| SSH_CONFIG(5) | File Formats Manual | SSH_CONFIG(5) |
ssh_config —
ssh obtains configuration data from the following
sources in the following order:
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.
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 keywords are case-insensitive and arguments are case-sensitive):
HostHost keyword) 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).AddressFamilyBatchModeBindAddressUsePrivilegedPort is set to
“yes”.ChallengeResponseAuthenticationCheckHostIPCipherssh 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
``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
aes192-ctr,aes256-ctr''
ClearAllForwardingsssh 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”.CompressionCompressionLevelConnectionAttemptsConnectTimeoutControlMasterssh 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 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 connecting to a master
instance.ControlPathControlMaster section above. In
the path, ‘%h’ will be substituted
by the target host name, ‘%p’ the
port and ‘%r’ by the remote login
username.DynamicForwardssh 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.EnableSSHKeysignHostbasedAuthentication. 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~’). 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).ForwardAgentAgent 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.
ForwardX11DISPLAY 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.
ForwardX11TrustedIf 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 “no”.
See the X11 SECURITY extension specification for full details on the restrictions imposed on untrusted clients.
GatewayPortsssh 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”.GlobalKnownHostsFileGSSAPIAuthenticationGSSAPIDelegateCredentialsHashKnownHostsssh 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 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).HostbasedAuthenticationRhostsRSAAuthentication.HostKeyAlgorithmsHostKeyAliasHostNameHostName specifications).IdentityFileIdentitiesOnlyssh 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”.KbdInteractiveDevicesLocalForwardGatewayPorts 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.LogLevelssh. 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.MACsNoHostAuthenticationForLocalhostNumberOfPasswordPromptsPasswordAuthenticationPortPreferredAuthenticationskeyboard-interactive) over another method (e.g.
password) The default for this option is:
“hostbased,publickey,keyboard-interactive,password”.Protocolssh should support
in order of preference. The possible values are “1” and
“2”. Multiple versions 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%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 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
PubkeyAuthenticationRemoteForwardIf 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)).
RhostsRSAAuthenticationssh to be
setuid root.RSAAuthenticationSendEnvAcceptEnv 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 variables.ServerAliveIntervalssh 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.ServerAliveCountMaxssh 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 (above) is set to 15, and
ServerAliveCountMax is left at the default, if
the server becomes unresponsive ssh will disconnect after approximately
45 seconds.
SmartcardDevicessh 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.StrictHostKeyCheckingssh
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, 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 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”.TCPKeepAliveThe 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”.
UsePrivilegedPortssh must be setuid root. Note that this option
must be set to “yes” for
RhostsRSAAuthentication with older servers.UserUserKnownHostsFileVerifyHostKeyDNSStrictHostKeyChecking option. The argument must be
“yes”, “no” or “ask”. The
default is “no”. Note that this option applies to protocol
version 2 only.XAuthLocationssh
client. Because of the potential for abuse, this file must have strict
permissions: read/write for the user, and not accessible by others.| September 25, 1999 | BSD |