<!-- Creator : groff version 1.22.4 -->
<!-- CreationDate: Fri Feb 8 18:31:09 2019 -->
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta name="generator" content="groff -Thtml, see www.gnu.org">
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<meta name="Content-Style" content="text/css">
<style type="text/css">
p { margin-top: 0; margin-bottom: 0; vertical-align: top }
pre { margin-top: 0; margin-bottom: 0; vertical-align: top }
table { margin-top: 0; margin-bottom: 0; vertical-align: top }
h1 { text-align: center }
</style>
<title></title>
</head>
<body>
<hr>
<p>SSH_CONFIG(5) BSD File Formats Manual SSH_CONFIG(5)</p>
<p style="margin-top: 1em"><b>NAME</b></p>
<p style="margin-left:6%;"><b>ssh_config</b> —
OpenSSH SSH client configuration files</p>
<p style="margin-top: 1em"><b>DESCRIPTION</b></p>
<p style="margin-left:6%;">ssh(1) obtains configuration
data from the following sources in the following order:</p>
<p style="margin-top: 1em">1.</p>
<p style="margin-left:21%;">command-line options</p>
<p>2.</p>
<p style="margin-left:21%;">user’s configuration file
(<i>~/.ssh/config</i>)</p>
<p>3.</p>
<p style="margin-left:21%;">system-wide configuration file
(<i>/etc/ssh/ssh_config</i>)</p>
<p style="margin-left:6%; margin-top: 1em">For each
parameter, the first obtained value will be used. The
configuration files contain sections separated by
<b>Host</b> 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
<b>CanonicalizeHostname</b> option for exceptions).</p>
<p style="margin-left:6%; margin-top: 1em">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.</p>
<p style="margin-left:6%; margin-top: 1em">Note that the
Debian <b>openssh-client</b> package sets several options as
standard in <i>/etc/ssh/ssh_config</i> which are not the
default in ssh(1):</p>
<p style="margin-top: 1em"><b>•</b></p>
<p style="margin-left:19%;"><b>SendEnv</b> LANG LC_*</p>
<p><b>•</b></p>
<p style="margin-left:19%;"><b>HashKnownHosts</b> yes</p>
<p><b>•</b></p>
<p style="margin-left:19%;"><b>GSSAPIAuthentication</b>
yes</p>
<p style="margin-left:6%; margin-top: 1em">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 <b>ssh</b>, <b>scp</b>, and <b>sftp -o</b>
option.</p>
<p style="margin-left:6%; margin-top: 1em">The possible
keywords and their meanings are as follows (note that
keywords are case-insensitive and arguments are
case-sensitive):</p>
<p style="margin-top: 1em"><b>Host</b></p>
<p style="margin-left:17%; margin-top: 1em">Restricts the
following declarations (up to the next <b>Host</b> or
<b>Match</b> 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 <i>hostname</i> argument given on the command
line (see the <b>CanonicalizeHostname</b> keyword for
exceptions).</p>
<p style="margin-left:17%; margin-top: 1em">A pattern entry
may be negated by prefixing it with an exclamation mark
(’!’). If a negated entry is matched, then the
<b>Host</b> entry is ignored, regardless of whether any
other patterns on the line match. Negated matches are
therefore useful to provide exceptions for wildcard
matches.</p>
<p style="margin-left:17%; margin-top: 1em">See
<i>PATTERNS</i> for more information on patterns.</p>
<p style="margin-top: 1em"><b>Match</b></p>
<p style="margin-left:17%; margin-top: 1em">Restricts the
following declarations (up to the next <b>Host</b> or
<b>Match</b> keyword) to be used only when the conditions
following the <b>Match</b> keyword are satisfied. Match
conditions are specified using one or more criteria or the
single token <b>all</b> which always matches. The available
criteria keywords are: <b>canonical</b>, <b>exec</b>,
<b>host</b>, <b>originalhost</b>, <b>user</b>, and
<b>localuser</b>. The <b>all</b> criteria must appear alone
or immediately after <b>canonical</b>. Other criteria may be
combined arbitrarily. All criteria but <b>all</b> and
<b>canonical</b> require an argument. Criteria may be
negated by prepending an exclamation mark
(’!’).</p>
<p style="margin-left:17%; margin-top: 1em">The
<b>canonical</b> keyword matches only when the configuration
file is being re-parsed after hostname canonicalization (see
the <b>CanonicalizeHostname</b> option.) This may be useful
to specify conditions that work with canonical host names
only. The <b>exec</b> keyword executes the specified command
under the user’s shell. If the command returns a zero
exit status then the condition is considered true. Commands
containing whitespace characters must be quoted. Arguments
to <b>exec</b> accept the tokens described in the
<i>TOKENS</i> section.</p>
<p style="margin-left:17%; margin-top: 1em">The other
keywords’ criteria must be single entries or
comma-separated lists and may use the wildcard and negation
operators described in the <i>PATTERNS</i> section. The
criteria for the <b>host</b> keyword are matched against the
target hostname, after any substitution by the
<b>Hostname</b> or <b>CanonicalizeHostname</b> options. The
<b>originalhost</b> keyword matches against the hostname as
it was specified on the command-line. The <b>user</b>
keyword matches against the target username on the remote
host. The <b>localuser</b> keyword matches against the name
of the local user running ssh(1) (this keyword may be useful
in system-wide <b>ssh_config</b> files).</p>
<p style="margin-top: 1em"><b>AddKeysToAgent</b></p>
<p style="margin-left:17%;">Specifies whether keys should
be automatically added to a running ssh-agent(1). If this
option is set to <b>yes</b> 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 <b>ask</b>, ssh(1) will require confirmation using the
SSH_ASKPASS program before adding a key (see ssh-add(1) for
details). If this option is set to <b>confirm</b>, each use
of the key must be confirmed, as if the <b>-c</b> option was
specified to ssh-add(1). If this option is set to <b>no</b>,
no keys are added to the agent. The argument must be
<b>yes</b>, <b>confirm</b>, <b>ask</b>, or <b>no</b> (the
default).</p>
<p style="margin-top: 1em"><b>AddressFamily</b></p>
<p style="margin-left:17%;">Specifies which address family
to use when connecting. Valid arguments are <b>any</b> (the
default), <b>inet</b> (use IPv4 only), or <b>inet6</b> (use
IPv6 only).</p>
<p style="margin-top: 1em"><b>BatchMode</b></p>
<p style="margin-left:17%;">If set to <b>yes</b>,
passphrase/password querying will be disabled. In addition,
the <b>ServerAliveInterval</b> option will be set to 300
seconds by default (Debian-specific). 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 <b>yes</b> or
<b>no</b> (the default).</p>
<p style="margin-top: 1em"><b>BindAddress</b></p>
<p style="margin-left:17%;">Use the specified address on
the local machine as the source address of the connection.
