SSH_CONFIG(5)

Configure the OpenSSH client. Static snapshot from Ubuntu 24.04.

SSH_CONFIG(5) 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 following or‐
der:

1. command-line options
2. user's configuration file (~/.ssh/config)
3. system-wide configuration file (/etc/ssh/ssh_config)

Unless noted otherwise, for each parameter, the first obtained value will be
used. The configuration files contain sections separated by Host specifications,
and that section is only applied for hosts that match one of the patterns given
in the specification. The matched host name is usually the one given on the com‐
mand line (see the CanonicalizeHostname option for exceptions).

Since the first obtained value for each parameter is used, more host-specific de‐
clarations 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 en‐
closed 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 white‐
space when specifying configuration options using the ssh, scp, and sftp -o op‐
tion.

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 key‐
word) to be only for those hosts that match one of the patterns given af‐
ter the keyword. If more than one pattern is provided, they should be
separated by whitespace. A single ‘*’ as a pattern can be used to pro‐
vide global defaults for all hosts. The host is usually the hostname ar‐
gument 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 key‐
word) 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 key‐
words are: canonical, final, exec, localnetwork, host, originalhost, Tag,
user, and localuser. The all criteria must appear alone or immediately
after canonical or final. Other criteria may be combined arbitrarily.
All criteria but all, canonical, and final require an argument. Criteria
may be negated by prepending an exclamation mark (‘!’).

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 canoni‐
cal host names only.

The final keyword requests that the configuration be re-parsed (regard‐
less 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 consid‐
ered true. Commands containing whitespace characters must be quoted.
Arguments to exec accept the tokens described in the “TOKENS” section.

The localnetwork keyword matches the addresses of active local network
interfaces against the supplied list of networks in CIDR format. This
may be convenient for varying the effective configuration on devices that
roam between networks. Note that network address is not a trustworthy
criteria in many situations (e.g. when the network is automatically con‐
figured using DHCP) and so caution should be applied if using it to con‐
trol security-sensitive configuration.

The other keywords' criteria must be single entries or comma-separated
lists and may use the wildcard and negation operators described in the
“PATTERNS” section. The criteria for the host keyword are matched
against the target hostname, after any substitution by the Hostname or
CanonicalizeHostname options. The originalhost keyword matches against
the hostname as it was specified on the command-line. The tagged keyword
matches a tag name specified by a prior Tag directive or on the ssh(1)
command-line using the -P flag. The user keyword matches against the
target username on the remote host. The localuser keyword matches
against the name of the local user running ssh(1) (this keyword may be
useful in system-wide ssh_config files).

AddKeysToAgent
Specifies whether keys should be automatically added to a running
ssh-agent(1). If this option is set to yes and a key is loaded from a
file, the key and its passphrase are added to the agent with the default
lifetime, as if by ssh-add(1). If this option is set to ask, ssh(1) will
require confirmation using the SSH_ASKPASS program before adding a key
(see ssh-add(1) for details). If this option is set to confirm, each use
of the key must be confirmed, as if the -c option was specified to
ssh-add(1). If this option is set to no, no keys are added to the agent.
Alternately, this option may be specified as a time interval using the
format described in the “TIME FORMATS” section of sshd_config(5) to spec‐
ify the key's lifetime in ssh-agent(1), after which it will automatically
be removed. The argument must be no (the default), yes, confirm (option‐
ally followed by a time interval), ask or a time interval.

AddressFamily
Specifies which address family to use when connecting. Valid arguments
are any (the default), inet (use IPv4 only), or inet6 (use IPv6 only).

BatchMode
If set to yes, user interaction such as password prompts and host key
confirmation requests will be disabled. 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 de‐
fault).

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 host‐
name using the system resolver's search rules. A value of no will cause
ssh(1) to fail instantly if CanonicalizeHostname is enabled and the tar‐
get 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 re‐
solver handle all hostname lookups. If set to yes then, for connections
that do not use a ProxyCommand or ProxyJump, ssh(1) will attempt to
canonicalize the hostname specified on the command line using the
CanonicalDomains suffixes and CanonicalizePermittedCNAMEs rules. If
CanonicalizeHostname is set to always, then canonicalization is applied
to proxied connections too.

