NAME¶
ssh_config
—
OpenSSH SSH client configuration files
SYNOPSIS¶
DESCRIPTION¶
ssh(1) obtains configuration data from the
following sources in the following order:
- command-line options
- user's configuration file
(~/.ssh/config)
- system-wide configuration file
(/etc/ssh/ssh_config)
For each parameter, the first obtained value will be used. The configuration
files contain sections separated by “Host” specifications, and
that section is only applied for hosts that match one of the patterns given in
the specification. The matched host name is the one given on the command line.
Since the first obtained value for each parameter is used, more host-specific
declarations should be given near the beginning of the file, and general
defaults at the end.
Note that the Debian
openssh-client
package
sets several options as standard in
/etc/ssh/ssh_config which are not the
default in
ssh(1):
The configuration file has the following format:
Empty lines and lines starting with ‘
#
’
are comments. Otherwise a line is of the format “keyword
arguments”. Configuration options may be separated by whitespace or
optional whitespace and exactly one ‘
=
’;
the latter format is useful to avoid the need to quote whitespace when
specifying configuration options using the
ssh
,
scp
, and
sftp
-o
option. Arguments may optionally be enclosed in double quotes (") in
order to represent arguments containing spaces.
The possible keywords and their meanings are as follows (note that keywords are
case-insensitive and arguments are case-sensitive):
Host
- Restricts the following declarations (up to the next
Host
or
Match
keyword) to be only for those
hosts that match one of the patterns given after the keyword. If more than
one pattern is provided, they should be separated by whitespace. A single
‘*
’ as a pattern can be used to
provide global defaults for all hosts. The host is the
hostname argument given on the command
line (i.e. the name is not converted to a canonicalized host name before
matching).
A pattern entry may be negated by prefixing it with an exclamation mark
(‘!’). If a negated entry is matched, then the
Host
entry is ignored, regardless of
whether any other patterns on the line match. Negated matches are
therefore useful to provide exceptions for wildcard matches.
See PATTERNS for more
information on patterns.
Match
- Restricts the following declarations (up to the next
Host
or
Match
keyword) to be used only when the
conditions following the Match
keyword
are satisfied. Match conditions are specified using one or more
keyword/criteria pairs or the single token
all
which matches all criteria. The
available keywords are: exec
,
host
,
originalhost
,
user
, and
localuser
.
The exec
keyword executes the specified
command under the user's shell. If the command returns a zero exit status
then the condition is considered true. Commands containing whitespace
characters must be quoted. The following character sequences in the
command will be expanded prior to execution:
‘%L
’ will be substituted by the
first component of the local host name,
‘%l
’ will be substituted by the
local host name (including any domain name),
‘%h
’ will be substituted by the
target host name, ‘%n
’ will be
substituted by the original target host name specified on the
command-line, ‘%p
’ the destination
port, ‘%r
’ by the remote login
username, and ‘%u
’ by the username
of the user running ssh(1).
The other keywords' criteria must be single entries or comma-separated lists
and may use the wildcard and negation operators described in the
PATTERNS section. The
criteria for the host
keyword are
matched against the target hostname, after any substitution by the
Hostname
option. The
originalhost
keyword matches against
the hostname as it was specified on the command-line. The
user
keyword matches against the target
username on the remote host. The
localuser
keyword matches against the
name of the local user running ssh(1) (this
keyword may be useful in system-wide
ssh_config
files).
AddressFamily
- Specifies which address family to use when connecting. Valid arguments are
“any”, “inet” (use IPv4 only), or
“inet6” (use IPv6 only).
BatchMode
- If set to “yes”, passphrase/password querying will be
disabled. In addition, the
ServerAliveInterval
option will be set
to 300 seconds by default. This option is useful in scripts and other
batch jobs where no user is present to supply the password, and where it
is desirable to detect a broken network swiftly. The argument must be
“yes” or “no”. The default is
“no”.
BindAddress
- Use the specified address on the local machine as the source address of
the connection. Only useful on systems with more than one address. Note
that this option does not work if
UsePrivilegedPort
is set to
“yes”.
CanonicalDomains
- When
CanonicalizeHostname
is enabled,
this option specifies the list of domain suffixes in which to search for
the specified destination host.
