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Name
ssh_config - OpenSSH SSH client configuration files
Synopsis
~/.ssh/config
/etc/ssh/ssh_config
Description
ssh(1) obtains configuration data from the following sources in the following order:
# $OpenBSD: sshdconfig,v 1.89 2013/02/06 00:20:42 dtucker Exp $ # This is the sshd server system-wide configuration file. See # sshdconfig(5) for more information. # This sshd was compiled with PATH=/usr/bin:/bin:/usr/sbin:/sbin # The strategy used for options in the default sshdconfig shipped with # OpenSSH is to specify options with their default value where # possible, but leave them. The /etc/ssh/sshdconfig file is the system-wide configuration file for OpenSSH which allows you to set options that modify the operation of the daemon. This file contains keyword-value pairs, one per line, with keywords being case insensitive. Sshd-mina, sshd-netty - replacements for the default NIO2 connector used to establish and manage network connections using MINA and/or Netty libraries respectively. Sshd-sftp - contains the server side SFTP subsystem and the SFTP client code. Sshd-spring-sftp - contains a Spring Integration compatible SFTP adapter.
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 configuration files contain sections separated by 'Host' specifications, and that sectionis only applied for hosts that match one of the patterns given in the specification. The matched host name is the one given on the command line.
Since the first obtained value for each parameter is used, more host-specific declarations should be given near the beginning of the file, and generaldefaults at the end.
The configuration file has the following format:
Empty lines and lines starting with '#' are comments. Otherwise a line is of the format 'keyword arguments'. Configuration options may be separated bywhitespace or optional whitespace and exactly one '='; the latter format is useful to avoid the need to quote whitespace when specifying configuration optionsusing the ssh, scp, and sftp -o option. Arguments may optionally be enclosed in double quotes (') in order to represent argumentscontaining spaces.
The possible keywords and their meanings are as follows (note that keywords are case-insensitive and arguments are case-sensitive):
Host' Restricts the following declarations (up to the next Host keyword) to be only for those hosts that match one of the patterns given afterthe 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 forall hosts. The host is the hostname argument given on the command line (i.e. the name is not converted to a canonicalized host name before matching).
See PATTERNS for more information on patterns.
AddressFamily
Specifies which address family to use when connecting. Valid arguments are 'any', 'inet' (use IPv4 only), or 'inet6' (use IPv6 only).
BatchMode
If set to 'yes', passphrase/password querying will be disabled. This option is useful in scripts and other batch jobs where no user is present to supply thepassword. The argument must be 'yes' or 'no'. The default is 'no'.
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 thisoption does not work if UsePrivilegedPort is set to 'yes'.
ChallengeResponseAuthentication
Specifies whether to use challenge-response authentication. The argument to this keyword must be 'yes' or 'no'. The default is 'yes'.
CheckHostIP
If this flag is set to 'yes', ssh(1) will additionally check the host IP address in the known_hosts file. This allows ssh to detect if a host keychanged due to DNS spoofing. If the option is set to 'no', the check will not be executed. The default is 'yes'.
Cipher
Specifies the cipher to use for encrypting the session in protocol version 1. Currently, 'blowfish', '3des', and 'des' are supported. des is onlysupported in the ssh(1) client for interoperability with legacy protocol 1 implementations that do not support the 3des cipher. Its use is stronglydiscouraged due to cryptographic weaknesses. The default is '3des'.
Ciphers
Specifies the ciphers allowed for protocol version 2 in order of preference. Multiple ciphers must be comma-separated. The supported ciphers are '3des-cbc','aes128-cbc', 'aes192-cbc', 'aes256-cbc', 'aes128-ctr', 'aes192-ctr', 'aes256-ctr', 'arcfour128', 'arcfour256', 'arcfour', 'blowfish-cbc',and 'cast128-cbc'. The default is:
aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
aes256-cbc,arcfour
Compression
Specifies whether to use compression. The argument must be 'yes' or 'no'. The default is 'no'.
CompressionLevel
Specifies the compression level to use if compression is enabled. The argument must be an integer from 1 (fast) to 9 (slow, best). The default level is 6,which is good for most applications. The meaning of the values is the same as in gzip(1). Note that this option applies to protocol version 1 only.
ConnectionAttempts
Specifies the number of tries (one per second) to make before exiting. The argument must be an integer. This may be useful in scripts if the connectionsometimes 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 thetarget 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 socketspecified using the ControlPath argument. Additional sessions can connect to this socket using the same ControlPath with ControlMaster setto 'no' (the default). These sessions will try to reuse the master instance's network connection rather than initiating new ones, but will fall back toconnecting normally if the control socket does not exist, or is not listening.
Setting this to 'ask' will cause ssh to listen for control connections, but require confirmation using the SSH_ASKPASS program before they are accepted(see ssh-add(1) for details). If the ControlPath cannot be opened, ssh will continue without connecting to a master instance.
X11 and ssh-agent(1) forwarding is supported over these multiplexed connections, however the display and agent forwarded will be the one belonging to themaster 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 alreadyexist. 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 disableconnection sharing. In the path, '%l' will be substituted by the local host name, '%h' will be substituted by the target host name, '%p' the port, and '%r' bythe remote login username. It is recommended that any ControlPath used for opportunistic connection sharing include at least %h, %p, and %r. Thisensures that shared connections are uniquely identified.