If this option is enabled, then the configuration files are processed
again using the new target name to pick up any new configuration in
matching Host and Match stanzas. A value of none disables the use of a
ProxyJump host.

CanonicalizeMaxDots
Specifies the maximum number of dot characters in a hostname before
canonicalization is disabled. The default, 1, allows a single dot (i.e.
hostname.subdomain).

CanonicalizePermittedCNAMEs
Specifies rules to determine whether CNAMEs should be followed when
canonicalizing hostnames. The rules consist of one or more arguments of
source_domain_list:target_domain_list, where source_domain_list is a pat‐
tern-list of domains that may follow CNAMEs in canonicalization, and
target_domain_list is a pattern-list of domains that they may resolve to.

For example, "*.a.example.com:*.b.example.com,*.c.example.com" will allow
hostnames matching "*.a.example.com" to be canonicalized to names in the
"*.b.example.com" or "*.c.example.com" domains.

A single argument of "none" causes no CNAMEs to be considered for canoni‐
calization. This is the default behaviour.

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

ssh-ed25519,ecdsa-sha2-nistp256,
ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
rsa-sha2-512,rsa-sha2-256

If the specified list begins with a ‘+’ character, then the specified al‐
gorithms will be appended to the default set instead of replacing them.
If the specified list begins with a ‘-’ character, then the specified al‐
gorithms (including wildcards) will be removed from the default set in‐
stead of replacing them.

ssh(1) will not accept host certificates signed using algorithms other
than those specified.

CertificateFile
Specifies a file from which the user's certificate is read. A corre‐
sponding private key must be provided separately in order to use this
certificate either from an IdentityFile directive or -i flag to ssh(1),
via ssh-agent(1), or via a PKCS11Provider or SecurityKeyProvider.

Arguments to CertificateFile may use the tilde syntax to refer to a
user's home directory, the tokens described in the “TOKENS” section and
environment variables as described in the “ENVIRONMENT VARIABLES” sec‐
tion.

It is possible to have multiple certificate files specified in configura‐
tion files; these certificates will be tried in sequence. Multiple
CertificateFile directives will add to the list of certificates used for
authentication.

ChannelTimeout
Specifies whether and how quickly ssh(1) should close inactive channels.
Timeouts are specified as one or more “type=interval” pairs separated by
whitespace, where the “type” must be a channel type name (as described in
the table below), optionally containing wildcard characters.

The timeout value “interval” is specified in seconds or may use any of
the units documented in the “TIME FORMATS” section. For example,
“session=5m” would cause the interactive session to terminate after five
minutes of inactivity. Specifying a zero value disables the inactivity
timeout.

The available channel types include:

agent-connection
Open connections to ssh-agent(1).

direct-tcpip, direct-streamlocal@openssh.com
Open TCP or Unix socket (respectively) connections that have been
established from a ssh(1) local forwarding, i.e. LocalForward or
DynamicForward.

forwarded-tcpip, forwarded-streamlocal@openssh.com
Open TCP or Unix socket (respectively) connections that have been
established to a sshd(8) listening on behalf of a ssh(1) remote
forwarding, i.e. RemoteForward.

session
The interactive main session, including shell session, command
execution, scp(1), sftp(1), etc.

tun-connection
Open TunnelForward connections.

x11-connection
Open X11 forwarding sessions.

Note that in all the above cases, terminating an inactive session does
not guarantee to remove all resources associated with the session, e.g.
shell processes or X11 clients relating to the session may continue to
execute.

Moreover, terminating an inactive channel or session does not necessarily
close the SSH connection, nor does it prevent a client from requesting
another channel of the same type. In particular, expiring an inactive
forwarding session does not prevent another identical forwarding from be‐
ing subsequently created.

The default is not to expire channels of any type for inactivity.

CheckHostIP
If set to yes, ssh(1) will additionally check the host IP address in the
known_hosts file. This allows it to detect if a host key changed due to
DNS spoofing and will add addresses of destination hosts to
~/.ssh/known_hosts in the process, regardless of the setting of
StrictHostKeyChecking. If the option is set to no (the default), the
check will not be executed.

