NAME¶
ssh_config —
OpenSSH SSH client
configuration files
SYNOPSIS¶
DESCRIPTION¶
ssh(1) obtains configuration data from the following sources
in the following order:
- command-line options
- user's configuration file
(~/.ssh/config)
- system-wide configuration file
(/etc/ssh/ssh_config)
For each parameter, the first obtained value will be used. The configuration
files contain sections separated by “Host” specifications, and
that section is only applied for hosts that match one of the patterns given in
the specification. The matched host name is the one given on the command line.
Since the first obtained value for each parameter is used, more host-specific
declarations should be given near the beginning of the file, and general
defaults at the end.
Note that the Debian
openssh-client package sets several
options as standard in
/etc/ssh/ssh_config which are not the
default in
ssh(1):
- SendEnv
LANG LC_*
- HashKnownHosts
yes
- GSSAPIAuthentication
yes
The configuration file has the following format:
Empty lines and lines starting with ‘
#
’ are
comments. Otherwise a line is of the format “keyword arguments”.
Configuration options may be separated by whitespace or optional whitespace
and exactly one ‘
=
’; the latter format is
useful to avoid the need to quote whitespace when specifying configuration
options using the
ssh,
scp, and
sftp -o option. Arguments may optionally
be enclosed in double quotes (") in order to represent arguments
containing spaces.
The possible keywords and their meanings are as follows (note that keywords are
case-insensitive and arguments are case-sensitive):
- Host
- Restricts the following declarations (up to the next
Host 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 exclamation mark
(‘!’). If a negated entry is matched, then the
Host entry is ignored, regardless of whether any other
patterns on the line match. Negated matches are therefore useful to
provide exceptions for wildcard matches.
See PATTERNS for more information on
patterns.
- 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. In addition, the ServerAliveInterval
option will be set to 300 seconds by default. This option is useful in
scripts and other batch jobs where no user is present to supply the
password, and where it is desirable to detect a broken network swiftly.
The argument must be “yes” or “no”. The default is
“no”.
- BindAddress
- Use the specified address on the local machine as the
source address of the connection. Only useful on systems with more than
one address. Note that this option does not work if
UsePrivilegedPort is set to “yes”.
- 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
“yes”.
- Cipher
- Specifies the cipher to use for encrypting the session in
protocol version 1. Currently, “blowfish”, “3des”,
and “des” are supported. des is only
supported in the ssh(1) client for interoperability with
legacy protocol 1 implementations that do not support the
3des cipher. Its use is strongly discouraged due to
cryptographic weaknesses. The default is “3des”.
- Ciphers
- Specifies the ciphers allowed for protocol version 2 in
order of preference. Multiple ciphers must be comma-separated. The
supported ciphers are “3des-cbc”, “aes128-cbc”,
“aes192-cbc”, “aes256-cbc”,
“aes128-ctr”, “aes192-ctr”,
“aes256-ctr”, “arcfour128”,
“arcfour256”, “arcfour”,
“blowfish-cbc”, and “cast128-cbc”. The default is:
aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
aes256-cbc,arcfour
- ClearAllForwardings
- Specifies that all local, remote, and dynamic port
forwardings specified in the configuration files or on the command line be
cleared. This option is primarily useful when used from the
ssh(1) command line to clear port forwardings set in
configuration files, and is automatically set by scp(1)
and sftp(1). The argument must be “yes” or
“no”. The default is “no”.
- Compression
- Specifies whether to use compression. The argument must be
“yes” or “no”. The default is
“no”.
- CompressionLevel
- Specifies the compression level to use if compression is
enabled. The argument must be an integer from 1 (fast) to 9 (slow, best).
The default level is 6, which is good for most applications. The meaning
of the values is the same as in gzip(1). Note that this
option applies to protocol version 1 only.
- ConnectionAttempts
- Specifies the number of tries (one per second) to make
before exiting. The argument must be an integer. This may be useful in
scripts if the connection sometimes fails. The default is 1.
- ConnectTimeout
- Specifies the timeout (in seconds) used when connecting to
the SSH server, instead of using the default system TCP timeout. This
value is used only when the target is down or really unreachable, not when
it refuses the connection.
