NAME
ssh_config
—
OpenSSH client configuration
file
DESCRIPTION
ssh(1) obtains configuration data from the following sources in the following order:
- command-line options
- user's configuration file (~/.ssh/config)
- system-wide configuration file (/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 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):
Host
- Restricts the following declarations (up to the next
Host
orMatch
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 theCanonicalizeHostname
keyword for exceptions).A pattern entry may be negated by prefixing it with an exclamation mark (‘!’). If a negated entry is matched, then the
Host
entry is ignored, regardless of whether any other patterns on the line match. Negated matches are therefore useful to provide exceptions for wildcard matches.See PATTERNS for more information on patterns.
Match
- Restricts the following declarations (up to the next
Host
orMatch
keyword) to be used only when the conditions following theMatch
keyword are satisfied. Match conditions are specified using one or more criteria or the single tokenall
which always matches. The available criteria keywords are:canonical
,final
,exec
,host
,originalhost
,user
, andlocaluser
. Theall
criteria must appear alone or immediately aftercanonical
orfinal
. Other criteria may be combined arbitrarily. All criteria butall
,canonical
, andfinal
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 theCanonicalizeHostname
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 whetherCanonicalizeHostname
is enabled), and matches only during this final pass. IfCanonicalizeHostname
is enabled, thencanonical
andfinal
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 toexec
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 theHostname
orCanonicalizeHostname
options. Theoriginalhost
keyword matches against the hostname as it was specified on the command-line. Theuser
keyword matches against the target username on the remote host. Thelocaluser
keyword matches against the name of the local user running ssh(1) (this keyword may be useful in system-widessh_config
files). AddKeysToAgent
- Specifies whether keys should be automatically added to a running
ssh-agent(1). If this option is set to
yes
and a key is loaded from a file, the key and its passphrase are added to the agent with the default lifetime, as if by ssh-add(1). If this option is set toask
, ssh(1) will require confirmation using theSSH_ASKPASS
program before adding a key (see ssh-add(1) for details). If this option is set toconfirm
, each use of the key must be confirmed, as if the-c
option was specified to ssh-add(1). If this option is set tono
, no keys are added to the agent. The argument must beyes
,confirm
,ask
, orno
(the default). AddressFamily
- Specifies which address family to use when connecting. Valid arguments are
any
(the default),inet
(use IPv4 only), orinet6
(use IPv6 only). BatchMode
- If set to
yes
, 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 beyes
orno
(the default). BindAddress
- Use the specified address on the local machine as the source address of the connection. Only useful on systems with more than one address.
BindInterface
- Use the address of the specified interface on the local machine as the source address of the connection.
CanonicalDomains
- When
CanonicalizeHostname
is enabled, this option specifies the list of domain suffixes in which to search for the specified destination host. CanonicalizeFallbackLocal
- Specifies whether to fail with an error when hostname canonicalization
fails. The default,
yes
, will attempt to look up the unqualified hostname using the system resolver's search rules. A value ofno
will cause ssh(1) to fail instantly ifCanonicalizeHostname
is enabled and the target hostname cannot be found in any of the domains specified byCanonicalDomains
. CanonicalizeHostname
- Controls whether explicit hostname canonicalization is performed. The
default,
no
, is not to perform any name rewriting and let the system resolver handle all hostname lookups. If set toyes
then, for connections that do not use aProxyCommand
orProxyJump
, ssh(1) will attempt to canonicalize the hostname specified on the command line using theCanonicalDomains
suffixes andCanonicalizePermittedCNAMEs
rules. IfCanonicalizeHostname
is set toalways
, 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
andMatch
stanzas. CanonicalizeMaxDots
- Specifies the maximum number of dot characters in a hostname before canonicalization is disabled. The default, 1, allows a single dot (i.e. hostname.subdomain).
CanonicalizePermittedCNAMEs
- Specifies rules to determine whether CNAMEs should be followed when
canonicalizing hostnames. The rules consist of one or more arguments of
source_domain_list:target_domain_list,
where source_domain_list is a pattern-list of
domains that may follow CNAMEs in canonicalization, and
target_domain_list is a pattern-list of domains that
they may resolve to.