Ciphers
Specifies the ciphers allowed and their order of preference. Multiple
ciphers must be comma-separated. If the specified list begins with a ‘+’
character, then the specified ciphers will be appended to the default set
instead of replacing them. If the specified list begins with a ‘-’ char‐
acter, 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 op‐
tion 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 connec‐
tion. 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 in‐
stance'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 con‐
nections, however the display and agent forwarded will be the one belong‐
ing 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 al‐
ready 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 de‐
scribed in the ControlMaster section above or the string none to disable
connection sharing. Arguments to ControlPath may use the tilde syntax to
refer to a user's home directory, the tokens described in the “TOKENS”
section and environment variables as described in the “ENVIRONMENT
VARIABLES” section. It is recommended that any ControlPath used for op‐
portunistic connection sharing include at least %h, %p, and %r (or alter‐
natively %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 connec‐
tion is closed. If set to yes or 0, then the master connection will re‐
main in the background indefinitely (until killed or closed via a mecha‐
nism 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 se‐
cure 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 speci‐
fied 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 addi‐
tional forwardings can be given on the command line. Only the superuser
can forward privileged ports.

EnableEscapeCommandline
Enables the command line option in the EscapeChar menu for interactive
sessions (default ‘~C’). By default, the command line is disabled.

EnableSSHKeysign
Setting this option to yes in the global client configuration file
/etc/ssh/ssh_config enables the use of the helper program ssh-keysign(8)
during HostbasedAuthentication. The argument must be yes or no (the de‐
fault). 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 en‐
tirely (making the connection transparent for binary data).

ExitOnForwardFailure
Specifies whether ssh(1) should terminate the connection if it cannot set
up all requested dynamic, tunnel, local, and remote port forwardings,
(e.g. if either end is unable to bind and listen on a specified port).
Note that ExitOnForwardFailure does not apply to connections made over
port forwardings and will not, for example, cause ssh(1) to exit if TCP
connections to the ultimate forwarding destination fail. The argument
must be yes or no (the default).

FingerprintHash
Specifies the hash algorithm used when displaying key fingerprints.
Valid options are: md5 and sha256 (the default).

ForkAfterAuthentication
Requests ssh to go to background just before command execution. This is
useful if ssh is going to ask for passwords or passphrases, but the user
wants it in the background. This implies the StdinNull configuration op‐
tion being set to “yes”. The recommended way to start X11 programs at a
remote site is with something like ssh -f host xterm, which is the same
as ssh host xterm if the ForkAfterAuthentication configuration option is
set to “yes”.

If the ExitOnForwardFailure configuration option is set to “yes”, then a
client started with the ForkAfterAuthentication configuration option be‐
ing set to “yes” will wait for all remote port forwards to be success‐
fully established before placing itself in the background. The argument
to this keyword must be yes (same as the -f option) or no (the default).

ForwardAgent
Specifies whether the connection to the authentication agent (if any)
will be forwarded to the remote machine. The argument may be yes, no
(the default), an explicit path to an agent socket or the name of an en‐
vironment 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-do‐
main 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 authoriza‐
tion 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 re‐
ceived by ssh(1) after this time will be refused. Setting
ForwardX11Timeout to zero will disable the timeout and permit X11 for‐
warding for the life of the connection. The default is to disable un‐
trusted 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. Re‐
mote clients will be refused access after this time.

See the X11 SECURITY extension specification for full details on the re‐
strictions 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 de‐
fault).

GlobalKnownHostsFile
Specifies one or more files to use for the global host key database, sep‐
arated 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 de‐
fault 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 ex‐
pected GSSAPI server identity will be determined from the target host‐
name.

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 de‐
fault is “no”.

GSSAPIKexAlgorithms
The list of key exchange algorithms that are offered for GSSAPI key ex‐
change. 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 reveal identifying informa‐
tion 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 be‐
ing able to read unhashed host names from ~/.ssh/known_hosts.

HostbasedAcceptedAlgorithms
Specifies the signature algorithms that will be used for hostbased au‐
thentication as a comma-separated list of patterns. Alternately if the
specified list begins with a ‘+’ character, then the specified signature
algorithms will be appended to the default set instead of replacing them.
If the specified list begins with a ‘-’ character, then the specified
signature algorithms (including wildcards) will be removed from the de‐
fault set instead of replacing them. If the specified list begins with a
‘^’ character, then the specified signature algorithms will be placed at
the head of the default set. The default for this option is:

ssh-ed25519-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
rsa-sha2-512,rsa-sha2-256

The -Q option of ssh(1) may be used to list supported signature algo‐
rithms. This was formerly named HostbasedKeyTypes.

