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 fol-
lowing 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_* COLORTERM NO_COLOR
• HashKnownHosts yes
• GSSAPIAuthentication yes
/etc/ssh/ssh_config.d/*.conf files are included at the start of the sys-
tem-wide configuration file, so options set there will override those in
/etc/ssh/ssh_config.
The file contains keyword-argument pairs, one per line. Lines starting
with ‘#’ and empty lines are interpreted as comments. Arguments may op-
tionally be enclosed in double quotes (") in order to represent argu-
ments containing spaces. Configuration options may be separated by
whitespace or optional whitespace and exactly one ‘=’; the latter format
is useful to avoid the need to quote whitespace when specifying configu-
ration 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 provide
exceptions for wildcard matches.
See “PATTERNS” for more information on patterns.
Match Restricts the following declarations (up to the next Host or
Match keyword) to be used only when the conditions following the
Match keyword are satisfied. Match conditions are specified us-
ing one or more criteria or the single token all which always
matches. The available criteria keywords are: canonical, final,
exec, localnetwork, host, originalhost, tagged, command, user,
localuser, and version. 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 prepend-
ing 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 con-
dition is considered true. Commands containing whitespace char-
acters must be quoted. Arguments to exec accept the tokens de-
scribed 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 con-
figuration on devices that roam between networks. Note that
network address is not a trustworthy criteria in many situations
(e.g. when the network is automatically configured using DHCP)
and so caution should be applied if using it to control secu-
rity-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
command keyword matches the remote command that has been re-
quested, or the subsystem name that is being invoked (e.g.
"sftp" for an SFTP session). The empty string will match the
case where a command or tag has not been specified, i.e. ‘Match
tag ""’. The version keyword matches against the version string
of ssh(1), for example “OpenSSH_10.0”.
The user keyword matches against the target username on the re-
mote 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).
Finally, the sessiontype keyword matches the requested session
type, which may be one of shell for interactive sessions, exec
for command execution sessions, subsystem for subsystem invoca-
tions such as sftp(1), or none for transport-only sessions, such
as when ssh(1) is started with the -N flag.
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 de-
tails). If this option is set to confirm, each use of the key
must be confirmed, as if the -c option was specified to
ssh-add(1). If this option is set to no, no keys are added to
the agent. 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 fol-
lowed 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 de-
fault (Debian-specific). This option is useful in scripts and
other batch jobs where no user is present to interact with
ssh(1), and where it is desirable to detect a broken network
swiftly. The argument must be yes or no (the default).
BindAddress
Use the specified address on the local machine as the source ad-
dress of the connection. Only useful on systems with more than
one address.
BindInterface
Use the address of the specified interface on the local machine
as the source address of the connection.
CanonicalDomains
When CanonicalizeHostname is enabled, this option specifies the
list of domain suffixes in which to search for the specified
destination host.
CanonicalizeFallbackLocal
Specifies whether to fail with an error when hostname canonical-
ization 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 be-
fore canonicalization is disabled. The default, 1, allows a
single dot (i.e. hostname.subdomain).
CanonicalizePermittedCNAMEs
Specifies rules to determine whether CNAMEs should be followed
when canonicalizing hostnames. The rules consist of one or more
arguments of source_domain_list:target_domain_list, where
source_domain_list is a pattern-list of domains that may follow
CNAMEs in canonicalization, and target_domain_list is a pattern-
list of domains that they may resolve to.
For example, "*.a.example.com:*.b.example.com,*.c.example.com"
will allow hostnames matching "*.a.example.com" to be canonical-
ized to names in the "*.b.example.com" or "*.c.example.com" do-
mains.
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 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.
ssh(1) will not accept host certificates signed using algorithms
other than those specified.
CertificateFile
Specifies a file from which the user's certificate is read. A
corresponding private key must be provided separately in order
to use this certificate either from an IdentityFile directive or
-i flag to ssh(1), via ssh-agent(1), or via a PKCS11Provider or
SecurityKeyProvider.
Arguments to CertificateFile may use the tilde syntax to refer
to a user's home directory, the tokens described in the “TOKENS”
section and environment variables as described in the
“ENVIRONMENT VARIABLES” section.