For example, "*.a.example.com:*.b.example.com,*.c.example.com" will allow hostnames matching "*.a.example.com" to be canonicalized to names in the "*.b.example.com" or "*.c.example.com" domains.
CASignatureAlgorithms
- Specifies which algorithms are allowed for signing of certificates by
certificate authorities (CAs). The default is:
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
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 aPKCS11Provider
orSecurityKeyProvider
.Arguments to
CertificateFile
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 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. ChallengeResponseAuthentication
- Specifies whether to use challenge-response authentication. The argument
to this keyword must be
yes
(the default) orno
. CheckHostIP
- If set to
yes
(the default), 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 ofStrictHostKeyChecking
. If the option is set tono
, the check will not be executed. Ciphers
- Specifies the ciphers allowed and their order of preference. Multiple
ciphers must be comma-separated. If the specified list begins with a
‘+’ character, then the specified ciphers will be appended
to the default set instead of replacing them. If the specified list begins
with a ‘-’ character, then the specified ciphers (including
wildcards) will be removed from the default set instead of replacing them.
If the specified list begins with a ‘^’ character, then the
specified ciphers will be placed at the head of the default set.
The supported ciphers are:
3des-cbc aes128-cbc aes192-cbc aes256-cbc aes128-ctr aes192-ctr aes256-ctr aes128-gcm@openssh.com aes256-gcm@openssh.com chacha20-poly1305@openssh.com
The default is:
chacha20-poly1305@openssh.com, aes128-ctr,aes192-ctr,aes256-ctr, aes128-gcm@openssh.com,aes256-gcm@openssh.com
The list of available ciphers may also be obtained using "ssh -Q cipher".
ClearAllForwardings
- Specifies that all local, remote, and dynamic port forwardings specified
in the configuration files or on the command line be cleared. This option
is primarily useful when used from the
ssh(1) command line to clear port forwardings set in configuration
files, and is automatically set by
scp(1) and
sftp(1). The argument must be
yes
orno
(the default). Compression
- Specifies whether to use compression. The argument must be
yes
orno
(the default). ConnectionAttempts
- Specifies the number of tries (one per second) to make before exiting. The argument must be an integer. This may be useful in scripts if the connection sometimes fails. The default is 1.
ConnectTimeout
- Specifies the timeout (in seconds) used when connecting to the SSH server, instead of using the default system TCP timeout. This timeout is applied both to establishing the connection and to performing the initial SSH protocol handshake and key exchange.
ControlMaster
- Enables the sharing of multiple sessions over a single network connection.
When set to
yes
, ssh(1) will listen for connections on a control socket specified using theControlPath
argument. Additional sessions can connect to this socket using the sameControlPath
withControlMaster
set tono
(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 theControlPath
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
andautoask
. The latter requires confirmation like theask
option. ControlPath
- Specify the path to the control socket used for connection sharing as
described in the
ControlMaster
section above or the stringnone
to disable connection sharing. Arguments toControlPath
may use the tilde syntax to refer to a user's home directory or the tokens described in the TOKENS section. It is recommended that anyControlPath
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. ControlPersist
- When used in conjunction with
ControlMaster
, specifies that the master connection should remain open in the background (waiting for future client connections) after the initial client connection has been closed. If set tono
(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 toyes
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 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 oflocalhost
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.
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) duringHostbasedAuthentication
. The argument must beyes
orno
(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, ornone
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 listen on a specified port). Note that
ExitOnForwardFailure
does not apply to connections made over port forwardings and will not, for example, cause ssh(1) to exit if TCP connections to the ultimate forwarding destination fail. The argument must beyes
orno
(the default). FingerprintHash
- Specifies the hash algorithm used when displaying key fingerprints. Valid
options are:
md5
andsha256
(the default). ForwardAgent
- Specifies whether the connection to the authentication agent (if any) will
be forwarded to the remote machine. The argument may be
yes
,no
(the default), an explicit path to an agent socket or the name of an environment variable (beginning with ‘$’) in which to find the path.Agent forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the agent's Unix-domain socket) can access the local agent through the forwarded connection. An attacker cannot obtain key material from the agent, however they can perform operations on the keys that enable them to authenticate using the identities loaded into the agent.
