| SSH_CONFIG(5) | File Formats Manual | SSH_CONFIG(5) |
ssh_config —
~/.ssh/config |
/etc/ssh/ssh_config |
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 configuration file has the following format:
Empty lines and lines starting with
‘#’ are comments. Otherwise a line is
of the format “keyword arguments”. 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. Arguments
may optionally be enclosed in double quotes (") in order to represent
arguments containing spaces.
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 option 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 critera or the single token
all which always matches. The available criteria
keywords are: canonical,
exec, host,
originalhost, user, and
localuser. The all
criteria must appear alone or immediately after
canonical. Other criteria may be combined
arbitrarily. All criteria but all and
canonical 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 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. The following
character sequences in the command will be expanded prior to execution:
‘%L’ will be substituted by the
first component of the local host name,
‘%l’ will be substituted by the
local host name (including any domain name),
‘%h’ will be substituted by the
target host name, ‘%n’ will be
substituted by the original target host name specified on the
command-line, ‘%p’ the destination
port, ‘%r’ by the remote login
username, and ‘%u’ by the username
of the user running ssh(1).
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).
AddressFamilyBatchModeBindAddressUsePrivilegedPort is set to
“yes”.CanonicalDomainsCanonicalizeHostname is enabled, this option
specifies the list of domain suffixes in which to search for the specified
destination host.CanonicalizeFallbackLocalCanonicalizeHostname is enabled and the target
hostname cannot be found in any of the domains specified by
CanonicalDomains.CanonicalizeHostnameProxyCommand, 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.
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.
ChallengeResponseAuthenticationCheckHostIPStrictHostKeyChecking. If the option is set to
“no”, the check will not be executed. The default is
“yes”.CipherCiphersThe supported ciphers are:
The default is:
chacha20-poly1305@openssh.com,
aes128-ctr,aes192-ctr,aes256-ctr,
aes128-gcm@openssh.com,aes256-gcm@openssh.com,
arcfour256,arcfour128,
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,
aes192-cbc,aes256-cbc,arcfour
The list of available ciphers may also be obtained using the
-Q option of ssh(1) with an
argument of “cipher”.
ClearAllForwardingsCompressionCompressionLevelConnectionAttemptsConnectTimeoutControlMasterControlPath 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 to listen
for control connections, but require confirmation using
ssh-askpass(1). If the
ControlPath cannot be opened, ssh 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. In the
path, ‘%L’ will be substituted by
the first component of the local host name,
‘%l’ will be substituted by the
local host name (including any domain name),
‘%h’ will be substituted by the
target host name, ‘%n’ will be
substituted by the original target host name specified on the command
line, ‘%p’ the destination port,
‘%r’ by the remote login username,
‘%u’ by the username and
‘%i’ by the numeric user id (uid) of
the user running ssh(1), and
‘%C’ by a hash of the concatenation:
%l%h%p%r. 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”, 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(1) “-O
exit” option). 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.
EnableSSHKeysignHostbasedAuthentication. The argument must be
“yes” or “no”. The default is
“no”. 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 is
“no”.FingerprintHashForwardAgentAgent 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 is
“no”.
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.
ForwardX11TimeoutForwardX11TrustedIf this option is set to “no”, 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.
The default is “no”.
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 is
“no”.GlobalKnownHostsFileGSSAPIAuthenticationGSSAPIDelegateCredentialsHashKnownHostsHostbasedAuthenticationRhostsRSAAuthentication.HostbasedKeyTypes
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa
The -Q option of
ssh(1) may be used to list supported key types.
HostKeyAlgorithms
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa
If hostkeys are known for the destination host then this default is modified to prefer their algorithms.
The list of available key types may also be obtained using the
-Q option of ssh(1) with an
argument of “key”.
HostKeyAliasHostName%h’, then this
will be replaced with the host name specified on the command line (this is
useful for manipulating unqualified names). The character sequence
‘%%’ will be replaced by a single
‘%’ character, which may be used
when specifying IPv6 link-local addresses.