Only useful on systems with more than one address.</p>
<p style="margin-top: 1em"><b>BindInterface</b></p>
<p style="margin-left:17%;">Use the address of the
specified interface on the local machine as the source
address of the connection.</p>
<p style="margin-top: 1em"><b>CanonicalDomains</b></p>
<p style="margin-left:17%;">When
<b>CanonicalizeHostname</b> is enabled, this option
specifies the list of domain suffixes in which to search for
the specified destination host.</p>
<p style="margin-top: 1em"><b>CanonicalizeFallbackLocal</b></p>
<p style="margin-left:17%;">Specifies whether to fail with
an error when hostname canonicalization fails. The default,
<b>yes</b>, will attempt to look up the unqualified hostname
using the system resolver’s search rules. A value of
<b>no</b> will cause ssh(1) to fail instantly if
<b>CanonicalizeHostname</b> is enabled and the target
hostname cannot be found in any of the domains specified by
<b>CanonicalDomains</b>.</p>
<p style="margin-top: 1em"><b>CanonicalizeHostname</b></p>
<p style="margin-left:17%;">Controls whether explicit
hostname canonicalization is performed. The default,
<b>no</b>, is not to perform any name rewriting and let the
system resolver handle all hostname lookups. If set to
<b>yes</b> then, for connections that do not use a
<b>ProxyCommand</b> or <b>ProxyJump</b>, ssh(1) will attempt
to canonicalize the hostname specified on the command line
using the <b>CanonicalDomains</b> suffixes and
<b>CanonicalizePermittedCNAMEs</b> rules. If
<b>CanonicalizeHostname</b> is set to <b>always</b>, then
canonicalization is applied to proxied connections too.</p>
<p style="margin-left:17%; margin-top: 1em">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 <b>Host</b> and <b>Match</b> stanzas.</p>
<p style="margin-top: 1em"><b>CanonicalizeMaxDots</b></p>
<p style="margin-left:17%;">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).</p>
<p style="margin-top: 1em"><b>CanonicalizePermittedCNAMEs</b></p>
<p style="margin-left:17%;">Specifies rules to determine
whether CNAMEs should be followed when canonicalizing
hostnames. The rules consist of one or more arguments of
<i>source_domain_list</i>:<i>target_domain_list</i>, where
<i>source_domain_list</i> is a pattern-list of domains that
may follow CNAMEs in canonicalization, and
<i>target_domain_list</i> is a pattern-list of domains that
they may resolve to.</p>
<p style="margin-left:17%; margin-top: 1em">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.</p>
<p style="margin-top: 1em"><b>CASignatureAlgorithms</b></p>
<p style="margin-left:17%;">Specifies which algorithms are
allowed for signing of certificates by certificate
authorities (CAs). The default is:</p>
<p style="margin-left:24%; margin-top: 1em">ecdsa-sha2-nistp256.ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
<br>
ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa</p>
<p style="margin-left:17%; margin-top: 1em">ssh(1) will not
accept host certificates signed using algorithms other than
those specified.</p>
<p style="margin-top: 1em"><b>CertificateFile</b></p>
<p style="margin-left:17%;">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 <b>IdentityFile</b> directive or
<b>-i</b> flag to ssh(1), via ssh-agent(1), or via a
<b>PKCS11Provider</b>.</p>
<p style="margin-left:17%; margin-top: 1em">Arguments to
<b>CertificateFile</b> may use the tilde syntax to refer to
a user’s home directory or the tokens described in the
<i>TOKENS</i> section.</p>
<p style="margin-left:17%; margin-top: 1em">It is possible
to have multiple certificate files specified in
configuration files; these certificates will be tried in
sequence. Multiple <b>CertificateFile</b> directives will
add to the list of certificates used for authentication.</p>
<p style="margin-top: 1em"><b>ChallengeResponseAuthentication</b></p>
<p style="margin-left:17%;">Specifies whether to use
challenge-response authentication. The argument to this
keyword must be <b>yes</b> (the default) or <b>no</b>.</p>
<p style="margin-top: 1em"><b>CheckHostIP</b></p>
<p style="margin-left:17%;">If set to <b>yes</b> (the
default), ssh(1) will additionally check the host IP address
in the <i>known_hosts</i> file. This allows it to detect if
a host key changed due to DNS spoofing and will add
addresses of destination hosts to <i>~/.ssh/known_hosts</i>
in the process, regardless of the setting of
<b>StrictHostKeyChecking</b>. If the option is set to
<b>no</b>, the check will not be executed.</p>
<p style="margin-top: 1em"><b>Ciphers</b></p>
<p style="margin-left:17%;">Specifies the ciphers allowed
and their 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. If
the specified value begins with a ’-’ character,
then the specified ciphers (including wildcards) will be
removed from the default set instead of replacing them.</p>
<p style="margin-left:17%; margin-top: 1em">The supported
ciphers are:</p>
<p style="margin-left:24%; margin-top: 1em">3des-cbc <br>
aes128-cbc <br>
aes192-cbc <br>
aes256-cbc <br>
aes128-ctr <br>
aes192-ctr <br>
aes256-ctr <br>
aes128-gcm@openssh.com <br>
aes256-gcm@openssh.com <br>
chacha20-poly1305@openssh.com</p>
<p style="margin-left:17%; margin-top: 1em">The default
is:</p>
<p style="margin-left:24%; margin-top: 1em">chacha20-poly1305@openssh.com,
<br>
aes128-ctr,aes192-ctr,aes256-ctr, <br>
aes128-gcm@openssh.com,aes256-gcm@openssh.com</p>
<p style="margin-left:17%; margin-top: 1em">The list of
available ciphers may also be obtained using "ssh -Q
cipher".</p>
<p style="margin-top: 1em"><b>ClearAllForwardings</b></p>
<p style="margin-left:17%;">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 <b>yes</b> or <b>no</b> (the default).</p>
<p style="margin-top: 1em"><b>Compression</b></p>
<p style="margin-left:17%;">Specifies whether to use
compression. The argument must be <b>yes</b> or <b>no</b>
(the default).</p>
<p style="margin-top: 1em"><b>ConnectionAttempts</b></p>
<p style="margin-left:17%;">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.</p>
<p style="margin-top: 1em"><b>ConnectTimeout</b></p>
<p style="margin-left:17%;">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.</p>
<p style="margin-top: 1em"><b>ControlMaster</b></p>
<p style="margin-left:17%;">Enables the sharing of multiple
sessions over a single network connection. When set to
<b>yes</b>, ssh(1) will listen for connections on a control
socket specified using the <b>ControlPath</b> argument.