- ControlMaster
- Enables the sharing of multiple sessions over a single
network connection. When set to “yes”,
ssh(1) will listen for connections on a control socket
specified using the ControlPath argument. Additional
sessions can connect to this socket using the same
ControlPath with ControlMaster set to
“no” (the default). These sessions will try to reuse the
master instance's network connection rather than initiating new ones, but
will fall back to connecting normally if the control socket does not
exist, or is not listening.
Setting this to “ask” will cause ssh to listen for control
connections, but require confirmation using the
SSH_ASKPASS
program before they are accepted (see
ssh-add(1) for details). If the
ControlPath cannot be opened, ssh will continue without
connecting to a master instance.
X11 and ssh-agent(1) forwarding is supported over these
multiplexed connections, however the display and agent forwarded will be
the one belonging to the master connection i.e. it is not possible to
forward multiple displays or agents.
Two additional options allow for opportunistic multiplexing: try to use a
master connection but fall back to creating a new one if one does not
already exist. These options are: “auto” and
“autoask”. The latter requires confirmation like the
“ask” option.
- ControlPath
- Specify the path to the control socket used for connection
sharing as described in the ControlMaster section above
or the string “none” to disable connection sharing. In the
path, ‘
%L
’ will be substituted by the
first component of the local host name,
‘%l
’ will be substituted by the local
host name (including any domain name),
‘%h
’ will be substituted by the target
host name, ‘%n
’ will be substituted by
the original target host name specified on the command line,
‘%p
’ the port,
‘%r
’ by the remote login username, and
‘%u
’ by the username of the user
running 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 connection has been closed. If set
to “no”, then the master connection will not be placed into
the background, and will close as soon as the initial client connection is
closed. If set to “yes”, then the master connection will
remain in the background indefinitely (until killed or closed via a
mechanism such as the ssh(1) “-O
exit” option). If set to a time in seconds,
or a time in any of the formats documented in
sshd_config(5), then the backgrounded master connection
will automatically terminate after it has remained idle (with no client
connections) for the specified time.
- DynamicForward
- Specifies that a TCP port on the local machine be forwarded
over the secure channel, and the application protocol is then used to
determine where to connect to from the remote machine.
The argument must be
[bind_address:]port.
IPv6 addresses can be specified by enclosing addresses in square brackets.
By default, the local port is bound in accordance with the
GatewayPorts setting. However, an explicit
bind_address may be used to bind the connection to a
specific address. The bind_address of
“localhost” indicates that the listening port be bound for
local use only, while an empty address or ‘*’ indicates that
the port should be available from all interfaces.
Currently the SOCKS4 and SOCKS5 protocols are supported, and
ssh(1) will act as a SOCKS server. Multiple forwardings
may be specified, and additional forwardings can be given on the command
line. Only the superuser can forward privileged ports.
- EnableSSHKeysign
- Setting this option to “yes” in the global
client configuration file /etc/ssh/ssh_config enables
the use of the helper program ssh-keysign(8) during
HostbasedAuthentication. The argument must be
“yes” or “no”. The default is “no”.
This option should be placed in the non-hostspecific section. See
ssh-keysign(8) for more information.
- EscapeChar
- Sets the escape character (default:
‘
~
’). The escape character can also be
set on the command line. The argument should be a single character,
‘^
’ followed by a letter, or
“none” to disable the escape character entirely (making the
connection transparent for binary data).
- ExitOnForwardFailure
- Specifies whether ssh(1) should terminate
the connection if it cannot set up all requested dynamic, tunnel, local,
and remote port forwardings. 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
redirected over the secure channel and
DISPLAY
set. The argument must be “yes” or “no”. The
default is “no”.
X11 forwarding should be enabled with caution. Users with the ability to
bypass file permissions on the remote host (for the user's X11
authorization database) can access the local X11 display through the
forwarded connection. An attacker may then be able to perform activities
such as keystroke monitoring if the ForwardX11Trusted
option is also enabled.
- ForwardX11Timeout
- Specify a timeout for untrusted X11 forwarding using the
format described in the TIME
FORMATS section of sshd_config(5). X11 connections
received by ssh(1) after this time will be refused. The
default is to disable untrusted X11 forwarding after twenty minutes has
elapsed.
- ForwardX11Trusted
- If this option is set to “yes”, remote X11
clients will have full access to the original X11 display.
If this option is set to “no”, remote X11 clients will be
considered untrusted and prevented from stealing or tampering with data
belonging to trusted X11 clients. Furthermore, the
xauth(1) token used for the session will be set to
expire after 20 minutes. Remote clients will be refused access after this
time.
