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Vendor import of OpenSSH 4.6p1 for posterity's sake
2008-07-23 09:15:38 +00:00
SSH_CONFIG(5) OpenBSD Programmer's Manual SSH_CONFIG(5)
NAME
ssh_config - OpenSSH SSH client configuration files
SYNOPSIS
~/.ssh/config
/etc/ssh/ssh_config
DESCRIPTION
ssh(1) obtains configuration data from the following sources in the fol-
lowing order:
1. command-line options
2. user's 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 whitespace when speci-
fying configuration options using the ssh, scp, and sftp -o option. Ar-
guments may optionally be enclosed in double quotes (") in order to rep-
resent arguments containing spaces.
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. 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).
See PATTERNS for more information on patterns.
AddressFamily
Specifies which address family to use when connecting. Valid ar-
guments 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
Use the specified address on the local machine as the source ad-
dress 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 proto-
col 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 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(1) command line to clear port forwardings set in configura-
tion 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 ex-
iting. 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 un-
reachable, 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 connec-
tions on a control socket specified using the ControlPath argu-
ment. 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 con-
nections, 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 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, `%l' will
be substituted by the local host name, `%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 at least %h, %p, and %r.
This ensures that shared connections 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 lo-
cal 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(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 dis-
able the escape character entirely (making the connection trans-
parent for binary data).
ExitOnForwardFailure
Specifies whether ssh(1) should terminate the connection if it
cannot set up all requested dynamic, local, and remote port for-
wardings. 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 redirect-
ed 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'', remote X11 clients will have
full access to the original X11 display.
If this option is set to ``no'', remote X11 clients will be con-
sidered 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 min-
utes. 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.
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 spec-
ify 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 on-
ly.
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 iden-
tifying 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).
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 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. The 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(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 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 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 con-
figuration files; all these identities will be tried in sequence.
KbdInteractiveDevices
Specifies the list of methods to use in keyboard-interactive au-
thentication. 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''.
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 di-
rective 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 ad-
dress. 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 in-
terfaces.
LogLevel
Gives the verbosity level that is used when logging messages from
ssh(1). 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 or-
der 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,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 disables 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 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 us-
ing 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. The de-
fault is 22.
PreferredAuthentications
Specifies the order in which the client should try protocol 2 au-
thentication 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: ``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 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 us-
er). Setting the command to ``none'' disables this option en-
tirely. 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 be-
fore 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 ap-
plies 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
server's 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(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 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 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 char-
acters. Multiple environment variables may be separated by
whitespace or spread across multiple SendEnv directives. The de-
fault 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 be-
ing 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 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 applies to protocol
version 2 only.
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. This option applies to protocol version 2 only.
SmartcardDevice
Specifies which smartcard device to use. The argument to this
keyword is the device ssh(1) should use to communicate with a
smartcard used for storing the user's private RSA key. By de-
fault, no device is specified and smartcard support is not acti-
vated.
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 con-
nect to hosts whose host key has changed. This provides maximum
protection against trojan horse attacks, though it can be annoy-
ing when the /etc/ssh/ssh_known_hosts file is poorly maintained
or when connections to new hosts are frequently made. This op-
tion 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''.
Tunnel Request tun(4) device forwarding between the client and the serv-
er. 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 de-
fault 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''.
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(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 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.
See also VERIFYING HOST KEYS in ssh(1).
XAuthLocation
Specifies the full pathname of the xauth(1) program. The default
is /usr/X11R6/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 decla-
rations for any host in the ``.co.uk'' set of domains, the following pat-
tern 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 au-
thorized_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. Be-
cause of the potential for abuse, this file must have strict per-
missions: read/write for the user, and not accessible 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 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.
OpenBSD 4.1 September 25, 1999 10