Additional sessions can connect to this socket using the
same <b>ControlPath</b> with <b>ControlMaster</b> set to
<b>no</b> (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.</p>
<p style="margin-left:17%; margin-top: 1em">Setting this to
<b>ask</b> will cause ssh(1) to listen for control
connections, but require confirmation using ssh-askpass(1).
If the <b>ControlPath</b> cannot be opened, ssh(1) will
continue without connecting to a master instance.</p>
<p style="margin-left:17%; margin-top: 1em">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.</p>
<p style="margin-left:17%; margin-top: 1em">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: <b>auto</b> and
<b>autoask</b>. The latter requires confirmation like the
<b>ask</b> option.</p>
<p style="margin-top: 1em"><b>ControlPath</b></p>
<p style="margin-left:17%;">Specify the path to the control
socket used for connection sharing as described in the
<b>ControlMaster</b> section above or the string <b>none</b>
to disable connection sharing. Arguments to
<b>ControlPath</b> may use the tilde syntax to refer to a
user’s home directory or the tokens described in the
<i>TOKENS</i> section. It is recommended that any
<b>ControlPath</b> 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.</p>
<p style="margin-top: 1em"><b>ControlPersist</b></p>
<p style="margin-left:17%;">When used in conjunction with
<b>ControlMaster</b>, 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 <b>no</b>, 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
<b>yes</b> 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.</p>
<p style="margin-top: 1em"><b>DynamicForward</b></p>
<p style="margin-left:17%;">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.</p>
<p style="margin-left:17%; margin-top: 1em">The argument
must be [</p>
<p><i>bind_address</i>: ]<i>port</i>. IPv6 addresses can be
specified by enclosing addresses in square brackets. By
default, the local port is bound in accordance with the
<b>GatewayPorts</b> setting. However, an explicit
<i>bind_address</i> may be used to bind the connection to a
specific address. The <i>bind_address</i> of
<b>localhost</b> 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.</p>
<p style="margin-left:17%; margin-top: 1em">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.</p>
<p style="margin-top: 1em"><b>EnableSSHKeysign</b></p>
<p style="margin-left:17%;">Setting this option to
<b>yes</b> in the global client configuration file
<i>/etc/ssh/ssh_config</i> enables the use of the helper
program ssh-keysign(8) during
<b>HostbasedAuthentication</b>. The argument must be
<b>yes</b> or <b>no</b> (the default). This option should be
placed in the non-hostspecific section. See ssh-keysign(8)
for more information.</p>
<p style="margin-top: 1em"><b>EscapeChar</b></p>
<p style="margin-left:17%;">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
<b>none</b> to disable the escape character entirely (making
the connection transparent for binary data).</p>
<p style="margin-top: 1em"><b>ExitOnForwardFailure</b></p>
<p style="margin-left:17%;">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 <b>ExitOnForwardFailure</b> 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
<b>yes</b> or <b>no</b> (the default).</p>
<p style="margin-top: 1em"><b>FingerprintHash</b></p>
<p style="margin-left:17%;">Specifies the hash algorithm
used when displaying key fingerprints. Valid options are:
<b>md5</b> and <b>sha256</b> (the default).</p>
<p style="margin-top: 1em"><b>ForwardAgent</b></p>
<p style="margin-left:17%;">Specifies whether the
connection to the authentication agent (if any) will be
forwarded to the remote machine. The argument must be
<b>yes</b> or <b>no</b> (the default).</p>
<p style="margin-left:17%; margin-top: 1em">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.</p>
<p style="margin-top: 1em"><b>ForwardX11</b></p>
<p style="margin-left:17%;">Specifies whether X11
connections will be automatically redirected over the secure
channel and DISPLAY set. The argument must be <b>yes</b> or
<b>no</b> (the default).</p>
<p style="margin-left:17%; margin-top: 1em">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 <b>ForwardX11Trusted</b> option
is also enabled.</p>
<p style="margin-top: 1em"><b>ForwardX11Timeout</b></p>
<p style="margin-left:17%;">Specify a timeout for untrusted
X11 forwarding using the format described in the <i>TIME
FORMATS</i> section of sshd_config(5). X11 connections
received by ssh(1) after this time will be refused. Setting
<b>ForwardX11Timeout</b> 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.</p>
<p style="margin-top: 1em"><b>ForwardX11Trusted</b></p>
<p style="margin-left:17%;">If this option is set to
<b>yes</b>, (the Debian-specific default), remote X11
clients will have full access to the original X11
display.</p>
<p style="margin-left:17%; margin-top: 1em">If this option
is set to <b>no</b> (the upstream 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.</p>
<p style="margin-left:17%; margin-top: 1em">See the X11
SECURITY extension specification for full details on the
restrictions imposed on untrusted clients.</p>
<p style="margin-top: 1em"><b>GatewayPorts</b></p>
<p style="margin-left:17%;">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. <b>GatewayPorts</b> 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 <b>yes</b> or <b>no</b> (the
default).</p>
<p style="margin-top: 1em"><b>GlobalKnownHostsFile</b></p>
<p style="margin-left:17%;">Specifies one or more files to
use for the global host key database, separated by
whitespace. The default is <i>/etc/ssh/ssh_known_hosts</i>,
<i>/etc/ssh/ssh_known_hosts2</i>.</p>
<p style="margin-top: 1em"><b>GSSAPIAuthentication</b></p>
<p style="margin-left:17%;">Specifies whether user
authentication based on GSSAPI is allowed. The default is
<b>no</b>.</p>
<p style="margin-top: 1em"><b>GSSAPIKeyExchange</b></p>
<p style="margin-left:17%;">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
<b>no</b>.</p>
<p style="margin-top: 1em"><b>GSSAPIClientIdentity</b></p>
<p style="margin-left:17%;">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.</p>
<p style="margin-top: 1em"><b>GSSAPIServerIdentity</b></p>
<p style="margin-left:17%;">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.</p>
