|
NAME | SYNOPSIS | DESCRIPTION | PATTERNS | TOKENS | FILES | SEE ALSO | AUTHORS | COLOPHON |
SSH_CONFIG(5) BSD File Formats Manual SSH_CONFIG(5)
ssh_config — OpenSSH SSH client configuration files
~/.ssh/config
/etc/ssh/ssh_config
ssh(1) obtains configuration data from the following sources in the
following 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 config‐
uration 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 usually the one
given on the command line (see the CanonicalizeHostname option for
exceptions).
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 file contains keyword-argument pairs, one per line. Lines starting
with ‘#’ and empty lines are interpreted as comments. Arguments may
optionally be enclosed in double quotes (") in order to represent argu‐
ments containing spaces. Configuration options may be separated by
whitespace or optional whitespace and exactly one ‘=’; the latter for‐
mat 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 or
Match keyword) to be only for those hosts that match one of the
patterns given after the keyword. If more than one pattern is
provided, they should be separated by whitespace. A single ‘*’
as a pattern can be used to provide global defaults for all
hosts. The host is usually the hostname argument given on the
command line (see the CanonicalizeHostname keyword for excep‐
tions).
A pattern entry may be negated by prefixing it with an exclama‐
tion mark (‘!’). If a negated entry is matched, then the Host
entry is ignored, regardless of whether any other patterns on
the line match. Negated matches are therefore useful to pro‐
vide exceptions for wildcard matches.
See PATTERNS for more information on patterns.
Match Restricts the following declarations (up to the next Host or
Match keyword) to be used only when the conditions following
the Match keyword are satisfied. Match conditions are speci‐
fied using one or more criteria or the single token all which
always matches. The available criteria keywords are:
canonical, exec, host, originalhost, user, and localuser. The
all criteria must appear alone or immediately after canonical.
Other criteria may be combined arbitrarily. All criteria but
all and canonical require an argument. Criteria may be negated
by prepending an exclamation mark (‘!’).
The canonical keyword matches only when the configuration file
is being re-parsed after hostname canonicalization (see the
CanonicalizeHostname option.) This may be useful to specify
conditions that work with canonical host names only. The exec
keyword executes the specified command under the user's shell.
If the command returns a zero exit status then the condition is
considered true. Commands containing whitespace characters
must be quoted. Arguments to exec accept the tokens described
in the TOKENS section.
The other keywords' criteria must be single entries or comma-
separated lists and may use the wildcard and negation operators
described in the PATTERNS section. The criteria for the host
keyword are matched against the target hostname, after any sub‐
stitution by the Hostname or CanonicalizeHostname options. The
originalhost keyword matches against the hostname as it was
specified on the command-line. The user keyword matches
against the target username on the remote host. The localuser
keyword matches against the name of the local user running
ssh(1) (this keyword may be useful in system-wide ssh_config
files).
AddKeysToAgent
Specifies whether keys should be automatically added to a run‐
ning ssh-agent(1). If this option is set to yes and a key is
loaded from a file, the key and its passphrase are added to the
agent with the default lifetime, as if by ssh-add(1). If this
option is set to ask, ssh(1) will require confirmation using
the SSH_ASKPASS program before adding a key (see ssh-add(1) for
details). If this option is set to confirm, each use of the
key must be confirmed, as if the -c option was specified to
ssh-add(1). If this option is set to no, no keys are added to
the agent. The argument must be yes, confirm, ask, or no (the
default).
AddressFamily
Specifies which address family to use when connecting. Valid
arguments are any (the default), 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).
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.
CanonicalDomains
When CanonicalizeHostname is enabled, this option specifies the
list of domain suffixes in which to search for the specified
destination host.
CanonicalizeFallbackLocal
Specifies whether to fail with an error when hostname canoni‐
calization fails. The default, yes, will attempt to look up
the unqualified hostname using the system resolver's search
rules. A value of no will cause ssh(1) to fail instantly if
CanonicalizeHostname is enabled and the target hostname cannot
be found in any of the domains specified by CanonicalDomains.
