Ubuntu Manpage: ssh_config — OpenSSH SSH client configuration files

Provided by: openssh-client_7.2p2-4ubuntu2.10_amd64

NAME

     ssh_config — OpenSSH SSH client configuration files

SYNOPSIS

     ~/.ssh/config
     /etc/ssh/ssh_config

DESCRIPTION

ssh(1) obtains configuration data from the following sources in the following order:

1. command-line options
2. user's configuration file (~/.ssh/config)
3. system-wide configuration file (/etc/ssh/ssh_config)

For each parameter, the first obtained value will be used. The configuration files contain
sections separated by “Host” specifications, and that section is only applied for hosts that
match one of the patterns given in the specification. The matched host name is usually the
one given on the command line (see the CanonicalizeHostname option for exceptions.)

Since the first obtained value for each parameter is used, more host-specific declarations
should be given near the beginning of the file, and general defaults at the end.

Note that the Debian openssh-client package sets several options as standard in
/etc/ssh/ssh_config which are not the default in ssh(1):

• SendEnv LANG LC_*
• HashKnownHosts yes
• GSSAPIAuthentication yes

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):

Host Restricts the following declarations (up to the next Host or Match keyword) to be
only for those hosts that match one of the patterns given after the keyword. If
more than one pattern is provided, they should be separated by whitespace. A single
‘*’ as a pattern can be used to provide global defaults for all hosts. The host is
usually the hostname argument given on the command line (see the
CanonicalizeHostname 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.

Match Restricts the following declarations (up to the next Host or Match keyword) to be
used only when the conditions following the Match keyword are satisfied. Match
conditions are specified using one or more criteria 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).

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 to “ask”, ssh 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. The
argument must be “yes”, “confirm”, “ask”, or “no”. The default is “no”.

AddressFamily
Specifies which address family to use when connecting. Valid arguments are “any”,
“inet” (use IPv4 only), or “inet6” (use IPv6 only). The default is “any”.

BatchMode
If set to “yes”, passphrase/password querying will be disabled. In addition, the
ServerAliveInterval option will be set to 300 seconds by default. This option is
useful in scripts and other batch jobs where no user is present to supply the
password, and where it is desirable to detect a broken network swiftly. The
argument must be “yes” or “no”. The default is “no”.

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. Note that this
option does not work if UsePrivilegedPort is set to “yes”.

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 of “no” will cause ssh(1) to fail instantly if
CanonicalizeHostname is enabled and the target hostname cannot be found in any of
the domains specified by CanonicalDomains.

CanonicalizeHostname
Controls whether explicit hostname canonicalization is performed. The default,
“no”, is not to perform any name rewriting and let the system resolver handle all
hostname lookups. If set to “yes” then, for connections that do not use a
ProxyCommand, 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.

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.

CertificateFile
Specifies a file from which the user's certificate is read. A corresponding private
key must be provided separately in order to use this certificate either from an
IdentityFile directive or -i flag to ssh(1), via ssh-agent(1), or via a
PKCS11Provider.

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 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” or “no”. The default is “yes”.

CheckHostIP
If this flag is set to “yes”, ssh(1) will additionally check the host IP address in
the known_hosts file. This allows ssh 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 check will not be executed. The default is “yes”.

Cipher Specifies the cipher to use for encrypting the session in protocol version 1.
Currently, “blowfish”, “3des”, and “des” are supported. des is only supported in
the ssh(1) client for interoperability with legacy protocol 1 implementations that
do not support the 3des cipher. Its use is strongly discouraged due to
cryptographic weaknesses. The default is “3des”.

Ciphers
Specifies the ciphers allowed for protocol version 2 in order of preference.
Multiple ciphers must be comma-separated. If the specified value begins with a ‘+’
character, then the specified ciphers will be appended to the default set instead of
replacing them.

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
arcfour
arcfour128
arcfour256
blowfish-cbc
cast128-cbc
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,
aes128-cbc,aes192-cbc,aes256-cbc,3des-cbc

The list of available ciphers may also be obtained using the -Q option of ssh(1)
with an argument of “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” or “no”. The default is “no”.

