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 order:
1. command-line options
2. user's configuration file (~/.ssh/config)
3. system-wide configuration file (/etc/ssh/ssh_config)
Unless noted otherwise, for each parameter, the first obtained value
will be used. The configuration files contain sections separated by
Host specifications, and that section is only applied for hosts that
match one of the patterns given in the specification. The matched host
name is usually the one given on the command line (see the
CanonicalizeHostname option for exceptions).
Since the first obtained value for each parameter is used, more host-
specific declarations should be given near the beginning of the file,
and general defaults at the end.
Note that the Debian openssh-client package sets several options as
standard in /etc/ssh/ssh_config which are not the default in ssh(1):
• Include /etc/ssh/ssh_config.d/*.conf
• SendEnv LANG LC_*
• HashKnownHosts yes
• GSSAPIAuthentication yes
/etc/ssh/ssh_config.d/*.conf files are included at the start of the
system-wide configuration file, so options set there will override
those in /etc/ssh/ssh_config.
The file contains keyword-argument pairs, one per line. Lines starting
with ‘#’ and empty lines are interpreted as comments. Arguments may
optionally be enclosed in double quotes (") in order to represent argu-
ments containing spaces. Configuration options may be separated by
whitespace or optional whitespace and exactly one ‘=’; the latter for-
mat is useful to avoid the need to quote whitespace when specifying
configuration options using the ssh, scp, and sftp -o option.
The possible keywords and their meanings are as follows (note that 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 pro-
vide exceptions for wildcard matches.
See “PATTERNS” for more information on patterns.
Match Restricts the following declarations (up to the next Host or
Match keyword) to be used only when the conditions following
the Match keyword are satisfied. Match conditions are speci-
fied using one or more criteria or the single token all which
always matches. The available criteria keywords are:
canonical, final, exec, localnetwork, host, originalhost, Tag,
user, and localuser. The all criteria must appear alone or im-
mediately after canonical or final. Other criteria may be com-
bined arbitrarily. All criteria but all, canonical, and final
require an argument. Criteria may be negated by prepending an
exclamation mark (‘!’).
The canonical keyword matches only when the configuration file
is being re-parsed after hostname canonicalization (see the
CanonicalizeHostname option). This may be useful to specify
conditions that work with canonical host names only.
The final keyword requests that the configuration be re-parsed
(regardless of whether CanonicalizeHostname is enabled), and
matches only during this final pass. If CanonicalizeHostname
is enabled, then canonical and final match during the same
pass.
The exec keyword executes the specified command under the
user's shell. If the command returns a zero exit status then
the condition is considered true. Commands containing white-
space 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 situa-
tions (e.g. when the network is automatically configured using
DHCP) and so caution should be applied if using it to control
security-sensitive configuration.
The other keywords' criteria must be single entries or comma-
separated lists and may use the wildcard and negation operators
described in the “PATTERNS” section. The criteria for the host
keyword are matched against the target hostname, after any sub-
stitution 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 run-
ning 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” sec-
tion of sshd_config(5) to specify the key's lifetime in
ssh-agent(1), after which it will automatically be removed.
The argument must be no (the default), yes, confirm (optionally
followed by a time interval), ask or a time interval.
AddressFamily
Specifies which address family to use when connecting. Valid
arguments are any (the default), inet (use IPv4 only), or inet6
(use IPv6 only).
BatchMode
If set to yes, user interaction such as password prompts and
host key confirmation requests will be disabled. In addition,
the ServerAliveInterval option will be set to 300 seconds by
default (Debian-specific). This option is useful in scripts
and other batch jobs where no user is present to interact with
ssh(1), and where it is desirable to detect a broken network
swiftly. The argument must be yes or no (the default).
BindAddress
Use the specified address on the local machine as the source
address of the connection. Only useful on systems with more
than one address.
BindInterface
Use the address of the specified interface on the local machine
as the source address of the connection.
CanonicalDomains
When CanonicalizeHostname is enabled, this option specifies the
list of domain suffixes in which to search for the specified
destination host.