CanonicalizeFallbackLocal
- Specifies whether to fail with an error when hostname canonicalization
fails. The default, “yes”, will attempt to look up the
unqualified hostname using the system resolver's search rules. A value of
“no” will cause ssh(1) to fail
instantly if
CanonicalizeHostname
is
enabled and the target hostname cannot be found in any of the domains
specified by CanonicalDomains
.
CanonicalizeHostname
- Controls whether explicit hostname canonicalization is performed. The
default, “no”, is not to perform any name rewriting and let
the system resolver handle all hostname lookups. If set to
“yes” then, for connections that do not use a
ProxyCommand
,
ssh(1) will attempt to canonicalize the
hostname specified on the command line using the
CanonicalDomains
suffixes and
CanonicalizePermittedCNAMEs
rules. If
CanonicalizeHostname
is set to
“always”, then canonicalization is applied to proxied
connections too.
If this option is enabled and canonicalisation results in the target
hostname changing, then the configuration files are processed again using
the new target name to pick up any new configuration in matching
Host
stanzas.
CanonicalizeMaxDots
- Specifies the maximum number of dot characters in a hostname before
canonicalization is disabled. The default, “1”, allows a
single dot (i.e. hostname.subdomain).
CanonicalizePermittedCNAMEs
- Specifies rules to determine whether CNAMEs should be followed when
canonicalizing hostnames. The rules consist of one or more arguments of
source_domain_list:target_domain_list,
where source_domain_list is a
pattern-list of domains that may follow CNAMEs in canonicalization, and
target_domain_list is a pattern-list of
domains that they may resolve to.
For example, “*.a.example.com:*.b.example.com,*.c.example.com”
will allow hostnames matching “*.a.example.com” to be
canonicalized to names in the “*.b.example.com” or
“*.c.example.com” domains.
ChallengeResponseAuthentication
- Specifies whether to use challenge-response authentication. The argument
to this keyword must be “yes” or “no”. The
default is “yes”.
CheckHostIP
- If this flag is set to “yes”,
ssh(1) will additionally check the host IP
address in the known_hosts file. This
allows ssh to detect if a host key changed due to DNS spoofing. If the
option is set to “no”, the check will not be executed. The
default is “yes”.
Cipher
- Specifies the cipher to use for encrypting the session in protocol version
1. Currently, “blowfish”, “3des”, and
“des” are supported. des is
only supported in the ssh(1) client for
interoperability with legacy protocol 1 implementations that do not
support the 3des cipher. Its use is
strongly discouraged due to cryptographic weaknesses. The default is
“3des”.
Ciphers
- Specifies the ciphers allowed for protocol version 2 in order of
preference. 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
- arcfour
- arcfour128
- arcfour256
- blowfish-cbc
- cast128-cbc
- chacha20-poly1305@openssh.com
The default is:
aes128-ctr,aes192-ctr,aes256-ctr,
aes128-gcm@openssh.com,aes256-gcm@openssh.com,
chacha20-poly1305@openssh.com,
arcfour256,arcfour128,
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).
ClearAllForwardings
- Specifies that all local, remote, and dynamic port forwardings specified
in the configuration files or on the command line be cleared. This option
is primarily useful when used from the ssh(1)
command line to clear port forwardings set in configuration files, and is
automatically set by scp(1) and
sftp(1). The argument must be
“yes” or “no”. The default is
“no”.
Compression
- Specifies whether to use compression. The argument must be
“yes” or “no”. The default is
“no”.
CompressionLevel
- Specifies the compression level to use if compression is enabled. The
argument must be an integer from 1 (fast) to 9 (slow, best). The default
level is 6, which is good for most applications. The meaning of the values
is the same as in gzip(1). Note that this
option applies to protocol version 1 only.
ConnectionAttempts
- Specifies the number of tries (one per second) to make before exiting. The
argument must be an integer. This may be useful in scripts if the
connection sometimes fails. The default is 1.
ConnectTimeout
- Specifies the timeout (in seconds) used when connecting to the SSH server,
instead of using the default system TCP timeout. This value is used only
when the target is down or really unreachable, not when it refuses the
connection.
ControlMaster
- Enables the sharing of multiple sessions over a single network connection.