The default is the name given on the command line. Numeric IP
addresses are also permitted (both on the command line and in
HostName specifications).
IdentitiesOnlyssh_config files,
even if ssh-agent(1) or a
PKCS11Provider offers more identities. The
argument to this keyword must be “yes” or
“no”. This option is intended for situations where ssh-agent
offers many different identities. The default is “no”.IdentityFileIdentitiesOnly
is set. ssh(1) will try to load certificate information
from the filename obtained by appending -cert.pub
to the path of a specified IdentityFile.
The file name may use the tilde syntax to refer to a user's
home directory or one of the following escape characters:
‘%d’ (local user's home
directory), ‘%u’ (local user
name), ‘%l’ (local host name),
‘%h’ (remote host name) or
‘%r’ (remote user name).
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.
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.IPQoSKbdInteractiveAuthenticationKbdInteractiveDevicesKexAlgorithms
curve25519-sha256@libssh.org,
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group-exchange-sha1,
diffie-hellman-group14-sha1
The list of available key exchange algorithms may also be
obtained using the -Q option of
ssh(1) with an argument of “kex”.
LocalCommand%d’ (local user's home directory),
‘%h’ (remote host name),
‘%l’ (local host name),
‘%n’ (host name as provided on the
command line), ‘%p’ (remote port),
‘%r’ (remote user name) or
‘%u’ (local user name) or
‘%C’ by a hash of the concatenation:
%l%h%p%r.
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.
LocalForwardGatewayPorts 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.LogLevelMACsThe 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,
umac-64@openssh.com,umac-128@openssh.com,
hmac-sha2-256,hmac-sha2-512,
hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
hmac-ripemd160-etm@openssh.com,
hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,
hmac-md5,hmac-sha1,hmac-ripemd160,
hmac-sha1-96,hmac-md5-96
The list of available MAC algorithms may also be obtained
using the -Q option of ssh(1)
with an argument of “mac”.
NoHostAuthenticationForLocalhostNumberOfPasswordPromptsPasswordAuthenticationPermitLocalCommandLocalCommand
option or using the !command
escape sequence in ssh(1). The argument must be
“yes” or “no”. The default is
“no”.PKCS11ProviderPortPreferredAuthenticationskeyboard-interactive) over another method (e.g.
password). The default is:
gssapi-with-mic,hostbased,publickey,
keyboard-interactive,password
Protocolssh will try
version 2 and fall back to version 1 if version 2 is not available. The
default is ‘2’.ProxyCommandexec’ directive to avoid a
lingering shell process.
In the command string, any occurrence of
‘%h’ will be substituted by the
host name to connect, ‘%p’ by the
port, and ‘%r’ by the remote user
name. 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
ProxyUseFdpassProxyCommand will pass a connected
file descriptor back to ssh(1) instead of continuing to
execute and pass data. The default is “no”.PubkeyAcceptedKeyTypes
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa
The -Q option of
ssh(1) may be used to list supported key types.
PubkeyAuthenticationRekeyLimitRekeyLimit 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. This option
applies to protocol version 2 only.RemoteForwardIf 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-t and -T flags for
ssh(1).RevokedHostKeysRhostsRSAAuthenticationRSAAuthenticationSendEnvTERM 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. The default is not to send any
environment variables.
See PATTERNS for more information on patterns.
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 inactive.
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. This option applies to protocol version 2 only.
ServerAliveIntervalStreamLocalBindMaskThe 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 is “no”.
StrictHostKeyCheckingTCPKeepAliveThe 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”.
TunnelTunnelDeviceThe 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” (the default) or
“ask”. Enabling 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 explicity accepted by the user. 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.
UsePrivilegedPortRhostsRSAAuthentication with older servers.UserUserKnownHostsFileVerifyHostKeyDNSStrictHostKeyChecking option. The argument must be
“yes”, “no”, or “ask”. The
default is “no”. Note that this option applies to protocol
version 2 only.
See also VERIFYING HOST KEYS in ssh(1).
VisualHostKeyXAuthLocationHost *.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"| September 11, 2015 | BSD |