SSH_CONFIG(5) BSD File Formats Manual SSH_CONFIG(5)NAMEssh_config — OpenSSH SSH client configuration files
DESCRIPTIONssh(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 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-spe‐
cific declarations should be given near the beginning of the file, and
general defaults at the end.
Note that the Debian openssh-client package sets several options as stan‐
dard in /etc/ssh/ssh_config which are not the default in ssh(1):
· SendEnv LANG LC_*
· HashKnownHosts yes
· GSSAPIAuthentication yes
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 format
is useful to avoid the need to quote whitespace when specifying configu‐
ration 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 provide
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 specified
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 con‐
ditions that work with canonical host names only. The exec key‐
word executes the specified command under the user's shell. If
the command returns a zero exit status then the condition is con‐
sidered 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-sep‐
arated 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 spec‐
ified 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 running
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. In
addition, the ServerAliveInterval option will be set to 300 sec‐
onds by default (Debian-specific). This option is useful in
scripts and other batch jobs where no user is present to supply
the password, and where it is desirable to detect a broken net‐
work swiftly. 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 des‐
tination host.
CanonicalizeFallbackLocal
Specifies whether to fail with an error when hostname canonical‐
ization 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 performed.
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 suffixes and
CanonicalizePermittedCNAMEs rules. If CanonicalizeHostname is
set to always, then canonicalization is applied to proxied con‐
nections too.
If this option is enabled, then the configuration files are pro‐
cessed again using the new target name to pick up any new config‐
uration 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 pattern-
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 canonical‐
ized 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.
Ciphers
Specifies the ciphers allowed and their order of preference.
Multiple ciphers must be comma-separated. If the specified value
begins with a ‘+’ character, then the specified ciphers will be
appended to the default set instead of replacing them. If the
specified value begins with a ‘-’ character, 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
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).
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 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 try to reuse the master instance's network connec‐
tion rather than initiating new ones, but will fall back to con‐
necting normally if the control socket does not exist, or is not
listening.
Setting this to ask will cause ssh(1) to listen for control con‐
nections, but require confirmation using ssh-askpass(1). If the
ControlPath cannot be opened, ssh(1) will continue without con‐
necting 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. 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 connection 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 identified.
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 indefi‐
nitely (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 backgrounded mas‐
ter connection will automatically terminate after it has remained
idle (with no client connections) for the specified 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 set‐
ting. 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.
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). This option should be placed in the
non-hostspecific section. See ssh-keysign(8) for more informa‐
tion.
EscapeChar
Sets the escape character (default: ‘~’). The escape character
can also be set on the command line. The argument should be a
single character, ‘^’ followed by a letter, or none to disable
the escape character entirely (making the connection transparent
for binary data).
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 lis‐
ten 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 ulti‐
mate 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 operations on the keys
that enable them to authenticate using the identities loaded into
the agent.
ForwardX11
Specifies whether X11 connections will be automatically redi‐
rected over the secure channel and DISPLAY set. The argument
must be yes or no (the default).
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.
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, (the Debian-specific default),
remote X11 clients will have full access to the original X11 dis‐
play.
If this option is set to no (the upstream default), remote X11
clients will be considered untrusted and prevented from stealing
or tampering with data belonging to trusted X11 clients. Fur‐
thermore, 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 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).
GlobalKnownHostsFile
Specifies one or more files to use for the global host key data‐
base, 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.
GSSAPIKeyExchange
Specifies whether key exchange based on GSSAPI may be used. When
using GSSAPI key exchange the server need not have a host key.
The default is no.
GSSAPIClientIdentity
If set, specifies the GSSAPI client identity that ssh should use
when connecting to the server. The default is unset, which means
that the default identity will be used.
GSSAPIServerIdentity
If set, specifies the GSSAPI server identity that ssh should
expect when connecting to the server. The default is unset, which
means that the expected GSSAPI server identity will be determined
from the target hostname.
GSSAPIDelegateCredentials
Forward (delegate) credentials to the server. The default is no.
GSSAPIRenewalForcesRekey
If set to yes then renewal of the client's GSSAPI credentials
will force the rekeying of the ssh connection. With a compatible
server, this can delegate the renewed credentials to a session on
the server. The default is no.
GSSAPITrustDns
Set to yes to indicate that the DNS is trusted to securely canon‐
icalize the name of the host being connected to. If no, the host‐
name entered on the command line will be passed untouched to the
GSSAPI library. 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 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 manu‐
ally hashed using ssh-keygen(1). Use of this option may break
facilities such as tab-completion that rely on being able to read
unhashed host names from ~/.ssh/known_hosts.
HostbasedAuthentication
Specifies whether to try rhosts based authentication with public
key authentication. The argument must be yes or no (the
default).