It is possible to have multiple certificate files specified in
configuration files; these certificates will be tried in 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 the spe-
cial keyword “global” or a channel type name from the list be-
low, 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 interactive sessions to termi-
nate after five minutes of inactivity. Specifying a zero value
disables the inactivity timeout.
The special timeout “global” applies to all active channels,
taken together. Traffic on any active channel will reset the
timeout, but when the timeout expires then all open channels
will be closed. Note that this global timeout is not matched by
wildcards and must be specified explicitly.
The available channel type names 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-gcm@openssh.com,aes256-gcm@openssh.com,
aes128-ctr,aes192-ctr,aes256-ctr
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. SetupTimeOut is a Debian-specific compatibility alias
for this option.
ControlMaster
Enables the sharing of multiple sessions over a single network
connection. When set to yes, ssh(1) will listen for connections
on a control socket specified using the ControlPath argument.
Additional sessions can connect to this socket using the same
ControlPath with ControlMaster set to no (the default). These
sessions will try to reuse the master instance's network connec-
tion rather than initiating new ones, but will fall back to con-
necting normally if the control socket does not exist, or is not
listening.
Setting this to ask will cause ssh(1) to listen for control con-
nections, but require confirmation using ssh-askpass(1). If the
ControlPath cannot be opened, ssh(1) will continue without con-
necting to a master instance.
X11 and ssh-agent(1) forwarding is supported over these multi-
plexed connections, however the display and agent forwarded will
be the one belonging to the master connection i.e. it is not
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 shar-
ing as described in the ControlMaster section above or the
string none to disable connection sharing. Arguments to
ControlPath may use the tilde syntax to refer to a user's home
directory, 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 oppor-
tunistic connection sharing include at least %h, %p, and %r (or
alternatively %C) and be placed in a directory that is not
writable by other users. This ensures that shared connections
are uniquely identified.
ControlPersist
When used in conjunction with ControlMaster, specifies that the
master connection should remain open in the background (waiting
for future client connections) after the initial client connec-
tion has been closed. If set to no (the default), then the mas-
ter connection will not be placed into the background, and will
close as soon as the initial client connection is closed. If
set to yes or 0, then the master connection will remain in the
background indefinitely (until killed or closed via a mechanism
such as the "ssh -O exit"). If set to a time in seconds, or a
time in any of the formats documented in sshd_config(5), then
the backgrounded master connection will automatically terminate
after it has remained idle (with no client connections) for the
specified time.
DynamicForward
Specifies that a TCP port on the local machine be forwarded over
the secure channel, and the application protocol is then used to
determine where to connect to from the remote machine.
The argument must be [bind_address:]port. IPv6 addresses can be
specified by enclosing addresses in square brackets. By 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 transparent
for binary data).
ExitOnForwardFailure
Specifies whether ssh(1) should terminate the connection if it
cannot set up all requested dynamic, tunnel, local, and remote
port forwardings, (e.g. if either end is unable to bind and lis-
ten on a specified port). Note that ExitOnForwardFailure does
not apply to connections made over port forwardings and will
not, for example, cause ssh(1) to exit if TCP connections to the
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 remote
port forwards to be successfully established before placing it-
self 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 mater-
ial from the agent, however they can perform operations on the
keys that enable them to authenticate using the identities
loaded into the agent.
ForwardX11
Specifies whether X11 connections will be automatically redi-
rected over the secure channel and DISPLAY set. The argument
must be yes or no (the default).
X11 forwarding should be enabled with caution. Users with the
ability to bypass file permissions on the remote host (for the
user's X11 authorization database) can access the local X11 dis-
play through the forwarded connection. An attacker may then be
able to perform activities such as keystroke monitoring if the
ForwardX11Trusted option is also enabled.
ForwardX11Timeout
Specify a timeout for untrusted X11 forwarding using the format
described in the “TIME FORMATS” section of sshd_config(5). X11
connections received by ssh(1) after this time will be refused.
Setting ForwardX11Timeout to zero will disable the timeout and
permit X11 forwarding for the life of the connection. The de-
fault is to disable untrusted X11 forwarding after twenty min-
utes has elapsed.
ForwardX11Trusted
If this option is set to yes, (the Debian-specific default), re-
mote X11 clients will have full access to the original X11 dis-
play.