The default is “yes” (Debian-specific).
See the X11 SECURITY extension specification for full details on the
restrictions imposed on untrusted clients.
- GatewayPorts
- Specifies whether remote hosts are allowed to connect to
local forwarded ports. By default, ssh(1) binds local
port forwardings to the loopback address. This prevents other remote hosts
from connecting to forwarded ports. GatewayPorts can be
used to specify that ssh should bind local port forwardings to the
wildcard address, thus allowing remote hosts to connect to forwarded
ports. The argument must be “yes” or “no”. The
default is “no”.
- GlobalKnownHostsFile
- Specifies one or more files to use for the global host key
database, separated by whitespace. The default is
/etc/ssh/ssh_known_hosts,
/etc/ssh/ssh_known_hosts2.
- GSSAPIAuthentication
- Specifies whether user authentication based on GSSAPI is
allowed. The default is “no”. Note that this option applies to
protocol version 2 only.
- GSSAPIKeyExchange
- Specifies whether key exchange based on GSSAPI may be used.
When using GSSAPI key exchange the server need not have a host key. The
default is “no”. Note that this option applies to protocol
version 2 only.
- GSSAPIClientIdentity
- If set, specifies the GSSAPI client identity that ssh
should use when connecting to the server. The default is unset, which
means that the default identity will be used.
- GSSAPIServerIdentity
- If set, specifies the GSSAPI server identity that ssh
should expect when connecting to the server. The default is unset, which
means that the expected GSSAPI server identity will be determined from the
target hostname.
- GSSAPIDelegateCredentials
- Forward (delegate) credentials to the server. The default
is “no”. Note that this option applies to protocol version 2
connections using GSSAPI.
- GSSAPIRenewalForcesRekey
- If set to “yes” then renewal of the client's
GSSAPI credentials will force the rekeying of the ssh connection. With a
compatible server, this can delegate the renewed credentials to a session
on the server. The default is “no”.
- GSSAPITrustDns
- Set to “yes to indicate that the DNS is trusted to
securely canonicalize” the name of the host being connected to. If
“no, the hostname entered on the” command line will be passed
untouched to the GSSAPI library. The default is “no”. This
option only applies to protocol version 2 connections using GSSAPI.
- HashKnownHosts
- Indicates that ssh(1) should hash host
names and addresses when they are added to
~/.ssh/known_hosts. These hashed names may be used
normally by ssh(1) and sshd(8), but
they do not reveal identifying information should the file's contents be
disclosed. The default is “no”. Note that existing names and
addresses in known hosts files will not be converted automatically, but
may be manually hashed using ssh-keygen(1). Use of this
option may break facilities such as tab-completion that rely on being able
to read unhashed host names from
~/.ssh/known_hosts.
- HostbasedAuthentication
- Specifies whether to try rhosts based authentication with
public key authentication. The argument must be “yes” or
“no”. The default is “no”. 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:
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
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 database
files. This option is useful for tunneling SSH connections or for multiple
servers running on a single host.
- HostName
- Specifies the real host name to log into. This can be used
to specify nicknames or abbreviations for hosts. If the hostname contains
the character sequence ‘
%h
’, then this
will be replaced with the host name specified on the command line (this is
useful for manipulating unqualified names). The default is the name given
on the command line. Numeric IP addresses are also permitted (both on the
command line and in HostName specifications).
- IdentitiesOnly
- Specifies that ssh(1) should only use the
authentication identity files configured in the
ssh_config files, even if ssh-agent(1)
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 or DSA
authentication identity is read. The default is
~/.ssh/identity for protocol version 1, and
~/.ssh/id_dsa, ~/.ssh/id_ecdsa and
~/.ssh/id_rsa for protocol version 2. Additionally, any
identities represented by the authentication agent will be used for
authentication. ssh(1) will try to load certificate
information from the filename obtained by appending
-cert.pub to the path of a specified
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 configuration
files; all these identities will be tried in sequence. Multiple
IdentityFile directives will add to the list of
identities tried (this behaviour differs from that of other configuration
directives).