When set to “yes”, ssh(1) will
listen for connections on a control socket specified using the
ControlPath
argument. Additional
sessions can connect to this socket using the same
ControlPath
with
ControlMaster
set to “no”
(the default). These sessions will try to reuse the master instance's
network connection rather than initiating new ones, but will fall back to
connecting normally if the control socket does not exist, or is not
listening.
Setting this to “ask” will cause ssh to listen for control
connections, but require confirmation using the
SSH_ASKPASS
program before they are
accepted (see ssh-add(1) for details). If the
ControlPath
cannot be opened, ssh will
continue without connecting to a master instance.
X11 and ssh-agent(1) forwarding is supported
over these multiplexed connections, however the display and agent
forwarded will be the one belonging to the master connection i.e. it is
not possible to forward multiple displays or agents.
Two additional options allow for opportunistic multiplexing: try to use a
master connection but fall back to creating a new one if one does not
already exist. These options are: “auto” and
“autoask”. The latter requires confirmation like the
“ask” option.
ControlPath
- Specify the path to the control socket used for connection sharing as
described in the
ControlMaster
section
above or the string “none” to disable connection sharing. In
the path, ‘%L
’ will be substituted
by the first component of the local host name,
‘%l
’ will be substituted by the
local host name (including any domain name),
‘%h
’ will be substituted by the
target host name, ‘%n
’ will be
substituted by the original target host name specified on the command
line, ‘%p
’ the destination port,
‘%r
’ by the remote login username,
‘%u
’ by the username of the user
running ssh(1), and
‘%C
’ by a hash of the concatenation:
%l%h%p%r. It is recommended that any
ControlPath
used for opportunistic
connection sharing include at least %h, %p, and %r (or alternatively %C).
This ensures that shared connections are uniquely identified.
ControlPersist
- When used in conjunction with
ControlMaster
, specifies that the
master connection should remain open in the background (waiting for future
client connections) after the initial client connection has been closed.
If set to “no”, then the master connection will not be
placed into the background, and will close as soon as the initial client
connection is closed. If set to “yes”, then the master
connection will remain in the background indefinitely (until killed or
closed via a mechanism such as the ssh(1)
“-O
exit” option). If set to a time in seconds,
or a time in any of the formats documented in
sshd_config(5), then the backgrounded master
connection will automatically terminate after it has remained idle (with
no client connections) for the specified time.
DynamicForward
- Specifies that a TCP port on the local machine be forwarded over the
secure channel, and the application protocol is then used to determine
where to connect to from the remote machine.
The argument must be
[
bind_address:
]port.
IPv6 addresses can be specified by enclosing addresses in square brackets.
By default, the local port is bound in accordance with the
GatewayPorts
setting. However, an
explicit bind_address may be used to bind
the connection to a specific address. The
bind_address of “localhost”
indicates that the listening port be bound for local use only, while an
empty address or ‘*’ indicates that the port should be
available from all interfaces.
Currently the SOCKS4 and SOCKS5 protocols are supported, and
ssh(1) will act as a SOCKS server. Multiple
forwardings may be specified, and additional forwardings can be given on
the command line. Only the superuser can forward privileged ports.
EnableSSHKeysign
- Setting this option to “yes” in the global client
configuration file /etc/ssh/ssh_config
enables the use of the helper program
ssh-keysign(8) during
HostbasedAuthentication
. The argument
must be “yes” or “no”. The default is
“no”. This option should be placed in the non-hostspecific
section. See ssh-keysign(8) for more
information.
EscapeChar
- Sets the escape character (default:
‘
~
’). The escape character can also
be set on the command line. The argument should be a single character,
‘^
’ followed by a letter, or
“none” to disable the escape character entirely (making the
connection transparent for binary data).
ExitOnForwardFailure
- Specifies whether ssh(1) should terminate the
connection if it cannot set up all requested dynamic, tunnel, local, and
remote port forwardings. The argument must be “yes” or
“no”. The default is “no”.
ForwardAgent
- Specifies whether the connection to the authentication agent (if any) will
be forwarded to the remote machine. The argument must be
“yes” or “no”. The default is
“no”.