<p style="margin-top: 1em"><b>GSSAPIDelegateCredentials</b></p>
<p style="margin-left:17%;">Forward (delegate) credentials
to the server. The default is <b>no</b>.</p>
<p style="margin-top: 1em"><b>GSSAPIRenewalForcesRekey</b></p>
<p style="margin-left:17%;">If set to <b>yes</b> 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 <b>no</b>.</p>
<p style="margin-top: 1em"><b>GSSAPITrustDns</b></p>
<p style="margin-left:17%;">Set to <b>yes</b> to indicate
that the DNS is trusted to securely canonicalize the name of
the host being connected to. If <b>no</b>, the hostname
entered on the command line will be passed untouched to the
GSSAPI library. The default is <b>no</b>.</p>
<p style="margin-top: 1em"><b>HashKnownHosts</b></p>
<p style="margin-left:17%;">Indicates that ssh(1) should
hash host names and addresses when they are added to
<i>~/.ssh/known_hosts</i>. 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 <b>no</b>. 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 <i>~/.ssh/known_hosts</i>.</p>
<p style="margin-top: 1em"><b>HostbasedAuthentication</b></p>
<p style="margin-left:17%;">Specifies whether to try rhosts
based authentication with public key authentication. The
argument must be <b>yes</b> or <b>no</b> (the default).</p>
<p style="margin-top: 1em"><b>HostbasedKeyTypes</b></p>
<p style="margin-left:17%;">Specifies the key types that
will be used for hostbased authentication as a
comma-separated list of patterns. 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. If the specified value begins
with a ’-’ character, then the specified key
types (including wildcards) will be removed from the default
set instead of replacing them. The default for this option
is:</p>
<p style="margin-left:21%; margin-top: 1em">ecdsa-sha2-nistp256-cert-v01@openssh.com,
<br>
ecdsa-sha2-nistp384-cert-v01@openssh.com, <br>
ecdsa-sha2-nistp521-cert-v01@openssh.com, <br>
ssh-ed25519-cert-v01@openssh.com, <br>
rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
<br>
ssh-rsa-cert-v01@openssh.com, <br>
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
<br>
ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa</p>
<p style="margin-left:17%; margin-top: 1em">The <b>-Q</b>
option of ssh(1) may be used to list supported key
types.</p>
<p style="margin-top: 1em"><b>HostKeyAlgorithms</b></p>
<p style="margin-left:17%;">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. If
the specified value begins with a ’-’ character,
then the specified key types (including wildcards) will be
removed from the default set instead of replacing them. The
default for this option is:</p>
<p style="margin-left:21%; margin-top: 1em">ecdsa-sha2-nistp256-cert-v01@openssh.com,
<br>
ecdsa-sha2-nistp384-cert-v01@openssh.com, <br>
ecdsa-sha2-nistp521-cert-v01@openssh.com, <br>
ssh-ed25519-cert-v01@openssh.com, <br>
rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
<br>
ssh-rsa-cert-v01@openssh.com, <br>
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
<br>
ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa</p>
<p style="margin-left:17%; margin-top: 1em">If hostkeys are
known for the destination host then this default is modified
to prefer their algorithms.</p>
<p style="margin-left:17%; margin-top: 1em">The list of
available key types may also be obtained using "ssh -Q
key".</p>
<p style="margin-top: 1em"><b>HostKeyAlias</b></p>
<p style="margin-left:17%;">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.</p>
<p style="margin-top: 1em"><b>HostName</b></p>
<p style="margin-left:17%;">Specifies the real host name to
log into. This can be used to specify nicknames or
abbreviations for hosts. Arguments to <b>HostName</b> accept
the tokens described in the <i>TOKENS</i> section. Numeric
IP addresses are also permitted (both on the command line
and in <b>HostName</b> specifications). The default is the
name given on the command line.</p>
<p style="margin-top: 1em"><b>IdentitiesOnly</b></p>
<p style="margin-left:17%;">Specifies that ssh(1) should
only use the authentication identity and certificate files
explicitly configured in the <b>ssh_config</b> files or
passed on the ssh(1) command-line, even if ssh-agent(1) or a
<b>PKCS11Provider</b> offers more identities. The argument
to this keyword must be <b>yes</b> or <b>no</b> (the
default). This option is intended for situations where
ssh-agent offers many different identities.</p>
<p style="margin-top: 1em"><b>IdentityAgent</b></p>
<p style="margin-left:17%;">Specifies the UNIX-domain
socket used to communicate with the authentication
agent.</p>
<p style="margin-left:17%; margin-top: 1em">This option
overrides the SSH_AUTH_SOCK environment variable and can be
used to select a specific agent. Setting the socket name to
<b>none</b> disables the use of an authentication agent. If
the string "SSH_AUTH_SOCK" is specified, the
location of the socket will be read from the SSH_AUTH_SOCK
environment variable. Otherwise if the specified value
begins with a ’$’ character, then it will be
treated as an environment variable containing the location
of the socket.</p>
<p style="margin-left:17%; margin-top: 1em">Arguments to
<b>IdentityAgent</b> may use the tilde syntax to refer to a
user’s home directory or the tokens described in the
<i>TOKENS</i> section.</p>
<p style="margin-top: 1em"><b>IdentityFile</b></p>
<p style="margin-left:17%;">Specifies a file from which the
user’s DSA, ECDSA, Ed25519 or RSA authentication
identity is read. The default is <i>~/.ssh/id_dsa</i>,
<i>~/.ssh/id_ecdsa</i>, <i>~/.ssh/id_ed25519</i> and
<i>~/.ssh/id_rsa</i>. Additionally, any identities
represented by the authentication agent will be used for
authentication unless <b>IdentitiesOnly</b> is set. If no
certificates have been explicitly specified by
<b>CertificateFile</b>, ssh(1) will try to load certificate
information from the filename obtained by appending
<i>-cert.pub</i> to the path of a specified
<b>IdentityFile</b>.</p>
<p style="margin-left:17%; margin-top: 1em">Arguments to
<b>IdentityFile</b> may use the tilde syntax to refer to a
user’s home directory or the tokens described in the
<i>TOKENS</i> section.</p>
<p style="margin-left:17%; margin-top: 1em">It is possible
to have multiple identity files specified in configuration
files; all these identities will be tried in sequence.