If this option is set to no (the upstream default), remote X11
clients will be considered untrusted and prevented from stealing
or tampering with data belonging to trusted X11 clients. Fur-
thermore, the xauth(1) token used for the session will be set to
expire after 20 minutes. Remote clients will be refused access
after this time.
See the X11 SECURITY extension specification for full details on
the restrictions imposed on untrusted clients.
GatewayPorts
Specifies whether remote hosts are allowed to connect to local
forwarded ports. By default, ssh(1) binds local port 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 wild-
card address, thus allowing remote hosts to connect to forwarded
ports. The argument must be yes or no (the default).
GlobalKnownHostsFile
Specifies one or more files to use for the global host key data-
base, separated by whitespace. The default is
/etc/ssh/ssh_known_hosts, /etc/ssh/ssh_known_hosts2.
GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is 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 compatible
server, this will delegate the renewed credentials to a session
on the server.
Checks are made to ensure that credentials are only propagated
when the new credentials match the old ones on the originating
client and where the receiving server still has the old set in
its cache.
The default is “no”.
For this to work GSSAPIKeyExchange needs to be enabled in the
server and also used by the client.
GSSAPIServerIdentity
If set, specifies the GSSAPI server identity that ssh should ex-
pect when connecting to the server. The default is unset, which
means that the expected GSSAPI server identity will be deter-
mined 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 visually
reveal identifying information if the file's contents are dis-
closed. The default is no. Note that existing names and ad-
dresses in known hosts files will not be converted 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. Al-
ternately 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 public
key authentication. The argument must be yes or no (the de-
fault).
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 signa-
ture algorithms will be appended to the default set instead of
replacing them. If the specified list begins with a ‘-’ charac-
ter, then the specified signature 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 signature algorithms will be placed at the head of
the default set. The default for this option is:
ssh-ed25519-cert-v01@openssh.com,
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
sk-ssh-ed25519-cert-v01@openssh.com,
sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
rsa-sha2-512-cert-v01@openssh.com,
rsa-sha2-256-cert-v01@openssh.com,
ssh-ed25519,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
sk-ecdsa-sha2-nistp256@openssh.com,
sk-ssh-ed25519@openssh.com,
rsa-sha2-512,rsa-sha2-256
If hostkeys are known for the destination host then this default
is modified to prefer their algorithms.
The list of available signature algorithms may also be obtained
using "ssh -Q HostKeyAlgorithms".
HostKeyAlias
Specifies an alias that should be used instead of the real host
name when looking up or saving the host key in the host key
database files and when validating host certificates. This 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 authentica-
tion identity and certificate files (either the default files,
or those explicitly configured in the ssh_config files or passed
on the ssh(1) command-line), even if ssh-agent(1) or a
PKCS11Provider or SecurityKeyProvider offers more identities.
The argument to this keyword must be yes or no (the default).
This option is intended for situations where ssh-agent offers
many different identities.
IdentityAgent
Specifies the Unix-domain socket used to communicate with the
authentication agent.
This option overrides the SSH_AUTH_SOCK environment variable and
can be used to select a specific agent. Setting the socket name
to none disables the use of an authentication agent. If the
string "SSH_AUTH_SOCK" is specified, the location of the socket
will be read from the SSH_AUTH_SOCK environment variable. Oth-
erwise if the specified value begins with a ‘$’ character, then
it will be treated as an environment variable containing the lo-
cation 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 ECDSA, authenticator-
hosted ECDSA, Ed25519, authenticator-hosted Ed25519 or RSA au-
thentication 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 and ~/.ssh/id_ed25519_sk.
Additionally, any identities represented by the authentication
agent will be used for authentication unless IdentitiesOnly is
set. If no certificates have been explicitly specified by
CertificateFile, ssh(1) will try to load certificate information
from the filename obtained by appending -cert.pub to the path of
a specified IdentityFile.
Arguments to IdentityFile may use the tilde syntax to refer to a
user's home directory or the tokens described in the “TOKENS”
section. Alternately an argument of none may be used to indi-
cate no identity files should be loaded.
It is possible to have multiple identity files specified in con-
figuration 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 authenti-
cation. IdentityFile may also be used in conjunction with
CertificateFile in order to provide any certificate also needed
for authentication with the identity.