CanonicalizeFallbackLocal
Specifies whether to fail with an error when hostname canoni-
calization fails. The default, yes, will attempt to look up
the unqualified hostname using the system resolver's search
rules. A value of no will cause ssh(1) to fail instantly if
CanonicalizeHostname is enabled and the target hostname cannot
be found in any of the domains specified by CanonicalDomains.
CanonicalizeHostname
Controls whether explicit hostname canonicalization is per-
formed. 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 suf-
fixes and CanonicalizePermittedCNAMEs rules. If
CanonicalizeHostname is set to always, then canonicalization is
applied to proxied connections too.
If this option is enabled, then the configuration files are
processed again using the new target name to pick up any new
configuration in matching Host and Match stanzas. A value of
none disables the use of a ProxyJump host.
CanonicalizeMaxDots
Specifies the maximum number of dot characters in a hostname
before canonicalization is disabled. The default, 1, allows a
single dot (i.e. hostname.subdomain).
CanonicalizePermittedCNAMEs
Specifies rules to determine whether CNAMEs should be followed
when canonicalizing hostnames. The rules consist of one or
more arguments of source_domain_list:target_domain_list, where
source_domain_list is a pattern-list of domains that may follow
CNAMEs in canonicalization, and target_domain_list is a pat-
tern-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 canoni-
calized to names in the "*.b.example.com" or "*.c.example.com"
domains.
A single argument of "none" causes no CNAMEs to be considered
for canonicalization. This is the default behaviour.
CASignatureAlgorithms
Specifies which algorithms are allowed for signing of certifi-
cates 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 algorithms will be appended to the default set in-
stead of replacing them. If the specified list begins with a
‘-’ character, then the specified algorithms (including wild-
cards) will be removed from the default set instead of replac-
ing them.
ssh(1) will not accept host certificates signed using algo-
rithms other than those specified.
CertificateFile
Specifies a file from which the user's certificate is read. A
corresponding private key must be provided separately in order
to use this certificate either from an IdentityFile directive
or -i flag to ssh(1), via ssh-agent(1), or via a PKCS11Provider
or SecurityKeyProvider.
Arguments to CertificateFile may use the tilde syntax to refer
to a user's home directory, the tokens described in the
“TOKENS” section and environment variables as described in the
“ENVIRONMENT VARIABLES” section.
It is possible to have multiple certificate files specified in
configuration files; these certificates will be tried in se-
quence. 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 ses-
sion does not guarantee to remove all resources associated with
the session, e.g. shell processes or X11 clients relating to
the session may continue to execute.
Moreover, terminating an inactive channel or session does not
necessarily close the SSH connection, nor does it prevent a
client from requesting another channel of the same type. In
particular, expiring an inactive forwarding session does not
prevent another identical forwarding from being subsequently
created.
The default is not to expire channels of any type for inactiv-
ity.
CheckHostIP
If set to yes, ssh(1) will additionally check the host IP ad-
dress 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, regard-
less 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 ‘-’ 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
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 ex-
change.
ControlMaster
Enables the sharing of multiple sessions over a single network
connection. When set to yes, ssh(1) will listen for connec-
tions on a control socket specified using the ControlPath argu-
ment. Additional sessions can connect to this socket using the
same ControlPath with ControlMaster set to no (the default).
These sessions will try to reuse the master instance's network
connection rather than initiating new ones, but will fall back
to connecting normally if the control socket does not exist, or
is not listening.
Setting this to ask will cause ssh(1) to listen for control
connections, but require confirmation using ssh-askpass(1). If
the ControlPath cannot be opened, ssh(1) will continue without
connecting to a master instance.
X11 and ssh-agent(1) forwarding is supported over these multi-
plexed connections, however the display and agent forwarded
will be the one belonging to the master connection i.e. it is
not possible to forward multiple displays or agents.
Two additional options allow for opportunistic multiplexing:
try to use a master connection but fall back to creating a new
one if one does not already exist. These options are: auto and
autoask. The latter requires confirmation like the ask option.
ControlPath
Specify the path to the control socket used for connection
sharing as described in the ControlMaster section above or the
string none to disable connection sharing. Arguments to
ControlPath may use the tilde syntax to refer to a user's home
directory, the tokens described in the “TOKENS” section and en-
vironment 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 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
master connection will not be placed into the background, and
will close as soon as the initial client connection is closed.