HostbasedKeyTypes
Specifies the key types that will be used for hostbased authenti‐
cation 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 replac‐
ing them. If the specified value begins with a ‘-’ character,
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 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
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 data‐
base files and when validating host certificates. This option is
useful for tunneling SSH connections or for multiple servers run‐
ning 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 identity
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 variable.
Arguments to IdentityAgent may use the tilde syntax to refer to a
user's home directory or the tokens described in the TOKENS sec‐
tion.
IdentityFile
Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA
authentication identity is read. The default is ~/.ssh/id_dsa,
~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 and ~/.ssh/id_rsa. Addition‐
ally, 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 certificate 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 sec‐
tion.
It is possible to have multiple identity files specified in con‐
figuration files; all these identities will be tried in sequence.
Multiple IdentityFile directives will add to the list of identi‐
ties tried (this behaviour differs from that of other configura‐
tion directives).
IdentityFile may be used in conjunction with IdentitiesOnly to
select which identities in an agent are offered during authenti‐
cation. 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 unrecog‐
nised 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 pathnames
may be specified and each pathname may contain glob(3) wildcards
and, for user configurations, shell-like ‘~’ references 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 connections.
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, a numeric value, or
none to use the operating system default. This option may take
one or two arguments, separated by whitespace. If one argument
is specified, it is used as the packet class unconditionally. If
two values are specified, the first is automatically selected for
interactive sessions 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. Multiple
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 replacing 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 forwardings can be
given on the command line. Only the superuser can forward privi‐
leged 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, 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
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 wild‐
cards) 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 warn‐
ings about changed host keys. However, this option disables host
authentication for localhost. The argument to this keyword must
be yes or no (the default).
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 commu‐
nicate 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 authentication
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
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 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 manage‐
ment 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
ProxyJump
Specifies one or more jump proxies as [user@]host[:port]. Multi‐
ple proxies may be separated by comma characters and will be vis‐
ited sequentially. Setting this option will cause ssh(1) to con‐
nect to the target host by first making a ssh(1) connection to
the specified ProxyJump host and then establishing a TCP forward‐
ing 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 descriptor
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 authen‐
tication 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 ‘-’ character, 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 argument
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 docu‐
mented 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.
RemoteCommand
Specifies a command to execute on the remote 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 RemoteCommand accept the tokens described in the
TOKENS section.
RemoteForward
Specifies that a TCP port on the remote machine be forwarded over
the secure channel. The remote port may either be fowarded to a
specified host and port from the local machine, or may act as a
SOCKS 4/5 proxy that allows a remote client to connect to arbi‐
trary destinations from the local machine. The first argument
must be [bind_address:]port If forwarding to a specific destina‐
tion then the second argument must be host:hostport, otherwise if
no destination argument is specified then the remote forwarding
will be established as a SOCKS proxy.
IPv6 addresses can be specified by enclosing addresses in square
brackets. Multiple forwardings may be specified, and additional
forwardings 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 inter‐
faces. 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 authentication 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).
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 variables. 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 wild‐
card characters. Multiple environment variables 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 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 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.
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, or 300 if the BatchMode option is set (Debian-spe‐
cific). ProtocolKeepAlives and SetupTimeOut are Debian-specific
compatibility aliases for this option.
StreamLocalBindMask
Sets the octal file creation mode mask (umask) used when creating
a Unix-domain socket file for local or remote port forwarding.
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 connect
to hosts whose host key has changed. This provides maximum pro‐
tection against trojan horse attacks, though it can be annoying
when the /etc/ssh/ssh_known_hosts file is poorly maintained 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 “accept-new” then ssh will automatically
add new host keys to the user known hosts files, but will not
permit connections to hosts with changed host keys. If this flag
is set to “no” or “off”, ssh will automatically add new host keys
to the user known hosts files and allow connections to hosts with
changed hostkeys to proceed, subject to some restrictions. 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 con‐
firmed 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.
SyslogFacility
Gives the facility code that is used when logging messages from
ssh(1). The possible values are: DAEMON, USER, AUTH, LOCAL0,
LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The
default is USER.
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. This
option only uses TCP keepalives (as opposed to using ssh level
keepalives), so takes a long time to notice when the connection
dies. As such, you probably want the ServerAliveInterval option
as well. However, 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 disabled 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.
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 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 finger‐
print 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/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
organization except from the "dialup" pool, the following entry (in
authorized_keys) could be used:
from="!*.dialup.example.com,*.example.com"
TOKENS
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.
RemoteCommand accepts the tokens %%, %C, %d, %h, %l, %n, %p, %r, and %u.
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.
Because of the potential for abuse, this file must have strict
permissions: read/write for the user, and not accessible by oth‐
ers. It may be group-writable provided that the group in ques‐
tion contains only the user.
/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 ALSOssh(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.
BSD April 19, 2024 BSD