IgnoreUnknown
Specifies a pattern-list of unknown options to be ignored if
they are encountered in configuration parsing. This may be used
to suppress errors if ssh_config contains options that are un-
recognised 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) wild-
cards, tokens as described in the “TOKENS” section, environment
variables as described in the “ENVIRONMENT VARIABLES” section
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 configuration file or /etc/ssh
if included from the system configuration file. Include direc-
tive may appear inside a Match or Host block to perform condi-
tional inclusion.
IPQoS Specifies the Differentiated Services Field Codepoint (DSCP)
value for connections. Accepted values are af11, af12, af13,
af21, af22, af23, af31, af32, af33, af41, af42, af43, cs0, cs1,
cs2, cs3, cs4, cs5, cs6, cs7, ef, le, 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 ses-
sions. The default is ef (Expedited Forwarding) for interactive
sessions and cs1 (Lower Effort) for non-interactive sessions.
KbdInteractiveAuthentication
Specifies whether to use keyboard-interactive authentication.
The argument to this keyword must be yes (the default) or no.
ChallengeResponseAuthentication is a deprecated alias for this.
KbdInteractiveDevices
Specifies the list of methods to use in keyboard-interactive au-
thentication. Multiple method names must be comma-separated.
The default is to use the server specified list. The methods
available vary depending on what the server supports. For an
OpenSSH server, it may be zero or more of: bsdauth and pam.
KexAlgorithms
Specifies the permitted KEX (Key Exchange) algorithms that will
be used and their preference order. The selected algorithm will
be the first algorithm in this list that the server also sup-
ports. Multiple algorithms must be comma-separated.
If the specified list begins with a ‘+’ character, then the
specified algorithms will be appended to the default set instead
of replacing them. If the specified list begins with a ‘-’
character, then the specified algorithms (including wildcards)
will be removed from the default set instead of replacing them.
If the specified list begins with a ‘^’ character, then the
specified algorithms will be placed at the head of the default
set.
The default is:
mlkem768x25519-sha256,
sntrup761x25519-sha512,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 supported 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 ad-
dition 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 command
exits abnormally or returns a non-zero exit status then the con-
nection 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 en-
abled.
LocalForward
Specifies that a TCP port or Unix-domain socket on the local ma-
chine be forwarded over the secure channel to the specified host
and port (or Unix-domain socket) from the remote machine. For a
TCP port, the first argument must 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 re-
mote host supports it.
IPv6 addresses can be specified by enclosing addresses in square
brackets.
If either argument contains a '/' in it, that argument will be
interpreted as a Unix-domain socket (on the corresponding host)
rather than a TCP port.
Multiple forwardings may be specified, and additional forward-
ings can be given on the command line. Only the superuser can
forward privileged ports. By default, the local port is bound
in accordance with the GatewayPorts setting. However, an ex-
plicit bind_address may be used to bind the connection to a spe-
cific address. The bind_address of localhost indicates that the
listening port be bound for local use only, while an empty ad-
dress or ‘*’ indicates that the port should be available from
all interfaces. Unix domain socket paths may use the tokens de-
scribed in the “TOKENS” section and environment variables as de-
scribed in the “ENVIRONMENT VARIABLES” section.
LogLevel
Gives the verbosity level that is used when logging messages
from ssh(1). The possible values are: QUIET, FATAL, ERROR,
INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default
is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3
each specify higher levels of verbose output.
LogVerbose
Specify one or more overrides to LogLevel. An override consists
of one or more pattern lists that matches the source file, func-
tion and line number to force detailed logging for. For exam-
ple, 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 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 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 replacing
them. If the specified list begins with a ‘^’ character, then
the specified algorithms will be placed at the head of the de-
fault 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 argu-
ment to this keyword must be yes, no or an interval specifier of
the form interval:milliseconds (e.g. interval:80 for 80 mil-
liseconds). 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 us-
ing 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 wild-
card ‘*’ 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 com-
mand string extends to the end of the line, and is executed us-
ing the user's shell ‘exec’ directive to avoid a lingering shell
process.
Arguments to ProxyCommand accept the tokens described in the
“TOKENS” section. The command can be basically anything, and
should read from its standard input and write to its standard
output. It should eventually connect an sshd(8) server running
on some machine, or execute sshd -i somewhere. Host key manage-
ment will be done using the Hostname of the host being connected
(defaulting to the name typed by the user). Setting the command
to none disables this option entirely. Note that CheckHostIP is
not available for connects with a proxy command.