CanonicalizeHostname
Controls whether explicit hostname canonicalization is per‐
formed. The default, no, is not to perform any name rewriting
and let the system resolver handle all hostname lookups. If
set to yes then, for connections that do not use a
ProxyCommand, ssh(1) will attempt to canonicalize the hostname
specified on the command line using the CanonicalDomains suf‐
fixes and CanonicalizePermittedCNAMEs rules. If
CanonicalizeHostname is set to always, then canonicalization is
applied to proxied connections too.
If this option is enabled, then the configuration files are
processed again using the new target name to pick up any new
configuration in matching Host and Match stanzas.
CanonicalizeMaxDots
Specifies the maximum number of dot characters in a hostname
before canonicalization is disabled. The default, 1, allows a
single dot (i.e. hostname.subdomain).
CanonicalizePermittedCNAMEs
Specifies rules to determine whether CNAMEs should be followed
when canonicalizing hostnames. The rules consist of one or
more arguments of source_domain_list:target_domain_list, where
source_domain_list is a pattern-list of domains that may follow
CNAMEs in canonicalization, and target_domain_list is a pat‐
tern-list of domains that they may resolve to.
For example, "*.a.example.com:*.b.example.com,*.c.example.com"
will allow hostnames matching "*.a.example.com" to be canoni‐
calized to names in the "*.b.example.com" or "*.c.example.com"
domains.
CertificateFile
Specifies a file from which the user's certificate is read. A
corresponding private key must be provided separately in order
to use this certificate either from an IdentityFile directive
or -i flag to ssh(1), via ssh-agent(1), or via a
PKCS11Provider.
Arguments to CertificateFile may use the tilde syntax to refer
to a user's home directory or the tokens described in the
TOKENS section.
It is possible to have multiple certificate files specified in
configuration files; these certificates will be tried in
sequence. Multiple CertificateFile directives will add to the
list of certificates used for authentication.
ChallengeResponseAuthentication
Specifies whether to use challenge-response authentication.
The argument to this keyword must be yes (the default) or no.
CheckHostIP
If set to yes (the default), ssh(1) will additionally check the
host IP address in the known_hosts file. This allows it to
detect if a host key changed due to DNS spoofing and will add
addresses of destination hosts to ~/.ssh/known_hosts in the
process, regardless of the setting of StrictHostKeyChecking.
If the option is set to no, the check will not be executed.
Cipher Specifies the cipher to use for encrypting the session in pro‐
tocol version 1. Currently, blowfish, 3des (the default), and
des are supported, though des is only supported in the ssh(1)
client for interoperability with legacy protocol 1 implementa‐
tions; its use is strongly discouraged due to cryptographic
weaknesses.
Ciphers
Specifies the ciphers allowed for protocol version 2 in order
of preference. Multiple ciphers must be comma-separated. If
the specified value begins with a ‘+’ character, then the spec‐
ified ciphers will be appended to the default set instead of
replacing them. If the specified value begins with a ‘-’ char‐
acter, then the specified ciphers (including wildcards) will be
removed from the default set instead of replacing them.
The supported ciphers are:
3des-cbc
aes128-cbc
aes192-cbc
aes256-cbc
aes128-ctr
aes192-ctr
aes256-ctr
aes128-gcm@openssh.com
aes256-gcm@openssh.com
arcfour
arcfour128
arcfour256
blowfish-cbc
cast128-cbc
chacha20-poly1305@openssh.com
The default is:
chacha20-poly1305@openssh.com,
aes128-ctr,aes192-ctr,aes256-ctr,
aes128-gcm@openssh.com,aes256-gcm@openssh.com,
aes128-cbc,aes192-cbc,aes256-cbc
The list of available ciphers may also be obtained using "ssh
-Q cipher".
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).
Compression
Specifies whether to use compression. The argument must be yes
or no (the default).
