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SSH_CONFIG(5) File Formats Manual SSH_CONFIG(5)
NAME
ssh_config M-bM-^@M-^S OpenSSH client configuration file
DESCRIPTION
ssh(1) obtains configuration data from the following sources in the
following order:
1. command-line options
2. user's configuration file (~/.ssh/config)
3. system-wide configuration file (/etc/ssh/ssh_config)
Unless noted otherwise, 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 M-bM-^@M-^X#M-bM-^@M-^Y 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 M-bM-^@M-^X=M-bM-^@M-^Y; 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 M-bM-^@M-^X*M-bM-^@M-^Y
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 (M-bM-^@M-^X!M-bM-^@M-^Y). 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, final,
exec, localnetwork, host, originalhost, Tag, user, and localuser.
The all criteria must appear alone or immediately after canonical
or final. Other criteria may be combined arbitrarily. All
criteria but all, canonical, and final require an argument.
Criteria may be negated by prepending an exclamation mark (M-bM-^@M-^X!M-bM-^@M-^Y).
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 final keyword requests that the configuration be re-parsed
(regardless of whether CanonicalizeHostname is enabled), and
matches only during this final pass. If CanonicalizeHostname is
enabled, then canonical and final match during the same pass.
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 localnetwork keyword matches the addresses of active local
network interfaces against the supplied list of networks in CIDR
format. This may be convenient for varying the effective
configuration on devices that roam between networks. Note that
network address is not a trustworthy criteria in many situations
(e.g. when the network is automatically configured using DHCP)
and so caution should be applied if using it to control security-
sensitive configuration.
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 tagged keyword matches a tag
name specified by a prior Tag directive or on the ssh(1) command-
line using the -P flag. 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. Alternately, this option may be specified as a time
interval using the format described in the TIME FORMATS section
of sshd_config(5) to specify the key's lifetime in ssh-agent(1),
after which it will automatically be removed. The argument must
be no (the default), yes, confirm (optionally followed by a time
interval), ask or a time interval.
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, user interaction such as password prompts and host
key confirmation requests will be disabled. This option is
useful in scripts and other batch jobs where no user is present
to interact with ssh(1). 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.
BindInterface
Use the address of the specified interface on the local machine
as the source address of the connection.
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 or ProxyJump,
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. A value of
none disables the use of a ProxyJump host.
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.
A single argument of "none" causes no CNAMEs to be considered for
canonicalization. This is the default behaviour.
CASignatureAlgorithms
Specifies which algorithms are allowed for signing of
certificates by certificate authorities (CAs). The default is:
ssh-ed25519,ecdsa-sha2-nistp256,
ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
rsa-sha2-512,rsa-sha2-256
If the specified list begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the
specified algorithms will be appended to the default set instead
of replacing them. If the specified list begins with a M-bM-^@M-^X-M-bM-^@M-^Y
character, then the specified algorithms (including wildcards)
will be removed from the default set instead of replacing them.
ssh(1) will not accept host certificates signed using algorithms
other than those specified.
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 or
SecurityKeyProvider.
Arguments to CertificateFile may use the tilde syntax to refer to
a user's home directory, the tokens described in the TOKENS
section and environment variables as described in the ENVIRONMENT
VARIABLES 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.
ChannelTimeout
Specifies whether and how quickly ssh(1) should close inactive
channels. Timeouts are specified as one or more M-bM-^@M-^\type=intervalM-bM-^@M-^]
pairs separated by whitespace, where the M-bM-^@M-^\typeM-bM-^@M-^] must be a channel
type name (as described in the table below), optionally
containing wildcard characters.
The timeout value M-bM-^@M-^\intervalM-bM-^@M-^] is specified in seconds or may use
any of the units documented in the TIME FORMATS section. For
example, M-bM-^@M-^\session=5mM-bM-^@M-^] would cause the interactive session to
terminate after five minutes of inactivity. Specifying a zero
value disables the inactivity timeout.