ForwardX11
- Specifies whether X11 connections will be automatically redirected over
the secure channel and
DISPLAY
set. The argument must beyes
orno
(the default).X11 forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the user's X11 authorization database) can access the local X11 display through the forwarded connection. An attacker may then be able to perform activities such as keystroke monitoring if the
ForwardX11Trusted
option is also enabled. ForwardX11Timeout
- Specify a timeout for untrusted X11 forwarding using the format described
in the TIME FORMATS section of
sshd_config(5). X11 connections received by
ssh(1) after this time will be refused. Setting
ForwardX11Timeout
to zero will disable the timeout and permit X11 forwarding for the life of the connection. The default is to disable untrusted X11 forwarding after twenty minutes has elapsed. ForwardX11Trusted
- If this option is set to
yes
, 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.
GatewayPorts
- Specifies whether remote hosts are allowed to connect to local forwarded
ports. By default,
ssh(1) binds local port forwardings to the loopback address. This
prevents other remote hosts from connecting to forwarded ports.
GatewayPorts
can be used to specify that ssh should bind local port forwardings to the wildcard address, thus allowing remote hosts to connect to forwarded ports. The argument must beyes
orno
(the default). GlobalKnownHostsFile
- Specifies one or more files to use for the global host key database, separated by whitespace. The default is /etc/ssh/ssh_known_hosts, /etc/ssh/ssh_known_hosts2.
GSSAPIAuthentication
- Specifies whether user authentication based on GSSAPI is allowed. The
default is
no
. GSSAPIDelegateCredentials
- Forward (delegate) credentials to the server. The default is
no
. 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 disclosed. The default is
no
. Note that existing names and addresses in known hosts files will not be converted automatically, but may be manually hashed using ssh-keygen(1). HostbasedAuthentication
- Specifies whether to try rhosts based authentication with public key
authentication. The argument must be
yes
orno
(the default). HostbasedKeyTypes
- Specifies the key types that will be used for hostbased authentication as
a comma-separated list of patterns. Alternately if the specified list
begins with a ‘+’ character, then the specified key types
will be appended to the default set instead of replacing them. If the
specified list begins with a ‘-’ character, then the
specified key types (including wildcards) will be removed from the default
set instead of replacing them. If the specified list begins with a
‘^’ character, then the specified key types will be placed
at the head of the default set. The default for this option is:
ecdsa-sha2-nistp256-cert-v01@openssh.com, ecdsa-sha2-nistp384-cert-v01@openssh.com, ecdsa-sha2-nistp521-cert-v01@openssh.com, sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, ssh-ed25519-cert-v01@openssh.com, sk-ssh-ed25519-cert-v01@openssh.com, rsa-sha2-512-cert-v01@openssh.com, rsa-sha2-256-cert-v01@openssh.com, ssh-rsa-cert-v01@openssh.com, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, sk-ecdsa-sha2-nistp256@openssh.com, ssh-ed25519,sk-ssh-ed25519@openssh.com, rsa-sha2-512,rsa-sha2-256,ssh-rsa
The
-Q
option of ssh(1) may be used to list supported key types. HostKeyAlgorithms
- Specifies the host key algorithms that the client wants to use in order of
preference. Alternately if the specified list begins with a
‘+’ character, then the specified key types will be appended
to the default set instead of replacing them. If the specified list begins
with a ‘-’ character, then the specified key types
(including wildcards) will be removed from the default set instead of
replacing them. If the specified list begins with a ‘^’
character, then the specified key types will be placed at the head of the
default set. The default for this option is:
ecdsa-sha2-nistp256-cert-v01@openssh.com, ecdsa-sha2-nistp384-cert-v01@openssh.com, ecdsa-sha2-nistp521-cert-v01@openssh.com, sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, ssh-ed25519-cert-v01@openssh.com, sk-ssh-ed25519-cert-v01@openssh.com, rsa-sha2-512-cert-v01@openssh.com, rsa-sha2-256-cert-v01@openssh.com, ssh-rsa-cert-v01@openssh.com, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, sk-ecdsa-sha2-nistp256@openssh.com, ssh-ed25519,sk-ssh-ed25519@openssh.com, rsa-sha2-512,rsa-sha2-256,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 "ssh -Q HostKeyAlgorithms".