Compression
Specifies whether to use compression. The argument must be “yes” or “no”. The
default is “no”.

CompressionLevel
Specifies the compression level to use if compression is enabled. The argument must
be an integer from 1 (fast) to 9 (slow, best). The default level is 6, which is
good for most applications. The meaning of the values is the same as in gzip(1).
Note that this option applies to protocol version 1 only.

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 value is used only when the target is
down or really unreachable, not when it refuses the connection.

ControlMaster
Enables the sharing of multiple sessions over a single network connection. When set
to “yes”, ssh(1) will listen for connections on a control socket specified using the
ControlPath argument. Additional sessions can connect to this socket using the same
ControlPath with ControlMaster set to “no” (the default). These sessions will try
to reuse the master instance's network 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.

ControlPath
Specify the path to the control socket used for connection sharing as described in
the ControlMaster section above or the string “none” to disable connection sharing.
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.

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 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.

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 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.

EnableSSHKeysign
Setting this option to “yes” in the global client configuration file
/etc/ssh/ssh_config enables the use of the helper program ssh-keysign(8) during
HostbasedAuthentication. The argument must be “yes” or “no”. The default is “no”.
This option should be placed in the non-hostspecific section. See ssh-keysign(8)
for more information.

EscapeChar
Sets the escape character (default: ‘~’). The escape character can also be set on
the command line. The argument should be a single character, ‘^’ followed by a
letter, or “none” to disable the escape character entirely (making the connection
transparent for binary data).

ExitOnForwardFailure
Specifies whether ssh(1) should terminate the connection if it cannot set up all
requested dynamic, tunnel, local, and remote port forwardings, (e.g. if either end
is unable to bind and listen on a specified port). Note that ExitOnForwardFailure
does not apply to connections made over port forwardings and will not, for example,
cause ssh(1) to exit if TCP connections to the ultimate forwarding destination fail.
The argument must be “yes” or “no”. The default is “no”.

FingerprintHash
Specifies the hash algorithm used when displaying key fingerprints. Valid options
are: “md5” and “sha256”. The default is “sha256”.

ForwardAgent
Specifies whether the connection to the authentication agent (if any) will be
forwarded to the remote machine. The argument must be “yes” or “no”. The default
is “no”.

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 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.

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. 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”, 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 “yes” (Debian-specific).

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 be “yes” or
“no”. The default is “no”.

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”.

GSSAPIKeyExchange
Specifies whether key exchange based on GSSAPI may be used. When using GSSAPI key
exchange the server need not have a host key. The default is “no”.

GSSAPIClientIdentity
If set, specifies the GSSAPI client identity that ssh should use when connecting to
the server. The default is unset, which means that the default identity will be
used.

GSSAPIServerIdentity
If set, specifies the GSSAPI server identity that ssh should expect when connecting
to the server. The default is unset, which means that the expected GSSAPI server
identity will be determined from the target hostname.

GSSAPIDelegateCredentials
Forward (delegate) credentials to the server. The default is “no”.

GSSAPIRenewalForcesRekey
If set to “yes” then renewal of the client's GSSAPI credentials will force the
rekeying of the ssh connection. With a compatible server, this can delegate the
renewed credentials to a session on the server. The default is “no”.

GSSAPITrustDns
Set to “yes” to indicate that the DNS is trusted to securely canonicalize the name
of the host being connected to. If “no”, the hostname entered on the command line
will be passed untouched to the GSSAPI library. 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 reveal identifying information should the file's contents be
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). Use of this option may break facilities such as tab-completion that
rely on being able to read unhashed host names from ~/.ssh/known_hosts.

HostbasedAuthentication
Specifies whether to try rhosts based authentication with public key authentication.
The argument must be “yes” or “no”. The default is “no”.