Agent forwarding should be enabled with caution. Users with the ability to
bypass file permissions on the remote host (for the agent's Unix-domain
socket) can access the local agent through the forwarded connection. An
attacker cannot obtain key material from the agent, however they can
perform operations on the keys that enable them to authenticate using the
identities loaded into the agent.
ForwardX11
- Specifies whether X11 connections will be automatically redirected over
the secure channel and
DISPLAY
set. The
argument must be “yes” or “no”. The default is
“no”.
X11 forwarding should be enabled with caution. Users with the ability to
bypass file permissions on the remote host (for the user's X11
authorization database) can access the local X11 display through the
forwarded connection. An attacker may then be able to perform activities
such as keystroke monitoring if the
ForwardX11Trusted
option is also
enabled.
ForwardX11Timeout
- Specify a timeout for untrusted X11 forwarding using the format described
in the TIME FORMATS section of
sshd_config(5). X11 connections received by
ssh(1) after this time will be refused. The
default is to disable untrusted X11 forwarding after twenty minutes has
elapsed.
ForwardX11Trusted
- If this option is set to “yes”, remote X11 clients will have
full access to the original X11 display.
If this option is set to “no”, remote X11 clients will be
considered untrusted and prevented from stealing or tampering with data
belonging to trusted X11 clients. Furthermore, the
xauth(1) token used for the session will be
set to expire after 20 minutes. Remote clients will be refused access
after this time.
The default is “yes” (Debian-specific).
See the X11 SECURITY extension specification for full details on the
restrictions imposed on untrusted clients.
GatewayPorts
- Specifies whether remote hosts are allowed to connect to local forwarded
ports. By default, ssh(1) binds local port
forwardings to the loopback address. This prevents other remote hosts from
connecting to forwarded ports.
GatewayPorts
can be used to specify
that ssh should bind local port forwardings to the wildcard address, thus
allowing remote hosts to connect to forwarded ports. The argument must be
“yes” or “no”. The default is
“no”.
GlobalKnownHostsFile
- Specifies one or more files to use for the global host key database,
separated by whitespace. The default is
/etc/ssh/ssh_known_hosts,
/etc/ssh/ssh_known_hosts2.
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 may be used. When using
GSSAPI key exchange the server need not have a host key. The default is
“no”. Note that this option applies to protocol version 2
only.
GSSAPIClientIdentity
- If set, specifies the GSSAPI client identity that ssh should use when
connecting to the server. The default is unset, which means that the
default identity will be used.
GSSAPIServerIdentity
- If set, specifies the GSSAPI server identity that ssh should expect when
connecting to the server. The default is unset, which means that the
expected GSSAPI server identity will be determined from the target
hostname.
GSSAPIDelegateCredentials
- Forward (delegate) credentials to the server. The default is
“no”. Note that this option applies to protocol version 2
connections using GSSAPI.
GSSAPIRenewalForcesRekey
- If set to “yes” then renewal of the client's GSSAPI
credentials will force the rekeying of the ssh connection. With a
compatible server, this can delegate the renewed credentials to a session
on the server. The default is “no”.
GSSAPITrustDns
- Set to “yes to indicate that the DNS is trusted to securely
canonicalize” the name of the host being connected to. If
“no, the hostname entered on the” command line will be
passed untouched to the GSSAPI library. The default is “no”.
This option only applies to protocol version 2 connections using
GSSAPI.
HashKnownHosts
- Indicates that ssh(1) should hash host names
and addresses when they are added to
~/.ssh/known_hosts. These hashed names
may be used normally by ssh(1) and
sshd(8), but they do not reveal identifying
information should the file's contents be disclosed. The default is
“no”. Note that existing names and addresses in known hosts
files will not be converted automatically, but may be manually hashed
using ssh-keygen(1). Use of this option may
break facilities such as tab-completion that rely on being able to read
unhashed host names from
~/.ssh/known_hosts.
HostbasedAuthentication
- Specifies whether to try rhosts based authentication with public key
authentication. The argument must be “yes” or
“no”. The default is “no”. This option applies
to protocol version 2 only and is similar to
RhostsRSAAuthentication
.
HostKeyAlgorithms
- Specifies the protocol version 2 host key algorithms that the client wants
to use in order of preference. The default for this option is:
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,
ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa,ssh-dss
If hostkeys are known for the destination host then this default is modified
to prefer their algorithms.