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

HostKeyAlgorithms
Specifies the host key signature algorithms that the client wants to use
in order of preference. Alternately if the specified list begins with a
‘+’ character, then the specified signature algorithms will be appended
to the default set instead of replacing them. If the specified list be‐
gins with a ‘-’ character, then the specified signature algorithms (in‐
cluding wildcards) will be removed from the default set instead of re‐
placing them. If the specified list begins with a ‘^’ character, then
the specified signature algorithms will be placed at the head of the de‐
fault set. The default for this option is:

ssh-ed25519-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ecdsa-sha2-nistp256@openssh.com,
sk-ssh-ed25519@openssh.com,
rsa-sha2-512,rsa-sha2-256

If hostkeys are known for the destination host then this default is modi‐
fied to prefer their algorithms.

The list of available signature algorithms may also be obtained using
"ssh -Q HostKeyAlgorithms".

HostKeyAlias
Specifies an alias that should be used instead of the real host name when
looking up or saving the host key in the host key database files and when
validating host certificates. This option is useful for tunneling SSH
connections or for multiple servers running on a single host.

Hostname
Specifies the real host name to log into. This can be used to specify
nicknames or abbreviations for hosts. Arguments to Hostname accept the
tokens described in the “TOKENS” section. Numeric IP addresses are also
permitted (both on the command line and in Hostname specifications). The
default is the name given on the command line.

IdentitiesOnly
Specifies that ssh(1) should only use the configured authentication iden‐
tity 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 de‐
fault). This option is intended for situations where ssh-agent offers
many different identities.

IdentityAgent
Specifies the Unix-domain socket used to communicate with the authentica‐
tion 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 dis‐
ables 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 be‐
gins 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, the tokens described in the “TOKENS” section and environ‐
ment variables as described in the “ENVIRONMENT VARIABLES” section.

IdentityFile
Specifies a file from which the user's DSA, ECDSA, authenticator-hosted
ECDSA, Ed25519, authenticator-hosted Ed25519 or RSA authentication iden‐
tity is read. You can also specify a public key file to use the corre‐
sponding private key that is loaded in ssh-agent(1) when the private key
file is not present locally. The default is ~/.ssh/id_rsa,
~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519,
~/.ssh/id_ed25519_sk and ~/.ssh/id_dsa. Additionally, any identities
represented by the authentication agent will be used for authentication
unless IdentitiesOnly is set. If no certificates have been explicitly
specified by CertificateFile, ssh(1) will try to load certificate infor‐
mation 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. Alter‐
nately an argument of none may be used to indicate no identity files
should be loaded.

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

IdentityFile may be used in conjunction with IdentitiesOnly to select
which identities in an agent are offered during authentication.
IdentityFile may also be used in conjunction with CertificateFile in or‐
der 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 en‐
countered 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.
Wildcards will be expanded and processed in lexical order. Files without
absolute paths are assumed to be in ~/.ssh if included in a user configu‐
ration file or /etc/ssh if included from the system configuration file.
Include directive may appear inside a Match or Host block to perform con‐
ditional inclusion.

IPQoS Specifies the IPv4 type-of-service or DSCP class for connections. Ac‐
cepted 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 argu‐
ment to this keyword must be yes (the default) or no.
ChallengeResponseAuthentication is a deprecated alias for this.

KbdInteractiveDevices
Specifies the list of methods to use in keyboard-interactive authentica‐
tion. 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 algo‐
rithms 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 default is:

sntrup761x25519-sha512@openssh.com,
curve25519-sha256,curve25519-sha256@libssh.org,
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group16-sha512,
diffie-hellman-group18-sha512,
diffie-hellman-group14-sha256

The list of available key exchange algorithms may also be obtained using
"ssh -Q kex".

