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.

AUTHORS