DynamicForward
Specifies that a TCP port on the local machine be forwarded over the secure channel, and the application protocol is then used to determine where to connect tofrom the remote machine.
The argument must be [
bind_address:]port. IPv6 addresses can be specified by enclosing addresses in square brackets or by using an alternative syntax: [
bind_address/]port. By default, the local port is bound in accordance with the GatewayPorts setting. However, an explicit bind_addressmay be used to bind the connection to a specific address. The bind_address of 'localhost' indicates that the listening port be bound for local useonly, 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 additionalforwardings 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) duringHostbasedAuthentication. The argument must be 'yes' or 'no'. The default is 'no'. This option should be placed in the non-hostspecific section.See ssh-keysign(8) for more information.
EscapeChar
Sets the escape character (default: '~'). The escape character can also be set on the command line. The argument should be a single character, '^' followed bya 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. The argumentmust be 'yes' or 'no'. The default is 'no'.
ForwardAgent
Specifies whether the connection to the authentication agent (if any) will be forwarded to the remote machine. The argument must be 'yes' or 'no'. Thedefault is 'no'.
Agent forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the agent's Unix-domain socket)can access the local agent through the forwarded connection. An attacker cannot obtain key material from the agent, however they can perform operations on thekeys that enable them to authenticate using the identities loaded into the agent.
ForwardX11
Specifies whether X11 connections will be automatically redirected over the secure channel and DISPLAY set. The argument must be 'yes' or 'no'. The defaultis 'no'.
X11 forwarding should be enabled with caution. Users with the ability to bypass file permissions on the remote host (for the user's X11 authorizationdatabase) can access the local X11 display through the forwarded connection. An attacker may then be able to perform activities such as keystroke monitoring ifthe ForwardX11Trusted option is also enabled.
ForwardX11Trusted
If this option is set to 'yes', remote X11 clients will have full access to the original X11 display.
If this option is set to 'no', remote X11 clients will be considered untrusted and prevented from stealing or tampering with data belonging to trusted X11clients. Furthermore, the xauth(1) token used for the session will be set to expire after 20 minutes. Remote clients will be refused access after this time.
The default is 'no'.
See the X11 SECURITY extension specification for full details on the restrictions imposed on untrusted clients.
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. Thisprevents other remote hosts from connecting to forwarded ports. GatewayPorts can be used to specify that ssh should bind local port forwardings to thewildcard address, thus allowing remote hosts to connect to forwarded ports. The argument must be 'yes' or 'no'. The default is 'no'.
GlobalKnownHostsFile
Specifies a file to use for the global host key database instead of /etc/ssh/ssh_known_hosts.
GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is allowed. The default is 'no'. Note that this option applies to protocol version 2 only.
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'. Notethat this option applies to protocol version 2 only.
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 identitywill be used.
GSSAPIDelegateCredentials
Forward (delegate) credentials to the server. The default is 'no'. Note that this option applies to protocol version 2 connections using GSSAPI.
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 delegatethe 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 canonicalize' the name of the host being connected to. If 'no, the hostname entered on the'command line will be passed untouched to the GSSAPI library. The default is 'no'. This option only applies to protocol version 2 connections using GSSAPI.
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 disclosed. The default is 'no'. Note that existing names andaddresses in known hosts files will not be converted automatically, but may be manually hashed using ssh-keygen(1).
HostbasedAuthentication
Specifies whether to try rhosts based authentication with public key authentication. The argument must be 'yes' or 'no'. The default is 'no'. This optionapplies to protocol version 2 only and is similar to RhostsRSAAuthentication.
HostKeyAlgorithms
Specifies the protocol version 2 host key algorithms that the client wants to use in order of preference. The default for this option is: 'ssh-rsa,ssh-dss'.
HostKeyAlias
Specifies an alias that should be used instead of the real host name when looking up or saving the host key in the host key database files. This option isuseful for tunneling SSH connections or for multiple servers running on a single host.
HostName
Specifies the real host name to log into. This can be used to specify nicknames or abbreviations for hosts. The default is the name given on the command line.Numeric IP addresses are also permitted (both on the command line and in HostName specifications).
IdentitiesOnly
Specifies that ssh(1) should only use the authentication identity files configured in the ssh_config files, even if ssh-agent(1) offers more identities.The argument to this keyword must be 'yes' or 'no'. This option is intended for situations where ssh-agent offers many different identities. The default is'no'.
IdentityFile
Specifies a file from which the user's RSA or DSA authentication identity is read. The default is ~/.ssh/identity for protocol version 1, and~/.ssh/id_rsa and ~/.ssh/id_dsa for protocol version 2. Additionally, any identities represented by the authentication agent will be used forauthentication.
The file name may use the tilde syntax to refer to a user's home directory or one of the following escape characters: '%d' (local user's home directory),'%u' (local user name), '%l' (local host name), '%h' (remote host name) or '%r' (remote user name).
It is possible to have multiple identity files specified in configuration files; all these identities will be tried in sequence.
KbdInteractiveAuthentication
Specifies whether to use keyboard-interactive authentication. The argument to this keyword must be 'yes' or 'no'. The default is 'yes'.