KnownHostsCommand
Specifies a command to use to obtain a list of host keys, in addition to
those listed in UserKnownHostsFile and GlobalKnownHostsFile. This com‐
mand is executed after the files have been read. It may write host key
lines to standard output in identical format to the usual files (de‐
scribed in the “VERIFYING HOST KEYS” section in ssh(1)). Arguments to
KnownHostsCommand accept the tokens described in the “TOKENS” section.
The command may be invoked multiple times per connection: once when
preparing the preference list of host key algorithms to use, again to ob‐
tain the host key for the requested host name and, if CheckHostIP is en‐
abled, one more time to obtain the host key matching the server's ad‐
dress. If the command exits abnormally or returns a non-zero exit status
then the connection is terminated.

LocalCommand
Specifies a command to execute on the local machine after successfully
connecting to the server. The command string extends to the end of the
line, and is executed with the user's shell. Arguments to LocalCommand
accept the tokens described in the “TOKENS” section.

The command is run synchronously and does not have access to the session
of the ssh(1) that spawned it. It should not be used for interactive
commands.

This directive is ignored unless PermitLocalCommand has been enabled.

LocalForward
Specifies that a TCP port on the local machine be forwarded over the se‐
cure channel to the specified host and port from the remote machine. The
first argument specifies the listener and may be [bind_address:]port or a
Unix domain socket path. The second argument is the destination and may
be host:hostport or a Unix domain socket path if the remote host supports
it.

IPv6 addresses can be specified by enclosing addresses in square brack‐
ets. Multiple forwardings may be specified, and additional forwardings
can be given on the command line. Only the superuser can forward privi‐
leged 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 may use the tokens described in the
“TOKENS” section and environment variables as described in the
“ENVIRONMENT VARIABLES” section.

LogLevel
Gives the verbosity level that is used when logging messages from ssh(1).
The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DE‐
BUG1, DEBUG2, and DEBUG3. The default is INFO. DEBUG and DEBUG1 are
equivalent. DEBUG2 and DEBUG3 each specify higher levels of verbose out‐
put.

LogVerbose
Specify one or more overrides to LogLevel. An override consists of a
pattern lists that matches the source file, function and line number to
force detailed logging for. For example, an override pattern of:

kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*

would enable detailed logging for line 1000 of kex.c, everything in the
kex_exchange_identification() function, and all code in the packet.c
file. This option is intended for debugging and no overrides are enabled
by default.

MACs Specifies the MAC (message authentication code) algorithms in order of
preference. The MAC algorithm is used for data integrity protection.
Multiple algorithms must be comma-separated. If the specified list be‐
gins with a ‘+’ character, then the specified algorithms will be appended
to the default set instead of replacing them. If the specified list be‐
gins with a ‘-’ character, then the specified algorithms (including wild‐
cards) will be removed from the default set instead of replacing them.
If the specified list begins with a ‘^’ character, then the specified al‐
gorithms 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 recom‐
mended.

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 ar‐
gument to this keyword must be yes or no (the default).

NumberOfPasswordPrompts
Specifies the number of password prompts before giving up. The argument
to this keyword must be an integer. The default is 3.

ObscureKeystrokeTiming
Specifies whether ssh(1) should try to obscure inter-keystroke timings
from passive observers of network traffic. If enabled, then for interac‐
tive sessions, ssh(1) will send keystrokes at fixed intervals of a few
tens of milliseconds and will send fake keystroke packets for some time
after typing ceases. The argument to this keyword must be yes, no or an
interval specifier of the form interval:milliseconds (e.g. interval:80
for 80 milliseconds). The default is to obscure keystrokes using a 20ms
packet interval. Note that smaller intervals will result in higher fake
keystroke packet rates.

PasswordAuthentication
Specifies whether to use password authentication. The argument to this
keyword must be yes (the default) or no.

PermitLocalCommand
Allow local command execution via the LocalCommand option or using the
!command escape sequence in ssh(1). The argument must be yes or no (the
default).

PermitRemoteOpen
Specifies the destinations to which remote TCP port forwarding is permit‐
ted when RemoteForward is used as a SOCKS proxy. The forwarding specifi‐
cation must be one of the following forms:

PermitRemoteOpen host:port
PermitRemoteOpen IPv4_addr:port
PermitRemoteOpen [IPv6_addr]:port

Multiple forwards may be specified by separating them with whitespace.
An argument of any can be used to remove all restrictions and permit any
forwarding requests. An argument of none can be used to prohibit all
forwarding requests. The wildcard ‘*’ can be used for host or port to
allow all hosts or ports respectively. Otherwise, no pattern matching or
address lookups are performed on supplied names.

