ssh 的参数 -o + 检查是否是无密码访问

Posted 2021-01-14 kinge

tags:

篇首语：本文由小常识网(cha138.com)小编为大家整理，主要介绍了ssh 的参数 -o + 检查是否是无密码访问相关的知识，希望对你有一定的参考价值。

转自：http://blog.chinaunix.net/xmlrpc.php?r=blog/article&uid=29578485&id=5300984

写脚本想检查服务器之间是否建立了公钥实现了无密码访问
查找过程中找到了ssh 的一些参数，贴出来，怕下次找不到了

这些参数可用在 ssh -o 后面，每个参数前面都必须有 -o
脚本如下：

ssh -o NumberOfPasswordPrompts=0 -o StrictHostKeyChecking=yes 192.168.61.218 "echo OK"

这个是 ssh_config(5) 的配置，也就是 -o 后面可以跟的参数

http://www.freebsd.org/cgi/man.cgi?query=ssh_config&sektion=5#end

SSH_CONFIG(5) FreeBSD File Formats Manual SSH_CONFIG(5)

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 fol-
lowing 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 configu-
ration 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 the one given on
the command line.

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

The configuration file has the following format:

Empty lines and lines starting with `#‘ are comments. Otherwise a line
is of the format ``keyword arguments‘‘. Configuration options may be
separated by whitespace or optional whitespace and exactly one `=‘; the
latter format is useful to avoid the need to quote whitespace when speci-
fying 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 key-
words 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 the hostname argument given on the command
line (i.e. the name is not converted to a canonicalized host name
before matching).

A pattern entry may be negated by prefixing it with an exclama-
tion 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 keyword/criteria pairs or the single token all
which matches all criteria. The available keywords are: exec,
host, originalhost, user, and localuser.

The exec keyword executes the specified command under the user‘s
shell. If the command returns a zero exit status then the condi-
tion is considered true. Commands containing whitespace charac-
ters must be quoted. The following character sequences in the
command will be expanded prior to execution: `%L‘ will be substi-
tuted 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-sep-
arated 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 sub-
stitution by the Hostname option. 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).

AddressFamily
Specifies which address family to use when connecting. Valid
arguments are ``any‘‘, ``inet‘‘ (use IPv4 only), or ``inet6‘‘
(use IPv6 only).

BatchMode
If set to ``yes‘‘, passphrase/password querying will be disabled.
This option is useful in scripts and other batch jobs where no
user is present to supply the password. 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 des-
tination host.

CanonicalizeFallbackLocal
Specifies whether to fail with an error when hostname canonical-
ization 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 and canonicalisation results in the
target hostname changing, then the configuration files are pro-
cessed again using the new target name to pick up any new config-
uration in matching Host 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 canoni-
calized to names in the ``*.b.example.com‘‘ or
``*.c.example.com‘‘ domains.

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. If the option
is set to ``no‘‘, the check will not be executed. The default is
``no‘‘.

Cipher Specifies the cipher to use for encrypting the session in proto-
col 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. The sup-
ported ciphers are:

``3des-cbc‘‘, ``aes128-cbc‘‘, ``aes192-cbc‘‘, ``aes256-cbc‘‘,
``aes128-ctr‘‘, ``aes192-ctr‘‘, ``aes256-ctr‘‘,
``[email protected]‘‘, ``[email protected]‘‘,
``arcfour128‘‘, ``arcfour256‘‘, ``arcfour‘‘, ``blowfish-cbc‘‘,
``cast128-cbc‘‘, and ``[email protected]‘‘.

The default is:

aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
[email protected],[email protected],
[email protected],
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
aes256-cbc,arcfour

The list of available ciphers may also be obtained using the -Q
option of ssh(1).

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 configura-
tion 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 connec-
tions on a control socket specified using the ControlPath argu-
ment. 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 con-
nections, but require confirmation using the SSH_ASKPASS program
before they are accepted (see ssh-add(1) for details). If the
ControlPath cannot be opened, ssh will continue without connect-
ing to a master instance.

X11 and ssh-agent(1) forwarding is supported over these multi-
plexed connections, however the display and agent forwarded will
be the one belonging to the master connection i.e. it is not pos-
sible 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 shar-
ing 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 speci-
fied on the command line, `%p‘ the destination port, `%r‘ by the
remote login username, and `%u‘ by the username of the user run-
ning ssh(1). It is recommended that any ControlPath used for
opportunistic connection sharing include at least %h, %p, and %r.
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 connec-
tion has been closed. If set to ``no‘‘, then the master connec-
tion will not be placed into the background, and will close as
soon as the initial client connection is closed. If set to
``yes‘‘, 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 speci-
fied 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 set-
ting. 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 dis-
able the escape character entirely (making the connection trans-
parent 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. The argument must be ``yes‘‘ or ``no‘‘. The
default is ``no‘‘.

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 redi-
rected 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 dis-
play 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 con-
sidered 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 min-
utes. Remote clients will be refused access after this time.

The default is ``no‘‘.

See the X11 SECURITY extension specification for full details on
the restrictions imposed on untrusted clients.

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 spec-
ify 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 data-
base, 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‘‘. Note that this option applies to protocol
version 2 only.