HostKeyAlias
- Specifies an alias that should be used instead of the real host name when looking up or saving the host key in the host key database files and when validating host certificates. This option is useful for tunneling SSH connections or for multiple servers running on a single host.
Hostname
- Specifies the real host name to log into. This can be used to specify
nicknames or abbreviations for hosts. Arguments to
Hostname
accept the tokens described in the TOKENS section. Numeric IP addresses are also permitted (both on the command line and inHostname
specifications). The default is the name given on the command line. IdentitiesOnly
- Specifies that ssh(1) should only use the configured authentication
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 aPKCS11Provider
orSecurityKeyProvider
offers more identities. The argument to this keyword must beyes
orno
(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 tonone
disables the use of an authentication agent. If the string "SSH_AUTH_SOCK" is specified, the location of the socket will be read from theSSH_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 or the tokens described in the TOKENS section. IdentityFile
- Specifies a file from which the user's DSA, ECDSA, authenticator-hosted
ECDSA, Ed25519, authenticator-hosted Ed25519 or RSA authentication
identity is read. The default is ~/.ssh/id_dsa,
~/.ssh/id_ecdsa,
~/.ssh/id_ecdsa_sk,
~/.ssh/id_ed25519,
~/.ssh/id_ed25519_sk and
~/.ssh/id_rsa. Additionally, any identities
represented by the authentication agent will be used for authentication
unless
IdentitiesOnly
is set. If no certificates have been explicitly specified byCertificateFile
, ssh(1) will try to load certificate information from the filename obtained by appending -cert.pub to the path of a specifiedIdentityFile
.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 withIdentitiesOnly
to select which identities in an agent are offered during authentication.IdentityFile
may also be used in conjunction withCertificateFile
in order to provide any certificate also needed for authentication with the identity. IgnoreUnknown
- Specifies a pattern-list of unknown options to be ignored if they are
encountered in configuration parsing. This may be used to suppress errors
if
ssh_config
contains options that are unrecognised by ssh(1). It is recommended thatIgnoreUnknown
be listed early in the configuration file as it will not be applied to unknown options that appear before it. Include
- Include the specified configuration file(s). Multiple pathnames may be
specified and each pathname may contain
glob(7) wildcards and, for user configurations, shell-like
‘~’ references to user home directories. Files without
absolute paths are assumed to be in ~/.ssh if
included in a user configuration file or /etc/ssh
if included from the system configuration file.
Include
directive may appear inside aMatch
orHost
block to perform conditional inclusion. IPQoS
- Specifies the IPv4 type-of-service or DSCP class 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
,lowdelay
,throughput
,reliability
, a numeric value, ornone
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 isaf21
(Low-Latency Data) for interactive sessions andcs1
(Lower Effort) for non-interactive sessions. KbdInteractiveAuthentication
- Specifies whether to use keyboard-interactive authentication. The argument
to this keyword must be
yes
(the default) orno
. KbdInteractiveDevices
- Specifies the list of methods to use in keyboard-interactive
authentication. Multiple method names must be comma-separated. The default
is to use the server specified list. The methods available vary depending
on what the server supports. For an OpenSSH server, it may be zero or more
of:
bsdauth
,pam
, andskey
. KexAlgorithms
- Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms
must be comma-separated. If the specified list begins with a
‘+’ character, then the specified methods will be appended
to the default set instead of replacing them. If the specified list begins
with a ‘-’ character, then the specified methods (including
wildcards) will be removed from the default set instead of replacing them.
If the specified list begins with a ‘^’ character, then the
specified methods will be placed at the head of the default set. The
default is:
curve25519-sha256,curve25519-sha256@libssh.org, ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, diffie-hellman-group-exchange-sha256, diffie-hellman-group16-sha512, diffie-hellman-group18-sha512, diffie-hellman-group14-sha256
The list of available key exchange algorithms may also be obtained using "ssh -Q kex".