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 serverspecified list. The methods available vary depending on what the server supports. For an OpenSSH server, it may be zero or more of: 'bsdauth', 'pam', and'skey'.
LocalCommand
Specifies a command to execute on the local machine after successfully connecting to the server. The command string extends to the end of the line, and isexecuted with the user's shell. The following escape character substitutions will be performed: '%d' (local user's home directory), '%h' (remote host name),'%l' (local host name), '%n' (host name as provided on the command line), '%p' (remote port), '%r' (remote user name) or '%u' (local user name). This directiveis 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 argumentmust be [
bind_address:]port and the second argument must be host:hostport. IPv6 addresses can be specified by enclosing addresses in squarebrackets or by using an alternative syntax: [
bind_address/]port and host/hostport. Multiple forwardings may be specified, and additional forwardings can be given on the commandline. Only the superuser can forward privileged ports. By default, the local port is bound in accordance with the GatewayPorts setting. However, anexplicit bind_address may be used to bind the connection to a specific address. The bind_address of 'localhost' indicates that the listeningport 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 in protocol version 2 for dataintegrity protection. Multiple algorithms must be comma-separated. The default is:
hmac-md5,hmac-sha1,umac-64@openssh.com,
hmac-ripemd160,hmac-sha1-96,hmac-md5-96
NumberOfPasswordPrompts
Specifies the number of password prompts before giving up. The argument to this keyword must be an integer. The default is 3.
PasswordAuthentication
Specifies whether to use password authentication. The argument to this keyword must be 'yes' or 'no'. The default is 'yes'.
PermitLocalCommand
Allow local command execution via the LocalCommand option or using the !command escape sequence in ssh(1). The argument must be 'yes' or'no'. The default is 'no'.
Port' Specifies the port number to connect on the remote host. The default is 22.
PreferredAuthentications
Specifies the order in which the client should try protocol 2 authentication methods. This allows a client to prefer one method (e.g.keyboard-interactive) over another method (e.g. password) The default for this option is: '
gssapi-with-mic, hostbased, publickey, keyboard-interactive, password'.
Protocol
Specifies the protocol versions ssh(1) should support in order of preference. The possible values are '1' and '2'. Multiple versions must be comma-separated.The default is '2,1'. This means that ssh tries version 2 and falls back to version 1 if version 2 is not available.
ProxyCommand
Specifies the command to use to connect to the server. The command string extends to the end of the line, and is executed with the user's shell. In the commandstring, '%h' will be substituted by the host name to connect and '%p' by the port. The command can be basically anything, and should read from its standardinput and write to its standard output. It should eventually connect an sshd(8) server running on some machine, or execute sshd -i somewhere. Host keymanagement will be done using the HostName of the host being connected (defaulting to the name typed by the user). Setting the command to 'none' disablesthis 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
Specifies whether to try public key authentication. The argument to this keyword must be 'yes' or 'no'. The default is 'yes'. This option applies toprotocol version 2 only.RekeyLimit
Specifies the maximum amount of data that may be transmitted before the session key is renegotiated. The argument is the number of bytes, with an optionalsuffix of 'K', 'M', or 'G' to indicate Kilobytes, Megabytes, or Gigabytes, respectively. The default is between '1G' and '4G', depending on the cipher. Thisoption applies to protocol version 2 only.
RemoteForward
Specifies that a TCP port on the remote machine be forwarded over the secure channel to the specified host and port from the local machine. The first argumentmust be [
bind_address:]port and the second argument must be host:hostport. IPv6 addresses can be specified by enclosing addresses in squarebrackets or by using an alternative syntax: [
bind_address/]port and host/hostport. Multiple forwardings may be specified, and additional forwardings can be given on the commandline. 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, thenthe forwarding is requested to listen on all interfaces. Specifying a remote bind_address will only succeed if the server's GatewayPorts optionis enabled (see sshd_config(5)).
RhostsRSAAuthentication
Specifies whether to try rhosts based authentication with RSA host authentication. The argument must be 'yes' or 'no'. The default is 'no'. This optionapplies to protocol version 1 only and requires ssh(1) to be setuid root.
RSAAuthentication
Specifies whether to try RSA authentication. The argument to this keyword must be 'yes' or 'no'. RSA authentication will only be attempted if the identityfile exists, or an authentication agent is running. The default is 'yes'. Note that this option applies to protocol version 1 only.
SendEnv
Specifies what variables from the local environ(7) should be sent to the server. Note that environment passing is only supported for protocol 2. The servermust also support it, and the server must be configured to accept these environment variables. Refer to AcceptEnv in sshd_config(5) for how to configurethe server. Variables are specified by name, which may contain wildcard characters. Multiple environment variables may be separated by whitespace or spreadacross 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 reachedwhile 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 alivemessages is very different from TCPKeepAlive (below). The server alive messages are sent through the encrypted channel and therefore will not bespoofable. The TCP keepalive option enabled by TCPKeepAlive is spoofable. How to remove interlace lines from video in premiere. The server alive mechanism is valuable when the client or server depend onknowing 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 theserver becomes unresponsive, ssh will disconnect after approximately 45 seconds. This option applies to protocol version 2 only.