HostKeyAlias
- Specifies an alias that should be used instead of the real host name when
looking up or saving the host key in the host key database files. This
option is useful for tunneling SSH connections or for multiple servers
running on a single host.
HostName
- Specifies the real host name to log into. This can be used to specify
nicknames or abbreviations for hosts. If the hostname contains the
character sequence ‘
%h
’, then this
will be replaced with the host name specified on the command line (this is
useful for manipulating unqualified names). The character sequence
‘%%
’ will be replaced by a single
‘%
’ character, which may be used
when specifying IPv6 link-local addresses.
The default is the name given on the command line. Numeric IP addresses are
also permitted (both on the command line and in
HostName
specifications).
IdentitiesOnly
- Specifies that ssh(1) should only use the
authentication identity files configured in the
ssh_config
files, even if
ssh-agent(1) or a
PKCS11Provider
offers more identities.
The argument to this keyword must be “yes” or
“no”. This option is intended for situations where ssh-agent
offers many different identities. The default is “no”.
IdentityFile
- Specifies a file from which the user's DSA, ECDSA, ED25519 or RSA
authentication identity is read. The default is
~/.ssh/identity for protocol version 1,
and ~/.ssh/id_dsa,
~/.ssh/id_ecdsa,
~/.ssh/id_ed25519 and
~/.ssh/id_rsa for protocol version 2.
Additionally, any identities represented by the authentication agent will
be used for authentication unless
IdentitiesOnly
is set.
ssh(1) will try to load certificate
information from the filename obtained by appending
-cert.pub to the path of a specified
IdentityFile
.
The file name may use the tilde syntax to refer to a user's home directory
or one of the following escape characters:
‘%d
’ (local user's home directory),
‘%u
’ (local user name),
‘%l
’ (local host name),
‘%h
’ (remote host name) or
‘%r
’ (remote user name).
It is possible to have multiple identity files specified in configuration
files; all these identities will be tried in sequence. Multiple
IdentityFile
directives will add to the
list of identities tried (this behaviour differs from that of other
configuration directives).
IdentityFile
may be used in conjunction
with IdentitiesOnly
to select which
identities in an agent are offered during authentication.
IgnoreUnknown
- Specifies a pattern-list of unknown options to be ignored if they are
encountered in configuration parsing. This may be used to suppress errors
if
ssh_config
contains options that are
unrecognised by ssh(1). It is recommended
that IgnoreUnknown
be listed early in
the configuration file as it will not be applied to unknown options that
appear before it.
IPQoS
- Specifies the IPv4 type-of-service or DSCP class for connections. 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 use keyboard-interactive authentication. The argument
to this keyword must be “yes” or “no”. The
default is “yes”.
KbdInteractiveDevices
- Specifies the list of methods to use in keyboard-interactive
authentication. Multiple method names must be comma-separated. The default
is to use the server specified list. The methods available vary depending
on what the server supports. For an OpenSSH server, it may be zero or more
of: “bsdauth”, “pam”, and
“skey”.
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-group14-sha1,
diffie-hellman-group-exchange-sha1,
diffie-hellman-group1-sha1
LocalCommand
- Specifies a command to execute on the local machine after successfully
connecting to the server. The command string extends to the end of the
line, and is executed with the user's shell. The following escape
character substitutions will be performed:
‘
%d
’ (local user's home directory),
‘%h
’ (remote host name),
‘%l
’ (local host name),
‘%n
’ (host name as provided on the
command line), ‘%p
’ (remote port),
‘%r
’ (remote user name) or
‘%u
’ (local user name) or
‘%C
’ by a hash of the concatenation:
%l%h%p%r.
The command is run synchronously and does not have access to the session of
the ssh(1) that spawned it. It should not be
used for interactive commands.
This directive is ignored unless
PermitLocalCommand
has been
enabled.
LocalForward
- Specifies that a TCP port on the local machine be forwarded over the
secure channel to the specified host and port from the remote machine. The
first argument must be
[
bind_address:
]port
and the second argument must be
host:hostport.
IPv6 addresses can be specified by enclosing addresses in square brackets.
