| SSH_CONFIG(5) | File Formats Manual | SSH_CONFIG(5) |
ssh_config —
OpenSSH client configuration file
ssh(1) obtains configuration data from the following sources in the following order:
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.
The file contains keyword-argument pairs, one per line. Lines
starting with ‘#’ and empty lines are
interpreted as comments. Arguments may optionally be enclosed in double
quotes (") in order to represent arguments containing spaces.
Configuration options may be separated by whitespace or optional whitespace
and exactly one ‘=’; the latter format
is useful to avoid the need to quote whitespace when specifying
configuration options using the ssh,
scp, and sftp
-o option.
The possible keywords and their meanings are as follows (note that keywords are case-insensitive and arguments are case-sensitive):
HostHost or Match keyword) to
be only for those hosts that match one of the patterns given after the
keyword. If more than one pattern is provided, they should be separated by
whitespace. A single ‘*’ as a
pattern can be used to provide global defaults for all hosts. The host is
usually the hostname argument given on the command
line (see the CanonicalizeHostname keyword for
exceptions).
A pattern entry may be negated by prefixing it with an
exclamation mark (‘!’). If a negated entry is matched,
then the Host entry is ignored, regardless of
whether any other patterns on the line match. Negated matches are
therefore useful to provide exceptions for wildcard matches.
See PATTERNS for more information on patterns.
MatchHost or Match keyword) to
be used only when the conditions following the
Match keyword are satisfied. Match conditions are
specified using one or more criteria or the single token
all which always matches. The available criteria
keywords are: canonical,
final, exec,
host, originalhost,
user, and localuser. The
all criteria must appear alone or immediately
after canonical or final.
Other criteria may be combined arbitrarily. All criteria but
all, canonical, and
final require an argument. Criteria may be negated
by prepending an exclamation mark (‘!’).
The canonical keyword matches only
when the configuration file is being re-parsed after hostname
canonicalization (see the CanonicalizeHostname
option). This may be useful to specify conditions that work with
canonical host names only.
The final keyword requests that the
configuration be re-parsed (regardless of whether
CanonicalizeHostname is enabled), and matches
only during this final pass. If
CanonicalizeHostname is enabled, then
canonical and final
match during the same pass.
The exec keyword executes the
specified command under the user's shell. If the command returns a zero
exit status then the condition is considered true. Commands containing
whitespace characters must be quoted. Arguments to
exec accept the tokens described in the
TOKENS section.
The other keywords' criteria must be single entries or
comma-separated lists and may use the wildcard and negation operators
described in the PATTERNS section.
The criteria for the host keyword are matched
against the target hostname, after any substitution by the
Hostname or
CanonicalizeHostname options. The
originalhost keyword matches against the
hostname as it was specified on the command-line. The
user keyword matches against the target username
on the remote host. The localuser keyword
matches against the name of the local user running
ssh(1) (this keyword may be useful in
system-wide ssh_config files).
AddKeysToAgentyes and a key is loaded from a file, the key
and its passphrase are added to the agent with the default lifetime, as if
by ssh-add(1). If this option is set
to ask, ssh(1)
will require confirmation using the SSH_ASKPASS
program before adding a key (see
ssh-add(1) for details). If this
option is set to confirm, each use of the key must
be confirmed, as if the -c option was specified to
ssh-add(1). If this option is set to
no, no keys are added to the agent. Alternately,
this option may be specified as a time interval using the format described
in the TIME FORMATS section of
sshd_config(5) to 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.AddressFamilyany (the default), inet
(use IPv4 only), or inet6 (use IPv6 only).BatchModeyes, user interaction such as password
prompts and host key confirmation requests will be disabled. This option
is useful in scripts and other batch jobs where no user is present to
interact with ssh(1). The argument must be
yes or no (the
default).BindAddressBindInterfaceCanonicalDomainsCanonicalizeHostname is enabled, this option
specifies the list of domain suffixes in which to search for the specified
destination host.CanonicalizeFallbackLocalyes, 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.CanonicalizeHostnameno, is not to perform any name rewriting
and let the system resolver handle all hostname lookups. If set to
yes then, for connections that do not use a
ProxyCommand or ProxyJump,
ssh(1) will attempt to canonicalize the
hostname specified on the command line using the
CanonicalDomains suffixes and
CanonicalizePermittedCNAMEs rules. If
CanonicalizeHostname is set to
always, then canonicalization is applied to
proxied connections too.
If this option is enabled, then the configuration files are
processed again using the new target name to pick up any new
configuration in matching Host and
Match stanzas. A value of
none disables the use of a
ProxyJump host.