ServerAliveInterval
Sets a timeout interval in seconds after which if no data has been received from the server, ssh(1) will send a message through the encrypted channel torequest a response from the server. The default is 0, indicating that these messages will not be sent to the server. This option applies to protocol version 2only.
SmartcardDevice
Specifies which smartcard device to use. The argument to this keyword is the device ssh(1) should use to communicate with a smartcard used for storing theuser's private RSA key. By default, no device is specified and smartcard support is not activated.
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 hostkey has changed. This provides maximum protection against trojan horse attacks, though it can be annoying when the /etc/ssh/ssh_known_hosts file ispoorly 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'no', ssh will automatically add new host keys to the user known hosts files. If this flag is set to 'ask', new host keys will be added to the user knownhost 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 hostkeys of known hosts will be verified automatically in all cases. The argument must be 'yes', 'no', or 'ask'. The default is 'ask'.
TCPKeepAlive
Specifies whether the system should send TCP keepalive messages to the other side. If they are sent, death of the connection or crash of one of the machineswill be properly noticed. 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 inscripts, 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'. Specifying 'yes' requests the default tunnel mode, which is 'point-to-point'. The default is 'no'.
TunnelDevice
Specifies the tun(4) devices to open on the client (local_tun) and the server (remote_tun).
The argument must be local_tun[:remote_tun]. The devices may be specified by numerical ID or the keyword 'any', which uses the nextavailable tunnel device. If remote_tun is not specified, it defaults to 'any'. The default is 'any:any'.
UsePrivilegedPort
Specifies whether to use a privileged port for outgoing connections. The argument must be 'yes' or 'no'. The default is 'no'. If set to 'yes', ssh(1)must be setuid root. Note that this option must be set to 'yes' for RhostsRSAAuthentication with older servers.
User' Specifies the user to log in as. This can be useful when a different user name is used on different machines. This saves the trouble of havingto remember to give the user name on the command line.
UserKnownHostsFile
Specifies a file to use for the user host key database instead of ~/.ssh/known_hosts.
VerifyHostKeyDNS
Specifies whether to verify the remote key using DNS and SSHFP resource records. If this option is set to 'yes', the client will implicitly trust keys thatmatch 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', informationon fingerprint match will be displayed, but the user will still need to confirm new host keys according to the StrictHostKeyChecking option. Theargument must be 'yes', 'no', or 'ask'. The default is 'no'. Note that this option applies to protocol version 2 only.
See also VERIFYING HOST KEYS in ssh(1).
VisualHostKey
If this flag is set to 'yes', an ASCII art representation of the remote host key fingerprint is printed in addition to the hex fingerprint string at loginand for unknown host keys. If this flag is set to 'no', no fingerprint strings are printed at login and only the hex fingerprint string will be printed forunknown host keys. The default is 'no'.
XAuthLocation
Specifies the full pathname of the xauth How to get virtual dj pro for free. (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 matchesexactly one character). For example, to specify a set of declarations for any host in the '.co.uk' set of domains, the following pattern could beused:
- Host 192.168.0.?
A pattern-list is a comma-separated list of patterns. Patterns within pattern-lists may be negated by preceding them with an exclamation mark ('!').For example, to allow a key to be used from anywhere within an organisation except from the 'dialup' pool, the following entry (in authorized_keys) could beused:
Files
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 forabuse, 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 donot have a configuration file. This file must be world-readable.
See Also
ssh(1)
Authors
OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt andDug 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.
BSD April 14, 2013 BSD
Referenced By
brlapi_tty(3),darcs(1),duplicity(1),gsissh(1),gsissh-keysign(8),gsisshd(8),gsisshd_config(5),pssh(1),task-sync(5)sshd_config(4)
Name
sshd_config - sshd configuration fileSynopsis
Description
The sshd(1M) daemon reads configuration data from /etc/ssh/sshd_config (or the file specified with sshd–f on the command line). The file contains keyword-value pairs,one per line. A line starting with a hash mark (#)and empty lines are interpreted as comments.
The sshd_config file supports the followingkeywords. Unless otherwise noted, keywords and their arguments arecase-insensitive.
This keyword can be followed by a number of groupnames, separated by spaces. If specified, login is allowed only forusers whose primary group or supplementary group list matches oneof the patterns. Asterisk (*) and question mark(?) can be used as wildcards in the patterns. Onlygroup names are valid; a numerical group ID is not recognized. Bydefault, login is allowed regardless of the primary group.
Specifies whether TCP forwarding is permitted. Thedefault is yes. Disabling TCP forwarding does notimprove security unless users are also denied shell access, as theycan always install their own forwarders.
This keyword can be followed by a number of user names,separated by spaces. If specified, login is allowed only for usernames that match one of the patterns. Asterisk (*)and question mark (?) can be used as wildcardsin the patterns. Only user names are valid; a numerical user ID isnot recognized. By default login is allowed regardless of the username.
If a specified pattern takes the form user@host then user and host are checked separately, restricting logins to particularusers from particular hosts.
Specifies the file that contains the public keys thatcan be used for user authentication. AuthorizedKeysFile cancontain tokens of the form %T, which are substitutedduring connection set-up. The following tokens are defined: %% isreplaced by a literal %, %h isreplaced by the home directory of the user being authenticated and %u is replaced by the username of that user. After expansion, AuthorizedKeysFile is taken to be an absolute path or onerelative to the user's home directory. The default is .ssh/authorized_keys.