PKCS11Provider
Specifies which PKCS#11 provider to use or none to indicate that no
provider should be used (the default). The argument to this keyword is a
path to the PKCS#11 shared library ssh(1) should use to communicate with
a PKCS#11 token providing keys for user authentication.

Port Specifies the port number to connect on the remote host. The default is
22.

PreferredAuthentications
Specifies the order in which the client should try authentication meth‐
ods. 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. Setting the host to none disables this option en‐
tirely.

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 sup‐
plied via the command-line or the configuration file) is not generally
applied to jump hosts. ~/.ssh/config should be used if specific configu‐
ration is required for jump hosts.

ProxyUseFdpass
Specifies that ProxyCommand will pass a connected file descriptor back to
ssh(1) instead of continuing to execute and pass data. The default is
no.

PubkeyAcceptedAlgorithms
Specifies the signature algorithms that will be used for public key au‐
thentication as a comma-separated list of patterns. If the specified
list begins with a ‘+’ character, then the algorithms after it will be
appended to the default instead of replacing it. 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 speci‐
fied algorithms will be placed at the head of the default set. The de‐
fault for this option is:

The list of available signature algorithms may also be obtained using
"ssh -Q PubkeyAcceptedAlgorithms".

PubkeyAuthentication
Specifies whether to try public key authentication. The argument to this
keyword must be yes (the default), no, unbound or host-bound. The final
two options enable public key authentication while respectively disabling
or enabling the OpenSSH host-bound authentication protocol extension re‐
quired for restricted ssh-agent(1) forwarding.

RekeyLimit
Specifies the maximum amount of data that may be transmitted or received
before the session key is renegotiated, optionally followed by a maximum
amount of time that may pass before the session key is renegotiated. The
first argument is specified in bytes and may have a suffix of ‘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 docu‐
mented 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 se‐
cure channel. The remote port may either be forwarded to a specified
host and port from the local machine, or may act as a SOCKS 4/5 proxy
that allows a remote client to connect to arbitrary destinations from the
local machine. The first argument is the listening specification and may
be [bind_address:]port or, if the remote host supports it, a Unix domain
socket path. If forwarding to a specific destination then the second ar‐
gument must be host:hostport or a Unix domain socket path, otherwise if
no destination argument is specified then the remote forwarding will be
established as a SOCKS proxy. When acting as a SOCKS proxy, the destina‐
tion of the connection can be restricted by PermitRemoteOpen.

IPv6 addresses can be specified by enclosing addresses in square brack‐
ets. Multiple forwardings may be specified, and additional forwardings
can be given on the command line. Privileged ports can be forwarded only
when logging in as root on the remote machine. Unix domain socket paths
may use the tokens described in the “TOKENS” section and environment
variables as described in the “ENVIRONMENT VARIABLES” section.

If the port argument is 0, the listen port will be dynamically allocated
on the server and reported to the client at run time.

If the bind_address is not specified, the default is to only bind to
loopback addresses. If the bind_address is ‘*’ or an empty string, then
the forwarding is requested to listen on all interfaces. Specifying a
remote bind_address will only succeed if the server's GatewayPorts option
is enabled (see sshd_config(5)).

RequestTTY
Specifies whether to request a pseudo-tty for the session. The argument
may be one of: no (never request a TTY), yes (always request a TTY when
standard input is a TTY), force (always request a TTY) or auto (request a
TTY when opening a login session). This option mirrors the -t and -T
flags for ssh(1).

RequiredRSASize
Specifies the minimum RSA key size (in bits) that ssh(1) will accept.
User authentication keys smaller than this limit will be ignored.
Servers that present host keys smaller than this limit will cause the
connection to be terminated. The default is 1024 bits. Note that this
limit may only be raised from the default.

RevokedHostKeys
Specifies revoked host public keys. Keys listed in this file will be re‐
fused for host authentication. Note that if this file does not exist or
is not readable, then host authentication will be refused for all hosts.
Keys may be specified as a text file, listing one public key per line, or
as an OpenSSH Key Revocation List (KRL) as generated by ssh-keygen(1).
For more information on KRLs, see the KEY REVOCATION LISTS section in
ssh-keygen(1). Arguments to RevokedHostKeys may use the tilde syntax to
refer to a user's home directory, the tokens described in the “TOKENS”
section and environment variables as described in the “ENVIRONMENT
VARIABLES” section.