CanonicalizeMaxDotsCanonicalizePermittedCNAMEsFor example, "*.a.example.com:*.b.example.com,*.c.example.com" will allow hostnames matching "*.a.example.com" to be canonicalized to names in the "*.b.example.com" or "*.c.example.com" domains.
A single argument of "none" causes no CNAMEs to be considered for canonicalization. This is the default behaviour.
CASignatureAlgorithmsssh-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.
CertificateFileIdentityFile 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 sequence.
Multiple CertificateFile directives will add to
the list of certificates used for authentication.
CheckHostIPyes
ssh(1) will additionally check the host IP
address in the known_hosts file. This allows it to
detect if a host key changed due to DNS spoofing and will add addresses of
destination hosts to ~/.ssh/known_hosts in the
process, regardless of the setting of
StrictHostKeyChecking. If the option is set to
no (the default), the check will not be
executed.CiphersThe 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".
ClearAllForwardingsyes or no (the
default).Compressionyes or no (the
default).ConnectionAttemptsConnectTimeoutControlMasteryes,
ssh(1) will listen for connections on a
control socket specified using the ControlPath
argument. Additional sessions can connect to this socket using the same
ControlPath with
ControlMaster set to no
(the default). These sessions will try to reuse the master instance's
network connection rather than initiating new ones, but will fall back to
connecting normally if the control socket does not exist, or is not
listening.
Setting this to ask will cause
ssh(1) to listen for control
connections, but require confirmation using
ssh-askpass(1). If the
ControlPath cannot be opened,
ssh(1) will continue without connecting
to a master instance.
X11 and ssh-agent(1) forwarding is supported over these multiplexed connections, however the display and agent forwarded will be the one belonging to the master connection i.e. it is not possible to forward multiple displays or agents.
Two additional options allow for opportunistic multiplexing:
try to use a master connection but fall back to creating a new one if
one does not already exist. These options are:
auto and autoask. The
latter requires confirmation like the ask
option.
ControlPathControlMaster section above or
the string none to disable connection sharing.
Arguments to ControlPath may use the tilde syntax
to refer to a user's home directory, the tokens described in the
TOKENS section and environment variables
as described in the
ENVIRONMENT VARIABLES
section. It is recommended that any ControlPath
used for opportunistic connection sharing include at least %h, %p, and %r
(or alternatively %C) and be placed in a directory that is not writable by
other users. This ensures that shared connections are uniquely
identified.ControlPersistControlMaster,
specifies that the master connection should remain open in the background
(waiting for future client connections) after the initial client
connection has been closed. If set to no (the
default), then the master connection will not be placed into the
background, and will close as soon as the initial client connection is
closed. If set to yes or 0, then the master
connection will remain in the background indefinitely (until killed or
closed via a mechanism such as the "ssh -O exit"). If set to a
time in seconds, or a time in any of the formats documented in
sshd_config(5), then the
backgrounded master connection will automatically terminate after it has
remained idle (with no client connections) for the specified time.DynamicForwardThe argument must be
[bind_address:]port. IPv6
addresses can be specified by enclosing addresses in square brackets. By
default, the local port is bound in accordance with the
GatewayPorts setting. However, an explicit
bind_address may be used to bind the connection to
a specific address. The bind_address of
localhost indicates that the listening port be
bound for local use only, while an empty address or ‘*’
indicates that the port should be available from all interfaces.
Currently the SOCKS4 and SOCKS5 protocols are supported, and ssh(1) will act as a SOCKS server. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the superuser can forward privileged ports.
EnableSSHKeysignyes 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~’). 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).ExitOnForwardFailureExitOnForwardFailure 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).FingerprintHashmd5 and
sha256 (the default).ForkAfterAuthenticationssh to go to background just before
command execution. This is useful if ssh is going
to ask for passwords or passphrases, but the user wants it in the
background. This implies the StdinNull
configuration 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
itself in the background. The argument to this keyword must be
yes (same as the -f
option) or no (the default).
ForwardAgentyes, no (the default), an
explicit path to an agent socket or the name of an environment variable
(beginning with ‘$’) in which to find the path.
Agent forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the agent's Unix-domain socket) can access the local agent through the forwarded connection. An attacker cannot obtain key material from the agent, however they can perform operations on the keys that enable them to authenticate using the identities loaded into the agent.
ForwardX11DISPLAY 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.
ForwardX11TimeoutForwardX11Timeout to zero will
disable the timeout and permit X11 forwarding for the life of the
connection. The default is to disable untrusted X11 forwarding after
twenty minutes has elapsed.ForwardX11Trustedyes, remote X11 clients
will have full access to the original X11 display.