Multiple <b>IdentityFile</b> directives will add to the list
of identities tried (this behaviour differs from that of
other configuration directives).</p>
<p style="margin-left:17%; margin-top: 1em"><b>IdentityFile</b>
may be used in conjunction with <b>IdentitiesOnly</b> to
select which identities in an agent are offered during
authentication. <b>IdentityFile</b> may also be used in
conjunction with <b>CertificateFile</b> in order to provide
any certificate also needed for authentication with the
identity.</p>
<p style="margin-top: 1em"><b>IgnoreUnknown</b></p>
<p style="margin-left:17%;">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 <b>ssh_config</b> contains options that are unrecognised
by ssh(1). It is recommended that <b>IgnoreUnknown</b> be
listed early in the configuration file as it will not be
applied to unknown options that appear before it.</p>
<p style="margin-top: 1em"><b>Include</b></p>
<p style="margin-left:17%;">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 <i>~/.ssh</i> if included in a user
configuration file or <i>/etc/ssh</i> if included from the
system configuration file. <b>Include</b> directive may
appear inside a <b>Match</b> or <b>Host</b> block to perform
conditional inclusion.</p>
<p style="margin-top: 1em"><b>IPQoS</b></p>
<p style="margin-left:17%; margin-top: 1em">Specifies the
IPv4 type-of-service or DSCP class for connections. Accepted
values are <b>af11</b>, <b>af12</b>, <b>af13</b>,
<b>af21</b>, <b>af22</b>, <b>af23</b>, <b>af31</b>,
<b>af32</b>, <b>af33</b>, <b>af41</b>, <b>af42</b>,
<b>af43</b>, <b>cs0</b>, <b>cs1</b>, <b>cs2</b>, <b>cs3</b>,
<b>cs4</b>, <b>cs5</b>, <b>cs6</b>, <b>cs7</b>, <b>ef</b>,
<b>lowdelay</b>, <b>throughput</b>, <b>reliability</b>, a
numeric value, or <b>none</b> to use the operating system
default. This option may take one or two arguments,
separated by whitespace. If one argument is specified, it is
used as the packet class unconditionally. If two values are
specified, the first is automatically selected for
interactive sessions and the second for non-interactive
sessions. The default is <b>af21</b> (Low-Latency Data) for
interactive sessions and <b>cs1</b> (Lower Effort) for
non-interactive sessions.</p>
<p style="margin-top: 1em"><b>KbdInteractiveAuthentication</b></p>
<p style="margin-left:17%;">Specifies whether to use
keyboard-interactive authentication. The argument to this
keyword must be <b>yes</b> (the default) or <b>no</b>.</p>
<p style="margin-top: 1em"><b>KbdInteractiveDevices</b></p>
<p style="margin-left:17%;">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: <b>bsdauth</b> and
<b>pam</b>.</p>
<p style="margin-top: 1em"><b>KexAlgorithms</b></p>
<p style="margin-left:17%;">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. If the specified value begins with a ’-’
character, then the specified methods (including wildcards)
will be removed from the default set instead of replacing
them. The default is:</p>
<p style="margin-left:24%; margin-top: 1em">curve25519-sha256,curve25519-sha256@libssh.org,
<br>
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
<br>
diffie-hellman-group-exchange-sha256, <br>
diffie-hellman-group16-sha512, <br>
diffie-hellman-group18-sha512, <br>
diffie-hellman-group-exchange-sha1, <br>
diffie-hellman-group14-sha256, <br>
diffie-hellman-group14-sha1</p>
<p style="margin-left:17%; margin-top: 1em">The list of
available key exchange algorithms may also be obtained using
"ssh -Q kex".</p>
<p style="margin-top: 1em"><b>LocalCommand</b></p>
<p style="margin-left:17%;">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
<b>LocalCommand</b> accept the tokens described in the
<i>TOKENS</i> section.</p>
<p style="margin-left:17%; margin-top: 1em">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.</p>
<p style="margin-left:17%; margin-top: 1em">This directive
is ignored unless <b>PermitLocalCommand</b> has been
enabled.</p>
<p style="margin-top: 1em"><b>LocalForward</b></p>
<p style="margin-left:17%;">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 [</p>
<p><i>bind_address</i>: ]<i>port</i> and the second
argument must be <i>host</i>:<i>hostport</i>. 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
<b>GatewayPorts</b> setting. However, an explicit
<i>bind_address</i> may be used to bind the connection to a
specific address. The <i>bind_address</i> of
<b>localhost</b> 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.</p>
<p style="margin-top: 1em"><b>LogLevel</b></p>
<p style="margin-left:17%;">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.</p>
<p style="margin-top: 1em"><b>MACs</b></p>
<p style="margin-left:17%; margin-top: 1em">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. If the specified
value begins with a ’-’ character, then the
specified algorithms (including wildcards) will be removed
from the default set instead of replacing them.</p>
<p style="margin-left:17%; margin-top: 1em">The algorithms
that contain "-etm" calculate the MAC after
encryption (encrypt-then-mac). These are considered safer
and their use recommended.</p>
<p style="margin-left:17%; margin-top: 1em">The default
is:</p>
<p style="margin-left:24%; margin-top: 1em">umac-64-etm@openssh.com,umac-128-etm@openssh.com,
<br>
hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
<br>
hmac-sha1-etm@openssh.com, <br>
umac-64@openssh.com,umac-128@openssh.com, <br>
hmac-sha2-256,hmac-sha2-512,hmac-sha1</p>
<p style="margin-left:17%; margin-top: 1em">The list of
available MAC algorithms may also be obtained using
"ssh -Q mac".</p>
<p style="margin-top: 1em"><b>NoHostAuthenticationForLocalhost</b></p>
<p style="margin-left:17%;">Disable host authentication for
localhost (loopback addresses). The argument to this keyword
must be <b>yes</b> or <b>no</b> (the default).</p>
<p style="margin-top: 1em"><b>NumberOfPasswordPrompts</b></p>
<p style="margin-left:17%;">Specifies the number of
password prompts before giving up. The argument to this
keyword must be an integer. The default is 3.</p>