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(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(1) to listen for control
connections, but require confirmation using ssh-askpass(1). If
the ControlPath cannot be opened, ssh(1) will continue without
connecting 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 possible 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
sharing as described in the ControlMaster section above or the
string none to disable connection sharing. Arguments to
ControlPath may use the tilde syntax to refer to a user's home
directory or the tokens described in the TOKENS section. It is
recommended that any ControlPath used for opportunistic connec‐
tion sharing include at least %h, %p, and %r (or alternatively
%C) and be placed in a directory that is not writable by other
users. This ensures that shared connections are uniquely iden‐
tified.
ControlPersist
When used in conjunction with ControlMaster, specifies that the
master connection should remain open in the background (waiting
for future client connections) after the initial client connec‐
tion has been closed. If set to no, then the master connection
will not be placed into the background, and will close as soon
as the initial client connection is closed. If set to yes or
0, then the master connection will remain in the background
indefinitely (until killed or closed via a mechanism such as
the "ssh -O exit"). If set to a time in seconds, or a time in
any of the formats documented in sshd_config(5), then the back‐
grounded master connection will automatically terminate after
it has remained idle (with no client connections) for the spec‐
ified time.
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. By
default, the local port is bound in accordance 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 listening port be
bound for local use only, while an empty address or ‘*’ indi‐
cates 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 com‐
mand 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). 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 transpar‐
ent for binary data).
ExitOnForwardFailure
Specifies whether ssh(1) should terminate the connection if it
cannot set up all requested dynamic, tunnel, local, and remote
port forwardings, (e.g. if either end is unable to bind and
listen on a specified port). Note that ExitOnForwardFailure
does not apply to connections made over port forwardings and
will not, for example, cause ssh(1) to exit if TCP connections
to the ultimate forwarding destination fail. The argument must
be yes or no (the default).
FingerprintHash
Specifies the hash algorithm used when displaying key finger‐
prints. Valid options are: md5 and sha256 (the default).
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).
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 opera‐
tions 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).
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.
ForwardX11Timeout
Specify a timeout for untrusted X11 forwarding using the format
described in the TIME FORMATS section of sshd_config(5). X11
connections received by ssh(1) after this time will be refused.
The default is to disable untrusted X11 forwarding after twenty
minutes has elapsed.
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 (the default), remote X11 clients
will be considered untrusted and prevented from stealing or
tampering with data belonging to trusted X11 clients. Further‐
more, 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.
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 forward‐
ings 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 for‐
warded ports. The argument must be yes or no (the default).
GlobalKnownHostsFile
Specifies one or more files to use for the global host key
database, separated by whitespace. The default is
/etc/ssh/ssh_known_hosts, /etc/ssh/ssh_known_hosts2.
GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is
allowed. The default is no.
GSSAPIDelegateCredentials
Forward (delegate) credentials to the server. The default is
no.
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
identifying information should the file's contents be dis‐
closed. The default is no. Note that existing names and
addresses in known hosts files will not be converted automati‐
cally, but may be manually hashed using ssh-keygen(1).
HostbasedAuthentication
Specifies whether to try rhosts based authentication with pub‐
lic key authentication. The argument must be yes or no (the
default).
HostbasedKeyTypes
Specifies the key types that will be used for hostbased authen‐
tication as a comma-separated pattern list. Alternately if the
specified value begins with a ‘+’ character, then the specified
key types will be appended to the default set instead of
replacing them. If the specified value begins with a ‘-’ char‐
acter, then the specified key types (including wildcards) will
be removed from the default set instead of replacing them. The
default for this option is:
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa
The -Q option of ssh(1) may be used to list supported key
types.
HostKeyAlgorithms
Specifies the host key algorithms that the client wants to use
in order of preference. Alternately if the specified value
begins with a ‘+’ character, then the specified key types will
be appended to the default set instead of replacing them. If
the specified value begins with a ‘-’ character, then the spec‐
ified key types (including wildcards) will be removed from the
default set instead of replacing them. The default for this
option is:
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa
If hostkeys are known for the destination host then this
default is modified to prefer their algorithms.