If this option is set to no (the
default), remote X11 clients will be considered untrusted and prevented
from stealing or tampering with data belonging to trusted X11 clients.
Furthermore, the xauth(1) token used
for the session will be set to expire after 20 minutes. Remote clients
will be refused access after this time.
See the X11 SECURITY extension specification for full details on the restrictions imposed on untrusted clients.
GatewayPortsGatewayPorts can be
used to specify that ssh should bind local port forwardings to the
wildcard address, thus allowing remote hosts to connect to forwarded
ports. The argument must be yes or
no (the default).GlobalKnownHostsFileGSSAPIAuthenticationno.GSSAPIDelegateCredentialsno.HashKnownHostsno. Note that existing names and addresses in
known hosts files will not be converted automatically, but may be manually
hashed using ssh-keygen(1).HostbasedAcceptedAlgorithmsssh-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.
HostbasedAuthenticationyes or
no (the default).HostKeyAlgorithmsssh-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".
HostKeyAliasHostnameHostname 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.IdentitiesOnlyssh_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.IdentityAgentThis option overrides the
SSH_AUTH_SOCK environment variable and can be
used to select a specific agent. Setting the socket name to
none disables the use of an authentication
agent. If the string "SSH_AUTH_SOCK" is specified, the
location of the socket will be read from the
SSH_AUTH_SOCK environment variable. Otherwise if
the specified value begins with a ‘$’ character, then it
will be treated as an environment variable containing the location of
the socket.
Arguments to IdentityAgent may use the
tilde syntax to refer to a user's home directory, the tokens described
in the TOKENS section and environment
variables as described in the
ENVIRONMENT VARIABLES
section.
IdentityFileIdentitiesOnly is set. If no certificates
have been explicitly specified by CertificateFile,
ssh(1) will try to load certificate
information from the filename obtained by appending
-cert.pub to the path of a specified
IdentityFile.
Arguments to IdentityFile may use the
tilde syntax to refer to a user's home directory or the tokens described
in the TOKENS section.
It is possible to have multiple identity files specified in
configuration files; all these identities will be tried in sequence.
Multiple IdentityFile directives will add to the
list of identities tried (this behaviour differs from that of other
configuration directives).
IdentityFile may be used in
conjunction with IdentitiesOnly to select which
identities in an agent are offered during authentication.
IdentityFile may also be used in conjunction
with CertificateFile in order to provide any
certificate also needed for authentication with the identity.
IgnoreUnknownssh_config contains options that are
unrecognised by ssh(1). It is recommended
that IgnoreUnknown be listed early in the
configuration file as it will not be applied to unknown options that
appear before it.IncludeInclude directive may appear inside a
Match or Host block to
perform conditional inclusion.IPQoSaf11, af12,
af13, af21,
af22, af23,
af31, af32,
af33, af41,
af42, af43,
cs0, cs1,
cs2, cs3,
cs4, cs5,
cs6, cs7,
ef, le,
lowdelay, throughput,
reliability, a numeric value, or
none to use the operating system default. This
option may take one or two arguments, separated by whitespace. If one
argument is specified, it is used as the packet class unconditionally. If
two values are specified, the first is automatically selected for
interactive sessions and the second for non-interactive sessions. The
default is af21 (Low-Latency Data) for interactive
sessions and cs1 (Lower Effort) for
non-interactive sessions.KbdInteractiveAuthenticationyes (the default) or
no.
ChallengeResponseAuthentication is a deprecated
alias for this.KbdInteractiveDevicesbsdauth, pam, and
skey.KexAlgorithmscurve25519-sha256,curve25519-sha256@libssh.org, ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, sntrup761x25519-sha512@openssh.com, diffie-hellman-group-exchange-sha256, diffie-hellman-group16-sha512, diffie-hellman-group18-sha512, diffie-hellman-group14-sha256
The list of available key exchange algorithms may also be obtained using "ssh -Q kex".
KnownHostsCommandUserKnownHostsFile 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 connection: once when preparing the preference
list of host key algorithms to use, again to obtain the host key for the
requested host name and, if CheckHostIP is
enabled, one more time to obtain the host key matching the server's
address. If the command exits abnormally or returns a non-zero exit status
then the connection is terminated.LocalCommandLocalCommand 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.
LocalForwardIPv6 addresses can be specified by enclosing addresses in
square brackets. Multiple forwardings may be specified, and additional
forwardings can be given on the command line. Only the superuser can
forward privileged ports. By default, the local port is bound in
accordance with the GatewayPorts setting.
