SSH_CONFIG(5)

SSH_CONFIG(5) BSD File Formats Manual SSH_CONFIG(5)

NAME
ssh_config -- OpenSSH client configuration file

DESCRIPTION
ssh(1) obtains configuration data from the following sources in the fol-
lowing 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 configu-
ration 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-spe-
cific 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 stan-
dard in /etc/ssh/ssh_config which are not the default in ssh(1):

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

/etc/ssh/ssh_config.d/*.conf files are included at the start of the sys-
tem-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 op-
tionally 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 op-
tions using the ssh, scp, and sftp -o option.

The possible keywords and their meanings are as follows (note that key-
words 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 excep-
tions).

A pattern entry may be negated by prefixing it with an exclama-
tion 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 ex-
ceptions 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 us-
ing 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 con-
ditions 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 condi-
tion is considered true. Commands containing whitespace charac-
ters must be quoted. Arguments to exec accept the tokens de-
scribed in the TOKENS section.

The other keywords' criteria must be single entries or comma-sep-
arated lists and may use the wildcard and negation operators de-
scribed in the PATTERNS section. The criteria for the host key-
word are matched against the target hostname, after any substitu-
tion by the Hostname or CanonicalizeHostname options. The
originalhost keyword matches against the hostname as it was spec-
ified 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 de-
tails). 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 ar-
guments 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 ad-
dress 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 des-
tination host.

CanonicalizeFallbackLocal
Specifies whether to fail with an error when hostname canonical-
ization fails. The default, yes, will attempt to look up the un-
qualified 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 con-
nections too.

If this option is enabled, then the configuration files are pro-
cessed again using the new target name to pick up any new config-
uration in matching Host and Match stanzas.

CanonicalizeMaxDots
Specifies the maximum number of dot characters in a hostname be-
fore canonicalization is disabled. The default, 1, allows a sin-
gle 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 canonical-
ized to names in the "*.b.example.com" or "*.c.example.com" do-
mains.

CASignatureAlgorithms
Specifies which algorithms are allowed for signing of certifi-
cates by certificate authorities (CAs). The default is:

ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa

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 se-
quence. 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 de-
tect if a host key changed due to DNS spoofing and will add ad-
dresses 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 configura-
tion 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 ex-
iting. 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 ex-
change.

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 connec-
tion rather than initiating new ones, but will fall back to con-
necting normally if the control socket does not exist, or is not
listening.

Setting this to ask will cause ssh(1) to listen for control con-
nections, but require confirmation using ssh-askpass(1). If the
ControlPath cannot be opened, ssh(1) will continue without con-
necting to a master instance.

X11 and ssh-agent(1) forwarding is supported over these multi-
plexed connections, however the display and agent forwarded will
be the one belonging to the master connection i.e. it is not pos-
sible 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 shar-
ing 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 connec-
tion has been closed. If set to no (the default), then the mas-
ter 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 back-
ground 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 back-
grounded 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 set-
ting. 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 informa-
tion.

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 lis-
ten 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 ulti-
mate forwarding destination fail. The argument must be yes or no
(the default).

FingerprintHash
Specifies the hash algorithm used when displaying key finger-
prints. 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 redi-
rected 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 dis-
play 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 de-
fault is to disable untrusted X11 forwarding after twenty minutes
has elapsed.

ForwardX11Trusted
If this option is set to yes, (the Debian-specific default), re-
mote X11 clients will have full access to the original X11 dis-
play.

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. Fur-
thermore, 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 spec-
ify 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 data-
base, 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 ex-
pect 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-group14-sha256-,gss-group16-sha512-,gss-nistp256-sha256-,gss-curve25519-sha256-,gss-gex-sha1-,gss-group14-sha1-".
This option only applies to 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 re-
veal identifying information if the file's contents are dis-
closed. The default is no. Note that existing names and ad-
dresses in known hosts files will not be converted automatically,
but may be manually hashed using ssh-keygen(1). Use of this op-
tion may break facilities such as tab-completion that rely on be-
ing 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 de-
fault).