If set to yes or 0, then the master connection will remain in
the background indefinitely (until killed or closed via a mech-
anism 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 ter-
minate 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 de-
fault, 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 ‘*’ indi-
cates 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 com-
mand line. Only the superuser can forward privileged ports.
EnableEscapeCommandline
Enables the command line option in the EscapeChar menu for in-
teractive 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 default). This option should be placed
in the non-hostspecific section. See ssh-keysign(8) for more
information.
EscapeChar
Sets the escape character (default: ‘~’). The escape character
can also be set on the command line. The argument should be a
single character, ‘^’ followed by a letter, or none to disable
the escape character entirely (making the connection transpar-
ent 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 finger-
prints. 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 im-
plies the StdinNull configuration option 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 being set to “yes” will wait for all re-
mote port forwards to be successfully established before plac-
ing itself in the background. The argument to this keyword
must be yes (same as the -f option) or no (the default).
ForwardAgent
Specifies whether the connection to the authentication agent
(if any) will be forwarded to the remote machine. The argument
may be yes, no (the default), an explicit path to an agent
socket or the name of an environment variable (beginning with
‘$’) 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 opera-
tions 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
display through the forwarded connection. An attacker may then
be able to perform activities such as keystroke monitoring if
the ForwardX11Trusted option is also enabled.
ForwardX11Timeout
Specify a timeout for untrusted X11 forwarding using the format
described in the “TIME FORMATS” section of sshd_config(5). X11
connections received by ssh(1) after this time will be refused.
Setting ForwardX11Timeout to zero will disable the timeout and
permit X11 forwarding for the life of the connection. The de-
fault is to disable untrusted X11 forwarding after twenty min-
utes 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 steal-
ing or tampering with data belonging to trusted X11 clients.
Furthermore, the xauth(1) token used for the session will be
set to expire after 20 minutes. Remote clients will be refused
access after this time.
See the X11 SECURITY extension specification for full details
on the restrictions imposed on untrusted clients.
GatewayPorts
Specifies whether remote hosts are allowed to connect to local
forwarded ports. By default, ssh(1) binds local port forward-
ings 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 for-
warded ports. The argument must be yes or no (the default).
GlobalKnownHostsFile
Specifies one or more files to use for the global host key
database, separated by whitespace. The default is
/etc/ssh/ssh_known_hosts, /etc/ssh/ssh_known_hosts2.
GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is al-
lowed. 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 compati-
ble server, this will delegate the renewed credentials to a
session on the server.
Checks are made to ensure that credentials are only propagated
when the new credentials match the old ones on the originating
client and where the receiving server still has the old set in
its cache.
The default is “no”.
For this to work GSSAPIKeyExchange needs to be enabled in the
server and also used by the client.
GSSAPIServerIdentity
If set, specifies the GSSAPI server identity that ssh should
expect when connecting to the server. The default is unset,
which means that the expected GSSAPI server identity will be
determined from the target hostname.
GSSAPITrustDns
Set to “yes” to indicate that the DNS is trusted to securely
canonicalize the name of the host being connected to. If “no”,
the hostname entered on the command line will be passed un-
touched 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 visu-
ally reveal identifying information if the file's contents are
disclosed. The default is no. Note that existing names and
addresses in known hosts files will not be converted automati-
cally, but may be manually hashed using ssh-keygen(1). Use of
this option may break facilities such as tab-completion that
rely on being able to read unhashed host names from
~/.ssh/known_hosts.
HostbasedAcceptedAlgorithms
Specifies the signature algorithms that will be used for host-
based authentication 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 algo-
rithms (including wildcards) will be removed from the default
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
algorithms. This was formerly named HostbasedKeyTypes.
HostbasedAuthentication
Specifies whether to try rhosts based authentication with pub-
lic key authentication. The argument must be yes or no (the
default).