LocalCommand
- Specifies a command to execute on the local machine after successfully
connecting to the server. The command string extends to the end of the
line, and is executed with the user's shell. Arguments to
LocalCommand
accept the tokens described in the TOKENS section.The command is run synchronously and does not have access to the session of the ssh(1) that spawned it. It should not be used for interactive commands.
This directive is ignored unless
PermitLocalCommand
has been enabled. LocalForward
- Specifies that a TCP port on the local machine be forwarded over the
secure channel to the specified host and port from the remote machine. The
first argument specifies the listener and may be
[bind_address:]port or a Unix
domain socket path. The second argument is the destination and may be
host:hostport or a Unix domain
socket path if the remote host supports it.
IPv6 addresses can be specified by enclosing addresses in square brackets. Multiple forwardings may be specified, and additional forwardings can be given on the command line. Only the superuser can forward privileged ports. By default, the local port is bound in accordance with the
GatewayPorts
setting. However, an explicit bind_address may be used to bind the connection to a specific address. The bind_address oflocalhost
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 accept the tokens described in the TOKENS 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.
MACs
- Specifies the MAC (message authentication code) algorithms in order of
preference. The MAC algorithm is used for data integrity protection.
Multiple algorithms must be comma-separated. If the specified list 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 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".
NoHostAuthenticationForLocalhost
- Disable host authentication for localhost (loopback addresses). The
argument to this keyword must be
yes
orno
(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.
PasswordAuthentication
- Specifies whether to use password authentication. The argument to this
keyword must be
yes
(the default) orno
. PermitLocalCommand
- Allow local command execution via the
LocalCommand
option or using the!
command escape sequence in ssh(1). The argument must beyes
orno
(the default). PKCS11Provider
- Specifies which PKCS#11 provider to use or
none
to indicate that no provider should be used (the default). The argument to this keyword is a path to the PKCS#11 shared library ssh(1) should use to communicate with a PKCS#11 token providing keys for user authentication. Port
- Specifies the port number to connect on the remote host. The default is 22.
PreferredAuthentications
- Specifies the order in which the client should try authentication methods.
This allows a client to prefer one method (e.g.
keyboard-interactive
) over another method (e.g.password
). The default is:gssapi-with-mic,hostbased,publickey, keyboard-interactive,password
ProxyCommand
- Specifies the command to use to connect to the server. The command string
extends to the end of the line, and is executed using the user's shell
‘
exec
’ directive to avoid a lingering shell process.Arguments to
ProxyCommand
accept the tokens described in the TOKENS section. The command can be basically anything, and should read from its standard input and write to its standard output. It should eventually connect an sshd(8) server running on some machine, or executesshd -i
somewhere. Host key management will be done using theHostname
of the host being connected (defaulting to the name typed by the user). Setting the command tonone
disables this option entirely. Note thatCheckHostIP
is not available for connects with a proxy command.This directive is useful in conjunction with nc(1) and its proxy support. For example, the following directive would connect via an HTTP proxy at 192.0.2.0:
ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
ProxyJump
- Specifies one or more jump proxies as either
[user@]host[:port]
or an ssh URI. Multiple proxies may be separated by comma characters and
will be visited sequentially. Setting this option will cause
ssh(1) to connect to the target host by first making a
ssh(1) connection to the specified
ProxyJump
host and then establishing a TCP forwarding to the ultimate target from there.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.
ProxyUseFdpass
- Specifies that
ProxyCommand
will pass a connected file descriptor back to ssh(1) instead of continuing to execute and pass data. The default isno
. PubkeyAcceptedKeyTypes
- Specifies the key types 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 key types after it will be appended
to the default instead of replacing it. If the specified list begins with
a ‘-’ character, then the specified key types (including
wildcards) will be removed from the default set instead of replacing them.