SecurityKeyProvider
Specifies a path to a library that will be used when loading any FIDO au‐
thenticator-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 config‐
ured to accept these environment variables. Note that the TERM environ‐
ment 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 sepa‐
rated 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 prefix‐
ing patterns with -. The default is not to send any environment vari‐
ables.

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 en‐
crypted 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 re‐
ceived from the server, ssh(1) will send a message through the encrypted
channel to request a response from the server. The default is 0, indi‐
cating 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.

SessionType
May be used to either request invocation of a subsystem on the remote
system, or to prevent the execution of a remote command at all. The lat‐
ter is useful for just forwarding ports. The argument to this keyword
must be none (same as the -N option), subsystem (same as the -s option)
or default (shell or command execution).

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

StdinNull
Redirects stdin from /dev/null (actually, prevents reading from stdin).
Either this or the equivalent -n option must be used when ssh is run in
the background. The argument to this keyword must be yes (same as the -n
option) or no (the default).

StreamLocalBindMask
Sets the octal file creation mode mask (umask) used when creating a Unix-
domain socket file for local or remote port forwarding. This option is
only used for port forwarding to a Unix-domain socket file.

The default value is 0177, which creates a Unix-domain socket file that
is readable and writable only by the owner. Note that not all operating
systems honor the file mode on Unix-domain socket files.

StreamLocalBindUnlink
Specifies whether to remove an existing Unix-domain socket file for local
or remote port forwarding before creating a new one. If the socket file
already exists and StreamLocalBindUnlink is not enabled, ssh will be un‐
able to forward the port to the Unix-domain socket file. This option is
only used for port forwarding to a Unix-domain socket file.

The argument must be yes or no (the default).

StrictHostKeyChecking
If this flag is set to yes, ssh(1) will never automatically add host keys
to the ~/.ssh/known_hosts file, and refuses to connect to hosts whose
host key has changed. This provides maximum protection against man-in-
the-middle (MITM) attacks, though it can be annoying when the
/etc/ssh/ssh_known_hosts file is poorly maintained or when connections to
new hosts are frequently made. This option forces the user to manually
add all new hosts.

If this flag is set to accept-new then ssh will automatically add new
host keys to the user's known_hosts file, but will not permit connections
to hosts with changed host keys. If this flag is set to no or off, ssh
will automatically add new host keys to the user known hosts files and
allow connections to hosts with changed hostkeys to proceed, subject to
some restrictions. If this flag is set to ask (the default), new host
keys will be added to the user known host files only after the user has
confirmed that is what they really want to do, and ssh will refuse to
connect to hosts whose host key has changed. The host keys of known
hosts will be verified automatically in all cases.

SyslogFacility
Gives the facility code that is used when logging messages from ssh(1).
The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LO‐
CAL3, 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 annoy‐
ing.

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 impor‐
tant in scripts, and many users want it too.

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

Tag Specify a configuration tag name that may be later used by a Match direc‐
tive to select a block of configuration.

Tunnel Request tun(4) device forwarding between the client and the server. The
argument must be yes, point-to-point (layer 3), ethernet (layer 2), or no
(the default). Specifying yes requests the default tunnel mode, which is
point-to-point.

TunnelDevice
Specifies the tun(4) devices to open on the client (local_tun) and the
server (remote_tun).

The argument must be local_tun[:remote_tun]. The devices may be speci‐
fied by numerical ID or the keyword any, which uses the next available
tunnel device. If remote_tun is not specified, it defaults to any. The
default is any:any.

UpdateHostKeys
Specifies whether ssh(1) should accept notifications of additional
hostkeys from the server sent after authentication has completed and add
them to UserKnownHostsFile. The argument must be yes, no or ask. This
option allows learning alternate hostkeys for a server and supports
graceful key rotation by allowing a server to send replacement public
keys before old ones are removed.