<p style="margin-top: 1em"><b>PasswordAuthentication</b></p>
<p style="margin-left:17%;">Specifies whether to use
password authentication. The argument to this keyword must
be <b>yes</b> (the default) or <b>no</b>.</p>
<p style="margin-top: 1em"><b>PermitLocalCommand</b></p>
<p style="margin-left:17%;">Allow local command execution
via the <b>LocalCommand</b> option or using the
<b>!</b><i>command</i> escape sequence in ssh(1). The
argument must be <b>yes</b> or <b>no</b> (the default).</p>
<p style="margin-top: 1em"><b>PKCS11Provider</b></p>
<p style="margin-left:17%;">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.</p>
<p style="margin-top: 1em"><b>Port</b></p>
<p style="margin-left:17%; margin-top: 1em">Specifies the
port number to connect on the remote host. The default is
22.</p>
<p style="margin-top: 1em"><b>PreferredAuthentications</b></p>
<p style="margin-left:17%;">Specifies the order in which
the client should try authentication methods. This allows a
client to prefer one method (e.g.
<b>keyboard-interactive</b>) over another method (e.g.
<b>password</b>). The default is:</p>
<p style="margin-left:24%; margin-top: 1em">gssapi-with-mic,hostbased,publickey,
<br>
keyboard-interactive,password</p>
<p style="margin-top: 1em"><b>ProxyCommand</b></p>
<p style="margin-left:17%;">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.</p>
<p style="margin-left:17%; margin-top: 1em">Arguments to
<b>ProxyCommand</b> accept the tokens described in the
<i>TOKENS</i> section. The command can be basically
anything, and should read from its standard input and write
to its standard output. It should eventually connect an
sshd(8) server running on some machine, or execute <b>sshd
-i</b> 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 <b>none</b>
disables this option entirely. Note that <b>CheckHostIP</b>
is not available for connects with a proxy command.</p>
<p style="margin-left:17%; margin-top: 1em">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:</p>
<p style="margin-left:21%; margin-top: 1em">ProxyCommand
/usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p</p>
<p style="margin-top: 1em"><b>ProxyJump</b></p>
<p style="margin-left:17%;">Specifies one or more jump
proxies as either</p>
<p>[<i>user</i>@]<i>host</i>[:<i>port</i>] 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 <b>ProxyJump</b> host and
then establishing a TCP forwarding to the ultimate target
from there.</p>
<p style="margin-left:17%; margin-top: 1em">Note that this
option will compete with the <b>ProxyCommand</b> option -
whichever is specified first will prevent later instances of
the other from taking effect.</p>
<p style="margin-top: 1em"><b>ProxyUseFdpass</b></p>
<p style="margin-left:17%;">Specifies that
<b>ProxyCommand</b> will pass a connected file descriptor
back to ssh(1) instead of continuing to execute and pass
data. The default is <b>no</b>.</p>
<p style="margin-top: 1em"><b>PubkeyAcceptedKeyTypes</b></p>
<p style="margin-left:17%;">Specifies the key types that
will be used for public key authentication as a
comma-separated list of patterns. 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. If the specified value begins with
a ’-’ character, then the specified key types
(including wildcards) will be removed from the default set
instead of replacing them. The default for this option
is:</p>
<p style="margin-left:21%; margin-top: 1em">ecdsa-sha2-nistp256-cert-v01@openssh.com,
<br>
ecdsa-sha2-nistp384-cert-v01@openssh.com, <br>
ecdsa-sha2-nistp521-cert-v01@openssh.com, <br>
ssh-ed25519-cert-v01@openssh.com, <br>
rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
<br>
ssh-rsa-cert-v01@openssh.com, <br>
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
<br>
ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa</p>
<p style="margin-left:17%; margin-top: 1em">The list of
available key types may also be obtained using "ssh -Q
key".</p>
<p style="margin-top: 1em"><b>PubkeyAuthentication</b></p>
<p style="margin-left:17%;">Specifies whether to try public
key authentication. The argument to this keyword must be
<b>yes</b> (the default) or <b>no</b>.</p>
<p style="margin-top: 1em"><b>RekeyLimit</b></p>
<p style="margin-left:17%;">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 <i>TIME FORMATS</i> section of
sshd_config(5). The default value for <b>RekeyLimit</b> is
<b>default none</b>, 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.</p>
<p style="margin-top: 1em"><b>RemoteCommand</b></p>
<p style="margin-left:17%;">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
<b>RemoteCommand</b> accept the tokens described in the
<i>TOKENS</i> section.</p>
<p style="margin-top: 1em"><b>RemoteForward</b></p>
<p style="margin-left:17%;">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 must
be [</p>
<p><i>bind_address</i>: ]<i>port</i> If forwarding to a
specific destination then the second argument must be
<i>host</i>:<i>hostport</i>, otherwise if no destination
argument is specified then the remote forwarding will be
established as a SOCKS proxy.</p>
<p style="margin-left:17%; margin-top: 1em">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.</p>
<p style="margin-left:17%; margin-top: 1em">If the
<i>port</i> argument is 0, the listen port will be
dynamically allocated on the server and reported to the
client at run time.</p>
<p style="margin-left:17%; margin-top: 1em">If the
<i>bind_address</i> is not specified, the default is to only
bind to loopback addresses. If the <i>bind_address</i> is
’*’ or an empty string, then the forwarding is
requested to listen on all interfaces. Specifying a remote
<i>bind_address</i> will only succeed if the server’s
<b>GatewayPorts</b> option is enabled (see
sshd_config(5)).</p>
<p style="margin-top: 1em"><b>RequestTTY</b></p>
<p style="margin-left:17%;">Specifies whether to request a
pseudo-tty for the session. The argument may be one of:
<b>no</b> (never request a TTY), <b>yes</b> (always request
a TTY when standard input is a TTY), <b>force</b> (always