If the specified list begins with a ‘^’ character, then the
specified key types will be placed at the head of the default set. The
default for this option is:
ecdsa-sha2-nistp256-cert-v01@openssh.com, ecdsa-sha2-nistp384-cert-v01@openssh.com, ecdsa-sha2-nistp521-cert-v01@openssh.com, sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, ssh-ed25519-cert-v01@openssh.com, sk-ssh-ed25519-cert-v01@openssh.com, rsa-sha2-512-cert-v01@openssh.com, rsa-sha2-256-cert-v01@openssh.com, ssh-rsa-cert-v01@openssh.com, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, sk-ecdsa-sha2-nistp256@openssh.com, ssh-ed25519,sk-ssh-ed25519@openssh.com, rsa-sha2-512,rsa-sha2-256,ssh-rsa
The list of available key types may also be obtained using "ssh -Q PubkeyAcceptedKeyTypes".
PubkeyAuthentication
- Specifies whether to try public key authentication. The argument to this
keyword must be
yes
(the default) orno
. RekeyLimit
- Specifies the maximum amount of data that may be transmitted before the
session key is renegotiated, optionally followed a maximum amount of time
that may pass before the session key is renegotiated. The first argument
is specified in bytes and may have a suffix of ‘K’,
‘M’, or ‘G’ to indicate Kilobytes, Megabytes,
or Gigabytes, respectively. The default is between ‘1G’ and
‘4G’, depending on the cipher. The optional second value is
specified in seconds and may use any of the units documented in the TIME
FORMATS section of
sshd_config(5). The default value for
RekeyLimit
isdefault none
, which means that rekeying is performed after the cipher's default amount of data has been sent or received and no time based rekeying is done. RemoteCommand
- Specifies a command to execute on the remote machine after successfully
connecting to the server. The command string extends to the end of the
line, and is executed with the user's shell. Arguments to
RemoteCommand
accept the tokens described in the TOKENS section. RemoteForward
- Specifies that a TCP port on the remote machine be forwarded over the
secure channel. The remote port may either be forwarded to a specified
host and port from the local machine, or may act as a SOCKS 4/5 proxy that
allows a remote client to connect to arbitrary destinations from the local
machine. The first argument is the listening specification and may be
[bind_address:]port or, if the
remote host supports it, a Unix domain socket path. If forwarding to a
specific destination then the second argument must be
host:hostport or a Unix domain
socket path, otherwise if no destination argument is specified then the
remote forwarding will be established as a SOCKS proxy.
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 accept the tokens described in the TOKENS 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'sGatewayPorts
option is enabled (see sshd_config(5)). RequestTTY
- Specifies whether to request a pseudo-tty for the session. The argument
may be one of:
no
(never request a TTY),yes
(always request a TTY when standard input is a TTY),force
(always request a TTY) orauto
(request a TTY when opening a login session). This option mirrors the-t
and-T
flags for ssh(1). 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).
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 protocol. Refer toAcceptEnv
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 multipleSendEnv
directives.See PATTERNS for more information on patterns.
It is possible to clear previously set
SendEnv
variable names by prefixing patterns with -. The default is not to send any environment variables. ServerAliveCountMax
- Sets the number of server alive messages (see below) which may be sent
without ssh(1) receiving any messages back from the server. If this
threshold is reached while server alive messages are being sent, ssh will
disconnect from the server, terminating the session. It is important to
note that the use of server alive messages is very different from
TCPKeepAlive
(below). The server alive messages are sent through the encrypted channel and therefore will not be spoofable. The TCP keepalive option enabled byTCPKeepAlive
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 andServerAliveCountMax
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.
SetEnv
- Directly specify one or more environment variables and their contents to
be sent to the server. Similarly to
SendEnv
, the server must be prepared to accept the environment variable. StreamLocalBindMask
- Sets the octal file creation mode mask (umask) used when creating a
Unix-domain socket file for local or remote port forwarding. This option
is only used for port forwarding to a Unix-domain socket file.
The default value is 0177, which creates a Unix-domain socket file that is readable and writable only by the owner. Note that not all operating systems honor the file mode on Unix-domain socket files.