HostKeyAlgorithms
Specifies the host key signature algorithms that the client
wants to use in order of preference. Alternately if the speci-
fied list begins with a ‘+’ character, then the specified sig-
nature 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 default set instead of re-
placing them. If the specified list begins with a ‘^’ charac-
ter, then the specified signature algorithms will be placed at
the head of the default set. The default for this option is:
ssh-ed25519-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ecdsa-sha2-nistp256@openssh.com,
sk-ssh-ed25519@openssh.com,
rsa-sha2-512,rsa-sha2-256
If hostkeys are known for the destination host then this de-
fault is modified to prefer their algorithms.
The list of available signature algorithms may also be obtained
using "ssh -Q HostKeyAlgorithms".
HostKeyAlias
Specifies an alias that should be used instead of the real host
name when looking up or saving the host key in the host key
database files and when validating host certificates. This op-
tion 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 authenti-
cation identity and certificate files (either the default
files, or those explicitly configured in the ssh_config files
or passed on the ssh(1) command-line), even if ssh-agent(1) or
a PKCS11Provider or SecurityKeyProvider offers more identities.
The argument to this keyword must be yes or no (the default).
This option is intended for situations where ssh-agent offers
many different identities.
IdentityAgent
Specifies the Unix-domain socket used to communicate with the
authentication agent.
This option overrides the SSH_AUTH_SOCK environment variable
and can be used to select a specific agent. Setting the socket
name to none disables the use of an authentication agent. If
the string "SSH_AUTH_SOCK" is specified, the location of the
socket will be read from the SSH_AUTH_SOCK environment vari-
able. Otherwise if the specified value begins with a ‘$’ char-
acter, then it will be treated as an environment variable con-
taining the location of the socket.
Arguments to IdentityAgent may use the tilde syntax to refer to
a user's home directory, the tokens described in the “TOKENS”
section and environment variables as described in the
“ENVIRONMENT VARIABLES” section.
IdentityFile
Specifies a file from which the user's DSA, ECDSA, authentica-
tor-hosted ECDSA, Ed25519, authenticator-hosted Ed25519 or RSA
authentication identity is read. You can also specify a public
key file to use the corresponding private key that is loaded in
ssh-agent(1) when the private key file is not present locally.
The default is ~/.ssh/id_rsa, ~/.ssh/id_ecdsa,
~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519, ~/.ssh/id_ed25519_sk and
~/.ssh/id_dsa. Additionally, any identities represented by the
authentication agent will be used for authentication unless
IdentitiesOnly is set. If no certificates have been explicitly
specified by CertificateFile, ssh(1) will try to load certifi-
cate information from the filename obtained by appending
-cert.pub to the path of a specified IdentityFile.
Arguments to IdentityFile may use the tilde syntax to refer to
a user's home directory or the tokens described in the “TOKENS”
section. Alternately an argument of none may be used to indi-
cate 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 se-
quence. 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 authen-
tication. IdentityFile may also be used in conjunction with
CertificateFile in order to provide any certificate also needed
for authentication with the identity.
IgnoreUnknown
Specifies a pattern-list of unknown options to be ignored if
they are encountered in configuration parsing. This may be
used to suppress errors if ssh_config contains options that are
unrecognised by ssh(1). It is recommended that IgnoreUnknown
be listed early in the configuration file as it will not be ap-
plied to unknown options that appear before it.
Include
Include the specified configuration file(s). Multiple path-
names may be specified and each pathname may contain glob(7)
wildcards and, for user configurations, shell-like ‘~’ refer-
ences to user home directories. Wildcards will be expanded and
processed in lexical order. Files without absolute paths are
assumed to be in ~/.ssh if included in a user configuration
file or /etc/ssh if included from the system configuration
file. Include directive may appear inside a Match or Host
block to perform conditional inclusion.
IPQoS Specifies the IPv4 type-of-service or DSCP class for connec-
tions. 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 nu-
meric 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 au-
tomatically selected for interactive sessions and the second
for non-interactive sessions. The default is lowdelay for in-
teractive 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.
ChallengeResponseAuthentication is a deprecated alias for this.
KbdInteractiveDevices
Specifies the list of methods to use in keyboard-interactive
authentication. Multiple method names must be comma-separated.