However, an explicit bind_address may be used to
bind the connection to a specific address. The
bind_address of localhost
indicates that the listening port be bound for local use only, while an
empty address or ‘*’ indicates that the port should be
available from all interfaces. Unix domain socket paths may use the
tokens described in the TOKENS section
and environment variables as described in the
ENVIRONMENT VARIABLES
section.
LogLevelLogVerbosekex.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.
MACsThe algorithms that contain "-etm" calculate the MAC after encryption (encrypt-then-mac). These are considered safer and their use recommended.
The default is:
umac-64-etm@openssh.com,umac-128-etm@openssh.com, hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, hmac-sha1-etm@openssh.com, umac-64@openssh.com,umac-128@openssh.com, hmac-sha2-256,hmac-sha2-512,hmac-sha1
The list of available MAC algorithms may also be obtained using "ssh -Q mac".
NoHostAuthenticationForLocalhostyes or
no (the default).NumberOfPasswordPromptsPasswordAuthenticationyes (the default) or
no.PermitLocalCommandLocalCommand
option or using the !command
escape sequence in ssh(1). The argument
must be yes or no (the
default).PermitRemoteOpenRemoteForward is used as a SOCKS
proxy. The forwarding specification must be one of the following forms:
PermitRemoteOpen
host:portPermitRemoteOpen
IPv4_addr:portPermitRemoteOpen
[IPv6_addr]:portMultiple forwards may be specified by separating them with
whitespace. An argument of any can be used to
remove all restrictions and permit any forwarding requests. An argument
of none can be used to prohibit all forwarding
requests. The wildcard ‘*’ can be used for host or port to
allow all hosts or ports respectively. Otherwise, no pattern matching or
address lookups are performed on supplied names.
PKCS11Providernone 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.PortPreferredAuthenticationskeyboard-interactive) over another method (e.g.
password). The default is:
gssapi-with-mic,hostbased,publickey, keyboard-interactive,password
ProxyCommandexec’ directive to avoid a
lingering shell process.
Arguments to ProxyCommand accept the
tokens described in the TOKENS section.
The command can be basically anything, and should read from its standard
input and write to its standard output. It should eventually connect an
sshd(8) server running on some machine,
or execute sshd -i somewhere. Host key
management will be done using the Hostname of
the host being connected (defaulting to the name typed by the user).
Setting the command to none disables this option
entirely. Note that CheckHostIP is not available
for connects with a proxy command.
This directive is useful in conjunction with nc(1) and its proxy support. For example, the following directive would connect via an HTTP proxy at 192.0.2.0:
ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
ProxyJumpProxyJump 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 (either supplied via the command-line or the configuration file) is not generally applied to jump hosts. ~/.ssh/config should be used if specific configuration is required for jump hosts.
ProxyUseFdpassProxyCommand will pass a connected
file descriptor back to ssh(1) instead of
continuing to execute and pass data. The default is
no.PubkeyAcceptedAlgorithmsssh-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".
PubkeyAuthenticationyes (the default) or
no.RekeyLimitRekeyLimit 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.RemoteCommandRemoteCommand accept the tokens described in the
TOKENS section.RemoteForwardPermitRemoteOpen.
IPv6 addresses can be specified by enclosing addresses in square brackets. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Privileged ports can be forwarded only when logging in as root on the remote machine. 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)).
RequestTTYno (never request a TTY),
yes (always request a TTY when standard input is a
TTY), force (always request a TTY) or
auto (request a TTY when opening a login session).
This option mirrors the -t and
-T flags for
ssh(1).RevokedHostKeysSecurityKeyProviderIf the specified value begins with a ‘$’ character, then it will be treated as an environment variable containing the path to the library.
SendEnvTERM environment variable is always sent whenever
a pseudo-terminal is requested as it is required by the protocol. Refer to
AcceptEnv in
sshd_config(5) for how to
configure the server. Variables are specified by name, which may contain
wildcard characters. Multiple environment variables may be separated by
whitespace or spread across multiple SendEnv
directives.
See PATTERNS for more information on patterns.
It is possible to clear previously set
SendEnv variable names by prefixing patterns
with -. The default is not to send any
environment variables.
ServerAliveCountMaxTCPKeepAlive (below). The
server alive messages are sent through the encrypted channel and therefore
will not be spoofable. The TCP keepalive option enabled by
TCPKeepAlive is spoofable. The server alive
mechanism is valuable when the client or server depend on knowing when a
connection has become unresponsive.
The default value is 3. If, for example,
ServerAliveInterval (see below) is set to 15 and
ServerAliveCountMax is left at the default, if
the server becomes unresponsive, ssh will disconnect after approximately
45 seconds.
