$HOME/.ssh/config
/etc/ssh/ssh_config
DESCRIPTION
ssh obtains configuration data from the following sources in
the follow-
ing order:
1. command-line options
2. user's configuration file ($HOME/.ssh/config)
3. system-wide configuration file (/etc/ssh/ssh_con-
fig)
For each parameter, the first obtained value will be used.
The configu-
ration files contain sections bracketed by ``Host'' specifi-
cations, 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 white-
space 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-sensi-
tive):
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
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 argu-
ment 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 op-
tion does not
work if UsePrivilegedPort is set to ``yes''.
ChallengeResponseAuthentication
Specifies whether to use challenge response authen-
tication. The
argument to this keyword must be ``yes'' or ``no''.
The default
is ``yes''.
CheckHostIP
If this flag is set to ``yes'', ssh will additional-
ly check the
host IP address in the known_hosts file. This al-
lows ssh to de-
tect 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 ses-
sion in proto-
col version 1. Currently, ``blowfish'', ``3des'',
and ``des''
are supported. des is only supported in the ssh
client for in-
teroperability with legacy protocol 1 implementa-
tions that do not
support the 3des cipher. Its use is strongly dis-
couraged 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-separat-
command line to clear port forwardings set in con-
figuration
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 compres-
sion 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 ap-
plications. 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 ex-
iting. The argument must be an integer. This may
be useful in
scripts if the connection sometimes fails. The de-
fault is 1.
ConnectTimeout
Specifies the timeout (in seconds) used when con-
necting to the
ssh server, instead of using the default system TCP
timeout.
This value is used only when the target is down or
really un-
reachable, not when it refuses the connection.
DynamicForward
Specifies that a TCP/IP port on the local machine be
forwarded
over the secure channel, and the application proto-
col is then
used to determine where to connect to from the re-
mote machine.
The argument must be a port number. Currently the
SOCKS4 and
SOCKS5 protocols are supported, and ssh will act as
a SOCKS serv-
er. Multiple forwardings may be specified, and ad-
be placed in the non-hostspecific section. See ssh-
keysign(8)
for more information.
EscapeChar
Sets the escape character (default: `~'). The es-
cape character
can also be set on the command line. The argument
should be a
single character, `^' followed by a letter, or
``none'' to dis-
able the escape character entirely (making the con-
nection trans-
parent for binary data).
ForwardAgent
Specifies whether the connection to the authentica-
tion 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 identi-
ties loaded into
the agent.
ForwardX11
Specifies whether X11 connections will be automati-
cally redirect-
ed over the secure channel and DISPLAY set. The ar-
gument 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 moni-
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 con-
nect 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 ad-
dress, thus allowing remote hosts to connect to for-
warded 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 GSS-
API is allowed.
The default is ``no''. Note that this option ap-
plies 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 on-
ly.
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 on-
ly and is similar to RhostsRSAAuthentication.
HostKeyAlgorithms
Specifies the protocol version 2 host key algorithms
HostName
Specifies the real host name to log into. This can
be used to
specify nicknames or abbreviations for hosts. De-
fault is the
name given on the command line. Numeric IP address-
es 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/identi-
ty 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 authenti-
cation. The
file name may use the tilde syntax to refer to a us-
er's home di-
rectory. It is possible to have multiple identity
files speci-
fied in configuration files; all these identities
will be tried
in sequence.
IdentitiesOnly
Specifies that ssh should only use the authentica-
tion identity
files configured in the ssh_config files, even if
the ssh-agent
offers more identities. The argument to this key-
word must be
``yes'' or ``no''. This option is intented for sit-
uations where
ssh-agent offers many different identities. The de-
fault 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
is INFO.
DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3
each specify
higher levels of verbose output.
MACs Specifies the MAC (message authentication code) al-
gorithms in or-
der of preference. The MAC algorithm is used in
protocol version
2 for data integrity protection. Multiple algo-
rithms 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 ma-
chine on each of the machines and the user will get
many warnings
about changed host keys. However, this option dis-
ables host au-
thentication for localhost. The argument to this
keyword must be
``yes'' or ``no''. The default is to check the host
key for lo-
calhost.
NumberOfPasswordPrompts
Specifies the number of password prompts before giv-
ing up. The
argument to this keyword must be an integer. De-
fault is 3.
PasswordAuthentication
Specifies whether to use password authentication.
The argument
to this keyword must be ``yes'' or ``no''. The de-
fault 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 au-
thentication methods. This allows a client to pre-
fer one method
version 1 if
version 2 is not available.
ProxyCommand
Specifies the command to use to connect to the serv-
er. The com-
mand string extends to the end of the line, and is
executed with
/bin/sh. In the command string, `%h' will be sub-
stituted by the
host name to connect and `%p' by the port. The com-
mand can be
basically anything, and should read from its stan-
dard 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 us-
er). Setting the command to ``none'' disables this
option en-
tirely. Note that CheckHostIP is not available for
connects with
a proxy command.
PubkeyAuthentication
Specifies whether to try public key authentication.
The argument
to this keyword must be ``yes'' or ``no''. The de-
fault 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 forward-
ings may be
specified, and additional forwardings can be given
on the command
line. Only the superuser can forward privileged
ports.
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.
ServerAliveInterval
Sets a timeout interval in seconds after which if no
data has
been received from the server, ssh will send a mes-
sage 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 mes-
sages are being
sent, ssh will disconnect from the server, terminat-
ing the ses-
sion. It is important to note that the use of serv-
er alive mes-
sages is very different from TCPKeepAlive (below).
The server
alive messages are sent through the encrypted chan-
nel and there-
fore will not be spoofable. The TCP keepalive op-
tion enabled by
TCPKeepAlive is spoofable. The server alive mecha-
nism 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, Server-
AliveInterval
(above) is set to 15, and ServerAliveCountMax is
left at the de-
fault, if the server becomes unresponsive ssh will
disconnect af-
ter approximately 45 seconds.
SmartcardDevice
Specifies which smartcard device to use. The argu-
ment to this
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 af-
ter 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 no-
ticed. 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 mes-
sages), 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 out-
going connec-
tions. The argument must be ``yes'' or ``no''. The
default is
``no''. If set to ``yes'' ssh must be setuid root.
Specifies a file to use for the user host key data-
base 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 fin-
gerprint 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 Stric-
tHostKeyChecking
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. This
file does not usually contain any sensitive informa-
tion, but the
recommended permissions are read/write for the user,
and not ac-
cessible by others.
/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 configu-
ration file.
This file must be world-readable.
SEE ALSO
8
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