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 usually the one given on the
command line (see the
CanonicalizeHostname option for
exceptions).
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.
The file contains keyword-argument pairs, one per line. Lines starting with
‘
#
’ and empty lines are interpreted as
comments. Arguments may optionally be enclosed in double quotes (") in
order to represent arguments containing spaces. 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.
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 usually the
hostname argument given on the command line (see the
CanonicalizeHostname keyword for exceptions).
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 criteria or
the single token all which always matches. The available
criteria keywords are: canonical,
exec, host,
originalhost, user, and
localuser. The all criteria must
appear alone or immediately after canonical. Other
criteria may be combined arbitrarily. All criteria but
all and canonical require an argument.
Criteria may be negated by prepending an exclamation mark
(‘!’).
The canonical keyword matches only when the configuration
file is being re-parsed after hostname canonicalization (see the
CanonicalizeHostname option.) This may be useful to
specify conditions that work with canonical host names only. 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. Arguments to exec accept the tokens described in
the TOKENS section.
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 or
CanonicalizeHostname options. 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).
-
-
- AddKeysToAgent
- Specifies whether keys should be automatically added to a
running ssh-agent(1). If
this option is set to yes and a key is loaded from a
file, the key and its passphrase are added to the agent with the default
lifetime, as if by
ssh-add(1). If this option
is set to ask,
ssh(1) will require
confirmation using the
SSH_ASKPASS
program before
adding a key (see
ssh-add(1) for details). If
this option is set to confirm, each use of the key must
be confirmed, as if the -c option was specified to
ssh-add(1). If this option
is set to no, no keys are added to the agent. The
argument must be yes, confirm,
ask, or no (the default).
-
-
- AddressFamily
- Specifies which address family to use when connecting.
Valid arguments are any (the default),
inet (use IPv4 only), or inet6 (use
IPv6 only).
-
-
- BatchMode
- If set to yes, passphrase/password
querying will be disabled. This option is useful in scripts and other
batch jobs where no user is present to supply the password. The argument
must be yes or no (the default).
-
-
- 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, then the configuration files are processed again
using the new target name to pick up any new configuration in matching
Host and Match 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.
-
-
- CertificateFile
- Specifies a file from which the user's certificate is read.
A corresponding private key must be provided separately in order to use
this certificate either from an IdentityFile directive
or -i flag to
ssh(1), via
ssh-agent(1), or via a
PKCS11Provider.
Arguments to CertificateFile may use the tilde syntax to
refer to a user's home directory or the tokens described in the
TOKENS section.
It is possible to have multiple certificate files specified in configuration
files; these certificates will be tried in sequence. Multiple
CertificateFile directives will add to the list of
certificates used for authentication.
-
-
- ChallengeResponseAuthentication
- Specifies whether to use challenge-response authentication.
The argument to this keyword must be yes (the default)
or no.
-
-
- CheckHostIP
- If set to yes (the default),
ssh(1) will additionally check
the host IP address in the known_hosts file. This allows
it to detect if a host key changed due to DNS spoofing and will add
addresses of destination hosts to ~/.ssh/known_hosts in
the process, regardless of the setting of
StrictHostKeyChecking. If the option is set to
no, the check will not be executed.
-
-
- Cipher
- Specifies the cipher to use for encrypting the session in
protocol version 1. Currently, blowfish,
3des (the default), and des are
supported, though des is only supported in the
ssh(1) client for
interoperability with legacy protocol 1 implementations; its use is
strongly discouraged due to cryptographic weaknesses.
-
-
- Ciphers
- Specifies the ciphers allowed for protocol version 2 in
order of preference. Multiple ciphers must be comma-separated. If the
specified value begins with a ‘+’ character, then the
specified ciphers will be appended to the default set instead of replacing
them. If the specified value begins with a ‘-’ character, then
the specified ciphers (including wildcards) will be removed from the
default set instead of replacing them.
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:
chacha20-poly1305@openssh.com,
aes128-ctr,aes192-ctr,aes256-ctr,
aes128-gcm@openssh.com,aes256-gcm@openssh.com,
aes128-cbc,aes192-cbc,aes256-cbc
The list of available ciphers may also be obtained using “ssh -Q
cipher”.
-
-
- 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).
-
-
- Compression
- Specifies whether to use compression. The argument must be
yes or no (the default).
-
-
- 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(1) to listen for control
connections, but require confirmation using
ssh-askpass(1). If the
ControlPath cannot be opened,
ssh(1) 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.