Multiple forwardings may be specified, and additional forwardings can be
given on the command line. Only the superuser can forward privileged
ports. By default, the local port is bound in accordance with the
GatewayPorts
setting. However, an
explicit bind_address may be used to bind
the connection to a specific address. The
bind_address of “localhost”
indicates that the listening port be bound for local use only, while an
empty address or ‘*’ indicates that the port should be
available from all interfaces.
LogLevel
- Gives the verbosity level that is used when logging messages from
ssh(1). 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 verbose output.
MACs
- Specifies the MAC (message authentication code) algorithms in order of
preference. 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:
umac-64-etm@openssh.com,umac-128-etm@openssh.com,
hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
umac-64@openssh.com,umac-128@openssh.com,
hmac-sha2-256,hmac-sha2-512,
hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
hmac-ripemd160-etm@openssh.com,
hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,
hmac-md5,hmac-sha1,hmac-ripemd160,
hmac-sha1-96,hmac-md5-96
NoHostAuthenticationForLocalhost
- This option can be used if the home directory is shared across machines.
In this case localhost will refer to a different machine on each of the
machines and the user will get many warnings about changed host keys.
However, this option disables host authentication for localhost. The
argument to this keyword must be “yes” or
“no”. The default is to check the host key for
localhost.
NumberOfPasswordPrompts
- Specifies the number of password prompts before giving up. The argument to
this keyword must be an integer. The default is 3.
PasswordAuthentication
- Specifies whether to use password authentication. The argument to this
keyword must be “yes” or “no”. The default is
“yes”.
PermitLocalCommand
- Allow local command execution via the
LocalCommand
option or using the
!
command
escape sequence in ssh(1). The argument must
be “yes” or “no”. The default is
“no”.
PKCS11Provider
- Specifies which PKCS#11 provider to use. The argument to this keyword is
the PKCS#11 shared library ssh(1) should use
to communicate with a PKCS#11 token providing the user's private RSA
key.
Port
- Specifies the port number to connect on the remote host. The default is
22.
PreferredAuthentications
- Specifies the order in which the client should try protocol 2
authentication methods. This allows a client to prefer one method (e.g.
keyboard-interactive
) over another
method (e.g. password
). The default is:
gssapi-with-mic,hostbased,publickey,
keyboard-interactive,password
Protocol
- Specifies the protocol versions ssh(1) should
support in order of preference. The possible values are ‘1’
and ‘2’. Multiple versions must be comma-separated. When
this option is set to “2,1”
ssh
will try version 2 and fall back to
version 1 if version 2 is not available. The default is
‘2’.
ProxyCommand
- Specifies the command to use to connect to the server. The command string
extends to the end of the line, and is executed using the user's shell
‘
exec
’ directive to avoid a
lingering shell process.
In the command string, any occurrence of
‘%h
’ will be substituted by the host
name to connect, ‘%p
’ by the port,
and ‘%r
’ by the remote user name.
The command can be basically anything, and should read from its standard
input and write to its standard output. It should eventually connect an
sshd(8) server running on some machine, or
execute sshd -i
somewhere. Host key
management will be done using the HostName of the host being connected
(defaulting to the name typed by the user). Setting the command to
“none” disables this option entirely. Note that
CheckHostIP
is not available for
connects with a proxy command.
This directive is useful in conjunction with
nc(1) and its proxy support. For example, the
following directive would connect via an HTTP proxy at 192.0.2.0:
ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
ProxyUseFdpass
- Specifies that
ProxyCommand
will pass a
connected file descriptor back to ssh(1)
instead of continuing to execute and pass data. The default is
“no”.
PubkeyAuthentication
- Specifies whether to try public key authentication. The argument to this
keyword must be “yes” or “no”. The default is
“yes”. 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 of sshd_config(5). 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.
RemoteForward
- Specifies that a TCP port on the remote machine be forwarded over the
secure channel to the specified host and port from the local machine. The
first argument must be
[
bind_address:
]port
and the second argument must be
host:hostport.
IPv6 addresses can be specified by enclosing addresses in square brackets.
Multiple forwardings may be specified, and additional forwardings can be
given on the command line. Privileged ports can be forwarded only when
logging in as root on the remote machine.