The available channel types include:
agent-connection
Open connections to ssh-agent(1).
direct-tcpip, direct-streamlocal@openssh.com
Open TCP or Unix socket (respectively) connections that
have been established from a ssh(1) local forwarding,
i.e. LocalForward or DynamicForward.
forwarded-tcpip, forwarded-streamlocal@openssh.com
Open TCP or Unix socket (respectively) connections that
have been established to a sshd(8) listening on behalf of
a ssh(1) remote forwarding, i.e. RemoteForward.
session
The interactive main session, including shell session,
command execution, scp(1), sftp(1), etc.
tun-connection
Open TunnelForward connections.
x11-connection
Open X11 forwarding sessions.
Note that in all the above cases, terminating an inactive session
does not guarantee to remove all resources associated with the
session, e.g. shell processes or X11 clients relating to the
session may continue to execute.
Moreover, terminating an inactive channel or session does not
necessarily close the SSH connection, nor does it prevent a
client from requesting another channel of the same type. In
particular, expiring an inactive forwarding session does not
prevent another identical forwarding from being subsequently
created.
The default is not to expire channels of any type for inactivity.
CheckHostIP
If set to yes, 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 default), the check will not be executed.
Ciphers
Specifies the ciphers allowed and their order of preference.
Multiple ciphers must be comma-separated. If the specified list
begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the specified ciphers will be
appended to the default set instead of replacing them. If the
specified list begins with a M-bM-^@M-^X-M-bM-^@M-^Y character, then the specified
ciphers (including wildcards) will be removed from the default
set instead of replacing them. If the specified list begins with
a M-bM-^@M-^X^M-bM-^@M-^Y character, then the specified ciphers will be placed at the
head of the default set.
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
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
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).
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 timeout is applied both to establishing the connection and
to performing the initial SSH protocol handshake and key
exchange.
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, the tokens described in the TOKENS section and
environment variables as described in the ENVIRONMENT VARIABLES
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 (the default), 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 M-bM-^@M-^X*M-bM-^@M-^Y 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.
EnableEscapeCommandline
Enables the command line option in the EscapeChar menu for
interactive sessions (default M-bM-^@M-^X~CM-bM-^@M-^Y). By default, the command
line is disabled.
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: M-bM-^@M-^X~M-bM-^@M-^Y). The escape character
can also be set on the command line. The argument should be a
single character, M-bM-^@M-^X^M-bM-^@M-^Y 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).
ForkAfterAuthentication
Requests ssh to go to background just before command execution.
This is useful if ssh is going to ask for passwords or
passphrases, but the user wants it in the background. This
implies the StdinNull configuration option being set to M-bM-^@M-^\yesM-bM-^@M-^].
The recommended way to start X11 programs at a remote site is
with something like ssh -f host xterm, which is the same as ssh
host xterm if the ForkAfterAuthentication configuration option is
set to M-bM-^@M-^\yesM-bM-^@M-^].
If the ExitOnForwardFailure configuration option is set to M-bM-^@M-^\yesM-bM-^@M-^],
then a client started with the ForkAfterAuthentication
configuration option being set to M-bM-^@M-^\yesM-bM-^@M-^] will wait for all remote
port forwards to be successfully established before placing
itself in the background. The argument to this keyword must be
yes (same as the -f option) or no (the default).
ForwardAgent
Specifies whether the connection to the authentication agent (if
any) will be forwarded to the remote machine. The argument may
be yes, no (the default), an explicit path to an agent socket or
the name of an environment variable (beginning with M-bM-^@M-^X$M-bM-^@M-^Y) in which
to find the path.
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.
Setting ForwardX11Timeout to zero will disable the timeout and
permit X11 forwarding for the life of the connection. 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 visually
reveal identifying information if the file's contents are
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).
HostbasedAcceptedAlgorithms
Specifies the signature algorithms that will be used for
hostbased authentication as a comma-separated list of patterns.