The list of available key types may also be obtained using "ssh
-Q key".
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 con‐
nections 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. Arguments to
HostName accept the tokens described in the TOKENS section.
Numeric IP addresses are also permitted (both on the command
line and in HostName specifications). The default is the name
given on the command line.
IdentitiesOnly
Specifies that ssh(1) should only use the authentication iden‐
tity and certificate files explicitly configured in the
ssh_config files or passed on the ssh(1) command-line, even if
ssh-agent(1) or a PKCS11Provider offers more identities. The
argument to this keyword must be yes or no (the default). This
option is intended for situations where ssh-agent offers many
different identities.
IdentityAgent
Specifies the UNIX-domain socket used to communicate with the
authentication agent.
This option overrides the SSH_AUTH_SOCK environment variable
and can be used to select a specific agent. Setting the socket
name to none disables the use of an authentication agent. If
the string "SSH_AUTH_SOCK" is specified, the location of the
socket will be read from the SSH_AUTH_SOCK environment vari‐
able.
Arguments to IdentityAgent may use the tilde syntax to refer to
a user's home directory or the tokens described in the TOKENS
section.
IdentityFile
Specifies a file from which the user's DSA, ECDSA, Ed25519 or
RSA authentication identity is read. The default is
~/.ssh/identity for protocol version 1, and ~/.ssh/id_dsa,
~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 and ~/.ssh/id_rsa for proto‐
col version 2. Additionally, any identities represented by the
authentication agent will be used for authentication unless
IdentitiesOnly is set. If no certificates have been explicitly
specified by CertificateFile, ssh(1) will try to load certifi‐
cate information from the filename obtained by appending
-cert.pub to the path of a specified IdentityFile.
Arguments to IdentityFile may use the tilde syntax to refer to
a user's home directory or the tokens described in the TOKENS
section.
It is possible to have multiple identity files specified in
configuration files; all these identities will be tried in
sequence. Multiple IdentityFile directives will add to the
list of identities tried (this behaviour differs from that of
other configuration directives).
IdentityFile may be used in conjunction with IdentitiesOnly to
select which identities in an agent are offered during authen‐
tication. IdentityFile may also be used in conjunction with
CertificateFile in order to provide any certificate also needed
for authentication with the identity.
IgnoreUnknown
Specifies a pattern-list of unknown options to be ignored if
they are encountered in configuration parsing. This may be
used to suppress errors if ssh_config contains options that are
unrecognised by ssh(1). It is recommended that IgnoreUnknown
be listed early in the configuration file as it will not be
applied to unknown options that appear before it.
Include
Include the specified configuration file(s). Multiple path‐
names may be specified and each pathname may contain glob(3)
wildcards and, for user configurations, shell-like ‘~’ refer‐
ences to user home directories. Files without absolute paths
are assumed to be in ~/.ssh if included in a user configuration
file or /etc/ssh if included from the system configuration
file. Include directive may appear inside a Match or Host
block to perform conditional inclusion.
IPQoS Specifies the IPv4 type-of-service or DSCP class for connec‐
tions. Accepted values are af11, af12, af13, af21, af22, af23,
af31, af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3, cs4,
cs5, cs6, cs7, ef, lowdelay, throughput, reliability, or a
numeric value. This option may take one or two arguments, sep‐
arated by whitespace. If one argument is specified, it is used
as the packet class unconditionally. If two values are speci‐
fied, the first is automatically selected for interactive ses‐
sions and the second for non-interactive sessions. The default
is lowdelay for interactive sessions and throughput for non-
interactive sessions.
KbdInteractiveAuthentication
Specifies whether to use keyboard-interactive authentication.
The argument to this keyword must be yes (the default) or no.
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. 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.
KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms. Multi‐
ple algorithms must be comma-separated. Alternately if the
specified value begins with a ‘+’ character, then the specified
methods will be appended to the default set instead of replac‐
ing them. If the specified value begins with a ‘-’ character,
then the specified methods (including wildcards) will be
removed from the default set instead of replacing them. The
default is:
curve25519-sha256,curve25519-sha256@libssh.org,
ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
diffie-hellman-group-exchange-sha256,
diffie-hellman-group-exchange-sha1,
diffie-hellman-group14-sha1
The list of available key exchange algorithms may also be
obtained using "ssh -Q kex".
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 the user's shell.
Arguments to LocalCommand accept the tokens described in the
TOKENS section.
The command is run synchronously and does not have access to
the session of the ssh(1) that spawned it. It should not be
used for interactive commands.
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.
Multiple forwardings may be specified, and additional forward‐
ings can be given on the command line. Only the superuser can
forward privileged ports. By default, the local port is bound
in accordance 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 listening 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(1). 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 for data
integrity protection. Multiple algorithms must be comma-sepa‐
rated. If the specified value begins with a ‘+’ character,
then the specified algorithms will be appended to the default
set instead of replacing them. If the specified value begins
with a ‘-’ character, then the specified algorithms (including
wildcards) will be removed from the default set instead of
replacing them.
The algorithms that contain "-etm" calculate the MAC after
encryption (encrypt-then-mac). These are considered safer and
their use recommended.
The default is:
umac-64-etm@openssh.com,umac-128-etm@openssh.com,
hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
hmac-sha1-etm@openssh.com,
umac-64@openssh.com,umac-128@openssh.com,
hmac-sha2-256,hmac-sha2-512,hmac-sha1
The list of available MAC algorithms may also be obtained using
"ssh -Q mac".
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
warnings about changed host keys. However, this option dis‐
ables host authentication for localhost. The argument to this
keyword must be yes or no (the default).
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 (the default) or no.
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).
PKCS11Provider
Specifies which PKCS#11 provider to use. The argument to this
keyword is the PKCS#11 shared library ssh(1) should use to com‐
municate with a PKCS#11 token providing the user's private RSA
key.
Port Specifies the port number to connect on the remote host. The
default is 22.
PreferredAuthentications
Specifies the order in which the client should try authentica‐
tion methods. This allows a client to prefer one method (e.g.
keyboard-interactive) over another method (e.g. password). The
default 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. When this option is set to 2,1
ssh will try version 2 and fall back to version 1 if version 2
is not available. The default is version 2. Protocol 1 suf‐
fers from a number of cryptographic weaknesses and should not
be used. It is only offered to support legacy devices.
ProxyCommand
Specifies the command to use to connect to the server. The
command string extends to the end of the line, and is executed
using the user's shell ‘exec’ directive to avoid a lingering
shell process.
Arguments to ProxyCommand accept the tokens described in the
TOKENS section. 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 man‐
agement will be done using the HostName of the host being con‐
nected (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 con‐
nect via an HTTP proxy at 192.0.2.0:
ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
ProxyJump
Specifies one or more jump proxies as [user@]host[:port]. Mul‐
tiple proxies may be separated by comma characters and will be
visited sequentially. Setting this option will cause ssh(1) to
connect to the target host by first making a ssh(1) connection
to the specified ProxyJump host and then establishing a TCP
forwarding to the ultimate target from there.
Note that this option will compete with the ProxyCommand option
- whichever is specified first will prevent later instances of
the other from taking effect.
ProxyUseFdpass
Specifies that ProxyCommand will pass a connected file descrip‐
tor back to ssh(1) instead of continuing to execute and pass
data. The default is no.
PubkeyAcceptedKeyTypes
Specifies the key types that will be used for public key
authentication as a comma-separated pattern list. Alternately
if the specified value begins with a ‘+’ character, then the
key types after it will be appended to the default instead of
replacing it. If the specified value begins with a ‘-’ charac‐
ter, then the specified key types (including wildcards) will be
removed from the default set instead of replacing them. The
default for this option is:
ecdsa-sha2-nistp256-cert-v01@openssh.com,
ecdsa-sha2-nistp384-cert-v01@openssh.com,
ecdsa-sha2-nistp521-cert-v01@openssh.com,
ssh-ed25519-cert-v01@openssh.com,
ssh-rsa-cert-v01@openssh.com,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
ssh-ed25519,ssh-rsa
The list of available key types may also be obtained using "ssh
-Q key".