In some jurisdictions, sending a warning message beforeauthentication can be relevant for getting legal protection. The contentsof the specified file are sent to the remote user before authenticationis allowed. This option is only available for protocol version 2.By default, no banner is displayed.
Specifies a path to chroot(2) to after authentication.This path, and all its components, must be root-owned directoriesthat are not writable by any other user or group.
The server always tries to change to the user's home directorylocally under the chrooted environment but a failure to do so is notconsidered an error. In addition, the path might contain the followingtokens that are expanded at runtime once the connecting user has beenauthenticated: %% is replaced by a literal %, %h is replaced by the home directory of the user being authenticated,and %u is replaced by the username of that user.
The ChrootDirectory must contain the necessaryfiles and directories to support the user's session. For an interactiveSSH session this requires at least a user's shell, shared librariesneeded by the shell, dynamic linker, and possibly basic /dev nodessuch as null, zero, stdin, stdout, stderr, random,and tty. Additionally, terminal databases are neededfor screen oriented applications. For file transfer sessions using sftp with the SSH protocol version 2, no additional configurationof the environment is necessary if the in-process sftp serveris used. See Subsystem for details.
The default is not to chroot(2).
Specifies the ciphers allowed for protocol version2. Cipher ordering on the server side is not relevant. Multiple ciphersmust be comma separated.
Valid ciphers are: aes128-ctr, aes192-ctr, aes256-ctr,aes128-cbc, aes192-cbc, aes256-cbc, arcfour, arcfour128, arcfour256,3des-cbc, and blowfish-cbc.
The default cipher list is:
Using CBC modes on the server side is not recommended due topotential security issues in connection with the SSH protocol version2.
When configuring sshd to run OpenSSL in FIPS-140mode, the default cipher list is: aes128-cbc, aes192-cbc, aes256-cbc. Specifying anon-FIPS approved cipher will return an error.
For the case of ssh with FIPS-140 enabledlogging into a non-FIPS-140 sshd, the supportedand approved FIPS ciphers must be explicitly specified in sshd_config using “Ciphers” for this scenario.
Sets the number of client alive messages, (see ClientAliveInterval), that can be sent without sshd receivingany messages back from the client. If this threshold is reached whileclient alive messages are being sent, sshd disconnectsthe client, terminating the session. The use of client alive messagesis very different from KeepAlive. The client alivemessages are sent through the encrypted channel and therefore arenot spoofable. The TCP keepalive option enabledby KeepAlive is spoofable. The client alive mechanismis valuable when a client or server depend on knowing when a connectionhas become inactive.
The default value is 3. If ClientAliveInterval isset to 15, and ClientAliveCountMax is left at thedefault, unresponsive ssh clients are disconnectedafter approximately 45 seconds.
Sets a timeout interval in seconds after which, ifno data has been received from the client, sshd sendsa message through the encrypted channel to request a response fromthe client. The default is 0, indicating that these messages are notsent to the client. This option applies only to protocol version 2.
Controls whether the server allows the client to negotiatethe use of compression. The default is yes.
Can be followed by a number of group names, separatedby spaces. Users whose primary or supplementary group matches oneof the patterns are not allowed to log in. Asterisk (*)and question mark (?) can be used as wildcardsin the patterns. Only group names are valid; a numerical group IDis not recognized. By default, login is allowed regardless of theprimary group.
Can be followed by a number of user names, separatedby spaces. Login is disallowed for user names that match one of thepatterns. Asterisk (*) and question mark (?)can be used as wildcards in the patterns. Only user names are valid;a numerical user ID is not recognized. By default, login is allowedregardless of the user name.
If a specified pattern takes the form user@host then user and host are checked separately, disallowing logins to particularusers from particular hosts.
Forces the execution of the command specified by ForceCommand, ignoring any command supplied by the client,and, if present, ~/.ssh/rc. The command is invokedby using the user's login shell with the –c option.This applies to shell, command, or subsystem execution. It is mostuseful inside a Match block. The command originally supplied by theclient is available in the SSH_ORIGINAL_COMMAND environmentvariable. Specifying a command of internal-sftp forcesthe use of an in-process sftp server that requiresno support files when used with ChrootDirectory.
Specifies whether remote hosts are allowed to connectto ports forwarded for the client. By default, sshd bindsremote port forwardings to the loopback address. This prevents otherremote hosts from connecting to forwarded ports. GatewayPorts canbe used to specify that sshd should bind remoteport forwardings to the wildcard address, thus allowing remote hoststo connect to forwarded ports.
The argument can be no to force remote portforwardings to be available to the local host only, yes toforce remote port forwardings to bind to the wildcard address, or clientspecified to allow the client to select the addressto which the forwarding is bound. The default is no.See also RemoteForward in ssh_config(4).
Enables/disables GSS-API user authentication. Thedefault is yes.
Currently sshd authorizes client user principalsto user accounts as follows: if the principal name matches the requesteduser account, then the principal is authorized. Otherwise, GSS-APIauthentication fails.
Enables/disables GSS-API-authenticated key exchanges.The default is yes.