GSSAPIDelegateCredentials
Forward (delegate) credentials to the server. The default is
``no‘‘. Note that this option applies to protocol version 2
only.

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 iden-
tifying 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).

HostbasedAuthentication
Specifies whether to try rhosts based authentication with public
key authentication. The argument must be ``yes‘‘ or ``no‘‘. The
default is ``no‘‘. This option applies to protocol version 2
only and is similar to RhostsRSAAuthentication.

HostKeyAlgorithms
Specifies the protocol version 2 host key algorithms that the
client wants to use in order of preference. The default for this
option is:

[email protected],
[email protected],
[email protected],
[email protected],
[email protected],[email protected],
[email protected],[email protected],
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa,ssh-dss

If hostkeys are known for the destination host then this default
is modified to prefer their algorithms.

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 data-
base 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 default is the name
given on the command line. Numeric IP addresses are also permit-
ted (both on the command line and in HostName specifications).

IdentitiesOnly
Specifies that ssh(1) should only use the authentication identity
files configured in the ssh_config files, even if ssh-agent(1) or
a PKCS11Provider offers more identities. The argument to this
keyword must be ``yes‘‘ or ``no‘‘. This option is intended for
situations where ssh-agent offers many different identities. The
default is ``no‘‘.

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. ssh(1) will try to load certificate information from the
filename obtained by appending -cert.pub to the path of a speci-
fied IdentityFile.

The file name may use the tilde syntax to refer to a user‘s home
directory or one of the following escape characters: `%d‘ (local
user‘s home directory), `%u‘ (local user name), `%l‘ (local host
name), `%h‘ (remote host name) or `%r‘ (remote user name).

It is possible to have multiple identity files specified in con-
figuration files; all these identities will be tried in sequence.
Multiple IdentityFile directives will add to the list of identi-
ties tried (this behaviour differs from that of other configura-
tion directives).

IdentityFile may be used in conjunction with IdentitiesOnly to
select which identities in an agent are offered during authenti-
cation.

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 unrecog-
nised 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 spec-
ified, 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. The default is:

[email protected],
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group-exchange-sha1,
diffie-hellman-group14-sha1,
diffie-hellman-group1-sha1

LocalCommand
Specifies a command to execute on the local machine after suc-
cessfully 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).

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 privi-
leged 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 `*‘ indi-
cates 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, VER-
BOSE, 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 in protocol ver-
sion 2 for data integrity protection. Multiple algorithms must
be comma-separated. The algorithms that contain ``-etm‘‘ calcu-
late the MAC after encryption (encrypt-then-mac). These are con-
sidered safer and their use recommended. The default is:

[email protected],[email protected],
[email protected],[email protected],
[email protected],[email protected],
[email protected],[email protected],
[email protected],
hmac-md5,hmac-sha1,[email protected],[email protected],
hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,
hmac-sha1-96,hmac-md5-96

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 warn-
ings 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 commu-
nicate 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 protocol 2
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 ver-
sions must be comma-separated. When this option is set to
``2,1‘‘ ssh will try version 2 and fall back to version 1 if ver-
sion 2 is not available. The default is `2‘.

ProxyCommand
Specifies the command to use to connect to the server. The com-
mand string extends to the end of the line, and is executed with
the user‘s shell. 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 basi-
cally 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). Set-
ting the command to ``none‘‘ disables this option entirely. Note
that CheckHostIP is not available for connects with a proxy com-
mand.

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‘‘.

PubkeyAuthentication
Specifies whether to try public key authentication. The argument
to this keyword must be ``yes‘‘ or ``no‘‘. The default is
``yes‘‘. This option applies to protocol version 2 only.

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 docu-
mented 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. This option applies to protocol version 2 only.

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 inter-
faces. 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).

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 authentica-
tion 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. Note that environment passing is only supported
for protocol 2. The server must also support it, and the server
must be configured to accept these environment variables. Refer
to AcceptEnv in sshd_config(5) for how to configure the server.
Variables are specified by name, which may contain wildcard char-
acters. 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 there-
fore will not be spoofable. The TCP keepalive option enabled by
TCPKeepAlive is spoofable. The server alive mechanism is valu-
able when the client or server depend on knowing when a connec-
tion has become inactive.

The default value is 3. If, for example, ServerAliveInterval
(see below) is set to 15 and ServerAliveCountMax is left at the
default, if the server becomes unresponsive, ssh will disconnect
after approximately 45 seconds. This option applies to protocol
version 2 only.

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. This option applies to protocol version 2 only.

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 con-
nect to hosts whose host key has changed. This provides maximum
protection against trojan horse attacks, though it can be annoy-
ing 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 automati-
cally 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. However,
this means that connections will die if the route is down tempo-
rarily, 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‘‘.

UsePrivilegedPort
Specifies whether to use a privileged port for outgoing connec-
tions. 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 dif-
ferent user name is used on different machines. This saves the
trouble of having to remember to give the user name on the com-
mand line.

UserKnownHostsFile
Specifies one or more files to use for the user host key data-
base, 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 ``yes‘‘ if compiled with LDNS and ``no‘‘ otherwise.
Note that this option applies to protocol version 2 only.