is
10.MaxStartups
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
yes
.PermitEmptyPasswords
no
.PermitListen
PermitListen
portPermitListen
host:portMultiple permissions may be specified by separating them with
whitespace. An argument of any
can be used to
remove all restrictions and permit any listen requests. An argument of
none
can be used to prohibit all listen
requests. The host name may contain wildcards as described in the
PATTERNS section in ssh_config(5). The wildcard
‘*’ can also be used in place of a port number to allow
all ports. By default all port forwarding listen requests are permitted.
Note that the GatewayPorts
option may further
restrict which addresses may be listened on. Note also that
ssh(1) will request a listen host of
“localhost” if no listen host was specifically requested,
and this this name is treated differently to explicit localhost
addresses of “127.0.0.1” and “::1”.
PermitOpen
PermitOpen
host:portPermitOpen
IPv4_addr:portPermitOpen
[IPv6_addr]:portMultiple 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. The wildcard ‘*’ can be used for host or port to
allow all hosts or ports, respectively. By default all port forwarding
requests are permitted.
PermitRootLogin
yes
,
prohibit-password
,
forced-commands-only
, or
no
. The default is
prohibit-password
.
If this option is set to
prohibit-password
(or its deprecated alias,
without-password
), password and
keyboard-interactive authentication are 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.
PermitTTY
yes
.PermitTunnel
yes
,
point-to-point
(layer 3),
ethernet
(layer 2), or no
.
Specifying yes
permits both
point-to-point
and
ethernet
. The default is
no
.
Independent of this setting, the permissions of the selected tun(4) device must allow access to the user.
PermitUserEnvironment
environment=
options in
~/.ssh/authorized_keys are processed by
sshd(8). Valid options are yes
,
no
or a pattern-list specifying which environment
variable names to accept (for example "LANG,LC_*"). The default
is no
. Enabling environment processing may enable
users to bypass access restrictions in some configurations using
mechanisms such as LD_PRELOAD
.PermitUserRC
yes
.PidFile
none
to not write one. The default is
/run/sshd.pid.Port
ListenAddress
.PrintLastLog
yes
.PrintMotd
yes
.PubkeyAcceptedKeyTypes
ecdsa-sha2-nistp256-cert-v01@openssh.com, ecdsa-sha2-nistp384-cert-v01@openssh.com, ecdsa-sha2-nistp521-cert-v01@openssh.com, ssh-ed25519-cert-v01@openssh.com, rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com, ssh-rsa-cert-v01@openssh.com, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
The list of available key types may also be obtained using "ssh -Q key".
PubkeyAuthentication
yes
.RekeyLimit
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.RevokedKeys
none
to not
use one. 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).RDomain
%D
, then the domain in
which the incoming connection was received will be applied.SetEnv
SetEnv
override the default environment and any variables specified by the user
via AcceptEnv
or
PermitUserEnvironment
.StreamLocalBindMask
The default value is 0177, which creates a Unix-domain socket file that is readable and writable only by the owner. Note that not all operating systems honor the file mode on Unix-domain socket files.
StreamLocalBindUnlink
StreamLocalBindUnlink
is not
enabled, sshd
will be unable to forward the port
to the Unix-domain socket file. This option is only used for port
forwarding to a Unix-domain socket file.
The argument must be yes
or
no
. The default is
no
.
StrictModes
yes
. Note that this does not apply to
ChrootDirectory
, whose permissions and ownership
are checked unconditionally.Subsystem
The command sftp-server
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.
SyslogFacility
TCPKeepAlive
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
none
to not use one. 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
If this option is set to no
(the
default) then only addresses and not host names may be used in
~/.ssh/authorized_keys
from
and sshd_config
Match
Host
directives.
UsePAM
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
.
VersionAddendum
none
.X11DisplayOffset
X11Forwarding
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
), 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.
X11UseLocalhost
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
none
to not use one. The default is
/usr/bin/xauth.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:
Each member of the sequence is added together to calculate the total time value.
Time format examples:
Arguments to some keywords can make use of tokens, which are expanded at runtime:
AuthorizedKeysCommand
accepts the tokens
%%, %f, %h, %k, %t, %U, and %u.
AuthorizedKeysFile
accepts the tokens %%,
%h, %U, and %u.
AuthorizedPrincipalsCommand
accepts the
tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u.
AuthorizedPrincipalsFile
accepts the
tokens %%, %h, %U, and %u.
ChrootDirectory
accepts the tokens %%, %h,
%U, and %u.
RoutingDomain
accepts the token %D.
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.
September 20, 2018 | Debian |