The default is to use the server specified list. The methods
available vary depending on what the server supports. For an
OpenSSH server, it may be zero or more of: bsdauth and pam.
KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms. Multi-
ple algorithms must be comma-separated. If the specified list
begins with a ‘+’ character, then the specified algorithms will
be appended to the default set instead of replacing them. If
the specified list begins with a ‘-’ character, then the speci-
fied 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 ob-
tained using "ssh -Q kex".
KnownHostsCommand
Specifies a command to use to obtain a list of host keys, in
addition to those listed in UserKnownHostsFile and
GlobalKnownHostsFile. This command is executed after the files
have been read. It may write host key lines to standard output
in identical format to the usual files (described in the
“VERIFYING HOST KEYS” section in ssh(1)). Arguments to
KnownHostsCommand accept the tokens described in the “TOKENS”
section. The command may be invoked multiple times per connec-
tion: once when preparing the preference list of host key algo-
rithms to use, again to obtain the host key for the requested
host name and, if CheckHostIP is enabled, one more time to ob-
tain the host key matching the server's address. If the com-
mand 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 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
enabled.
LocalForward
Specifies that a TCP port on the local machine be forwarded
over the secure channel to the specified host and port from the
remote machine. The first argument specifies the listener and
may be [bind_address:]port or a Unix domain socket path. The
second argument is the destination and may be host:hostport or
a Unix domain socket path if the remote host supports it.
IPv6 addresses can be specified by enclosing addresses in
square brackets. Multiple forwardings may be specified, and
additional forwardings can be given on the command line. Only
the superuser can forward privileged ports. By default, the
local port is bound in accordance with the GatewayPorts 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. Unix domain socket
paths may use the tokens described in the “TOKENS” section and
environment variables as described in the “ENVIRONMENT
VARIABLES” section.
LogLevel
Gives the verbosity level that is used when logging messages
from ssh(1). The possible values are: QUIET, FATAL, ERROR,
INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default
is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3
each specify higher levels of verbose output.
LogVerbose
Specify one or more overrides to LogLevel. An override con-
sists 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, every-
thing in the kex_exchange_identification() function, and all
code in the packet.c file. This option is intended for debug-
ging 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 in-
tegrity protection. Multiple algorithms must be comma-sepa-
rated. 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 wild-
cards) will be removed from the default set instead of replac-
ing 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.
ObscureKeystrokeTiming
Specifies whether ssh(1) should try to obscure inter-keystroke
timings from passive observers of network traffic. If enabled,
then for interactive sessions, ssh(1) will send keystrokes at
fixed intervals of a few tens of milliseconds and will send
fake keystroke packets for some time after typing ceases. The
argument to this keyword must be yes, no or an interval speci-
fier of the form interval:milliseconds (e.g. interval:80 for 80
milliseconds). The default is to obscure keystrokes using a
20ms packet interval. Note that smaller intervals will result
in higher fake keystroke packet rates.
PasswordAuthentication
Specifies whether to use password authentication. The argument
to this keyword must be yes (the default) or no.
PermitLocalCommand
Allow local command execution via the LocalCommand option or
using the !command escape sequence in ssh(1). The argument
must be yes or no (the default).
PermitRemoteOpen
Specifies the destinations to which remote TCP port forwarding
is permitted when RemoteForward is used as a SOCKS proxy. The
forwarding specification must be one of the following forms:
PermitRemoteOpen host:port
PermitRemoteOpen IPv4_addr:port
PermitRemoteOpen [IPv6_addr]:port
Multiple forwards may be specified by separating them with
whitespace. An argument of any can be used to remove all re-
strictions 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 authentica-
tion methods. This allows a client to prefer one method (e.g.
keyboard-interactive) over another method (e.g. password). The
default is:
gssapi-with-mic,hostbased,publickey,
keyboard-interactive,password
ProxyCommand
Specifies the command to use to connect to the server. The
command string extends to the end of the line, and is executed
using the user's shell ‘exec’ directive to avoid a lingering
shell process.