If the port argument is
‘0
’, the listen port will be
dynamically allocated on the server and reported to the client at run
time.
If the bind_address is not specified, the
default is to only bind to loopback addresses. If the
bind_address is
‘*
’ or an empty string, then the
forwarding is requested to listen on all interfaces. Specifying a remote
bind_address will only succeed if the
server's GatewayPorts
option is enabled
(see sshd_config(5)).
RequestTTY
- Specifies whether to request a pseudo-tty for the session. The argument
may be one of: “no” (never request a TTY),
“yes” (always request a TTY when standard input is a TTY),
“force” (always request a TTY) or “auto”
(request a TTY when opening a login session). This option mirrors the
-t
and
-T
flags for
ssh(1).
RhostsRSAAuthentication
- Specifies whether to try rhosts based authentication with RSA host
authentication. The argument must be “yes” or
“no”. The default is “no”. This option applies
to protocol version 1 only and requires
ssh(1) to be setuid root.
RSAAuthentication
- Specifies whether to try RSA authentication. The argument to this keyword
must be “yes” or “no”. RSA authentication will
only be attempted if the identity file exists, or an authentication agent
is running. The default is “yes”. Note that this option
applies to protocol version 1 only.
SendEnv
- Specifies what variables from the local
environ(7) should be sent to the server. Note
that environment passing is only supported for protocol 2. The server must
also support it, and the server must be configured to accept these
environment variables. Refer to
AcceptEnv
in
sshd_config(5) for how to configure the
server. Variables are specified by name, which may contain wildcard
characters. Multiple environment variables may be separated by whitespace
or spread across multiple SendEnv
directives. The default is not to send any environment variables.
See PATTERNS for more
information on patterns.
ServerAliveCountMax
- Sets the number of server alive messages (see below) which may be sent
without ssh(1) receiving any messages back
from the server. If this threshold is reached while server alive messages
are being sent, ssh will disconnect from the server, terminating the
session. It is important to note that the use of server alive messages is
very different from
TCPKeepAlive
(below). The server alive messages are sent through the encrypted channel
and therefore will not be spoofable. The TCP keepalive option enabled by
TCPKeepAlive
is spoofable. The server
alive mechanism is valuable when the client or server depend on knowing
when a connection has become inactive.
The default value is 3. If, for example,
ServerAliveInterval
(see below) is set
to 15 and ServerAliveCountMax
is left
at the default, if the server becomes unresponsive, ssh will disconnect
after approximately 45 seconds. This option applies to protocol version 2
only; in protocol version 1 there is no mechanism to request a response
from the server to the server alive messages, so disconnection is the
responsibility of the TCP stack.
ServerAliveInterval
- Sets a timeout interval in seconds after which if no data has been
received from the server, ssh(1) will send a
message through the encrypted channel to request a response from the
server. The default is 0, indicating that these messages will not be sent
to the server, or 300 if the
BatchMode
option is set. This option applies to protocol version 2 only.
ProtocolKeepAlives
and
SetupTimeOut
are Debian-specific
compatibility aliases for this option.
StreamLocalBindMask
- Sets the octal file creation mode mask (umask) used when creating a
Unix-domain socket file for local or remote port forwarding. This option
is only used for port forwarding to a Unix-domain socket file.
The default value is 0177, which creates a Unix-domain socket file that is
readable and writable only by the owner. Note that not all operating
systems honor the file mode on Unix-domain socket files.
StreamLocalBindUnlink
- Specifies whether to remove an existing Unix-domain socket file for local
or remote port forwarding before creating a new one. If the socket file
already exists and
StreamLocalBindUnlink
is not enabled,
ssh
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”.
StrictHostKeyChecking
- If this flag is set to “yes”,
ssh(1) will never automatically add host keys
to the ~/.ssh/known_hosts file, and
refuses to connect to hosts whose host key has changed. This provides
maximum protection against trojan horse attacks, though it can be annoying
when the /etc/ssh/ssh_known_hosts file
is poorly maintained or when connections to new hosts are frequently made.