request a TTY) or <b>auto</b> (request a TTY when opening a
login session). This option mirrors the <b>-t</b> and
<b>-T</b> flags for ssh(1).</p>
<p style="margin-top: 1em"><b>RevokedHostKeys</b></p>
<p style="margin-left:17%;">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).</p>
<p style="margin-top: 1em"><b>SendEnv</b></p>
<p style="margin-left:17%;">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 <b>AcceptEnv</b> 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 <b>SendEnv</b> directives.</p>
<p style="margin-left:17%; margin-top: 1em">See
<i>PATTERNS</i> for more information on patterns.</p>
<p style="margin-left:17%; margin-top: 1em">It is possible
to clear previously set <b>SendEnv</b> variable names by
prefixing patterns with <i>-</i>. The default is not to send
any environment variables.</p>
<p style="margin-top: 1em"><b>ServerAliveCountMax</b></p>
<p style="margin-left:17%;">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 <b>TCPKeepAlive</b>
(below). The server alive messages are sent through the
encrypted channel and therefore will not be spoofable. The
TCP keepalive option enabled by <b>TCPKeepAlive</b> is
spoofable. The server alive mechanism is valuable when the
client or server depend on knowing when a connection has
become inactive.</p>
<p style="margin-left:17%; margin-top: 1em">The default
value is 3. If, for example, <b>ServerAliveInterval</b> (see
below) is set to 15 and <b>ServerAliveCountMax</b> is left
at the default, if the server becomes unresponsive, ssh will
disconnect after approximately 45 seconds.</p>
<p style="margin-top: 1em"><b>ServerAliveInterval</b></p>
<p style="margin-left:17%;">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 <b>BatchMode</b> option is set
(Debian-specific). <b>ProtocolKeepAlives</b> and
<b>SetupTimeOut</b> are Debian-specific compatibility
aliases for this option.</p>
<p style="margin-top: 1em"><b>SetEnv</b></p>
<p style="margin-left:17%; margin-top: 1em">Directly
specify one or more environment variables and their contents
to be sent to the server. Similarly to <b>SendEnv</b>, the
server must be prepared to accept the environment
variable.</p>
<p style="margin-top: 1em"><b>StreamLocalBindMask</b></p>
<p style="margin-left:17%;">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.</p>
<p style="margin-left:17%; margin-top: 1em">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.</p>
<p style="margin-top: 1em"><b>StreamLocalBindUnlink</b></p>
<p style="margin-left:17%;">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 <b>StreamLocalBindUnlink</b> is not
enabled, <b>ssh</b> 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.</p>
<p style="margin-left:17%; margin-top: 1em">The argument
must be <b>yes</b> or <b>no</b> (the default).</p>
<p style="margin-top: 1em"><b>StrictHostKeyChecking</b></p>
<p style="margin-left:17%;">If this flag is set to
<b>yes</b>, ssh(1) will never automatically add host keys to
the <i>~/.ssh/known_hosts</i> 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 <i>/etc/ssh/ssh_known_hosts</i>
file is poorly maintained or when connections to new hosts
are frequently made. This option forces the user to manually
add all new hosts.</p>
<p style="margin-left:17%; margin-top: 1em">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 <b>ask</b> (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.</p>
<p style="margin-top: 1em"><b>SyslogFacility</b></p>
<p style="margin-left:17%;">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.</p>
<p style="margin-top: 1em"><b>TCPKeepAlive</b></p>
<p style="margin-left:17%;">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 <b>ServerAliveInterval</b>
option as well. However, this means that connections will
die if the route is down temporarily, and some people find
it annoying.</p>
<p style="margin-left:17%; margin-top: 1em">The default is
<b>yes</b> (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.</p>
<p style="margin-left:17%; margin-top: 1em">To disable TCP
keepalive messages, the value should be set to <b>no</b>.
See also <b>ServerAliveInterval</b> for protocol-level
keepalives.</p>
<p style="margin-top: 1em"><b>Tunnel</b></p>
<p style="margin-left:17%; margin-top: 1em">Request tun(4)
device forwarding between the client and the server. The
argument must be <b>yes</b>, <b>point-to-point</b> (layer
3), <b>ethernet</b> (layer 2), or <b>no</b> (the default).
Specifying <b>yes</b> requests the default tunnel mode,
which is <b>point-to-point</b>.</p>
<p style="margin-top: 1em"><b>TunnelDevice</b></p>
<p style="margin-left:17%;">Specifies the tun(4) devices to
open on the client (<i>local_tun</i>) and the server
(<i>remote_tun</i>).</p>
<p style="margin-left:17%; margin-top: 1em">The argument
must be <i>local_tun</i>[:<i>remote_tun</i>]. The devices
may be specified by numerical ID or the keyword <b>any</b>,
which uses the next available tunnel device. If
<i>remote_tun</i> is not specified, it defaults to
<b>any</b>. The default is <b>any:any</b>.</p>
<p style="margin-top: 1em"><b>UpdateHostKeys</b></p>
<p style="margin-left:17%;">Specifies whether ssh(1) should
accept notifications of additional hostkeys from the server
sent after authentication has completed and add them to
<b>UserKnownHostsFile</b>. The argument must be <b>yes</b>,
<b>no</b> (the default) or <b>ask</b>. 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 <b>UpdateHostKeys</b> is set to
<b>ask</b>, then the user is asked to confirm the
modifications to the known_hosts file. Confirmation is
currently incompatible with <b>ControlPersist</b>, and will
be disabled if it is enabled.</p>
<p style="margin-left:17%; margin-top: 1em">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.</p>
<p style="margin-top: 1em"><b>User</b></p>
<p style="margin-left:17%; margin-top: 1em">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.</p>
<p style="margin-top: 1em"><b>UserKnownHostsFile</b></p>
<p style="margin-left:17%;">Specifies one or more files to
use for the user host key database, separated by whitespace.