Arguments to ProxyCommand accept the tokens described in the
“TOKENS” section. The command can be basically anything, and
should read from its standard input and write to its standard
output. It should eventually connect an sshd(8) server running
on some machine, or execute sshd -i somewhere. Host key man-
agement will be done using the Hostname of the host being con-
nected (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 con-
nect 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 op-
tion will cause ssh(1) to connect to the target host by first
making a ssh(1) connection to the specified ProxyJump host and
then establishing a TCP forwarding to the ultimate target from
there. Setting the host to none disables this option entirely.
Note that this option will compete with the ProxyCommand option
- whichever is specified first will prevent later instances of
the other from taking effect.
Note also that the configuration for the destination host (ei-
ther 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 descrip-
tor back to ssh(1) instead of continuing to execute and pass
data. The default is no.
PubkeyAcceptedAlgorithms
Specifies the signature algorithms that will be used for public
key authentication as a comma-separated list of patterns. If
the specified list begins with a ‘+’ character, then the algo-
rithms after it will be appended to the default instead of re-
placing it. 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
default for this option is:
ssh-ed25519-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ssh-ed25519@openssh.com,
sk-ecdsa-sha2-nistp256@openssh.com,
rsa-sha2-512,rsa-sha2-256
The list of available signature algorithms may also be obtained
using "ssh -Q PubkeyAcceptedAlgorithms".
PubkeyAuthentication
Specifies whether to try public key authentication. The argu-
ment to this keyword must be yes (the default), no, unbound or
host-bound. The final two options enable public key authenti-
cation while respectively disabling or enabling the OpenSSH
host-bound authentication protocol extension required for re-
stricted 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 op-
tional second value is specified in seconds and may use any of
the units documented in the TIME FORMATS section of
sshd_config(5). The default value for RekeyLimit is default
none, which means that rekeying is performed after the cipher's
default amount of data has been sent or received and no time
based rekeying is done.
RemoteCommand
Specifies a command to execute on the remote machine after 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 for-
warded to a specified host and port from the local machine, or
may act as a SOCKS 4/5 proxy that allows a remote client to
connect to arbitrary destinations from the local machine. The
first argument is the listening specification and may be
[bind_address:]port or, if the remote host supports it, a Unix
domain socket path. If forwarding to a specific destination
then the second argument must be host:hostport or a Unix domain
socket path, otherwise if no destination argument is specified
then the remote forwarding will be established as a SOCKS
proxy. When acting as a SOCKS proxy, the destination of the
connection can be restricted by PermitRemoteOpen.
IPv6 addresses can be specified by enclosing addresses in
square brackets. Multiple forwardings may be specified, and
additional forwardings can be given on the command line. Priv-
ileged ports can be forwarded only when logging in as root on
the remote machine. Unix domain socket paths may use the to-
kens described in the “TOKENS” section and environment vari-
ables 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 re-
quest a TTY) or auto (request a TTY when opening a login ses-
sion). This option mirrors the -t and -T flags for ssh(1).
RequiredRSASize
Specifies the minimum RSA key size (in bits) that ssh(1) will
accept. User authentication keys smaller than this limit will
be ignored. Servers that present host keys smaller than this
limit will cause the connection to be terminated. The default
is 1024 bits. Note that this limit may only be raised from the
default.
RevokedHostKeys
Specifies revoked host public keys. Keys listed in this file
will be refused for host authentication. Note that if this
file does not exist or is not readable, then host authentica-
tion will be refused for all hosts. Keys may be specified as a
text file, listing one public key per line, or as an OpenSSH
Key Revocation List (KRL) as generated by ssh-keygen(1). For
more information on KRLs, see the KEY REVOCATION LISTS section
in ssh-keygen(1). Arguments to RevokedHostKeys may use the
tilde syntax to refer to a user's home directory, the tokens
described in the “TOKENS” section and environment variables as
described in the “ENVIRONMENT VARIABLES” section.
SecurityKeyProvider
Specifies a path to a library that will be used when loading
any FIDO authenticator-hosted keys, overriding the default of
using the built-in USB HID support.
If the specified value begins with a ‘$’ 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 vari-
ables. Note that the TERM environment variable is always sent
whenever a pseudo-terminal is requested as it is required by
the protocol. Refer to AcceptEnv in sshd_config(5) for how to
configure the server. Variables are specified by name, which
may contain wildcard characters. Multiple environment vari-
ables may be separated by whitespace or spread across multiple
SendEnv directives.