Alternately if the specified list begins with a M-bM-^@M-^X+M-bM-^@M-^Y character,
then the specified signature algorithms will be appended to the
default set instead of replacing them. If the specified list
begins with a M-bM-^@M-^X-M-bM-^@M-^Y character, then the specified signature
algorithms (including wildcards) will be removed from the default
set instead of replacing them. If the specified list begins with
a M-bM-^@M-^X^M-bM-^@M-^Y character, then the specified signature algorithms will be
placed at the head of the default set. The default for this
option is:
ssh-ed25519-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
rsa-sha2-512,rsa-sha2-256
The -Q option of ssh(1) may be used to list supported signature
algorithms. This was formerly named HostbasedKeyTypes.
HostbasedAuthentication
Specifies whether to try rhosts based authentication with public
key authentication. The argument must be yes or no (the
default).
HostKeyAlgorithms
Specifies the host key signature algorithms that the client wants
to use in order of preference. Alternately if the specified list
begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the specified signature
algorithms will be appended to the default set instead of
replacing them. If the specified list begins with a M-bM-^@M-^X-M-bM-^@M-^Y
character, then the specified signature algorithms (including
wildcards) will be removed from the default set instead of
replacing them. If the specified list begins with a M-bM-^@M-^X^M-bM-^@M-^Y
character, then the specified signature algorithms will be placed
at the head of the default set. The default for this option is:
ssh-ed25519-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ecdsa-sha2-nistp256@openssh.com,
sk-ssh-ed25519@openssh.com,
rsa-sha2-512,rsa-sha2-256
If hostkeys are known for the destination host then this default
is modified to prefer their algorithms.
The list of available signature algorithms may also be obtained
using "ssh -Q HostKeyAlgorithms".
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 and when validating host certificates. 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 configured
authentication identity and certificate files (either the default
files, or those explicitly configured in the ssh_config files or
passed on the ssh(1) command-line), even if ssh-agent(1) or a
PKCS11Provider or SecurityKeyProvider 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.
Otherwise if the specified value begins with a M-bM-^@M-^X$M-bM-^@M-^Y character,
then it will be treated as an environment variable containing the
location of the socket.
Arguments to IdentityAgent may use the tilde syntax to refer to a
user's home directory, the tokens described in the TOKENS section
and environment variables as described in the ENVIRONMENT
VARIABLES section.
IdentityFile
Specifies a file from which the user's DSA, ECDSA, authenticator-
hosted ECDSA, Ed25519, authenticator-hosted Ed25519 or RSA
authentication identity is read. You can also specify a public
key file to use the corresponding private key that is loaded in
ssh-agent(1) when the private key file is not present locally.
The default is ~/.ssh/id_rsa, ~/.ssh/id_ecdsa,
~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519, ~/.ssh/id_ed25519_sk and
~/.ssh/id_dsa. 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. Alternately an argument of none may be used to indicate
no identity files should be loaded.
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(7) wildcards
and, for user configurations, shell-like M-bM-^@M-^X~M-bM-^@M-^Y references to user
home directories. Wildcards will be expanded and processed in
lexical order. 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, le, 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 af21 (Low-Latency
Data) for interactive sessions and cs1 (Lower Effort) for non-
interactive sessions.
KbdInteractiveAuthentication
Specifies whether to use keyboard-interactive authentication.
The argument to this keyword must be yes (the default) or no.
ChallengeResponseAuthentication is a deprecated alias for this.
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 and pam.
KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms. Multiple
algorithms must be comma-separated. If the specified list begins
with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the specified algorithms will be
appended to the default set instead of replacing them. If the
specified list begins with a M-bM-^@M-^X-M-bM-^@M-^Y character, then the specified
algorithms (including wildcards) will be removed from the default
set instead of replacing them. If the specified list begins with
a M-bM-^@M-^X^M-bM-^@M-^Y character, then the specified algorithms will be placed at
the head of the default set. The default is:
sntrup761x25519-sha512@openssh.com,
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
The list of available key exchange algorithms may also be
obtained using "ssh -Q kex".
KnownHostsCommand
Specifies a command to use to obtain a list of host keys, in
addition to those listed in UserKnownHostsFile and
GlobalKnownHostsFile. This command is executed after the files
have been read. It may write host key lines to standard output
in identical format to the usual files (described in the
VERIFYING HOST KEYS section in ssh(1)). Arguments to
KnownHostsCommand accept the tokens described in the TOKENS
section. The command may be invoked multiple times per
connection: once when preparing the preference list of host key
algorithms to use, again to obtain the host key for the requested
host name and, if CheckHostIP is enabled, one more time to obtain
the host key matching the server's address. If the command exits
abnormally or returns a non-zero exit status then the connection
is terminated.