- IPQoS
- Specifies the IPv4 type-of-service or DSCP class for
connections. Accepted values are “af11”, “af12”,
“af13”, “af21”, “af22”,
“af23”, “af31”, “af32”,
“af33”, “af41”, “af42”,
“af43”, “cs0”, “cs1”,
“cs2”, “cs3”, “cs4”,
“cs5”, “cs6”, “cs7”, “ef”,
“lowdelay”, “throughput”,
“reliability”, or a numeric value. This option may take one or
two arguments, separated by whitespace. If one argument is specified, it
is used as the packet class unconditionally. If two values are specified,
the first is automatically selected for interactive sessions and the
second for non-interactive sessions. The default is “lowdelay”
for interactive sessions and “throughput” for non-interactive
sessions.
- KbdInteractiveAuthentication
- Specifies whether to use keyboard-interactive
authentication. The argument to this keyword must be “yes” or
“no”. The default is “yes”.
- KbdInteractiveDevices
- Specifies the list of methods to use in
keyboard-interactive authentication. Multiple method names must be
comma-separated. The default is to use the server specified list. The
methods available vary depending on what the server supports. For an
OpenSSH server, it may be zero or more of: “bsdauth”,
“pam”, and “skey”.
- KexAlgorithms
- Specifies the available KEX (Key Exchange) algorithms.
Multiple algorithms must be comma-separated. The default is:
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
successfully connecting to the server. The command string extends to the
end of the line, and is executed with the user's shell. The following
escape character substitutions will be performed:
‘
%d
’ (local user's home directory),
‘%h
’ (remote host name),
‘%l
’ (local host name),
‘%n
’ (host name as provided on the
command line), ‘%p
’ (remote port),
‘%r
’ (remote user name) or
‘%u
’ (local user name).
The command is run synchronously and does not have access to the session of
the ssh(1) that spawned it. It should not be used for
interactive commands.
This directive is ignored unless PermitLocalCommand has
been enabled.
- LocalForward
- Specifies that a TCP port on the local machine be forwarded
over the secure channel to the specified host and port from the remote
machine. The first argument must be
[bind_address:]port
and the second argument must be
host:hostport. IPv6 addresses
can be specified by enclosing addresses in square brackets. Multiple
forwardings may be specified, and additional forwardings can be given on
the command line. Only the superuser can forward privileged ports. By
default, the local port is bound in accordance with the
GatewayPorts setting. However, an explicit
bind_address may be used to bind the connection to a
specific address. The bind_address of
“localhost” indicates that the listening port be bound for
local use only, while an empty address or ‘*’ indicates that
the port should be available from all interfaces.
- LogLevel
- Gives the verbosity level that is used when logging
messages from ssh(1). The possible values are: QUIET,
FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The
default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each
specify higher levels of verbose output.
- MACs
- Specifies the MAC (message authentication code) algorithms
in order of preference. The MAC algorithm is used in protocol version 2
for data integrity protection. Multiple algorithms must be
comma-separated. The default is:
hmac-md5,hmac-sha1,umac-64@openssh.com,
hmac-ripemd160,hmac-sha1-96,hmac-md5-96,
hmac-sha2-256,hmac-sha2-256-96,hmac-sha2-512,
hmac-sha2-512-96
- 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 warnings about changed
host keys. However, this option disables host authentication for
localhost. The argument to this keyword must be “yes” or
“no”. The default is to check the host key for localhost.
- NumberOfPasswordPrompts
- Specifies the number of password prompts before giving up.
The argument to this keyword must be an integer. The default is 3.
- PasswordAuthentication
- Specifies whether to use password authentication. The
argument to this keyword must be “yes” or “no”.
The default is “yes”.
- PermitLocalCommand
- Allow local command execution via the
LocalCommand option or using the
!command escape sequence in
ssh(1). The argument must be “yes” or
“no”. The default is “no”.
- PKCS11Provider
- Specifies which PKCS#11 provider to use. The argument to
this keyword is the PKCS#11 shared library ssh(1) should
use to communicate with a PKCS#11 token providing the user's private RSA
key.
- Port
- Specifies the port number to connect on the remote host.
The default is 22.
- PreferredAuthentications
- Specifies the order in which the client should try 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 versions must be
comma-separated. When this option is set to “2,1”
ssh will try version 2 and fall back to version 1 if
version 2 is not available. The default is ‘2’.
- ProxyCommand
- Specifies the command to use to connect to the server. The
command 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 basically anything, and should read from its standard input
and write to its standard output. It should eventually connect an
sshd(8) server running on some machine, or execute
sshd -i somewhere. Host key management will be done
using the HostName of the host being connected (defaulting to the name
typed by the user). Setting the command to “none” disables
this option entirely. Note that CheckHostIP is not
available for connects with a proxy command.