HostbasedKeyTypes
Specifies the key types that will be used for hostbased authentication as a comma-
separated pattern list. Alternately if the specified value begins with a ‘+’
character, then the specified key types will be appended to the default set instead
of replacing them. 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,
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
Specifies the host key algorithms that the client wants to use in order of
preference. Alternately if the specified value begins with a ‘+’ character, then
the specified key types will be appended to the default set instead of replacing
them. The default for this option is:

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”.

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. 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. If the hostname contains the character sequence ‘%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).

IdentitiesOnly
Specifies that ssh(1) should only use the authentication identity and certificate
files explicitly configured in the ssh_config files or passed on the ssh(1) command-
line, 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”.

IdentityFile
Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
identity is read. The default is ~/.ssh/identity for protocol version 1, and
~/.ssh/id_dsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 and ~/.ssh/id_rsa for protocol
version 2. Additionally, any identities represented by the authentication agent
will be used for authentication unless IdentitiesOnly is set. If no certificates
have been explicitly specified by CertificateFile, ssh(1) will try to load
certificate information from the filename obtained by appending -cert.pub to the
path of a specified IdentityFile.

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.

IgnoreUnknown
Specifies a pattern-list of unknown options to be ignored if they are encountered in
configuration parsing. This may be used to suppress errors if ssh_config contains
options that are unrecognised by ssh(1). It is recommended that IgnoreUnknown be
listed early in the configuration file as it will not be applied to unknown options
that appear before it.

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”,
“lowdelay”, “throughput”, “reliability”, or a numeric value. 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 “lowdelay” for interactive sessions and “throughput” for
non-interactive sessions.

KbdInteractiveAuthentication
Specifies whether to use keyboard-interactive authentication. The argument to this
keyword must be “yes” or “no”. The default is “yes”.

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”, and “skey”.

KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms must be
comma-separated. Alternately if the specified value begins with a ‘+’ character,
then the specified methods will be appended to the default set instead of replacing
them. The default is:

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
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. The following escape character substitutions will be performed:
‘%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.

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 must be
[bind_address:]port and the second argument must be host:hostport. 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 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.

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 value begins with a ‘+’ character, then the
specified algorithms will be appended to the default set instead of replacing them.

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 the -Q option of
ssh(1) with an argument of “mac”.

NoHostAuthenticationForLocalhost
This option can be used if the home directory is shared across machines. In this
case localhost will refer to a different machine on each of the machines and the
user will get many warnings about changed host keys. However, this option disables
host authentication for localhost. The argument to this keyword must be “yes” or
“no”. The default is to check the host key for localhost.

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” or “no”. The default is “yes”.

PermitLocalCommand
Allow local command execution via the LocalCommand option or using the !command
escape sequence in ssh(1). The argument must be “yes” or “no”. The default is
“no”.

PKCS11Provider
Specifies which PKCS#11 provider to use. The argument to this keyword is the
PKCS#11 shared library ssh(1) should use to communicate with a PKCS#11 token
providing the user's private RSA key.

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

Protocol
Specifies the protocol versions ssh(1) should support in order of preference. The
possible values are ‘1’ and ‘2’. Multiple versions must be comma-separated. When
this option is set to “2,1” ssh will try version 2 and fall back to version 1 if
version 2 is not available. The default is ‘2’. Protocol 1 suffers from a number
of cryptographic weaknesses and should not be used. It is only offered to support
legacy devices.

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.

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

ProxyUseFdpass
Specifies that ProxyCommand will pass a connected file descriptor back to ssh(1)
instead of continuing to execute and pass data. The default is “no”.

PubkeyAcceptedKeyTypes
Specifies the key types that will be used for public key authentication as a comma-
separated pattern list. Alternately if the specified value begins with a ‘+’
character, then the key types after it will be appended to the default instead of
replacing it. The default for this option is:

The -Q option of ssh(1) may be used to list supported key types.

PubkeyAuthentication
Specifies whether to try public key authentication. The argument to this keyword
must be “yes” or “no”. The default is “yes”.