The default is <i>~/.ssh/known_hosts</i>,
<i>~/.ssh/known_hosts2</i>.</p>
<p style="margin-top: 1em"><b>VerifyHostKeyDNS</b></p>
<p style="margin-left:17%;">Specifies whether to verify the
remote key using DNS and SSHFP resource records. If this
option is set to <b>yes</b>, 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 <b>ask</b>. If this option is set to <b>ask</b>,
information on fingerprint match will be displayed, but the
user will still need to confirm new host keys according to
the <b>StrictHostKeyChecking</b> option. The default is
<b>no</b>.</p>
<p style="margin-left:17%; margin-top: 1em">See also
<i>VERIFYING HOST KEYS</i> in ssh(1).</p>
<p style="margin-top: 1em"><b>VisualHostKey</b></p>
<p style="margin-left:17%;">If this flag is set to
<b>yes</b>, an ASCII art representation of the remote host
key fingerprint is printed in addition to the fingerprint
string at login and for unknown host keys. If this flag is
set to <b>no</b> (the default), no fingerprint strings are
printed at login and only the fingerprint string will be
printed for unknown host keys.</p>
<p style="margin-top: 1em"><b>XAuthLocation</b></p>
<p style="margin-left:17%;">Specifies the full pathname of
the xauth(1) program. The default is
<i>/usr/bin/xauth</i>.</p>
<p style="margin-top: 1em"><b>PATTERNS</b></p>
<p style="margin-left:6%;">A <i>pattern</i> 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:</p>
<p style="margin-left:14%; margin-top: 1em">Host
*.co.uk</p>
<p style="margin-left:6%; margin-top: 1em">The following
pattern would match any host in the 192.168.0.[0-9] network
range:</p>
<p style="margin-left:14%; margin-top: 1em">Host
192.168.0.?</p>
<p style="margin-left:6%; margin-top: 1em">A
<i>pattern-list</i> 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:</p>
<p style="margin-left:14%; margin-top: 1em">from="!*.dialup.example.com,*.example.com"</p>
<p style="margin-left:6%; margin-top: 1em">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:</p>
<p style="margin-left:14%; margin-top: 1em">from="!host1,!host2"</p>
<p style="margin-left:6%; margin-top: 1em">The solution
here is to include a term that will yield a positive match,
such as a wildcard:</p>
<p style="margin-left:14%; margin-top: 1em">from="!host1,!host2,*"</p>
<p style="margin-top: 1em"><b>TOKENS</b></p>
<p style="margin-left:6%;">Arguments to some keywords can
make use of tokens, which are expanded at runtime:</p>
<p style="margin-top: 1em">%%</p>
<p style="margin-left:22%; margin-top: 1em">A literal
’%’.</p>
<p>%C</p>
<p style="margin-left:22%; margin-top: 1em">Hash of
%l%h%p%r.</p>
<p>%d</p>
<p style="margin-left:22%; margin-top: 1em">Local
user’s home directory.</p>
<p>%h</p>
<p style="margin-left:22%; margin-top: 1em">The remote
hostname.</p>
<p>%i</p>
<p style="margin-left:22%; margin-top: 1em">The local user
ID.</p>
<p>%L</p>
<p style="margin-left:22%; margin-top: 1em">The local
hostname.</p>
<p>%l</p>
<p style="margin-left:22%; margin-top: 1em">The local
hostname, including the domain name.</p>
<p>%n</p>
<p style="margin-left:22%; margin-top: 1em">The original
remote hostname, as given on the command line.</p>
<p>%p</p>
<p style="margin-left:22%; margin-top: 1em">The remote
port.</p>
<p>%r</p>
<p style="margin-left:22%; margin-top: 1em">The remote
username.</p>
<p>%T</p>
<p style="margin-left:22%; margin-top: 1em">The local
tun(4) or tap(4) network interface assigned if tunnel
forwarding was requested, or "NONE" otherwise.</p>
<p>%u</p>
<p style="margin-left:22%; margin-top: 1em">The local
username.</p>
<p style="margin-left:6%; margin-top: 1em"><b>Match
exec</b> accepts the tokens %%, %h, %i, %L, %l, %n, %p, %r,
and %u.</p>
<p style="margin-left:6%; margin-top: 1em"><b>CertificateFile</b>
accepts the tokens %%, %d, %h, %i, %l, %r, and %u.</p>
<p style="margin-left:6%; margin-top: 1em"><b>ControlPath</b>
accepts the tokens %%, %C, %h, %i, %L, %l, %n, %p, %r, and
%u.</p>
<p style="margin-left:6%; margin-top: 1em"><b>HostName</b>
accepts the tokens %% and %h.</p>
<p style="margin-left:6%; margin-top: 1em"><b>IdentityAgent</b>
and <b>IdentityFile</b> accept the tokens %%, %d, %h, %i,
%l, %r, and %u.</p>
<p style="margin-left:6%; margin-top: 1em"><b>LocalCommand</b>
accepts the tokens %%, %C, %d, %h, %i, %l, %n, %p, %r, %T,
and %u.</p>
<p style="margin-left:6%; margin-top: 1em"><b>ProxyCommand</b>
accepts the tokens %%, %h, %p, and %r.</p>
<p style="margin-left:6%; margin-top: 1em"><b>RemoteCommand</b>
accepts the tokens %%, %C, %d, %h, %i, %l, %n, %p, %r, and
%u.</p>
<p style="margin-top: 1em"><b>FILES</b> <br>
~/.ssh/config</p>
<p style="margin-left:17%;">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.</p>
<p style="margin-top: 1em">/etc/ssh/ssh_config</p>
<p style="margin-left:17%;">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.</p>
<p style="margin-top: 1em"><b>SEE ALSO</b></p>
<p style="margin-left:6%;">ssh(1)</p>
<p style="margin-top: 1em"><b>AUTHORS</b></p>
<p style="margin-left:6%;">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.</p>
<p style="margin-left:6%; margin-top: 1em">BSD
October 3, 2018 BSD</p>
<hr>
</body>
</html>