HostbasedKeyTypes
Specifies the key types that will be used for hostbased authenti-
cation 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 replac-
ing them. If the specified list begins with a '-' character,
then the specified key types (including wildcards) will be re-
moved 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 de-
fault 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 ap-
pended 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 data-
base files and when validating host certificates. This option is
useful for tunneling SSH connections or for multiple servers run-
ning 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. Nu-
meric 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 authentica-
tion 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 au-
thentication 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. Other-
wise if the specified value begins with a '$' character, then it
will be treated as an environment variable containing the loca-
tion 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 sec-
tion.

IdentityFile
Specifies a file from which the user's DSA, ECDSA, authenticator-
hosted ECDSA, Ed25519, authenticator-hosted Ed25519 or RSA au-
thentication 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 iden-
tities 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 sec-
tion.

It is possible to have multiple identity files specified in con-
figuration files; all these identities will be tried in sequence.
Multiple IdentityFile directives will add to the list of identi-
ties tried (this behaviour differs from that of other configura-
tion directives).

IdentityFile may be used in conjunction with IdentitiesOnly to
select which identities in an agent are offered during authenti-
cation. 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 unrecog-
nised by ssh(1). It is recommended that IgnoreUnknown be listed
early in the configuration file as it will not be applied to un-
known 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 argu-
ment is specified, it is used as the packet class uncondition-
ally. If two values are specified, the first is automatically
selected for interactive sessions and the second for non-interac-
tive 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 au-
thentication. 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 (in-
cluding wildcards) will be removed from the default set instead
of replacing them. If the specified list begins with a '^' char-
acter, 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 ob-
tained using "ssh -Q kex".

LocalCommand
Specifies a command to execute on the local machine after suc-
cessfully 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 en-
abled.

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 ar-
gument is the destination and may be host:hostport or a Unix do-
main 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 '*' indicates that the port should be available from
all interfaces. Unix domain socket paths accept the tokens de-
scribed in the TOKENS section.

LogLevel
Gives the verbosity level that is used when logging messages from
ssh(1). The possible values are: QUIET, FATAL, ERROR, INFO, VER-
BOSE, 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 or-
der 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 replac-
ing them. If the specified list begins with a '-' character,
then the specified algorithms (including wildcards) will be re-
moved 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 en-
cryption (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 us-
ing 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 au-
thentication.

Port Specifies the port number to connect on the remote host. The de-
fault 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 com-
mand 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 manage-
ment 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 char-
acters 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 estab-
lishing 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 authen-
tication 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 be-
fore the session key is renegotiated, optionally followed a maxi-
mum amount of time that may pass before the session key is rene-
gotiated. 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 per-
formed 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 suc-
cessfully 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 arbi-
trary 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 for-
warding 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.

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 ma-
chine. Unix domain socket paths accept the tokens described in
the TOKENS 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 '*' or an empty
string, then the forwarding is requested to listen on all inter-
faces. 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 re-
quest 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 wild-
card 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 envi-
ronment 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 be-
ing 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 there-
fore will not be spoofable. The TCP keepalive option enabled by
TCPKeepAlive is spoofable. The server alive mechanism is valu-
able when the client or server depend on knowing when a connec-
tion 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-spe-
cific). ProtocolKeepAlives and SetupTimeOut are Debian-specific
compatibility aliases for this option.

SetEnv Directly specify one or more environment variables and their con-
tents 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 pro-
tection 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 con-
firmed 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, LO-
CAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The de-
fault 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 op-
tion 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 de-
faults to any. The default is any:any.

UpdateHostKeys
Specifies whether ssh(1) should accept notifications of addi-
tional 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 al-
lowing 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 overrid-
den the default UserKnownHostsFile setting, otherwise
UpdateHostKeys will be set to ask.

If UpdateHostKeys is set to ask, then the user is asked to con-
firm 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 dif-
ferent user name is used on different machines. This saves the
trouble of having to remember to give the user name on the com-
mand line.

UserKnownHostsFile
Specifies one or more files to use for the user host key data-
base, 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.