Arguments to ControlPath may use the tilde syntax to
refer to a user's home directory or the tokens described in the
TOKENS section. It is recommended that
any ControlPath used for opportunistic connection
sharing include at least %h, %p, and %r (or alternatively %C) and be
placed in a directory that is not writable by other users. 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 or 0, then the
master connection will remain in the background indefinitely (until killed
or closed via a mechanism such as the “ssh -O exit”). 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). 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, (e.g. if either end is unable to bind and listen
on a specified port). Note that ExitOnForwardFailure
does not apply to connections made over port forwardings and will not, for
example, cause ssh(1) to exit
if TCP connections to the ultimate forwarding destination fail. The
argument must be yes or no (the
default).
-
-
- FingerprintHash
- Specifies the hash algorithm used when displaying key
fingerprints. Valid options are: md5 and
sha256 (the default).
-
-
- 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).
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).
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 (the default), 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.
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).
-
-
- 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.
-
-
- GSSAPIDelegateCredentials
- Forward (delegate) credentials to the server. The default
is no.
-
-
- 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).
-
-
- HostbasedAuthentication
- Specifies whether to try rhosts based authentication with
public key authentication. The argument must be yes or
no (the default).
-
-
- HostbasedKeyTypes
- Specifies the key types that will be used for hostbased
authentication as a comma-separated pattern list. Alternately if the
specified value begins with a ‘+’ character, then the
specified key types will be appended to the default set instead of
replacing them. If the specified value begins with a ‘-’
character, then the specified key types (including wildcards) will be
removed from the default set instead of replacing them. 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,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa
The -Q option of
ssh(1) may be used to list
supported key types.
-
-
- HostKeyAlgorithms
- Specifies the host key algorithms that the client wants to
use in order of preference. Alternately if the specified value begins with
a ‘+’ character, then the specified key types will be appended
to the default set instead of replacing them. If the specified value
begins with a ‘-’ character, then the specified key types
(including wildcards) will be removed from the default set instead of
replacing them. 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,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa
If hostkeys are known for the destination host then this default is modified
to prefer their algorithms.
The list of available key types may also be obtained using “ssh -Q
key”.
-
-
- 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. Arguments to
HostName accept the tokens described in the
TOKENS section. Numeric IP addresses are
also permitted (both on the command line and in HostName
specifications). The default is the name given on the command line.
-
-
- IdentitiesOnly
- Specifies that
ssh(1) should only use the
authentication identity and certificate files explicitly configured in the
ssh_config files or passed on the
ssh(1) command-line, even if
ssh-agent(1) or a
PKCS11Provider offers more identities. The argument to
this keyword must be yes or no (the
default). This option is intended for situations where ssh-agent offers
many different identities.
-
-
- IdentityAgent
- Specifies the UNIX-domain socket
used to communicate with the authentication agent.
This option overrides the
SSH_AUTH_SOCK
environment
variable and can be used to select a specific agent. Setting the socket
name to none disables the use of an authentication
agent. If the string “SSH_AUTH_SOCK” is specified, the
location of the socket will be read from the
SSH_AUTH_SOCK
environment variable.
Arguments to IdentityAgent may use the tilde syntax to
refer to a user's home directory or the tokens described in the
TOKENS section.
-
-
- 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. If no certificates have been
explicitly specified by CertificateFile,
ssh(1) will try to load
certificate information from the filename obtained by appending
-cert.pub to the path of a specified
IdentityFile.
Arguments to IdentityFile may use the tilde syntax to
refer to a user's home directory or the tokens described in the
TOKENS section.
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. IdentityFile may also
be used in conjunction with CertificateFile in order to
provide any certificate also needed for authentication with the
identity.
-
-
- 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.
-
-
- Include
- Include the specified configuration file(s). Multiple
pathnames may be specified and each pathname may contain
glob(3) wildcards and, for
user configurations, shell-like ‘~’ references to user home
directories. Files without absolute paths are assumed to be in
~/.ssh if included in a user configuration file or
/etc/ssh if included from the system configuration file.
Include directive may appear inside a
Match or Host block to perform
conditional inclusion.
-
-
- 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
(the default) or no.
-
-
- 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. Alternately if the specified
value begins with a ‘+’ character, then the specified methods
will be appended to the default set instead of replacing them. If the
specified value begins with a ‘-’ character, then the
specified methods (including wildcards) will be removed from the default
set instead of replacing them. 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-group-exchange-sha1,
diffie-hellman-group14-sha1
The list of available key exchange algorithms may also be obtained using
“ssh -Q kex”.
-
-
- 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. Arguments to
LocalCommand accept the tokens described in the
TOKENS section.
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 for data integrity
protection. Multiple algorithms must be comma-separated. If the specified
value begins with a ‘+’ character, then the specified
algorithms will be appended to the default set instead of replacing them.
If the specified value begins with a ‘-’ character, then the
specified algorithms (including wildcards) will be removed from the
default set instead of replacing them.
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,
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”.
-
-
- 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).
-
-
- 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 (the default) or
no.
-
-
- 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).
-
-
- 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
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 version 2. Protocol 1 suffers
from a number of cryptographic weaknesses and should not be used. It is
only offered to support legacy devices.