StreamLocalBindUnlink
- Specifies whether to remove an existing Unix-domain socket file for local
or remote port forwarding before creating a new one. If the socket file
already exists and
StreamLocalBindUnlink
is not enabled,ssh
will be 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
orno
(the default). StrictHostKeyChecking
- If this flag is set to
yes
, ssh(1) will never automatically add host keys to the ~/.ssh/known_hosts file, and refuses to connect to hosts whose host key has changed. This provides maximum protection against man-in-the-middle (MITM) attacks, though it can be annoying when the /etc/ssh/ssh_known_hosts file is poorly maintained or when connections to new hosts are frequently made. This option forces the user to manually add all new hosts.If this flag is set to “accept-new” then ssh will automatically add new host keys to the user known hosts files, but will not permit connections to hosts with changed host keys. If this flag is set to “no” or “off”, ssh will automatically add new host keys to the user known hosts files and allow connections to hosts with changed hostkeys to proceed, subject to some restrictions. If this flag is set to
ask
(the default), new host keys will be added to the user known host files only after the user has confirmed that is what they really want to do, and ssh will refuse to connect to hosts whose host key has changed. The host keys of known hosts will be verified automatically in all cases. SyslogFacility
- Gives the facility code that is used when logging messages from ssh(1). The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is USER.
TCPKeepAlive
- Specifies whether the system should send TCP keepalive messages to the
other side. If they are sent, death of the connection or crash of one of
the machines will be properly noticed. 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 alsoServerAliveInterval
for protocol-level keepalives. 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), orno
(the default). Specifyingyes
requests the default tunnel mode, which ispoint-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 toany
. The default isany:any
. UpdateHostKeys
- Specifies whether ssh(1) should accept notifications of additional hostkeys
from the server sent after authentication has completed and add them to
UserKnownHostsFile
. The argument must beyes
,no
orask
. 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.UpdateHostKeys
is enabled by default if the user has not overridden the defaultUserKnownHostsFile
setting, otherwiseUpdateHostKeys
will be set toask
.If
UpdateHostKeys
is set toask
, then the user is asked to confirm the modifications to the known_hosts file. Confirmation is currently incompatible withControlPersist
, and will be disabled if it is enabled.Presently, only sshd(8) from OpenSSH 6.8 and greater support the "hostkeys@openssh.com" protocol extension used to inform the client of all the server's hostkeys.
User
- Specifies the user to log in as. This can be useful when a different user name is used on different machines. This saves the trouble of having to remember to give the user name on the command line.
UserKnownHostsFile
- Specifies one or more files to use for the user host key database, separated by whitespace. 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 toask
. If this option is set toask
, information on fingerprint match will be displayed, but the user will still need to confirm new host keys according to theStrictHostKeyChecking
option. The default isno
.See also VERIFYING HOST KEYS in ssh(1).
VisualHostKey
- If this flag is set to
yes
, an ASCII art representation of the remote host key fingerprint is printed in addition to the fingerprint string at login and for unknown host keys. If this flag is set tono
(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 default is /usr/X11R6/bin/xauth.
PATTERNS
A pattern consists of zero or more non-whitespace characters, ‘*’ (a wildcard that matches zero or more characters), or ‘?’ (a wildcard that matches exactly one character). For example, to specify a set of declarations for any host in the ".co.uk" set of domains, the following pattern could be used:
Host *.co.uk
The following pattern would match any host in the 192.168.0.[0-9] network range:
Host 192.168.0.?
A pattern-list is a comma-separated list of patterns. Patterns within pattern-lists may be negated by preceding them with an exclamation mark (‘!’). For 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,*"
TOKENS
Arguments to some keywords can make use of tokens, which are expanded at runtime:
- %%
- A literal ‘%’.
- %C
- Hash of %l%h%p%r.
- %d
- Local user's home directory.
- %h
- The remote hostname.
- %i
- The local user ID.
- %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.
- %u
- The local username.
CertificateFile
,
ControlPath
, IdentityAgent
,
IdentityFile
, LocalForward
,
Match exec
, RemoteCommand
,
and RemoteForward
accept the tokens %%, %C, %d, %h,
%i, %L, %l, %n, %p, %r, and %u.
Hostname
accepts the tokens %% and %h.
LocalCommand
accepts all tokens.
ProxyCommand
accepts the tokens %%, %h,
%n, %p, and %r.
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 others.
- /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
AUTHORS
OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer features and created OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0.