Ubuntu Manpage: ssh_config — OpenSSH client configuration file

Provided by: openssh-client_8.2p1-4ubuntu0.13_amd64

NAME

       ssh_config — 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)

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.

Note that the Debian openssh-client package sets several options as standard in /etc/ssh/ssh_config which
are not the default in ssh(1):

• Include /etc/ssh/ssh_config.d/*.conf
• SendEnv LANG LC_*
• HashKnownHosts yes
• GSSAPIAuthentication yes

/etc/ssh/ssh_config.d/*.conf files are included at the start of the system-wide configuration file, so
options set there will override those in /etc/ssh/ssh_config.

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, final, exec, host, originalhost, 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 (‘!’).

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 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, user interaction such as password prompts and host key confirmation requests will
be disabled. In addition, the ServerAliveInterval option will be set to 300 seconds by default
(Debian-specific). This option is useful in scripts and other batch jobs where no user is
present to interact with ssh(1), and where it is desirable to detect a broken network swiftly.
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.

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.

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

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 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.

Ciphers
Specifies the ciphers allowed and their order of preference. Multiple ciphers must be comma-
separated. If the specified list begins with a ‘+’ character, then the specified ciphers will be
appended to the default set instead of replacing them. If the specified list begins with a ‘-’
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 ‘^’ 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 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 (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 ‘*’ 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 may be yes, no (the default), an explicit path to an agent socket
or the name of an environment variable (beginning with ‘$’) 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, (the Debian-specific default), remote X11 clients will have full
access to the original X11 display.

If this option is set to no (the upstream 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.

GSSAPIClientIdentity
If set, specifies the GSSAPI client identity that ssh should use when connecting to the server.
The default is unset, which means that the default identity will be used.

GSSAPIDelegateCredentials
Forward (delegate) credentials to the server. The default is no.

GSSAPIKeyExchange
Specifies whether key exchange based on GSSAPI may be used. When using GSSAPI key exchange the
server need not have a host key. The default is “no”.

GSSAPIRenewalForcesRekey
If set to “yes” then renewal of the client's GSSAPI credentials will force the rekeying of the
ssh connection. With a compatible server, this will delegate the renewed credentials to a session
on the server.

Checks are made to ensure that credentials are only propagated when the new credentials match the
old ones on the originating client and where the receiving server still has the old set in its
cache.

The default is “no”.

For this to work GSSAPIKeyExchange needs to be enabled in the server and also used by the client.

GSSAPIServerIdentity
If set, specifies the GSSAPI server identity that ssh should expect when connecting to the
server. The default is unset, which means that the expected GSSAPI server identity will be
determined from the target hostname.

GSSAPITrustDns
Set to “yes” to indicate that the DNS is trusted to securely canonicalize the name of the host
being connected to. If “no”, the hostname entered on the command line will be passed untouched to
the GSSAPI library. The default is “no”.

GSSAPIKexAlgorithms
The list of key exchange algorithms that are offered for GSSAPI key exchange. Possible values are

gss-gex-sha1-,
gss-group1-sha1-,
gss-group14-sha1-,
gss-group14-sha256-,
gss-group16-sha512-,
gss-nistp256-sha256-,

gss-curve25519-sha256-
The default is “gss-gex-sha1-,gss-group14-sha1-”. This option only applies to protocol version 2
connections using GSSAPI.

HashKnownHosts
Indicates that ssh(1) should hash host names and addresses when they are added to
~/.ssh/known_hosts. These hashed names may be used normally by ssh(1) and sshd(8), but they do
not 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). Use of this option may break
facilities such as tab-completion that rely on being able to read unhashed host names from
~/.ssh/known_hosts.

HostbasedAuthentication
Specifies whether to try rhosts based authentication with public key authentication. The
argument must be yes or no (the default).