See “PATTERNS” for more information on patterns.
It is possible to clear previously set SendEnv variable names
by prefixing patterns with -. The default is not to send any
environment variables.
ServerAliveCountMax
Sets the number of server alive messages (see below) which may
be sent without ssh(1) receiving any messages back from the
server. If this threshold is reached while server alive mes-
sages are being sent, ssh will disconnect from the server, ter-
minating the session. It is important to note that the use of
server alive messages is very different from TCPKeepAlive (be-
low). The server alive messages are sent through the encrypted
channel and therefore will not be spoofable. The TCP keepalive
option enabled by TCPKeepAlive is spoofable. The server alive
mechanism is valuable when the client or server depend on know-
ing 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 discon-
nect after approximately 45 seconds.
ServerAliveInterval
Sets a timeout interval in seconds after which if no data has
been received from the server, ssh(1) will send a message
through the encrypted channel to request a response from the
server. The default is 0, indicating that these messages will
not be sent to the server, or 300 if the BatchMode option is
set (Debian-specific). ProtocolKeepAlives and SetupTimeOut are
Debian-specific compatibility aliases for this option.
SessionType
May be used to either request invocation of a subsystem on the
remote system, or to prevent the execution of a remote command
at all. The latter is useful for just forwarding ports. The
argument to this keyword must be none (same as the -N option),
subsystem (same as the -s option) or default (shell or command
execution).
SetEnv Directly specify one or more environment variables and their
contents to be sent to the server. Similarly to SendEnv, with
the exception of the TERM variable, the server must be prepared
to accept the environment variable.
StdinNull
Redirects stdin from /dev/null (actually, prevents reading from
stdin). Either this or the equivalent -n option must be used
when ssh is run in the background. The argument to this key-
word must be yes (same as the -n option) or no (the default).
StreamLocalBindMask
Sets the octal file creation mode mask (umask) used when creat-
ing a Unix-domain socket file for local or remote port forward-
ing. 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-do-
main 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 con-
nect to hosts whose host key has changed. This provides maxi-
mum 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 fre-
quently 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 re-
strictions. If this flag is set to ask (the default), new host
keys will be added to the user known host files only after the
user has confirmed that is what they really want to do, and ssh
will refuse to connect to hosts whose host key has changed.
The host keys of known hosts will be verified automatically in
all cases.
SyslogFacility
Gives the facility code that is used when logging messages from
ssh(1). The possible values are: DAEMON, USER, AUTH, LOCAL0,
LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The
default is USER.
TCPKeepAlive
Specifies whether the system should send TCP keepalive messages
to the other side. If they are sent, death of the connection
or crash of one of the machines will be properly noticed. This
option only uses TCP keepalives (as opposed to using ssh level
keepalives), so takes a long time to notice when the connection
dies. As such, you probably want the ServerAliveInterval op-
tion 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 important in scripts, and many users want it
too.
To disable TCP keepalive messages, the value should be set to
no. See also ServerAliveInterval for protocol-level
keepalives.
Tag Specify a configuration tag name that may be later used by a
Match directive to select a block of configuration.
Tunnel Request tun(4) device forwarding between the client and the
server. The argument must be yes, point-to-point (layer 3),
ethernet (layer 2), or no (the default). Specifying yes re-
quests the default tunnel mode, which is point-to-point.
TunnelDevice
Specifies the tun(4) devices to open on the client (local_tun)
and the server (remote_tun).
The argument must be local_tun[:remote_tun]. The devices may
be specified by numerical ID or the keyword any, which uses the
next available tunnel device. If remote_tun is not specified,
it defaults to any. The default is any:any.
UpdateHostKeys
Specifies whether ssh(1) should accept notifications of 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 au-
thenticate 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 over-
ridden the default UserKnownHostsFile setting and has not en-
abled VerifyHostKeyDNS, otherwise UpdateHostKeys will be set to
no.
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 dis-
abled if it is enabled.