This option also enables the use of the GSS-API to authenticatethe user to server after the key exchange. GSS-API key exchange cansucceed but the subsequent authentication using the GSS-API fail ifthe server does not authorize the user's GSS principal name to thetarget user account.
Currently sshd authorizes client user principalsto user accounts as follows: if the principal name matches the requesteduser account, then the principal is authorized. Otherwise, GSS-APIauthentication fails.
Enables/disables the use of delegated GSS-API credentialson the server-side. The default is yes.
Specifically, this option, when enabled, causes the server tostore delegated GSS-API credentials in the user's default GSS-APIcredential store (which for the Kerberos V mechanism means /tmp/krb5cc_<uid>).
sshd does not take any steps to explicitlydestroy stored delegated GSS-API credentials upon logout. It is theresponsibility of PAM modules to destroy credentials associated witha session.
Default Sshd_config Ubuntu 18.04
Specifies whether to try rhosts-basedauthentication with public key authentication. The argument must be yes or no. The default is no.This option applies to protocol version 2 only and is similar to RhostsRSAAuthentication. See sshd(1M) for guidelines onsetting up host-based authentication.
Controls which hostname is searched for in the files ~/.shosts, /etc/shosts.equiv, and /etc/hosts.equiv. If this parameter is set to yes,the server uses the name the client claimed for itself and signedwith that host's key. If set to no, the default,the server uses the name to which the client's IP address resolves.
Setting this parameter to no disables host-basedauthentication when using NAT or when the client gets to the serverindirectly through a port-forwarding firewall.
Specifies the file containing the private host keyused by SSH. The default is /etc/ssh/ssh_host_rsa_key and/etc/ssh/ssh_host_dsa_key for protocol version 2. sshd refusesto use a file if it is group/world-accessible. It is possible to have multiple host key files. dsa or rsa keys are used for version2 of the SSH protocol.
Specifies that .rhosts and .shosts files are not used in authentication. /etc/hosts.equiv and /etc/shosts.equiv are still used.The default is yes.
Specifies whether sshd should ignorethe user's $HOME/.ssh/known_hosts during RhostsRSAAuthentication. The default is no.
Specifies whether authentication by means of the keyboard-interactive authentication method is allowed. Defaultsto yes
Specifies whether the system should send keepalive messages to the other side. If they are sent, death ofthe connection or crash of one of the machines is properly noticed.However, this means that connections die if the route is down temporarily,which can be an annoyance. On the other hand, if keepalives arenot sent, sessions can hang indefinitely on the server, leaving ghostusers and consuming server resources.
The default is yes (to send keepalives),and the server notices if the network goes down or the client hostreboots. This avoids infinitely hanging sessions.
To disable keepalives, the value should beset to no in both the server and the client configurationfiles.
A filename for the KMF policy database. If not set,KMF defaults to /etc/security/kmfpolicy.xml.See the Using X.509 Certificates section in the sshd(1M) man page.
A name of the KMF policy to be used. If not set, default is used. See the Using X.509 Certificates sectionin the sshd(1M) man page.
Specifies what local address sshd shouldlisten on. The following forms can be used:
If port is not specified, sshd listenson the address and all prior Port options specified.The default is to listen on all local addresses. Multiple ListenAddress options are permitted. Additionally, any Port optionsmust precede this option for non-port qualified addresses.
The default is to listen on all local addresses. Multiple optionsof this type are permitted. Additionally, the Ports optionsmust precede this option.
The server disconnects after this time (in seconds)if the user has not successfully logged in. If the value is 0, thereis no time limit. The default is 120 (seconds).
Gives the verbosity level that is used when loggingmessages from sshd. The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.The default is INFO. DEBUG2 and DEBUG3 each specifyhigher levels of debugging output. Logging with level DEBUG violatesthe privacy of users and is not recommended.
Specifies whether or not to reverse lookup the namesof client's addresses. Setting this parameter to no canbe useful where name resolution might be broken and thus cause sshd to spend a lot of time trying to resolve the client's IPaddress to a name. Defaults to yes. See VerifyReverseMapping.
Specifies the available MAC (message authentication code) algorithms. The MAC algorithm isused in protocol version 2 for data integrity protection. Multiple algorithms must becomma-separated. The default is hmac-md5,hmac-sha1,hmac-sha1-96,hmac-md5-96, hmac-sha2-256,hmac-sha2-256-96, hmac-sha2-512, and hmac-sha2-512-96.
When configuring sshd to run OpenSSL in FIPS-140mode, the default is hmac-sha1, hmac-sha1-96, hmac-sha2-256, hmac-sha2-256-96, hmac-sha2-512, and hmac-sha2-512-96. Specifying anon-FIPS approved MAC algorithm will return an error.
Introduces a conditional block. If all of the criteriaon the Match line are satisfied, the keywords on the following linesoverride those set in the global section of the config file,until either another Match line or the end of the file. Match blocksmust be located at the end of the file, after all the global settings.
The arguments to Match are one or more criteria-pattern pairs.The available criteria are User, Group, Host, and Address. The match patternscan consist of single entries or comma-separated lists and can usethe wildcard (Asterisk * and question mark ?)and negation (!) operators.