PubkeyAuthentication
Specifies whether to try public key authentication. The argu‐
ment to this keyword must be yes (the default) or no.
RekeyLimit
Specifies the maximum amount of data that may be transmitted
before the session key is renegotiated, optionally followed a
maximum amount of time that may pass before the session key is
renegotiated. The first argument is specified in bytes and may
have a suffix of ‘K’, ‘M’, or ‘G’ to indicate Kilobytes,
Megabytes, or Gigabytes, respectively. The default is between
‘1G’ and ‘4G’, depending on the cipher. The optional second
value is specified in seconds and may use any of the units doc‐
umented in the TIME FORMATS section of sshd_config(5). The
default value for RekeyLimit is default none, which means that
rekeying is performed after the cipher's default amount of data
has been sent or received and no time based rekeying is done.
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.
Multiple forwardings may be specified, and additional forward‐
ings can be given on the command line. Privileged ports can be
forwarded only when logging in as root on the remote machine.
If the port argument is 0, the listen port will be dynamically
allocated on the server and reported to the client at run time.
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
interfaces. Specifying a remote bind_address will only succeed
if the server's GatewayPorts option is enabled (see
sshd_config(5)).
RequestTTY
Specifies whether to request a pseudo-tty for the session. The
argument may be one of: no (never request a TTY), yes (always
request a TTY when standard input is a TTY), force (always
request a TTY) or auto (request a TTY when opening a login ses‐
sion). This option mirrors the -t and -T flags for ssh(1).
RevokedHostKeys
Specifies revoked host public keys. Keys listed in this file
will be refused for host authentication. Note that if this
file does not exist or is not readable, then host authentica‐
tion will be refused for all hosts. Keys may be specified as a
text file, listing one public key per line, or as an OpenSSH
Key Revocation List (KRL) as generated by ssh-keygen(1). For
more information on KRLs, see the KEY REVOCATION LISTS section
in ssh-keygen(1).
RhostsRSAAuthentication
Specifies whether to try rhosts based authentication with RSA
host authentication. The argument must be yes or no (the
default). 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 (the default) or no. RSA authentica‐
tion will only be attempted if the identity file exists, or an
authentication agent is running. 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. The server must also support it, and the
server must be configured to accept these environment vari‐
ables. Note that the TERM environment variable is always sent
whenever a pseudo-terminal is requested as it is required by
the protocol. Refer to AcceptEnv in sshd_config(5) for how to
configure the server. Variables are specified by name, which
may contain wildcard characters. Multiple environment vari‐
ables may be separated by whitespace or spread across multiple
SendEnv directives. The default 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 mes‐
sages are being sent, ssh will disconnect from the server, ter‐
minating 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
(see below) is set to 15 and ServerAliveCountMax is left at the
default, if the server becomes unresponsive, ssh will discon‐
nect after approximately 45 seconds.
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.
StreamLocalBindMask
Sets the octal file creation mode mask (umask) used when creat‐
ing a Unix-domain socket file for local or remote port forward‐
ing. This option is only used for port forwarding to a Unix-
domain socket file.
The default value is 0177, which creates a Unix-domain socket
file that is readable and writable only by the owner. Note
that not all operating systems honor the file mode on Unix-
domain socket files.
StreamLocalBindUnlink
Specifies whether to remove an existing Unix-domain socket file
for local or remote port forwarding before creating a new one.
If the socket file already exists and StreamLocalBindUnlink is
not enabled, ssh will be unable to forward the port to the
Unix-domain socket file. This option is only used for port
forwarding to a Unix-domain socket file.
The argument must be yes or no (the default).