HostbasedKeyTypes
Specifies the key types that will be used for hostbased authentication as a comma-separated list
of patterns. Alternately if the specified list begins with a ‘+’ character, then the specified
key types will be appended to the default set instead of replacing them. If the specified list
begins with a ‘-’ character, then the specified key types (including wildcards) will be removed
from the default set instead of replacing them. If the specified list begins with a ‘^’
character, then the specified key types will be placed at the head of the default set. 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,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ecdsa-sha2-nistp256@openssh.com,
ssh-ed25519,sk-ssh-ed25519@openssh.com,
rsa-sha2-512,rsa-sha2-256,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 list begins with a ‘+’ character, then the specified key types will
be appended to the default set instead of replacing them. If the specified list begins with a
‘-’ character, then the specified key types (including wildcards) will be removed from the
default set instead of replacing them. If the specified list begins with a ‘^’ character, then
the specified key types will be placed at the head of the default set. The default for this
option is:

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 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 ‘$’
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 or the
tokens described in the “TOKENS” 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. The default is
~/.ssh/id_dsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519, ~/.ssh/id_ed25519_sk and
~/.ssh/id_rsa. 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(7) 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, 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 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 and pam.

KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms must be comma-
separated. If the specified list begins with a ‘+’ character, then the specified methods will be
appended to the default set instead of replacing them. If the specified list begins with a ‘-’
character, then the specified methods (including wildcards) will be removed from the default set
instead of replacing them. If the specified list begins with a ‘^’ character, then the specified
methods will be placed at the head of the default set. The default is:

curve25519-sha256,curve25519-sha256@libssh.org,
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group16-sha512,
diffie-hellman-group18-sha512,
diffie-hellman-group14-sha256

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 list begins with a ‘+’ character, then the specified algorithms will be appended
to the default set instead of replacing them. If the specified list begins with a ‘-’ 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 ‘^’ 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.

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 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 ‘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 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.

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.

PubkeyAcceptedKeyTypes
Specifies the key types that will be used for public key authentication as a comma-separated list
of patterns. If the specified list begins with a ‘+’ character, then the key types after it will
be appended to the default instead of replacing it. If the specified list begins with a ‘-’
character, then the specified key types (including wildcards) will be removed from the default
set instead of replacing them. If the specified list begins with a ‘^’ character, then the
specified key types will be placed at the head of the default set. The default for this option
is:

The list of available key types may also be obtained using "ssh -Q PubkeyAcceptedKeyTypes".

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.

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 must be [bind_address:]port If forwarding to a specific destination
then the second argument must be host:hostport, otherwise if no destination argument is specified
then the remote forwarding will be established as a SOCKS proxy.

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).

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 ‘$’ 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, or 300 if the
BatchMode option is set (Debian-specific). ProtocolKeepAlives and SetupTimeOut are Debian-
specific compatibility aliases for this option.

SetEnv Directly specify one or more environment variables and their contents to be sent to the server.
Similarly to SendEnv, the server must be prepared to accept the environment variable.

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
known hosts files, 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. This
option only uses TCP keepalives (as opposed to using ssh level keepalives), so takes a long time
to notice when the connection dies. As such, you probably want the ServerAliveInterval option as
well. However, this means that connections will die if the route is down temporarily, and some
people find it annoying.

The default is yes (to send TCP keepalive messages), and the client will notice if the network
goes down or the remote host dies. This is important in scripts, and many users want it too.

To disable TCP keepalive messages, the value should be set to no. See also ServerAliveInterval
for protocol-level keepalives.

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.

UpdateHostKeys is enabled by default if the user has not overridden the default
UserKnownHostsFile setting, otherwise UpdateHostKeys will be set to ask.

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. 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/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"

       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 ‘%’.
             %C    Hash of %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.
             %T    The  local tun(4) or tap(4) network interface assigned if tunnel forwarding was requested, or
                   "NONE" otherwise.
             %u    The local username.

       Match exec accepts the tokens %%, %h, %i, %L, %l, %n, %p, %r, and %u.

       CertificateFile accepts the tokens %%, %d, %h, %i, %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, %i, %l, %r, and %u.

       LocalCommand accepts the tokens %%, %C, %d, %h, %i, %l, %n, %p, %r, %T, and %u.

       ProxyCommand accepts the tokens %%, %h, %n, %p, and %r.

       RemoteCommand accepts the tokens %%, %C, %d, %h, %i, %l, %n, %p, %r, and %u.

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.   It  may  be  group-writable
               provided that the group in question contains only the user.

       /etc/ssh/ssh_config
               Systemwide  configuration  file.   This  file  provides  defaults  for  those values that are not
               specified in the user's configuration file, and for those users who do not have  a  configuration
               file.  This file must be world-readable.

AUTHORS