The patterns in a Host criteria should be hostname. The patternsin an Address criteria should be an IP address, which can additionallycontain addresses to match in CIDR address/masklen format, for example, 192.0.2.0/24 or 2001:DB8::/32. The masklength provided must be consistent with the address - it is an errorto specify a mask length that is too long for the address or one withbits set in this host portion of the address. For example, 192.0.2.0/33 and 192.0.2.0/8 respectively.
Only a subset of keywords can be used on the lines followinga Match keyword. Available keywords are AllowTcpForwarding,AuthorizedKeysFile, Banner, ChrootDirectory, ForceCommand, GatewayPorts,GSSAPIAuthentication, HostbasedAuthentication, HostbasedUsesNameFromPacketOnly,KbdInteractiveAuthentication,MaxAuthTries, PasswordAuthentication, PermitEmptyPasswords, PermitRootLogin,PubkeyAuthentication, RhostsRSAAuthentication, RSAAuthentication,X11DisplayOffset, X11Forwarding, and X11UseLocalhost.
The following are four examples of using Match:
Disallowing user testuser to useTCP forwarding:
Displaying a special banner for users not in the staff group:
Allowing root login from host rootallowed.example.com:
Allowing anyone to use GatewayPorts fromthe local net:
Specifies the maximum number of concurrent unauthenticatedconnections to the sshd daemon. Additional connectionsare dropped until authentication succeeds or the LoginGraceTime expiresfor a connection. The default is 10:30:100.
Alternatively, random early drop can be enabled by specifyingthe three colon-separated values start:rate:full (forexample, 10:30:60). Referring to this example, sshd refuse connection attempts with a probability of rate/100 (30% in our example) if there are currently10 (from the start field) unauthenticatedconnections. The probability increases linearly and all connectionattempts are refused if the number of unauthenticated connectionsreaches full (60 in our example).
The default is 10:30:100.
Specifies the PAM service name for the PAM session.The PAMServiceName and PAMServicePrefix optionsare mutually exclusive and if both set, sshd doesnot start. If this option is set the service name is the same forall user authentication methods. The option has no default value.See PAMServicePrefix for more information.
Specifies the PAM service name prefix for servicenames used for individual user authentication methods. The defaultis sshd. The PAMServiceName and PAMServicePrefix options are mutually exclusive and if bothset, sshd does not start.
For example, if this option is set to admincli,the service name for the keyboard-interactive authentication methodis admincli-kbdint instead of the default sshd-kbdint.
Specifies whether password authentication is allowed.The default is yes.
When password or keyboard-interactive authenticationis allowed, it specifies whether the server allows login to accountswith empty password strings.
If not set then the /etc/default/loginPASSREQ value is used instead.
PASSREQ=no is equivalent to PermitEmptyPasswordsyes. PASSREQ=yes is equivalent to PermitEmptyPasswordsno. If neither PermitEmptyPasswords or PASSREQ are set the default is no.
Specifies whether the root can log in using ssh(1). The argument mustbe yes, without-password, forced-commands-only, or no. without-password means that root cannot be authenticated using the “password”or “keyboard-interactive” methods (see description of KbdInteractiveAuthentication). forced-commands-only meansthat authentication is allowed only for publickey (forSSHv2, or RSA, for SSHv1) and only if the matching authorized_keysentry for root has a command=<cmd> option.
In Solaris, the default /etc/ssh/sshd_config fileis shipped with PermitRootLogin set to no.If unset by the administrator, then CONSOLE parameterfrom /etc/default/login supplies the defaultvalue as follows: if the CONSOLE parameter is notcommented out (it can even be empty, that is, “CONSOLE=”),then without-password is used as default value.If CONSOLE is commented out, then the default for PermitRootLogin is yes.
The without-password and forced-commands-only settings are useful for, for example, performing remoteadministration and backups using trusted public keys for authenticationof the remote client, without allowing access to the root accountusing passwords.
Specifies whether a user's ~/.ssh/environment onthe server side and environment options in the AuthorizedKeysFile file are processed by sshd.The default is no. Enabling environment processingcan enable users to bypass access restrictions in some configurationsusing mechanisms such as LD_PRELOAD.
Environment setting from a relevant entry in AuthorizedKeysFile file is processed only if the user was authenticated usingthe public key authentication method. Of the two files used, valuesof variables set in ~/.ssh/environment are ofhigher priority.
Allows you to specify an alternative to /var/run/sshd.pid, the default file for storing the PID of the sshd listeningfor connections. See sshd(1M).
Specifies the port number that sshd listenson. The default is 22. Multiple options of this type are permitted.See also ListenAddress.
Specifies an executable which is run prior to anyof the processed authentication methods. The executable can be usedto synchronize user information with a remote user-management facilityusing an arbitrary communication protocol.
The executable is run before any user validation is conductedby SSHD so the user is not required to be existentbefore she tries to log in.
The executable is invoked with two arguments in the followingorder: the name of the current authentication method and the username.The environment variable SSH_CONNECTION is alsopassed to the executable. If the executable returns a zero exit status,the current authentication method is processed as normal. See sshd(1M).
If the exit status is 1, the current authenticationmethod is ignored and can not be used to validate the user. The executablemust be owned by root and have permissions of 0500,otherwise it is treated as if it has exited with status 1.
There is no default value for this property.