This directive is useful in conjunction with nc(1) and its proxy
support. For example, the following directive would connect via
an HTTP proxy at 192.0.2.0:
ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
ProxyJump
Specifies one or more jump proxies as either [user@]host[:port]
or an ssh URI. Multiple proxies may be separated by comma char-
acters and will be visited sequentially. Setting this option
will cause ssh(1) to connect to the target host by first making
a ssh(1) connection to the specified ProxyJump host and then es-
tablishing 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 authentica-
tion while respectively disabling or enabling the OpenSSH host-
bound authentication protocol extension required for restricted
ssh-agent(1) forwarding.
RekeyLimit
Specifies the maximum amount of data that may be transmitted or
received before the session key is renegotiated, optionally fol-
lowed by a maximum amount of time that may pass before the ses-
sion key is renegotiated. The first argument is specified in
bytes and may have a suffix of ‘K’, ‘M’, or ‘G’ to indicate
Kilobytes, Megabytes, or Gigabytes, respectively. The default
is between ‘1G’ and ‘4G’, depending on the cipher. The optional
second value is specified in seconds and may use any of the
units documented in the TIME FORMATS section of sshd_config(5).
The default value for RekeyLimit is default none, which means
that rekeying is performed after the cipher's default amount of
data has been sent or received and no time based rekeying is
done.
RemoteCommand
Specifies a command to execute on the remote machine after 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 or Unix-domain socket on the remote
machine be forwarded over the secure channel. The remote port
may either be forwarded to a specified host and port or Unix-do-
main socket from the local machine, or may act as a SOCKS 4/5
proxy that allows a remote client to connect to arbitrary desti-
nations from the local machine. The first argument is the lis-
tening specification and may be [bind_address:]port or, if the
remote host supports it, a Unix domain socket path. If forward-
ing to a specific destination then the second argument must be
host:hostport or a Unix domain socket path, otherwise if no des-
tination 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.
If either argument contains a '/' in it, that argument will be
interpreted as a Unix-domain socket (on the corresponding host)
rather than a TCP port.
Multiple forwardings may be specified, and additional forward-
ings 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 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 authentication will
be refused for all hosts. Keys may be specified as a text file,
listing one public key per line, or as an OpenSSH Key Revocation
List (KRL) as generated by ssh-keygen(1). For more information
on KRLs, see the KEY REVOCATION LISTS section in ssh-keygen(1).
Arguments to RevokedHostKeys may use the tilde syntax to refer
to a user's home directory, the tokens described in the “TOKENS”
section and environment variables as described in the
“ENVIRONMENT VARIABLES” section.
SecurityKeyProvider
Specifies a path to a library that will be used when loading any
FIDO authenticator-hosted keys, overriding the default of using
the built-in USB HID support.
If the specified value begins with a ‘$’ character, then it will
be treated as an environment variable containing the path to the
library.
SendEnv
Specifies what variables from the local environ(7) should be
sent to the server. The server must also support it, and the
server must be configured to accept these environment variables.
Note that the TERM environment variable is always sent whenever
a pseudo-terminal is requested as it is required by the proto-
col. 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 sep-
arated by whitespace or spread across multiple SendEnv direc-
tives.
See “PATTERNS” for more information on patterns.
It is possible to clear previously set SendEnv variable names by
prefixing patterns with -. The default is not to send any envi-
ronment variables.
ServerAliveCountMax
Sets the number of server alive messages (see below) which may
be sent without ssh(1) receiving any messages back from the
server. If this threshold is reached while server alive 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 disconnect
after approximately 45 seconds.
ServerAliveInterval
Sets a timeout interval in seconds after which if no data has
been received from the server, ssh(1) will send a message
through the encrypted channel to request a response from the
server. The default is 0, indicating that these messages will
not be sent to the server, or 300 if the BatchMode option is set
(Debian-specific). ProtocolKeepAlives is a Debian-specific com-
patibility alias 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 in the form “NAME=VALUE”.
Similarly to SendEnv, with the exception of the TERM variable,
the server must be prepared to accept the environment variable.
The “VALUE” may use the tokens described in the “TOKENS” section
and environment variables as described in the “ENVIRONMENT
VARIABLES” section.
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 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-do-
main socket file.