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 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.

RemoteForward
Specifies that a TCP port on the remote machine be forwarded over the secure channel
to the specified host and port from the local machine. The first argument must be
[bind_address:]port and the second argument must be host:hostport. 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.

If the port argument is ‘0’, the listen port will be dynamically allocated on the
server and reported to the client at run time.

If the bind_address is not specified, the default is to only bind to loopback
addresses. If the bind_address is ‘*’ or an empty string, then the forwarding is
requested to listen on all interfaces. Specifying a remote bind_address will only
succeed if the server's GatewayPorts option is enabled (see sshd_config(5)).

RequestTTY
Specifies whether to request a pseudo-tty for the session. The argument may be one
of: “no” (never request a TTY), “yes” (always request a TTY when standard input is a
TTY), “force” (always request a TTY) or “auto” (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).

RhostsRSAAuthentication
Specifies whether to try rhosts based authentication with RSA host authentication.
The argument must be “yes” or “no”. The default is “no”. This option applies to
protocol version 1 only and requires ssh(1) to be setuid root.

RSAAuthentication
Specifies whether to try RSA authentication. The argument to this keyword must be
“yes” or “no”. RSA authentication will only be attempted if the identity file
exists, or an authentication agent is running. The default is “yes”. Note that
this option applies to protocol version 1 only.

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 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.

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 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.

ServerAliveInterval
Sets a timeout interval in seconds after which if no data has been received from the
server, ssh(1) will send a message through the encrypted channel to request a
response from the server. The default is 0, indicating that these messages will not
be sent to the server, or 300 if the BatchMode option is set. ProtocolKeepAlives
and SetupTimeOut are Debian-specific compatibility aliases for this option.

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” or “no”. The default is “no”.

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 trojan horse 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 “no”, ssh will automatically add
new host keys to the user known hosts files. If this flag is set to “ask”, 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. The argument must be “yes”, “no”, or “ask”. The default is “ask”.

TCPKeepAlive
Specifies whether the system should send TCP keepalive messages to the other side.
If they are sent, death of the connection or crash of one of the machines will be
properly noticed. This option only uses TCP keepalives (as opposed to using ssh
level keepalives), so takes a long time to notice when the connection dies. As
such, you probably want the ServerAliveInterval option as well. However, this means
that connections will die if the route is down temporarily, and some people find it
annoying.

The default is “yes” (to send TCP keepalive messages), and the client will notice if
the network goes down or the remote host dies. This is important in scripts, and
many users want it too.

To disable TCP keepalive messages, the value should be set to “no”.

Tunnel Request tun(4) device forwarding between the client and the server. The argument
must be “yes”, “point-to-point” (layer 3), “ethernet” (layer 2), or “no”.
Specifying “yes” requests the default tunnel mode, which is “point-to-point”. The
default is “no”.

TunnelDevice
Specifies the tun(4) devices to open on the client (local_tun) and the server
(remote_tun).

The argument must be local_tun[:remote_tun]. The devices may be specified by
numerical ID or the keyword “any”, which uses the next available tunnel device. If
remote_tun is not specified, it defaults to “any”. The default is “any:any”.

UpdateHostKeys
Specifies whether ssh(1) should accept notifications of additional hostkeys from the
server sent after authentication has completed and add them to UserKnownHostsFile.
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 explicitly 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.

UsePrivilegedPort
Specifies whether to use a privileged port for outgoing connections. The argument
must be “yes” or “no”. The default is “no”. If set to “yes”, ssh(1) must be setuid
root. Note that this option must be set to “yes” for RhostsRSAAuthentication with
older servers.

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 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 argument must be “yes”, “no”, or
“ask”. The default is “no”.

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"

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 accessible
             by others.  It may be group-writable provided that the group in question contains
             only the user.

     /etc/ssh/ssh_config
             Systemwide configuration file.  This file provides defaults for those values that
             are not specified in the user's configuration file, and for those users who do not
             have a configuration file.  This file must be world-readable.

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