This option forces the user to manually add all new hosts. If this flag is
set to “no”, ssh will automatically add new host keys to the
user known hosts files. If this flag is set to “ask”, new
host keys will be added to the user known host files only after the user
has confirmed that is what they really want to do, and ssh will refuse to
connect to hosts whose host key has changed. The host keys of known hosts
will be verified automatically in all cases. The argument must be
“yes”, “no”, or “ask”. The
default is “ask”.
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. This option only uses TCP
keepalives (as opposed to using ssh level keepalives), so takes a long
time to notice when the connection dies. As such, you probably want the
ServerAliveInterval
option as well.
However, this means that connections will die if the route is down
temporarily, and some people find it annoying.
The default is “yes” (to send TCP keepalive messages), and the
client will notice if the network goes down or the remote host dies. This
is important in scripts, and many users want it too.
To disable TCP keepalive messages, the value should be set to
“no”.
Tunnel
- Request tun(4) device forwarding between the
client and the server. The argument must be “yes”,
“point-to-point” (layer 3), “ethernet” (layer
2), or “no”. Specifying “yes” requests the
default tunnel mode, which is “point-to-point”. The default
is “no”.
TunnelDevice
- Specifies the tun(4) devices to open on the
client (local_tun) and the server
(remote_tun).
The argument must be
local_tun[
:remote_tun
].
The devices may be specified by numerical ID or the keyword
“any”, which uses the next available tunnel device. If
remote_tun is not specified, it defaults
to “any”. The default is “any:any”.
UsePrivilegedPort
- Specifies whether to use a privileged port for outgoing connections. The
argument must be “yes” or “no”. The default is
“no”. If set to “yes”,
ssh(1) must be setuid root. Note that this
option must be set to “yes” for
RhostsRSAAuthentication
with older
servers.
User
- Specifies the user to log in as. This can be useful when a different user
name is used on different machines. This saves the trouble of having to
remember to give the user name on the command line.
UserKnownHostsFile
- Specifies one or more files to use for the user host key database,
separated by whitespace. The default is
~/.ssh/known_hosts,
~/.ssh/known_hosts2.
VerifyHostKeyDNS
- Specifies whether to verify the remote key using DNS and SSHFP resource
records. If this option is set to “yes”, the client will
implicitly trust keys that match a secure fingerprint from DNS. Insecure
fingerprints will be handled as if this option was set to
“ask”. If this option is set to “ask”,
information on fingerprint match will be displayed, but the user will
still need to confirm new host keys according to the
StrictHostKeyChecking
option. The
argument must be “yes”, “no”, or
“ask”. The default is “no”. Note that this
option applies to protocol version 2 only.
See also VERIFYING HOST KEYS in ssh(1).
VisualHostKey
- If this flag is set to “yes”, an ASCII art representation of
the remote host key fingerprint is printed in addition to the hex
fingerprint string at login and for unknown host keys. If this flag is set
to “no”, no fingerprint strings are printed at login and
only the hex fingerprint string will be printed for unknown host keys. The
default is “no”.
XAuthLocation
- Specifies the full pathname of the xauth(1)
program. The default is
/usr/bin/xauth.
PATTERNS¶
A
pattern consists of zero or more non-whitespace
characters, ‘*’ (a wildcard that matches zero or more
characters), or ‘?’ (a wildcard that matches exactly one
character). For example, to specify a set of declarations for any host in the
“.co.uk” set of domains, the following pattern could be used:
Host *.co.uk
The following pattern would match any host in the 192.168.0.[0-9] network range:
Host 192.168.0.?
A
pattern-list is a comma-separated list of
patterns. Patterns within pattern-lists may be negated by preceding them with
an exclamation mark (‘!’). For example, to allow a key to be
used from anywhere within an organization except from the
“dialup” pool, the following entry (in authorized_keys) could be
used:
from="!*.dialup.example.com,*.example.com"
FILES¶
- ~/.ssh/config
- This is the per-user configuration file. The format of this file is
described above. This file is used by the SSH client. Because of the
potential for abuse, this file must have strict permissions: read/write
for the user, and not accessible by others. It may be group-writable
provided that the group in question contains only the user.
- /etc/ssh/ssh_config
- Systemwide configuration file. This file provides defaults for those
values that are not specified in the user's configuration file, and for
those users who do not have a configuration file. This file must be
world-readable.
SEE ALSO¶
ssh(1)
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.