ServerAliveIntervalSessionTypenone (same as the -N
option), subsystem (same as the
-s option) or default
(shell or command execution).SetEnvSendEnv, with
the exception of the TERM variable, the server
must be prepared to accept the environment variable.StdinNull-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).StreamLocalBindMaskThe 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.
StreamLocalBindUnlinkStreamLocalBindUnlink 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).
StrictHostKeyCheckingyes,
ssh(1) will never automatically add host
keys to the ~/.ssh/known_hosts file, and refuses
to connect to hosts whose host key has changed. This provides maximum
protection against man-in-the-middle (MITM) attacks, though it can be
annoying when the /etc/ssh/ssh_known_hosts file is
poorly maintained or when connections to new hosts are frequently made.
This option forces the user to manually add all new hosts.
If this flag is set to accept-new then
ssh will automatically add new host keys to the user's
known_hosts file, but will not permit
connections to hosts with changed host keys. If this flag is set to
no or off, ssh will
automatically add new host keys to the user known hosts files and allow
connections to hosts with changed hostkeys to proceed, subject to some
restrictions. If this flag is set to ask (the
default), new host keys will be added to the user known host files only
after the user has confirmed that is what they really want to do, and
ssh will refuse to connect to hosts whose host key has changed. The host
keys of known hosts will be verified automatically in all cases.
SyslogFacilityTCPKeepAliveThe 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.
Tunnelyes, point-to-point (layer
3), ethernet (layer 2), or
no (the default). Specifying
yes requests the default tunnel mode, which is
point-to-point.TunnelDeviceThe 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.
UpdateHostKeysUserKnownHostsFile. The argument must be
yes, no or
ask. This option allows learning alternate
hostkeys for a server and supports graceful key rotation by allowing a
server to send replacement public keys before old ones are removed.
Additional hostkeys are only accepted if the key used to
authenticate the host was already trusted or explicitly accepted by the
user, the host was authenticated via
UserKnownHostsFile (i.e. not
GlobalKnownHostsFile) and the host was
authenticated using a plain key and not a certificate.
UpdateHostKeys is enabled by default
if the user has not overridden the default
UserKnownHostsFile setting and has not enabled
VerifyHostKeyDNS, otherwise
UpdateHostKeys will be set to
no.
If UpdateHostKeys is set to
ask, then the user is asked to confirm the
modifications to the known_hosts file. Confirmation is currently
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.
UserUserKnownHostsFileVerifyHostKeyDNSyes, 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).
VisualHostKeyyes, an ASCII art
representation of the remote host key fingerprint is printed in addition
to the fingerprint string at login and for unknown host keys. If this flag
is set to no (the default), no fingerprint strings
are printed at login and only the fingerprint string will be printed for
unknown host keys.XAuthLocationA pattern consists of zero or more non-whitespace characters, ‘*’ (a wildcard that matches zero or more characters), or ‘?’ (a wildcard that matches exactly one character). For example, to specify a set of declarations for any host in the ".co.uk" set of domains, the following pattern could be used:
Host *.co.ukThe following pattern would match any host in the 192.168.0.[0-9] network range:
Host 192.168.0.?A pattern-list is a comma-separated list of patterns. Patterns within pattern-lists may be negated by preceding them with an exclamation mark (‘!’). For example, to allow a key to be used from anywhere within an organization except from the "dialup" pool, the following entry (in authorized_keys) could be used:
from="!*.dialup.example.com,*.example.com"Note that a negated match will never produce a positive result by itself. For example, attempting to match "host3" against the following pattern-list will fail:
from="!host1,!host2"The solution here is to include a term that will yield a positive match, such as a wildcard:
from="!host1,!host2,*"Arguments to some keywords can make use of tokens, which are expanded at runtime:
KnownHostsCommand execution: 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.ssh-ed25519.CertificateFile,
ControlPath, IdentityAgent,
IdentityFile,
KnownHostsCommand,
LocalForward, Match exec,
RemoteCommand,
RemoteForward, and
UserKnownHostsFile accept the tokens %%, %C, %d, %h,
%i, %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 accepts the tokens %%, %h,
%n, %p, and %r.
Arguments to some keywords can be expanded at runtime from
environment variables on the client by enclosing them in
${}, for example
${HOME}/.ssh would refer to the user's .ssh
directory. If a specified environment variable does not exist then an error
will be returned and the setting for that keyword will be ignored.
The keywords CertificateFile,
ControlPath, IdentityAgent,
IdentityFile,
KnownHostsCommand, and
UserKnownHostsFile support environment variables.
The keywords LocalForward and
RemoteForward support environment variables only for
Unix domain socket paths.
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.
| November 10, 2021 | OpenBSD-current |