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 specifies the listener and may be
[bind_address:]port or a Unix domain socket path. The second
argument is the destination and may be host:hostport or a Unix
domain socket path if the remote host supports it.
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 M-bM-^@M-^X*M-bM-^@M-^Y indicates that the port should be available from
all interfaces. Unix domain socket paths may use the tokens
described in the TOKENS section and environment variables as
described in the ENVIRONMENT VARIABLES section.
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.
LogVerbose
Specify one or more overrides to LogLevel. An override consists
of a pattern lists that matches the source file, function and
line number to force detailed logging for. For example, an
override pattern of:
kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
would enable detailed logging for line 1000 of kex.c, everything
in the kex_exchange_identification() function, and all code in
the packet.c file. This option is intended for debugging and no
overrides are enabled by default.
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 list begins with a M-bM-^@M-^X+M-bM-^@M-^Y character,
then the specified algorithms will be appended to the default set
instead of replacing them. If the specified list begins with a
M-bM-^@M-^X-M-bM-^@M-^Y character, then the specified algorithms (including
wildcards) will be removed from the default set instead of
replacing them. If the specified list begins with a M-bM-^@M-^X^M-bM-^@M-^Y
character, then the specified algorithms will be placed at the
head of the default set.
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
Disable host authentication for localhost (loopback addresses).
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.
ObscureKeystrokeTiming
Specifies whether ssh(1) should try to obscure inter-keystroke
timings from passive observers of network traffic. If enabled,
then for interactive sessions, ssh(1) will send keystrokes at
fixed intervals of a few tens of milliseconds and will send fake
keystroke packets for some time after typing ceases. The
argument to this keyword must be yes, no or an interval specifier
of the form interval:milliseconds (e.g. interval:80 for 80
milliseconds). The default is to obscure keystrokes using a 20ms
packet interval. Note that smaller intervals will result in
higher fake keystroke packet rates.
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).
PermitRemoteOpen
Specifies the destinations to which remote TCP port forwarding is
permitted when RemoteForward is used as a SOCKS proxy. The
forwarding specification must be one of the following forms:
PermitRemoteOpen host:port
PermitRemoteOpen IPv4_addr:port
PermitRemoteOpen [IPv6_addr]:port
Multiple forwards may be specified by separating them with
whitespace. An argument of any can be used to remove all
restrictions and permit any forwarding requests. An argument of
none can be used to prohibit all forwarding requests. The
wildcard M-bM-^@M-^X*M-bM-^@M-^Y can be used for host or port to allow all hosts or
ports respectively. Otherwise, no pattern matching or address
lookups are performed on supplied names.
PKCS11Provider
Specifies which PKCS#11 provider to use or none to indicate that
no provider should be used (the default). The argument to this
keyword is a path to the PKCS#11 shared library ssh(1) should use
to communicate with a PKCS#11 token providing keys for user
authentication.
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
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 M-bM-^@M-^XexecM-bM-^@M-^Y 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 either [user@]host[:port]
or an ssh URI. 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.
Setting the host to none disables this option entirely.
Note that this option will compete with the ProxyCommand option -
whichever is specified first will prevent later instances of the
other from taking effect.
Note also that the configuration for the destination host (either
supplied via the command-line or the configuration file) is not
generally applied to jump hosts. ~/.ssh/config should be used if
specific configuration is required for jump hosts.
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.
PubkeyAcceptedAlgorithms
Specifies the signature algorithms that will be used for public
key authentication as a comma-separated list of patterns. If the
specified list begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the algorithms
after it will be appended to the default instead of replacing it.
