SSH_CONFIG(5) BSD File Formats Manual SSH_CONFIG(5)
NAME
ssh_config - OpenSSH SSH client configuration files
SYNOPSIS
~/.ssh/config
/etc/ssh/ssh_config
DESCRIPTION
ssh obtains configuration data from the following sources in the follow
ing order:
1. command-line options
2. users 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 whites
pace when specifying configuration options using the ssh, scp and sftp -o
option.
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 key
word) to be only for those hosts that match one of the patterns
given after the keyword. * and ? can be used as wildcards in
the patterns. 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).
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 and SetupTimeOut options
will both be set to 300 seconds by default. This option is use
ful 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 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 proto
col version 1. Currently, blowfish, 3des, and des are sup
ported. des is only supported in the ssh client for interoper
ability with legacy protocol 1 implementations that do not sup
port 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,
arcfour128, arcfour256, arcfour, blowfish-cbc, and
cast128-cbc. The default is
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
aes192-ctr,aes256-ctr
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
command line to clear port forwardings set in configuration
files, and is automatically set by scp(1) and sftp(1). The argu
ment 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 will listen for connections on
a control socket specified using the ControlPath argument. Addi
tional 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 instances network connec
tion rather than initiating new ones, but will fall back to con
necting 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 can not 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, %h will be
substituted by the target host name, %p the port and %r by
the remote login username. It is recommended that any
ControlPath used for opportunistic connection sharing include all
three of these escape sequences. This ensures that shared con
nections are uniquely identified.
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 or by using
an alternative syntax: [bind_address/]port. By default, the
local port is bound in accordance with the GatewayPorts setting.
However, an explicit bind_address may be used to bind the connec
tion 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
will act as a SOCKS server. Multiple forwardings may be speci
fied, 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).
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
agents 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
users 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.
ForwardX11Trusted
If this option is set to yes then remote X11 clients will have
full access to the original X11 display.
If this option is set to no then 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 binds local port forwardings to
the loopback address. This prevents other remote hosts from con
necting 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 a file to use for the global host key database instead
of /etc/ssh/ssh_known_hosts.
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.
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 GSS
API key exchange.
HashKnownHosts
Indicates that ssh should hash host names and addresses when they
are added to ~/.ssh/known_hosts. These hashed names may be used
normally by ssh and sshd, but they do not reveal identifying
information should the files contents be disclosed. The default
is no. Note that hashing of names and addresses will not be
retrospectively applied to existing known hosts files, but these
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: ssh-rsa,ssh-dss.
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 connec
tions 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. Default is the
name given on the command line. Numeric IP addresses are also
permitted (both on the command line and in HostName specifica
tions).
IdentitiesOnly
Specifies that ssh should only use the authentication identity
files configured in the ssh_config files, even if the ssh-agent
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 users RSA or DSA authentication
identity is read. The default is ~/.ssh/identity for protocol
version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for protocol ver
sion 2. Additionally, any identities represented by the authen
tication agent will be used for authentication. The file name
may use the tilde syntax to refer to a users home directory. It
is possible to have multiple identity files specified in configu
ration files; all these identities will be tried in sequence.
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.
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 /bin/sh. 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 or by using
an alternative syntax: [bind_address/]port and host/hostport.
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 accor
dance 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 lis
tening 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. 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 default is
hmac-md5,hmac-sha1,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. 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.
Port Specifies the port number to connect on the remote host. 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 for this option is:
hostbased,publickey,keyboard-interactive,password.
Protocol
Specifies the protocol versions ssh should support in order of
preference. The possible values are 1 and 2. Multiple ver
sions must be comma-separated. The default is 2,1. This means
that ssh tries version 2 and falls back to version 1 if version 2
is not available.
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
/bin/sh. In the command string, %h will be substituted by the
host name to connect and %p by the port. 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 some
where. 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 num
ber 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 or by using
an alternative syntax: [bind_address/]port and host/hostport.
Multiple forwardings may be specified, and additional forwardings
can be given on the command line. Only the superuser can forward
privileged ports.
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
servers GatewayPorts option is enabled (see sshd_config(5)).
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 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 the wildcard
characters * and ?. Multiple environment variables may be
separated by whitespace or spread across multiple SendEnv direc
tives. The default is not to send any environment variables.
ServerAliveCountMax
Sets the number of server alive messages (see below) which may be
sent without ssh 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 ses
sion. It is important to note that the use of server alive mes
sages 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 works when using
protocol version 2 only; in protocol version 1 there is no mecha
nism 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 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 is a
Debian-specific compatibility alias for this option.
SetupTimeOut
Normally, ssh blocks indefinitely whilst waiting to receive the
ssh banner and other setup protocol from the server, during the
session setup. This can cause ssh to hang under certain circum
stances. If this option is set, ssh will give up if no data from
the server is received for the specified number of seconds. The
argument must be an integer. The default is 0 (disabled), or 300
if BatchMode is set. This is a Debian-specific option.
SmartcardDevice
Specifies which smartcard device to use. The argument to this
keyword is the device ssh should use to communicate with a smart
card used for storing the users private RSA key. By default, no
device is specified and smartcard support is not activated.
StrictHostKeyChecking
If this flag is set to yes, ssh 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 pro
tection against trojan horse attacks, however, can be annoying
when the /etc/ssh/ssh_known_hosts file is poorly maintained, or
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 con
firmed 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 starting tun(4) device forwarding between the client and
the server. This option also allows requesting layer 2 (ether
net) instead of layer 3 (point-to-point) tunneling from the
server. The argument must be yes, point-to-point, ethernet
or no. The default is no.
TunnelDevice
Force a specified tun(4) device on the client. Without this
option, the next available device will be used.
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 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 a file to use for the user host key database instead of
~/.ssh/known_hosts.
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.
XAuthLocation
Specifies the full pathname of the xauth(1) program. The default
is /usr/bin/X11/xauth.
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 oth
ers. It may be group-writable provided that the group in ques
tion contains only the user.
/etc/ssh/ssh_config
Systemwide configuration file. This file provides defaults for
those values that are not specified in the users 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 cre
ated OpenSSH. Markus Friedl contributed the support for SSH protocol
versions 1.5 and 2.0.
BSD September 25, 1999 BSD
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