This directive is useful in conjunction with nc(1) and its
proxy support. For example, the following directive would connect via an
HTTP proxy at 192.0.2.0:
ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
- 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. The argument is the
number of bytes, with an optional suffix of ‘K’,
‘M’, or ‘G’ to indicate Kilobytes, Megabytes, or
Gigabytes, respectively. The default is between ‘1G’ and
‘4G’, depending on the cipher. 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 interfaces. Specifying a remote
bind_address will only succeed if the server's
GatewayPorts option is enabled (see
sshd_config(5)).
- RequestTTY
- Specifies whether to request a pseudo-tty for the session.
The argument may be one of: “no” (never request a TTY),
“yes” (always request a TTY when standard input is a TTY),
“force” (always request a TTY) or “auto” (request
a TTY when opening a login session). This option mirrors the
-t and -T flags for
ssh(1).
- RhostsRSAAuthentication
- Specifies whether to try rhosts based authentication with
RSA host authentication. The argument must be “yes” or
“no”. The default is “no”. This option applies to
protocol version 1 only and requires ssh(1) to be setuid
root.
- RSAAuthentication
- Specifies whether to try RSA authentication. The argument
to this keyword must be “yes” or “no”. RSA
authentication will only be attempted if the identity file exists, or an
authentication agent is running. The default is “yes”. Note
that this option applies to protocol version 1 only.
- SendEnv
- Specifies what variables from the local
environ(7) should be sent to the server. 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 characters.
Multiple environment variables may be separated by whitespace or spread
across multiple SendEnv directives. The default is not
to send any environment variables.
See PATTERNS for more information on
patterns.
- ServerAliveCountMax
- Sets the number of server alive messages (see below) which
may be sent without ssh(1) receiving any messages back
from the server. If this threshold is reached while server alive messages
are being sent, ssh will disconnect from the server, terminating the
session. It is important to note that the use of server alive messages is
very different from TCPKeepAlive (below). The server
alive messages are sent through the encrypted channel and therefore will
not be spoofable. The TCP keepalive option enabled by
TCPKeepAlive is spoofable. The server alive mechanism is
valuable when the client or server depend on knowing when a connection has
become inactive.
The default value is 3. If, for example,
ServerAliveInterval (see below) is set to 15 and
ServerAliveCountMax is left at the default, if the
server becomes unresponsive, ssh will disconnect after approximately 45
seconds. This option applies to protocol version 2 only; in protocol
version 1 there is no mechanism to request a response from the server to
the server alive messages, so disconnection is the responsibility of the
TCP stack.
- ServerAliveInterval
- Sets a timeout interval in seconds after which if no data
has been received from the server, ssh(1) will send a
message through the encrypted channel to request a response from the
server. The default is 0, indicating that these messages will not be sent
to the server, or 300 if the BatchMode option is set.
This option applies to protocol version 2 only.
ProtocolKeepAlives and SetupTimeOut
are Debian-specific compatibility aliases for this option.
- StrictHostKeyChecking
- If this flag is set to “yes”,
ssh(1) will never automatically add host keys to the
~/.ssh/known_hosts file, and refuses to connect to hosts
whose host key has changed. This provides maximum protection against
trojan horse attacks, though it can be annoying when the
/etc/ssh/ssh_known_hosts file is poorly maintained or
when connections to new hosts are frequently made. This option forces the
user to manually add all new hosts. If this flag is set to
“no”, ssh will automatically add new host keys to the user
known hosts files. If this flag is set to “ask”, new host keys
will be added to the user known host files only after the user has
confirmed that is what they really want to do, and ssh will refuse to
connect to hosts whose host key has changed. The host keys of known hosts
will be verified automatically in all cases. The argument must be
“yes”, “no”, or “ask”. The default is
“ask”.
- TCPKeepAlive
- Specifies whether the system should send TCP keepalive
messages to the other side. If they are sent, death of the connection or
crash of one of the machines will be properly noticed. This option only
uses TCP keepalives (as opposed to using ssh level keepalives), so takes a
long time to notice when the connection dies. As such, you probably want
the ServerAliveInterval option as well. However, this
means that connections will die if the route is down temporarily, and some
people find it annoying.