Specifies whether sshd should displaythe date and time when the user last logged in. The default is yes.
Specifies whether sshd should displaythe contents of /etc/motd when a user logs ininteractively. (On some systems it is also displayed by the shellor a shell startup file, such as /etc/profile.)The default is yes.
Specifies the protocol versions sshd shouldsupport in order of preference. The option is left for backwards compatibilityand only value 2 is supported. The default is 2.
Specifies whether public key authentication is allowed.The default is yes. This option applies to protocolversion 2 only.
Specifies whether sshd should checkfile modes and ownership of the user's files and home directory beforeaccepting login. This is normally desirable because novices sometimesaccidentally leave their directory or files world-writable. The defaultis yes.
Configures an external subsystem (for example, a filetransfer daemon). Arguments should be a subsystem name and a commandto execute upon subsystem request. The command sftp-server(1M) implements the sftp file transfer subsystem.
Alternately, the name internal-sftp implementsan in-process sftp server. This can simplify configurationsusing ChrootDirectory to force a different filesystemroot on clients.
To specify an option to the internal sftp server,simply add the sftp-server option to the end ofline. For example,
By default, no subsystems are defined. This option applies toprotocol version 2 only.
Specifies a directory where certificates of trustedanchors are located. Those certificates are used to validate hostcertificates if used as host keys.
Sshd Pubkeyauthentication Default
Currently only one level certificate chains are supported. Thismeans that certificates must be signed by a private key that correspondsto a certificate located in the directory set by this option. Hostkey certificates can be self-signed as well. See the UsingX.509 Certificates section in the sshd(1M) man page.
Gives the facility code that is used when loggingmessages from sshd. The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, and LOCAL7. The default is AUTH.
Specifies whether sshd will runOpenSSL in FIPS-140 mode. The default is no. A yes setting will allow sshd to use theFIPS capable crypto modules in OpenSSL.
Specifies whether sshd should usethe OpenSSL PKCS#11 engine for offloading cryptographic operationsto the Cryptographic Framework. Cryptographic operations are acceleratedaccording to the available installed plug-ins. When no suitable plug-insare present this option does not have an effect. The default is yes.
2015dn driver. If sshd is configured to run OpenSSL in FIPS-140mode, the default is no. The setting of UseOpenSSLEngine to yes does not have an effect in FIPS-140mode.
Specifies whether sshd should tryto verify the remote host name and check that the resolved host namefor the remote IP address maps back to the very same IP address.
A yes setting means verify. This featureis useful for Internet-facing servers. The default is no.
The option is only usable if LookupClientHostnames isset to yes.
Specifies the first display number available for sshd's X11 forwarding. This prevents sshd frominterfering with real X11 servers. The default is 10.
Specifies whether X11 forwarding is permitted. Thedefault is yes. Disabling X11 forwarding does notimprove security in any way, as users can always install their ownforwarders.
When X11 forwarding is enabled, there can be additional exposureto the server and to client displays if the sshd proxydisplay is configured to listen on the wildcard address (see X11UseLocalhost). However, this is not the default. Additionally, the authenticationspoofing and authentication data verification and substitution occuron the client side. The security risk of using X11 forwarding is thatthe client's X11 display server can be exposed to attack when the ssh client requests forwarding (see the warnings for ForwardX11 in ssh_config(4)). A system administrator who wants to protectclients that expose themselves to attack by unwittingly requestingX11 forwarding, should specify a no setting.
Disabling X11 forwarding does not prevent users from forwardingX11 traffic, as users can always install their own forwarders.
Specifies whether sshd should bindthe X11 forwarding server to the loopback address or to the wildcardaddress. By default, sshd binds the forwardingserver to the loopback address and sets the hostname part of the DISPLAY environment variable to localhost. Thisprevents remote hosts from connecting to the proxy display. However,some older X11 clients might not function with this configuration. X11UseLocalhost can be set to no to specifythat the forwarding server should be bound to the wildcard address.The argument must be yes or no.The default is yes.
Specifies the location of the xauth(1) program. The default is /usr/X11/bin/xauth and sshd attempts to open it when X11 forwarding is enabled.
Time Formats
sshd command-line arguments and configurationfile options that specify time can be expressed using a sequence ofthe form: time[qualifier,]where time is a positive integer valueand qualifier is one of the following:
seconds
seconds
minutes
hours
days
weeks
Each element of the sequence is added together to calculatethe total time value. For example:
600 seconds (10 minutes)
10 minutes
1 hour, 30 minutes (90 minutes)
Files
Contains configuration data for sshd.This file should be writable by root only, but it is recommended (thoughnot necessary) that it be world-readable.
Attributes
See attributes(5) for descriptions of the following attributes:
See also
Sshd_config Default Directory
login(1), sshd(1M), chroot(2), ssh_config(4), attributes(5), kerberos(5)
AUTHORS
AUTHORSOpen Sshd Config
OpenSSH is a derivative of the original and free ssh 1.2.12release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, NielsProvos, Theo de Raadt, and Dug Song removed many bugs, re-added recentfeatures, and created OpenSSH. Markus Friedl contributed the supportfor SSH protocol versions 1.5 and 2.0. Niels Provos and Markus Friedlcontributed support for privilege separation.
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