Presently, only sshd(8) from OpenSSH 6.8 and greater support
the "hostkeys@openssh.com" protocol extension used to inform
the client of all the server's hostkeys.
User Specifies the user to log in as. This can be useful when a
different user name is used on different machines. This saves
the trouble of having to remember to give the user name on the
command line.
UserKnownHostsFile
Specifies one or more files to use for the user host key data-
base, separated by whitespace. Each filename may use tilde no-
tation to refer to the user's home directory, the tokens de-
scribed 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 fin-
gerprint string at login and for unknown host keys. If this
flag is set to no (the default), no fingerprint strings are
printed at login and only the fingerprint string will be
printed for unknown host keys.
XAuthLocation
Specifies the full pathname of the xauth(1) program. The de-
fault 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 dec-
larations 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] net-
work range:
Host 192.168.0.?
A pattern-list is a comma-separated list of patterns. Patterns within
pattern-lists may be negated by preceding them with an exclamation mark
(‘!’). For example, to allow a key to be used from anywhere within an
organization except from the "dialup" pool, the following entry (in au-
thorized_keys) could be used:
from="!*.dialup.example.com,*.example.com"
Note that a negated match will never produce a positive result by it-
self. 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 exe-
cution: either ADDRESS when looking up a host by address
(only when CheckHostIP is enabled), HOSTNAME when searching
by hostname, or ORDER when preparing the host key algorithm
preference list to use for the destination host.
%i The local user ID.
%j The contents of the ProxyJump option, or the empty string
if this option is unset.
%K The base64 encoded host key.
%k The host key alias if specified, otherwise the original re-
mote hostname given on the command line.
%L The local hostname.
%l The local hostname, including the domain name.
%n The original remote hostname, as given on the command line.
%p The remote port.
%r The remote username.
%T The local tun(4) or tap(4) network interface assigned if
tunnel forwarding was requested, or "NONE" otherwise.
%t The type of the server host key, e.g. ssh-ed25519.
%u The local username.
CertificateFile, ControlPath, IdentityAgent, IdentityFile,
KnownHostsCommand, LocalForward, Match exec, RemoteCommand,
RemoteForward, RevokedHostKeys, and UserKnownHostsFile accept the to-
kens %%, %C, %d, %h, %i, %j, %k, %L, %l, %n, %p, %r, and %u.
KnownHostsCommand additionally accepts the tokens %f, %H, %I, %K and
%t.
Hostname accepts the tokens %% and %h.
LocalCommand accepts all tokens.
ProxyCommand and ProxyJump accept the tokens %%, %h, %n, %p, and %r.
Note that some of these directives build commands for execution via the
shell. Because ssh(1) performs no filtering or escaping of characters
that have special meaning in shell commands (e.g. quotes), it is the
user's responsibility to ensure that the arguments passed to ssh(1) do
not contain such characters and that tokens are appropriately quoted
when used.
ENVIRONMENT VARIABLES
Arguments to some keywords can be expanded at runtime from environment
variables on the client by enclosing them in ${}, for example
${HOME}/.ssh would refer to the user's .ssh directory. If a specified
environment variable does not exist then an error will be returned and
the setting for that keyword will be ignored.
The keywords CertificateFile, ControlPath, IdentityAgent, IdentityFile,
KnownHostsCommand, and UserKnownHostsFile support environment vari-
ables. The keywords LocalForward and RemoteForward support environment
variables only for Unix domain socket paths.
FILES
~/.ssh/config
This is the per-user configuration file. The format of this
file is described above. This file is used by the SSH client.
Because of the potential for abuse, this file must have strict
permissions: read/write for the user, and not writable by oth-
ers. It may be group-writable provided that the group in ques-
tion 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.
SEE ALSO
ssh(1)
AUTHORS
OpenSSH is a derivative of the original and free ssh 1.2.12 release by
Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
Theo de Raadt and Dug Song removed many bugs, re-added newer features
and created OpenSSH. Markus Friedl contributed the support for SSH
protocol versions 1.5 and 2.0.
Debian October 12, 2023 SSH_CONFIG(5)
Generated by dwww version 1.16 on Tue Dec 16 06:18:23 CET 2025.