The default is “yes” (to send TCP keepalive messages), and the
client will notice if the network goes down or the remote host dies. This
is important in scripts, and many users want it too.
To disable TCP keepalive messages, the value should be set to
“no”.
- Tunnel
- Request tun(4) device forwarding between
the client and the server. The argument must be “yes”,
“point-to-point” (layer 3), “ethernet” (layer 2),
or “no”. Specifying “yes” requests the default
tunnel mode, which is “point-to-point”. The default is
“no”.
- TunnelDevice
- Specifies the tun(4) devices to open on
the client (local_tun) and the server
(remote_tun).
The argument must be
local_tun[:remote_tun].
The devices may be specified by numerical ID or the keyword
“any”, which uses the next available tunnel device. If
remote_tun is not specified, it defaults to
“any”. The default is “any:any”.
- UseBlacklistedKeys
- Specifies whether ssh(1) should use keys
recorded in its blacklist of known-compromised keys (see
ssh-vulnkey(1)) for authentication. If
“yes”, then attempts to use compromised keys for
authentication will be logged but accepted. It is strongly recommended
that this be used only to install new authorized keys on the remote
system, and even then only with the utmost care. If “no”, then
attempts to use compromised keys for authentication will be prevented. The
default is “no”.
- UsePrivilegedPort
- Specifies whether to use a privileged port for outgoing
connections. The argument must be “yes” or “no”.
The default is “no”. If set to “yes”,
ssh(1) must be setuid root. Note that this option must
be set to “yes” for RhostsRSAAuthentication
with older servers.
- User
- Specifies the user to log in as. This can be useful when a
different user name is used on different machines. This saves the trouble
of having to remember to give the user name on the command line.
- UserKnownHostsFile
- Specifies one or more files to use for the user host key
database, separated by whitespace. The default is
~/.ssh/known_hosts,
~/.ssh/known_hosts2.
- VerifyHostKeyDNS
- Specifies whether to verify the remote key using DNS and
SSHFP resource records. If this option is set to “yes”, the
client will implicitly trust keys that match a secure fingerprint from
DNS. Insecure fingerprints will be handled as if this option was set to
“ask”. If this option is set to “ask”, information
on fingerprint match will be displayed, but the user will still need to
confirm new host keys according to the
StrictHostKeyChecking option. The argument must be
“yes”, “no”, or “ask”. The default is
“no”. Note that this option applies to protocol version 2
only.
See also VERIFYING HOST KEYS
in ssh(1).
- VisualHostKey
- If this flag is set to “yes”, an ASCII art
representation of the remote host key fingerprint is printed in addition
to the hex fingerprint string at login and for unknown host keys. If this
flag is set to “no”, no fingerprint strings are printed at
login and only the hex fingerprint string will be printed for unknown host
keys. The default is “no”.
- XAuthLocation
- Specifies the full pathname of the
xauth(1) program. The default is
/usr/bin/xauth.
PATTERNS¶
A
pattern consists of zero or more non-whitespace characters,
‘*’ (a wildcard that matches zero or more characters), or
‘?’ (a wildcard that matches exactly one character). For example,
to specify a set of declarations for any host in the “.co.uk” set
of domains, the following pattern could be used:
Host *.co.uk
The following pattern would match any host in the 192.168.0.[0-9] network range:
Host 192.168.0.?
A
pattern-list is a comma-separated list of patterns. Patterns
within pattern-lists may be negated by preceding them with an exclamation mark
(‘!’). For example, to allow a key to be used from anywhere within
an organisation except from the “dialup” pool, the following entry
(in authorized_keys) could be used:
from="!*.dialup.example.com,*.example.com"
FILES¶
- ~/.ssh/config
- This is the per-user configuration file. The format of this
file is described above. This file is used by the SSH client. Because of
the potential for abuse, this file must have strict permissions:
read/write for the user, and not accessible by others. It may be
group-writable provided that the group in question contains only the
user.
- /etc/ssh/ssh_config
- Systemwide configuration file. This file provides defaults
for those values that are not specified in the user's configuration file,
and for those users who do not have a configuration file. This file must
be world-readable.
SEE ALSO¶
ssh(1)
AUTHORS¶
OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu
Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt
and Dug Song removed many bugs, re-added newer features and created OpenSSH.
Markus Friedl contributed the support for SSH protocol versions 1.5 and
2.0.