If the specified list begins with a M-bM-^@M-^X-M-bM-^@M-^Y character, then the
specified algorithms (including wildcards) will be removed from
the default set instead of replacing them. If the specified list
begins with a M-bM-^@M-^X^M-bM-^@M-^Y character, then the specified algorithms will
be placed at the head of the default set. The default for this
option is:
ssh-ed25519-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
rsa-sha2-512,rsa-sha2-256
The list of available signature algorithms may also be obtained
using "ssh -Q PubkeyAcceptedAlgorithms".
PubkeyAuthentication
Specifies whether to try public key authentication. The argument
to this keyword must be yes (the default), no, unbound or
host-bound. The final two options enable public key
authentication while respectively disabling or enabling the
OpenSSH host-bound authentication protocol extension required for
restricted ssh-agent(1) forwarding.
RekeyLimit
Specifies the maximum amount of data that may be transmitted or
received before the session key is renegotiated, optionally
followed by 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 M-bM-^@M-^XKM-bM-^@M-^Y, M-bM-^@M-^XMM-bM-^@M-^Y, or M-bM-^@M-^XGM-bM-^@M-^Y to indicate
Kilobytes, Megabytes, or Gigabytes, respectively. The default is
between M-bM-^@M-^X1GM-bM-^@M-^Y and M-bM-^@M-^X4GM-bM-^@M-^Y, 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.
RemoteCommand
Specifies a command to execute on the remote 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 RemoteCommand accept the tokens described in
the TOKENS section.
RemoteForward
Specifies that a TCP port on the remote machine be forwarded over
the secure channel. The remote port may either be forwarded to a
specified host and port from the local machine, or may act as a
SOCKS 4/5 proxy that allows a remote client to connect to
arbitrary destinations from the local machine. The first
argument is the listening specification and may be
[bind_address:]port or, if the remote host supports it, a Unix
domain socket path. If forwarding to a specific destination then
the second argument must be host:hostport or a Unix domain socket
path, otherwise if no destination argument is specified then the
remote forwarding will be established as a SOCKS proxy. When
acting as a SOCKS proxy, the destination of the connection can be
restricted by PermitRemoteOpen.
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. Unix domain socket paths may use the tokens described
in the TOKENS section and environment variables as described in
the ENVIRONMENT VARIABLES section.
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 M-bM-^@M-^X*M-bM-^@M-^Y 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).
RequiredRSASize
Specifies the minimum RSA key size (in bits) that ssh(1) will
accept. User authentication keys smaller than this limit will be
ignored. Servers that present host keys smaller than this limit
will cause the connection to be terminated. The default is 1024
bits. Note that this limit may only be raised from the default.
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).
Arguments to RevokedHostKeys may use the tilde syntax to refer to
a user's home directory, the tokens described in the TOKENS
section and environment variables as described in the ENVIRONMENT
VARIABLES section.
SecurityKeyProvider
Specifies a path to a library that will be used when loading any
FIDO authenticator-hosted keys, overriding the default of using
the built-in USB HID support.
If the specified value begins with a M-bM-^@M-^X$M-bM-^@M-^Y character, then it will
be treated as an environment variable containing the path to the
library.
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.
See PATTERNS for more information on patterns.
It is possible to clear previously set SendEnv variable names by
prefixing patterns with -. The default is not to send any
environment variables.
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 unresponsive.
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.
SessionType
May be used to either request invocation of a subsystem on the
remote system, or to prevent the execution of a remote command at
all. The latter is useful for just forwarding ports. The
argument to this keyword must be none (same as the -N option),
subsystem (same as the -s option) or default (shell or command
execution).
SetEnv Directly specify one or more environment variables and their
contents to be sent to the server. Similarly to SendEnv, with
the exception of the TERM variable, the server must be prepared
to accept the environment variable.
StdinNull
Redirects stdin from /dev/null (actually, prevents reading from
stdin). Either this or the equivalent -n option must be used
when ssh is run in the background. The argument to this keyword
must be yes (same as the -n option) or no (the default).
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 man-in-the-middle (MITM) 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 accept-new then ssh will automatically add
new host keys to the user's known_hosts file, but will not permit
connections to hosts with changed host keys. If this flag is set
to no or off, ssh will automatically add new host keys to the
user known hosts files and allow connections to hosts with
changed hostkeys to proceed, subject to some restrictions. 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.