Additional hostkeys are only accepted if the key used to authenticate the
host was already trusted or explicitly accepted by the user, the host was
authenticated via UserKnownHostsFile (i.e. not GlobalKnownHostsFile) and
the host was authenticated using a plain key and not a certificate.

UpdateHostKeys is enabled by default if the user has not overridden the
default UserKnownHostsFile setting and has not enabled VerifyHostKeyDNS,
otherwise UpdateHostKeys will be set to no.

If UpdateHostKeys is set to ask, then the user is asked to confirm the
modifications to the known_hosts file. Confirmation is currently incom‐
patible 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 hav‐
ing 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, sepa‐
rated by whitespace. Each filename may use tilde notation to refer to
the user's home directory, the tokens described in the “TOKENS” section
and environment variables as described in the “ENVIRONMENT VARIABLES”
section. A value of none causes ssh(1) to ignore any user-specific known
hosts files. The default is ~/.ssh/known_hosts, ~/.ssh/known_hosts2.

VerifyHostKeyDNS
Specifies whether to verify the remote key using DNS and SSHFP resource
records. If this option is set to yes, the client will implicitly trust
keys that match a secure fingerprint from DNS. Insecure fingerprints
will be handled as if this option was set to ask. If this option is set
to ask, information on fingerprint match will be displayed, but the user
will still need to confirm new host keys according to the
StrictHostKeyChecking option. The default is no.

See also “VERIFYING HOST KEYS” in ssh(1).

VisualHostKey
If this flag is set to yes, an ASCII art representation of the remote
host key fingerprint is printed in addition to the fingerprint string at
login and for unknown host keys. If this flag is set to no (the de‐
fault), no fingerprint strings are printed at login and only the finger‐
print 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 exam‐
ple, 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%j.
%d Local user's home directory.
%f The fingerprint of the server's host key.
%H The known_hosts hostname or address that is being searched for.
%h The remote hostname.
%I A string describing the reason for a KnownHostsCommand execution: ei‐
ther ADDRESS when looking up a host by address (only when CheckHostIP
is enabled), HOSTNAME when searching by hostname, or ORDER when
preparing the host key algorithm preference list to use for the des‐
tination host.
%i The local user ID.
%j The contents of the ProxyJump option, or the empty string if this op‐
tion is unset.
%K The base64 encoded host key.
%k The host key alias if specified, otherwise the original remote host‐
name given on the command line.
%L The local hostname.
%l The local hostname, including the domain name.
%n The original remote hostname, as given on the command line.
%p The remote port.
%r The remote username.
%T The local tun(4) or tap(4) network interface assigned if tunnel for‐
warding was requested, or "NONE" otherwise.
%t The type of the server host key, e.g. ssh-ed25519.
%u The local username.

CertificateFile, ControlPath, IdentityAgent, IdentityFile, KnownHostsCommand,
LocalForward, Match exec, RemoteCommand, RemoteForward, RevokedHostKeys, and
UserKnownHostsFile accept the tokens %%, %C, %d, %h, %i, %j, %k, %L, %l, %n, %p,
%r, and %u.

KnownHostsCommand additionally accepts the tokens %f, %H, %I, %K and %t.

Hostname accepts the tokens %% and %h.

LocalCommand accepts all tokens.

ProxyCommand and ProxyJump accept the tokens %%, %h, %n, %p, and %r.

Note that some of these directives build commands for execution via the shell.
Because ssh(1) performs no filtering or escaping of characters that have special
meaning in shell commands (e.g. quotes), it is the user's responsibility to en‐
sure that the arguments passed to ssh(1) do not contain such characters and that
tokens are appropriately quoted when used.

ENVIRONMENT VARIABLES
Arguments to some keywords can be expanded at runtime from environment variables
on the client by enclosing them in ${}, for example ${HOME}/.ssh would refer to
the user's .ssh directory. If a specified environment variable does not exist
then an error will be returned and the setting for that keyword will be ignored.

The keywords CertificateFile, ControlPath, IdentityAgent, IdentityFile,
KnownHostsCommand, and UserKnownHostsFile support environment variables. The
keywords LocalForward and RemoteForward support environment variables only for
Unix domain socket paths.

FILES
~/.ssh/config
This is the per-user configuration file. The format of this file is de‐
scribed above. This file is used by the SSH client. Because of the po‐
tential 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.