The default value is 0177, which creates a Unix-domain socket
file that is readable and writable only by the owner. Note that
not all operating systems honor the file mode on Unix-domain
socket files.
StreamLocalBindUnlink
Specifies whether to remove an existing Unix-domain socket file
for local or remote port forwarding before creating a new one.
If the socket file already exists and StreamLocalBindUnlink is
not enabled, ssh will be unable to forward the port to the Unix-
domain socket file. This option is only used for port forward-
ing to a Unix-domain socket file.
The argument must be yes or no (the default).
StrictHostKeyChecking
If this flag is set to yes, ssh(1) will never automatically add
host keys to the ~/.ssh/known_hosts file, and refuses to connect
to hosts whose host key has changed. This provides maximum pro-
tection against man-in-the-middle (MITM) attacks, though it can
be annoying when the /etc/ssh/ssh_known_hosts file is poorly
maintained or when connections to new hosts are frequently made.
This option forces the user to manually add all new hosts.
If this flag is set to accept-new then ssh will automatically
add new host keys to the user'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 restric-
tions. 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 de-
fault is USER.
TCPKeepAlive
Specifies whether the system should send TCP keepalive messages
to the other side. If they are sent, death of the connection or
crash of one of the machines will be properly noticed. This op-
tion only uses TCP keepalives (as opposed to using ssh level
keepalives), so takes a long time to notice when the connection
dies. As such, you probably want the ServerAliveInterval option
as well. However, this means that connections will die if the
route is down temporarily, and some people find it annoying.
The default is yes (to send TCP keepalive messages), and the
client will notice if the network goes down or the remote host
dies. This is important in scripts, and many users want it too.
To disable TCP keepalive messages, the value should be set to
no. See also ServerAliveInterval for protocol-level keepalives.
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 authen-
ticate 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 disabled
if it is enabled.
Presently, only sshd(8) from OpenSSH 6.8 and greater support the
"hostkeys@openssh.com" protocol extension used to inform the
client of all the server's hostkeys.
User Specifies the user to log in as. This can be useful when a dif-
ferent user name is used on different machines. This saves the
trouble of having to remember to give the user name on the com-
mand line. Arguments to User may use the tokens described in
the “TOKENS” section (with the exception of %r and %C) and envi-
ronment variables as described in the “ENVIRONMENT VARIABLES”
section.
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 de-
scribed 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 finger-
print match will be displayed, but the user will still need to
confirm new host keys according to the StrictHostKeyChecking op-
tion. The default is no.
See also “VERIFYING HOST KEYS” in ssh(1).
VersionAddendum
Optionally specifies additional text to append to the SSH proto-
col banner sent by the client upon connection. The default is
none.
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 decla-
rations for any host in the ".co.uk" set of domains, the following pat-
tern 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, Include,
KnownHostsCommand, LocalForward, Match exec, RemoteCommand,
RemoteForward, RevokedHostKeys, UserKnownHostsFile and VersionAddendum
accept the tokens %%, %C, %d, %h, %i, %j, %k, %L, %l, %n, %p, %r, and
%u.
KnownHostsCommand additionally accepts the tokens %f, %H, %I, %K and %t.
Hostname accepts the tokens %% and %h.
LocalCommand accepts all tokens.
ProxyCommand and ProxyJump accept the tokens %%, %h, %n, %p, and %r.
Note that some of these directives build commands for execution via the
shell. Because ssh(1) performs no filtering or escaping of characters
that have special meaning in shell commands (e.g. quotes), it is the
user's responsibility to ensure that the arguments passed to ssh(1) do
not contain such characters and that tokens are appropriately quoted
when used.
ENVIRONMENT VARIABLES
Arguments to some keywords can be expanded at runtime from environment
variables on the client by enclosing them in ${}, for example
${HOME}/.ssh would refer to the user's .ssh directory. If a specified
environment variable does not exist then an error will be returned and
the setting for that keyword will be ignored.
The keywords CertificateFile, ControlPath, IdentityAgent, IdentityFile,
Include, KnownHostsCommand, and UserKnownHostsFile support environment
variables. The keywords LocalForward and RemoteForward support environ-
ment 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 pro-
tocol versions 1.5 and 2.0.
Debian March 3, 2025 SSH_CONFIG(5)
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