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 maxi‐
mum protection against trojan horse attacks, though it can be
annoying when the /etc/ssh/ssh_known_hosts file is poorly main‐
tained or when 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
(the default), 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.
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. How‐
ever, 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 tun(4) device forwarding between the client and the
server. The argument must be yes, point-to-point (layer 3),
ethernet (layer 2), or no (the default). Specifying yes
requests the default tunnel mode, which is point-to-point.
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.
UpdateHostKeys
Specifies whether ssh(1) should accept notifications of addi‐
tional hostkeys from the server sent after authentication has
completed and add them to UserKnownHostsFile. The argument
must be yes, no (the default) or ask. Enabling this option
allows learning alternate hostkeys for a server and supports
graceful key rotation by allowing a server to send replacement
public keys before old ones are removed. Additional hostkeys
are only accepted if the key used to authenticate the host was
already trusted or explicitly accepted by the user. If
UpdateHostKeys is set to ask, then the user is asked to confirm
the modifications to the known_hosts file. Confirmation is
currently incompatible with ControlPersist, and will be dis‐
abled if it is enabled.
Presently, only sshd(8) from OpenSSH 6.8 and greater support
the "hostkeys@openssh.com" protocol extension used to inform
the client of all the server's hostkeys.
UsePrivilegedPort
Specifies whether to use a privileged port for outgoing connec‐
tions. The argument must be yes or no (the default). 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
different user name is used on different machines. This saves
the trouble of having to remember to give the user name on the
command line.
UserKnownHostsFile
Specifies one or more files to use for the user host key data‐
base, separated by whitespace. The default is
~/.ssh/known_hosts, ~/.ssh/known_hosts2.
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 default is no.
See also VERIFYING HOST KEYS in ssh(1).
VisualHostKey
If this flag is set to yes, an ASCII art representation of the
remote host key fingerprint is printed in addition to the fin‐
gerprint string at login and for unknown host keys. If this
flag is set to no (the default), no fingerprint strings are
printed at login and only the fingerprint string will be
printed for unknown host keys.
XAuthLocation
Specifies the full pathname of the xauth(1) program. The
default is /usr/X11R6/bin/xauth.
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 dec‐
larations for any host in the ".co.uk" set of domains, the following
pattern could be used:
Host *.co.uk
The following pattern would match any host in the 192.168.0.[0-9] net‐
work 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
organization except from the "dialup" pool, the following entry (in
authorized_keys) could be used:
from="!*.dialup.example.com,*.example.com"
Arguments to some keywords can make use of tokens, which are expanded
at runtime:
%% A literal ‘%’.
%C Shorthand for %l%h%p%r.
%d Local user's home directory.
%h The remote hostname.
%i The local user ID.
%L The local hostname.
%l The local hostname, including the domain name.
%n The original remote hostname, as given on the command line.
%p The remote port.
%r The remote username.
%u The local username.
Match exec accepts the tokens %%, %h, %L, %l, %n, %p, %r, and %u.
CertificateFile accepts the tokens %%, %d, %h, %l, %r, and %u.
ControlPath accepts the tokens %%, %C, %h, %i, %L, %l, %n, %p, %r, and
%u.
HostName accepts the tokens %% and %h.
IdentityAgent and IdentityFile accept the tokens %%, %d, %h, %l, %r,
and %u.
LocalCommand accepts the tokens %%, %C, %d, %h, %l, %n, %p, %r, and %u.
ProxyCommand accepts the tokens %%, %h, %p, and %r.
~/.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
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.
ssh(1)
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
protocol versions 1.5 and 2.0.
This page is part of the openssh (Portable OpenSSH) project. Informa‐
tion about the project can be found at
http://www.openssh.com/portable.html. If you have a bug report for
this manual page, see http://www.openssh.com/report.html. This page
was obtained from the tarball openssh-7.5p1.tar.gz fetched from
http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/ on 2017-07-05.
If you discover any rendering problems in this HTML version of the
page, or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the information in
this COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
BSD February 27, 2017 BSD