SyslogFacility
Gives the facility code that is used when logging messages from
ssh(1). The possible values are: DAEMON, USER, AUTH, LOCAL0,
LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The
default is USER.
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.
See also ServerAliveInterval for protocol-level keepalives.
Tag Specify a configuration tag name that may be later used by a
Match directive to select a block of configuration.
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 or ask. 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, the host was authenticated via UserKnownHostsFile
(i.e. not GlobalKnownHostsFile) and the host was authenticated
using a plain key and not a certificate.
UpdateHostKeys is enabled by default if the user has not
overridden the default UserKnownHostsFile setting and has not
enabled VerifyHostKeyDNS, otherwise UpdateHostKeys will be set to
no.
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.
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. Each filename may use tilde
notation to refer to the user's home directory, the tokens
described in the TOKENS section and environment variables as
described in the ENVIRONMENT VARIABLES section. A value of none
causes ssh(1) to ignore any user-specific known hosts files. 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, M-bM-^@M-^X*M-bM-^@M-^Y (a
wildcard that matches zero or more characters), or M-bM-^@M-^X?M-bM-^@M-^Y (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
(M-bM-^@M-^X!M-bM-^@M-^Y). 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"
Note that a negated match will never produce a positive result by itself.
For example, attempting to match "host3" against the following pattern-
list will fail:
from="!host1,!host2"
The solution here is to include a term that will yield a positive match,
such as a wildcard:
from="!host1,!host2,*"
TOKENS
Arguments to some keywords can make use of tokens, which are expanded at
runtime:
%% A literal M-bM-^@M-^X%M-bM-^@M-^Y.
%C Hash of %l%h%p%r%j.
%d Local user's home directory.
%f The fingerprint of the server's host key.
%H The known_hosts hostname or address that is being searched
for.
%h The remote hostname.
%I A string describing the reason for a KnownHostsCommand
execution: either ADDRESS when looking up a host by address
(only when CheckHostIP is enabled), HOSTNAME when searching
by hostname, or ORDER when preparing the host key algorithm
preference list to use for the destination host.
%i The local user ID.
%j The contents of the ProxyJump option, or the empty string if
this option is unset.
%K The base64 encoded host key.
%k The host key alias if specified, otherwise the original
remote hostname given on the command line.
%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.
%T The local tun(4) or tap(4) network interface assigned if
tunnel forwarding was requested, or "NONE" otherwise.
%t The type of the server host key, e.g. ssh-ed25519.
%u The local username.
CertificateFile, ControlPath, IdentityAgent, IdentityFile,
KnownHostsCommand, LocalForward, Match exec, RemoteCommand,
RemoteForward, RevokedHostKeys, and UserKnownHostsFile accept the tokens
%%, %C, %d, %h, %i, %j, %k, %L, %l, %n, %p, %r, and %u.
KnownHostsCommand additionally accepts the tokens %f, %H, %I, %K and %t.
Hostname accepts the tokens %% and %h.
LocalCommand accepts all tokens.
ProxyCommand and ProxyJump accept the tokens %%, %h, %n, %p, and %r.
Note that some of these directives build commands for execution via the
shell. Because ssh(1) performs no filtering or escaping of characters
that have special meaning in shell commands (e.g. quotes), it is the
user's responsibility to ensure that the arguments passed to ssh(1) do
not contain such characters and that tokens are appropriately quoted when
used.
ENVIRONMENT VARIABLES
Arguments to some keywords can be expanded at runtime from environment
variables on the client by enclosing them in ${}, for example
${HOME}/.ssh would refer to the user's .ssh directory. If a specified
environment variable does not exist then an error will be returned and
the setting for that keyword will be ignored.
The keywords CertificateFile, ControlPath, IdentityAgent, IdentityFile,
KnownHostsCommand, and UserKnownHostsFile support environment variables.
The keywords LocalForward and RemoteForward support environment variables
only for Unix domain socket paths.
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 writable 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.
OpenBSD 7.3 October 12, 2023 OpenBSD 7.3
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