-
-
- 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.
Arguments to ProxyCommand accept the tokens described in
the TOKENS section. 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
-
-
- ProxyJump
- Specifies one or more jump proxies as
[user@]host[:port].
Multiple proxies may be separated by comma characters and will be visited
sequentially. Setting this option will cause
ssh(1) to connect to the target
host by first making a ssh(1)
connection to the specified ProxyJump host and then
establishing a TCP forwarding to the ultimate target from there.
Note that this option will compete with the ProxyCommand
option - whichever is specified first will prevent later instances of the
other from taking effect.
-
-
- 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.
-
-
- PubkeyAcceptedKeyTypes
- Specifies the key types that will be used for public key
authentication as a comma-separated pattern list. Alternately if the
specified value begins with a ‘+’ character, then the key
types after it will be appended to the default instead of replacing it. If
the specified value begins with a ‘-’ character, then the
specified key types (including wildcards) will be removed from the default
set instead of replacing them. 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,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa
The list of available key types may also be obtained using “ssh -Q
key”.
-
-
- PubkeyAuthentication
- Specifies whether to try public key authentication. The
argument to this keyword must be yes (the default) or
no.
-
-
- 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.
-
-
- 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).
-
-
- RevokedHostKeys
- Specifies revoked host public keys. Keys listed in this
file will be refused for host authentication. Note that if this file does
not exist or is not readable, then host authentication will be refused for
all hosts. Keys may be specified as a text file, listing one public key
per line, or as an OpenSSH Key Revocation List (KRL) as generated by
ssh-keygen(1). For more
information on KRLs, see the KEY REVOCATION LISTS section in
ssh-keygen(1).
-
-
- RhostsRSAAuthentication
- Specifies whether to try rhosts based authentication with
RSA host authentication. The argument must be yes or
no (the default). 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 (the default) or
no. RSA authentication will only be attempted if the
identity file exists, or an authentication agent is running. 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. The server must also support it, and the server must be
configured to accept these environment variables. Note that the
TERM
environment variable is always sent whenever
a pseudo-terminal is requested as it is required by the protocol. 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.
-
-
- 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.
-
-
- 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).
-
-
- 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 (the
default), 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.
-
-
- 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.
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 (the
default). Specifying yes requests the default tunnel
mode, which is point-to-point.
-
-
- 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.
-
-
- UpdateHostKeys
- Specifies whether
ssh(1) should accept
notifications of additional hostkeys from the server sent after
authentication has completed and add them to
UserKnownHostsFile. The argument must be
yes, no (the default) or
ask. Enabling this option allows learning alternate
hostkeys for a server and supports graceful key rotation by allowing a
server to send replacement public keys before old ones are removed.
Additional hostkeys are only accepted if the key used to authenticate the
host was already trusted or explicitly accepted by the user. If
UpdateHostKeys is set to ask, then the
user is asked to confirm the modifications to the known_hosts file.
Confirmation is currently incompatible with
ControlPersist, and will be disabled if it is enabled.
Presently, only sshd(8) from
OpenSSH 6.8 and greater support the “hostkeys@openssh.com”
protocol extension used to inform the client of all the server's
hostkeys.
-
-
- UsePrivilegedPort
- Specifies whether to use a privileged port for outgoing
connections. The argument must be yes or
no (the default). 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 default is
no.
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 fingerprint string at login and for unknown host keys. If this flag
is set to no (the default), no fingerprint strings are
printed at login and only the fingerprint string will be printed for
unknown host keys.
-
-
- XAuthLocation
- Specifies the full pathname of the
xauth(1) program. The default
is /usr/X11R6/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"
TOKENS
Arguments to some keywords can make use of tokens, which are expanded at
runtime:
- %%
- A literal ‘%’.
- %C
- Shorthand for %l%h%p%r.
- %d
- Local user's home directory.
- %h
- The remote hostname.
- %i
- The local user ID.
- %L
- The local hostname.
- %l
- The local hostname, including the domain name.
- %n
- The original remote hostname, as given on the command
line.
- %p
- The remote port.
- %r
- The remote username.
- %u
- The local username.
Match exec accepts the tokens %%, %h, %L, %l, %n, %p, %r, and
%u.
CertificateFile accepts the tokens %%, %d, %h, %l, %r, and %u.
ControlPath accepts the tokens %%, %C, %h, %i, %L, %l, %n, %p,
%r, and %u.
HostName accepts the tokens %% and %h.
IdentityAgent and
IdentityFile accept the
tokens %%, %d, %h, %l, %r, and %u.
LocalCommand accepts the tokens %%, %C, %d, %h, %l, %n, %p,
%r, and %u.
ProxyCommand accepts the tokens %%, %h, %p, and %r.
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.
-
-
- /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.