ssh_config
SSH_CONFIG(5) BSD File Formats Manual SSH_CONFIG(5)
NAME
ssh_config - OpenSSH SSH client configuration files
SYNOPSIS
$HOME/.ssh/config
/etc/ssh/ssh_config
DESCRIPTION
ssh obtains configuration data from the following sources in the follow-
ing order:
1. command-line options
2. user’s configuration file ($HOME/.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 bracketed 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.
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.
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 pro-
vide global defaults for all hosts. The host is the hostname
argument given on the command line (i.e., the name is not con-
verted 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.
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
Specify the interface to transmit from on machines with multiple
interfaces or aliased addresses. 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
supported. des is only supported in the ssh client for interop-
erability 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”,
“arcfour”, “blowfish-cbc”, and “cast128-cbc”. The default is
‘‘aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
aes192-cbc,aes256-cbc’’
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
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 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 reuse the master instance’s network connection
rather than initiating new ones. Setting this to “ask” will
cause ssh to listen for control connections, but require confir-
mation using the SSH_ASKPASS program before they are accepted
(see ssh-add(1) for details).
ControlPath
Specify the path to the control socket used for connection shar-
ing. See ControlMaster above.
DynamicForward
Specifies that a TCP/IP 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 a port number. Currently the SOCKS4 and
SOCKS5 protocols are supported, and ssh will act as a SOCKS
server. Multiple forwardings may be specified, and additional
forwardings can be given on the command line. Only the supe-
ruser 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
agent’s Unix-domain socket) can access the local agent through
the forwarded connection. An attacker cannot obtain key mate-
rial 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.
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.
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 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 wild-
card 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.
HashKnownHosts
Indicates that ssh should hash host names and addresses when
they are added to $HOME/.ssh/known_hosts. These hashed names
may be used normally by ssh and sshd, but they do not reveal
identifying information should the file’s 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).
IdentityFile
Specifies a file from which the user’s RSA or DSA authentication
identity is read. The default is $HOME/.ssh/identity for proto-
col version 1, and $HOME/.ssh/id_rsa and $HOME/.ssh/id_dsa for
protocol version 2. Additionally, any identities represented by
the authentication agent will be used for authentication. The
file name may use the tilde syntax to refer to a user’s home
directory. It is possible to have multiple identity files spec-
ified in configuration files; all these identities will be tried
in sequence.
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 intented for situations where
ssh-agent offers many different identities. The default is
“no”.
LocalForward
Specifies that a TCP/IP 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 a port number, and
the second must be host:port. IPv6 addresses can be specified
with an alternative syntax: host/port. Multiple forwardings may
be specified, and additional forwardings can be given on the
command line. Only the superuser can forward privileged ports.
LogLevel
Gives the verbosity level that is used when logging messages
from ssh. 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 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”.
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.
PubkeyAuthentication
Specifies whether to try public key authentication. The argu-
ment to this keyword must be “yes” or “no”. The default is
“yes”. This option applies to protocol version 2 only.
RemoteForward
Specifies that a TCP/IP 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 a port number, and
the second must be host:port. IPv6 addresses can be specified
with an alternative syntax: host/port. Multiple forwardings may
be specified, and additional forwardings can be given on the
command line. Only the superuser can forward privileged ports.
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 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 sup-
ported 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
directives. The default is not to send any environment vari-
ables.
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. This option applies to protocol version 2
only.
ServerAliveCountMax
Sets the number of server alive messages (see above) 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
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 mecha-
nism 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
(above) is set to 15, and ServerAliveCountMax is left at the
default, if the server becomes unresponsive ssh will disconnect
after approximately 45 seconds.
SmartcardDevice
Specifies which smartcard device to use. The argument to this
keyword is the device ssh should use to communicate with a
smartcard used for storing the user’s 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 $HOME/.ssh/known_hosts file, and refuses to
connect to hosts whose host key has changed. This provides max-
imum protection against trojan horse attacks, however, can be
annoying when the /etc/ssh/ssh_known_hosts file is poorly main-
tained, 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 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. However,
this means that connections will die if the route is down tem-
porarily, 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”.
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 $HOME/.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/X11R6/bin/xauth.
FILES
$HOME/.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.
/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 pro-
tocol versions 1.5 and 2.0.
BSD September 25, 1999 BSD
